THE TUTORIALS NOTE: The blue links do not work on this version of The Tutorials. They only work on the CD version In the following section we will be looking at overviews of several pump and seal subjects: ● ● ● ●
Centrifugal pump selection. Centrifugal pump installation. Centrifugal pump modifications you can make to increase the pump's performance. Mechanical seal selection.
In these narratives I am attempting to put each of the subjects into perspective. You will use the narratives for multiple purposes: ● ● ● ●
To learn the terms we use for each of these individual subjects. To see how the various subjects fit together. To find out how much you know about any one of the subjects. And you can use the narratives as an outline to teach the subjects to other people.
I suggest that you read the entire narrative and then go back and look up the details of any unfamiliar words or subjects. Any word or phrase in blue and underlined is a link to a detailed explanation of the subject. Most of the time I have tried to use the link only for the first mention of the word or phrase; otherwise the narrative would be full of links. CENTRIFUGAL PUMP SELECTION. HOW TO PICK THE CORRECT SIZE PUMP FOR YOUR APPLICATION. We will begin by deciding what operating conditions our pump has to meet and then we will approach pump suppliers to see how closely they can satisfy these needs. Unfortunately no comprehensive theory which would permit the complete hydrodynamic design of a centrifugal pump has evolved in the many years that pumps have been around, so the pump manufacturer will be doing the best he can with the information you supply to him. To clearly define the capacity and pressure needs of our system we will construct a type of graph called a system curve. This system curve will then be given to the pump suppliers and they will try to match it with a pump curve that satisfies these needs as closely as possible. To start the construction of the system curve I will assume you want to pump some fluid from point "A" to point "B". To do that efficiently you must make a couple of decisions: ●
Decide the capacity you will need. This means the gallons per minute or cubic meters per hour. You must also consider if this capacity will change with the operation of your process. A boiler feed pump is an example of an application that needs a constant pressure with varying capacities
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to meet a changing steam demand. The demand for boiler water is regulated by opening and closing a control valve on the discharge side of the pump with a discharge re-circulation line returning the unneeded portion back to a convenient storage place, or the suction side of the pump. Remember that with a centrifugal pump if you change its capacity you change the pressure also. A rotary or positive displacement pump is different. It puts out a constant capacity regardless of the pressure. For other centrifugal pump applications, you are going to have to calculate how much pressure will be needed to deliver different capacities to the place where you will need them. You will need enough pressure to : ❍ Reach the maximum static head or height the fluid will have to attain. ❍ Enough discharge pressure to over come any pressure that might be in the vessel where the fluid is discharging, such as the boiler we just discussed. This is called the pressure head. ❍ Overcome friction resistance in the lines, fittings and any valves or hardware that might be in the system. As an example: high-pressure nozzles can be tricky, especially if they clog up. This resistance is called the friction head. Will you need any special materials for the pump components? ❍ The pump manufacturer will try to choose pump metal components that are chemically compatible with what you are pumping as well as any cleaners or solvents that might be flushed through the lines. If the temperature of the pumpage changes the corrosion rate can change also. His choice of materials could have a serious affect on your spare parts inventory. Will he be selecting universal and easily obtainable materials? Unless you have a great deal of experience with the product you are pumping do not select the metal components by using a compatibility chart. Metal selection is a job for metallurgists or your own experience. ❍ If the product you are pumping is explosive or a fire hazard, you should be looking at nonsparking materials for the pump components. Do not depend totally upon the pump manufacturer to make this decision for you. If you are not sure what materials are compatible with your product, how will the pump man know? Also, keep in mind that some of the fluids you will be pumping could be proprietary products known only by their trade name. ❍ Dangerous and radioactive materials will dictate special materials. ❍ Food products require high-density seal and pump materials that are easy to clean. ❍ If there are abrasive solids in the pumpage you will need materials with good wearing capabilities. Hard surfaces and chemically resistant materials are often incompatible. You may have to go to some type of coating on the pump wetted parts or select an expensive duplex metal. Occasionally you will find an application where metal is either not compatible or not practical. There are many monomer and polymer materials available for these applications, but their cost is generally higher than comparable metal parts. Be aware that if you are using a mechanical seal in a non-metallic pump, the seal cannot have metal parts in contact with the fluid for the same reasons the pump was manufactured from non-metallic materials. Use a non-metallic seal in these applications
Since we are just getting into the subject, one of the first things we should learn is that centrifugal pump people do not use the word pressure. As mentioned in an earlier paragraph they substitute the word
"head", so you will have to calculate the three kinds of head that will be combined together to give you the total head of the system required to deliver the needed capacity. Here are the three kinds of head you will be calculating: ●
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The static head or maximum height that the liquid will reach. We must also learn how to compensate for the siphon affect from down running pipes on the discharge side of the pump. Remember that if you fill a tank from the bottom instead of the top the static head will continually increase. This is not a good application for a centrifugal pump because the capacity is decreasing with an increasing head. If you must fill from the bottom, or if you will be using the pump as an accumulator, a rotary positive displacement pump will be your best choice as long as it can meet the needed capacities. The pressure heads are next if the container we are pumping to, or from, is pressurized. We will have to learn how to convert pressure units to head units because later on we will need this conversion knowledge to read the manufacturers pump curve. Pump gages are labeled in psi or bar. Pump curves are labeled in feet of head, or meters of head. The friction head is the last one that we will have to calculate. This head tells us how much friction or resistance head there is in both the suction and discharge piping, along with the fittings and valves in the piping system. And to make the job a little tougher this head changes dramatically as the pump capacity changes.
You will be calculating these heads on both the suction and discharge side of the pump. To get the total head you will subtract the suction head from the discharge head and that will be the head that the pump must produce to satisfy the application. It will become obvious in the calculations, but I should mention here, that if the suction head is a negative number, the suction and discharge heads will be added together to get the total head. If you subtract a minus number from a positive number you must add the numbers together. As an example: 4 - (-2) = + 6 The total head of a pump seldom remains static. There are a number of factors that can change the head of a pump while it is operating, and you should become familiar with most of them. All of this head information is calculated from piping, valve, and fitting, friction graphs you will find in the index.This head data will be plotted on a set of coordinates called a system curve. Since we will not be operating at a single point all of the time we will make the calculations for a range of different capacities and heads that we might expect to encounter. This range is described as the operating window we will need to satisfy the application. Making these calculations is not an exact science because the piping is seldom new, pipe inside diameters are not exact, and the graphs you will be consulting cannot compensate for corrosion and solids built up on the piping, valve and fitting walls. Life is never simple. This is the point where most people start adding in safety factors to compensate for some of the unknowns. These safety factors will almost always guarantee the selection of an oversized pump that will run off of its best efficiency point (BEP) most of the time. The final calculations are then plotted on the system curve that describes what the pump has to do to
satisfy the requirements of the application. You can learn to do all of this by referencing the following subjects: ● ● ●
Calculating the total head in metric units Calculating the total head in USCS (inch) units Making a system curve, S111
The pump manufacturer requires a certain amount of net positive suction head required (NPSHR) to prevent the pump from cavitating. He shows that number on his pump curve. When you look at the curve you will also note that the net positive suction head required (NPSHR) increases with any increase in the pump's capacity. You will also be calculating the net positive suction head available (NPSHA) to be sure that the pump you select will not cavitate. Cavitation is caused by cavities or bubbles in the fluid collapsing on the impeller and volute. In the pump business we recognize several different types of cavitation. : ● ● ● ● ●
Vaporization cavitation. Air ingestion cavitation. Internal recirculation cavitation. Flow turbulence cavitation. Vane Passing Syndrome cavitation.
Pump cavitation is recognized in several different ways ● ● ●
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We can hear cavitation because it sounds like the pump is pumping rocks or ball bearings. We can see the damage from cavitation on the pump's impeller and volute. The operator can sometimes tell if the pump is cavitating because of a reduction in the pump's capacity. The main problem with cavitation is that it shakes and bends the shaft causing both seal and bearing problems. We call all of this shaking and bending shaft deflection.
Remember that the net positive suction head required (NPSHR) number shown on the pump curve is for fresh water at 68° Fahrenheit (20°C) and not the fluid or combinations of fluids you will be pumping. When you make your calculations for net positive suction head available (NPSHA) the formula you will be using will adjust for the specific gravity of your fluid. ●
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In some cases you can reduce the NPSH required. This is especially true if you are pumping hot water or mixed hydrocarbons. You may have to install an inducer on the pump, add a booster pump, or go to a double suction pump design if you do not have enough net positive suction head available (NPSHA)
When the pump supplier has all of this in-exact information in his possession he can then hopefully
select the correct size pump and driver for the job. Since he wants to quote a competitive price he is now going to make some critical decisions: He might begin with the type of pump he will recommend: ●
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If the capacity were going to be very low he would recommend a rotary, or positive displacement (PD) pump. Between 25 and 500 gpm (5 m3 /hr - 115 m3/hr) he will probably select a single stage end suction centrifugal pump. It all depends upon the supplier. At higher capacities he may go to a double suction design with a wide impeller, two pumps in parallel or maybe a high-speed pump. You might need a high head, low capacity pump. The pump supplier has several options you should know about. Will he recommend a self-priming pump? These pumps remove air from the impeller eye and suction side of the pump. Some operating conditions dictate the need for a self-priming design. If you do not have a self-priming pump and you are on intermittent service, will priming become a problem the next time you start the pump? How will the pump be operated? ❍ If the pump is going to run twenty-four hours a day, seven days a week and you are not going to open and close valves; you will not need a heavy-duty pump. It is easy to select a pump that will run at its best efficiency point and at the best efficiency point (BEP) there is very little shaft displacement and vibration. ❍ Intermittent service is the more difficult application because of changing temperatures, vibration levels, thrust direction, etc. Intermittent pumps require a more robust, heavyduty design with a low L3/D4 shaft. How important is efficiency in your application? High efficiency is desirable, but you pay a price for efficiency in higher maintenance costs and a limited operating window. You should be looking for performance, reliability, and efficiency in that order. Too often the engineer specifies efficiency and loses the other two. The following designs solve some operation and maintenance problems, but their efficiency is lower than conventional centrifugal pumps. ❍ A magnetic drive or canned pump may be your best choice if you can live with the several limitations they impose. ❍ A vortex or slurry pump design may be needed if there are lot of solids or "stringy" material in the pumpage. ❍ A double volute centrifugal pump can eliminate many of the seal problems we experience when we operate off the pump's best efficiency point. The problem is trying to find a supplier that will supply one for your application. Although readily available for impellers larger than 14 inches (355 mm) in diameter they have become very scarce in the smaller diameters because of their less efficient design. The supplier should recommend a centerline design to avoid the problems caused by thermal expansion of the wet end if you are operating at temperatures over 200°F (100°C)? Will you need a volute or circular casing? Volute casings build a higher head; circular casing are used for low head and high capacity. Do you need a pump that meets a standard? ANSI, API, DIN, VDMA or ISO are some of the current standards. You should be aware of pump standards problems that contribute to premature
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seal and bearing failures. An ANSI (American National Standards Institute) standard back pullout design pump has many advantages but presents problems with mechanical seals when the impeller clearance is adjusted, unless you are purchasing cartridge seals. The decision to use either a single or multistage pump will be determined by the head the pump must produce to meet the capacities you need. Some suppliers like to recommend a high speed small pump to be competitive, other suppliers might recommend a more expensive low speed large pump to lessen NPSH and wear problems.
There are additional decisions that have to be made about the type of pump the supplier will recommend: ●
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Will the pump be supplied with a mechanical seal or packing? If the stuffing box is at negative pressure (vacuum) a seal will be necessary to prevent air ingestion. If he is going to supply a mechanical seal will he also supply an oversized stuffing box and any environmental controls that might be needed? Will he specify a jacketed stuffing box so that the temperature of the sealed fluid can be regulated? How does he intend to control the stuffing box temperature? Will he be using water, steam or maybe a combination of both? Electric heating is sometimes an option. How will the open or semi-open impeller be adjusted to the volute casing or back plate? Can the mechanical seal face loading be adjusted at the same time? If not, the seal face load will change and the seal life will be shortened. If the pump is going to be supplied with a closed impeller you should have some means of knowing when the wear rings have to be replaced. If the wear ring clearance becomes too large the pumps efficiency will be lowered causing heat and vibration problems. Most manufacturers require that you disassemble the pump to check the wear ring clearance and replace the rings when this clearance doubles. Will he supply a "C" or "D" frame adapter, or will the pump to motor alignment have to be done manually using dual indicators or a laser aligner to get the readings? A closed-coupled design can eliminate the need for an alignment between the pump and driver. What type of coupling will he select to connect the pump to its driver? Couplings can compensate for axial growth of the shaft and transmit torque to the impeller. They cannot compensate for pump to driver misalignment as much as we would like them to. Universal joints are especially bad because they have to be misaligned to be lubricated. He may decide to run two pumps in parallel operation if he needs a real high capacity, or two pumps in series operation if he needs a high head. Pumps that run in parallel or series require that they are running at the same speed. This can be a problem for some induction motors.. An inline pump design can solve many pipe strain and thermal growth problems. The pump supplier must insure that the pump will not be operating at a critical speed or passing through a critical speed at start up. If he has decided to use a variable speed drive or motor this becomes a possibility. We all want pumps with a low net positive suction head required to prevent cavitation problems but sometimes it is not practical. The manufacturer has the option of installing an inducer or altering the pump design to lower the net positive suction head required, but if he goes too far all of the internal clearances will have to be perfect to prevent cavitation problems. This modification of the impeller to get the low net positive suction head required (NPSHR) and its affects will be explained when you learn about suction specific speed.
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The difference between specific speed and suction specific speed can be confusing but you should know the difference. Shaft speed is an important decision. Speed affects pump component wear and NPSH requirements, along with the head, capacity, and the pump size. High speed pumps cost less initially, but the maintenance costs can be staggering. Speed is especially critical if you are going to be specifying a slurry pump. The ratio of the shaft diameter to its length is called the shaft L3/D4number. This ratio will have a major affect on the operating window of the pump and its inital cost. The lower the number the better, but any thing below 60 (2 in the metric system) is acceptable when you are using mechanical seals. A low L3/D4 can be costly in a standard long shaft pump design because it dictates a large diameter shaft that is usually found only on expensive heavy-duty pumps. A short shaft with a smaller outside diameter would accomplish the same goal, but then the pump would no longer conform to the ANSI or ISO standard. We often run into L3/D4problems when you specify, or the pump supplier sells you a low cost, corrosion resistant sleeve, mounted on a steel shaft rather than a more expensive solid, corrosion resistant shaft.
There are multiple decisions to be made about the impeller selection and not all pump suppliers are qualified to make them: ●
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The impeller shape or specific speed number will dictate the shape of the pump curve, the NPSH required and influence the efficiency of the pump. Has the impeller configuration been iterated in recent years? Impeller design is improving with some of the newer computer programs that have become available to the design engineer. The suction specific speed number of the impeller will often predict if you are going to experience a cavitation problem. The impeller material must be chosen for both chemical compatibility and wear resistance. You should consider one of the duplex metals because most corrosion resistant materials are too soft for the demands of a pump impeller. The decision to use a closed impeller, open impeller, semi-open, or vortex design is another decision to be made. Closed impellers require wear rings and these wear rings present another maintenance problem. Open and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back-plate to get the proper impeller setting and prevent internal recirculation. Vortex pump impellers are great for solids and "stringy" materials but they are up to 50% less efficient than conventional designs. Investment cast impellers are usually superior to sand cast versions because you can cast compound curves with the investment casting process. The compound curve allows the impeller to pump abrasive fluids with less vane wear. If you are going to pump low specific gravity fluids with an open impeller, a non-sparking type metal may be needed to prevent a fire or explosion. You will be better off choosing a closed impeller design with soft wear rings in these applications. The affinity laws will predict the affect of changing the impeller speed or diameter. You will want to be familiar with these laws for both centrifugal and PD pumps..
Either you or the supplier must select the correct size electric motor, or some other type of driver for the pump. The decision will be dictated by the specific gravity of the liquid you will be pumping along with the specific gravity of any cleaners or solvents that might be flushed through the lines. The selection will also be influenced by how far you will venture off the best efficiency point (BEP) on the capacity side of the pump curve. If this number is under-estimated there is a danger of burning out some electric motors. ●
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How are you going to vary the pump's capacity? Are you going to open and close a valve or maybe you will be using a variable speed drive like a gasoline or diesel engine. Will the regulating valve open and close automatically like a boiler feed valve or will it be operated manually? The variable speed motor might be an alternative if the major part of the system head is friction head rather than static or pressure head. The viscosity of the fluid is another consideration because it will affect the head, capacity, efficiency and power requirement of the pump. You should know about viscosity and how the viscosity of the pumpage will affect the performance of the pump. There are some viscosity corrections you should make to the pump curve when you pump viscous fluids. After carefully considering all of the above, the pump supplier will select a pump type and size, present his quote and give you a copy of his pump curve. Hopefully you will be getting his best pump technology. To be sure that is true you should know what the best pumping technology is. At this stage it is important for you to be able to read the pump curve. To do that you must understand: ❍ Efficiency ❍ Best efficiency point (BEP) ❍ Shut off head. ❍ How to convert pressure to head so you can reference pump gage readings to the pump curve. When you learn the three formulas you will get the conversion information. ❍ Brake horsepower (BHP) ❍ Water horsepower (WHP) ❍ Capacity ❍ Net positive suction head required (NPSHR) ❍ How to calculate the net positive suction head available (NPSHA) to the pump to insure you will not have a cavitation problem.
If all of the above decisions were made correctly the pump supplier will place his pump curve on top of your system curve and the required operating window will fall within the pump's operating window on either side of the best efficiency point (BEP). Additionally, the motor will not overheat and the pump should not cavitate. If the decisions were made incorrectly the pump will operate where the pump and system curves intersect and that will not be close to, or at the best efficiency point, producing radial impeller loading problems that will cause shaft deflection, resulting in premature seal and bearing failures. Needless to say the motor or driver will be adversely affected also. With few exceptions pump manufacturers are generally not involved in mechanical sealing. You will probably be contacting separate seal suppliers for their recommendation about the mechanical seal.
Recent mergers between pump and seal companies unfortunately does not produce the instant expertise we would like sales and service people to posses. CENTRIFUGAL PUMP INSTALLATION Some one has to install the pump and all of its associated hardware. The quality of this pump and driver installation will have a major affect on the performance and reliability of the pump, especially if it is equipped with a mechanical seal. The pump will be installed on a baseplate. The baseplate will be attached to a foundation and grout will be placed between the baseplate and the foundation to transmit any vibrations from the pump to the foundation. Once the pump and driver are firmly on the foundation it will be time to connect the piping. Be sure to pipe from the pump to the pipe rack and not the other way, so as to avoid pipe strain that will interfere with the operation of the mechanical seal and bearings. There are many piping recommendations that you should be familiar with. The leveling, and pump to driver alignment can be made at this point, but you should check the alignment after the pump has come up to its operating temperature because metal parts expand and contract with a change in temperature. If this is a new piping system some people like to install packing in the pump and run on packing until the new piping has been cleaned of slag or any junk that might be left in the piping system. If it is not a new installation, and there is a mechanical seal in the stuffing box, then installing the mechanical seal environmental controls will come next. If the pump has an open or semi-open impeller it is time to make the initial impeller clearance setting. The final clearance can be set when the pump comes up to its operating temperature. It is important to note that if you do not have a cartridge seal installed in the pump the seal face loading will change as you make both the initial and subsequent impeller settings and there is nothing you can do about it. You will now want to do a proper venting of the pump. If it is a vertical installation you will have to pay particular attention to keeping air vented from the stuffing box while the pump is running and be sure to vent the space between dual seals if they have been installed. After you have done all of the above, it is time to check out the mechanical seal environmental controls to be sure they are working properly. In most cases the environmental control will continue to run after the pump has stopped. Be sure the operators understand this or they might be tempted to shut the control off when the pump is between batches. Seal quench is always a problem with operators because the steam or water dripping out of the seal gland looks like the seal is leaking. A constant monitoring of the pump is a good idea. Are you familiar with some of the more popular monitoring methods? Unlike vibration analysis, monitoring can tell you if some part of the pump is getting into trouble before the vibration starts.
CENTRIFUGAL PUMP MODIFICATION If you find that your present centrifugal pump is not satisfying the application and running as trouble free as you would like, and you have checked: ● ● ●
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All of the internal tolerances are correct. There is no excessive pipe strain. The open impeller has been adjusted to the volute or backplate after the pump came up to operating temperature. The pump to driver alignment was made. The rotating parts were dynamically balanced. The wear ring clearance is within manufacturers specifications. The pump is running at the correct speed, in the right direction, with the correct size impeller.
Then you may have to purchase a different centrifugal pump or you might want to consider modifying the existing pump to get the performance and reliability you are looking for. Here are a few modifications and pump upgrades you can consider: ●
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Modifying the impeller diameter could get you closer to the best efficiency point. The affinity laws will predict the affect the trimming will have on the pump's head; capacity, net positive suction head required (NPSHR), and horsepower requirement. Converting to an impeller with a different specific speed number will change the shape of the pump curve, power consumption and the NPSH required. Changing to a heavy-duty power end can stop a lot of shaft deflection, and with some pump manufacturers get you the pilot diameter you need to install a "C or D" frame adapter to eliminate pump alignment. Converting from a sleeved to a solid, corrosion resistant shaft will often reduce or stop shaft deflection problems caused by operating off the best efficiency point (BEP). If you are using mechanical seals be sure that you are using the type that prevents fretting corrosion. Most original equipment manufactured (OEM) seals damage shafts, and that is one of the main reasons they supply a sacrificial sleeve. Reducing the overhung shaft length can solve many shaft deflection problems. You should be able to get the L3/D4 number down to a desirable 15-20 (0,5 &endash; 0,6 metric) by either reducing the shaft length or increasing the shaft diameter. Changing the wet end to a double volute configuration will allow the pump to operate in a larger window without the danger of deflecting the shaft too much. You can drill a hole in the end of the stuffing box, at the top, to increase stuffing box venting. Change the flushing or recirculation connection from the top lantern ring connection to the bottom of the stuffing box to insure a better fluid flow through the stuffing box. Try to get close to the seal faces. Enlarging the inside diameter of the stuffing box or going to an oversize stuffing box can solve some persistent seal problems. Converting the wet end of the pump to a centerline design might solve some pipe strain problems
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by compensating for radial thermal growth. Increasing the impeller to cutwater clearance could stop a cavitation problem Installing a sight glass in the bearing case can help you maintain the correct oil level and prevent overheating problems in the bearings. Replacing the bearing case grease or lip seals with either labyrinth or positive face seals for bearings will keep moisture out of the bearing case and eliminate a lot of premature bearing failure. Converting the radial bearing retention snap ring to a more rugged holding device will eliminate many of he problems associated with axial movement of the shaft. Converting the packed pump to a good mechanical seal will reduce power consumption and product leakage. Converting solid mechanical seals to split mechanical seals can reduce the time it takes to change seals and eliminate the need for other trades to become involved in the process of disassembling a pump and bringing it into the shop.
MECHANICAL SEAL SELECTION ●
In the following pages I will be using the word "pump" to describe the piece of equipment that you will be sealing. If your equipment is anything other than a single stage centrifugal pump with an over hung impeller, the information still applies with a couple of exceptions: ❍ Mixers, agitators and similar pieces of equipment sometimes have severe axial thrust and shaft deflection problems due to their high L3/D4 numbers (The ratio of the shaft length to its diameter). ❍ Sleeve or journal bearing equipment allows more axial movement of the shaft than those pieces of equipment provided with precision bearings. Axial movement is a problem for mechanical seals because of the changing face load; especially at start up when the axial thrust reverses in a centrifugal pump. ❍ Open impeller pumps require impeller adjustment that could cause excessive axial movement of the shaft that will affect the seal face loading. Depending upon the severity of the abrasives being pumped, this could be a frequent occurrence. ❍ Multi-stage pumps are seldom as sensitive to operating off the best efficiency point (BEP) as single stage centrifugal pumps. The opposing cutwaters in these pumps tend to cancel out the radial forces created when the pump is operating off of its best efficiency point (BEP). ❍ Centrifugal pumps equipped with double volutes are not too sensitive to operating off the best efficiency point (BEP), but do experience all of the other types of shaft deflection. ❍ Specialized equipment such as a refiner in a paper mill will experience a great deal of axial travel as the internal clearances are adjusted.
Whenever I use the word fluid, I am talking about either a liquid or a gas. If I say either liquid or gas, I am limiting my discussion to that one phase of the fluid. Any discussion of mechanical face seals requires that you have many different types of knowledge. The first is, "should you be converting packed pumps to a mechanical seal?" Seals cost a lot more money than conventional packing and unless you are using split seals, they can be a lot more difficult to install.
There is a packing conversion down side. Assuming you have made the decision that the mechanical seal is your best choice for sealing, you must know how to select the correct design for your application. There are many different kinds of seals to choose from: ● ● ●
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Rotating seals where the springs or bellows rotate with the shaft. Stationary seals where the springs or bellows do not rotate with the shaft. Metal bellows seals used to eliminate elastomers that can have trouble with temperature extremes or fluid compatibility. Elastomer type seals utilizing O-rings and other shape elastomers. Single seals for most applications. Dual seal designs for dangerous and expensive products or any time back up protection is needed. Inside mounted designs that take advantage of centrifugal force to throw solids away from the lapped seal faces. Outside seals. Usually the non-metallic variety for pumps manufactured from non-metallic materials. Cartridge seals to ease installation and allow you to make impeller adjustments without disturbing the seal face loading. Split seal designs that allow you to install and change seals without taking the pump apart and disturbing the alignment. Hydrodynamic or non-contacting seals used for the sealing of gases. Hydrostatic designs are another version of non-contacting vapor seals.
There are some very desirable design features that you should specify for your mechanical seals: ●
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The ability to seal fugitive emissions without the use of dual seals, other than having the dual seal installed as a "back-up" or spare seal. Will the seal dynamic elastomer damage or cause fretting corrosion of the pump shaft? Almost all-original equipment designs do. Spring-loaded Teflon® and graphite are notorious for shaft destruction. There are many seal designs available that will not cause fretting corrosion or damage shafts and sleeves, and that is the kind you should be using. The seal should have built in non-clogging features such as springs out of the fluid. The seal should be able to compensate for a reasonable amount of both radial and axial movement of the shaft. There are special mixer seal designs that can compensate for axial and radial travel in excess of 0.125 inches (3 mm) and you should know about them The seal should be designed to be positioned as close to the bearings as possible to lessen the affects of shaft deflection. Ideally the seal would be located between the stuffing box face and the bearing case with a large diameter seal gland allowing plenty of internal radial clearance for the seal. The seal should generate only a small amount of heat. Seal face heat generation can be a problem with many fluids and there is no advantage in letting the seal faces, or the fluid surrounding them get hot ❍ Any heat generation between the seal faces should be efficiently removed by conduction
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away from the lapped faces and dynamic elastomer. Check to see if your design does it efficiently. Any dynamic elastomer (an O-ring is typical) should have the ability to flex and then roll, or slide to a clean surface as the carbon face wears. The seal face load should be adjustable to compensate for open impeller adjustments and axial growth of the shaft. Cartridge seals do this very well. Can you use universal materials to lower your inventory costs and avoid mix-up problems? All of the seal materials should be clearly identified by type and grade. You will need this information if you have to analyze a premature seal failure. Some seal companies try to make everything a secret, do not tolerate it! Will the seals be hydraulically balanced to prevent the generation of unwanted heat between the lapped faces? What is the percentage of balance? If you are using dual seals will the inner seal be a double balanced seal that is hydraulically balanced in both directions? Pressures can reverse in dual seal applications. You will want to become familiar with the effects of heat on: ❍ The seal faces, especially the carbon and plated or coated hard faces ❍ The elastomers, especially the dynamic elastomer ❍ Excessive corrosion of the seal components. ❍ The product. It can change with heat. It can vaporize, solidify, crystallize, coke or build a film with an increase in the product's temperature. ❍ Internal tolerances of the seal especially face flatness and elastomer squeeze. Heat causes thermal growth of these components that will alter their critical tolerances.
We would like to be able to install the seal without having to modify the pump. The seal should be the shortest, thinnest design that will satisfy all of the operating conditions. Once you have the shortest, thinnest design that will satisfy the operating conditions there is seldom a need to modify any seal design. The specific sealing application will dictate which seal design you should choose. If your seal application falls within the following parameters any stationary or rotating, "off the shelf" balanced Oring seal should be able to handle the application without any serious problems. ●
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Stuffing box pressures from a one Torr vacuum to 400 psi. (28 bar). Note that stuffing box pressure is normally closer to suction than discharge pressure Stuffing box temperature from -40°F to 400°F. (-40°C to 200°C) Shaft speed within electric motor speeds. If the surface speed at the seal faces exceeds 5000 fpm. (25 m/sec) you will have to select the stationary version of the seal. Shaft sizes from 1 inch to 4 inches. (25 mm to 100 mm)
You may have to go to a special seal design if your application falls into any of the following categories: ● ●
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Stuffing box pressures in excess of 400 psi. (28 bar) require heavy duty seals. Excessive shaft movement of the type you find in mixers, agitators, and some types of sleeve or journal bearing equipment. The seal must meet fugitive emission standards. No metal parts are allowed in the system. You need a non-metallic seal.
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Nothing black is allowed in the system because of a fear of color contamination. You cannot use any form of carbon face; you must use two hard faces. There is not enough room to install a standard seal. You are not allowed to use an environmental control or no environmental control is available. Odd shaft sizes often dictate special seals. If the seal components must be manufactured from an exotic metal.
If any of the following are part of the application, you may need a metal bellows design that eliminates all elastomers. ●
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You are sealing a non-petroleum fluid and the stuffing box temperature exceeds 400°F (200°C) Petroleum fluids have coking problems that require cooling in the seal area. Cryogenic temperatures.
You should go to a dual seal application if your product falls into any of the following categories: ● ●
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You need two seals to control the seal environment outside the stuffing box. To control the temperature at a seal face to stop a product from vaporizing, solidifying, crystallizing, or building a film. To prevent a pressure drop across a seal face that can cause a liquid to vaporize. To eliminate atmospheric conditions outboard of a mechanical seal when there is a possibility of freezing water vapor in the air. To break down the pressure in a high-pressure application, by inserting an intermediate pressure between the seals. Two lower pressure seals can then be used to seal a high-pressure fluid that would normally require a very expensive high-pressure mechanical seal. To provide a lubricant if one is needed to prevent slip stick between lapped seal faces. This is always a problem when you are sealing a gas or non-lubricating liquid.
You need dual seals as a protection for personnel in the area if your product is any of the following categories: ● ● ● ● ● ● ●
A toxic liquid or gas. A fire hazard A pollutant A carcinogen A radioactive fluid An explosive fluid Etc.
The other places we use dual seals are: ● ● ●
Expensive products that are too valuable to let leak. You cannot afford to be shut down in the middle of a batch operation. You do not have a standby pump and experience shows that the seal failure is your highest probability of an unexpected shut down.
In the Sealing Application section you will learn: ● ● ●
How to choose the correct seal materials. How to classify the fluid into specific sealing categories The environmental controls you might need to insure the seal will not fail prematurely.
Link to Mc Nally home page
IMPELLER SELECTION NOTE: The blue links do not work on this version of the tutorials. They only work on the CD version There are multiple decisions to be made about the impeller selection and not all pump suppliers are qualified to make them: ●
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The impeller shape or specific speed number will dictate the shape of the pump curve, the NPSH required and influence the efficiency of the pump. Has the impeller configuration been iterated in recent years? Impeller design is improving with some of the newer computer programs that have become available to the design engineer. The suction specific speed number of the impeller will often predict if you are going to experience a cavitation problem. The impeller material must be chosen for both chemical compatibility and wear resistance. You should consider one of the duplex metals because most corrosion resistant materials are too soft for the demands of a pump impeller. The decision to use a closed impeller, open impeller, semi-open, or vortex design is another decision to be made. Closed impellers require wear rings and these wear rings present another maintenance problem. Open and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back-plate to get the proper impeller setting and prevent internal recirculation. Vortex pump impellers are great for solids and "stringy" materials but they are up to 50% less efficient than conventional designs. Investment cast impellers are usually superior to sand cast versions because you can cast compound curves with the investment casting process. The compound curve allows the impeller to pump abrasive fluids with less vane wear. If you are going to pump low specific gravity fluids with an open impeller, a non-sparking type metal may be needed to prevent a fire or explosion. You will be better off choosing a closed impeller design with soft wear rings in these applications. The affinity laws will predict the affect of changing the impeller speed or diameter. You will want to be familiar with these laws for both centrifugal and PD pumps.
Link to Mc Nally home page
CENTRIFUGAL PUMP SELECTION. HOW TO PICK THE CORRECT SIZE PUMP FOR YOUR APPLICATION. NOTE: The blue links do not work on this version of the tutorials. . They only work on the CD version We will begin by deciding what operating conditions our pump has to meet and then we will approach pump suppliers to see how closely they can satisfy these needs. Unfortunately no comprehensive theory which would permit the complete hydrodynamic design of a centrifugal pump has evolved in the many years that pumps have been around, so the pump manufacturer will be doing the best he can with the information you supply to him. To clearly define the capacity and pressure needs of our system we will construct a type of graph called a system curve. This system curve will then be given to the pump suppliers and they will try to match it with a pump curve that satisfies these needs as closely as possible. To start the construction of the system curve I will assume you want to pump some fluid from point "A" to point "B". To do that efficiently you must make a couple of decisions: ●
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Decide the capacity you will need. This means the gallons per minute or cubic meters per hour. You must also consider if this capacity will change with the operation of your process. A boiler feed pump is an example of an application that needs a constant pressure with varying capacities to meet a changing steam demand. The demand for boiler water is regulated by opening and closing a control valve on the discharge side of the pump with a discharge re-circulation line returning the unneeded portion back to a convenient storage place, or the suction side of the pump. Remember that with a centrifugal pump if you change its capacity you change the pressure also. A rotary or positive displacement pump is different. It puts out a constant capacity regardless of the pressure. For other centrifugal pump applications, you are going to have to calculate how much pressure will be needed to deliver different capacities to the place where you will need them. You will need enough pressure to : ❍ Reach the maximum static head or height the fluid will have to attain. ❍ Over come any pressure that might be in the vessel where the fluid is discharging, such as the boiler we just discussed. This is called the pressure head. ❍ Overcome friction resistance in the lines, fittings and any valves or hardware that might be in the system. As an example: high-pressure nozzles can be tricky, especially if they clog up. This resistance is called the friction head. Will you need any special materials for the pump components? ❍ The pump manufacturer will try to choose pump metal components that are chemically compatible with what you are pumping as well as any cleaners or solvents that might be flushed through the lines. If the temperature of the pumpage changes the corrosion rate can change also. His choice of materials could have a serious affect on your spare parts inventory. Will he be selecting universal and easily obtainable materials? Unless you have a great deal of experience with the product you are pumping do not select the metal
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components by using a compatibility chart. Metal selection is a job for metallurgists or your own experience. ❍ If the product you are pumping is explosive or a fire hazard, you should be looking at nonsparking materials for the pump components. Do not depend totally upon the pump manufacturer to make this decision for you. If you are not sure what materials are compatible with your product, how will the pump man know? Also, keep in mind that some of the fluids you will be pumping could be proprietary products known only by their trade name. ❍ Dangerous and radioactive materials will dictate special materials. ❍ Food products require high-density seal and pump materials that are easy to clean. ❍ If there are abrasive solids in the pumpage you will need materials with good wearing capabilities. Hard surfaces and chemically resistant materials are often incompatible. You may have to go to some type of coating on the pump wetted parts or select an expensive duplex metal. Occasionally you will find an application where metal is either not compatible or not practical. There are many monomer and polymer materials available for these applications, but their cost is generally higher than comparable metal parts. Be aware that if you are using a mechanical seal in a non-metallic pump, the seal cannot have metal parts in contact with the fluid for the same reasons the pump was manufactured from non-metallic materials. Use a non-metallic seal in these applications
Since we are just getting into the subject, one of the first things we should learn is that centrifugal pump people do not use the word pressure. As mentioned in an earlier paragraph they substitute the word "head", so you will have to calculate the three kinds of head that will be combined together to give you the total head of the system required to deliver the needed capacity. Here are the three kinds of head you will be calculating: ●
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The static head or maximum height that the liquid will reach. We must also learn how to compensate for the siphon affect from down running pipes on the discharge side of the pump. Remember that if you fill a tank from the bottom instead of the top the static head will continually increase. This is not a good application for a centrifugal pump because the capacity is decreasing with an increasing head. If you must fill from the bottom, or if you will be using the pump as an accumulator, a rotary positive displacement pump will be your best choice as long as it can meet the needed capacities. The pressure heads are next if the container we are pumping to, or from, is pressurized. We will have to learn how to convert pressure units to head units because later on we will need this conversion knowledge to read the manufacturers pump curve. Pump gages are labeled in psi or bar. Pump curves are labeled in feet of head, or meters of head. The friction head is the last one that we will have to calculate. This head tells us how much friction or resistance head there is in both the suction and discharge piping, along with the fittings and valves in the piping system. And to make the job a little tougher this head changes dramatically as the pump capacity changes.
You will be calculating these heads on both the suction and discharge side of the pump. To get the total head you will subtract the suction head from the discharge head and that will be the head that the pump
must produce to satisfy the application. It will become obvious in the calculations, but I should mention here, that if the suction head is a negative number, the suction and discharge heads will be added together to get the total head. If you subtract a minus number from a positive number you must add the numbers together. As an example: 4 - (-2) = + 6 The total head of a pump seldom remains static. There are a number of factors that can change the head of a pump while it is operating, and you should become familiar with most of them. All of this head information is calculated from piping, valve, and fitting, friction graphs you will find in the index.This head data will be plotted on a set of coordinates called a system curve. Since we will not be operating at a single point all of the time we will make the calculations for a range of different capacities and heads that we might expect to encounter. This range is described as the operating window we will need to satisfy the application. Making these calculations is not an exact science because the piping is seldom new, pipe inside diameters are not exact, and the graphs you will be consulting cannot compensate for corrosion and solids built up on the piping, valve and fitting walls. Life is never simple. This is the point where most people start adding in safety factors to compensate for some of the unknowns. These safety factors will almost always guarantee the selection of an oversized pump that will run off of its best efficiency point (BEP) most of the time. The final calculations are then plotted on the system curve that describes what the pump has to do to satisfy the requirements of the application. You can learn to do all of this by referencing the following subjects: ● ● ●
Calculating the total head in metric units Calculating the total head in USCS (inch) units Making a system curve, S111
The pump manufacturer requires a certain amount of net positive suction head required (NPSHR) to prevent the pump from cavitating. He shows that number on his pump curve. When you look at the curve you will also note that the net positive suction head required (NPSHR) increases with any increase in the pump's capacity. You will also be calculating the net positive suction head available (NPSHA) to be sure that the pump you select will not cavitate. Cavitation is caused by cavities or bubbles in the fluid collapsing on the impeller and volute. In the pump business we recognize several different types of cavitation. : ● ● ● ●
Vaporization cavitation. Air ingestion cavitation. Internal recirculation cavitation. Flow turbulence cavitation.
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Vane Passing Syndrome cavitation.
Pump cavitation is recognized in several different ways ● ● ●
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We can hear cavitation because it sounds like the pump is pumping rocks or ball bearings. We can see the damage from cavitation on the pump's impeller and volute. The operator can sometimes tell if the pump is cavitating because of a reduction in the pump's capacity. The main problem with cavitation is that it shakes and bends the shaft causing both seal and bearing problems. We call all of this shaking and bending shaft deflection.
Remember that the net positive suction head required (NPSHR) number shown on the pump curve is for fresh water at 68° Fahrenheit (20°C) and not the fluid or combinations of fluids you will be pumping. When you make your calculations for net positive suction head available (NPSHA) the formula you will be using will adjust for the specific gravity of your fluid. ●
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In some cases you can reduce the NPSH required. This is especially true if you are pumping hot water or mixed hydrocarbons. You may have to install an inducer on the pump, add a booster pump, or go to a double suction pump design if you do not have enough net positive suction head available (NPSHA)
When the pump supplier has all of this in-exact information in his possession he can then hopefully select the correct size pump and driver for the job. Since he wants to quote a competitive price he is now going to make some critical decisions: He might begin with the type of pump he will recommend: ●
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If the capacity were going to be very low he would recommend a rotary, or positive displacement (PD) pump. Between 25 and 500 gpm (5 m3 /hr - 115 m3/hr) he will probably select a single stage end suction centrifugal pump. It all depends upon the supplier. At higher capacities he may go to a double suction design with a wide impeller, two pumps in parallel or maybe a high-speed pump. You might need a high head, low capacity pump. The pump supplier has several options you should know about. Will he recommend a self-priming pump? These pumps remove air from the impeller eye and suction side of the pump. Some operating conditions dictate the need for a self-priming design. If you do not have a self-priming pump and you are on intermittent service, will priming become a problem the next time you start the pump? How will the pump be operated? ❍ If the pump is going to run twenty-four hours a day, seven days a week and you are not going to open and close valves; you will not need a heavy-duty pump. It is easy to select a pump that will run at its best efficiency point and at the best efficiency point (BEP) there is very little shaft displacement and vibration.
Intermittent service is the more difficult application because of changing temperatures, vibration levels, thrust direction, etc. Intermittent pumps require a more robust, heavyduty design with a low L3/D4 shaft. How important is efficiency in your application? High efficiency is desirable, but you pay a price for efficiency in higher maintenance costs and a limited operating window. You should be looking for performance, reliability, and efficiency in that order. Too often the engineer specifies efficiency and loses the other two. The following designs solve some operation and maintenance problems, but their efficiency is lower than conventional centrifugal pumps. ❍ A magnetic drive or canned pump may be your best choice if you can live with the several limitations they impose. ❍ A vortex or slurry pump design may be needed if there are lot of solids or "stringy" material in the pumpage. ❍ A double volute centrifugal pump can eliminate many of the seal problems we experience when we operate off the pump's best efficiency point. The problem is trying to find a supplier that will supply one for your application. Although readily available for impellers larger than 14 inches (355 mm) in diameter they have become very scarce in the smaller diameters because of their less efficient design. The supplier should recommend a centerline design to avoid the problems caused by thermal expansion of the wet end if you are operating at temperatures over 200°F (100°C)? Will you need a volute or circular casing? Volute casings build a higher head; circular casing are used for low head and high capacity. Do you need a pump that meets a standard? ANSI, API, DIN, VDMA or ISO are some of the current standards. You should be aware of pump standards problems that contribute to premature seal and bearing failures. An ANSI (American National Standards Institute) standard back pullout design pump has many advantages but presents problems with mechanical seals when the impeller clearance is adjusted, unless you are purchasing cartridge seals. The decision to use either a single or multistage pump will be determined by the head the pump must produce to meet the capacities you need. Some suppliers like to recommend a high speed small pump to be competitive, other suppliers might recommend a more expensive low speed large pump to lessen NPSH and wear problems. ❍
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There are additional decisions that have to be made about the type of pump the supplier will recommend: ●
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Will the pump be supplied with a mechanical seal or packing? If the stuffing box is at negative pressure (vacuum) a seal will be necessary to prevent air ingestion. If he is going to supply a mechanical seal will he also supply an oversized stuffing box and any environmental controls that might be needed? Will he specify a jacketed stuffing box so that the temperature of the sealed fluid can be regulated? How does he intend to control the stuffing box temperature? Will he be using water, steam or maybe a combination of both? Electric heating is sometimes an option. How will the open or semi-open impeller be adjusted to the volute casing or back plate? Can the mechanical seal face loading be adjusted at the same time? If not, the seal face load will change and the seal life will be shortened. If the pump is going to be supplied with a closed impeller you should have some means of knowing when the wear rings have to be replaced. If the wear ring clearance becomes too large
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the pumps efficiency will be lowered causing heat and vibration problems. Most manufacturers require that you disassemble the pump to check the wear ring clearance and replace the rings when this clearance doubles. Will he supply a "C" or "D" frame adapter, or will the pump to motor alignment have to be done manually using dual indicators or a laser aligner to get the readings? A closed-coupled design can eliminate the need for an alignment between the pump and driver. What type of coupling will he select to connect the pump to its driver? Couplings can compensate for axial growth of the shaft and transmit torque to the impeller. They cannot compensate for pump to driver misalignment as much as we would like them to. Universal joints are especially bad because they have to be misaligned to be lubricated. He may decide to run two pumps in parallel operation if he needs a real high capacity, or two pumps in series operation if he needs a high head. Pumps that run in parallel or series require that they are running at the same speed. This can be a problem for some induction motors.. An inline pump design can solve many pipe strain and thermal growth problems. The pump supplier must insure that the pump will not be operating at a critical speed or passing through a critical speed at start up. If he has decided to use a variable speed drive or motor this becomes a possibility. We all want pumps with a low net positive suction head required to prevent cavitation problems but sometimes it is not practical. The manufacturer has the option of installing an inducer or altering the pump design to lower the net positive suction head required, but if he goes too far all of the internal clearances will have to be perfect to prevent cavitation problems. This modification of the impeller to get the low net positive suction head required (NPSHR) and its affects will be explained when you learn about suction specific speed. The difference between specific speed and suction specific speed can be confusing but you should know the difference. Shaft speed is an important decision. Speed affects pump component wear and NPSH requirements, along with the head, capacity, and the pump size. High speed pumps cost less initially, but the maintenance costs can be staggering. Speed is especially critical if you are going to be specifying a slurry pump. The ratio of the shaft diameter to its length is called the shaft L3/D4number. This ratio will have a major affect on the operating window of the pump and its inital cost. The lower the number the better, but any thing below 60 (2 in the metric system) is acceptable when you are using mechanical seals. A low L3/D4 can be costly in a standard long shaft pump design because it dictates a large diameter shaft that is usually found only on expensive heavy-duty pumps. A short shaft with a smaller outside diameter would accomplish the same goal, but then the pump would no longer conform to the ANSI or ISO standard. We often run into L3/D4problems when you specify, or the pump supplier sells you a low cost, corrosion resistant sleeve, mounted on a steel shaft rather than a more expensive solid, corrosion resistant shaft.
There are multiple decisions to be made about the impeller selection and not all pump suppliers are qualified to make them: ●
The impeller shape or specific speed number will dictate the shape of the pump curve, the NPSH required and influence the efficiency of the pump.
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Has the impeller configuration been iterated in recent years? Impeller design is improving with some of the newer computer programs that have become available to the design engineer. The suction specific speed number of the impeller will often predict if you are going to experience a cavitation problem. The impeller material must be chosen for both chemical compatibility and wear resistance. You should consider one of the duplex metals because most corrosion resistant materials are too soft for the demands of a pump impeller. The decision to use a closed impeller, open impeller, semi-open, or vortex design is another decision to be made. Closed impellers require wear rings and these wear rings present another maintenance problem. Open and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back-plate to get the proper impeller setting and prevent internal recirculation. Vortex pump impellers are great for solids and "stringy" materials but they are up to 50% less efficient than conventional designs. Investment cast impellers are usually superior to sand cast versions because you can cast compound curves with the investment casting process. The compound curve allows the impeller to pump abrasive fluids with less vane wear. If you are going to pump low specific gravity fluids with an open impeller, a non-sparking type metal may be needed to prevent a fire or explosion. You will be better off choosing a closed impeller design with soft wear rings in these applications. The affinity laws will predict the affect of changing the impeller speed or diameter. You will want to be familiar with these laws for both centrifugal and PD pumps..
Either you or the supplier must select the correct size electric motor, or some other type of driver for the pump. The decision will be dictated by the specific gravity of the liquid you will be pumping along with the specific gravity of any cleaners or solvents that might be flushed through the lines. The selection will also be influenced by how far you will venture off the best efficiency point (BEP) on the capacity side of the pump curve. If this number is under-estimated there is a danger of burning out some electric motors. ●
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How are you going to vary the pump's capacity? Are you going to open and close a valve or maybe you will be using a variable speed drive like a gasoline or diesel engine. Will the regulating valve open and close automatically like a boiler feed valve or will it be operated manually? The variable speed motor might be an alternative if the major part of the system head is friction head rather than static or pressure head. The viscosity of the fluid is another consideration because it will affect the head, capacity, efficiency and power requirement of the pump. You should know about viscosity and how the viscosity of the pumpage will affect the performance of the pump. There are some viscosity corrections you should make to the pump curve when you pump viscous fluids. After carefully considering all of the above, the pump supplier will select a pump type and size, present his quote and give you a copy of his pump curve. Hopefully you will be getting his best pump technology. To be sure that is true you should know what the best pumping technology is. At this stage it is important for you to be able to read the pump curve. To do that you must understand: ❍ Efficiency
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Best efficiency point (BEP) Shut off head. How to convert pressure to head so you can reference pump gage readings to the pump curve. When you learn the three formulas you will get the conversion information. Brake horsepower (BHP) Water horsepower (WHP) Capacity Net positive suction head required (NPSHR) How to calculate the net positive suction head available (NPSHA) to the pump to insure you will not have a cavitation problem.
If all of the above decisions were made correctly the pump supplier will place his pump curve on top of your system curve and the required operating window will fall within the pump's operating window on either side of the best efficiency point (BEP). Additionally, the motor will not overheat and the pump should not cavitate. If the decisions were made incorrectly the pump will operate where the pump and system curves intersect and that will not be close to, or at the best efficiency point, producing radial impeller loading problems that will cause shaft deflection, resulting in premature seal and bearing failures. Needless to say the motor or driver will be adversely affected also. With few exceptions pump manufacturers are generally not involved in mechanical sealing. You will probably be contacting separate seal suppliers for their recommendation about the mechanical seal. Recent mergers between pump and seal companies unfortunately does not produce the instant expertise we would like sales and service people to posses. Link to Mc Nally home page
CENTRIFUGAL PUMP INSTALLATION NOTE: The blue links do not work on this version of the tutorials. . They only work on the CD version Some one has to install the pump and all of its associated hardware. The quality of this pump and driver installation will have a major affect on the performance and reliability of the pump, especially if it is equipped with a mechanical seal. The pump will be installed on a baseplate. The baseplate will be attached to a foundation and grout will be placed between the baseplate and the foundation to transmit any vibrations from the pump to the foundation. Once the pump and driver are firmly on the foundation it will be time to connect the piping. Be sure to pipe from the pump to the pipe rack and not the other way, so as to avoid pipe strain that will interfere with the operation of the mechanical seal and bearings. There are many piping recommendations that you should be familiar with. The leveling, and pump to driver alignment can be made at this point, but you should check the alignment after the pump has come up to its operating temperature because metal parts expand and contract with a change in temperature. If this is a new piping system some people like to install packing in the pump and run on packing until the new piping has been cleaned of slag or any junk that might be left in the piping system. If it is not a new installation, and there is a mechanical seal in the stuffing box, then installing the mechanical seal environmental controls will come next. If the pump has an open or semi-open impeller it is time to make the initial impeller clearance setting. The final clearance can be set when the pump comes up to its operating temperature. It is important to note that if you do not have a cartridge seal installed in the pump the seal face loading will change as you make both the initial and subsequent impeller settings and there is nothing you can do about it. You will now want to do a proper venting of the pump. If it is a vertical installation you will have to pay particular attention to keeping air vented from the stuffing box while the pump is running and be sure to vent the space between dual seals if they have been installed. After you have done all of the above, it is time to check out the mechanical seal environmental controls to be sure they are working properly. In most cases the environmental control will continue to run after the pump has stopped. Be sure the operators understand this or they might be tempted to shut the control off when the pump is between batches. Seal quench is always a problem with operators because the steam or water dripping out of the seal gland looks like the seal is leaking. A constant monitoring of the pump is a good idea. Are you familiar with some of the more popular monitoring methods? Unlike vibration analysis, monitoring can tell you if some part of the pump is
getting into trouble before the vibration starts. Link to Mc Nally home page
CENTRIFUGAL PUMP MODIFICATION NOTE: The blue links do not work on this version of the tutorials. . They only work on the CD version If you find that your present centrifugal pump is not satisfying your application and running as trouble free as you would like, and you have checked: ● ● ●
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All of the internal tolerances are correct. There is no excessive pipe strain. The open impeller has been adjusted to the volute or backplate after the pump came up to operating temperature. The pump to driver alignment was made. The rotating parts were dynamically balanced. The wear ring clearance is within manufacturers specifications. The pump is running at the correct speed, in the right direction, with the correct size impeller.
Then you may have to purchase a different centrifugal pump, or you might want to consider modifying the existing pump to get the performance and reliability you are looking for. Here are a few modifications and pump upgrades you can consider: ●
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Modifying the impeller diameter could get you closer to the best efficiency point. The affinity laws will predict the affect the trimming will have on the pump's head; capacity, net positive suction head required (NPSHR), and horsepower requirement. Converting to an impeller with a different specific speed number will change the shape of the pump curve, power consumption and the NPSH required. Changing to a heavy-duty power end can stop a lot of shaft deflection, and with some pump manufacturers get you the pilot diameter you need to install a "C or D" frame adapter to eliminate pump alignment. Converting from a sleeved to a solid, corrosion resistant shaft will often reduce or stop shaft deflection problems caused by operating off the best efficiency point (BEP). If you are using mechanical seals be sure that you are using the type that prevents fretting corrosion. Most original equipment manufactured (OEM) seals damage shafts, and that is one of the main reasons they supply a sacrificial sleeve. Reducing the overhung shaft length can solve many shaft deflection problems. You should be able to get the L3/D4 number down to a desirable 15-20 (0,5 &endash; 0,6 metric) by either reducing the shaft length or increasing the shaft diameter. Changing the wet end to a double volute configuration will allow the pump to operate in a larger window without the danger of deflecting the shaft too much. You can drill a hole in the end of the stuffing box, at the top, to increase stuffing box venting. Change the flushing or recirculation connection from the top lantern ring connection to the
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bottom of the stuffing box to insure a better fluid flow through the stuffing box. Try to get close to the seal faces. Enlarging the inside diameter of the stuffing box or going to an oversize stuffing box can solve some persistent seal problems. Converting the wet end of the pump to a centerline design might solve some pipe strain problems by compensating for radial thermal growth. Increasing the impeller to cutwater clearance could stop a cavitation problem Installing a sight glass in the bearing case can help you maintain the correct oil level and prevent overheating problems in the bearings. Replacing the bearing case grease or lip seals with either labyrinth or positive face seals for bearings will keep moisture out of the bearing case and eliminate a lot of premature bearing failure. Converting the radial bearing retention snap ring to a more rugged holding device will eliminate many of he problems associated with axial movement of the shaft. Converting the packed pump to a good mechanical seal will reduce power consumption and product leakage. Converting solid mechanical seals to split mechanical seals can reduce the time it takes to change seals and eliminate the need for other trades to become involved in the process of disassembling a pump and bringing it into the shop.
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MECHANICAL SEAL SELECTION NOTE: The blue links do not work on this version of the tutorials. . They only work on the CD version In the following pages I will be using the word "pump" to describe the piece of equipment that you will be sealing. If your equipment is anything other than a single stage centrifugal pump with an over hung impeller, the information still applies with a couple of exceptions: ●
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Mixers, agitators and similar pieces of equipment sometimes have severe axial thrust and shaft deflection problems due to their high L3/D4 numbers (The ratio of the shaft length to its diameter). Sleeve or journal bearing equipment allows more axial movement of the shaft than those pieces of equipment provided with precision bearings. Axial movement is a problem for mechanical seals because of the changing face load; especially at start up when the axial thrust reverses in a centrifugal pump. Open impeller pumps require impeller adjustment that could cause excessive axial movement of the shaft that will affect the seal face loading. Depending upon the severity of the abrasives being pumped, this could be a frequent occurrence. Multi-stage pumps are seldom as sensitive to operating off the best efficiency point (BEP) as single stage centrifugal pumps. The opposing cutwaters in these pumps tend to cancel out the radial forces created when the pump is operating off of its best efficiency point (BEP). Centrifugal pumps equipped with double volutes are not too sensitive to operating off the best efficiency point (BEP), but do experience all of the other types of shaft deflection. Specialized equipment such as a refiner in a paper mill will experience a great deal of axial travel as the internal clearances are adjusted.
Whenever I use the word fluid, I am talking about either a liquid or a gas. If I say either liquid or gas, I am limiting my discussion to that one phase of the fluid. Any discussion of mechanical face seals requires that you have many different types of knowledge. The first is, "should you be converting packed pumps to a mechanical seal?" Seals cost a lot more money than conventional packing and unless you are using split seals, they can be a lot more difficult to install. There is a packing conversion down side. Assuming you have made the decision that the mechanical seal is your best choice for sealing, you must know how to select the correct design for your application. There are many different kinds of seals to choose from: ● ● ●
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Rotating seals where the springs or bellows rotate with the shaft. Stationary seals where the springs or bellows do not rotate with the shaft. Metal bellows seals used to eliminate elastomers that can have trouble with temperature extremes or fluid compatibility. Elastomer type seals utilizing O-rings and other shape elastomers.
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Single seals for most applications. Dual seal designs for dangerous and expensive products or any time back up protection is needed. Inside mounted designs that take advantage of centrifugal force to throw solids away from the lapped seal faces. Outside seals. Usually the non-metallic variety for pumps manufactured from non-metallic materials. Cartridge seals to ease installation and allow you to make impeller adjustments without disturbing the seal face loading. Split seal designs that allow you to install and change seals without taking the pump apart and disturbing the alignment. Hydrodynamic or non-contacting seals used for the sealing of gases. Hydrostatic designs are another version of non-contacting vapor seals.
There are some very desirable design features that you should specify for your mechanical seals: ●
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The ability to seal fugitive emissions without the use of dual seals, other than having the dual seal installed as a "back-up" or spare seal. Will the seal dynamic elastomer damage or cause fretting corrosion of the pump shaft? Almost all-original equipment designs do. Spring-loaded Teflon® and graphite are notorious for shaft destruction. There are many seal designs available that will not cause fretting corrosion or damage shafts and sleeves, and that is the kind you should be using. The seal should have built in non-clogging features such as springs out of the fluid. The seal should be able to compensate for a reasonable amount of both radial and axial movement of the shaft. There are special mixer seal designs that can compensate for axial and radial travel in excess of 0.125 inches (3 mm) and you should know about them The seal should be designed to be positioned as close to the bearings as possible to lessen the affects of shaft deflection. Ideally the seal would be located between the stuffing box face and the bearing case with a large diameter seal gland allowing plenty of internal radial clearance for the seal. The seal should generate only a small amount of heat. Seal face heat generation can be a problem with many fluids and there is no advantage in letting the seal faces, or the fluid surrounding them get hot ❍ Any heat generation between the seal faces should be efficiently removed by conduction away from the lapped faces and dynamic elastomer. Check to see if your design does it efficiently. Any dynamic elastomer (an O-ring is typical) should have the ability to flex and then roll, or slide to a clean surface as the carbon face wears. The seal face load should be adjustable to compensate for open impeller adjustments and axial growth of the shaft. Cartridge seals do this very well. Can you use universal materials to lower your inventory costs and avoid mix-up problems? All of the seal materials should be clearly identified by type and grade. You will need this information if you have to analyze a premature seal failure. Some seal companies try to make everything a secret, do not tolerate it! Will the seals be hydraulically balanced to prevent the generation of unwanted heat between the lapped faces? What is the percentage of balance? If you are using dual seals will the inner seal be
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a double balanced seal that is hydraulically balanced in both directions? Pressures can reverse in dual seal applications. You will want to become familiar with the effects of heat on: ❍ The seal faces, especially the carbon and plated or coated hard faces ❍ The elastomers, especially the dynamic elastomer ❍ Excessive corrosion of the seal components. ❍ The product. It can change with heat. It can vaporize, solidify, crystallize, coke or build a film with an increase in the product's temperature. ❍ Internal tolerances of the seal especially face flatness and elastomer squeeze. Heat causes thermal growth of these components that will alter their critical tolerances.
We would like to be able to install the seal without having to modify the pump. The seal should be the shortest, thinnest design that will satisfy all of the operating conditions. Once you have the shortest, thinnest design that will satisfy the operating conditions there is seldom a need to modify any seal design. The specific sealing application will dictate which seal design you should choose. If your seal application falls within the following parameters any stationary or rotating, "off the shelf" balanced Oring seal should be able to handle the application without any serious problems. ●
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Stuffing box pressures from a one Torr vacuum to 400 psi. (28 bar). Note that stuffing box pressure is normally closer to suction than discharge pressure Stuffing box temperature from -40°F to 400°F. (-40°C to 200°C) Shaft speed within electric motor speeds. If the surface speed at the seal faces exceeds 5000 fpm. (25 m/sec) you will have to select the stationary version of the seal. Shaft sizes from 1 inch to 4 inches. (25 mm to 100 mm)
You may have to go to a special seal design if your application falls into any of the following categories: ● ●
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Stuffing box pressures in excess of 400 psi. (28 bar) require heavy duty seals. Excessive shaft movement of the type you find in mixers, agitators, and some types of sleeve or journal bearing equipment. The seal must meet fugitive emission standards. No metal parts are allowed in the system. You need a non-metallic seal. Nothing black is allowed in the system because of a fear of color contamination. You cannot use any form of carbon face; you must use two hard faces. There is not enough room to install a standard seal. You are not allowed to use an environmental control or no environmental control is available. Odd shaft sizes often dictate special seals. If the seal components must be manufactured from an exotic metal.
If any of the following are part of the application, you may need a metal bellows design that eliminates all elastomers. ●
You are sealing a non-petroleum fluid and the stuffing box temperature exceeds 400°F (200°C) Petroleum fluids have coking problems that require cooling in the seal area.
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Cryogenic temperatures.
You should go to a dual seal application if your product falls into any of the following categories: ● ●
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You need two seals to control the seal environment outside the stuffing box. To control the temperature at a seal face to stop a product from vaporizing, solidifying, crystallizing, or building a film. To prevent a pressure drop across a seal face that can cause a liquid to vaporize. To eliminate atmospheric conditions outboard of a mechanical seal when there is a possibility of freezing water vapor in the air. To break down the pressure in a high-pressure application, by inserting an intermediate pressure between the seals. Two lower pressure seals can then be used to seal a high-pressure fluid that would normally require a very expensive high-pressure mechanical seal. To provide a lubricant if one is needed to prevent slip stick between lapped seal faces. This is always a problem when you are sealing a gas or non-lubricating liquid.
You need dual seals as a protection for personnel in the area if your product is any of the following categories: ● ● ● ● ● ● ●
A toxic liquid or gas. A fire hazard A pollutant A carcinogen A radioactive fluid An explosive fluid Etc.
The other places we use dual seals are: ● ● ●
Expensive products that are too valuable to let leak. You cannot afford to be shut down in the middle of a batch operation. You do not have a standby pump and experience shows that the seal failure is your highest probability of an unexpected shut down.
In the Sealing Application section you will learn: ● ● ●
How to choose the correct seal materials. How to classify the fluid into specific sealing categories The environmental controls you might need to insure the seal will not fail prematurely.
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HOW TO SEAL VARIOUS FLUIDS ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
Sealing common fluids Classifying chemicals to assure effective sealing 2-12 Clearing up the confusion about flushing mechanical seals 3-6 Controlling the pressure in the stuffing box area 4-10 Controlling the temperature in the stuffing box area 4-6 Environmental controls and special seal designs 3-2 More about the sealing of condensate 10-7 Sealing high pressure and hard vacuum fluids 5-7 Sealing high temperature oil 3-5 Sealing hot water 3-3 Sealing liquid slurries 3-4 Sealing mixers and agitators 3-7 Sealing non lubricants including gases and dry powders 4-8 Sealing products sensitive to a change in temperature or pressure 8-9 Sealing products that are affected by agitation 6-12 Sealing to the OSHA 1910.119 standard 8-10
Selecting the correct seal materials ● ● ● ● ●
How the carbon graphite seal face is manufactured4-7 Limits of the super elastomer compounds. 10-6 Selecting the correct elastomer for your application 4-9 Selecting the correct hard face material for your mechanical seal 5-2 Selecting the correct metal for the mechanical seal parts 5-9
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SUBJECT : A new way of classifying chemicals to assure effective sealing 2-12 The most common question asked by seal salesmen is "what are you sealing?" This is usually followed by asking about shaft size, product, temperature, speed, stuffing box pressure and any other operating conditions they can think of. The problem with this simplistic approach is that you would have to have a very large data bank of information to reference a particular problem so as to be able to make a sensible seal recommendation. There is a much more logical approach to the problem that we will be discussing in the following paragraphs. A sensible approach to the sealing of various chemicals, mixtures, and compounds would be divided into three parts: ●
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You must know how to select mechanical seal components that will not corrode or be attacked in any way by the fluid you are sealing, or any other chemicals that might come into contact with the seal as a result of cleaning the system, flushing the stuffing box, using barrier fluids between double seals, quenching behind the seal etc.. You must understand the total range of operating conditions of the equipment and then select seal designs that can handle this range. You need a method of classifying chemicals that puts them into neat, logical categories that can be handled by the use of a special seal design and/or environmental controls. It is important to note that the sealing environment will affect the sealing fluid often preventing the lapped sealing faces from staying in contact..
In this paper we will concentrate on the classification of chemicals and leave the selection of seal materials, types of seals and use of various environmental controls to other papers on this site A fluid can be classified as either a liquid or a gas, and can be divided into seven categorizes: 1. Fluids sensitive to changes in temperature and/or pressure. 2. Fluids that require two mechanical seals. 3. Non lubricating liquids, gases and solids. 4. Slurries, classified as solids in liquid . The solids may or may not be abrasive. 5. Liquids sensitive to agitation. 6. Liquids that react with each other to form a solid. 7. Lubricating liquids.
We will be investigating each of these categories in detail and learn how they affect the life of a mechanical seal. In other papers on this site we will learn the detailed methods of sealing each of these problems Fluids that are sensitive to changes in temperature and/or pressure. ●
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Corrosive liquids - Most corrosives will double their corrosion rate with a 18 degree Fahrenheit (10 C.) rise in temperature. The temperature at the seal face is always hotter than the temperature recorded in the stuffing box or seal chamber. Keep in mind that any contact between the rotating shaft and a stationary component will cause high heat and will be detected as localized corrosion. Wear rings and throttle bushings are subject to this rubbing. If the equipment is provided with a cooling jacket. and it is not being utilized, the air inside can act as an insulation increasing the heat in the stuffing box considerably. Liquids that vaporize - Most any liquid will vaporize if it becomes hot enough, or if the stuffing box pressure gets too low. It is the product with a low specific gravity that give us the most trouble. If the product vaporizes between the lapped seal faces it will separate the faces as the gases expand. When hot water vaporizes it leaves behind any chemicals that were dissolved in the water. Most of these chemicals are left in a hard crystal form that will damage the lapped faces. Fluids such as benzene and others with a low specific gravity, will freeze as they vaporize. If any oil or lubricant was placed on the seal face it will freeze and possibly damage the lapped faces. Moisture on the outboard side of the seal will freeze also and restrict movement of the sliding or flexing seal components Liquids that solidify - Some solidify with an increase in temperature, others with a decrease. Solvents vaporize with lower pressure leaving any solids behind. Paint is a good example of a product where the solvent will vaporize at or below atmospheric pressure. In most cases you can reference a vapor pressure chart to learn when the solvent or carrier will vaporize in your application. Viscous Products - Their viscosity usually decreases with an increase in temperature and increases with a decrease in temperature. Oil is a good example of this type of fluid. High viscosity can interfere with free seal movement and cause seal face contact problems. Lowering the viscosity can often increase the seal face wear as there is not enough film thickness to keep the surfaces separated. You need a film thickness of at least one micron to keep the lapped seal faces separated. Film building liquids - Petroleum products will form a varnish when first heated and then gradually form a layer of coke as the temperature is elevated. These transformations are not reversible and the resultant hard film restricts sliding and/or flexing of the seal components. Hard water is another example of a film building fluid. Hot water systems pick up magnetite (Ferric Oxide) from the inside of the pipes. It is black or reddish in color and will be attracted by a magnet. This abrasive material will collect on the seal components and destroy the dynamic O-Ring as well as restrict the movement of the seal causing the lapped faces to open. Magnetite is a severe problem in new , hot water systems. The problem will lessen as the system ages and the protective film stabilizes. Liquids that crystallize - Sugar and salt solutions are two examples of these fluids. If the crystals form between the faces they can destroy the carbon. If they form in the sliding or flexing components they will open the seal faces as the shaft moves. Any leakage across the seal faces will form a solids build up on the other side of the seal causing interference as the seal tries to
move when it compensates for wear. The names of these chemicals is not important. If you knew how to seal any one of them you could seal all of them. It is just a matter of fitting the particular chemical into the right categories and learning how to seal that category. Common sense would dictate that the product temperature and/or pressure must be controlled in the seal area to prevent any of the above from occurring. In most cases you should try to avoid the use of two hard faces in these applications because of the additional heat that will be generated between the faces. Needless to say, only hydraulically balanced seals are acceptable in any temperature/ pressure sensitive fluid. Liquids that require two mechanical seals : These seals are installed with a circulating barrier fluid that can be a "forced circulation", or in many cases a convection system with a "pumping ring". The pressure of the barrier or buffer fluid can be regulated to indicate a failure in either of the mechanical seals allowing time for a pump shut down, isolation and no subsequent loss in the pumping fluid. ●
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Costly products - Some times the product costs so much you just cannot afford to have it leak. There are plenty of charts to show how much leakage you get from various sized drips or steady streams. The smallest steady stream you can produce will be between twenty five and thirty U.S. gallons per day (95 to 115 liters/day) Dangerous products - these fluids are given a special category because even small amounts of leakage are not acceptable. The danger could fall into many categories: radiation, toxic, fire, explosion, bacteria, etc.. The new United States' "right to know law" is having a major affect on how mechanical seals used in these type of products will be repaired. Pollutants - Usually there is a "penalty" involved and the bad publicity does no one any good. In this day and age a responsible company will not let pollutants leak to the atmosphere or to the earth for any reason. Fugitive emission legislation has increased the need for these type of mechanical seals. Any time an unexpected seal failure would be inconvenient - Down time can be a very costly time in many plants. Two seals prevents the unexpected seal failure shut down. This is especially important with batch operations or when there is no back up pump installed. On the atomic submarine NAUTILUS the back up shaft seal allowed us to get to the surface if a main shaft seal failed while we were submerged.
Sealing non lubricants. ●
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Dry solids - They can clog the sliding components and provide no lubrication for seal faces. Once the faces are open they penetrate between the faces and usually destroy the lapped surfaces. Pharmaceuticals, freeze dried coffee and cake mix are examples of this category. You can think of many more. Non lubricating fluids such as solvents and hot water. We experience more rapid face wear with these types of fluids. In most cases their film thickness is less than one micron and cannot support a load between two sliding surfaces Dry gases- unlike non lubricating liquids they will not conduct heat very well and often are dangerous at the same time. This is a common problem if you forget to vent the stuffing box of a vertical pump. A top entering mixer is another example of this type of application.
Slurries, especially abrasive slurries. Clog the seal components and destroy faces like the dry solids mentioned above. ●
The list of these products is without end. A slurry is defined as solids in liquid that cannot be dissolved by normal control of the temperature or pressure. The number of solids or their size is not important. They will collect on or in the sliding or flexing components of the seal causing the faces to open and then penetrate between the lapped faces causing leakage and damage. In some designs the springs or bellows (metallic or elastomer) will experience severe wear in a short period of time. In these designs it is important to rotate the fluid rather than have the bellows component rotate within the abrasive slurry.
Liquids sensitive to agitation : ●
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Dilatants - Their viscosity increases with agitation. This is how cream becomes butter. Some clay slurries have the same problem. The resulting high viscosities will restrict the free movement of the seal. When dealing with dilatants it is important that you do not continually rotate the fluid in the stuffing box area. Thixotropic fluids lower their viscosity with agitation. They seldom present a problem for mechanical seals except for an increase in seal face wear. Plastic fluids change their viscosity suddenly. Catsup is a good example of this type of fluid. Newtonian fluids do not change viscosity with agitation. They present no problem for mechanical seals.
Liquids that combine together to form a solid. ● ●
Epoxy is a combination of a Resin and a hardener. Styrofoam is formed by combining several liquids together.
We seldom have problems with these liquids in pumps because the blending takes place outside of the pump, but the problem sometimes comes up in mixer applications. You will note that I have not included anaerobic fluids (they solidify in the absence of air) in any of the categories (super glue is the product that first comes to mind). lubricating liquids ●
This is the ideal application for a mechanical seal but we seldom see it. More often than not we are sealing raw product that falls into one or more of the above categories. Back in the days when we were using packing in pumps we did not pay too much attention to these categories because we were either prepared to let the product leak on the ground or we would flush in clean liquid and concentrate on sealing the clean flush instead.
Now that leakage is no longer tolerable and product dilution is no longer desirable you must have knowledge of these categories to approach the job of effective sealing. In most cases the fluid you are sealing will fall into several of the above mentioned categories. Using Heat transfer oil as an example we note that it falls into the following :
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Hot - Normally pumped at 600 -700 Fahrenheit ( 315 -370 C.) the fluid is too hot for available elastomers. Film Building - The product "cokes" at these temperatures. Dangerous - You do not need this temperature oil leaking out. It is not only a fire hazard, but a personnel hazard as well. Recent information indicates that some of these oils are also carcinogenic. Costly - Most of these transfer oils cost between $12.00 to $20.00 per gallon (3,8 Ltrs.) Slurry - Because of the coking, solids are always present.
To successfully seal heat transfer oil you would have to address all of these problems at the same time. As is the case with all slurry applications, you would also have to recognize the problems with vibration (impeller imbalance), thermal growth, and frequent impeller adjustments. In addition to handling various chemicals we are often faced with extreme or severe operating conditions. These conditions would include: ●
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Hot products - Defined as too hot for one of the seal components, or hot enough to cause the fluid to change form. Heat transfer oil is a good example of a fluid that will "coke" at elevated temperature. Cryogenic fluids - They present a problem for elastomers and some carbon faces. Liquid Nitrogen or Oxygen would be an example. High Pressure - Defined as stuffing box, (not discharge) pressure in excess of 400 psi. (28 bar). Pipe line and boiler circulating pumps can have stuffing box pressures of this magnitude. Hard Vacuum - Defined as 10-2 Torr or below. This number is well below most condenser or evaporator applications, but does come up every once in a while. High Speed - Defined as the seal faces moving greater than 5000 Feet Per Minute or 25 meters per second. Most process pumps do not approach this speed. The Sundstrand "Sundyne" pump is typical of a high speed application. Excessive motion - defined as more than 0.005 inches (0,15 mm.) in a radial or axial direction. Mixers, agitators and specialized equipment have shaft movements up to 1/8 inch (3 mm). Long shaft vertical pumps and pumps equipped with sleeve or babbitt bearings, are another application for excessive motion. Excessive vibration - Unfortunately there are no reliable numbers for the vibration limits of mechanical seals. Most vibration studies have addressed the bearings. It is important to consider that excessive vibration can: ❍ Open the lapped seal faces. ❍ Chip the outside diameter of the carbon face. ❍ Break the metal bellows used in some seal designs. ❍ Wear the driving mechanism used to transmit torque from the set screws to the seal faces. ❍ Loosen drive screws. ❍ Shorten bearing life ❍ Most seal designs can damage (frett) expensive sleeves and shafts. ❍ Some, but not all designs have built in vibration dampers to relieve some of these problems.
In other papers on this site you can learn how to seal each of these fluid categories and learn how to
protect the mechanical seal against the affects of these extreme operating conditions.
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SUBJECT : Let's clear up the confusion about flushing seals 3-6 Consumers use the term "flushing" to describe six different methods of bringing fluid to the stuffing box area of a centrifugal pump. Experienced seal people use different terms to differentiate between the methods. DISCHARGE RECIRCULATION A line is connected between the discharge of the pump and the stuffing box. The high pressure fluid is then recirculated through the stuffing box to the back of the impeller and eventually to the pump discharge. This technique presents several problems for maintenance people: ●
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If the fluid contains solids ( and most of them do) the centrifugal action of the impeller will concentrate the solids on the inside diameter of the pump volute and it is this dirty fluid that is being recirculated to the stuffing box. Needless to say this will not be good for the mechanical seal because the solid particles will act as a "sand blaster" cutting into the lapped seal faces and clogging the sliding seal components. The pump wear rings, critical tolerances and close fitting bushings will experience rapid wear as the solids pass through the narrow clearances.
The only legitimate use of this technique is to pressurize the stuffing box to prevent a liquid from vaporizing. Be careful if you use this method in hot water applications especially if a heat exchanger is installed in the recirculation line. A high temperature water or steam leak in any of the fittings could be dangerous for any personnel in the area and the solids can clog up the heat exchanger. When this line is used to pressurize the stuffing box you should keep several additional things in mind: ●
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Install a close fitting bushing in the bottom of the stuffing box. It will look like the thermal bushing described in the above illustration. The clearance should be .002 inches/ inch (0,002 mm/ mm) of shaft diameter. Be sure to direct the line away from the lapped seal faces and the thin metal plates if you use a metal bellows seal. If you are using properly installed , balanced O-ring seals (and you should be), The sealed product will not flash between the faces as long as the stuffing box pressure is a least one atmosphere higher than the liquid vapor pressure. The discharge recirculation line should guarantee you will have this pressure difference.
SUCTION RECIRCULATION A line is connected between the suction of the pump and the bottom of the stuffing box or seal gland connection. Many pumps have a connection already tapped at the suction throat of the pump for a suction gage, but if none is available you can install one in the piping or a pipe flange if the piping is not thick enough to be drilled and tapped.
Stuffing box pressure is almost always higher than the suction pressure of the pump. Liquid from behind the impeller will be circulated through the stuffing box to the pump suction. This liquid has been centrifuged by the impeller and the result is that the liquid in the stuffing box is considerably cleaner than what you are pumping. In many cases you can eliminate the need for bringing in clean liquid and diluting your product. This environment control works very well in closed impeller pump designs and those open impeller designs that adjust towards the pump volute rather than the back plate, such as the Duriron pump. FLUSHING A clean liquid, from an outside source is brought into the stuffing box through a regulating valve at one atmosphere (15 psi. /1 bar) higher than stuffing box pressure. The liquid should be brought in at the bottom of the stuffing box to insure thorough cleaning. All of this liquid will eventually go into your product. If you are using balanced O-ring seals you will only need enough liquid to remove solids that might interfere with the seal movement. You will not need additional liquid to provide cooling because balanced seals do not generate enough heat to cause problems in most applications. Seal designs that have the springs out of the fluid require only one to two gallons per hour (4 to 8 Ltrs./ hour) of flush. NOTE: this is per hour, not per minute. If you are using designs with multiple springs in the fluid check with your manufacturer for his recommendations. The clean flush can come from several sources: ● ● ● ● ● ●
Clean water A compatible fluid A solvent One of the ingredients in the product Finished product will never hurt raw product. Finished product is almost always clean. An additive that is going to be put into the product down stream and can be added at the pump stuffing box instead.
If you are using shop water as the flush you must be careful or solids in the flushing water will clog up the flow control valve. The shop water pressure also tends to vary through out the day and in some instances it can fall below the pump stuffing box pressure. Most states require an air gap in the line if you want to use shop or city water as a flushing medium. A back flow presenter valve is used many times but it is illegal in most states. BARRIER OR BUFFER FLUID
Any time you use two seals in an application you will need a fluid between them. If the fluid between the seals is higher than stuffing box pressure we call it barrier fluid. If it is lower than stuffing box pressure we call it buffer fluid The liquid can be circulated either by forced circulation, a pumping ring or convection. The method that you will use will be dictated by the pressure, pump speed and shaft size. All seal manufacturers have charts available to give you the correct guidelines. If you elect to use a forced circulation system be sure to introduce the fluid into the bottom connection and out the top connection. This arrangement will insure that the space between the seals is vented and proper cooling will take place. Forced circulation is the recommended method with all vertical shaft applications, although it is possible to offset the centering of the seal gland and get a small amount of pumping action as the liquid circulating in the seal changes its velocity at the convection tank connections. Check with your local distributor for an explanation of this principle. Many of the latest seal designs utilize a built in pumping ring to enhance convection. This pumping arrangement is very necessary when ever oil is used as the barrier fluid. The following illustration shows a typical convection system that can be used with two balanced seals.
Water is one of the best barrier or buffer fluids because of its high specific heat and good conductivity. Petroleum oil is probably one of the worse because of its low specific heat and poor conductivity. Keep this in mind when you select a barrier or buffer fluid for your seals. The type of seal you select will determine if the barrier fluid has to be kept higher or lower than the stuffing box pressure. Fluctuating pressures are normal in this business so you should select seals that balance in both directions to eliminate any problems that might be caused when the barrier fluid or system pressure varies. Be sure to connect the convection tank or forced lubrication system so that the inlet is at the bottom of the double seal and the outlet discharges from the top of the seal. This arrangement will allow the seal to
vent, and insure that the passages are full of liquid. JACKETING FLUID (B)
High temperature pumps have a cooling/ heating jacket installed around the pump stuffing box. If a jacket has not been installed on your pump it can be purchased from the pump manufacturer or an "after market" supplier. The secret to using a jacketed stuffing box is to install a thermal bushing into the bottom of the stuffing box and then "dead end" the stuffing box liquid. Dead ending means that no suction or discharge recirculation lines should be installed. Any material that has poor thermal conducting properties will be satisfactory for the bushing provided it is compatible with what you are sealing. Carbon is an excellent choice because unlike Teflon it does not change dimensions too much with a change in temperature. A small amount of liquid or steam through the jacket can control the stuffing box to what ever temperature range you need. In some instances cool heat transfer oil is utilized. Keep in mind that this jacket is also providing cooling to the bearing case as well as the stuffing box. Be sure the jacketing fluid is free from calcium (hard water) or any substance that can build a film on the inside of the jacket surface and restrict the heat transfer. A number of cleaners are available if you experience this problem. Condensate is a good jacketing fluid that presents few problems and is usually available. QUENCHING - Often called vent and drain (Q connection on an A.P.I. gland)
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Some seal glands have a vent or quench connection provided behind the seal so that steam or some other fluid can be used to control temperature in the seal area. A close fitting carbon ( or any other non sparking material) bushing is installed outboard of this connection to provide a close clearance between the gland and the shaft. Refinery applications use a version of the quench gland and call it an A.P.I. ( American Petroleum Institute) gland.
Now that you know the names of the six different methods let's see how we use them in various sealing applications: DISCHARGE RECIRCULATION ● ● ● ●
You can use it to pressurize the stuffing box to prevent product vaporization. You can install a heat exchanger in this line but it is only effective when the pump is running. Do not install a filter into this line because it will clog up and restrict stuffing box recirculation. Many consumers install a "cyclone separator" type of device into the recirculation line. These separators have never proven to be very effective in removing solids from the stuffing box fluid.
SUCTION RECIRCULATION ● ●
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You will need this line to vent a vertical pump. Use the line for normal product recirculation and to replace the stuffing box fluid with cleaner fluid that has been recirculated from behind the impeller.. This connection provides a safe way to drain the stuffing box prior to seal removal.
FLUSHING ● ● ● ●
To introduce clean liquid into the stuffing box, to remove solids or any problem fluid. Cool a hot liquid by flushing in a cold one. Remove a liquid that is sensitive to changes in either temperature and/ or pressure. You can use this connection to cross connect the stuffing boxes in a double ended pump application, and thereby equalize the pressures in the stuffing boxes.
BARRIER OR BUFFER FLUID ● ● ● ● ● ● ● ●
To stop a pressure drop across the inboard seal. To protect the dynamic O-ring in Ethylene Oxide applications. To control temperature at the seal faces To stage pressure in a high pressure application. To keep air or oxygen away from a seal face. To detect inner seal leakage when used with a convection tank. To shift the load to the outboard seal when sealing a non lubricant with the inboard seal. This is the normal method of sealing a gas.
JACKETING FLUID ●
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The best method of controlling temperature in the stuffing box when the pump is shut down. Be sure to install the thermal bushing or it will not work very well. Make sure that there are no suction or discharge recirculation lines connected.
QUENCHING OR VENT & DRAIN - plus the disaster bushing.
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The disaster bushing will protect the seal from hitting the inside of the stuffing box if you have a bearing failure. This is a very important feature in those applications where the product will burn or explode if overheated. The disaster bushing will protect personnel if there is a massive seal failure. The majority of the leakage can be directed, down the drain connection, to a collecting tank or vent. To wash away solids from the outboard side of the seal that will prevent "hang up" as the seal face wears and the seal moves forward. To wash away toxic or corrosive vapors that might leak across the seal faces. To control the temperature in the seal area. As a back up to a heating/ cooling jacketing failure.
The rest of the world calls all of these techniques "FLUSHING". Try to acquire the habit of using the proper terminology so that you will avoid confusion when you communicate with seal people and your fellow workers.
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SUBJECT: Controlling the pressure in the stuffing box area 4-10 We have very little control over the products that we must seal, but we have a great deal of control over how we elect to seal them. Special seals are seldom the answer. In most cases satisfactory seal life can be obtained by carefully choosing the seal materials for temperature and chemical compatibility, selecting a balanced O- ring seal design and then controlling the environment in the stuffing box to prevent a change in the fluid characteristics from affecting the seal performance. Controlling stuffing box pressure is one of these controls and is extremely important in many seal applications. In the following paragraphs we will discuss several methods of controlling pressure but first we will learn where the environmental control is necessary. We can raise the pressure in the stuffing box to : ●
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Prevent a fluid from vaporizing in the stuffing box or across the seal faces. If the product vaporizes across the seal faces it can open the faces and possibly do some damage as the faces rapidly open and close. In many cases solids will be left between the faces as the fluid vaporizes. To destroy a vacuum in the stuffing box. A balanced O-ring seal can seal either vacuum or a positive pressure. Vacuum often implies higher heat at the seal faces and that is never good for a mechanical seal. Most split seal designs can accommodate either vacuum or a positive pressure application, but not one that alternates between them ( the reverse pressure is forcing the splits open). Raising the stuffing box pressure will keep it positive so that a split seal can be applied.
We can raise the pressure between two seals to : ●
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Stop a pressure differential across a dynamic elastomer from failing the elastomer. This is a serious problem when we seal ethylene oxide or any fluid capable of penetrating the elastomer and blowing out on the low pressure side. To prevent sub micron solids from penetrating between the lapped faces. Kaoline is a good example of this type of product. Sub micron products have no problem penetrating lapped seal faces when the pressure drop is from the outside diameter to the inside diameter of the seal face. To take the load off the inboard face that is sealing a non lubricating fluid and transfer the load to the outboard seal. This can make a dramatic difference in the life of the inboard seal To prevent a pressure drop across the faces that could cause a product to solidify. Many solids are dissolved in a liquid that will vaporize at atmospheric pressure. Paint is a good example of this application. To prevent a liquid from vaporizing between the inboard faces and blowing them open.
We can lower the pressure in the stuffing box : ● ●
If it is too high for a standard balanced O-ring seal To vent the air from a vertical pump.
Now to the actual techniques :
Discharge recirculation. In this application we connect a recirculation line from the discharge side of the pump to the stuffing box or a flush connection in the gland. You should install a close fitting bushing into the bottom of the stuffing box with a clearance of .002 inch/ inch (0.002 mm/mm) of shaft diameter. The bushing can be manufactured from any compatible material with the fluid you are sealing. Carbon is often selected as a first choice. A properly installed balanced O-ring seal will not generate enough heat to flash a liquid between the seal faces as long as the stuffing box pressure is at least one atmosphere (one bar or 15 psi.) higher than the product vapor point. You should have no problem in getting this additional pressure if you have installed the restriction bushing. Suction recirculation The technique is the same as discharge recirculation, but in this application we connect the stuffing box or seal gland to the suction side of the pump or a low pressure sump instead. We do not use a restriction bushing in this application because the differential pressure can cause the bushing to move and contact the mechanical seal. Suction recirculation works best with the proper gland connection but it can be used with the lantern ring connection if necessary. Be sure the connection is on the bottom or as close to the bottom as possible of the stuffing box or gland. Caution: Lowering the pressure in the stuffing box is sometimes a bad idea because of the danger of flashing the sealing fluid. The technique is commonly used, however, to remove air that might be trapped in the stuffing box of a vertical pump or to provide normal circulation through the stuffing box when the sealing fluid contains solids. By connecting to the suction side you will be pulling fluid from behind the impeller, through the stuffing box and then to the lower pressure on the suction side of the pump. Fluid behind the impeller usually contains less solids than fluid coming from the pump discharge side. It should be mentioned that this technique works very well on most closed impeller pumps and those open impeller designs that adjust to the volute of the pump. Open impeller designs that adjust to the back plate and double ended, single impeller designs have the stuffing box pressure just about at suction pressure so this application does not work very well. We go to larger diameter stuffing boxes in those applications. Another common application for this technique is to cross connect the stuffing boxes of a multi stage pump to equalize the pressures and balance the seal face wear. Using two or more seals with a lower pressure between them. Some seal companies use this as a technique to stage the pressure in a high pressure application. I do not approve of this method because the operator is lead to believe that there are multiple seals in the pump when in fact the multiple seals are acting as one and a failure in any one of the seals will fail them all. I believe you would be better off purchasing a high pressure seal for this application.
Using two seals with a convection tank installed between them. This is done to take the load off the seal that is sealing a non lubricant or to prevent a pressure drop across the seal faces. The convection tank is filled with a lubricating liquid and the pressure is adjusted to provide the necessary pressure differential. With the proper instrumentation you will be able to tell which seal wears out or fails first. The second seal will act as a back up until you can shut the valves and start the repair. As an example : In the pump stuffing box you have a 75 psi.(5 bar) non lubricating liquid. You cannot afford product dilution so you install two seals with a lubricating buffer fluid of 75 psi. (5 bar) between them. This will take the load off of the inboard seal that is sealing the non lubricant and shift the load to the outer seal that is containing the lubricating barrier fluid.
The convection tank can be purchased or manufactured from an appropriate corrosion resistant material. Some companies (Coca Cola as an example) ship their product in a similar tank and then scrap the tank because of sanitary or safety regulations. Many of these tanks can be purchased at a low price and modified for your needs. The air connection on the top will allow you to pressurize the tank to the correct pressure for your application. Keep in mind that convection tank applications are limited by the combination of seal size, face combination, barrier fluid pressure and shaft speed. Check with your seal supplier for a specific recommendation. When ever possible avoid selecting petroleum base liquids for the barrier fluid circulating in the tank and between the two seals. Petroleum fluids have a very low specific heat that will cause overheating and "coking" problems. If you have the choice. Water is the ideal heat barrier fluid because of its conductivity and high specific heat number. If water is not acceptable choose any compatible fluid with a high conductivity and high specific heat value.
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SUBJECT: Controlling the temperature in the stuffing box area 4-6 Many fluids are adversely affected by a change in their temperature, and when this reaction takes place seal failure is almost sure to follow. The reaction can take several forms: ●
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One of the seal components can be destroyed. The elastomer, seal faces, or metal parts will almost always be altered at some degree of change in ambient temperature. Coated hard faces can "heat check" (crack). Carbon fillers can melt and pits can form in carbon/ graphite faces as trapped air expands and blows out pieces of the carbon. Hydrocarbons can solidify (coke) between the seal faces and pull out pieces of carbon also, causing small pits that will prevent you from conforming to fugitive emission standards. Carbon graphite faces can lose their lubricating ability at cryogenic temperature and chip on the outside diameter as "slip stick" vibration takes place. Elastomers can take a "compression set" and crack at elevated temperature Cold temperatures can cause elastomers to harden. The liquid can crystallize restricting seal movement and opening the faces. The liquid can vaporize between the faces causing them to open. The viscosity of the fluid can change either restricting seal movement or making the fluid less of a lubricant. The liquid can solidify causing the seal to become inoperative. The liquid's corrosion rate will double with an 18° Fahrenheit (10° C) rise in temperature. The liquid can convert to a film between the sliding seal components, restricting their movements. The magnetite that forms in hot water is a good example of this. A film can form on the seal faces causing them to separate. Lapped seal faces can distort and go out of flat at elevated or cryogenic temperatures.
By keeping the stuffing box temperature within specified limits you can prevent all of the above from happening. These limits vary with each fluid, but they can be obtained from any one knowledgeable about the fluid that has to be sealed. A balanced mechanical seal incorporating the following features and installed at the proper compression, is your best insurance against a significant rise in stuffing box temperature: ● ● ● ●
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Proper face balance. 70/ 30 is the most common to 5000 fpm. (25 Meters per sec.) Low friction face materials. Carbon/ graphite vs. a hard face is the best. The correct spring compression to control face loading. Faces with good heat conductivity. Tungsten carbide and silicone carbide have excellent thermal conductivity compared to most other hard face materials. A small cross section carbon/ graphite face press fit into a metal holder is better than solid carbon/ graphite for removing heat from between the seal faces.
Sometimes, however, that is not good enough, so occasionally you will have to come up with some additional method of controlling the temperature in the stuffing box area and between the lapped seal faces.
THE HEATING / COOLING JACKET.
If your pump does not have a heating/ cooling jacket installed one is usually available from the pump distributor. If possible try to select an oversized stuffing box with a cooling/ heating jacket cast around it. This jacket can be used to heat a product, cool a product, or keep the product within close temperature limits. When using the jacket there are several important things to keep in mind: ●
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Install a carbon bushing into the bottom of the stuffing box to act as a thermal barrier. The clearance over the shaft should be about 0.002" per inch diameter of shaft (0,002 mm/ mm of shaft diameter). The length should be at least 3/8 inch (10 mm). Dead end the stuffing box. In other words no discharge, flush or suction recirculation lines connected to the stuffing box. Many hot fluids contain lots of solids; the dead ending feature will allow you to centrifuge the fluid and clean it up. If you elect to use water as the jacketing fluid make sure that it is not hard water as it will form a layer of calcium on the walls of the jacket restricting the heat transfer. This jacket is also used to cool the rotating shaft in hot applications. If the cooling is lost the heat will conduct back to the bearings causing their premature failure. Steam is an excellent medium to control heating or cooling. A regulating valve can be installed on the discharge side of the jacket for precision pressure control which will, in turn, control the stuffing box temperature within narrow limits. A mixing valve proportioning steam and water is another method of controlling temperature within precise limits.
THE QUENCH OR DRAIN CONNECTION
Quench and drain connections are available in American Petroleum Institute (API ) type glands. The quench and drain connection (Q) is used in conjunction with a close fitting, non sparking disaster bushing (DB) ●
When using the quench for temperature control keep in mind: ❍ Excessive fluid will be directed towards the bearing case. Be sure to use only small amounts of steam or water. It would be wise to replace the existing grease seals with
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mechanical bearing seals if you elect to use quench as your primary temperature control method. The drain connection should be connected to a suitable tank to save the condensed steam. The quench connection can be used to vent a volatile product to a flare where it can be burned. The quench fluid can also remove any solids that have built up outboard of the seal as well as remove any vapors that might leak across the seal faces. This is an important feature when sealing products that can crystallize at the seal faces and on the outboard side of the seal. The drain connection is used to direct the major amount of failed seal leakage away from the bearings or any personnel in the area. It should be connected to an appropriate tank for retention.
THE FLUSH CONNECTION In temperature control applications we flush in cooled product to control the stuffing box temperature. If you use the pump fluid cooled, or cooled finished product you will have no problem with product dilution. THE DUAL SEAL Another method of providing temperature control is to utilize two seals with the correct temperature liquid circulating between them as a barrier or buffer fluid. Look at the following illustration:
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When using a dual seal for heating/ cooling be sure to bring the fluid into the bottom of the seal gland and out the top of the gland to insure that the void between the seals is full of fluid. This is an excellent method of controlling the temperature at the seal faces if you are experiencing an over heating problem. In many instances a convection tank can be installed between the seals but it will seldom do an adequate job of lowering or raising the barrier fluid temperature. In almost every instance forced circulation will be necessary if you need any degree of heating or cooling. Convection tanks are satisfactory for removing the heat generated by balanced seal faces, but that is about all. In some instances a convection tank has been used with an installed cooling coil and a pumping ring built into the mechanical seal. The amount of barrier fluid circulation needed will be determined by the seal size, speed, and stuffing box pressure. Your seal supplier will gladly supply this information. Water should be selected for the barrier fluid when ever possible. Oil is a poor choice because of its low specific heat and poor conductivity.
THE HEAT EXCHANGER The normal procedure is to install the heat exchanger in the discharge recirculation line connected between the pump discharge and the stuffing box. If you elect to use this method be careful of the following: ●
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This can be dangerous in hot water applications because a leak in any of the fittings will direct high pressure, hot water into the atmosphere and some one may be standing close by and become injured. Many hot fluids also contain solids that will clog up the heat exchanger. The temperature control is effective only while the pump is running. Many seal failures occur at start up because of lack of proper temperature control while the pump was idle. If you want to use this method, and only a small amount of cooling is necessary, a commercial automotive, automatic transmission cooler can be used effectively in many applications. A heat exchanger can be used with a pumping ring. In this application low pressure fluid is circulated out of the top of the stuffing box, to the heat exchanger, and then back to the seal through the bottom connection on the seal gland.
OTHER CONSIDERATIONS ●
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Vertical pumps require venting or the seal will trap air in the stuffing box, causing high heat at the seal faces. To vent the stuffing box properly, connect a suction recirculation line between the seal flush connection and the pump suction. Vertical pump applications also present a problem for dual seal applications. You will need to provide some method of venting air trapped at the outside seal. Carbon/ metal composites are a good choice for heat dissipation across the carbon seal face. Try to avoid seal faces that are thermally isolated by elastomers. Silicone carbide is a good choice for the hard face because of its excellent thermal conductivity feature. Use the alpha sintered type to avoid chemical compatibility problems If you elect to use anti freeze as a barrier fluid between two seals, do not use the automotive brands as many of them contain an anti leak chemical that will clog up the mechanical seal. Water is the best barrier fluid because of its high specific heat (1.0) and good conductivity. Oil is a bad choice because of its low specific heat (0.25/0.30), but if you must use it, try to select a heat transfer oil. Heat pipes should have application in stuffing box cooling, but their application experience is very limited. Try to select seal designs that have the elastomer positioned away from the seal faces. The elastomer is the one seal component that is very sensitive to temperature change. Because elastomers usually have poor thermal conductivity, cooling one side of the elastomer has a minimal affect on the other side. Unfilled carbon/ graphite seal faces are absolutely necessary in higher temperature applications. Less dense seal faces experience trouble when air trapped below the surface of the carbon, expands and blows out pieces of carbon from the center of the seal face. The exception to this is high temperature oil that will coke at the seal faces and pull pieces of carbon away. These resultant pits will cause problems if you are trying to meet fugitive emission standards.
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In those pump designs where the open impeller is adjusted back against the back plate (Duriron), any impeller adjustment tends to over compress the seal faces causing high heat. Proper face load is essential to long seal life so cartridge designs should be specified any time you use open impellers and high heat is a problem. Keep in mind that the pump cooling jacket is also used to cool the shaft that is conducting heat back to the bearings. If you have a high heat application, you might consider a stainless steel shaft because of its' poor heat conductivity compared to steel. Some bellows seal manufacturers tend to tell people that they no longer need the stuffing box cooling and the result is premature bearing failure. A centerline design pump is always desirable in hot applications to prevent pipe strain at the pump suction and damage to the close clearance wear rings. Instead of supporting the volute at the bottom this design bolts the pump feet to the sides of the volute, allowing the volute to expand both up and down. The wet end off your pump can be modified to this configuration or a new wet end can be purchased. If the seal is going to be used in a hot oil application do not hydrostatically test the seal with water or a water based fluid. Moisture trapped in gaskets, elastomer clearances, and other small crevices will flash when it comes into contact with the hot oil, causing a potential damage to the equipment, seal and/ or the people that might be in the area. In cryogenic applications it is not practical to heat the seal area to protect the elastomer. A non elastomer seal with a special self lubricating cryogenic carbon is your best solution to this application. Be aware that the moisture laden atmosphere can freeze on the out board side of the seal restricting the seal movement as the faces wear. In most cases a dual, non elastomer seal with a non freezing barrier fluid between the seals is going to be your best choice.
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SUBJECT : Environmental controls and special seals 3-2 For any given seal application problem there are two generally accepted solutions : ● ●
Build a special seal that can compensate for the problem once it occurs. Control the environment surrounding the seal to prevent the problem from occurring in the first place. If you control the seal environment you will avoid the inventory and delivery problems associated with special seals.
In the following paragraphs I will be covering each of these environmental controls in detail. CONTROLLING THE TEMPERATURE IN THE STUFFING BOX AREA. ●
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Flush the stuffing box ( port "C" in the illustrations at the end of this paper) with a compatible, cool, clean liquid. Many seal glands have this connection available in a more convenient location than the lantern ring connection. Flush is a misunderstood term. It describes six very different functions: Discharge recirculation, where a line is connected from the discharge side of the pump to the lantern ring connection in the stuffing box (A), or an appropriate connection in the gland. Suction recirculation, The recirculation line is connected from the bottom of the stuffing box to the suction side of the pump (A). Jacketing fluid, The cooling or heating fluid flows through a jacket that is surrounding the stuffing box (B). Barrier or buffer fluid, The fluid is circulated between two seals either by convection or by a separate circulation system (E). Quench, The fluid is passed between the seal and a disaster bushing that has been installed in the rear of the seal gland (D). Flush, A liquid, from an outside source, is injected into the stuffing box at one atmosphere above stuffing box pressure and dilutes the product a small amount (C). Use two seals with a cool liquid circulating between them (E). A two way balanced cartridge seal would be an excellent choice. This arrangement provides cooling at the seal faces where it will often do the most good. Use the jacketed stuffing box that came installed on the pump (figure "B") or install one if it is missing. These jackets are available as a replacement part for the back plate on most popular pumps or as an after market bolt on accessory. To use the jacket properly: ❍ Dead end the fluid you are trying to control. This means no lines in or out of the stuffing box except those used to circulate the jacketing fluid. ❍ Install a thermal bushing in the bottom of the stuffing box. Carbon is a good choice because it is a poor conductor of heat compared to the metal pump components. A typical clearance over the shaft would be 0.002 inches per inch of shaft diameter ( 0,01 mm/mm of shaft diameter). ❍ Circulate the heating or cooling fluid, through the jacket, to control the temperature. Six to eight gpm. (25 to 30 liters /min.) is typical of the amount of cool water needed to cool down heat transfer fluid to the point where it will stop "coking" and Viton O-rings will be acceptable. If your water is too hard you can substitute condensate or low pressure steam.
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An A.P.I. Gland is available for most mechanical seals. The gland has several features to provide various functions. It can be used as: ❍ A quench connection (Q) to provide heating, cooling, or to remove any vapors that might escape between the seal faces. Steam can be injected to lower the seal temperature in the event of a fire. In the event of a major seal failure, this quench connection can be used, in conjunction with the gland disaster bushing, to direct seal fluid leakage to point where it can be collected. ❍ A flush connection (F) to provide clean fluid to the stuffing box or it can be used to vent the stuffing box and seal in a vertical pump application. ❍ A close fitting, non sparking, disaster bushing (DB) to provide shaft support in the event of a bearing failure or to protect personnel in the event of a massive seal failure. Heat tape or tracing lines can be installed around the stuffing box to provide a small amount of temperature control. Install a cooler in the line between the pump discharge and the stuffing box. Keep in mind that this system only works while the pump is operating so it would be of no value if the problem occurs during pump shut down Use only balanced seals in these applications to avoid the heat problems associated with unbalanced seal designs. Elastomers in the faces and two hard faces should also be avoided for the same reason.
CONTROLLING THE PRESSURE IN THE STUFFING BOX AREA ●
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Increase stuffing box pressure by installing a recirculation line from the pump discharge back to the stuffing box (figure "A") with a close fitting bushing in the bottom of the stuffing box. Try to avoid positioning the recirculation line so that it aimed at the lapped seal faces or thin bellows seal plate materials. Eliminate the pressure drop between seal faces by using two seals with a higher pressure barrier fluid circulating between them. This is very important in the sealing of chemicals such as ethylene oxide that will penetrate into the elastomer, expand and blow out the other side causing severe damage to the elastomer. Flush the stuffing box with a high pressure liquid. This is the best solution if the fluid contains solid particles that could interfere with the seal movement. The only reason to lower stuffing box pressure is because your seal does not have high pressure sealing capability. It is possible to lower stuffing box pressure by the use of environmental controls but a high pressure seal would be a much better choice. In an emergency you could lower the pressure by one of the following environmental controls: ❍ Equalize the pressure in the stuffing boxes, of a double ended pump, by connecting the stuffing boxes together to get even seal wear. This is a common application for a double ended centrifugal pump. ❍ It is possible to lower stuffing box pressure by installing a close fitting bushing in the bottom of the stuffing box and recirculate to the suction side of the pump. Be sure to "lock in" the position of this bushing with either a snap ring or some other retaining device to prevent it from moving towards the seal. Be careful of using this control on a vertical turbine pump because the high velocity liquid, recirculating to the suction, can heat up the line to the point where it can become "red hot". ❍ Lower the sealing pressure by utilizing an intermediate fluid pressure between two tandem or "two way balanced seals"
PROVIDING A LUBRICANT IF THE SEALING PRODUCT IS A NON LUBRICANT ( Non lubricants have a film thickness less than one micron) ●
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Use two seals with a higher pressure lubricant as the barrier fluid. This is an excellent choice in most gas applications or liquids that have little to no lubricating properties. This form of lubrication will often solve the problems associated with seal "slipstick" and some other types of vibration. Flush the stuffing box with a liquid lubricant. Cooling the product will sometimes turn a non lubricant such as hot water into a lubricating liquid. For some vacuum applications it makes sense to install a discharge recirculation line to destroy the vacuum in the stuffing box area.
NOTE. If the impeller has been adjusted too close to the back plate the "pump out vanes" can cause a vacuum to occur in the stuffing box. This often happens if the impeller adjustment has been made backwards (as is the case with Duriron pumps). The problem exists with those open impeller designs that adjust towards the volute (Goulds is an example). DECREASING THE AMOUNT OF LIQUID AGITATION IN THE STUFFING BOX. This becomes very important if you have to seal a liquid that increases its viscosity with agitation. We call these liquids DILATANTS. Connect the bottom of the stuffing box to the suction side of the pump to allow a single pass of the liquid through the stuffing box. Make sure the connection is very close to the seal faces. You will be better off using the seal gland flush connection rather than the stuffing box lantern ring connection. Some liquids decrease their viscosity with agitation. We call these liquids THIXOTROPHIC. In some instances the thinner liquid film can cause more face wear and seal "slip stick". If this problem exists use one of the environmental controls mentioned above. HANDLING SLURRY APPLICATIONS. ●
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Flush with a clean liquid. Check with your seal manufacture for the minimum amount of flush that is needed. Balanced seal designs with the springs located outside of the fluid and most metal bellows designs require only one to two gallons (4 to 8 liters) per hour. ❍ Note: this is per hour not per minute. At shaft speeds below 2900 rpm. filling the seal cavity with a compatible grease is usually satisfactory. Increase the seal clearance in the stuffing box area. Replacement back covers with extra large stuffing box designs are available for most pumps. Bolt on, large diameter stuffing boxes are also available in the after market. If your product is cool you can probably run the fluid "dead ended" with no connections coming into or out of the stuffing box. If you product contains sub micron particles as is the case with Kaoline (china clay), you will have to circulate a higher pressure clean liquid between two seals to prevent solids penetration between the faces. In some cases two hard faces also helps. Almost any dual seal design is acceptable with the exception of the "back to back" rotating design which is never acceptable in
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any application. Recirculate to the suction side of the pump when possible. This will circulate cleaner fluid from behind the impeller, through the stuffing box, and then back to the suction side of the pump. Original equipment manufacturers do just the opposite by having the stuffing box fitting connected to the discharge side of the pump. ❍ CAUTION! Do not connect to the suction side of the pump if the fluid is being pumped at or near its vapor point as this could cause flashing in the stuffing box location. If the solids have a low specific gravity (they float on the liquid) you may have to go to a clean liquid flush because centrifugal force will work against you. Any time that you deal with a slurry application you are going to have a couple of other problems as well so be prepared for them: ❍ Frequent impeller adjustment and excessive wear ring wear. You will need a cartridge seal or a sleeve mounted split seal to compensate for the impeller adjustment. Cartridge seals can generally be reused if the pump has been disassembled to replace the wear rings. ❍ Vibration will increase as the impeller goes out of balance due to abrasive wear. This can cause drive lug wear and carbon face chipping. Vibration damping will become very important. Seal designs that incorporate O- rings have a built in natural vibration damper. Metal bellows seals require another solution. ❍ Wear of the rotating components. This is especially true if the seal rotates in the fluid. Better seals are designed to cause rotation of the fluid in the seal chamber.
If you prefer to solve the application problem by using a special seal. the following thoughts might help in deciding your selection. SEAL DESIGN FEATURES THAT ADDRESS THE PROBLEMS OF EXTREMES IN HOT AND COLD. ●
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Metal bellows seals. An excellent choice for cryogenic and high temperature, non petroleum liquids. Petroleum products "coke" in the presence of high heat so cooling is necessary in these applications. Carbon / metal composites to conduct heat away from the seal faces. Do not use "glued in" versions. Elastomers located some distance from the seal face to protect the elastomer (rubber part) from the additional heat generated at the seal faces Low friction face combinations. Carbon / tungsten carbide or Carbon/ silicone Carbide are among the best. Some duplex material faces are showing good results in these applications. Carbon impregnated silicone carbide is an example of such a material. Elastomers that have a wide range of operating temperature. Kalrez® is a good example. Low expansion metals such as Carpenter 42 and Invar 36 that will still retain the carbon or hard face in the holder even though the temperature changes greatly. Be aware that low expansion metals have poor chemical resistance so be careful in using them. Stationary seal designs are subject to a differential temperature across the seal face and body if a recirculation line or flush is being used. This differential temperature can cause the face to go out of flat. You will be better off with a rotating design in this instance.
If you elect to solve only the sealing problem you must keep in mind that the extremes in heat and cold
will also affect the bearing seals as well as the bearing oil. Unless you address these problems separately you will be better off controlling the temperature in the stuffing box area and solving most of the bearing area problems at the same time. SEAL DESIGN FEATURES THAT ADDRESS THE PROBLEM WITH SLURRIES. ● ● ● ●
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Springs out of the fluid, the most common place to clog a seal. Vibration damping because the wear causes the rotating assembly to go out of balance. Be sure the dynamic elastomer moves to a clean surface as the seal carbon face wears. Use centrifugal force to clean the sliding seal components. Rotating seals (the spring loaded face rotates) should be your first choice. Non stick coatings on the metal parts to prevent a build up of solids on the sliding components. These coatings are porous so do not use them for corrosion resistance. If possible, rotate the slurry to reduce seal component wear. In future papers I will address the problem of sealing individual applications. In the mean time the above information should help you to get started.
®Dupont Dow elastomer
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SUBJECT: Some more about condensate 10-7 What is condensate? ●
Steam that has been condensed back into water by either raising its pressure or lowering its temperature. Not to be confused with demineralized, de-ionized, make up, or softened water. When the condensate enters the boiler feed pump additional chemicals are added and the product is now called boiler feed water.
Where does condensate come from? ● ● ●
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Condenser hotwells, the bottom part of the condenser Steam traps. They trap steam in the lines and let the condensate drain through. Heat exchangers. Condensate must be removed to allow the heat transfer. The condensate flows to the bottom where the steam trap will open and allow the condensate to flow to the receiver. There must be a positive differential pressure between the heat exchanger and the condensate line so that the condensate will flow out of the heat exchanger. If the differential pressure is not there a pump will have to be installed to remove the condensate. Or any other place that you are using steam.
We want to keep dissolved oxygen out of condensate. Why? ●
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It changes the pH of the water. This will contribute to corrosion problems in the system. Especially the boiler. ❍ Boilers like a ph of somewhere between 10 and 11. ❍ Hot water is almost the perfect solvent. Give it enough time and it will dissolve anything. Remember that boilers have to last thirty years or more. This means that water has plenty of time to do its damage. It is the oxygen in the condensate that makes condensate a strong oxidizing agent that can attack metals. Some carbon seal faces can be attacked by high oxygen levels in the condensate. The more gases entrained in the condensate the more likely the pump will experience cavitation problems. The condensate temperature determines the amount of dissolved oxygen. You are trying to conserve the energy (temperature) that was added to the steam to keep the amount of dissolved oxygen down
TEMPERATURE °F.
TEMPERATURE °C.
PPM DISSOLVED OXYGEN
30
1
10 ppm
90
32
5 ppm
120
50
4 ppm
150
65
3 ppm
180
82
2 ppm
210
100
0 ppm
The average level detected in condensate receivers is three parts per million. This is almost one thousand times greater than the five parts per billion level that can induce pitting corrosion. How does oxygen get into the condensate system? ●
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Through the packing of condensate pumps. The stuffing box is under a negative pressure, and air that is one third oxygen, leaks in. Valves above the water line can introduce oxygen as the velocity of the water lowers the pressure at the valve stem. Flanges can have the same problem as valves. Oxygen is dissolved in makeup water that was added to the boiler because of condensate leaks. Pumps with built in repellers that create a negative pressure in the pump stuffing box.
How do you get rid of the dissolved oxygen? ●
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Add chemicals to convert it. Hydrazine is an example. You are adding hydrogen that will combine with the oxygen to form water. In nuclear applications it is common to add hydrogen to the system for the same reason. Hydrogen and oxygen will combine to form water in a neutron flux. Deaerate the condensate. This is normally done by heating the condensate with steam in a deaerating tank that is located close to the suction of the boiler feed pump. Use balanced, O-ring mechanical seals that will prevent air from coming into the stuffing boxes of condensate pumps. Seal valves and flanges to prevent air from entering the system.
Why do we have to use so much "make up" water in our boiler ? ●
Because we lose so much of it. ❍ Condensate pump discharge recirculation lines that are trying to put a positive pressure on packing are a common source of condensate loss. ❍ Boiler blow down is a major problem. Some boilers run with a constant blow down because air that is entering the system is changing the pH of the water, causing chemical addition that increases the total solids, causing the need for additional blow down.. ❍ Steam tools. ❍ Air ejectors that are used to create a vacuum in receivers etc. ❍ Steam traps that drain to the ground.
What are some methods for conserving condensate? ●
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The discharge recirculation line used with packed pumps is a big waste. Convert to a balanced oring seal and save a pile of condensate. Stop air from entering the system. The air is causing frequent boiler blowdowns. You can easily seal flanges, valves and rotating shafts. Do not drain steam traps to the ground. Collect it in a tank that can be pumped back into the system. If condensate is being circulated through the cooling jacket on a pump, make sure it is not being discharged to a drain. There is no reason it cannot be returned to the condensate system. If condensate is being circulated between dual mechanical seals, it is not a good idea to return it to the condensate system. There is too a high probability of contaminating the condensate with product leakage.
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SUBJECT: The sealing of high pressure and hard vacuum 5-7 High pressure does three things that will damage any mechanical seal : ●
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It will create a high axial hydraulic load on the seal faces. This load will be in addition to the axial force created by the single spring, multiple springs, or metal bellows that are used to create the initial face loading in popular seal designs. This excessive axial loading can: ❍ Generate heat that could be detrimental to one or more of the seal components such as the elastomer or in some cases, the product that you are sealing. ❍ It can cause excessive wear in a short period of time. This will be a very important consideration when you are sealing non, or poor lubricating fluids. Thixotrophic fluids (they lose their viscosity when agitated) will also be affected. ❍ If the product is a poor lubricant or a non lubricant, a high axial load can cause "slip stick" problems that can chip the carbon outside diameter and possibly open the lapped seal faces. ❍ It can change critical dimensions, such as the lapped seal faces going out of flat. It can distort one or more of the seal components causing the lapped seal faces to go "out of flat." ❍ Seal faces are subjected to "hoop stresses" that attempt to shrink the material. Since these faces are seldom designed as a "solid block" the affect is to alter the lapped face flatness. Finite element analysis design techniques help, but are still limited in practice. High pressure can extrude the elastomer (rubber part) in many seal designs, either "locking up" the seal or causing leakage where the elastomer was extruded. In almost every case the elastomer suffers permanent damage.
The excessive hydraulic pressure can come from several sources that include: ●
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The normal system pressure. In most single stage pump applications the stuffing box pressure is slightly higher than suction pressure, but multi stage pump applications, boiler circulating pumps, and some pipe line service pumps can experience very high stuffing box pressures. Water hammer and pressure surges can cause a very high temporary pressure in the system. Unusual system operation is another cause. The rapid opening and closing of valves can cause these surges of pressure. A loss of power to a running pump can cause vacuum pockets in the lines. As the liquid rushes to fill up these vacuum voids, very high pressures can be experienced.
The solution to high pressure sealing falls into three separate categories. You must decide which of the approaches makes the best sense in any given application. The three approaches you can use are: ●
Build a seal that can handle the excessive pressure. ❍ Select hydraulic pressure balanced seal designs to lower the axial load. ❍ Higher modulus materials are seldom available. You will have to go to a finite element stress analyzed design. Look for seal components that have uniform thickness cross sections, or go to larger cross section seals that will require more stuffing box radial room. ❍ Laminated bellows are available for many higher pressure metal bellows applications (same principal as plywood).
Higher durometer O-rings with non-metallic back up rings are available to prevent elastomer extrusion. Stage the seals in an application so that several seals will be sharing the pressure. ❍ Tandem sealing with an intermediate lower barrier fluid pressure is the most common. In some nuclear applications three seals have been connected in tandem to handle the high pressure. Tandem and other types of multiple seal arrangements take a great deal of axial room. In every case you are moving the first seal further away from the bearings so shaft stabilization becomes very important. You should also remember that the multiple units are acting as a single seal. In other words if you fail one of the seals, you fail them all. Lower the pressure in the stuffing box. ❍ Locking a restriction bushing into the bottom of the stuffing box and then connecting a suction recirculation line from the bottom of the stuffing box to a lower pressure location in the system is the normal way to accomplish this. Watch out for erosion of this bushing, especially in abrasive applications. Be aware that if stuffing box pressure is near the product vapor pressure, flashing could occur in the stuffing box or between the lapped seal faces. ❍ You can cross-connect stuffing boxes in a multiple stage, double ended pump design. Keep in mind that this will not work with single stage centrifugal pumps. ❍
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Vacuum means less than atmospheric pressure and vacuum sealing falls into two categories: Normal vacuum. This vacuum is usually measured in inches or millimeters of mercury. ●
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This is the vacuum found in condensers, evaporators and at the suction side of the pump every time you use the centrifugal pump to lift liquid. Hydraulic balanced seal designs can handle this vacuum because vacuum only means one atmosphere of pressure (15 psi. or one bar) coming from the other side of the seal. O-rings are preferred for the elastomer design. Continuous O-rings can seal either vacuum or pressure. They also have the ability to flex and roll to compensate for shaft movement. Carbon metal composite seal faces are satisfactory as long as the carbon is sealed at the inside diameter to prevent the pressure from penetrating behind the carbon, upsetting the hydraulic face balance and possible blowing the carbon out of its holder.
Hard vacuum. This vacuum is normally measured in microns, micro inches, or portions of a millimeter of mercury (Torr). ●
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Elastomers are not acceptable for hard vacuums. The vacuum will cause the elastomer to "out gas" increasing the elastomers' density and reducing the volume to a point where leakage is possible. All metal seal designs will probably be your first choice. Seal face density and self lubrication can be a real problem in hard vacuum applications because of the lack of moisture to release the graphite from the carbon/ graphite compound. Conventional seal designs are seldom satisfactory in these applications. A great many materials exist that can solve the problem, so you will want to contact your seal supplier for the availability of higher density and self lubricating carbons for these special applications. Tandem dual seals with a higher pressure lubricating barrier fluid is the most common solution to hard vacuum sealing.
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SUBJECT : The sealing of hot oil 3-5 The largest user of hot oil pumps is the heat transfer oil customer. Many consumers use these products with oil temperatures exceeding 500° Fahrenheit (260° C.) and 600° to 700° F. ( 315° to 370° C.) becoming common. Some hotels have recently installed these systems in their laundry to dry clothing. Heat transfer oils have many advantages over the steam that was formally used in these applications. ● ● ● ● ● ● ● ● ● ●
The product does not flash. No boiler blow down. No deaeration heat loss. No high pressure. This means it is not only safer but also tends to leak less. No licensed boiler operator needed. The temperature can be kept uniform over a large processing area. You can heat and cool with the same system. These oils are excellent in systems that are water/ steam sensitive. The product is kept in a closed system. This means that all leakage can be stopped. There is less corrosion in the system.
In addition to these heat transfer oils you will encounter hot petroleum oil applications in refineries and hot organic oil applications in various other industries. There are several problems associated with sealing these hot oil products and each of them has to be solved if satisfactory seal life is ever to be obtained. ●
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High temperature oil is generally too hot for most commercially available elastomers. (the rubber parts) The product "cokes". These coke particles form at the elevated temperatures and coat them selves inside the system piping, hardware and on the mechanical seal working parts. These "coke" particles restrict the movement of sliding/flexing seal components causing the lapped seal faces to open. The amount of coke that forms is a function of time and temperature. In other words, coking will be a more severe problem in a closed loop system than it will be in the oil refining business. Contrary to popular opinion testing has shown that air or oxygen is not needed for the formation of coke. This means that seal designs that try to eliminate the oxygen by quenching or some other method will not work. The use of steam quenching is limited to its' cooling effect only. The product is always a fire hazard and depending upon the type and brand you purchase, there could be toxicological problems. Keep in mind that the seal is going to wear out or fail at some time and the product will leak out to the atmosphere. Thermal growth of the pump parts will cause problems in maintaining proper pump "wear ring" and impeller clearances, as well as the correct seal compression. Misalignment between the driver and the pump and between the piping and the pump suction is a serious problem at elevated temperatures. The product is costly. Leakage represents large monetary losses and personnel danger as well as environmental problems. Heat tracing must be provided throughout the system to prevent the product from becoming too
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viscous during periods of prolong shut down. No one ever heat traces the stuffing box. Vibration is always a problem with hot oil pumps because the coke attaches to rotating components interfering with the dynamic balance. You always end up pumping a slurry which means frequent impeller adjustments or wear ring replacement and excessive vibration due to the imbalance caused by wear of the rotating parts. As the coke builds up on the inside of the discharge piping the pump will operate further off of its best efficiency point (B.E.P.) causing shaft deflection, vibration, and excessive seal movement. Coking on the inside of the suction piping can cause cavitation problems
Although there are many techniques available to the seal man to address each of these problems the combination of these problems eliminates most of the common techniques and leaves the customer with very few options to get good seal life. Regardless of the seal selected you must address all of the problems or the seal life will be shortened. Oil refineries pump hot oil with closed impeller pumps and as a result have to put up with the additional problems associated with replacing "closed impeller" wear rings. Unlike the chemical industry they cannot take advantage of the features of an open impeller design that can be easily adjusted to maintain maximum efficiency. There are two reasons why oil refineries chose closed impeller designs with mechanical seals and A.P.I. glands : ●
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Fear of a bearing failure that could cause sparking as the metal impeller contacted the metal volute. The soft non sparking, metal wear ring on one end of the shaft and the carbon disaster bushing installed in the A.P.I. Gland on the other, would insure no hard metal contact if a bearing failed as the shaft was turning. Shaft expansion or impeller adjustment could cause the rotating, open impeller to contact the stationary volute. To prevent sparking the impeller or volute would have to be manufactured from a soft non sparking metal such as aluminum or bronze and this would not be very practical.
To insure long seal life you must do the following: The product has to be cooled in the seal chamber : ●
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The oil must be cooled to stop the coking. Coke is a function of heat. Many years ago it was believed that oxygen had to be present for coking to occur, but testing has shown that this is not true. You can coke any petroleum product in an inert atmosphere as long as the temperature is high enough. The finest lubricating oil available will start to coke at 300° F (150° C). The amount of coking that you get is determined by the oil temperature and time. The oil must be cooled to prevent damage to any elastomers that might be installed in the seal or shaft sleeve. Elastomers that are subjected to high heat will first take a compression set and then shrink in volume. They will eventually grow hard, crack and leak excessively. The oil must be cooled to reduce the amount of heat that will be transferred through the shaft to the bearing oil or grease. This heat will reduce the viscosity of the lubricating oil or grease and eventually cause premature bearing failure. The SKF bearing company states in their lubrication literature, that the life of bearing oil is cut in half for each ten degrees Centigrade (18° F) increase in bearing oil temperature. They recommend 60° C to 70° C (140° F to 158° F) as an ideal oil temperature.
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The grease or lip seals are sensitive to any increase in shaft temperature. A stainless steel shaft is a good choice in these applications because stainless steel is a poor conductor of heat compared to carbon steel. This is the reason there are no stainless steel frying pans unless they are clad with either aluminum or copper.
You must install a back up seal for the following reasons: ●
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The product is dangerous. Leaking hot oil can start a fire or injure any personnel in the area. Many brands are toxic and some have been identified as possibly carcinogenic. The product is too costly to tolerate even small amounts of leakage. Back up cooling is necessary if the primary cooling method fails. A back up seal, with a cool barrier fluid system, can provide this cooling If you elect not to use a back up seal, then be sure to install an American Petroleum Institute (A.P. I.) type gland. Look at the following illustration. The gland can perform several functions for you:
The disaster bushing can provide shaft support if you lose a bearing. The leakage will be directed to the quench and drain connection when the seal wears out or fails. The quench connection will allow you to use steam for product cooling, but do not use too much because it could penetrate into the bearing case. You can connect steam to the quench connection and use it to put out a fire, should it occur on the outboard side of the seal. In this application the flush connection is not used. The stuffing box is "dead ended" to take full advantage of the heating/ cooling jacket.
A large diameter cooled sealing chamber should be installed on the pump. ●
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To allow centrifugal force to throw solid coke particles away from the seal faces and sliding or flexing components Misalignment is always a problem in these pumps. This shaft displacement can cause the rotating seal to rub against stationary parts in a conventional stuffing box. Vibration means movement . The seal must be free to move within the seal chamber. When the pump stops gravity will pull solid particles to the bottom of the stuffing box. A large seal chamber will almost guarantee that the particles will not collect around the seal at this time.
A Cartridge seal is necessary. ●
Thermal growth will cause volute casing and shaft expansion. Only a cartridge seal will compensate for this movement and allow for the impeller adjustment that will be necessary.
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The wear caused by the slurry will cause frequent impeller adjustments. The average pump used in these applications has almost 0.250 inches (6 mm) of adjustment possible.
To compensate for misalignment you will have to : ●
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Use a "C" or "D" fame adapter to compensate for misalignment between the pump and its driver. ❍ These adapters are available from all good pump companies and will compensate for misalignment as the pump goes through its temperature transients. ❍ No other method of alignment works any where near as well. If you are going to do a conventional alignment with dual indicators or a laser aligned be sure your calculations compensate for thermal growth. Use a "centerline" wet end to prevent excessive wear ring wear and pipe strain at the pump suction. If your pump did not come equipped with this type of wet end it can easily be installed in the maintenance shop. Look at the following illustration:
The illustration shows the centerline design. It will allow the pump volute to thermally expand both up and down, and thereby eliminate strain on the suction piping.
Now that we have discussed these important points lets take a look at some solutions that are often offered, but that we should not adopt as our solution. Here are the things that do not work well : Bad solution #1. Use a metal bellows seal to eliminate the need for cooling in the seal area. Comment: Although the metal bellows does not have rubber parts that are sensitive to high temperature cooling is still needed for the coking. Most bellow suppliers offer an A.P.I. type gland to provide low pressure steam behind the seal for cooling purposes and thereby eliminate the option of backup sealing. This quenching should be limited to only a back up cooling status. If quenching is done with water rather than steam, watch out for a calcium build up outboard of the seal. This "hard water" build up can restrict the movement of the flexing portion of the seal as it tries to compensate for face wear. If you substitute condensate for the quenching fluid the build up can be eliminated almost entirely. Bad solution #2. Run a line from the discharge of the pump through a cooler and filter to cool down and clean up the oil going into the stuffing box. ●
Comment: The problems with this solution are obvious. The filter will clog and the cooler will
become inoperative as coke builds up on the tubes. Bad solution #3. Use two seals and run a cool oil between them. ●
Comment: You have addressed the cooling problem but you have not addressed the problem of the slurry with this solution.
What then is the best solution that addresses all of the problems? Look at the following illustration:
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Install a large jacketed sealing chamber. These bolt on accessories are available from your local pump/seal supplier. Many suppliers can provide a replaceable pump back plate with a large seal chamber cast into the plate Dead end the stuffing box. In other words no lines coming into or away from the inner seal chamber. Do not worry about the heat. With a six to eight gallon per minute( 20 to 30 liters/ minute) flow through the cooling chamber the cooling jacket can keep the temperature down to 200° to 250° Fahrenheit (95° to 120° C.) without any trouble. If you have hard water in your area, condensate may be the best choice to use as the cooling medium. In some cases low pressure stream is satisfactory. If you anticipate long periods of shut down, low pressure steam will be your best choice as it will keep the heat transfer oil at the proper low viscosity during these shut down periods. Install a cartridge dual seal that has the inner seal balanced in both directions. If the pump does not have precision bearings a double motion seal with the same features will work just as well. "Two way" balance is necessary because the system and barrier fluid pressure can and will vary. The dual seal is necessary to conserve the expensive product and to provide a safety feature when the inboard seal wears out or fails. It will also allow you time to schedule a seal replacement. Install a convection tank between the two seals and use cool heat transfer oil as the barrier or buffer fluid. A lower pressure or buffer fluid is preferred. A slight pressure on the tank will allow you determine which seal has worn out or failed first. A pumping ring or forced lubrication between the seals is necessary Install a carbon restrictive bushing into the bottom of the stuffing box to act as a thermal barrier. Applications have worked without this bushing but not as well as with it. Any material that has poor heat conductivity will work as well as carbon as long as it is non sparking and dimensionally stable.
That is all there is to the application. Centrifugal force will clean up the small amount of fluid in the sealing chamber while the cooling jacket holds the temperature low enough to prevent coking and injuring the seal elastomer. The only problem with this system is that it works so well we often forget to clean the cooling jacket on the pump. A small layer of calcium inside this jacket will provide an insulation and destroy the cooling affect of the jacket. Be sure to keep this jacket clean or substitute steam or condensate for the cooling water, and then don't worry about it. Here are a few additional thoughts: ●
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A cartridge dual bellows seal can be substituted as long as adequate vibration damping has been provided to prevent breakage of the bellows. With metal bellows seals try to rotate the fluid in the sealing chamber to prevent excessive wear of the thin bellows plates. The bearing grease or lip seals should be replaced with labyrinth or positive face seals. The O.E. M. lip seals have a design life of about two thousand hours (84 days) and they will cause costly shaft fretting damage. These grease or lip seals will also allow moisture to penetrate into the bearing case dramatically reducing bearing life. If you eliminate these lip seals, you will be able to convert to a solid shaft and improve the "stiffness ratio" enough to prevent some of the shaft bending and vibration that is experienced at start up, and as the pump runs off of its' best efficiency point. A cool oil flush with a restriction bushing installed into the bottom of the stuffing box, is another choice. Be sure that the flushing pressure remains at least one atmosphere (15 psi. or 1 bar) higher than the stuffing box pressure. Do not hydrostatically test the seal with water. Any moisture left in the seal or trapped in a gasket will flash to steam when the hot oil enters the seal. This could be dangerous. When using an A.P.I. type gland be sure to check that the quench and drain ports have not been confused with the flush ports. If these ports are connected incorrectly it could be very dangerous. If you are using stationary bellows seals with a cool oil flush be careful to direct the flushing fluid away from the seal face. Since the bellows is not rotating the cooling on one side and the hot system temperature on the other can cause the bellows seal face to go "out of flat". Recent tests show that carbon faces always experience some pitting in hot oil applications. In the past these pits were ignored, but fugitive emission standards dictate that two hard faces should be use in all hot oil applications.
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SUBJECT : The sealing of hot water 3-3 Water is normally considered a good lubricant and can do an adequate job of providing lubrication between the lapped faces of a mechanical seal, but there are a few problems: ●
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At elevated temperature the water lubricating film is not thick enough to separate the sliding surfaces of the seal faces. Cold water has a film thickness of about one micron which will keep lapped seal faces separated most of the time. Hot water has a film thickness of only one third to one half of that amount depending upon the temperature. At some combination of temperature and pressure the water will vaporize, expand, and open up the lapped seal faces. When this occurs: ❍ The carbon outside diameter can become chipped and damaged as the constant vaporizing and subsequent cooling vibrates the seal faces causing them to bang together. Drive lugs will wear, metal bellows can break and lug driven hard or soft, faces can crack. ❍ Solids dissolved or suspended in the water will be left between the seal faces when the water vaporizes. They will imbed into the softer face causing severe wear and damage to the hard face. ❍ A phonograph finish can form on the carbon if a large particle of scale or any foreign matter is blown across the two faces. The seal will leak through this damaged face. "Slip stick" can occur because the faces are trying to stick together due to a lack of lubrication between them. The alternating sticking and slipping will produce a vibration that will chip carbon, break bellows and crack lug driven faces unless some form of vibration damping has been installed. In many piping systems magnetite ( Fe304 ) forms on the inside surfaces as a corrosion resistant covering. This magnetite breaks loose from the piping walls and often collects on the seal components. It can be recognized by its black or reddish color and its attraction to a magnet. The magnetite affects the seal a couple of ways : ❍ Being an abrasive material, it will mechanically attack the seal sliding elastomer by penetrating into it This will cause "hang up" and eventual leakage. ❍ It will wear the sliding elastomer sealing surface. ❍ Loose magnetite is very common in new water systems. The problem will eventually clear its self up after the system has been in use for about a year and the ferric oxide has formed into a stable layer. Hot water is dangerous. The leakage will be invisible as it flashes to steam. ❍ If the hot water is part of a condensate system it may have to be sealed under vacuum conditions.
In order to seal this product effectively, you must address all five problems at the same time. We will begin by learning how to pick the correct materials for the seal components, then we will choose a seal design and finally apply the correct environmental controls to insure that the above problems are being addressed. Picking the correct seal materials: ●
The seal face combination should be an unfilled carbon graphite running against either solid
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silicon carbide or tungsten carbide as your first choice. Plated or coated faces should not be used in this application. A new face material made from graphite impregnated silicon carbide has become very popular in recent years because of its better heat conductivity. The elastomer. Use ethylene propylene to 275 degrees Fahrenheit (135 C.) If you seal at a higher temperature, either Dupont's Kalrez or an equivalent will be necessary. In most cases you should be trying to cool the water to increase the face life. If the water is cooled a high temperature elastomer is not necessary. Be sure that you do not put petroleum grease on the ethylene propylene. Any petroleum product will attack ethylene propylene rubber (epr) The metal components. 316 grade stainless steel is preferred. Metal bellows or springs should not be manufactured from stainless steel to avoid chloride stress corrosion problems. Hastelloy "C" is your best choice for the springs or metal bellows.
Choosing the correct mechanical seal ●
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A balanced, O-ring mechanical seal should be used. Both rotating and stationary versions are acceptable although stationary is preferred. The O-ring will allow sealing in both directions if the application alternates between vacuum and pressure. A cartridge seal should be used for ease of installation and in the case of open impeller pumps, to allow for impeller adjustment as the pump cycles between operating and ambient temperature. Do not use cartridge mounted stationary seals unless they have been fitted with some type of self aligning feature. A motion seal should be specified if the pump is equipped with sleeve or babbitt bearings. This is a very common arrangement with multiple stage boiler feed pumps. A high pressure seal should be used if the seal chamber pressure (not the pump discharge pressure) exceeds 350 psi. (24 bar). High pressure seals are of a more rugged construction that prevents face distortion and elastomer extrusion. Split seals can be used in some of these applications, but some designs have trouble when the pressure alternates between a positive pressure and vacuum. Sleeve mounting the split seal helps with impeller adjustment, or in the case of vacuum applications the seal can be installed backwards, or with a discharge recirculation line installed to keep a positive pressure in the stuffing box. Note: many hot water applications are dangerous so dual seals are recommended. Care must be exercised if you use a stationary metal bellows seal design. Flow through the normal flush or recirculation connection can cause a substantial temperature differential across the seal face that can cause the lapped seal faces to become distorted.
The environmental controls you will need to seal hot water: ●
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To insure the longest possible seal life, the water should be cooled as close to ambient temperature as possible. The cooler the water the better it will lubricate the faces. Install a carbon bushing into the bottom of the stuffing box to act as a thermal barrier. Utilize the jacketed stuffing box on the pump, to cool down the stuffing box fluid. Be sure there are no recirculation or flush lines coming into, or out of, the stuffing box. If there is no jacket installed on the stuffing box one can be purchased from the pump manufacturer or an outside vendor. If you purchase the jacket from an outside vendor be sure to order the enlarged, jacketed seal chamber or replacement back plate with the large, jacketed seal chamber cast into it. ❍ NOTE : Be sure the cooling jacket is functioning. If you are in an area that has hard water
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calcium can coat the jacket surfaces interfering with the heat transfer. In that instance you must provide for jacket cleaning on a regular basis or substitute condensate as the cooling medium. The cooling jacket is also necessary to prevent heat transfer to the bearing case. Each 18 degree Fahrenheit (10 C.) rise in oil temperature will cut the life of the oil in half. If cooling is not at all possible, another alternative is to pressurize the stuffing box to at least one atmosphere above the water vaporization pressure. This can be done by installing a close fitting bushing into the bottom of the stuffing box and using a recirculation line from the pump discharge to pressurize the box. As noted above, be careful of leaks in the fittings. This could be dangerous in some high pressure boiler feed pump or boiler circulating pump applications. Depending upon the pressures involved you may be better off with a special high pressure seal design. ❍ NOTE : You are going to have trouble when the heat transfers back to the bearing oil. Many pumps have a bearing oil cooler available to provide the necessary cooling. Check with the manufacturer for this accessory. At 200° Fahrenheit (100° C.) non contaminated oil has a useful life of only three months. The lip or grease seals used in these applications have a useful life of only three months also, even when the temperature is closely controlled. These seals should be replaced with labyrinth or positive face seals. It is not wise to install a cooler between the pump discharge and a conventional stuffing box. Although this arrangement will provide adequate cooling, in most cases it is too dangerous at elevated temperatures because of possible leaks in the piping and fittings. Tandem seals, with a pumping ring and cooler installed between the seals is another alternative, but this application takes a great deal of axial room. An A.P.I. type gland with a cool quench is not a good choice for this application. ❍ The quench water will vaporize when it hits the hot surfaces under the seal, causing solids to form that will restrict the seal movement and contribute to the corrosion of the seal sleeve and other components. ❍ Those designs that have the springs out of the sealing fluid can easily clog the springs in this solution. ❍ Excess quenching water can leak back into the bearings through the grease or lip seal.
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SUBJECT : The sealing of liquid slurries 3-4 A slurry is defined as solids suspended in liquid that cannot be dissolved by controlling the temperature and/ or pressure. The solids may or may not be abrasive. It does no good to try to identify the number of solids or their size because no one knows how these numbers relate to slurry related seal problems. Whenever you deal with slurries there are several problems you must consider: ●
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The slurry can clog the flexing parts of a mechanical seal causing the lapped faces to open as a result of both shaft and seal movement. If the slurry is abrasive it can wear the rotating components. This can be a serious problem with thin plate metal bellows seals. The pump rotating assembly will go out of balance as the slurry wears the impeller and other rotating components. This will cause excessive moving of the seal components. The pump will lose its efficiency as critical tolerances wear rapidly. This can cause vibration and internal recirculation problems. The wear will also cause the need for frequent impeller adjustments that will cause problems with mechanical seals
It is generally believed that the main problem with slurries is that they penetrate between the lapped seal faces and cause damage. Although this is true, it is also true that they cannot penetrate until the seal faces open. Seal faces should be lapped flat to within three helium light bands. That is a distance just a little bit shy of one micron. Compare this to the fact that the smallest object that can be seen with the human eye is forty microns in size, and you will appreciate the technology used in the manufacture of mechanical seals. As a matter of comparison look at a common coffee filter. It filters out particles larger than ten to fifteen microns. All of this means that the seal is in fact a superior filter and as long as you can keep the two lapped faces in contact there little chance for solids to penetrate the faces and do any type of damage. There are three approaches to the sealing of solids : ● ● ●
Design a seal with non clogging features. Create a clean sealing environment for the mechanical seal. Do a combination of both
Let's look at each of the approaches, and in the process learn a sensible method of sealing a slurry. Build a seal with non clogging features. ● ● ●
Take the springs out of the sealing fluid.They cannot clog if they are not in the slurry. Make sure the sliding or flexing components move towards a clean surface as the seal faces wear. Take advantage of centrifugal force to throw the solids away from the sliding/flexing components and lapped seal faces.
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Use a non stick coating to prevent the slurry from sticking to the sliding components. Use only balanced seal designs. Additional heat generated at the seal faces can cause many products to solidify, coke, and crystallize creating even more solids problem. Metal bellows designs can be used, but they must have extra thick plates to resist excessive wear. Extra convolutions will have to be provided to compensate for the higher spring rate caused by these thicker plates. Rotating the abrasive fluid with the bellows can be a big asset. Some commercial designs have this feature.
Create a clean sealing environment. Give the seal as much radial room as possible. You can either bore out the packing chamber or install a large bore sealing chamber. Try to give yourself at least 1 inch (25 mm. ) radial space if possible. The more room you can provide for the seal the better off you are going to be. Try to remove the solids from the sealing area. There are a number of techniques for doing this. Some work and some do not. Let's look at each of them. First we will look at the solutions that do not work very well and comment on their problems : ●
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Bad Solution #1. Connect a filter in the line from the pump discharge to the stuffing box. Since the discharge is a higher pressure the flow of liquid through the filter will clean up the fluid and then there will be clean liquid flowing to the stuffing box.. ❍ Comment : The problem with this idea is that the filter will clog and no one will clean it. Bad Solution #2. Install a cyclone separator into the line instead of a filter. Connect it between the pump discharge and suction with the third (the center) port connected to the stuffing box. ❍ Comment: This idea is just as bad. The cyclone was never intended to be a single pass device. They work well if used in a bank of several filters but there is not enough pressure differential between the suction side of a pump and the stuffing box for them to be effective. Bad Solution #3. Install the seal outside the stuffing box so the springs will not be located in the dirty fluid. ❍ Comment: The problem with this idea is that as the seal faces wear they must move forward and in doing so they will move into the dirty fluid. The result will be that the movable face will hang up in the solids and the faces will open. Another problem with this approach is that centrifugal force throws the solids into the seal faces and not away from them. Bad Solution #4. Install a double rotating seal in the "Back to Back" configuration with a higher pressure, clean liquid barrier between the seals. ❍ Comment: This is a very common approach to the problem and has all of the problems associated with installing the seal outside the stuffing box. In addition to a rapid failure you will also experience product dilution as the barrier fluid leaks into the pump. Bad Solution #5. Since we are discussing things that don't work we might as well try two hard faces. ❍ Comment: Needless to say they will not prevent the faces from opening and when they do open, experience shows that you are going to destroy both hard faces. Some seal salesmen may even try to convince you that the seal faces are designed to "grind up" the solid particles into a fine powder. In other words the seal is designed as some type of a "quasi-
milling machine" Now we will look at some methods that do work: Good Solution #1. ●
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Flushing with a clean liquid is a good method of cleaning up the pumping fluid. The amount of flushing you will need depends upon the design of your seal. If the design has multiple small springs in the fluid then more flushing will be required. There are various sources for the flushing liquid : ❍ Finished, clean product or one of the mixture's clean ingredients ❍ A compatible fluid. ❍ A solvent. ❍ An additive that is going to be added down stream and could be injected into the stuffing box location. ❍ Clean water. ❍ A compatible grease can be used with most balanced seals running at lower speeds NOTE : Never introduce live steam into the stuffing box as it could cause the product to flash and the pump to cavitate. Be sure to start with a flushing pressure that is at least one atmosphere (15 psi or 1 bar) higher than the stuffing box pressure. You can use a pressure gauge to determine stuffing box pressure. You can then use a flow meter to regulate the amount of flushing fluid. With intermittent service pumps it is a good idea to have an electrician install a solenoid valve with a delay switch that would allow the flushing fluid to come on thirty seconds prior to the pump starting and to leave the flushing valve open for a few minutes after the pump has stopped
Good Solution #2. ●
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Install an oversize, jacketed sealing chamber and "dead end" the fluid. "Dead ending" means that there are no circulation lines coming in or going out of the sealing chamber. You can use the cooling jacket to remove the heat being generated by the seal faces as centrifugal force cleans up the solids that are present in the small amount of fluid, trapped in the seal chamber. This solution works exceptionally well with fluids where temperature control is important. Heat Transfer Oil is a prime example. If the fluid you are sealing is not hot, the cooling jacket will not be necessary. Some times one shot of clean liquid into this oversize, dead ended stuffing box, is all that is necessary to seal even a severe slurry. Needless to say this application works best on a continuous running pump. If the specific gravity of the solids is less than the liquid they are mixed in, centrifugal force will not work for you. A clean flush will be necessary in this instance or one shot of a higher specific gravity compatible liquid.
Good Solution #3. ●
If the solid particles are sub micron in size two seals with a higher pressure barrier fluid becomes necessary. In some instances you might want to use two hard faces on the inner seal. Kaoline and some dyes are a good example of products with sub micron size particles.
Good Solution #4. ●
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Install a large seal chamber on the pump and connect a recirculation line from the bottom of the stuffing box back to the suction side of the pump. The size of this line will be determined by the size and number of solids that you are trying to remove. This will cause liquid to flow from behind the impeller to the stuffing box and then on to the suction of the pump. Fluid entering the stuffing box, from behind the impeller has been centrifuged and should be a lot cleaner than the fluid you are pumping. This solution works well with closed impeller pumps and those open impeller designs that adjust to the front of the pump volute. If your open impeller adjusts to the back plate (as is the case with the Duriron pump) this method is not as effective. Do not use this technique if: ❍ You are pumping close to the vapor point of the fluid, as lowering the pressure could cause the pumping fluid to vaporize in the stuffing box and in some cases between the seal faces. ❍ You are sealing a Duriron pump where the impeller adjusts to the back plate. ❍ You are sealing double ended pups where the stuffing boxes are at suction pressure. ❍ If the solids have a low specific gravity or density and float on the liquid.
Compensate for the fact that the rotating unit will go out of balance. ●
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The seal faces have to be vibration dampened. O-Ring type seals are equipped with a natural vibration damper because of the dynamic elastomer that has been installed. Metal bellows seals have to be provided with some other method. Letting the seal face holder rub and vibrate along the shaft is a normal approach used by most metal bellows seal manufacturers. The logic is questionable. Give the seal room to move. Shaft run out and vibration can cause the seal rotating components to contact the inside of the stuffing box unless you have installed an oversized sealing chamber. Use motion seals if the run out or vibration is excessive. Unlike pump seals, these seals have much wider hard faces and extra internal clearances. Most popular designs can compensate for plus or minus 1/8" (3 mm. ) in a radial direction and 1/8" (3 mm) in an axial direction. Move the seal closer to the bearings. Split seal designs are a logical choice because most of them come with a stuffing box extension gland that positions them next to the bearings. A support bushing or sleeve can be installed in the end of the stuffing box to minimize the affects of unbalance, vibration and shaft whip or wobble. A variety of materials are available for these support sleeves. Check with your supplier for availability in your area.
The pump will lose its efficiency and experience more shaft movement as close tolerances wear. ●
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If you are using open impellers it will mean frequent impeller adjustment. In this case a cartridge seal is your best approach as impeller adjustments can be made without disturbing the seal face loading. Split seals can compensate for the initial impeller setting and split seals mounted on a split sleeve will easily compensate for movement caused by temperature growth or impeller adjustment. Closed impeller pumps will have to be disassembled and the wear rings changed when the
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clearances become excessive. If you are fortunate enough to have adjustable wear rings on your pump then only an outside adjustment will be needed and the pump will not have to be put out of service. Cartridge seals can almost always be reused in these applications because the seal faces were not separated as the pump was disassembled. Remember that with closed impeller pumps the wear rings will have to be replaced when the normal clearance doubles. A typical normal clearance would be 0.008" to 0.015" ( 0,2 to 0,4 mm). A good rule of thumb is that the pump will lose 1% of its capacity for each .001 inch (0,025 mm.) of wear ring wear.
A few more thoughts about the sealing of slurries : ●
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Kaoline (China clay) is a product that is used in many industries including paper and pharmaceutical. Its' abrasive particles are less than one micron in size and as a result will penetrate lapped seal faces causing rapid carbon and hard face wear. In this application it is necessary to use two mechanical seals with a higher clean barrier pressure between the faces to prevent most of the penetration. In addition to one of the recommended solutions mentioned above, two hard seal faces can also be used as some particles will always penetrate the faces. Using a combination of packing and a split mechanical seal is proving to be an ideal solution in many applications. With the seal installed there is no pressure differential across the packing and therefore the solids do not try to penetrate. Move the packing flushing line to the bottom of the split seal housing and flush the packing through this connection instead of the lantern ring or seal cage. The flushing is necessary to remove the additional heat being generated by the packing. You should be able to cut the flushing fluid volume down to about one third of the amount you had been using. Since the packing is not being forced to the shaft only a small amount of cooling is necessary. ❍ CAUTION! It is important that the flushing fluid be kept at a higher pressure than the stuffing box pressure. If this pressure differential fails it could force the packing into the rear of the mechanical seal. A split adapter plate installed between the split seal and the stuffing box face can prevent the packing from blowing out if the flushing pressure is lost. If you elect to use a rotating metal bellows in a slurry application, remember that the bellows should rotate the fluid in the sealing chamber. Most bellows designs allow the thin bellows plates to cut through the abrasive slurry and experience severe wear and breakage in a short period of time.
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SUBJECT: The sealing of mixers and agitators 3-7 Mixer applications are the same as process pump applications except for a couple of things: ● ●
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The mixer seal is subject to much more radial and axial shaft movement. Dual seals are usually required (especially in top entering mixers) because the seal is frequently located above the fluid causing a single seal to run dry. Many mixers run under vacuum conditions requiring the use of balanced seals. Bottom entering designs usually have a lot of solids in the seal area.
Please refer to the following drawingfor the bearing arrangement of a typical mixer. You will note that the motor turns the shaft through some gearing that is well supported by bearings. The problem starts when the mixer shaft passes through the stuffing box area.
As the impeller turns in the liquid it can cause excessive shaft whip and radial movement. As shown in figure "A" the packing helps to stabilize the shaft and is acting as another support bearing. Packing never was designed to act as a bearing so the result is major shaft wear and excessive leakage problems. The shaft movement is aggravated by several factors: ●
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If the liquid level in the tank is too low it can cause "vortexing". To prevent this vortexing you should measure the diameter of the mixer blade and then insure that the fluid level is at least one and one half times this distance above the turning blade. In many instances we are mixing a combination of liquid and a solid. Since it is customary to add the solids first they can cause "skidding" of the blade as it rides on these solids. The length of the shaft is too long for the diameter. In pumping we call this the L3/D4 ratio and recommend that the number never go over 60 (2 in the metric scale). In most mixer applications the number runs into the hundreds.
If you attempt to convert mixer packing to a mechanical seal you usually run into trouble with the excessive shaft motion. Some consumers have attempted to solve the motion problem by installing another anti- friction bearing closer to the mechanical seal. Unless the equipment is "line bored" (and it never is) it is not wise to put three anti- friction bearings on any rotating shaft. Picture "B" is one method of solving the problem. The mixer shaft is placed inside a "Quill Shaft" that is well supported by bearings and this quill shaft is then coupled to the mixer shaft. This design will allow the manufacture to place the second shaft bearing closer to the stuffing box area where it should have been in the first place. Unfortunately this new design adds several thousand dollars to the mixer cost. The manufacture typically packages a double seal and bearing into a spool piece that can be inserted into the two piece shaft. This original equipment seal is a poor choice because most mixer manufacturers choose an inferior unbalanced "back to back" double seal that will damage the expensive stub shaft and clog easily if there are solids in the mixing liquid. The following illustration describes this " back to back" double seal:
As the seal faces wear and the inner seal attempts to move forward it will hang up on the solids collected in front of the inside rotating face. Centrifugal force will throw the solids into the inside faces causing excessive wear in a short period of time. The best, and lowest cost solution is to install a motion seal in the stuffing box and solve the problem once and for all. I do not have an illustration for a motion seal, but you can check with your seal supplier for a catalog that will explain the seal in great detail. Motion seals are different than pump seals: ●
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Because of the misalignment potential they are almost always a stationary design. (The springs do not rotate with the shaft) The inner seal should be balanced in both directions because many of these applications can fluctuate between pressure and vacuum. This feature will also prevent the inner seal from blowing open and allowing the barrier fluid to contaminate the product if you loose barrier fluid pressure between the seals. The rotating hard faces must be wider than normal to accommodate the excessive radial motion. A design movement of plus or minus 0.125 inches (3 mm) would be typical. The internal clearances must be greater than pump seals to prevent contact during excessive radial movement. The stationary faces must be spring loaded in some manner to prevent loading one seal and
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unloading the other during periods of axial movement. The easiest way to do this is to spring load the faces together. Cartridge designs are necessary for correct and easy installation and to allow you to change seals without emptying the mixer in "Side Entering" and "Bottom Entering" applications. Be sure the seal has some sort of self aligning feature to compensate for misalignment caused by the set screws in the cartridge.
Single split seals, mounted on a split sleeve have become very popular for mixer type applications (see the following illustration). The shaft movement can be partially stabilized by installing a split sleeve bearing in the stuffing box. A wide variety of materials are available for this split bearing. Check with your local distributor.
The split seal is also a good choice for those mixers that have a moveable shaft that seals a Teflon ring against the tank wall and allows you to change the seal without emptying the mixer. This is a major improvement over the original equipment design because you do not have to remove the shaft spool piece. It should be obvious that when the spool piece is removed there is usually no force to hold the Teflon® ring against the mixer wall. Bottom entering shafts create a different set of problems. Look at the following illustration:
Dirt and solids fall into the stuffing box and cannot be centrifuged out. Flushing liquid tends to channel through the solids and does a poor job of removing them from the stuffing box. There are a couple of possible solutions. ●
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Cut off the stuffing box and move the seal into the vessel so that centrifugal force will throw the solids away from the flexible seal components Use a higher specific gravity fluid in the stuffing box to prevent gravity from pulling the solids into the seal area. A high density grease could be a good solution. ❍ If you cannot tolerate grease you should be able to find some type of a compatible fluid that will work.
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SUBJECT: The sealing of non lubricants 4-8 When we are discussing mechanical seals, a lubricant is defined as a fluid that has a film thickness of at least one micron (0.000039 inches) at its operating temperature and load. If the product we are sealing is not a lubricant we are forced to use the self lubricating characteristics of the carbon/ graphite mixture used in the manufacture of the seal face. The key to this self lubrication is that carbon can form strong chemical bonds with gases such as water vapor. The adsorbed gas then weaken the interlacing bonding forces, which in turn reduce the rubbing friction. Many other types of vapors and gases can be readily adsorbed by carbon/ graphite and in some instances inorganic compounds can be added to the carbon/ graphite if adsorbable gases are not present or in short supply. Graphitizing of the carbon (heating it to 5000 degrees Fahrenheit or 2750 degree Centigrade) is another approach to self lubrication. In the seal business we are faced with the challenge of sealing three types of non lubricants. I will address the problems in order of their difficulty : THE NON LUBRICATING LIQUID Hot water and many solvents fit into this category. The lack of lubrication at the seal faces causes more rapid wear of the carbon face. This carbon face is really a combination of carbon and graphite with the graphite being a good dry lubricant. As the seal face wears, the graphite is deposited on the hard face (you can see the black ring) leaving the carbon behind. The function of the hard face is to give the graphite a place to deposit. Testing has shown that when we seal a lubricating fluid the lubricant becomes trapped between these asperities (the peaks the graphite leaves when it deposits on the hard face) and in many cases becomes a vapor, separating the two running surfaces. A lack of lubrication between the seal faces can also cause a destructive form of vibration called "slipstick". Without proper lubrication the lapped seal faces try to stick together, but "slip" when the seal drive mechanism engages the drive lugs and inertia accelerates the faces off of these lugs. The faces then slow down as a result of the poor lubrication. This alternating "slipping" and "sticking" causes severe vibration with a resultant "chipping" at the out side diameter of the carbon face, along with drive lug and slot wear. The amount of wear experienced by the carbon /graphite mixture is affected by: ●
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The surface speed of the seal faces. (a combination of shaft rpm. and seal face diameter). PV numbers are not really valid because the carbon is sensitive to "P" but not to "V" The spring load on the seal faces and the area of the seal faces. The stuffing box pressure. Keep in mind that this number can vary during pump operation. The quality and grade of the carbon/ graphite face. The surface finish and hardness of the hard face. The cleanliness of the sealing fluid.
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The accuracy of the initial installation dimension. The hydraulic balance designed into the face. The hardness of the carbon. The thickness of the lubricating film. The affect of centrifugal and hydrodynamic forces on the face loading.
There is little chance of excessive heat developing between the seal faces and in the stuffing box area because the generated heat can be carried away by the conductivity of the non lubricating liquid surrounding the seal. All of the above means that the elastomer (O-ring) will probably not be affected by the extra heat generated between the seal faces, as a result of the poor or no lubricating properties of the fluid you are sealing. THE NON LUBRICATING GAS This application has all of the problems associated with the sealing of non lubricating liquids, but now you have the additional problem of heat because gases are for the most part good insulators and do not let the heat generated between the faces dissipate to the surrounding product and metal stuffing box. Heat can affect a seal several ways: ●
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Filled carbon faces can be damaged depending on the filler or binder that was selected. There are special filled carbons manufactured if the gas can not be adsorbed into the carbon/ graphite releasing the graphite to provide dry lubrication. The elastomer (rubber part) is probably the most sensitive to an increase in heat. Its proximity to the seal faces is very important in dry running applications. Heat can cause an initial compression set off the elastomer and eventual complete destruction. Each elastomer compound has a temperature limit as well as sensitivity to certain chemicals and compounds. Most fluids are affected by an increase in heat. They can: crystallize, solidify, lose their viscosity, vaporize, or build a film. In each of these cases, seal life will be affected. The corrosion rate of most corrosive fluids will double with an 18° Fahrenheit (10°C) increase in temperature. Seal flatness, face load, carbon squeeze, elastomer interference and many other tolerances can be affected by a change in stuffing box temperature.
SEALING A DRY SOLID You now have all of the problems associated with the sealing of a gas with the additional problem of a bunch of solids thrown into the mix. This application is seldom associated with pumps but is commonly found in mixer applications. The application is very similar to sealing a slurry so you should try to select those seal designs that have non clogging features. These features would include: ● ●
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Springs out of the fluid. You might consider rotating the seal in the powder to take advantage of centrifugal force, to throw the solids away from the sliding components. The elastomer must move to a clean surface as the seal face wears.
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Select non fretting designs. They are especially important in dry solids applications. Teflon coating of the rotating parts helps to prevent the solids from sticking to the moving components.
The majority of mixers designed with bottom entering stuffing boxes are especially sensitive to this problem. Try to locate the seal inside of the mixer and out of the narrow stuffing box or you will have trouble with the solids packing around the outside diameter of the mechanical seal. A clean flush with air or a suitable gas seldom works in this application because the air channels through the dry solids, or the vessel pressure will equalize with the incoming air pressure stopping the flow. Most of these applications are slow speed (less than 500 rpm.) so a non clogging type seal works well. A non metallic, outside seal can be used if you are prepared to clean it out with air or some other gas between batches. A split seal with air introduced into the bottom of the gland is getting good results in many applications. If the seal does clog up, it is easy to disassemble the seal for cleaning between batches. In some applications it is acceptable to use a compatible grease in the stuffing box to prevent the ingress of solids. A balanced O-ring type seal, running at lower motor speeds should not generate enough heat to affect the lubricating qualities of the grease.
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SUBJECT: Sealing products sensitive to a change in temperature or pressure. 8-9 Liquids and gases are both called fluids and a fluid can shorten the life of a seal in two ways; ● ●
It can cause the seal faces to open allowing solids to penetrate. It can damage one of the seal materials. Elastomers are very sensitive to both temperature and pressure.
In this paper we will be considering how small changes in either temperature or pressure will cause one or both of these failures to occur, and learn how to prevent these changes especially when the pump is stopped and often subject to both temperature and pressure fluctuations. In another paper in this series we learned that a change in temperature could: ●
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Cause a fluid to crystallize, sticking the seal to the shaft and making the faces open as the shaft moves. Caustic and sugar solutions are examples of this. Cause a liquid to vaporize, blowing the lapped seal faces apart, letting solids penetrate between the faces or causing damage as the faces bounce open and shut. This happens any time water flashes to steam. Cause some liquids to become viscous, preventing the seal faces from staying in contact. Bunker fuel oil becomes very thick when it gets cold. Cause some liquids to solidify, either sticking the seal to the shaft, preventing the flexible seal parts from moving or causing the seal faces to stick together. Sugar syrups do this when they get hot, some fluids do it when they get cold. Cause a film to build on the seal sliding components or between the faces. Oil varnish or "coking" is as typical example of this problem. Hard water will build a film on the seal sliding components as the water temperature increases. If the system is new and has not been passivated (protective oxide film on the metal surface) Ferric oxide or a similar oxide can build up on the sealing components. This build up will accelerate with temperature. Cause a liquid to become a non-lubricant. Water becomes less of a lubricant as its temperature increases. This lack of lubrication can cause "slip stick" problems between the lapped faces. The corrosion rate of most corrosives increase with a rise in temperature. A general rule of thumb says that the corrosion rate of an acid will double with a 18°F (10°C) rise in temperature. This is the reason we avoid the use of packing in acid pumps. You will recall the packing generates almost six times the heat of a balanced mechanical seal
If you are not using a dual seal with a pressurized barrier fluid, then you will get some sort of a pressure drop across the seal face. A pressure drop could: ●
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Cause the fluid to vaporize and blow open the lapped faces. If this happens several problems might occur: Allow sub micron size solids to penetrate between the faces, imbed themselves into the softer carbon and destroy the lapped hard face. As the product passes across the faces a cooling occurs, causing the faces to close. When the faces close, the cycle repeats its self and the alternating closing and opening will probably crack the carbon as it bangs against the drive lugs or chip the carbon face on the outside diameter.
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If the product freezes when it evaporates it could freeze any oil or grease that was put on the seal face, causing damage to the carbon. This will also freeze the moisture on the outboard side of the seal causing ice that can restrict the movement of the seal (you can see the ice on the shaft). Cause the liquid to solidify. Paint is a mixture of a solid and a solvent. If the solvent evaporates, the paint will solidify between the faces. This can also occur if the suction of the pump is under a vacuum (negative suction head) because the pump is trying to lift the fluid. Some liquids will form a film at the faces if the pressure drops
If the temperature or pressure of the pumping fluid never changed we would seldom have any application problems. But since pumpage pressure and temperature changes are normal (especially at shut down) we are going to have to become skillful in controlling the temperature and pressure in the stuffing box area to prevent a premature seal failure. THE HEATING AND COOLING JACKET
If you use this technique be sure to check: ●
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A carbon thermal bushing is installed in the end of the stuffing box to reduce the heat transfer between the product you are pumping and the fluid in the stuffing box. The cooling jacket must be free from scale and calcium build up. There are many cleaning products on the market you can flush through the jacket to insure that it is clean with out having to disassemble the pump. Dead end the fluid; no recirculation lines either into or out of the stuffing box. Check carefully because some of these lines can be hidden by insulation. We are trying to trap a small amount of liquid in the stuffing box that will be easy to either heat or cool. The best fluids to circulate through this jacket are steam and condensate. Shop or city water is generally too hard and will form a calcium film on the inside of the jacket. Remember that steam will act as a coolant with hot oil applications. The steam temperature can be controlled by the use of a regulator on the outboard side of the jacket. The temperature of steam is directly related to its pressure. You can use a mixer valve that will blend the steam and condensate to give you a very precise control over the stuffing box temperature. The main advantage of this control is that it lets you regulate the stuffing box temperature when the pump is shut down. That far out weighs the disadvantage of having to provide circulation to the jacket. Be sure to bring the coolant into the bottom of the jacket and out the top. This will insure that
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there are no bubbles trapped to restrict heat transfer. Because you are "dead ending" the fluid, centrifugal force will throw the solids away from the seal components and very soon the seal will be in a clean environment at exactly the right temperature. If your pump is not equipped with a jacket, one is probably available from the pump manufacturer or an after market supplier.
THE QUENCH AND DRAIN CONNECTION is used to heat or cool the outboard side of a single seal and wash away any product the might leak across the faces or build up outboard of the seal.
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Use only low pressure steam or water at connection "D". You do not want these products to penetrate through the disaster bushing and get into the bearings. This is another reason to replace those bearing grease or lip seals with either a labyrinth or a positive face seal. The non sparking disaster bushing has two functions: ❍ To direct most of the seal leakage to a drain where it can be collected, or a flare where it can be burned. ❍ To prevent the shaft from hitting the seal if you have a bearing failure. If the product burns this could cause a fire or an explosion. In any case the damage would be severe without this non sparking disaster bushing. ❍ A steam line hooked up to this connection can be used to put out a fire in the stuffing box area. All you need is a solenoid valve and a melt switch that will open the solenoid when it senses high temperature (same as a fire sprinkler system).
A DISCHARGE RECIRCULATION CONNECTION connected between the pump discharge and the stuffing box can be used to pressurize the stuffing box area with the discharge pressure available at the pump. ●
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Do not aim this connection at the seal faces or sliding components. The abrasive action of entrained solids can injure the lapped faces or destroy a seal component. Thin wall metal bellows seals are very sensitive to this abrasive action. The high velocity fluid can also interfere with the seal movement so be very careful how you make the connection.
THE DUAL SEAL is another way to control either temperature, pressure at the seal faces, or both at the same time. Take a look at the following diagram:
You can: ●
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Circulate a fluid at the correct temperature between the seals. You can cool the area, heat the area or hold the temperature at precise limits if that is desirable. Bring the fluid in the bottom and out the top to avoid air pockets You can pressurize between the dual seals to prevent a pressure drop across the seal faces. If you use the two way balanced version of a dual seal you can choose either a higher barrier or lower pressure buffer fluid between the seals. Fill the system and convection tank with anti-freeze and you will prevent ice from forming out board the inner seal. This can happen any time you seal a product that can freeze the moisture in the atmosphere.
HERE ARE A FEW MORE CONSIDERATIONS ABOUT CONTROLLING PRESSURE AND TEMPERATURE IN THE SEAL AREA: ●
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A cooler in the line between the pump discharge and the stuffing box is not a good method of controlling stuffing box temperature because it functions only when the pump is running, and many problems with crystallization, solidifying, becoming viscous, etc. occur when the pump is shut down. Flushing the system between batches seldom cleans the stuffing box area and the mechanical seal. Flushing the stuffing box with an outside fluid is the universal environmental control. You can always replace the fluid that is giving you trouble by flushing in a clean liquid at the right temperature and pressure. It will cause product dilution, but maybe you can flush in finished product or a fluid that is compatible with the fluid you are trying to seal. Heat tracer lines are often used in piping systems, but are seldom placed on the stuffing box. Maybe you will find it practical to trace and insulate the stuffing box for your application. There is little need to lower the pressure in the stuffing box area. If you find that the stuffing box pressure is to high for your mechanical seal, you are better off purchasing a high pressure mechanical seal that will satisfy your application.
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SUBJECT: Sealing products that are sensitive to agitation. 6-12 This category of sealing is the one that is the least understood by most of the people that are involved in the process industry. It is easy to understand how temperature can change an "easy to seal liquid" into a difficult to seal crystallized product, a solid, or a gas, but it is hard to see how agitation alone can have much of an affect, because pumped liquids are continually being agitated. Whether or not you are going to have a problem often depends upon how long the fluid is going to be agitated, and how fast the agitation takes place. We all know that cream becomes butter with agitation and if you beat it fast enough, and long enough, an egg white (a fluid) will become a solid. The fluid we find in a pump stuffing box seldom gets the proper circulation. The stuffing box lantern ring connection is commonly used for this purpose and if you will look at the area closely you will see that the fluid is trapped in the seal face area where it is exposed to long periods of high speed agitation. If the fluid is not affected by agitation or mixing, we say it is a Newtonian fluid (you remember, the apple fell on his head and he discovered gravity). These fluids are not considered a sealing problem for us unless they are sensitive to temperature or pressure changes, or contain lots of solids. The fluids we are concerned about are the non Newtonian fluids, and the problem ones fall into three neat categories: Dilatants. The more you agitate them, the more viscous they become and in many cases they can solidify. Any time a fluid becomes viscous it can interfere with the ability of the mechanical seal to follow shaft "run out" or vibration. This hysteresis or delay will allow solids to penetrate between the lapped faces or allow fugitive emissions to escape to the atmosphere. ●
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Dilatants are commonly used in industries that manufacture cleaners. You need this increased viscosity to hold the cleaner on a vertical surface. Many sugar syrups and clay slurries fall into the same category. In the paper industry the product "Kaoline" is a common example. To insure proper sealing, you must insure that the product circulates through the stuffing box only one time as would be the case if you used a suction recirculation line connected from the face of the seal, at the bottom of the stuffing box, to the suction side of the pump, or some other low pressure point in the system. In this application it is important to use either seals that have no spring or springs in the fluid, or you can use metal bellows seals.
Thixothrophic fluids are the opposite of dilatants. Their viscosity decreases with agitation. ●
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Non drip paint is a good example of this type of application as would be automobile wax or most of the very viscous hand cleaners you find available on store shelves. The decreased viscosity can cause the product to become a non lubricant as the film thickness diminishes to less than one micron between the lapped seal faces. This will cause an increase in face wear and in the case of carbon/graphite seal faces, create a potential color contamination problem with some color sensitive products.
Plastic materials release their viscosity suddenly and present the very same problems as thixotrophic fluids.
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Ketchup or the tomato sauce product you find in restaurant bottles is a good example of a plastic fluid.
When dealing with any of these problems be sure to keep the agitation in the stuffing box to a minimum. In some isolated cases the seal hydraulic balance diameter could be lowered and/or the spring face load reduced to lower the amount of shear. If you are running at higher than conventional motor speeds this can be a real problem. The use of two seals with a pressurized lubricant as a barrier fluid can keep a lubricant between the faces and diminish the color contamination problem. If color contamination is a real problem, the use of two hard faces is recommended. As is the case with just about any fluid sealing problem the use of a reliable, clean, compatible, liquid flush is the universal solution. It is often the only solution if you find that none of the above suggestions are practical in your application. In another paper we will discuss the affect of this changing viscosity on the performance of the pump. As is the case with the mechanical seal the change is significant.
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SUBJECT: Sealing to the Osha 1910.119 process safety management standard and the clean air act of 1990. 8-10 The Process Safety Management Standard was created to prevent the unwanted releases of hazardous chemicals. The standard identifies more than 130 specific toxic and reactive chemicals covered in specific quantities and processes that involve flammable liquids and gases in quantities of 10,000 pounds or more. Hydrocarbon fuels may be excluded if used solely as a fuel. A process is covered if it involves the toxic or reactive highly hazardous chemicals at or above the specified threshold quantity of the standard. The threshold quantity is the amount of the chemical present at any given point in time, not aggregated over a period of time. If you look in the last pages of this paper you will find a list of these highly hazardous chemicals, toxins and reactives, along with their threshold quantities. The clean air act was created in 1990 to address the escape to atmosphere of Volatile Organic Compounds (VOC). The proposed amendments for chemical plants apply to any component in contact with a substance that is at least ten percent applicable VOC, and is in gaseous or light liquid VOC service more than 300 hour annually. To determine the amount of VOC in a gaseous leak, the VOC is measured at a distance no more than one centimeter (less than a half inch) from the source. PUMP STANDARDS Phase 1 at the onset
greater than 10000 ppm.
Phase 2 one year later .
greater than 5000 ppm.
Phase 3 two and one half years later
greater than 1000 ppm
Polymerizing polymer
greater than 5000 ppm.
Food/ Medical
greater than 2000 ppm.
All other pumps
greater than 1000 ppm.
For components with moving parts (pumps) the first attempt to repair a leak must be made within two days after the leak is detected. The standard also require the monthly visual inspection of all single mechanical seals.
Pumps with dual mechanical seals can be exempted from the monthly inspection if the barrier fluid pressure between the seals is at a higher pressure than the pump stuffing box pressure at all times and the barrier fluid is not a light liquid VHAP (Volatile hazardous air pollutant), or is equipped with one of the following three features designed to prevent VOC emissions from the outboard seal: ●
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A barrier degassing reservoir that transports the gas or vapor to a VOC control apparatus. That system prevents applicable VOC from accumulating where it can be emitted from the outboard seal. A closed-loop system that purges the barrier fluid into a process stream and returns process fluid to the process without venting to the atmosphere. A sensor that detects failure of the seal system, the barrier fluid system or both.
If a leak is detected between the seals the first attempt at repair must be no later than five days and the repair or replacement no later than 15 days. Single seals are available that can satisfy current standards. They must be monitored monthly (EPA Method 21) and visually inspected weekly. If they are detected leaking: ● ● ●
The first attempt at repair must take place within five days. Repair or replacement within fifteen days. Phase three pumps, when the leak is greater than 2000 ppm.
The only sensible approach to the sealing of fluids and gases identified in these acts is the use of dual seals designed with a two way hydraulic balance and the barrier fluid pressurized at least one atmosphere above maximum stuffing box pressure. The tandem configuration would be a logical choice for both rotating and stationary versions of a dual seal. In the following diagram I am showing a simple, unbalanced version of a dual tandem seal for demonstration purposes. The preferred configuration would be: ● ● ● ● ● ●
Hydraulically balanced seals with the inner seal balanced in two directions (two way balance.) The stationary version of the tandem configuration is the most desirable. A pressurized convection tank connected between the seals. A remote indication of the pressure in the convection tank. A pumping ring built into the seal. to increase the circulation between the seals. A cartridge version of the seal, with some sort of "self aligning" feature to prevent excessive face movement of the stationary seals.
The preferred configuration should not only satisfy the regulations, but also will provide additional safety features: ●
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The seal will not blow open if the barrier fluid pressure is lost. The two way balance will insure that the inner seal faces will stay closed when the pressure reverses. The seal will be less sensitive to solids in the fluid. Centrifugal force will work for you throwing the heavier solids away from the lapped faces. The higher barrier fluid pressure will help to lubricate the seal faces in some gas applications. You can easily detect if either of the seals fails prematurely. The convection tank pressure will either drop to system or atmospheric pressure depending upon which seal wears out or fails first. The higher barrier fluid pressure will prevent some fluids such as ethylene oxide from penetrating into and destroying the dynamic elastomer in the inboard seal. The barrier fluid can prevent the formation of ice outboard the seal in some low specific gravity applications.
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SUBJECT : How the Carbon/ Graphite seal face is manufactured and where carbon/graphite cannot be used. 4-7 Seal companies purchase carbon/ graphite molded faces from one of several carbon manufacturers. The seal companies pay for the necessary molds and then retain the exclusive use of them. A good seal face would be a mixture of carbon, graphite and nothing else. The carbon is purchased as a by product of a manufacturing process while the graphite is mined with the main sources being in Canada and Madagascar. The cost of these elements is determined by two things: ● ●
How finely is the product milled? A fine talc is desirable. How pure is the product?
A good mixture would be 80% carbon and 20% graphite. Graphite is a good conductor of heat, a natural lubricant, and has a laminar grain structure similar to a deck of cards allowing the individual grains to slide over one another. It is this laminar structure that allows the graphite to release from the carbon/ graphite face and deposit on the hard face in the same manner a graphite pencil will write on a sheet of paper. To manufacture the finished product we place this mixture in an oversized mold using a hydrocarbon as the glue to hold the powder together. Years ago "pitch" from a tree, was used for the same purpose. The mixture is then compressed and placed in an oven at 2000° Fahrenheit (1000° C) for a period of thirty to sixty days. The hydrocarbon will convert to carbon at this temperature. The piece must be heated slowly or otherwise the carbon will combine with oxygen to form carbon monoxide or carbon dioxide which will, in either case, ruin it. At the end of this time the piece has shrunk a small amount, but still resembles a real carbon face. The problem is : ● ● ●
It has poor tensile strength It has low heat conductivity because the mixture is very porous. It has low density which would be a problem in vacuum applications.
At this point any inorganic (it never lived) material can be imbedded into the carbon/graphite shape. If you should use such an impregnation you would have to be concerned about the chemical compatibility of the filler material with the product you are trying to seal. If you want a serious carbon you must place the component into an autoclave, where a vacuum will remove impurities that may have imbedded into the porous face. The autoclave will then be filled with a hydrocarbon and pressurized to force the hydrocarbon into the porous face under high pressure. This first impregnation will penetrate approximately 25 mm. (one inch) meaning that 50 mm (2 inches) will be impregnated if the hydrocarbon can penetrate from all sides of the shape. The face is then placed back into the oven and fired at 2000° Fahrenheit (1000 C.) for an additional 30 to 60 days.
You now have a more dense carbon/graphite, but you are a long way from a good one. Two more impregnations at 3,0 mm. (0.125 inches) and 0,5 mm (0.020 inches) will complete the impregnations, each taking 30 to 60 days in the oven. About this time you hit a point of diminishing returns, so the fourth impregnation is pushed into the carbon/graphite, but not fired in the furnace. This type of seal face is referred to as an "unfilled carbon and is available from several manufacturers both in the United States and abroad. If a seal manufacturer needs a only a few seal faces for test purposes he can machine them out of an unfilled carbon and then send them to the carbon manufacturer for the final impregnations. Small batch applications are handled like this also. This is the type of face that should be the standard in all of your mechanical seals. It can be used in any chemical or combination of chemicals except an oxidizing agent. As mentioned, the oxidizing agents will combine with the carbon to form carbon dioxide and carbon monoxide. Here is a list of some of the common oxidizers: ● ● ● ● ● ● ● ● ● ● ● ● ●
Aqua Regia (a combination of nitric and hydrochloric acid) used for dissolving metals. Oleum, used in the manufacture of detergents and explosives. Perchloric acid, used in the manufacture of medicine, explosives, and esters . Sulfur trioxide, used to manufacture sulphuric acid. Nitric acid, used in fertilizer, dyeing, explosives, drugs, etching and medicine. Hot sulphuric acid, the most widely used industrial chemical. Chloric acid, ignites organic material on contact. Chlorous acid, over 200 degrees Fahrenheit (100 C). Ferric chloride, used in sewage treatment photography, medicine and feed additives. Hydrofluoric acid, used for etching, cleaning castings and fermentation. Sodium hypochlorite, used in bleaching paper pulp, textiles, and tanning textiles. Methyl Ethyl Ketone (MEK) a common solvent. Perchloric Acid - 2N
Additionally look for any chemical whose name contains the word: ● ● ● ● ●
Peroxide Chlorate Perchlorate Nitrate Permanganate
The Halogens are another group of chemicals that will attack carbon: ● ● ● ● ●
Chlorine Fluorine Bromine Astintine Iodine
The degree of attack will be affected by the oxidizer's chemical concentration and temperature. If you are handling any of these chemicals it would pay to test an unfilled carbon for compatibility prior to installing a mechanical seal. Recent experience shows that all grades of carbon are no longer being recommended in the following applications: ●
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If there is a possibility of color contamination of the product. Some paper and paint applications have this problem. If you are sealing hot oil and have to meet fugitive emission standards. Some de-ionized water applications can attack carbon
Original equipment manufacturers (O.E.M.) use filled carbon in their seals and as a result you end up with a spare parts problem. It is not unusual to find five different seals, with five different part numbers and the only difference between them is the grades of carbon/ graphite. Cryogenic service uses a special carbon that has some inorganic compounds added to compensate for the fact that adsorbable gases are not present to weaken the interlacing bonding forces between the carbon and the graphite. It is these adsorbable gases and/ or vapors that allow the graphite to release from the compound and coat the hard surface with a low friction lubricating layer. Most sealing applications can be satisfied with an unfilled carbon running against one of several hard faces. The only exceptions being an oxidizing agent, those applications where carbon is not acceptable because of color contamination and hot petroleum . You should contact the carbon manufacturers for their catalog giving you the grades they have available and the "physicals " (specifications) of their unfilled carbon. You can then check with your seal supplier to be sure he is using the proper unfilled grade in your mechanical seals.
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SUBJECT : The super compounds 10-6 When you are selecting an O-ring, or any other elastomer shape for your mechanical seal application remember that with the exception of solvents, most chemicals and chemical compounds can be successfully sealed with either ethylene propylene or a good grade of Viton® as the dynamic elastomer. Most mechanical seal designs incorporate both dynamic and static elastomers. Dynamic O-rings are required to flex and roll with the shaft movement. This means that a very low shaft squeeze is important to prevent seal "hang up" or hysteresis. They must also be free to flex and roll to compensate for mechanical seal face wear. Static O-rings do not have to move. They are used as a gasket and are a lot more forgiving than dynamic O-rings because a small amount of swell can be tolerated that might even improve their sealing. There are many elastomer shapes available to you. Individual seal companies use wedges, V-rings, Ucups, Quad rings etc, but O-rings have a lot of advantages over these other elastomer shapes in mechanical seal design. As an example: ● ●
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They can seal both pressure and vacuum. They can flex 0.003 to 0.005 inches (0.08 to 0 0.13 mm). before they roll, and then they can roll up to half of their diameter making it a lot easier for the seal faces to follow shaft run out and end play. O-rings reduce shaft fretting dramatically because of this ability to flex and roll. They are available in a variety of compounds. They are the first shape available when a new compound is introduced. Most of the O-ring compounds are available in a wide range of durometer or hardness. The average mechanical seal uses a durometer of 75 to 80 (as measured on the shore A scale), but harder durometers are available for high pressure applications similar to those we find in pipe line sealing. The O-ring configuration is usually the first shape available when a new compound becomes available from the manufacturer. They are the most precision rubber part that you can purchase. O-rings are manufactured to a tolerance of ± 0.003 inches (0.08 mm) You can buy them anywhere. There are plenty of distributors. Unlike other shapes, most designers have settled on only a few O-ring cross sections, making spare parts and inventory a lot easier. Their cost is low compared to other shapes. Because they are self energizing there is no need to spring load them to the shaft or sleeve. This means that the seal spring or springs can be designed for face loading only. You cannot put them in backwards.
In recent years the elastomer industry has produced a variety of newer compounds that appear to be getting closer to the universal rubber that we are all seeking. Unfortunately we are not there yet, so this paper is an attempt to put these "super compounds" into a proper perspective. There are several of these compounds that you should know about.
KALREZ®, a Dupont product that is not a true elastomer so you will experience some "compression set" depending upon the compound you select. You have a few choices: ●
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Compound 4079, A "low compression set" compound (about 25% compression at 400°F) (205° C). Can be used to 600°F (316°C) Not recommended for hot water or steam applications, or in contact with certain hot aliphatic amines, ethylene oxide and propylene oxide. Compound 1050. Slightly harder than 4079. Can be used to 500°F (260°C) in non oxidizing environments. Not recommended for pure water or steam at higher temperatures. This compound is scheduled to be phased out of production. Compound 2035, To 425°F (218°C) It is the compound recommended for Ethylene Oxide and Propylene Oxide service. It also exhibits low swell in organic and inorganic acids, esters, ketones, and aldehydes. Compound 1018, To 550°F (288°C). It has better hot water/ steam resistance than all other compounds except 3018. Not recommended for use in organic or inorganic acids at high temperature or for rapid temperature cycling applications. Compound 3018, To 600°F (315°C). It has the best hot water/steam resistance and the best high pressure extrusion resistance. It is too hard for most mechanical seal applications at temperatures below 400°F (205°C) .
The following compounds are also exhibited on the Special Elastomers chart . ●
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CHEMRAZ is distributed by Greene, Tweed & Company. Telephone (714) 875 3301. It is similar to KALREZ® and can be used to 400°F (205°C). It is available in both black and white Orings. FLUORAZ - is another product distributed by Greene Tweed & Company. Telephone (714) 875 3301. It can be used to 400°F (205°C). Field experience indicates that in operation it appears t o be very similar to AFLAS. AFLAS is distributed through the 3M company. Telephone (612) 733 5353. It can be used to 400° F (205°C)
To be classified as a true elastomer you should be able to compress the O-ring and have it return to 90% of its original shape in less than five seconds after the compression force is removed. It is this elasticity that gives the compound its memory and eliminates the need for spring loading the elastomer to the seal shaft or sleeve. If the compound does not return to 90% of its original shape in five seconds or less it is called a "plastic" and becomes less desirable as a dynamic seal in mechanical seal design. Many of these "super compounds" are plastics and present sealing problems in some seal configurations. You are going to have to depend upon your experience to select individual seal designs that work well with these materials. Some distributors of these compounds recommend the use of mechanical seals with spring loaded dynamic O-rings. They do this to booster their sales of the compound. They forget to mention that when you spring load one of these compounds you will experience shaft fretting under the O-ring. This shaft fretting increases the probability of seal failure, and dictates the use of shaft sleeves that raise the L3/D4 rating of the shaft, contributing to excessive shaft deflection. There are many selection charts available to help you pick the correct elastomer compound for your
application. Unfortunately your fluid may not be shown on some of these charts and the temptation is to go to one of the super compounds for the solution. At other times you will tempted to standardize on a super compound to avoid the selection process altogether. The attached chart will help you to avoid a mistake in both of these instances. The Special Elastomers chart link at the bottom of this page is unique in that it shows you where these "super compounds" cannot be used. This does not imply that if the chemical is not listed, or if no notation is made, that the compound is suitable for your service. It means nothing more than what it says. These are the chemicals that each manufacturer has designated as not suitable for a dynamic O-ring application. ●
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n = According to the manufacturer this compound is not suitable for either dynamic or static mechanical seal O-ring service. In some cases a compound was given an "n" rating when field experience proved that the published compatibility information was incorrect. c = Caution. May be suitable for static service, but probably not for a dynamic application. The higher the fluid operating temperature the less acceptable. You may want to check for experience in your plant or test the O-ring in your fluid to be sure.
If there is any question about the use of one of these compounds in a given service you can test the compound by immersing the O-ring in the fluid to be tested for about ten days to two weeks. If the fluid is going to attack the compound, the O-ring it will change weight, shape, or appearance. If the application is going to be at a hot temperature, you might want to put the test container in an oven to duplicate the seal operating conditions. You can look to see if your chemical is listed in the charts. Just click on the appropriate box: A-C ®DuPont Dow elastomer
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D-I
J-Q
R-Z
SUBJECT : Selecting the correct elastomer (O- ring) for your mechanical seal application 4-9 This paper is to be used with the O-ring selection guide you will find in the chart and data section of this web site. The guide is an attempt to select the fewest number of elastomers that will give you satisfactory sealing of most of the chemicals we find in the process industry. As you can see from the selection, most of the chemicals can be handled by either fluorocarbon (Viton/ Fluorel) or Ethylene Propylene. The following paragraphs describe the codes used in the chart. ●
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V - Fluorocarbon. The compound specified is the specific one that has some water immersion capability. Dupont E60 Viton ®, 3M Fluorel 2174, Parker 747-75 and Parker V884-85 are typical examples . E - Ethylene propylene C - Perfluoroelastomers. Chemraz (a registered trademark of Greene, Tweed & Co.) or Kalrez ® are typical examples. These are very expensive compounds. N - Neoprene B - Buna N Bu- Butyl U - Unknown, or unreliable test data. Immersion testing or plant experience is your best bet. If no elastomer proves to be acceptable a non-elastomer seal may be your only answer.
Keep in mind that this O-ring selection chart is only a guide to help you in selecting the correct elastomer for your mechanical seal application. It was created from published information, various industry guide lines and many years of practical experience by field sales and engineering people. Most mechanical seals use at least one dynamic elastomer, so even small amounts of swelling or chemical attack is almost always un-acceptable. When using this chart please keep the following in mind: ●
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Chemical attack will usually double with a 10°C (18°F) increase in temperature. If the elastomer is located close to the seal face it will see the additional heat that is being generated by rubbing friction. Elastomers are poor conductors of heat. Cooling one side of the O-ring does not always allow the coolant to conduct to the hot side. If the chemical name is followed by (*), the chemical is called an oxidizer. Oxidizers spontaneously emit oxygen at either room temperature or under slight heating. The oxygen can then combine with the carbon in mechanical seal faces or the carbon black used to color O-rings, causing chemical attack. The largest group of oxidizing materials is comprised of peroxides. Hydrogen peroxide and benzoyl peroxide are typical. Permanganates, chlorates and some nitrates are also strong oxidizing agents. These materials additionally constitute a dangerous fire hazard so two seals may be required.
The degree of carbon and elastomer attack is determined by the chemical concentration and temperature. The higher the concentration and the higher the temperature the more likely the attack. Plant experience is your best protection, but if you have no experience in handling these chemicals it would be wise to immersion test both the black O-ring and carbon face prior to installing a mechanical
seal. You could duplicate the temperature by placing the test vessel in an oven or on a hot plate when practical. ●
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The product you are sealing is often a mixture of several chemicals and/ or may have a trade name. This chart normally shows only individual chemicals so you may have to rely upon plant experience or immersion test to determine compatibility. Most plants have prior experience in handling their chemicals so look for elastomers in other mechanical seals, valves, gages, filters, strainers, hoses, lined pipe, etc. In most cases Chemraz or Kalrez ® will handle the job if there is no plant experience or if immersion testing is not practical. It is always worth a try. Remember that each of these elastomers has an upper and lower temperature limit. Although the elastomer may be chemically compatible with the sealing fluid it could still fail if the temperature limit is exceeded. Excessive temperature is usually indicated by a change in weight, shape or appearance of the Oring. Compression set is often the first indication of high heat followed by a shrinking and hardening of the elastomer. If the stuffing box temperature is too high it is necessary to cool down the seal area. Using an installed pump jacket is the obvious solution. Keep in mind that quenching or the use of two seals with a cool barrier fluid between them, cools only one side of the O-ring. If cooling is not possible you will have to use a non-elastomer seal. Elastomer
Temperature range F.
Temperature range C.
Flurocarbon (Viton ®)
-15 to 400
-25 to 205
Ethyle propylene
-70 to 300
-55 to 150
Chemraz
-20 to 450
-30 to 230
Kalrez ®
0 to 500
-20 to 260
Neoprene
-45 to 300
-45 to 150
Buna N
-65 to 225
-55 to 105
Buna S
-75 to 250
-60 to 120
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Solvents, cleaners and steam are often used to flush lines and systems. Be sure the elastomer you choose is chemically and temperature compatible with these solvents, cleaners and steam. Some processes will not allow any thing "black" in the system. White colored O-rings are available for many compounds. Ethylene propylene rubber (EPR) is a very common elastomer mentioned in this chart. Be aware that EPR. is easily attacked by any petroleum product so be careful with the type of lubricant you
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use to lubricate this elastomer. For all practical purposes silicone grease is probably your safest lubricant but to be sure check for compatibility. There is a high temperature version of this compound available (500°F or 260°C), but it cannot be used if air or oxygen is present on one side of the O-ring. In other words, the application is limited to the dynamic elastomer on the inboard side of a double seal application. Many of the chemicals listed are dangerous. Be sure to use an A.P.I. gland or better still, two mechanical seals in these applications. Nuclear, food products, and pharmaceutical often specify specific grades of elastomers and require cure date information for certain products. If you are working in any of these areas check for a list of approved materials. The term water does not describe a single product. For instance: ❍ De-ionized and demineralized water has had various ions and minerals removed and as a result is constantly trying to replace them as the water moves through the pipes and around other hardware. The result is that sometimes the water can attack stainless steel and some seal face materials including carbon. You may have to do some immersion testing to be sure if your choices are satisfactory. ❍ Water treatment varies with each application. These treatment chemicals and additives can attack some elastomers ❍ Condensate often contains dissolved amines that could attack the elastomer. ❍ Water hardness varies with geographic locations. ❍ Waste water is liable to be any thing.
Ethylene Propylene Rubber (EPR) is the first choice in most water and water based applications, but the variance noted above can cause premature O-ring failure. If you have any doubt about your water, conduct an O-ring immersion test prior to installing the mechanical seal. The four step procedure for selecting the correct elastomer is: ●
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Look up the chemical in the O-ring compatibility guide in the CHARTS AND GRAPHS section of my home page. If your product is not on the list, or is a combination of several chemical on the list, go to step "2". Look around the plant for present or past experience. Look for elastomers in valves, other seals, gages, filters, strainers, etc. If you have no experience with elastomers in this fluid, go to step "3". Test is the next step. If possible, start with two elastomers of the same compound and immerse only one of them in the fluid and leave it there for one to two weeks. You can then compare that O-ring to the one that was not immersed. If the elastomer is not compatible with the fluid it will change weight, shape, or appearance. If the elastomer does not pass this test go to step "4". Chemraz or Kalrez ® is usually the end of the line. If neither is satisfactory you will have to use a non elastomer seal. If a reliable flush is available the elastomer may be compatible with the flush, but remember that if you lose the flushing fluid the product will attack the elastomer. ® Registered trademark of DuPont Dow Elastomers Link to Chemical listing guide Link to Mc Nally home page
SUBJECT: Selecting the correct hard face seal material 5-2 A good mechanical seal should run leak free until the carbon/graphite seal face wears away. This is the same way we decide if we are getting good life with our automobile tires. The tires should not go flat or the sidewalls "blow-out". The tire tread should wear at a rate that is consistent with our driving habits. An inspection of your used seals will show that 85% or more of mechanical seals fail long before the faces wear out. The seal starts to leak and an inspection shows that there is plenty of wearable face visible. Some of these failures are caused by the wrong choice of seal face materials so we have to be knowledgeable about those materials that are available to us. The ideal hard face material would incorporate many features including the following: ● ● ● ● ● ● ● ● ● ● ● ● ●
Excellent corrosion resistance. Self lubricating. High strength in compression, shear and tension. High modulus of elasticity to prevent face distortion. Good heat conductivity. Good wearing characteristics (hardness). Low friction. High temperature capability. Temperature cycling capability. Easy insertion into a metal holder Low coefficient of friction. The ability to be molded in thin cross sections. And of course, low cost
Needless to say all of these characteristics are not available in the same face material. The idea is to get as many of them as you can in a properly chosen face combination. With just a few exceptions, seal companies purchase hard face materials from outside vendors. Be sure the face component you chose is identified by material, type and grade so that you can check out the "physicals". Some companies change the generic name of the material to confuse you. Make sure you know exactly what you are purchasing, or you will never be able to trouble shoot a seal failure caused by a wrong material selection. Here is some information about the common hard face materials we use in the seal business: Reaction bonded silicon carbide ●
●
Reaction bonded silicon carbide is produced by adding molten silicon to a mixture of silicon carbide and carbon. A reaction between the silicon and carbon bonds the structure while the excess silicon metal fills the majority of the pits left in the resultant material. There is almost no shrinkage during the process. The silicon content is about 8% to 15%. High pH chemicals such as caustic can attack this grade of silicon carbide .
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As of this writing, carbon/ graphite vs. reaction bonded Silicon Carbide has been demonstrated to have the best wear characteristics of all the possible face combinations. Reaction bonded silicon carbide is difficult to insert into a metal holder so it is usually supplied in a solid rather than a composite configuration.
There are many manufacturers of reaction bonded Silicon Carbide. They include: COMPANY
DESIGNATION
BNFL
REFEL
COORS
SC-2
NORTON
DD-630
PURE CARBON
PS 9242
Shunk and Hoechst of West Germany are also manufacturers of reaction bonded silicon carbide . ● ●
Reaction bonded silicon carbide has proven to be more chip resistant than the sintered version Avoid the following chemicals when using reaction bonded silicon carbide : ❍ Sodium Hydroxide ❍ Potassium Hydroxide ❍ Nitric Acid * ❍ Green Sulfate Liquor * ❍ Calcium Hydroxide * ❍ Hydrofluoric Acid ❍ Caustics and strong acids ❍ Most high pH chemicals
* Results vary with temperature and concentration. These chemical can leach out the silicon leaving a weakened structure that can act like a grinding wheel against the softer carbon face. Self sintered silicon carbide (sometimes called direct sintered or pressure less sintered) ●
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This material begins as a mixture of silicon carbide grains and a sintering aid which is pressed and subsequently sintered as its name implies. Unlike Reaction bonded SiC, there is no free silicon present . These direct sintered materials have no metal phase and are therefore more resistant to chemical attack. There are two grain shapes available to the manufacturer. Alpha (Hexagonal Structure), and Beta (Cubic Structure). There does not appear to be any difference in the chemical resistance, wear or friction of these two grain shapes. These self sintered materials will not be attacked by most process chemicals.
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Here are a few of the bigger manufacturers: COMPANY
DESIGNATION
Carborundum
SA-80
Kyocera
SC-201
ESK
EKasicD (the standard)
ESK
Tribo 2000 (Controlled porosity)
ESK
Tribo 2000-1 (Controlled porosity + graphite)
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Sintered silicon carbide is impossible to shrink into a metal holder. Self sintered silicon carbide carries a slight price premium compared to the reaction bonded version. Although the preferred seal face material, it is sometimes too brittle for some designs.
Siliconized graphite ●
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The manufacturing process uses a permeable form of carbon graphite that is reaction sintered in silicon at elevated temperature. This forms an outer layer of silicon carbide on the graphite base. A resin impregnate is added to increase the density.
Tungsten Carbide ●
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Cobalt and nickel are the common binders. Each is susceptible to selective chemical attack of this metallic binder that will leave a skeletal surface structure of tungsten carbide particles. Galvanic corrosion can take place between a passivated stainless steel shaft or seal face holder and the active nickel in the nickel base tungsten carbide seal face. This can be a real problem in caustic and other high PH fluids. The temperature at the seal face is higher than the temperature of the sealing fluid so the attack takes place quicker. The metallic binders in tungsten carbide are also subject to galvanic attack near copper, brass or bronze. Tungsten carbide is easy to insert into a metal holder so it is the most common material used in metal bellows and other hard face&endash;metal composite designs.
Here are some additional thoughts about hard seal faces: ●
Many sales people promote two hard faces as the ideal face combination for slurry and similar services. Keep in mind that solids cannot penetrate between seal faces unless they open. Seal faces are lapped to a flatness of less than one micron (three helium light bands), and as long as
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they stay in contact solids are filtered out. Here are some of the main disadvantages of using two hard faces in a seal application: ❍ Higher cost compared to using carbon as a seal face. ❍ If either face is "out of flat" it is almost impossible for the faces to lap them selves back together again. ❍ Carbon graphite provides an additional lubricating film if you are sealing a poor or non lubricating fluid. It should be noted that many fluids fall into that category. It takes a film thickness of at least one micron at operating temperature and face load to be classified as a lubricating fluid. ❍ Carbon graphite can easily be inserted into a metal holder. ❍ In the event the equipment is run dry, carbon/ graphite is self lubricating. ❍ Use two hard faces in the following applications. or any place carbon is not acceptable: ■ If you are sealing hot oil or almost any hot hydrocarbon. Most oils coke between the seal faces and can pull out pieces of carbon , causing fugitive emissions problems. ■ If the product tends to stick the faces together. ■ Some DI water applications can attack any form of carbon. ■ Halogens can attack all forms of carbon. These chemicals include: ■ chlorine ■ fluorine ■ bromine ■ astintine ■ iodine ■ If the product you are sealing is an oxidizer that will attack all forms of carbon, including black O-rings. ■ If you are pumping a slurry and you cannot keep the two lapped faces together by flushing, suction recirculation, a large diameter stuffing box or some other method usually employed to seal a large percentage of solids. ■ If nothing black is allowed in the system because of a possible color contamination of the product you are pumping. Plated or coated faces can "heat check" and crack due to the differential expansion of the coating and the base material. PV factors as a design tool are unreliable because carbon is sensitive to "P" but not to "V". Water can cause cracking problems with both 85% and 99.5% ceramic. The cause is not fully understood, but hydrogen embrittlement is suspected as the main cause. Cracks have been observed after seven to eight temperature cycles.
Unfilled carbon should be your first choice for a material to run against the above mentioned hard faces. Use an unfilled carbon in all applications except an oxidizing agent, halogen, cryogenic fluid, or if color contamination is a potential problem. See another paper in this site for details about how carbon/graphite seal faces are manufactured.
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SUBJECT : Selecting the correct metal for the mechanical seal components 5-9 Selecting the best metal for the seal components is usually a simple task. If the pumps' wetted (they get wet from the pumpage) parts are manufactured from a non-metallic material such as Teflon, Kynar, Polyethylene, etc. we choose non-metallic seal components, but if the wetted parts are manufactured from iron, steel, stainless steel or bronze, and they are not showing signs of corrosion, the seal components (with the exception of the springs) can usually be manufactured from grade 316 Stainless Steel. The springs must be manufactured from Hastelloy C or a similar corrosion resistant material to avoid the problems associated with "Chloride Stress Corrosion" and the 300 series of stainless steel There are exceptions to all general rules, however, and it turns out that there are a number of places we cannot use grade 316 stainless steel seal components successfully and yet iron, steel, other grades of stainless steel or bronze are usually satisfactory. The following list describes some of those chemicals and identifies the metal normally selected by the equipment manufacturer for chemical resistance. Keep in mind that the chemical resistance of any material is affected by temperature, concentration, stress etc., so check with some one knowledgeable before you specify any material: CHEMICAL
METAL
Arocolor
Bronze
Barium Casrbonate
Bronze
Benzene
Carbon steel or Beronze
Benzene, hot
Bronze
Bromine gas
Bronze
Calcium carbonate
303/304 stainless steel
Phenol (carbolic acid)
303/304 stainless steel
Butyl phthalaate
Bronze
Dichlorodifluoromethane (F-12)
303/304 stainless steel
Diethyl ether
430 stainless
Ethanol
Bronze
Ethanolamine
303/304 stainless steel
Fluorine gas, dry
430 stainless steel
Hydrogen chloride gas, wet
Carbon steel
Magnesium sulfate
303/304 stainless steel
Monoethanolamine
303/304 stainless steel
Mixed acids
Bronze
Nickel chloride
303/304 stainless steel
Nuclear primary water systems
304 stainless
Potassium bicarbonate
303/304 stainless steel
Potassium chlorate
303/304 stainless steel
Potassium hydrate
303/304 stainless steel
Potassium Oxalate
Bronze
Potassium permanganate
Bronze
Pyrogallic acid
Bronze
Sodium benzoate
Bronze
Sodium bichromate
Bronze
Sodium bromide
Bronze
Sodium chlorate
Bronze
Sodium citrate
Bronze
Sodium dichromate
Bronze
Sodium ferricyanide
Bronze
Sodium fluoride
Bronze
Sulfuric acid
Carbon steel, 430 stainless steel or Alloy 20
Titanium tetrachloride
Carbon steel
Uricacid
Bronze
If you have any doubt about the compatibility of 316 Stainless Steel with your pump you can check your facility for any experience you might have with 316 stainless parts in a similar service. If no such experience exists and you are uncomfortable making the selection, contact a qualified metallurgist. As an additional matter of interest the material we refer to as grade 316 stainless steel is made from the following ingredients: MATERIAL
PERCENTAGE
Chrome
18-20
Nickel
8-12
Carbon
.08
Iron
64-70
Silicone
1
Manganese
2
Sulphur
.030
Phosphorous
.045
The designation 316 stainless steel is not used in all countries, The following list shows the designations used by some other nations for a similar product : COUNTRY
DESIGNATION
Germany
1.4571 or V4A
England
EN58J
Sweden
2343
Hungary
K035
Czechoslovakia
17246
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Troubleshooting pump failures ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
A different method of troubleshooting pumps and seals 5-6 A little bit more about centrifugal pump cavitation 9-10 A.P.I, and C.P.I. specifications merger 12-5 Analizing the rub marks in a centrifugal pump failure 5-05 Centrifugal pump shaft displacement 9-2 Maintenance practices that cause seal & bearing problems 6-8 Operating practices that cause maintenance problems 6-7 Positive displacement pump troubleshooting 12-4 Pump bearings, why do they fail? 4-12 Pump selection practices that cause maintenance problems 6-9 Shaft fretting. What causes the problem? 10-3 The different types of cavitation 1-3 The centrifugal pump is not producing enough head 10-9 The centrifugal pump is not producing enough capacity 10-10 The centrifugal pump is drawing too much amperage. 10-11 The centrifugal pump looses suction, the pump is cavitating 10-12 The reasons for premature bearing failure 5-3 Troubles with no apparent cause 4-5 Troubleshooting common centrifugal pump problems 1-02
Troubleshooting mechanical seal failure ● ● ● ● ● ● ● ● ●
Causes of overheating in cartridge seal designs 7-4 Corrosion problems associated with stainless steel 4-1 Design, operation, and maintenance problems associated with mechanical seals 8-11 Troubleshooting mechanical seal failure, a quick refrence guide 4-11 Troubleshooting mechanical seals, an overview of the subject. 3-1 Troubleshooting mechanical seals at equipment disassembly 3-9 Troubleshooting mechanical seals at the pump site 4-2 Why do most mechanical seals fail? 2-2 Why do not good seals wear out? 9-9
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SUBJECT : A little bit more about troubleshooting centrifugal pumps and mechanical seals. 5-6 One of the U. S. based Japanese automobile manufacturers has a unique method of troubleshooting any type of mechanical failure. The system is called the "Five Whys". It is a simple but powerful concept, nothing has been solved until the question "why ?" has been asked at least five times and a sensible answer has been given for each of the "why" questions. As an example: 1. Why did the seal fail? ●
The lapped faces opened and solids penetrated between them. (solids can't get in until the faces open)
2. Why did the faces open? ●
The set screws holding the rotary unit slipped due to a combination of vibration and system pressure.
3. Set screws are not supposed to slip. Why did the set screws slip? ●
The seal was installed on a hardened sleeve.
4. Why was the seal installed on a hardened sleeve? ●
This was a packing conversion and a stock sleeve was used.
5. Why couldn't the mechanic tell the difference between a hardened sleeve and a soft one? ●
They were both stored in the same bin.
6 Why were they stored in the same bin? ●
Because they had the same part number.
7. Why did they have the same part number? ●
They should have had different part numbers. Once that problem is corrected, the failures will stop.
Now you get the idea! Needless to say you may have to go further than just five "whys". Let's try another example: 1. Why did the seal fail?
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The pump was cavitating and the vibration caused the carbon face to crack.
2. Why was the pump cavitating? ●
It did not have enough suction head.
3. Why didn't it have enough suction head? ●
The level in the tank got too low.
4. Why did the level in the tank get too low? ●
I don't know.
You have not finished five "whys" so you better go find out why the level in the tank go too low or the problem is going to repeat its self. In the above example the float got stuck on a corroded rod, giving an incorrect level indication. One more example should do it: 1. Why did the seal start to leak? ●
The elastomer got hard and cracked.
2. Why did the elastomer get hard and crack? ●
It got too hot.
3. Why did it get too hot? ●
The pump stuffing box ran dry.
4. Why did the stuffing box run dry? ●
It was running under a vacuum and it was not supposed to.
5. Why was it running under a vacuum? ●
A Goulds pump impeller was adjusted backwards to the back plate and the impeller pump-out rings emptied the stuffing box.
6. Why was it adjusted backwards? ●
Most of the pumps in the facility are of the Duriron brand and they normally adjust to the back
plate. The mechanic confused the impeller adjustment method. He has since been retrained This is a powerful trouble shooting technique. I hope you make good use of it.
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SUBJECT: A little bit more about Cavitation 9-10 Cavitation means different things to different people. It has been described as: ● ● ● ● ●
A reduction in pump capacity. A reduction in the head of the pump. The formation of bubbles in a low pressure area of the pump volute. A noise that can be heard when the pump is running. Damaged that can be seen on the pump impeller and volute.
Just what then is this thing called cavitation? Actually it is all of the above. In another section of this series I described the several types of cavitation, so in this paper I want to talk about another side of cavitation and try to explain why the above happens. Cavitation implies cavities or holes in the fluid we are pumping. These holes can also be described as bubbles, so cavitation is really about the formation of bubbles and their collapse. Bubbles form when ever liquid boils. Be careful not to associate boiling with hot to the touch. Liquid oxygen will boil and no one would ever call that hot. Fluids boil when the temperature of the fluid gets too hot or the pressure on the fluid gets too low. At an ambient sea level pressure of 14.7 psia (one bar) water will boil at 212°F. (100°C) If you lower the pressure on the water it will boil at a much lower temperature and conversely if you raise the pressure the water will not boil until it gets to a higher temperature. There are charts available to give you the exact vapor pressure for any temperature of water. If you fall below this vapor pressure the water will boil. As an example: Fahrenheit
Centigrade
Vapor pressure lb/in2 A
Vapor pressure (Bar) A
40
4.4
0.1217
0.00839
100
37.8
0.9492
0.06546
180
82.2
7.510
0.5179
212
100
14.696
1.0135
300
148.9
67.01
4.62
Please note that I am using absolute not gauge pressure. It is common when discussing the suction side of a pump to keep everything in absolute numbers to avoid the use of minus signs. So instead of calling atmospheric pressure zero, we say one atmosphere is 14.7 psia at seal level and in the metric system the term commonly used is one bar, or 100 kPa if you are more comfortable with those units.
Now we will go back to the first paragraph and see if we can clear up some of the confusion: The capacity of the pump is reduced ●
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This happens because bubbles take up space and you cannot have bubbles and liquid in the same place at the same time. If the bubble gets big enough at the eye of the impeller, the pump will lose its suction and will require priming.
The head is often reduced ●
Bubbles unlike liquid are compressible. It is this compression that can change the head.
The bubbles form in a lower pressure area because they cannot form in a high pressure area. ●
You should keep in mind that as the velocity of a fluid increase, the pressure of the fluid decreases. This means that high velocity liquid is by definition a lower pressure area. This can be a problem any time a liquid flows through a restriction in the piping, volute, or changes direction suddenly. The fluid will accelerate as it changes direction. The same acceleration takes place as the fluid flows in the small area between the tip of the impeller and the volute cut water.
A noise is heard ●
Any time a fluid moves faster than the speed of sound, in the medium you are pumping, a sonic boom will be heard. The speed of sound in water is 4800 feet per second (1480 meters/sec) or 3,273 miles per hour (5,267 kilometers per hour).
Pump parts show damage ●
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The bubble tries to collapse on its self. This is called imploding, the opposite of exploding. The bubble is trying to collapse from all sides, but if the bubble is laying against a piece of metal such as the impeller or volute it cannot collapse from that side, so the fluid comes in from the opposite side at this high velocity proceeded by a shock wave that can cause all kinds of damage. There is a very characteristic round shape to the liquid as it bangs against the metal creating the impression that the metal was hit with a "ball peen hammer". This damage would normally occur at right angles to the metal, but experience shows that the high velocity liquid seems to come at the metal from a variety of angles. This can be explained by the fact that dirt particles get stuck on the surface of the bubble and are held there by the surface tension of the fluid. Since the dirt particle has weakened the surface tension of the bubble it becomes the weakest part and the section where the collapse will probably take place.
The higher the capacity of the pump the more likely cavitation will occur. Some plants inject air into the suction of the pump to reduce this capacity and lower the possibility of cavitation. They choose this solution because they fear that throttling the discharge of a high temperature application will heat the
fluid in the pump and almost guarantee the cavitation. In this case air injection is the better choice of two evils. High specific speed pumps have a different impeller shape that allows them to run at high capacity with less power and less chance of cavitating. You normally find this impeller in a pipe shaped casing rather than the volute type of casing that you commonly see.
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Subject : A.P.I. (American Petroleum Institute) and C.P.I. (Chemical Process Industry) merger 1206 Any prediction about the future of the pump and seal business would have to include the high probability that the CPI will adopt the API seal standard. The adoption of this standard will be enthusiastically supported by the CPI insurance companies and will dramatically increase the price of mechanical seals to the consumer as well as bring seals into a commodity status which has been the goal of some of the largest pump and seal manufacturers all along. Recent pump/seal mergers, buy outs, and alliances hint that the adoption of these new standards will also dramatically increase the profits of these highly competitive manufacturers. The API (American Petroleum Institute) standard is the one universal standard being used by oil refineries throughout the world. There is on going talk about combining this standard with the chemical industry ANSI (American National Standards Institute) standard for a single unified pump standard. The problem with all standards of this type is that they have produced a failure rate in mechanical seals that exceeds 85%. The only part of a mechanical seal that is sacrificial is the carbon face and in better than 85% of the cases there is plenty of carbon face left when the seal begins to leak. The A.P.I. specification addresses just about everything about mechanical seals. The subjects include: ● ● ● ● ● ●
Seal design Materials Accessories Instrumentation Inspection, testing and preparation for shipment. Manufacturing.
In this section we will be looking at just a few of those parts of the A.P.I. standard 682 that when combined with the C.P.I. standard, will be affecting your seal purchases in the near future. Most of this information was taken from A.P.I. Standard 682, First Edition, dated October 1994. I recommend you get hold of a copy of this and any future updates to learn the full particulars. 2.1.1 ●
All standard mechanical seals, regardless of type or arrangement, shall be of the cartridge design.
2.1.2 ●
The standard single arrangement pusher seal shall be an inside-mounted balanced cartridge seal.
2.1.5 ●
The standard, un-pressurized dual mechanical seal shall be an inside, balanced, cartridge
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mounted mechanical seal (with two rotating flexible elements and two mating rings in series). Outer seals shall be designed to the same operating pressure as the inner seal, but do not have to be balanced. Cooling for the inboard seal is achieved by a seal flush. Cooling for the outside seal is accomplished by a circulating device moving a buffer fluid through an external seal flush system.
2.1.6 ●
The standard pressurized dual mechanical seal shall be an inside, balanced, cartridge mounted mechanical seal (with two rotating flexible elements and two mating rings in series). The inner seal shall have an internal (reverse) balance feature designed and constructed to withstand reverse pressure differentials without opening.
2.1.7 ●
The standard configuration for API single pusher and all dual mechanical seals is for the flexible elements to rotate. For seals having a seal face surface speed greater than 25 meters per second (5000 feet per minute), the standard alternative of stationary flexible elements shall be provided.
2.2.6 ●
O-ring grooves shall be sized to accommodate perfluoroelastomer O-rings.
2.27 ●
For vacuum services, all seal components shall be designed with a positive means of retaining the sealing components to prevent them from being dislodged.
2.3.3.1 ●
Seal chambers shall conform to the minimum dimensions shown in Table 1 or Table 2. With these dimensions the minimum radial clearance between the rotating member of the seal and the stationary surfaces of the seal chamber and gland shall be 3 mm (1/8 inch).
2.3.5.1 ●
For horizontally split pumps, slotted glands shall be provided to make disassembly easier.
2.3.5.2 ●
Provisions shall be made for centering the seal gland and/or chamber with either an inside-or outside diameter register fit. The register fit surface shall be concentric to the shaft and shall have a total indicated run out of not more than 125 micrometers (0.005 inch). Shaft centering of mechanical seal components or the use of seal gland bolts is not acceptable.
2.3.10 ●
Seal chamber pressure for single seals, and for the inner un-pressurized dual seal, shall be a minimum of 3.5 bar (50 psi.) or 10 percent above the maximum fluid vapor pressure at seal chamber fluid temperature. This margin shall be achieved by raising the seal chamber pressure and/or lowering the seal chamber temperature. Lowering the temperature is always preferable. Pumps which develop less than 3.5 bar (50 psi) differential pressure may not meet this requirement and alternate requirements shall be agreed upon by the purchaser and the seal manufacturer
2.3.18.1 ●
On vertical pumps the seal chamber or gland plates shall have a port no less than 3 mm, (1/8") above the seal faces to allow the removal of trapped gas. The port must be orificed and valved.
2.3.20 ●
For single seals and when specified for dual seals, a non-sparking, floating-throttle bushing shall be installed in the seal gland or chamber and positively retained against blowout to minimize leakage if the seal fails.
2.4 ●
Shaft sleeves shall be supplied by the seal manufacturer.
2.4.1 ●
Unless otherwise specified a shaft sleeve of wear, corrosion, and erosion resistant material shall be provided to protect the shaft. The sleeve shall be sealed at one end. The shaft sleeve assembly shall extend beyond the outer face of the seal gland plate.
2.4.3 ●
Shaft sleeves shall have a shoulder or shoulders for positively locating the rotating element or elements.
2.4.4.4 ●
Shaft to sleeve sealing devices shall be elastomeric O-rings or flexible graphite rings.
2.4.5 ●
Standard seal sizes shall be in even increments of ten millimeters. It is preferred that alternate seals be sized in increments of 0.635 mm (0,25 inches) starting with 38.0 mm (1.5 inches).
2.4.6 ●
Sleeves shall have a minimum radial thickness of 2.5 mm (0.100 inches).
2.4.8 ●
Sleeves shall be relieved along their bore leaving a locating fit at or near each end.
2.4.9 ●
Shaft to sleeve diametral clearance shall be 25 micrometers to 75 micrometers (0.001 inch to 0.003 inch
2.4.10.2 ●
Drive collar set screws shall be of sufficient hardness to securely embed in the shaft.
2.4.9 ●
Shaft to sleeve diametrical clearance shall be 25 micrometers to 75 micrometers (0.001 inch to 0.003 inches)
2.5.1 ●
Seal and mating rings shall be of one homogeneous material. Overlays and coatings shall not be used as the sole source of wear resistant material. Materials such as silicone or tungsten carbide may be enhanced by applying additional coating.
2.6.1 ●
The type A standard pusher seal shall incorporate multiple springs with O-rings as the secondary sealing elements. When specified on the date sheet option, a single spring shall be furnished.
3.2.2 ●
One of the seal face rings shall be premium grade, blister resistant carbon graphite with suitable binders and impregnates to reduce wear and provide chemical resistance. Several grades are available; therefore, the manufacturer shall state the type of carbon offered for each service.
3.2.3 ●
The mating ring should be reaction bonded silicone carbide (RBSiC). When specified, self
sintered silicone carbide (SSSiC) shall be furnished. 3.2.4 ●
Abrasive service may require two hard materials. Unless otherwise specified for this service, the seal ring shall be reaction bonded silicone carbide and tungsten carbide (WC) with nickel binder
3.6 ●
Unless otherwise specified, metal bellows for the type B seal shall be Hastelloy C. For the type C seal, Inconel 718.
3.7.2 ●
Unless otherwise specified, gland plate to seal chamber seal shall be fluoroelastomer O-ring for services below 150°C (300°F). For temperatures over 150°C (300°F) or when specified, graphitefilled type 304 stainless steel spiral wound gaskets shall be used.
4.2.1 ●
If you are using dual mechanical seals, only mechanically forced seal flush and barrier/buffer fluid systems shall be provided. Systems that rely upon a thermo-syphon to maintain circulation during normal operation are not allowed.
4.2.3 ●
Seal systems that utilize internal circulating devices, such as a pumping ring, that rely upon the rotation of the mechanical seal to maintain circulation shall be designed to thermo-syphon when the seal is not running.
4.5.4.1.1 ●
If a dual seal buffer/barrier fluid reservoir is specified, a separate barrier/buffer fluid reservoir shall be furnished for each mechanical seal
Section 4.4.4 contains numerous references to dual seal system reservoirs. 4.5.5.1 ●
The purchaser will specify on the date sheets the characteristics of the buffer/barrier fluid.
Section 4.6 addresses the circulation of the buffer/barrier fluid.
There will be some benefits to the user when the API specification is adopted in to the CPI industry ●
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The decision to standardize on balanced seals is a wise one. It will reduce the seal inventory of most consumers and prevent a lot of premature seal failures. Allowing slotted glands for horizontally split pumps is a good idea. It should also extend to end suction centrifugal pumps. Requiring seal chamber vents on vertical pump installations makes sense. Banning coated or plated seal faces makes sense. Requiring the manufacturer to specify the carbon he is supplying is an excellent idea.
What is the problem with this API specification as a standard for the Chemical Process Industry? There are a lot of things I do not like about it in its present form. If combining with the CPI means a complete re-writing of the API specification that will be fine depending upon the final result. ●
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2.1.1 Some seal designs do not lend themselves to a cartridge design. Split seals as an example. You could mount a split seal on a split cartridge, but that would be "over kill" in most cases. 2.1.2 I do not like the definition of pusher seal in this standard. The term "pusher seal" is emotionally charged and misleading. It is used to describe a reliable O-ring seal in the same category as spring loaded Teflon® wedges, or chevrons, and non-elastomer "U" cup designs. The implication is that the "non-pusher" metal bellows seal is a better choice. The fact is that O-ring seals are usually a better choice because of their ability to flex and roll and the O-ring provides a built in vibration damper that eliminates the need for letting a bellows metal face holder bounce off the shaft or sleeve. 2.1.5 The dual seal specification recognizes only tandem or series mounted rotating seals. It ignores concentric and "face to face" designs that make sense in some applications where space is not available for tandem configurations. Over the years the API has failed to recognize that there are four ways to install dual seals in a pump. They have played with the terminology over the years but have never got it simplified. It should be: Face to face Tandem or series Back to back Concentric, or one inside of the other. 2.1.6 The specification calls for the inner seal of a dual seal to be either balanced or reverse balanced depending upon whether high pressure barrier fluid or lower pressure buffer fluid is circulated between the dual seals. It totally ignores two way balance of the inner seal that would allow the consumer his choice between barrier or buffer fluid. 2.1.6 The specification call for the dual seals to be mounted in series (tandem), but almost all gas dual seals supplied to refineries to date have been supplied in the "back to back" configuration which is the worst possible installation method for slurry and abrasive service. 2.1.7 The specification approves rotating seals only and recommends stationary seals for speeds above 5000 fpm (25 m/sec). The fact is that stationary seals are almost always a better choice for leak free and the more severe fugitive emission sealing. 2.1.7 Stationary seals (the spring or springs do not rotate with the shaft) can be cartridge mounted if you take precautions to insure that the rotating face stays square to the shaft when the cartridge sleeve is set screwed or tightened to the shaft. It is not an easy problem to solve, but there are several solutions to the problem. Please see "stationary cartridge seals".
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2.2.6 The specification calls for O-ring grooves with a larger groove dimension than normally used to accommodate perfluoroelastomer O-rings. 2.3.5.2 The specification assumes all pump manufacturers have provided a machined diameter concentric to the pump shaft so that the seal gland can be machined to register on an inside or outside diameter. The fact is that most pumps were manufactured for packing and do not have these concentric machined surfaces available to the seal manufacturer. In the CPI industry, shaft centering makes the most sense. 2.3.10 Maintaining a seal chamber 50 psi (3.5) bar above vapor pressure does not make any sense in the majority of balanced seal applications. 2.4.1 The specification calls for a shaft sleeve and allows the manufacturer to reduce the diameter of the solid shaft to accommodate the sleeve. This increasing of the pump shaft L3/D4 adversely affects the pump and seal performance. 2.4.1 The specification calls for sealing the sleeve on one end, but fails to specify the impeller end except in the case of O-ring seals. If the seal is on the outboard end, the space between the sleeve and shaft can fill with solids and hamper the removal of the sleeve. This can be a major concern in hot oil type applications where "coking" is always a problem. 2.4.9 A shaft to sleeve diametral clearance of 0.001 inch to 0.003 inch is not practical. You will never be able to remove the sleeve once some solids get between the sleeve and shaft, and they will get there! 2.6.1 The standard seal is equipped with multiple springs, but the standard does not specify the springs must be located outside the fluid. If located in the fluid they can easily clog with solids. 3.2.3 Reaction bonded silicone carbide is specified as the standard hard face even though it is sensitive to caustic and other high pH chemicals frequently used to clean lines and systems. In most cases alpha sintered would be a much better choice. 4.2.1 The term "flush" is misleading. Over the years the API has failed to recognize the differences in bringing liquid to the pump stuffing box area and lumped them all under the common term "Flush". There is better terminology: ❍ Discharge recirculation connects the discharge of the pump to the stuffing box to raise stuffing box pressure. ❍ Suction recirculation connects the bottom of the stuffing box to the suction side of the pump usually allowing clean fluid to circulate from behind the impeller into the stuffing box. ❍ Barrier fluid describes a higher-pressure fluid that is circulated between dual seals. ❍ Buffer fluid describes a low-pressure fluid circulating between dual seals. ❍ Quenching fluid is introduced into the seal gland outboard the seal to wash away leakage and control the environment outboard the seal. ❍ Jacketing fluid circulates around the outside the stuffing box to control stuffing box temperature. ❍ Flushing fluid is fluid from an outside source introduced into the stuffing box that dilutes the pumpage. It is seldom desirable, but sometimes necessary. The specification allows spring-loaded elastomers (O-rings) that do not have the ability to flex and roll. The specification allows a single spring seal design even if it is sensitive to the direction of rotation. The specification does not prohibit the use of mechanical seals that frett (damage) shafts and sleeves. The specification should call for the seal's dynamic O-ring to move towards a clean surface to
prevent "hang up".
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SUBJECT : Troubleshooting rub marks in a centrifugal pump 5-05 When a centrifugal pump is disassembled there are a couple of things visible to the trained trouble shooter. He can see either corrosion or evidence of rubbing, damage or wear. When ever a rotating piece of hardware hits a stationary piece it leaves a mark that is clearly visible and capable of being analyzed for cause. This type of rub mark should never be confused with the dull appearance we see on a piece of metal that has been rotating in an abrasive slurry. In strong corrosive applications the rub mark may not be visible. The contact will cause an increase in the metal temperature causing rapid chemical attack. This condition is easy to identify because the corrosion is localized at the rubbing location. Shaft fretting is another common rub mark that should not be confused with the rub marks we will be discussing in the following paragraphs. Fretting is visible between the dynamic elastomer in the mechanical seal and the sleeve or shaft that the elastomer is sealing against. You will also observe this type of damage immediately under the grease or lip seals that we find being used to seal most bearing applications. There are five possible rubbing combinations that can be seen: 1. All around the rotary and one spot on the stationary. 2. All around the stationary and one spot on the rotary. 3. All around both the rotary and stationary. 4. One spot on both the rotary and stationary. 5. One spot on the rotating component. You should look for the rub marks on those pieces that normally come in close contact. Common sense will dictate that the further the hardware is located from the bearings, the more likely the contact will occur. Here are some likely candidates for rubbing when the pump experiences shaft deflection, or any other type of radial displacement. Look for contact between : ●
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The stationary and rotary parts of the wear rings that are installed in most closed impeller pump designs. The shaft&endash;sleeve and the mechanical seal stationary face inside diameter. The shaft&endash;sleeve and the bottom of the pump stuffing box, or stuffing box restrictive bushing. The shaft&endash;sleeve and the A.P.I. gland disaster bushing. The outside diameter of the mechanical seal rotating element and the inside diameter of the stuffing box. You will need a mirror and flashlight to see the stuffing box inside diameter. The impeller and the volute casing or the pump back plate. The outside diameter of the rotating seal, and a protruding gasket or fitting.
In the following paragraphs I will list the observations, explain the causes and where practical list some of the conditions that can initiate the problem with centrifugal pumps. If you would like to learn more about how to trouble shoot the rubbing marks we normally find in ball bearings, please refer to another
paper in this series Observation - All around the rotary, one spot on the stationary. The shaft is being deflected from its true position or the hardware surrounding the rotating piece is being forced into the rotary unit. ●
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The pump is operating off of its B.E.P. The stationary mark will be visible at either 240° or 60° from the discharge "cut-water" as measured in the direction of shaft rotation. ❍ Some one has throttled the pump discharge valve. ❍ The capacity has increased. ❍ The discharge lines have a solids build up on the I.D. or there is a restriction in the discharge piping. ❍ The tank is being filled from the bottom. The head is increasing as the tank fills. ❍ The discharge by-pass line is not functioning. ❍ You have the wrong size pump. ❍ Two pumps are piped in parallel. The larger pump is shutting the discharge check valve of the smaller pump. ❍ The pump speed has changed. ❍ The system has been altered. Piping and fittings have been added or removed. ❍ The pump was started with the discharge valve fully open or shut. ❍ The viscosity of the liquid has changed. ❍ The impeller has been trimmed. ❍ The discharge piping or a fitting on the discharge has been damaged. ❍ The motor is running at the wrong speed. This could be caused by a change in the specific gravity of the pumped fluid. ❍ The suction head has changed & the discharge head changed to compensate. ❍ An in-line filter is clogged. The shaft is pulley driven. The off-set driver is causing the deflection. Misalignment between the pump and the driver. ❍ They never were aligned. ❍ Thermal growth. ❍ Vibration has loosened the hold down bolts. ❍ The seal was changed and the pump was not realigned. ❍ A universal joint has been installed between the pump and the driver. Pipe strain ❍ Thermal growth - no expansion joints. ❍ During the installation process the piping was forced to the pump suction&emdash; instead of piping from the suction to the pipe rack. ❍ A center line design pump was not specified for elevated temperatures. A protruding piece of stationary hardware is contacting the rotating part. ❍ A fitting is protruding into the stuffing box through the lantern ring connection. ❍ A gasket on the gland face is extruding into the stuffing box. A recirculation line aimed at the seal will give the appearance of rubbing marks if there is a lot of abrasives in the re-circulating fluid. The mechanical seal gland has slipped and is now contacting the rotating shaft. A bad foot bearing on a mixer. The stationary seal face was not centered on the shaft and now the inside diameter of the seal face is rubbing on the shaft. A severe cocking of the seal face can cause the same problem.
Observation - All around the stationary, one spot on the rotary. ●
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The shaft is bent. ❍ It never was straight. ❍ The shaft was damaged when it was dropped. ❍ The shaft was overheated and warped when the sleeve was removed. The rotary unit is out of balance. You must balance everything that rotates with the shaft such as the impeller, sleeve, sleeve gasket, drive key, seal, bearings, coupling, motor etc. ❍ It never was balanced. ❍ Cavitation damage. ❍ Some of the product has attached it self to the rotating assembly. ❍ The impeller is the most logical place to look for un balance problems, especially in the balancing holes. ❍ Erosion can remove metal from the rotating parts. ❍ Corrosion can do the same. ❍ Temperature distortion. ❍ A non concentric sleeve, seal, impeller, coupling, etc. ❍ The impeller was trimmed and not re&endash;balanced. ❍ A piece was damaged during the installation process. The rotary unit is dragging something around with it. ❍ A piece left over from the last seal change. No one notices that one of the springs has fallen out and is resting in the bottom of the stuffing box, getting ready to be picked up by the new seal. ❍ A piece of the seal has come loose. Look for set screws, springs, drive lugs and all of the obvious seal parts. The pump is running at a critical speed, or it has passed through a critical speed. The seal or sleeve is not concentric with the shaft.
Observation - The mark is all around both units. ● ●
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Look for a combination of the first two discussed. This is a very common condition. Thermal expansion. ❍ The shaft usually expands faster than restriction bushings placed in the bottom of the stuffing box. ❍ Hot oil applications use a thermal bushing in the bottom of the stuffing box to gain more efficiency from the cooling jacket. ❍ Steam is often used as a quench with an A.P.I. gland. This gland has a close fitting disaster bushing that can be overheated by the quench temperature. Excessive vibration. ❍ Bad bearings or a loose bearing fit. ❍ Cavitation - there are five types&emdash; see Volume 1 Number 3 ❍ Harmonic, from nearby equipment. ❍ Seal "slip stick".
Observation - One spot on both the stationary and rotary units.
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This is caused by a momentary deflection of the rotary unit. Just about the only time it happens is when some one drops the pump while it is being transported.
Observation &endash; One spot on the rotating unit. ●
Someone has hit the piece with a hammer.
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SUBJECT: Shaft displacement cheat sheet 9-2 This paper is the first in a series of "Cheat Sheets" that will summarize the information you have been learning through out this series. These sheets will not substitute for your learning of the subject. They are intended to outline the subject in a logical way so that you will be able to make an effective presentation on any of the subject material. We know that seals fail for only two reasons: ● ●
One of the seal components becomes damaged. The lapped faces open.
Common sense dictates that the more the shaft deflects from the center of the stuffing box, the more likely the lapped faces are to separate. Rotating seals (the spring loaded face rotates with the shaft) are very sensitive to this type of shaft displacement or any other form of misalignment between the stationary and rotating faces. THE PUMP IS OPERATING OFF THE BEST EFFICIENCY POINT (BEP) ● ● ● ● ● ● ●
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The wrong size pump was selected because safety factors were added to the computations. A discharge valve is throttled to decrease the excessive capacity . An orifice has been installed in the discharge piping to limit flow. Two pumps are being operated in a parallel mode with different diameter impellers. Two pumps are being operated in series with different width impellers. The suction tank level is increasing or decreasing dramatically. The centrifugal pump is discharging into the bottom of the tank instead of the top. The head is changing. The system pressure is being maintained by a head tank. The centrifugal pump is acting like an accumulator because it stars when the head tank pressure falls and stops when the pressure tank pressure is reached. You are using a variable speed motor, trying to maintain a flat system curve. The impeller diameter has been changed. The specific speed of the impeller is too high or too low for the application. The piping system has changed: There have been piping additions and deletions since the pump was originally sized. Extra pumps have been installed into the system. The piping inside diameter is reduced because of product build up. A globe valve has been substituted for a gate valve in the system. An oversized impeller was installed to satisfy a system requirement. The pipe was damaged when a truck ran over it.
THE PUMP IS CAVITATING ● ●
Suction vaporization. The suction temperature is too high or the suction pressure is too low. Vane passing syndrome. There is not enough clearance between the tip of the impeller and the
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pump cut water. The suction specific speed number is in excess of 8500 (5000 in the metric system) Air ingestion. The fluid is vortexing at the pump suction or air is entering the system through packing, valves above the water line, flange gaskets, etc... Turbulence&emdash;there is an elbow too close to the suction. A discharge bypass line is recirculating to the pump suction.
THE PUMP IS VIBRATING ● ● ● ● ● ● ● ● ● ● ● ● ●
Dynamic unbalance of the rotating assembly caused by erosion, corrosion, or damage. Harmonic vibration. The shaft is vibrating in harmony with something close by. Slip stick. The seal faces are slipping and sticking due to poor lubrication. Water hammer. The pump is hitting a critical speed. Bent shaft Bad bearings Poor lubrication. Contamination of the lubricant. Poor quality. Bad installation. Over lubrication. The bearing is being retained by a snap ring..
OTHER CAUSES OF SHAFT DISPLACEMENT ● ● ● ● ● ● ● ● ●
Pipe strain caused by either mechanical or thermal expansion. Misalignment between the pump and driver. Pulley driven designs. Start up thrust. Water hammer High L3/D4 number Thermal growth, both axial and radial. Impeller adjustment. The pump pedestal is not five times the mass of the hardware sitting on it.
THE STUFFING BOX IS CAUSING THE MISALIGNMENT PROBLEM ● ● ●
The face is not machined square to the shaft. The stuffing box is not concentric with the shaft. Some bolted on stuffing boxes can slip with vibration.
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SUBJECT: Maintenance practices that cause high seal and bearing maintenance problems 6-8 Maintenance departments seldom return savings to the company management. They fear that if they do not spend this year's budget, next year's will be reduced. Management views maintenance savings as bottom line money and works at reducing maintenance man power and inventory costs. Here are some of the maintenance practices that increase the pump failure rate: Problems with pump maintenance that can cause excessive shaft movement and deflection. This shaft displacement is a major cause of premature seal and bearing failure. ●
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Failure to align the pump and driver. Misalignment will cause the mechanical seal to move excessively, increasing the chance for the seal faces to open and fail the seal. Pipe strain is another cause of misalignment between the seal's stationary and rotating faces. Wear ring damage is common if pipe strain is present. Failure to dynamically balance the rotating assembly can result in "whip, wobble, and run-out problems." Damage to the shaft and bearings during the sleeve removal process. Banging on the sleeve with a large hammer or heating the shaft with a torch are common methods used to remove sleeves. Needless to say the seal and bearings stand a good chance of being destroyed in the process, along with the shaft that will be bent or warped. Using the coupling to compensate for misalignment. A coupling is used to transmit torque and compensate for axial growth of the shaft, nothing else! It cannot compensate for misalignment between the pump and its' driver. Trimming the impeller without dynamically re-balancing it. The impeller casting is not homogeneous, it must be re-balanced after any machining operation has taken place. Throttling the pump discharge to stop a cavitation problem. The more you pump the more N.P.S. H. you need, so throttling does work, but you may be now operating off the pumps' B.E.P. resulting in shaft deflection. Failure to machine the stuffing box square to the shaft will result in excessive seal movement. Repairing the cutwater to the wrong length can cause a cavitation problem known as the "Vane Passing Syndrome" that will damage the tips of the impeller blades and damage the volute just beyond the discharge nozzle. Failure to properly adjust the open impeller clearance or letting the closed impeller wear ring clearance become excessive can make the pump run inefficiently and vibrate. Turning down a shaft and repairing it with a polymer material will weaken the shaft making it more sensitive to deflection forces. That practice was common with packed pumps, but should be avoided when mechanical seals are being used. Substituting a globe valve for a gate valve will throw the pump off of its B.E.P. causing shaft deflection. Any alteration in the piping system or failure to prevent solids "build up" in the lines will have the same affect. Mounting the pump and motor on too light a foundation. The foundation should be at least five times the mass of the equipment sitting on it or vibration will become a problem. Proper grouting is also necessary to mate the base of the pump to the foundation.
Seal handling practices can also lead to premature seal failure. ●
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Installation problems: ❍ The seal is installed at the wrong length. There are a lot of ways to do this from reading the print wrong to the sleeve moving after the seal was attached to the sleeve. ❍ The wrong lubricant was used on the dynamic rubber part causing it to be chemically attacked. Petroleum grease on Ethylene Propylene O-rings is a good example of this problem. In salt water applications zinc oxide should be used on all rubber parts and metal components that clamp together. ❍ The seal was installed before the impeller setting was made or an impeller adjustment was made without resetting the mechanical seal. In most cases this will cause the seal faces to open prematurely. ❍ The shaft or sleeve is out of tolerance. This can cause serious problems with those seal designs that have a dynamic elastomer sliding on the shaft (most original equipment seals fit into this category). ❍ The sleeve was hardened to resist packing wear causing the seal set screws to slip and the faces to open. ❍ The elastomer (rubber part) exceeded its' shelf life. This is a real problem with the Buna "N" material found in most rubber bellows seals. ❍ Installing a stationary seal on a cartridge will cause the rotating face to "cock" when the set screws are attached to the shaft. An environmental control was lost while the seal was installed in the pump. Typical environmental controls include: ❍ Clean flushing liquid to keep solids away from the moving seal parts. ❍ Controlling stuffing box temperature with a cooling or heating jacket. If the circulating water is "hard" condensate may have to be substituted to prevent the cooling jacket from becoming coated with calcium and other solids that will interfere with the heat transfer. ❍ Barrier fluid is used to circulate between two mechanical seals. Sometimes the circulation is done by simple convection, but pumping rings and forced circulation are common also. Check to see if your convection tank has to be pressurized. This is a common problem with many original equipment seals. Feel the convection lines to make sure the convection is taking place in the right direction. ❍ A steam quench is often used to remove dangerous vapors and to keep the seal area warm when the pump is shut down. Metal bellows applications use the steam quench to cool down hot oil to prevent unwanted "coking". ❍ A stuffing box vent should be connected from the area of the seal faces to the suction side of the pump, or some other low pressure area to prevent air from being trapped at the seal faces. ❍ A discharge recirculation line, and a bushing in the bottom of the stuffing box are often used to pressurize the stuffing box to prevent the product from vaporizing at or between the lapped seal faces. Is there enough clearance between the seal outside diameter and the inside of the stuffing box? Solids build up in the stuffing box can interfere with the free movement of the seal. The seal was installed with unidentified materials, making troubleshooting almost impossible. Which carbon is being used? There are a hundred available and they are not all alike. Which elastomer? Do you know both the material and the grade? What material are the metal components manufactured from? Not all stainless steel grades are
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alike, and stainless steel springs or metal bellows should never be used because of potential problems with chloride stress corrosion. There are many hard seal faces in use. All ceramics, silicone carbides and tungsten carbides are not alike. The outside springs were painted on a double seal when the pump area was refurbished. The pump discharge recirculation line is handling abrasive solids. They are being directed at the lapped seal faces or at the thin metal bellows. If the open impeller is adjusted backwards (this is a common problem if a facility has both Duriron and Goulds pumps), it can create a vacuum in the stuffing box as the "pump out vanes" are running too close to the back plate. Do not shut off the stuffing box cooling jacket when a metal bellows seal is installed. The stuffing box is cooling down the shaft as well a the seal area. Shaft cooling is necessary to prevent heat conduction to the bearings.
Poor bearing maintenance practices are a major cause of premature bearing failure. ●
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If the oil level is too high or the bearings are over greased, the low specific heat of the lubrication and its poor conductivity will cause the bearing area to over heat. The inside of the bearing case must be protected against rust when it is stored as a spare. The bearings should be coated with an appropriate grease because they can rust also. During storage, or while in a standby condition, nearby equipment that is vibrating can induce vibration into the static bearings causing "false brinneling" or hardening of the bearing balls and races. If the oil becomes contaminated with water you will experience a very rapid bearing failure. The water can enter through the grease or lip seals from several sources: ❍ leakage through the packing or mechanical seal. ❍ From the water hose that is used to wash down the base plates and pump area. ❍ From moisture in the air as the moisture enters the bearing casing through "aspiration". ❍ From the quench gland on a mechanical seal. The bearing was installed improperly: ❍ The shaft outside diameter was the wrong tolerance. Remember that the tolerance is given in tenths of thousands of an inch or thousands of a millimeter. ❍ Too much pressure was put on the arbor press during the assembly sequence. ❍ The bearing was heated in contaminated oil that has deposited the contaminates in the races ❍ The oil was over heated and varnish particles are now in the race ways. ❍ The bearing was pushed too far up a tapered shaft. The thrust bearing is being retained by a simple snap ring. During operation the shaft thrust is usually toward the volute and this thin ring.
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SUBJECT: Operation practices that cause frequent seal and bearing maintenance problems 6-7 Wouldn't it be wonderful if the plant operation and maintenance departments could work independently? The fact of the matter is that there are three types of problems we encounter with centrifugal pumps and poor operation is one of them. If you are curious, the other two are design problems and poor maintenance practices. Seals and bearings account for over eighty five percent (85%) of premature centrifugal pump failure. In the following paragraphs we will be looking at only those operation practices that can, and will cause premature seal and bearing failure. Design and maintenance practices will be discussed in other papers in this series. When pumps were supplied with jam packing, the soft packing stabilized the shaft to prevent too much deflection. In an effort to save flushing water and to conserve power, many of these same pumps have since been converted to a mechanical seal and the radial stabilization the packing provided has been lost. The bad operating practices include: Running the pump dry will cause over-heating and excessive vibration problems that will shorten seal life. Here are some of the common reasons why a pump is run dry: ● ● ● ● ●
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Failing to vent the pump prior to start-up. Running the tank dry at the end of the operation cycle. Emptying the tank for steaming or introduction of the next product. Running on the steam that is being used to flush the tank. Starting the standby pump without venting it. Venting a hazardous product can cause a lot of problems with the liquid disposal. Many operators have stopped venting for that reason. Tank vents sometimes freeze during cold weather. This will cause a vacuum in the suction tank, and in some cases could collapse the tank. Sump fluids are often dirty, corrosive or both. The control rods for the float switch will often "gum up" or corrode and give a false reading to the operator. He may think that there is an adequate level, when in fact, the tank is empty.
Dead heading the pump can cause severe shaft deflection as the pump moves off of its best efficiency point (B.E.P.). This translates to excessive heat that will affect both the seal and the bearings as well as causing the seal faces to open, and the possibility of the impeller contacting the volute when the shaft deflects. ●
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Starting the centrifugal pump with a shut discharge valve is standard practice with many operation departments. The concern is to save power without realizing the damage that is being done to the mechanical seal, impeller, wear rings and bearings. Some pumps are equipped with a recirculation valve that must be opened to lessen the problem, but many times the valve is not opened, or the bypass line is clogged or not of the correct diameter to prevent the excessive head. Another point to remember is that if the bypass line is discharged to the suction side of the pump the increased temperature can cause cavitation.
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After a system has been blocked out the pump is started with one or more valves not opened. Discharge valves are shut before the pump has been stopped.
Operating off of the best efficiency point (B.E.P.). Changing the flow rate of the liquid causes shaft deflection that can fail the mechanical seal and over-load the bearings. ● ●
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Starting the pump with the discharge valve closed to save power. The level in the suction tank is changing. Remember that the pump pumps the difference between the discharge and suction heads. If the suction head varies, the pump moves to a different point on its curve. Any upset in the system such as closing, throttling or opening a valve will cause the pump to move to a new point on the curve as the tank fills. Pumping to the bottom of a tank will cause the pump to move to a different point on the curve as the tank fills. Some systems were designed for a low capacity positive displacement pump and have since been converted to a centrifugal design because of a need for higher capacity. Centrifugal pumps must discharge to the top of the tank to prevent this problem. If the discharge piping is restricted because of product build up on the inside walls, the pump will run throttled. This is one of the reasons that it is important to take periodic flow and amperage readings. Increasing the flow will often cause cavitation problems.
Seal environmental controls are necessary to insure long mechanical seal life. It is important that operations understand their function and need because many times we find the controls installed, but not functioning. ●
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Cooling-heating jackets should show a differential temperature between the inlet and outlet lines. If the jacket clogs up, this differential will be lost and seal failure will shortly follow. Barrier fluid is circulated between two mechanical seals. There may or may not be a differential temperature depending upon the flow rate. If a convection tank is installed, there should be a temperature differential between the inlet and outlet lines. The line coming out of the top of the seal to the side of the tank should be warmer than the line from the bottom of the tank to the bottom of the seals, otherwise the system is running backwards and may fail completely. The level in the tank is also critical. It should be above the tank inlet line or no convection will occur. Some convection tanks are pressurized with a gas of some type. Many original equipment (O.E. M.) seal designs will fail if this differential pressure is lost. Some seal glands (A.P.I. type) are equipped with a quench connection that looks like the seal is leaking water or steam. If there is too much steam pressure on this quench connection, the excessive leakage will get into the bearings causing premature failure. The steam is often used to keep the product warm to prevent it from solidifying, crystallizing, getting too viscous, building a film on the faces etc. Operating people frequently shut off the quench to stop the condensate from leaking. Flushing fluids are used for a variety of purposes, but most of the time they are used to get rid of unwanted solids. The flush can be closely controlled with a flow meter or throttling valve. The amount of flush is determined by the seal design. As an example, those designs that have springs in the product require more flush. It is important to check that the stuffing box has been vented in vertical pumps. The vent should
be coming out of the seal gland and not the stuffing box lantern ring connection. There are some additional things that all operators should know to insure longer rotating equipment life. As an example : ●
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Mechanical seals have an 85% or more failure rate that is normally correctable. This is causing unnecessary down time and excessive operating expense. Seals should run until the sacrificial carbon face is worn away, but in more that 85% of the cases the seal fails before this happens. There are five different causes of cavitation. You should know where the best efficiency point (B.E.P.) is on a particular pump, and how far it is safe to operate off the B.E.P. with a mechanical seal installed. You should be aware that washing down the pump area with a water hose will cause premature bearing failure when the water penetrates the bearing case. Learn about the affect of shaft L3/D4 on pump operation. Know how the pumped product affects the life of the mechanical seal and why environmental controls are necessary. If you are not using cartridge seals, adjusting the open impeller for efficiency will shorten the seal life. In most cases the seal will open as the impeller is being adjusted to the volute. Durco pumps are the best example of the exception to this rule. The popular Durco pumps adjust to the back plate causing a compression of the seal faces that can create mechanical seal "over heating" problems. Cycling pumps for test will often cause a mechanical seal failure unless an environmental control has been installed to prevent the failure. Mechanical seals should be positioned after the impeller has been adjusted for thermal growth. This is important on any pump that is operated above 200°F (100°C) or you will experience premature seal failure. Some elastomers will be affected by steaming the system. A great deal of caution must be exercised if a flushing fluid such as caustic is going to be circulated through the lines or used to clean a tank. Both the elastomer and some seal faces (reaction bonded silicone carbide is a good example) can be damaged. If the elastomer is attacked, the failure usually occurs within one week of the cleaning procedure. The stuffing box must be vented on all vertical centrifugal pumps or otherwise air will be trapped at the seal faces that can cause premature failure of many seal designs. Most original equipment seal designs cause shaft damage (fretting) necessitating the use of shaft sleeves that weaken the shaft and restrict pump operation to a narrow range at the B.E.P..
Here are a few common misconceptions that cause friction between maintenance and operation departments ● ● ● ● ●
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Shutting the pump discharge valve suddenly, will blow the seal open. All ceramics cold shock. High head, low capacity consumes a lot of power. The pump must come into the shop to change a mechanical seal. If you use two hard faces or dual mechanical seals in slurry applications, you will not need flushing water with its corresponding product dilution. If you use metal bellows seals for hot oil applications, you will not need the stuffing box cooling
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jacket operating. It is O.K. to use an oversized impeller because throttling back will save power.
A few more thoughts on the subject ●
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Operators should receive proper schooling on the trouble shooting and maintenance of pumps. In the military and many modern plants, the operator and the maintenance mechanic are often the same person. If the operator knows how the pump works he will have no trouble figuring out the solution to his problem. Too often he is told to keep the flow gage at a certain point, or between two values without understanding what is actually happening with the equipment. If the operator recognizes cavitation he can tell the maintenance department and help them with their trouble shooting. As you wander around the plant look out for painters that paint the springs of outside and double mechanical seals. There is a trend to putting two seals in a pump for environmental reasons and the painting of springs is becoming a common problem. If someone is adjusting the impeller make sure he is resetting the seal spring tension at the same time. If the pump is getting hot or making excessive noises, report it immediately. After the failure, it does no good to tell maintenance that it was making noise for two weeks. If you are the floor operator it is common knowledge that taking temperature and pressure readings is very boring, especially on those gages that are located in hot or awkward locations. Avoid the temptation to "radio" these readings. From hot to failure is a very short trip. Maintenance's favorite expression is "there is never time to do it right, but there is always time to fix it." Try to keep this in mind when the pressure is on to get the equipment running again. Do not let cleaning people direct their "wash down" hoses directly at the pump. Water entering the bearings through the lip or grease seals is a major cause of premature bearing failure. Most water wash downs are used to dilute and wash away seal leakage. Stop the leak and you have eliminated the reason for the hose. A great many motor and electrical problems are caused by these same wash down hoses. Cooling a bearing outside diameter will cause it to shrink and the bearing will get hotter as the radial load increases. Keep the water hose and all other forms of cooling off of the bearing casing.
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Subject: Troubleshooting the positive displacement rotary pump. 12-04 No liquid discharge. ●
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The pump is not primed. Prime it from the outlet side by keeping the outlet air vent open until liquid comes out the vent. The rotating unit is turning in the wrong direction. Valves are closed or there is an obstruction in the inlet or outlet line. Check that the flange gaskets have their center cut out. The end of the inlet pipe is not submerged. You can either increase the length of the inlet pipe into the liquid level or raise the level in the tank. The foot valve is stuck. A strainer or filter is clogged. The net inlet pressure is too low. A bypass valve is open. There is an air leak some where in the inlet line. Air can come in through gaskets or valves above the fluid line. The stuffing box is under negative pressure. Packing is allowing air to get into the system. You should convert the packing to a mechanical seal The pump is worn. The critical clearances have increased. Something is broken. Check the shaft, coupling, internal parts, etc. There is no power to the pump.
The pump is putting out a low capacity. ● ● ● ● ● ● ● ● ● ●
The pump's internal clearances have increased. It is time to change some parts. The net inlet pressure is too low; the pump is cavitating. A strainer or filter is partially clogged. The speed is too low. Check the voltage. The tank vent is partially frozen shut. A bypass line is partially open. A relief valve is stuck partially open. The inlet piping is damaged. Something ran over it A corrosion resistant liner has collapsed in the inlet piping. Air is leaking through the packing. You should go to a mechanical seal.
The pump looses its prime after it has been running for a while. ●
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The liquid supply is exhausted. Check the tank level; sometimes the float is stuck, giving an incorrect level reading. The liquid velocity has increased dramatically. The liquid is vaporizing at the pump inlet. A bypass line is heating the incoming fluid. An air leak has developed in the suction piping.
The pump is using too much power ● ● ● ● ●
The speed is too high. The liquid viscosity is higher than expected. The discharge pressure is higher than calculated The packing has been over tightened. You should convert to a mechanical seal. A rotating element is binding. Misalignment could be the problem or something is stuck in a close clearance and binding the rotating element.
Excessive noise and vibration. ● ● ● ● ●
Relief valve chatter. Foundation or anchor bolts have come loose. The pump and driver are misaligned. The piping is not supported properly. The liquid viscosity is too high. The pump is starving. Check the temperature of the incoming liquid. Check to see if the supply tank heater has failed.
Excessive noise or a loss of capacity is frequently caused by cavitation. Here is how the NPSH required was determined initially: With the pump initially operating with a 0 psig. inlet pressure and constant differential pressure, temperature, speed and viscosity; a valve in the inlet line is gradually closed until cavitation noise is clearly audible, there is a sudden drop off in capacity or there is a 5% overall reduction in output flow. Cavitation occurs with: ● ● ●
A loss of suction pressure. An increase in fluid velocity. An increase in inlet temperature.
Here are some common causes of cavitation problems: ● ●
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A foot valve or any valve in the suction piping is sticking. Something is occasionally plugging up the suction piping. If the pump suction is coming from a river, pond or the ocean, grass is a strong possibility. A loose rag is another common cause. A collapsed pipe liner. A filter or strainer is gradually clogging up. The tank vent partially freezes in cold weather. The sun is heating the suction piping, raising the product temperature close to its vapor point. The level in the open suction tank decreases causing vortex problems that allow air into the pump suction. Several pumps in the same sump are running, decreasing the level too much. The suction tank float is stuck. It will sometimes show a higher level than you really have. A discharge recirculation line, piped to the pump suction, opens and heats the incoming liquid. Sometimes the suction lift is too high. The increase in pipe friction will reduce the suction head.
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The vapor pressure of the product is very close to atmospheric pressure. The pump cavitates every time it rains because of a drop in atmospheric pressure. The tank is being heated to de-aerate the fluid. Sometimes it is being heated too much. The process fluid specific gravity is changing. This can happen with a change in product operating temperature or if a cleaner or solvent is being flushed through the lines. The source tank is changing from a positive pressure to a vacuum due to the process. A packed valve in the suction piping is at a negative pressure and air is leaking in through the packing. The tank is being pumped dry. The inlet piping has been moved or altered in some way. Has a foot valve, strainer, elbow, or some other type of hardware been installed in the suction piping? Has a layer of hard water calcium or some other type of solid formed on the inside of the suction piping reducing its inside diameter over some period of time?
You are experiencing rapid pump wear. ●
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There are abrasives in the liquid you are pumping causing erosion problems. You may have to go to a larger pump running at a slower speed. There is some corrosion in one or more of the pump elements. There is a lack of lubrication. You have a severe pipe strain problem. It could have been caused by thermal growth of the hardware. Too much misalignment. The pump is running dry. When all else fails the best way to reduce NPSH required is to select a larger pump and run it at a slower speed.
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SUBJECT : Why do pump bearings fail? 4-12 What do we mean by good bearing life? Most of us change the bearings every time we disassemble the equipment to replace the mechanical seal or the packing sleeve. Is this really a sensible thing to do? If you think about it for a minute there is nothing in a bearing to wear out, there are no sacrificial parts. Bearing life is determined by the number of hours it will take for the metal to "fatigue" and that is a function of the load on the bearing, the number of rotations, and the amount of lubrication that the bearing receives. Pump companies predict bearing life measured in years. As an example, the Duriron pump company anticipates a three hundred year life for the radial bearing on their 3 x 2 x 10 pump ( 75 mm. x 50 mm. x 250 mm.) when pumping a liquid with a specific gravity of "one" (fresh water). To understand the term "fatigue" we will conduct an experiment: ● ●
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Straighten out a standard paper clip. Flex it a little and then let it go. You will notice that it returns to the straightened position. You could repeat this cycle many times (many years actually) without breaking (fatiguing) the metal because you are cycling the metal in its "elastic range" ( it has a memory similar to piece of rubber). Now we will bend (stress) the paper clip a lot further and you will note that it did not return to the straightened position. This time you stressed the metal in its' "plastic range" where it did not have a memory. If you bend the metal back and forth in this plastic range it will crack and break in less than twenty cycles. The metal fatigued more quickly because it "work hardened" and became brittle. The more you stress the metal by flexing it the quicker it will work harden and break. You have just demonstrated that fatigue is a function of stress and cycles. When the bearing is pressed on a rotating shaft the load passes from the inner race( inside ring) through the balls to the bearing outer race (the outside ring). Each ball carries a portion of the stress as the balls roll under the load. It is this stress that will eventually fatigue the metal parts.
When a pump is operating at its best efficiency point (B.E.P.) the only load the bearing has to carry is: ● ● ●
The weight of the rotating assembly. The stress caused by the interference fit on the shaft. Any bearing preload specified by the manufacturer.
The fact is that most bearings become overloaded because of: ● ● ● ● ● ●
The wrong interference fit between the bearing and the shaft ( the shaft was out of tolerance). Misalignment between the pump and its' driver. Bent shafts. An unbalanced rotating element. Pushing the bearing too far up a tapered sleeve. Operating the pump off of its best efficiency point (B.E.P.).
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Shaft radial thermal expansion. A futile attempt to cool the bearings by cooling the bearing housing with a water hose or some other similar system. Cooling the outside diameter of a bearing causes it to shrink, increasing the interference and causing additional stress. Cavitation. Water hammer. Axial thrust. The bearing housing is sometimes out of round. Pulley driven designs. Vibration of almost any form. The impeller is located too far away from the bearing. This is a common problem in many mixer/ agitator applications. A bad bearing was supplied. This is becoming more of a problem with the increase in counterfeit parts we are finding in industry.
This overloading will cause heat to be generated, and heat is another common cause of premature bearing failure. Heat will cause the lubricant to: ● ●
Decrease in viscosity, causing more heat as it loses its ability to support the load. Form a "varnish" residue and then "coke" at the elevated temperature. This "coking" will destroy the ability of the grease or oil to lubricate the bearing. It will also introduce solid particles into the lubricant.
In addition to the heat generated by overloading we get additional heat from: ● ● ●
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The oil level is too high or too low. Too often pumps are aligned but not leveled. The bearing was over greased. The shaft material is conducting heat from the pumpage back to the bearing housing. This is a common problem in heat transfer oil pumps, or any time a metal bellows seal is used in an application and the stuffing box cooling jacket is shut off or inoperative. A loss of barrier fluid between double seals causing a temperature rise that conducts heat back to the bearings. A failed cooling jacket in the bearing housing around the stuffing box or built into the seal gland. Grease or lip seal contact on the shaft, right next to the bearings. A failed cooling "quench" in an A.P.I. type seal gland.
A leading bearing manufacturer states that the life of bearing oil is directly related to heat. Non contaminated oil cannot wear out and has a useful life of about thirty years at thirty degrees centigrade (86 F.). They further state that the life of the bearing oil is cut in half for each ten degree centigrade rise (18 F.) in temperature of the oil. This means that oil temperature regulation is critical in any attempt to increase the useful life of anti friction bearings.
Probably the major cause of premature bearing failure is the contamination of the bearing lubrication by moisture and solids. As little as 0.002% water in the lubricant can reduce bearing life by 48%. Six percent water can reduce bearing life by 83% percent. There are several methods used by pump companies to keep this water and moisture out of the bearing housing: ●
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A flinger ring to deflect packing or seal leakage away from the bearings. A silly arrangement at best. Keeping the bearing oil hot to prevent the forming of condensation inside the bearing case. A ridiculous system when you consider that bearing life is directly related to heat. The use of "so called" sealed bearings. You can call them any thing you want, but the seals will not seal anything, especially moisture or water. Grease or lip seals that have a useful life of about two thousand hours (84 days at 24 hours per day) and will cut the expensive shaft directly under the seal lip. Double lip seals will cut the shaft in two places. Labyrinth seals that are superior to lip seals but not totally effective because you are still trying to seal with non contacting surfaces that are useless Statically.
The moisture comes from multiple sources: ● ●
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Packing leakage flows back to the bearing area. Because of packing leakage a water hose is used to wash down the area. This washing splashes on to the pump bearing case also. Aspiration, moist air enters through the lip or labyrinth seals when the bearing case cools down. A seal quench gland that often has steam, condensate or cooling water leaking out and directed at the radial bearing.
The moisture causes several problems: ●
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Pitting and corrosion of the bearing races and rolling elements that will increase the fatigue of the metal components. Free atomic hydrogen, in the water, appears to cause hydrogen embrittlement of the bearing metal accelerating the fatigue. A water and oil emulsion does not provide a good lubricating film.
We get solids into the lubricant from several sources: ●
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Metal seal cage wear. This is the part the separates the balls that are held between the bearing races. It is often manufactured from brass or a non metallic material. Abrasive particles leach out of the bearing housing casting. Often solid particles were already contaminating the grease or oil we are using for the lubricant. Solids were introduced into the system during the assembly process because of a lack of cleanliness. Airborne particles penetrate the bearing seals. Particles worn off of the grease or lip seals penetrate into the bearings.
How to keep solids and moisture out of the bearing housing. ●
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Seal the inside of the bearing housing with epoxy or some other suitable material to stop rusting and to prevent solids from leaching out of the metal case. If you do this be careful about using some of the new high detergent lubricants. They might be powerful enough to remove this protective coating. Replace the grease or labyrinth seals with positive face seals. In the future, you are going to need these seals to prevent hydrocarbon fugitive emissions. Install an expansion chamber outside of the bearing casing to accept the air (approximately 16 oz. or 475 ml. in a typical process pump) that expands as the bearing casing increases in temperature. Without this expansion chamber approximately one atmosphere of pressure will build up in the bearing housing. This is not a problem for a mechanical seal, but during long periods of shut down the pressure could be lost. Clean the oil in the bearing casing by installing a simple oil circulating and filtering system or change the oil frequently.
When do you go from anti-friction ball and roller bearings to hydrodynamic (sleeve) bearings in a centrifugal pump? ● ●
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Any time the DN number exceeds 300,000 (Bearing bore times rpm) If the standard bearings fail to meet an L10 life of 25.000 hours in continuous operation or 16,000 hours at maximum axial and radial load and rated speed. If the product of the pump horsepower and speed in rpm is 2.7 million or greater.
The past several years have seen a decrease in the quality of the bearings available for rotating equipment. We find prepacked bearing being shipped with too much, or no grease at all. Stabilization temperatures have changed and overall quality has diminished. If you adopt the above suggestions you should not have to be changing your bearings as frequently as you are now.
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SUBJECT : Pump selection practices that cause high seal and bearing maintenance problems. 6-9 Purchasing well designed hardware does not bring automatic trouble free performance with it. The very best equipment will cause problems if it was not designed for your particular application. Here are a few of the more common selection problems we find with centrifugal pumps: ●
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Buying the same size pump as the one that came out of the application. That's O.K. If the old pump was the correct size, but the odds are that it was too big because of the safety factors that were added at the time of purchase. This will cause the pump to run off of its best efficiency point (B.E.P.) and you will spend a lot of production money for the additional power that is needed to run against a throttled discharge valve or orifice installed in the discharge piping. Buying to a standard, or making a decision based on efficiency, and believing that these two some how relate to quality. Standards were written for packed pumps. When a mechanical seal is being used the shaft L3/D4 number is almost always too large. Efficiency is always gained at the expense of maintenance. Efficiency means tight tolerances and smooth passages that will eliminate reliable double volute designs and keep the maintenance department busy adjusting tight tolerances to maintain the efficiency you paid for. Series and parallel installation problems. We often find pumps installed in parallel, but no one knows it because the second pump was installed at a much later date and no one has bothered to trace the piping. Pumps in parallel require that they have the same diameter impeller and that they run at the same speed, or the larger pump will throttle the smaller one causing it to run off the best efficiency point, deflecting the shaft. The capacity should be looked at if the higher capacity pump might exceed the N.P.S.H. available. When pumps are installed in series the impellers must be the same width and they must run at the same speed or the higher capacity pump will either cavitate because the smaller capacity pump can not feed liquid at the proper capacity, or it will run throttled if it is feeding the smaller pump. In either case the larger of the two pumps will be adversely affected. Purchasing a larger pump because it will be needed in the future. Will raise the operating cost to unacceptable levels (Power = head x capacity) as the pump is run against a throttled discharge valve. This inefficient use of power will translate to a higher heat environment for the seal along with all of the problems associated with shaft deflection. Using a variable speed motor to compensate for a pump curve that is not flat enough. Many boiler feed pumps require a flat curve so that the pump can put out varying capacities at a constant boiler pressure (head). We see this same need if we are pumping a varying amount of liquid to a very high constant height. Varying the speed of a pump is similar to changing the diameter of the impeller. If you look at a typical pump curve you will observe that the best efficiency point (B.E.P.) comes down with impeller size to form an angle with the base line (capacity line) of the graph. This means that if you vary the speed of the impeller, the pump always runs off the B.E.P. except in the case where the system curve intersects the pump curve, or in the case of an exponential system curve such as we find in a typical hot or cold water circulating system. Double ended pumps installed in a vertical position to save floor space. Makes seal replacement a nightmare unless you are using split or cartridge designs. Specifying a desired capacity without knowing the true system head. You can't guess with this one. Some one has to make the calculations and "walk the system". The present pump is not a
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reliable guide because we seldom know where it is pumping on its' curve. Chart recorders installed on both the suction and discharge side of the pump will give a more accurate reading of the present head if they are left on long enough to record the differences in flow. The trouble with this method is that it will also record a false head caused by a throttled valve, an orifice, or any other restriction that might be present in the piping. Requesting too low a required N.P.S.H. will cause you to end up with a different kind of cavitation problem. See another paper in this series for information about "Internal recirculation". Failure to request a "center line design" when pumping temperature exceeds 200°F (100°C) it will cause pipe strain that will translate to wear ring damage and excessive mechanical seal movement. The use of "inline" pumps to save floor space. Many of these designs are "close coupled" with the motor bearings carrying the radial and thrust loads. Because of typical L3/D4 numbers being very high, the wear rings act as "steady bearings" after the pump is converted to a mechanical seal. The pump should have been designed with a separate bearing case and a "C" or "D" frame adapter installed to connect a motor to the bearing case. Thrust bearings being retained by a simple snap ring. Beyond 65% of its rated efficiency most centrifugal pumps thrust towards the pump volute. The thin snap ring has to absorb all of this axial thrust and most of them can not do it very well . The mechanical seal has been installed in a packing stuffing box that is too narrow to allow free seal movement. If a mechanical seal was specified, the pump back plate should have been manufactured with a large diameter seal chamber. In most cases the stuffing box recirculation line should be installed from the bottom of this large seal chamber to the suction side of the pump or a low pressure point in the system. There are some exceptions to this, however: ❍ If you are pumping at or close to vapor point. ❍ If the entrained solids have a low specific gravity. ❍ If you are using a Duriron pump that adjusts to the back plate. ❍ If you are using a double suction pump where the stuffing boxes are at suction pressure. High temperature applications have several special needs: ❍ A jacketed stuffing box that isolates the pumpage from the stuffing box contents by a carbon bushing to retard heat transfer. ❍ A centerline design to compensate for thermal expansion. ❍ A cartridge seal design that allows open impeller adjustment after the pump has come up to operating temperature. ❍ A stainless steel shaft to retard heat transfer to the bearings. ❍ A method of cooling the bearing oil, but never the bearings. ❍ A coupling that will compensate for axial expansion.
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SUBJECT: Shaft fretting 10-3 The next time you remove a grease or lip seal (the rubber seal located next to the bearing) you will note that the shaft is grooved and damaged under the rubber lip. You will see this same damage in a few other locations also: ● ●
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On the sleeve under the stuffing box packing, if you are still using packing in your pumps. On the sleeve under the Teflon® wedge, "U' cup, or "V" rings if you are using original equipment type mechanical seals. Underneath the spring loaded o-ring found on many popular single and double mechanical seals. Underneath the rubber bellows of the type #1 seal if the rubber bellows did not vulcanize to the shaft.
This shaft or sleeve damage is called fretting and it will cause you several problems: ● ● ●
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Sleeve replacement is costly. The pump bearings are often destroyed in the process of removing the damaged shaft sleeve. The shaft diameter was reduced to accommodate the wear sleeve. This reduction weakened the shaft, raised the L3/D4 number, and increased shaft deflection problems. The seal can "hang up" in the fretted groove, opening the lapped seal faces. The fretted grove becomes an additional leak path for the fluid. This is a major cause of seal failure
What causes this fretting problem? How can a soft piece of rubber or a slick wedge of Teflon cut a hard shaft? It doesn't seem to make any sense. Surprisingly it has nothing to do with dirt in the air or abrasives in the fluid. The problem will occur even if you are pumping a filtered, clean lubricant in a sterile atmosphere. To understand fretting you must first understand the term "corrosion resistant". Some materials resistant corrosion others do not. What is the difference? We say that iron rusts, but aluminum oxidizes. A look at any dictionary will verify that these terms mean the same thing. So why do we use different terms to describe the same problem? The answer lies in the way a metal rusts or oxidizes. If the oxide layer is protective we say that the material is corrosion resistant. Take aluminum as an example: ●
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Aluminum protects its self by forming a layer of aluminum oxide on the surface when it is exposed to oxygen. It is very visible and looks almost white in color. A more common name for aluminum oxide is ceramic,a dense, hard, corrosion resistant material.
After this dense layer is formed on the surface of the aluminum the oxidation or rusting rate is slowed down to less than 0.002 inches (0,05 mm) per year, and this is the definition of corrosion resistant.
If this protective oxide layer is rubbed or polished off by the packing, lip seal or Teflon wedge the oxide will immediately reform to protect the base material. It is this constant oxide removal and reforming that causing the shaft grooving that is so visible. We get the same reaction when we polish silver. The "tarnish" replaces its self to protect the silver. Shaft vibration and end play causes a constant axial movement of the shaft through the mechanical seal dynamic rubber or Teflon® part. Bearing grease seals and stuffing box packing are stationary, so the rotating shaft is constantly being polished by these materials when the pump is running There is a second problem associated with fretting. The ceramic oxide that is removed imbeds its self into the rubber part causing a wear or grinding action on the base metal. Stainless steel protects its self by forming a protective oxide called chrome oxide, one of the hardest ceramics. When this oxide forms we say that the active stainless steel is now "passivated". It is this chrome oxide imbedded into the packing, Teflon®, or rubber lip that does so much damage to the shaft sleeve. So now we have two causes of fretting: ● ●
The removal of the passivated layer by the rubbing action of the rubber or Teflon®. The hard ceramic that we removed sticking into the rubber or Teflon causing a grinding action.
Now that we know the causes of fretting what is the solution? ●
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Replace bearing lip or grease seals with labyrinth or the newer positive face seals. Face seals are the better choice. Stop putting packing into pumps. You don't need that kind of leakage any more. Do not use mechanical seals that are designed with a dynamic elastomer positioned on the pump shaft or sleeve. Most original equipment seals are designed this way. Stationary cartridge seals, most balanced O-ring seals and all bellows seals eliminate the shaft dynamic elastomer and the fretting associated with it.
® DuPont Dow elastomer
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SUBJECT : Cavitation 1-3 Cavitation means that cavities are forming in the liquid that we are pumping. When these cavities form at the suction of the pump several things happen all at once. ● ● ● ●
We experience a loss in capacity. We can no longer build the same head (pressure) The efficiency drops. The cavities or bubbles will collapse when they pass into the higher regions of pressure causing noise, vibration, and damage to many of the components.
The cavities form for five basic reasons and it is common practice to lump all of them into the general classification of cavitation. This is an error because we will learn that to correct each of these conditions we must understand why they occur and how to fix them. Here they are in no particular order : ● ● ● ● ●
Vaporization Air ingestion Internal recirculation Flow turbulence The Vane Passing Syndrome
Vaporization . A fluid vaporizes when its pressure gets too low, or its temperature too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization. This head requirement is supplied to us by the pump manufacturer and is calculated with the assumption that fresh water at 68 degrees Fahrenheit (20 degrees Centigrade) is the fluid being pumped. Since there are losses in the piping leading from the source to the suction of the pump we must determine the head after these losses are calculated. Another way to say this is that a Net Positive Suction Head is Required (N.P.S.H.R.) to prevent the fluid from vaporizing. We take the Net Positive Suction Head Available (N.P.S.H.A.) subtract the Vapor Pressure of the product we are pumping, and this number must be equal to or greater than the Net Positive Suction Head Required. To cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the N.P.S.H. Required. We shall look at each possibility: Increase the suction head ● ● ●
Raise the liquid level in the tank Raise the tank Put the pump in a pit
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Reduce the piping losses. These losses occur for a variety of reasons that include : ❍ The system was designed incorrectly. There are too many fittings and/or the piping is too small in diameter. ❍ A pipe liner has collapsed. ❍ Solids have built up on the inside of the pipe. ❍ The suction pipe collapsed when it was run over by a heavy vehicle. ❍ A suction strainer is clogged. ❍ Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather ❍ Something is stuck in the pipe, It either grew there or was left during the last time the system was opened . Maybe a check valve is broken and the seat is stuck in the pipe. ❍ The inside of the pipe, or a fitting has corroded. ❍ A bigger pump has been installed and the existing system has too much loss for the increased capacity. ❍ A globe valve was used to replace a gate valve. ❍ A heating jacket has frozen and collapsed the pipe. ❍ A gasket is protruding into the piping. ❍ The pump speed has increased. Install a booster pump Pressurize the tank
Lower the fluid temperature ● ● ●
Injecting a small amount of cooler fluid at the suction is often practical. Insulate the piping from the sun's rays. Be careful of discharge recirculation lines, they can heat up the suction fluid.
Reduce the N.P.S.H. Required ●
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Use a double suction pump. This can reduce the N.P.S.H.R. by as much as 27% or in some cases it will allow you to raise the pump speed by 41% Use a lower speed pump Use a pump with a larger impeller eye opening. If possible install an Inducer. These inducers can cut N.P.S.H.R. by almost 50%. Use several smaller pumps. Three half capacity pumps can be cheaper than one large pump plus a spare. This will also conserve energy at lighter loads.
It is a general rule of thumb that hot water and gas free hydrocarbons can use up to 50% of normal cold water N.P.S.H. requirements, or 10 feet (3 meters), whichever is smaller. I would suggest you use this as a safety margin rather than design for it. Air ingestion A centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous. Air gets into as system in several ways that include : ●
Through the stuffing box. This occurs in any packed pump that lifts liquid, pumps from a
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condenser, evaporator or any piece of equipment that runs in vacuum. Valves above the water line. Through leaking flanges Vortexing fluid. A bypass line has been installed too close to the suction. The suction inlet pipe is out of fluid. This can occur when the level gets too low or there is a false reading on the gauge because the float is stuck on a corroded rod.
Both vaporization and air ingestion have an affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity. Although air ingestion and vaporization both occur they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump. Internal Recirculation This condition is visible on the leading edge of the impeller, and will usually be found at the discharge tip working its way back to the suction. It can also be found at the suction eye of the pump. As the name implies the fluid recirculates increasing its velocity until it vaporizes and then collapses in the surrounding higher pressure. This has always been a problem with low NPSH pumps and the term SPECIFIC SUCTION SPEED was coined to give you a guide in determining how close you have to operate to the B.E.P. of a pump to prevent the problem. The higher the number the smaller the window in which you have to operate. The numbers range between 3,000 and 20,000. Water pumps should stay between 3,000 and 12,000. Here is the formula to determine the suction specific speed number of your pump:
rpm = Pump speed gpm = Gallons per minute or liters per second of the largest impeller at its BEP Head= Net positive suction head required at that rpm ● ●
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For a double suction pump the flow is divided by 2 since there are 2 impeller eyes Try to buy pumps lower than 8500.(5200 metric ) forget those over 12000 (8000 metric) except for extreme circumstances. Mixed hydrocarbons and hot water at 9000 to 12000 (5500 to 7300 metric) or higher, can probably operate satisfactorily. High specific speed indicates the impeller eye is larger than normal, and efficiency may be compromised to obtain a low NPSH required. Higher values of specific speed may require special designs, and operate with some cavitation.
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Normally a pump operating 50% below its best efficiency point (B.E.P.) is less reliable.
With an open impeller pump you can usually correct the problem by adjusting the impeller clearance to the manufacturers specifications. Closed impeller pumps present a bigger problem and the most practical solution seems to be to contact the manufacturer for an evaluation of the impeller design and a possible change in the design of the impeller or the wear ring clearances. Turbulence We would prefer to have liquid flowing through the piping at a constant velocity. Corrosion or obstructions can change the velocity of the liquid and any time you change the velocity of a liquid you change its pressure. Good piping layouts would include : ● ●
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Ten diameters of pipe between the pump suction and the first elbow. In multiple pump arrangements we would prefer to have the suction bells in separate bays so that one pump suction will not interfere with another. If this is not practical a number of units can be installed in a single large sump provided that : The pumps are located in a line perpendicular to the approaching flow. There must be a minimum spacing of at least two suction diameters between pump center lines. All pumps are running. The upstream conditions should have a minimum straight run of ten pipe diameters to provide uniform flow to the suction bells. Each pump capacity must be less than 15,000 gpm.. Back wall clearance distance to the centerline of the pump must be at least 0.75 of the suction diameter. Bottom clearance should be approximately 0.30 (30%) of the suction diameter The minimum submergence should be as follows: FLOW
MINIMUM SUBMERGENCE
20,000 GPM
4 FEET
100,000 GPM
8 FEET
180,000 GPM
10 FEET
200,000 GPM
11 FEET
250,000 GPM
12 FEET
The metric numbers are :
FLOW
MINIMUM SUBMERGENCE
4,500 M3/HR
1.2 METERS
22,500 M3/HR
2.5 METERS
40,000 M3/HR
3.0 METERS
45,000 M3/HR
3.4 METERS
55,000 M3/HR
3.7 METERS
The Vane Passing Syndrome You will notice damage to the tip of the impeller caused by its passing too close to the pump cutwater. The velocity of the liquid increases if the clearance is too small lowering the pressure and causing local vaporization. The bubbles collapse just beyond the cutwater and there is where you should look for volute damage. You will need a flashlight and mirror to see the damage unless it has penetrated to the outside of the volute. The damage is limited to the center of the impeller and does not extend into the shrouds. You can prevent this problem if you keep a minimum impeller tip to cutwater clearance of 4 % of the impeller diameter in the smaller impeller sizes (less than 14' or 355 mm.) and 6% in the larger impeller sizes (greater than 14" or 355 mm.). To prevent excessive shaft movement bulkhead rings can be installed in the suction eye. At the discharge rings can be manufactured to extend from the walls to the impeller shrouds.
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SUBJECT: The pump is not producing enough head to satisfy the application 10-9 This is the first paper in a four part series about pump troubleshooting. Let me begin by pointing out that there are a couple of things you must keep in mind when troubleshooting centrifugal pump problems: ●
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The centrifugal pump always pumps the difference between the suction and discharge heads. If the suction head increases, the pump head will decrease to meet the system requirements. If the suction head decreases the pump head will increase to meet the system requirements. A centrifugal pump always pumps a combination of head and capacity. These two numbers multiplied together must remain a constant. In other words, if the head increases the capacity must decrease. Likewise if the head decreases, the capacity must increase. The pump will pump where the pump curve intersects the system curve. If the pump is not meeting the system curve requirements the problem could be in the pump, the suction side including the piping and source tank, or somewhere in the discharge system. Most pumps are oversized because of safety factors that were added at the time the pump was sized. This means that throttling is a normal condition in most plants, causing the pump to run on the left hand side of its curve.
THE PROBLEM COULD BE IN THE PUMP ITS SELF ●
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The impeller diameter is too small. ❍ The impeller is running at too slow a speed ❍ You are running an induction motor. Their speed is different than synchronous motors. It's always slower. The pump curve was created using a variable frequency motor that ran at a constant speed. Put a tachometer on your motor to see its actual speed. ❍ Your pulley driven pump is running on the wrong pulley diameter. ❍ A variable frequency motor is running at the wrong speed. ❍ Check the speed of the driver if the pump is driven by something other than an electric motor. There is something physically wrong with the motor. Check the bearings etc. Check the voltage of the electric motor. It may be too low. The impeller is damaged. The damage could be caused by excessive wear, erosion, corrosion or some type of physical damage. ❍ Physical damage often occurs during the assembly process when the impeller is driven on or off the shaft with a wooden block and a mallet. Many impeller designs do not have a nut cast into the impeller hub to ease removal. ❍ Erosion occurs when solids enter the eye of the impeller. The solids can chip off pieces of the ceramic that are passivating the impeller, causing localized corrosion. ❍ Damage can occur if the impeller to volute, or back plate clearance is too small and the shaft experiences some type of deflection. The original clearance could have diminished with thermal growth of the shaft. Keep in mind that some open impellers adjust to the volute (Goulds) while other designs adjust to the back plate (Duriron). In an ANSI and similar design centrifugal pumps, the normal thrust towards the volute has bent the snap ring designed for bearing retention. This can allow the rotating impeller to hit the stationary volute.
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Here are some examples of shaft displacement: ❍ Operating the pump too far off the BEP. ❍ Pulley driven applications. ❍ Pipe strain. ❍ Misalignment between the pump and driver. ❍ The shaft could be bent. ❍ The rotating assembly was probably not dynamically balanced. The impeller is clogged. This is a major problem with closed impellers. With the exception of finished product, most of what you will be pumping contains entrained solids. Remember also that some products can solidify, or they can crystallize with a change in fluid temperature or pressure. Impeller balance holes have been drilled between the eye and the wear rings of a closed impeller. The reverse flow is interfering with the product entering the impeller eye. A discharge recirculation line should have been used in place of the balance holes to reduce the axial thrust. The double volute casting is clogged with solids or solids have built up on the surface of the casting. The open impeller to volute clearance is too large. 0.017" (0,5 mm) is typical. This excessive clearance will cause internal recirculation problems. A bad installation, thermal growth, or normal impeller wear could be the cause. ❍ A large impeller to cutwater clearance can cause a problem called discharge recirculation. Wear is a common symptom of this condition. If the impeller is positioned too close to the cutwater you could have cavitation problems that will interfere with the head. The impeller specific speed number is too high. Lower specific speed numbered impellers are used to build higher heads. An impeller inducer was left off at the time of assembly. Inducers are almost always needed with high specific speed impellers. Leaving off the inducer can cause cavitation problems that will interfere with the head. The impeller is loose on the shaft. The impeller is running backwards The shaft is running backwards because of a wiring problem. The pump is running backwards because the discharge check valve is not holding and system pressure is causing the reverse rotation. This is a common problem with pumps installed in a parallel configuration. Check valves are notoriously unreliable. The impeller has been installed backwards. This can happen with closed impellers on double ended pumps The second stage of a two stage pump is wired backwards. The pump reverses when the second stage kicks in. You should have heard a loud noise when this happened. The wear ring clearance is too large. ❍ This is a common problem if the shaft L3/D4 number is greater than 60 (2 in the metric system). ❍ You should replace the rings when the original clearance doubles. Needless to say this can only be determined by inspection. If you are pumping a product at 200°F (100°C) or more you should use a centerline design volute to prevent excessive wear ring wear as the volute grows from the base straight up, engaging the wear rings. A wear ring is missing. It was probably left off during the installation process.
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A high suction tank level is reducing the differential pressure across the pump increasing its capacity. The pump pumps the difference between the suction and discharge heads. A bubble is trapped in the eye of the impeller. The eye is the lowest pressure area. When this bubble forms it shuts off all liquid coming into the pump suction. This could cause the pump to lose its prime. You cannot vent a running pump because centrifugal force will throw the liquid out the vent leaving the air trapped inside. Air is coming directly into the pump. This happens with a negative pressure at the suction side. Negative suction happens when the pump is lifting liquid, pumping from a condenser hot well etc. ❍ Air is coming into the stuffing box through the pump packing. ❍ Air is coming into the stuffing box through an unbalanced mechanical seal. As the carbon face wears the spring load holding the faces together diminishes. ❍ If you are using mechanical seals in vacuum service, they should be of the O-ring design. Unlike other designs, O-rings are the only shape that seals both pressure and vacuum. ❍ The pump was not primed prior to start up. With the exception of the self priming version, centrifugal pumps must be full of liquid at start up. ❍ Air can enter the stuffing box if the gasket between the two halves of a double ended pump is defective or does not extend to the stuffing box face. Any small gaps between the face of the stuffing box and the split at the side of the stuffing box will allow either air in, or product out. ❍ Air is coming into the suction side of the pump through a pin hole in the casing. ❍ Air is entering the stuffing box between the sleeve and the shaft. This happens if you convert a double ended pump from packing to a mechanical seal and fail to install a gasket or o-ring between the impeller hub and the sleeve. The open impeller was adjusted backwards and now the close fitting "pump out vanes" are creating a vacuum in the stuffing box. You need a volute casing instead of a concentric casing. Volute casings are much better for producing head. You have the wrong size pump. It cannot meet the system curve requirements: The pump was not selected to meet the system curve requirements because no system curve was given to the pump supplier. At replacement time the same size pump was purchased because no one had calculated losses in the system. The pump was sized from a piping diagram that was thirty five years old. There have been numerous piping changes and additions since the original layout. In many instances additional pumps have been installed and this pump is running in parallel with them, but nobody knows it.
THE PROBLEM IS ON THE SUCTION SIDE OF THE PUMP. THE PUMP COULD BE CAVITATING. ●
Air is entering the suction piping at some point. ❍ Air is being pumped into the suction piping to reduce cavitation problems ❍ Fluid returning to the sump is being aerated by too far a free fall. The return line should terminate below the liquid level. ❍ The fluid is vortexing at the pump inlet because the sump level is too low and the pump capacity is too high.
Air is coming into the system through valves above the water line or gaskets in the piping flanges. ❍ The liquid source is being pumped dry. If this is a problem in your application you might want to consider a self priming pump in the future. The vapor pressure of the fluid is too close to atmospheric pressure. When it rains the drop in atmospheric pressure causes the inlet fluid to vaporize. There is a problem with the piping layout. It is reducing the head on the suction side of the pump. ❍ There is too much piping between the pump suction and the source tank. You may need a booster pump or an inducer. The higher the pump speed the bigger the problem. ❍ There is an elbow too close to the pump suction. There should be at least ten diameters of pipe between the elbow and the pump suction. Suction piping should never run parallel with the pump shaft in a double ended pump installation. This can cause unnecessary shaft thrusting. ❍ A piece of pipe of reduced diameter has been installed in the suction piping. ❍ Piping was added on the inlet side of the pump to by-pass a piece of equipment that was installed on the floor. ❍ A piping to pump reducer has been installed upside down causing an air pocket. Concentric reducers can cause the same problem.. ❍ Multiple pump inlets are too close together. The pump inlet is too close to the tank floor. The suction lift is too high. A gasket with too small an inside diameter has been installed in the suction piping restricting the liquid flow. A gasket in the suction piping is not centered and is protruding into the product stream. A globe valve has been substituted for a gate valve in the suction piping. The loss of head in a globe valve is many times that of a gate valve. Two pumps are connected in series. The first pump is not sending enough capacity to the second pump. The piping inlet is clogged. A filter or strainer is clogged or covered with something. Intermittent plugging of the suction inlet. ❍ Loose rags can do this. ❍ If the suction is from a pond, river, or the sea, grass can be pulled into the suction inlet. A foot valve is stuck. A check valve is stuck partially closed The foot valve is too small. A small clam or marine animal cleared the suction screen, but has now grown large on the pump side of the screen. The suction piping diameter has been reduced. ❍ The suction piping collapsed when a heavy object either hit or ran over the piping. ❍ Solids have built up on the piping walls. Hard water is a good example of this problem ❍ A liner has broken away from the piping wall and has collapsed in the piping. Look for corrosion in the piping caused by a hole in the liner. ❍ A foreign object is stuck in the piping It was left there when the piping was repaired. ❍ The suction is being throttled to prevent the heating of the process fluid. This is a common operating procedure with fuel pumps where discharge throttling could cause a fire or explosion. ❍
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The pump inlet temperature is too high. ❍ The tank is being heated to deaerate the fluid, but it is heating the fluid up too much. Look for this problem in boiler feed pump applications. ❍ The sun is heating the inlet piping. The piping should be insulated to prevent this problem. ❍ The operating temperature of the pumped fluid has been increased to accommodate the process requirements. ❍ A discharge recirculation line is heating the incoming fluid. You should direct this line to a reservoir rather than the pump suction. ❍ Steam or some other hot cleaner is being circulated through the lines. The problem is in the tank connected to the suction of the pump. ❍ The pump capacity is too high for the tank volume. ❍ The tank float is stuck, showing a higher tank level that does not exist. ❍ The tank vent is partially shut or frozen, lowering the suction pressure. ❍ There is not enough NPSH available for the fluid you are pumping. Maybe you can use an inducer or booster pump to increase the suction pressure. ❍ A high suction tank level is reducing the differential pressure across the pump, increasing its capacity and lowering the head.
PROBLEMS ON THE DISCHARGE SIDE OF THE PUMP INCLUDING THE PIPING ●
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Two pumps are in connected in series. The first pump does not have enough capacity for the second pump. They should be running at the same speed with the same width impeller. The pump discharge is connected to the bottom of the tank. The head is low until the level in the tank increases. Units in the discharge piping should not normally be shut off, they should be by-passed to prevent too much of a change in the pump's capacity. If too many units are being by-passed in the discharge system the head will decrease as the capacity increases. This can happen if an extra storage tank farm is being by-passed because the storage capacity is no longer needed. A bypass line has been installed in the pump discharge increasing the capacity and lowering the head. Piping or fittings have been removed from the discharge side of the pump reducing piping resistance. Connections have been installed in the discharge piping that have increased the demand that increases capacity. The pump is acting as an accumulator, coming on when the tank level drops. The head will be low until the accumulator is recharged. Consider the possibility of a siphon affect in the discharge piping. This will occur if the pump discharge piping is entering into the top of a tank and discharging at a lower level The pump must build enough head initially to take advantage of the siphoning action. A discharge valve (manual or automatic) is opened too much.
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SUBJECT: The pump is not producing enough capacity to satisfy the application 10-10 This is the second paper in a four part series about pump troubleshooting. Let me begin by pointing out that there are a couple of things you must keep in mind when troubleshooting centrifugal pump problems: ●
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The centrifugal pump always pumps the difference between the suction and discharge heads. If the suction head increases, the pump head will decrease to meet the system requirements. If the suction head decreases the pump head will increase to meet the system requirements. A centrifugal pump always pumps a combination of head and capacity. These two numbers multiplied together must remain a constant. In other words, if the head increases the capacity must decrease. Likewise if the head decreases, the capacity must increase. The pump will pump where the pump curve intersects the system curve. If the pump is not meeting the system curve requirements the problem could be in the pump, the suction side including the piping and source tank, or somewhere in the discharge system. Most pumps are oversized because of safety factors that were added at the time the pump was sized. This means that throttling is a normal condition in most plants, causing the pump to run on the left hand side of its curve.
THE PROBLEM IS IN THE PUMP ITS SELF: ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
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The impeller diameter is too small The impeller width is too narrow The impeller speed is too slow. Check the voltage and frequency The impeller is damaged. The impeller is clogged. The open impeller clearance is too large. The impeller to cutwater clearance is too large. The impeller specific speed number is too low. The impeller has been installed backwards The shaft is running backwards. The wear ring clearance is too large. A wear ring is missing. The second stage of a two stage pump is wired backwards. A bubble is trapped in the eye of the impeller. A low suction tank level is increasing the differential pressure across the pump decreasing its capacity. Air is coming into the pump suction through the packing. Air is coming into the pump suction through an unbalanced mechanical seal. The pump was not primed prior to star up. You may need a concentric casing rather than the volute design. You are using a variable speed motor trying to produce a flat curve. Remember that both the head and capacity change with speed. The pump is the wrong size. Someone gave the pump distributor a wrong system curve
THE PROBLEM IS ON THE SUCTION SIDE OF THE PUMP ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
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There is too much piping between the pump suction and the source tank. There is an elbow too close to the pump suction. A filter or strainer is clogged. Intermittent plugging of the suction inlet. Loose rags can do this. A foot valve is stuck The tank float is stuck. Showing a higher tank level that does not exist. The tank vent is partially shut or frozen. A globe valve has been substituted for a gate valve. A check valve is stuck partially closed Solids have built up on the piping walls. A liner has broken away from the piping wall and has collapsed in the piping. The piping was collapsed by a heavy object that hit the outside of the piping. A foreign object is stuck in the piping It was left there when the piping was repaired. A small clam cleared the suction screen, but has now grown large on the pump side of the screen. The sun is heating the inlet piping. It should be insulated to prevent this problem. Piping was added on the inlet side of the pump to compensate for a piece of equipment that was installed in the shop. A reducer has been installed upside down. A discharge recirculation line is heating the incoming fluid. The pump capacity is too high for the tank volume. Multiple pump inlets are too close together. The suction lift is too high. There is not enough NPSH available for the fluid you are pumping. Maybe you can use an inducer to increase the suction pressure. Air is coming into the system through valves above the water line or gaskets in the piping. Air is being pumped into the suction piping to reduce cavitation problems Fluid returning to the sump is being aerated by too far a free fall. The fluid is vortexing at the pump inlet because the sump level is too low. The tank is being heated to deaerate the fluid, but it is heating the fluid up too much. Two pumps are connected in series. The first pump is not sending enough capacity to the second pump. The operating temperature of the pumped fluid has increased. The vapor pressure of the fluid is too close to atmospheric pressure. When it rains the drop in atmospheric pressure causes the inlet fluid to vaporize. The suction is being throttled to prevent the heating of the process fluid.
PROBLEMS ON THE DISCHARGE SIDE OF THE PUMP INCLUDING THE PIPING ● ● ● ●
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Extra piping has been added to the system to accommodate extra storage capacity. A bypass line has been installed in the pump discharge. Piping or fittings have been added to the discharge side of the pump. An orifice has been installed in the discharge piping to reduce the capacity or produce a false head. A gate valve has been substituted for a globe valve in the discharge piping.
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A check valve is stuck partially closed. An orifice has been installed into the piping to restrict flow. The piping was collapsed by a heavy object that hit the outside of the piping. The discharge valve is throttled too much. There is a restriction in the discharge piping. Extra pumps have been installed into the existing piping They are connected in parallel, but are not producing the same head. Two pumps are in parallel. The larger one is shutting the check valve of the smaller pump. Two pumps are in connected in series. The first pump does not have enough capacity for the second pump. They should be running at the same speed with the same width impeller The pump discharge is connected to the bottom of the tank. The head is increasing and the capacity is decreasing as the tank fills. The pump is acting as an accumulator&emdash;coming on when the tank level drops. The head is too high when the tank fills.
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SUBJECT: The centrifugal pump is drawing too much amperage. 10-11 This is the third paper in a four part series about pump troubleshooting. As mentioned in paper number ten, volume number ten, there are a couple of things you must keep in mind when troubleshooting centrifugal pump problems: ●
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The centrifugal pump always pumps the difference between the suction and discharge heads. If the suction head increases, the pump head will decrease to meet the system requirements. If the suction head decreases the pump head will increase to meet the system requirements. A centrifugal pump always pumps a combination of head and capacity. These two numbers multiplied together must remain a constant. In other words, if the head increases the capacity must decrease. Likewise if the head decreases, the capacity must increase. The pump will pump where the pump curve intersects the system curve. If the pump is not meeting the system curve requirements the problem could be in the pump, the suction side including the piping and source tank, or somewhere in the discharge system. Most pumps are oversized because of safety factors that were added at the time the pump was selected. This means that throttling is a normal condition in most plants, causing the pump to run on the left hand side of its curve.
The increased amperage can be caused by a pump that is too large for the application. ● ●
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A large pump was specified in anticipation of future needs. The pump was sized for the maximum operating condition, but does not run anywhere near that point most of the time. The capacity requirement has been lowered and the pump is being throttled rather than cut back the impeller diameter. The pump was oversized because of safety factors that were added at the time the pump was sized. Increasing the speed of the pump causes a dramatic change in the amperage required. The amperage changes by the cube of the change in speed or impeller diameter. If you double the speed of a pump you will need eight times the amperage.
The increased amperage can be caused by a change in the product. ●
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The motor was sized for a low specific gravity fluid, but the lines are being flushed or tested with water. The specific gravity of the fluid has increased for some reason. The viscosity of the liquid is increasing with a change in temperature. Some viscosities increase with a lower temperature, some with a higher temperature. The viscosity of a liquid can increase with agitation. That is how cream becomes butter.
The increased amperage is caused by two part rubbing together as a result of shaft displacement. Here are some common causes of shaft displacement: ●
Pipe strain
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Misalignment between the pump and driver. A bent shaft. The rotating assembly is not dynamically balanced. Cavitation. Water hammer. Operating off the BEP. Thermal growth. Pulley driven pumps. Different types of vibration including harmonic, slipstick, induced, etc...
There are many parts that can come into contact when the shaft displaces. ●
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The impeller can contact the pump volute or back plate. This can also happen with an improper impeller adjustment or thermal growth. The end of the stuffing box can be hit by the shaft or sleeve. There is often a close fitting bushing installed in this location. The outside diameter of the rotating mechanical seal and the inside of the stuffing box. A gasket or fitting protruding into the stuffing box that rubs against the mechanical seal. The rotating shaft and the stationary seal face. The shaft and the API gland disaster bushing. The closed impeller wear rings are a common source of rubbing.
The increased amperage can be caused by an increase in bearing loading. ● ● ●
Check the shaft and housing tolerances along with the installation method. Cooling a bearing outside diameter causes it to shrink and over compress. The wrong lubrication level. There is too much lubricant in the bearing
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The radial flow pump is being started with the discharge valve open. Radial flow pumps use the most horsepower at high capacity. The axial flow pump is being started with the discharge valve shut. Axial flow pumps use the most horsepower at high head.
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See if the impeller balance holes are clogged. If there is an elbow too close to the suction of a double ended pump, and the piping is running parallel with the shaft, The change in velocity of the incoming fluid will cause axial thrust. Converting packing to a mechanical seal can increase the axial loading on the bearing
Here are a few more reasons why you might be using too much amperage. ●
The stuffing box packing has been tightened too much.
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An unbalanced mechanical seal is being used in a high pressure application. There is too much face load The impeller has been installed backwards. The shaft is running in the wrong direction. The open impeller needs adjusting. You have too much clearance between the impeller and the volute, or back plate, depending upon the pump design.
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SUBJECT: The pump works for a while and then loses suction 10-12 This is the fourth paper in a four part series about pump troubleshooting. As mentioned in paper number eleven, volume number ten, there are a couple of things you must keep in mind when troubleshooting centrifugal pump problems: ●
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The centrifugal pump always pumps the difference between the suction and discharge heads. If the suction head increases, the pump head will decrease to meet the system requirements. If the suction head decreases the pump head will increase to meet the system requirements. A centrifugal pump always pumps a combination of head and capacity. These two numbers multiplied together must remain a constant. In other words, if the head increases the capacity must decrease. Likewise if the head decreases, the capacity must increase. The pump will pump where the pump curve intersects the system curve. If the pump is not meeting the system curve requirements the problem could be in the pump, the suction side including the piping and source tank, or somewhere in the discharge system. Most pumps are oversized because of safety factors that were added at the time the pump was sized. This means that throttling is a normal condition in most plants, causing the pump to run on the left hand side of its curve.
Cavitation is a main cause of losing pump suction, but remember that there are several different types of cavitation: ●
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Vaporization of the liquid within the pump caused by a loss of suction head or an increase in suction temperature. The "vane passing syndrome" caused by too small an impeller to cutwater clearance. Too high a suction specific speed number will cause internal recirculation problems resulting in cavitation. The suction specific speed number is obtained from a formula that can be found in paper 9-12 of this series. Air ingestion on the suction side of the pump allows air and bubbles into the suction of the pump. Turbulence of the fluid that releases entrained gases into the suction piping.
Each of these cavitations has been addressed in other papers in this Technical Series. In this paper we will be looking at only the intermittent loss of suction fluid. You will be looking at several possibilities: ●
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A recurring restriction in the suction piping that may or may not be causing a cavitation problem within the pump. Intermittent cavitation problems as opposed to a design or operation problem that causes a constant cavitation condition. A repetitive need for an increase in the pump's capacity.
Now we will take a look at each of these possibilities in detail: A re-occurring restriction in the suction piping that may or may not be causing a cavitation problem within the pump.
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A foot valve or any valve in the suction piping is sticking. Something is occasionally plugging up the suction piping. If the pump suction is coming from a river, pond or the ocean, grass is a strong possibility. A loose rag is another common cause. A collapsed pipe liner will restrict the piping at higher velocities. The suction is being throttled to prevent heating of the process fluid. This can happen with some volatile fuel applications. A filter or strainer is gradually clogging up. Air is being introduced into the suction side of the pump to reduce the capacity. This is sometimes done with low specific gravity fluids to avoid throttling the discharge that might overheat and flash the product.
Intermittent incidents that cause cavitation problems ● ● ●
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The tank vent partially freezes in cold weather. The sun is heating the suction piping, raising the product temperature close to its vapor point. The level in the suction tank increases, decreasing the differential head across the pump. This will increase the pump capacity until the level in the tank drops. The level in the open suction tank decreases causing vortex problems that allow air into the pump suction. Several pumps in the same sump are running, decreasing the level too much. The suction tank float is stuck. It will sometimes show a higher level than you really have. A discharge recirculation line, piped to the pump suction, opens and heats the incoming liquid. Sometimes the suction lift is too high. The increase in pipe friction will reduce the suction head. The vapor pressure of the product is very close to atmospheric pressure. The pump cavitates every time it rains because of a drop in atmospheric pressure. The tank is being heated to de-aerate the fluid. Sometimes it is being heated too much. The process fluid specific gravity is changing. This can happen with a change in product operating temperature, or if a cleaner or solvent is being flushed through the lines. A booster pump is malfunctioning or leaking excessively. The source tank is changing from a positive pressure to a vacuum due to the process. A packed valve in the suction piping is at a negative pressure and air is leaking in through the packing. The tank is being pumped dry.
A repetitive need for an increase in the pumps capacity. ● ●
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A bypass line, or relief valve opens, decreasing the discharge resistance, increasing the capacity. A break or leak in the line down stream of the pump will increase the capacity of the pump as the head drops. The pump is supplying many sources and too many valves are open at one time. The pump discharge is being directed to several different tank farm locations. The changing piping resistance is changing the pump's head and capacity.
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SUBJECT: Troubleshooting premature bearing failure 5-3 As discussed in a another technical paper on this web site, bearings have no wearable surfaces, they are instead designed to fatigue after many hours of service. In a properly operating bearing the race ways and rolling elements will become dull in appearance. This dullness is not an indication of wear and has no affect on the life of the bearing. These dull surfaces form the visible paths that I will be referring to in the following paragraphs, so their appearance and location is important in analyzing any type of bearing failure. When we install a bearing into a piece of rotating equipment the general rule is to have the interference fit on the race that is rotating and, therefore, carrying the load. Almost all centrifugal pumps, motors, and a high percentage of other types of rotating equipment have the bearings installed with the inner race an interference fit and rotating with the shaft . The outer race remains stationary or in a fixed position. In the following paragraphs I will be discussing various load conditions and the resultant appearance of the raceways and rotating elements in this type of an installation: The radial load is rotating with the shaft, This is caused by an unbalanced rotating assembly or a bent shaft. ●
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The inner ring appearance. The load acts all of the time at the same place in the race way. Here the path pattern is at its widest, tapering off at the ends. If the load is only radial, the pattern will be in the center of the race way and will extend around slightly less the half the race way circumference. The outer ring appearance. The path will extend around the entire race way. It will be uniform in width and if the load is only radial, it will be in the center of the race way.
The radial load is unidirectional. This is what we would expect to find with a properly operating piece of equipment. If the equipment is operating off of its best efficiency point, is misaligned, or if there is excessive pipe strain the pattern will be the same, only more pronounced. ●
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The inner ring appearance. The path will be in the center of the race way, uniform in width and visible around the entire circumference of the race way. The outer ring appearance. The pattern will be widest at the load point and tapering towards the ends. If the fit and clearances are normal the pattern will extend around to slightly less than one half of the raceway. It will be located in the center of the race way, if the load is only radial.
The radial load is multidirectional . Cavitation, too tight an interference fit, preloading, or cooling a bearing outside diameter are all common causes of this problem. ● ●
The inner ring appearance. All around the race way, widest where the load was the greatest. The outer ring appearance. All around the race way, widest where the load was the greatest.
The axial load is unidirectional. This is the normal condition of all end suction centrifugal pumps. ●
Both the inner and outer rings. The pattern will extend around both raceways and is displaced axially from the center. A centrifugal pump thrusts towards the thrust bearing until it reaches 65% of its efficiency and then it thrusts towards the volute or wet end during normal operation.
An oval compression of the outer ring. Caused by an out of round housing. ● ●
The inner ring appearance. The path extends around the entire ring and is uniform in width. The outer ring appearance. Two wider paths where the ring was distorted to the oval shape.
The inner ring was misaligned. Normally happens during the installation process. ●
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The inner ring appearance. The pattern extends around the entire ring and is uniform in appearance. The outer ring appearance. The ball path will be oval, extending from one side of the race way to the other, and wider in two diametrically opposite sections.
Now that we know what some typical wear paths look like, we will inspect the only two things that are visible to the trained trouble shooter. ● ●
Evidence of rubbing. Evidence of corrosion and damage.
Look for damage caused by solid particles. These particles will be rolled into the race ways and can: ● ●
Score, or cause small indentations in the precision races and rolling elements. Interfere with the transfer of heat within the tight tolerances, causing discoloration, thermal expansion, seizing etc.… The particles come from: ❍ Varnish and "coke" that forms where the lubricant overheated. ❍ Parts of the ball cage that have broken loose due to a lack of lubrication. Brass cage parts will turn the lubricant green. ❍ Pieces from a failed grease or lip seal. ❍ A contaminated lubricant. ❍ Lack of cleanliness during the installation process. The bearings are being installd next to the area where the mechanic is grinding a new edge on his lawnmower blade. ❍ The bearing lubricant could have been over heated during the installation process. ❍ Rust coming off the inside of the casting. ❍ Silica leaching out of the casting ❍ Particles of material flaking off of the protective coating put on the inside of the housing to prevent rust. ❍ Airborne - through the bearing seals or housing vent.
Look for lack of lubrication that can eventually cause the bearing to seize:
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You will see" mirror like" surfaces on the metal parts that look like the piece was "lapped". The metal will become discolored and soften as it anneals. Annealing can occur any time the temperature exceeds 300°F (150°C): ❍ Straw yellow 600° F. 315° C. ❍ Brown 700° F. 370° C. ❍ Blue 800° F. 425° C. ❍ Black 900° F. 480° C. If a pre- lubricated bearing was heated by immersing it in a hot oil bath (200°F or 100°C), the hot oil will wash out the grease and leave the bearing with little to no lubrication. ❍ Many pre-lubricated bearings actually have no lubricant at all installed. Check yours to be sure. Bearing quality is a serious maintenance problem. A clogged oil level gauge can give a false reading of lubrication level. If the bearing case has no expansion chamber installed, a build up of internal pressure, as the bearing case comes up to temperature, can blow out of the seals. At shut down, moisture laden air will return to the case through the same seals. A poorly designed labyrinth seal can pump hot oil out of the bearing case. The lubricating oil level should be at the middle of the lower bearing ball when the pump is at rest. Be sure the pump has been leveled to insure the correct lubrication height.
Look for smearing of the metal . When two non lubricated surfaces slide against each other, under load, the material transfers from one surface to the other. ●
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The metal melts and then re-hardens, causing localized stress that can produce cracks in the metal.. The load was too light for the speed. Centrifugal force threw the balls out. The outer race will smear on the outside diameter if it slides during operation due to an improper "slip fit". This slipping can also cause "fretting corrosion" as the protective oxide film is worn away from the metal surface.
Look for evidence of static vibration. You will see indents in the raceway that could be either shiny or rusted in the bottom. The frequency of the vibration has no affect, but greater energy causes greater damage. Roller bearings are more susceptible to this type of damage because the balls, in a ball bearing, can roll in many directions. Rollers, how ever, can roll in only one direction. Movement in the other directions takes the form of "sliding". ●
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The pump was located too close to another piece of equipment that was vibrating. This can be a big problem during storage. The shaft was not locked during shipment. In addition to vibration, equally spaced indents can be caused by: ❍ An induction heater was used during assembly, causing "false Duriron". ❍ The bearing was installed by pressing on the wrong race. ❍ The bearing was driven too far up a tapered shaft.
Look for electric current damage. It will show up on both the races and the rolling element. The bottom of the depression will be dark in color.
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The pump was used as a ground for a welding rig.
Look for flaking or spalling of the metal race way. Since there is nothing in a bearing to wear out, flaking or spalling is a sign of normal fatigue. Overloading however, can cause premature fatigue. Look for the following causes of bearing overloading: ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
The bearing housing is out of round. The shaft is over size. The bearing was driven up too far on a tapered shaft. Misalignment between the pump and its driver. The rotating assembly is out of balance. The shaft is bent. The pump is operating too far off of its best efficiency point (B.E.P.). Pipe strain. Water hammer in the lines. Cavitation. The bearing had a quality problem to start with. Shaft thermal expansion. The bearing housing is being cooled, causing the outer race to shrink, increasing the load. Excessive axial thrust. Pulley driven design. Hydrogen embrittlement of the metal caused by moisture entering the lubricant. Pumping a high specific gravity fluid such as sulfuric acid can almost double the radial load.
Overloading is often accompanied by a change in appearance of the lubricant. You will see varnish or coke as the lubricant is subjected to this high heat. In addition to overloading there are additional sources of heat that can destroy the lubricant : ●
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Soak temperatures through the shaft. This can be a big problem in either hot oil or hot water applications. Over lubrication of the bearing. Plugged oil return holes. Constant oil cups at the wrong level. Insufficient clearance in labyrinth seals. The oil gage breather hole is blocked and showing the wrong lubrication level. Bent lock washer prongs can rub against the bearing race. Grease or lip seals are too tight on the shaft. The pump stuffing box cooling jacket was shut off and drained when the metal bellows seal was installed in a high temperature oil application. Someone is cooling the power end case causing the bearing outer race to shrink. Friction with the seal cage. Sliding friction caused by small changes in the shaft speed. Inertia keeps the balls moving as the shaft slows down. The stuffing box packing has been over tightened.
Look for cracks in the metal. ● ● ●
Mishandling. The bearing was driven too far up a tapered shaft. Any type of flaking or smearing can cause a fracture notch that will lead to cracking.
Look for signs of corrosion. ●
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Moisture is in the lubricant. It came from: ❍ Packing or seal leakage. ❍ A water hose being used to wash down the area. ❍ Normal aspiration as the pump cooled down, and the moisture ladened atmosphere entered the bearing case. ❍ Steam or water from a seal quench gland. This is a common problem with the A.P.I. gland that is commonly used in oil refineries. Regardless of the protective coating put on the bearing races, (cadmium, chromium, zinc, etc.) the rolling elements are almost always fabricated from 52100 bearing steel, and it rusts.
The major bearing companies do a good job of providing the literature and photographs that you need to do effective "comparison troubleshooting". Check with your bearing supplier for the availability of this information.
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SUBJECT: Pump and seal problems with no apparent cause 4-5 These problems are the ones that drive you crazy. No matter how hard you look the solution keeps evading you. Over the years I have collected quite a few examples. I offer some of them for your enjoyment and maybe, in the process, they will help you solve the "un-solvable" CAVITATION The pump cavitated every time it rained. ●
Solution: The product temperature would cause it to vaporize very close to ambient pressure, and when it rained atmospheric pressure dropped enough to cause the problem.
The pump never cavitated in the summer months, only during the winter when everything was cooler. ●
Solution: The tank vent froze during the winter months causing the pump to pull a partial vacuum in the tank.
The cavitation started suddenly. ●
Solution: A plastic pipe liner collapsed at the suction side of the pump or the gate fell off a gate valve.
The cavitation started after the packing was converted to a mechanical seal. A careful inspection showed that the seal was not leaking air into the suction. ●
Solution: The pump had speeded up (increased the rpm) when the packing was removed. This increase in speed and capacity caused the cavitation.
The cavitation kept getting worse with time, nothing obvious had changed in the system. ●
Solution: The product had formed a coating on the inside of the suction pipe increasing the pressure drop and resulting in a loss of suction head.
The cavitation only occurred when there was a higher head at the suction of the pump and stopped cavitating when the level fell in the tank - just the opposite of what should have happened. ●
Solution: The pump was pumping to a fixed discharge head. The capacity of the pump increased when the suction level was higher, because the pump delivers the difference between the suction and discharge head. When the differential went down, the capacity increased.
Two pumps were installed in parallel, one cavitated the other did not. They had separate suction lines so that was not the problem.
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Solution: Some one had installed an oversized section of pipe on the discharge side of the pump that was cavitating. The lower discharge resistance caused an increase in capacity which caused the cavitation. When the proper sized pipe was installed the cavitation stopped.
The pump had been cavitating for some time, but after a visual check everything appeared normal. ●
Solution: A globe valve had been substituted for a gate valve on the suction of the pump. A globe valve can add the equivalent of another 100 feet (30,5 meters) of pipe to the system.
The pump started to cavitate when a flange gasket was replaced on the suction side of the pump. ●
Solution: The inside diameter of the gasket was too small. It was acting as an orifice, and restricting the flow.
The pump cavitated about one third of the time it was running. ●
Solution: A close inspection of the system revealed that there was no surge tank installed between the pump discharge and the multiple outlets that were using the product. The pump was acting like an accumulator and started to cavitate when the demand went up and the discharge head dropped.
The pump cavitated although here was excessive suction head available. ●
Solution: There was too much velocity on the suction side of the pump. I saw this problem in Scandanavia in an application where the pump was taking a suction on a flow of water coming off of a mountain.
THE SEAL WAS GETTING HOT The seal was showing evidence of running dry, but the fluid level was never lost in the pump. ●
Solution: Air was trapped in the stuffing box of a vertical pump after it was converted from packing to a mechanical seal. Most seal designs have no facility for venting the stuffing box in a vertical application
The seal showed evidence of running dry. ●
Solution: The open impeller had been adjusted backwards and the "pump out vanes" on the rear of the impeller were pumping the stuffing box dry. This happens if you are using several brands of pumps and the maintenance mechanics confuse the impeller adjustment method. Some pumps adjust towards the volute (Goulds), some adjust towards the back plate (Duriron). It is easy to mix them up.
There was little to no fluid circulating between the two seals.
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Solution: The pipe fitting had bottomed out in a gland inlet elbow shutting off the flow. This sometimes happens after the seal has been repaired several times and the pipe thread shows some wear letting it protrude further into the elbow fitting.
The mechanic had marked the seal location on the shaft sleeve before the impeller was installed. When the impeller was tightened against the shaft shoulder the sleeve moved and over compressed the seal. Durco pump impellers adjust to the pump back plate. When you make impeller adjustments you over compress the mechanical seal. A cooling jacket was being used, but the seal continued to get hot. I have seen multiple reasons for this: ●
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A discharge recirculation line had also been installed, but it was hidden by some insulation. The cooling jacket could not keep up with the heat being added by the recirculation line. The inside of the cooling jacket had become coated with a layer of calcium because hard water was being used as the cooling medium. Condensate should have been substituted. A thermal bushing had not been installed in the bottom of the stuffing box. The cooling jacket flow changed with fluctuations in shop water pressure. The inner seal of some double seal applications can get hot if the mechanic installs the cartridge seal by pushing on the gland and fails to reset the seal compression with the installation clips. The interference from the cartridge sleeve elastomer can cause enough resistance to compress the inner seal and unload the outer seal.
THE SEAL WOULD LEAK FOR NO APPARENT REASON The open impeller was being adjusted without resetting the seal. Many operators make their own impeller adjustments. The seal faces were opening because the equipment's sliding foot had been bolted to the floor allowing the shaft to grow through the stuffing box when the unit came up to temperature. The cartridge seal had been hydrostatically tested with water and then put into a hot oil application. It leaked almost immediately. ●
Solution: The trapped water vaporized when the unit was started. This could be a dangerous condition because water trapped in a gasket and then flashed to steam could blow the equipment apart.
The seal would start leaking about thirty minutes after the pump started. ●
Solution: The carbon insert would come loose in its holder when the seal came up to temperature. At shut down the metal holder would shrink and everything appeared normal.
The seal was tested in the shop, but leaked when it was installed in the pump that was operating at cryogenic (cold) temperature.
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Solution: The faces had to be lapped at cryogenic temperature to keep them flat at the seal operating temperature. The cryogenic temperature can also harden the O-ring and freeze any lubricant that was put on the seal face.
The seal was found to be leaking every Monday morning. ●
Solution: A utility man did not know about seals. He would loosen the gland on the weekend so that what he thought was packing would drip a little. The leak was found by the regular maintenance people every Monday morning.
The leakage occurred during the winter months. ●
Solution: Someone circulated commercial anti freeze between two seals to act as a barrier fluid. The brand they selected contained a chemical to plug up radiator leaks and it kept plugging up the seal.
The seal would fail only during the winter months. The problem was traced to swelling of the dynamic O-ring but no logical reason could be found for its failure. ●
Solution: During the winter months a worker decided to oil the bed of his dump truck to make the mined, raw product slip off easier. The petroleum oil he used attacked the Ethylene Propylene (EPR) O-ring in the mechanical seal, installed downstream in the system.
The seal area was wet, but no visible leakage could be seen. ●
Solution: It turned out that there was a flange leaking above the pump and dripping the product next to the shaft.
The problem was traced to the fact that the mechanic was installing the seal at the wrong dimension. The written instructions were clear and placed in the box and yet the mechanic continued to do the installation incorrectly. ●
Solution: The mechanic could not read. He had been faking it for many years and was quite good at it. The same problem occurs with older mechanics that refuse to wear glasses and as a result cannot see the funny little lines between the numbers on their measuring scale.
The centrifugal pump discharge was connected to the bottom of a surge tank. As the tank filled, the pump operating point shifted from too much capacity to too much head, deflecting the shaft in two directions. The outside seal in a double seal application failed suddenly. Nothing had changed in the system. ●
Solution: Routine maintenance included repainting the pump. The paint spray got into the outside seal springs and stopped them from moving.
The seal ran great for several days and then started to leak. It tested all right on the test bench after it had been removed from the pump. ●
Solution: It had been set screwed to a hardened sleeve and the set screws gradually loosened.
The seal was changed several times, but the steady leak persisted. ●
Solution: The leak was occurring between the pump sleeve and the shaft. This is a common problem in double ended pumps that have been converted to a mechanical seal. You often have to devise a method of sealing the sleeve to the shaft or the sleeve to the impeller because the manufacture has not provided one.
The seal started to leak after many months of service. A bench vacuum test showed that the seal was all right. ●
Solution: The seal was fretting the shaft below the Teflon wedge allowing the leak to come through this groove.
The seal ran approximately six months and then failed. ●
Solution: The lines were steam cleaned and the wrong grade of Viton® was in the seal. Most Viton® compounds will be attacked by steam, caustic or other water based solutions.
The seal was installed correctly, but it leaked immediately. ●
Solution: The solid, hard face is usually lapped on only one side. The face had been installed backwards and the rotary unit was running on a non lapped surface.
SEAL COMPONENT DAMAGE IS VISIBLE, BUT WHAT IS THE CAUSE? It looked like a seal part had come loose in the stuffing box, but all of the parts were there. ●
Solution: During a previous installation a small spring had been lost when it fell into a drain hole in the bottom of the seal gland. It came loose after a later installation. This is a problem when several people work on the same pump.
The bellows plates were breaking but there was no evidence of corrosion, excessive wear, or vibration. ●
Solution: A discharge recirculation line was directing high velocity abrasive particles at the thin metal section of a metal bellow seal.
The inner seal of a dual, rotating "Back to back" seal was showing excessive face wear in a short period of time.
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Solution: The inner seal stationary face was not locked in the bottom of the stuffing box and when the system pressure overcame the barrier fluid pressure, the stationary face was pushed into the inside rotating face. When the pump was stopped the spring pushed the stationary face back to its normal position.
The carbon showed massive damage in a cryogenic (cold) application. ●
Solution: The carbon had been lubricated at assembly and the lubricating oil froze in the cryogenic atmosphere.
The bellows plates showed massive wear. ●
Solution: The seal was rotating in an abrasive slurry. Metal bellows seals should be designed to rotate the fluid inside of the stuffing box, instead of rotating through the fluid.
OTHER PROBLEMS The pump had been recently overhauled and at start up the pump was reading high amperage, but low flow. ●
Solution: One of the wear rings had been left off of the suction side of the impeller and the fluid was recirculating to the pump suction.
The pump made a terrific racket during start up. It produced the proper head, but the capacity was less than anticipated. ●
Solution: It was a two speed pump and the second speed had been wired backwards.
In an acid application, a stationary seal showed localized corrosion only on the gland. ●
Solution: This was an older pump with a bolted on stuffing box that would slip because the bolts were worn. This caused the shaft to run against the gland causing it to overheat and, in an acid application, the corrosion rate of the acid doubles with an 18° F. (10° C) rise in temperature. It doesn't make any difference if the acid or the part gets hot, the affect is the same.
The dual seal convection tank was running backwards. ●
Solution: The seal was not centered in its gland, and as the shaft turned, the close tolerance between the seal and the gland outlet increased the velocity of the liquid enough to drop the pressure and cause the tank to convect backwards.
The pump was converted from packing to a mechanical seal and then started to break shafts. ●
Solution: The pump was operating way off of its best efficiency point, causing major shaft deflection. The packing was acting as a bearing and supported the shaft during this deflection.
The product was solidifying in the stuffing box. Steam was being used to heat a jacket around the pump. The header gauge showed adequate pressure. ●
Solution: The gage was located too far away from the pump jacket. The line was not insulated and this allowed the steam to experience a pressure drop between the header and the stuffing box heating jacket. The result was that the steam cooled down below the necessary heating temperature. The problem was only visible when the pump was stopped for a period of time.
The nickel base tungsten carbide face was being chemically attacked. ●
Solution: A galvanic action occurred between the passivated stainless steel and the active nickel contained in the tungsten carbide face.
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SUBJECT : Centrifugal Pump Troubleshooting 1-02 You have four opportunities to trouble shoot centrifugal pumps and each opportunity can offer you a clue as to what is wrong with the pump. Let's take a look at each of these conditions: The pump is hooked up to the piping and it is running : ●
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You can observe leakage from the stuffing box or some other area. This would include gaskets, bearing seals and cracks or holes in the piping. You can hear an abnormal noise. You can probably "pin point" the source if you try. There is evidence of excessive heat in one or more of the components. You can detect excessive vibration either from the use of instruments or one of your senses You can check if stuffing box environmental controls are hooked up properly, and in many cases tell if they are functioning correctly. You can check the position of control and isolation valves throughout the system. This is especially important to check while the pump is running. If there are meters available you can check : ❍ Flow ❍ Pressure ❍ Power consumption ❍ Temperature ❍ Speed You can estimate if the foundation is too weak. It should be five times the mass of the hardware sitting on it.
The pump is still hooked up to the piping, but it is not running. You will be present during the removal process : ● ● ●
You can check alignment between the pump and driver. During the removal process you can check for excessive pipe strain. You can check if the piping has been installed according to good engineering practices. This is a major factor in many cavitation problems.
The pump has been taken into the workshop, but has not yet been disassembled and you will be present at the disassembly. ● ● ●
You can check the seal installation dimension. You can feel if there is restricted movement of any of the rotating parts. You can see if there is any loose hardware in the assembly
The pump has been disassembled. You were not present, but the parts are available for your inspection. ● ● ●
You can see evidence of wear, rubbing or discoloration of the components. You can see evidence of corrosion. You can see if any parts are missing.
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You can see if any material or coating has attached its self to one of the components. As an example, calcium can build up on the inside of pipes and restrict flow, or magnetite (Fe304) build up on the seal components.
In this paper we will address the last condition. The pump has been disassembled. You were not present, but the parts are available for your inspection. When a rotating part such as a shaft seal, impeller, etc. comes in contact with a stationary part such as the inside of the stuffing box, a wear ring, stationary bushing etc., there will be evidence of this contact in the form of rubbing, wear, discoloration or damage to one or both of the components. There are four possibilities that we will be able to see : ●
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A rubbing mark, or evidence of wear all around the rotating part and one place on the stationary part. All around the stationary part and one place on the rotating part. Evidence of rubbing or wear all around both the rotating and stationary parts. One mark on both the rotating and stationary component.
The cause could be the result of a problem in design, operation or maintenance. I will attempt to isolate these three areas as we look into the problems. All around the rotating part, one spot on the stationary part. ●
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Design Problems: ❍ The pump is pulley driven and the shaft L3/D4 is too high. Maintenance Problems: ❍ The pump and driver are not aligned properly ❍ The shaft is not centered in the stuffing box. ❍ A gasket or fitting is protruding in, and touching the rotating part. ❍ Excessive pipe strain. This is a common problem when a Centerline Design is not specified for applications over 200 F (100 C) Operation problems: ❍ A major cause of this problem is the fact that the pump is operating too far from its best efficiency point (B.E.P.) and the shaft is not large enough to resist the bending.
All around the stationary, one spot on the rotary. ●
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Design Problems ❍ You have converted the pump to a mechanical seal. The unit was originally designed for the packing to act as a bearing and stabilize the shaft. This is a very big problem with mixers and agitators Maintenance Problems ❍ The rotating assembly is out of balance. ❍ Normal wear ❍ Damage ❍ Corrosion of the impeller
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Foreign material attached The impeller was trimmed and not re balanced. The seal, sleeve, or impeller is not concentric with the shaft. The unit never was balanced. The shaft is bent. Excessive heat or force was used during sleeve, seal, or bearing removal. The rotating unit is dragging something around with it.
All around both the rotating and stationary units. This problem could be caused by a combination ofthe first and second examples or: ●
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Design Problems: ❍ High temperature application. The shaft is expanding and a restriction bushing is growing in towards the shaft/sleeve. ❍ The pump is operating at a critical speed. This can happen with variable speed motors. Operation Problems: ❍ The pump is cavitating Maintenance Problems ❍ Bad bearings. ❍ The oil is contaminated with water, product, dirt, rust, casting leaching, etc.. ❍ Incorrect oil level. ❍ Poor fit because of shaft tolerances or the installation technique. ❍ Excessive load due to a variety of reasons. ❍ Oil temperature too high. Be sure to cool the oil not the bearings. Cooling the housing will cause it to shrink and thereby increase the squeeze on the bearing.
One mark on both the rotating and stationary component. ●
I have only seen this one time and that was when the pump fell off the back of a pick up truck.
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SUBJECT: Causes of overheating in cartridge mechanical seals 7-4 Too much heat can cause multiple problems with mechanical seals: ● ● ●
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The elastomer (rubber part) can be damaged. Some seal faces can be damaged. Carbon-graphite faces can pit as trapped air expands within the carbon, or the product carbonizes and pulls out pieces of the seal face. Plated faces can heat check and crack causing rapid carbon face wear. The filler in some carbon /graphite compounds can melt or oxidize at elevated temperatures. Critical dimensions can change causing the lapped seal faces to go out of flat and leak prematurely (especially fugitive emissions). The sealed product can change state and : ❍ Vaporize between the faces opening them. ❍ Crystallize on the moving components, restricting their movement. ❍ Change fluid viscosity restricting the ability of the seal to follow run out. ❍ Solidify, making the seal inoperable. ❍ Build a film on sliding components and the lapped seal faces. ❍ Carbonize or coke restricting the seal movement and opening the lapped faces. Corrosion always increases with increasing temperature.
Some heat problems are not seal design or seal installation related: ●
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An inefficient heating or cooling jacket on the pump. ❍ A layer of calcium or some other similar product has built up on the jacket walls, interfering with the heat transfer. ❍ The coolant is flowing too rapidly through the cooling jacket. ❍ A thermal bushing was not located in the bottom or end of the stuffing box. ❍ If steam is being used as the coolant, the pressure is too high. ❍ The fluid is not "dead ended" in the stuffing box. There is either suction or discharge recirculation of the pumping fluid. ❍ Clearance between the seal outside diameter and the stuffing box bore is not sufficient. The shaft material is conducting the product heat to the cartridge static elastomer and other components. As an example: carbon steel conducts heat much better than a stainless steel shaft. The dual seal convection tank is not convecting. ❍ The convection tank is running backwards. ❍ The dual seal barrier or buffer fluid has been shut off. The quench has failed. The product has a low specific heat and poor conductivity. Oil is a good example of such a product. The seal faces were over-compressed during the installation process. A wrong installation measurement was used. The mechanic did not read, or understand the print dimension. The pump sleeve moved as the impeller was tightened on the shaft. The measurement was taken at the wrong place. The stuffing box face is the only safe reference
point. The cartridge seal design has a major affect on heat generation and heat sensitivity: ● ● ●
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Unbalanced seals generate more heat than hydraulically balanced mechanical seals. Two hard faces generate more heat than carbon/graphite vs. a hard face. Silicone carbide and tungsten carbide dissipate heat faster than 99.5 ceramic or carbon&endash; graphite. The location as well as the grade of the elastomer can be critical in temperature sensitive applications. In dual seal applications, convection systems are not as efficient as pumping rings or forced circulation of the barrier fluid system. When oil is used as a barrier fluid forced circulation or the use of a pumping ring is mandatory.
The above problems are not unique to cartridge seals, there are however some problems that are unique: ●
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Pushing the seal gland along the shaft and against the stuffing box face can over compress the seal because of the friction between the shaft and the cartridge sleeve static elastomer. In dual seal applications the inner seal can over compress as the outside seal looses some of its compression. Be sure to reset the spacing device (usually "clips" of some type) prior to locking the seal to the shaft. Some open impeller pump designs (Duriron as an example) adjust to the back plate rather than the volute. Be sure to reset the cartridge seal after the impeller adjustment. Cartridge set screws can slip on a hardened sleeve. The system pressure can then over compress the seal. Higher pressure applications, or water hammer can move the set screws and over compress the seal faces. Be sure to re-tighten the adjusting nuts after making the impeller micrometer adjustment on those pump that uses that type of adjustment method. The Chesterton System #1 pump is a good example of this design. Make sure the centering-positioning clips are in place when installing or resetting the seal for proper face loading.
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SUBJECT: Corrossion problems associated with stainless steel 4-1 The rotating equipment business uses a great deal of 300 series stainless steel, and as a result we often experience corrosion that is described in a variety of technical terms that include: ● ● ● ● ● ● ● ● ● ●
General corrosion Galvanic corrosion Pitting Inter granular corrosion Stress corrosion cracking Erosion- corrosion Fretting Concentrated cell or crevice corrosion Selective leaching Micro organisms
The last page of this report is a list titled "The Galvanic Series Of Metals and alloys". I will be referring to this chart during our discussion. The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a "passive" film which is resistant to further "oxidation" or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or many other fluids that contain oxygen. Once the layer has formed we say that the metal has become "passivated" and the oxidation or "rusting" rate will slow down to less than 0.002" per year (0,05 mm. per year). Unlike aluminum or silver this passive film is invisible in stainless steel. It is due to the combining of oxygen with the chrome in the stainless to form chrome oxide which is more commonly called "ceramic". This protective oxide or ceramic coating is common to most corrosion resistant materials. Halogen salts, especially chlorides easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters "ine". Listed in order of their activity they are: fluorine, chlorine, bromine, iodine and astatine. These are the same chemicals that will penetrate Teflon and cause trouble with Teflon coated or encapsulated O-Rings and/ or similar coated materials. Chlorides are one of the most common elements in nature and if that isn't bad enough they are also soluble, active ions; the basis for good electrolytes, the best conditions for corrosion or chemical attack. GENERAL OR OVERALL CORROSION. This type of corrosion occurs when there is an overall breakdown of the passive film formed on the stainless steel. It is the easiest to recognize as the entire surface of the metal shows a uniform "sponge like" appearance. The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. As a general rule the rate of attack will double with an eighteen degree Fahrenheit rise in temperature (10° C.) of either the product or the metal part.
If the rotating portion of the seal is rubbing against some stationary component, such as a protruding gasket or fitting the protective oxide layer will be polished off and the heat generated will increase the corrosion as noted above. This explains why corrosion is often limited to only one portion of the metal case. There are many good publications available to help you select the proper metal for any given mechanical seal application. As a general rule, if the wetted parts of the equipment are manufactured from iron, steel, stainless steel or bronze, and they are showing no signs of corrosion, grade 316 stainless is acceptable as long as you do not use stainless steel springs. (see chloride stress corrosion) GALVANIC CORROSION If you put two dissimilar metals or alloys in a common electrolyte, and connect them with a voltmeter, it will show an electric current flowing between the two. (This is how the battery in your automobile works). When the current flows, material will be removed from one of the metals or alloys ( the ANODIC one) and dissolve into the electrolyte. The other metal (the CATHODIC one) will be protected. Now let's take a look at the Galvanic Series chart that is attached to this report. The further apart the materials are located on this chart the more likely that the one on the ANODIC end will corrode if they are both immersed in a common fluid considered to be an electrolyte. water, containing chlorides, is one of the best. Example #1. A ship has lots of bronze fittings and a steel hull. Note that steel is located seven lines from the ANODIC end, and bronze is listed at twenty seven rows from the same end. Sea water is a perfect electrolyte so the bronze fittings would immediately attack the steel hull unless something could be done to either protect the steel or give the bronze something else to attack. The classic way to solve this problem is to attach sacrificial zinc pieces to the hull and let the bronze go after them. Again, looking at the chart, you will note that zinc is found on line three from the top of the chart. In other words the zinc is further away from the bronze than the iron is, so the galvanic action takes place between the zinc and the bronze, rather than between the steel and the bronze. Zinc paint is used for the same reason in many applications. Example #2 Nickel base tungsten carbide contains active nickel. When this face material is used in dual seal it is common to circulate water or antifreeze containing water between the seals (as mentioned in the beginning of this report, water can be an excellent electrolyte because of the addition of chlorine and fluorine). You will note that active nickel is located twenty one rows from the top of the chart. Passivated 316 stainless steel is positioned nine rows from the bottom. This means that the stainless steel can attack the nickel in the tungsten carbide causing it to corrode. Many of you have run into this problem already. The rate at which corrosion takes place is determined by :
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The distance separating the metals on the galvanic series chart The temperature and concentration of the electrolyte. The higher the temperature, the faster it happens. Any stray electrical currents in the electrolyte will increase the corrosion also. The relative size of the metal pieces. A large cross section piece will not be affected as much as a smaller one. Many metal seal components are isolated from each other by the use of rubber O-Rings or similar materials and designs. Shaft movement that causes fretting of the 316 stainless steel rubs off the passivated layer and exposes the active stainless to the electrolyte until the metal part becomes passivated once more. This is one of the reasons we see corrosion under O-rings, Teflon, and similar materials. In the next paragraph I will be discussing another cause of corrosion under rubber parts.
PITTING This is an accelerated form of chemical attack in which the rate of corrosion is greater in some areas than others. It occurs when the corrosive environment penetrates the passivated film in only a few areas as opposed to the overall surface. As stated earlier the halogens will penetrate passivated stainless steel. Referring to the galvanic chart you will note that passivated 316 stainless steel is located nine lines from the bottom and active 316 stainless steel is located thirteen lines from the top. Pit type corrosion is therefore simple galvanic corrosion, as the small active area is being attacked by the large passivated area. This difference in relative areas accelerates the corrosion causing the pits to penetrate deeper. The electrolyte fills the pits and prevents the oxygen from passivating the active metal so the problem gets even worse. This type of corrosion is often called "concentrated cell corrosion". You will also see it under rubber parts that tend to keep oxygen away from the active metal parts, retarding its ability to form the passivated layer. INTERGRANULAR CORROSION All Austenitic stainless steels (the 300 series, the types that "work harden", is one of them) contain a small amount of carbon in solution in the austenite. Carbon is precipitated out at the grain boundaries, of the steel, in the temperature range of 1050° F. (565° C) to 1600° F. (870° C.). This is a normal temperature range during the welding of stainless steel. This carbon combines with the chrome in the stainless steel to form chromium carbide starving the adjacent areas of the chrome they need for corrosion protection. In the presence of some strong corrosives an electrochemical action is initiated between the chrome rich and chrome poor areas with the areas being low in chrome becoming attacked. The grain boundaries are then dissolved and become non existent. There are three ways to combat this: ●
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Anneal the stainless after it has been heated to this sensitive range. This means bringing it up to the proper annealing temperature and then quickly cooling it down through the sensitive temperature range to prevent the carbides from forming. When possible use low carbon content stainless if you intend to do any welding on it. A carbon content of less than 0.3% will not precipitate into a continuous film of chrome carbide at the grain boundaries. 316L is as good example of a low carbon stainless steel. Alloy the metal with a strong carbide former. The best is columbium, but sometimes titanium is
used. The carbon will now form columbium carbide rather than going after the chrome to form chrome carbide. The material is now said to be "stabilized" CHLORIDE STRESS CORROSION. If the metal piece is under tensile stress, either because of operation or residual stress left during manufacture, the pits mentioned in a previous paragraph will deepen even more. Since the piece is under tensile stress cracking will occur in the stressed piece. Usually there will be more than one crack present that causes the pattern to resemble a spider's web. Chloride stress cracking is a common problem in industry and not often recognized by the people involved. In the seal business it is a serious problem if you use stainless steel springs or stainless steel bellows material. This is the main reason that Hastelloy C is recommended for spring material. Here are some additional thoughts about chloride stress cracking that you will want to consider: ●
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Chlorides are the big problem when using the 300 series grades of stainless steel. The 300 series is the one most commonly used in the process industry because of its good corrosion resistant proprieties. Outside of water chloride is the most common chemical found in nature and remember that the most common water treatment is the addition of chlorine. Beware of insulating or painting stainless steel pipe. Most insulation contains plenty of chlorides and piping is frequently under tensile stress. The worst condition would be insulated steam traced, stainless steel piping.
If it is necessary to insulate stainless steel pipe a special chloride free insulation can be purchased or the pipe can be coated with a protective film prior to insulating. ●
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Stress cracking can be minimized by annealing the metal, after manufacture to remove residual manufactured stresses. Never replace a carbon steel bolt with a stainless steel one unless you are sure there are no chlorides present. Bolts can be under severe tensile stress. No one knows the threshold values for stress cracking to occur. We only know that you need tensile stress, chlorides, temperature and the 300 series of stainless steel. We do not know how much chloride, stress or temperature. Until I figured out what was happening I had trouble breaking stainless steel fishing hooks in the warm water we have in Florida. Many cleaning solutions and solvents contain chlorinated hydrocarbons. Be careful using them on or near stainless steel. Sodium hypochlorite, chlorethene. methylene chloride and trichlorethane are just a few in common use. The most common cleaner used with dye checking material is trichloroethane accounting for the reason we sometimes experience cracks after we weld stainless steel and die check it to check the quality of the weld.
EROSION CORROSION This is an accelerated attack resulting from the combination of mechanical and chemical wear. The liquid velocities in some pumps prevents the protective oxide passive layer from forming on the metal surface. The suspended solids also remove some of the passivated layer increasing the galvanic action. You see this type of corrosion very frequently at the eye of the pump impeller.
FRETTING CORROSION This type of corrosion is easily seen on the pump shaft or sleeve. You will see the damage beneath: ● ● ● ● ●
The grease or lip seal that is supposed to protect the bearings. The packing used to seal the fluid. The dynamic Teflon or elastomer used in most original equipment seals. The vibration damper used in rotating metal bellows seals. Under the rubber boot used in low cost seals, if they did not attach them selves to the shaft properly.
As mentioned earlier, 300 series stainless steel passivates its self by forming a protective chrome oxide layer when ever it is exposed to free oxygen. This oxide layer is very hard and when it imbeds into a soft elastomer it will cut and damage the shaft or sleeve rubbing against it. The mechanism works like this: ● ● ● ● ●
Oxygen passivates the active stainless steel forming a protective ceramic layer. The seal or packing removes the oxide layer as the shaft or sleeve rubs against it. The ceramic sticks into the soft elastomer turning it into a "grinding surface". The oxide reforms when the active metal is exposed and the process starts all over again. A visible groove is cut into the shaft or sleeve that will cause seal leakage and "hang up".
CONCENTRATED CELL OR CREVICE CORROSION This corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between nut and bolt surfaces, under O-rings and gaskets, and between the clamps and stainless steel shafts we find in many split seal applications. Salt water applications are the most severe problem because of the salt water low PH (8.0&endash;9.0). Here is the mechanism: ● ● ●
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Chlorides pit the passivated stainless steel surface. The low PH salt water attacks the active layer that is exposed Because of the lack of fluid flow over the attacked surface oxygen is not available to re passivate the stainless steel. Corrosion continues unhampered under the rubber and tight fitting clamp. The inside of the O-ring groove experiences the same corrosion as the shaft or sleeve.
SELECTIVE LEACHING The process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. The mechanism is: ● ● ●
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Metals are removed from the liquid during a de-ionization or de-mineralizing process. The liquid tries to replace the missing elements as it flows through the system. The un-dissolved metals often coat them selves on the mechanical seal faces or the sliding components and cause a premature seal failure. Heat accelerates the process.
MICRO ORGANISMS These organisms are commonly used in sewage treatment, oil spills and other cleaning processes. Although there are many different uses for these "bugs", one common one is for them to eat the carbon you find in waste and other hydrocarbons, and convert it to carbon dioxide. The "bugs" fall into three categories: ● ● ●
Aerobic, the kind that need oxygen. Anaerobic, the kind that do not need oxygen. Facultative, the type that goes both ways.
If the protective oxide layer is removed from stainless steel because of rubbing or damage, the "bugs" can penetrate through the damaged area and attack the carbon in the metal. Once in, the attack can continue on in a manner similar to that which happens when rust starts to spread under the paint on an automobile. GALVANIC SERIES OF METALS AND ALLOYS CORRODED END ( ANODIC OR LEAST NOBLE) MAGNESIUM MAGNESIUM ALLOYS ZINC ALUMINUM 5052, 3004, 3003, 1100, 6053 CADMIUM ALUMINUM 2117, 2017, 2024 MILD STEEL (1018), WROUGHT IRON CAST IRON, LOW ALLOY HIGH STRENGTH STEEL CHROME IRON (ACTIVE) STAINLESS STEEL, 430 SERIES (ACTIVE) 302, 303, 304, 321, 347, 410,416, STAINLESS STEEL (ACTIVE) NI - RESIST 316, 317, STAINLESS STEEL (ACTIVE) CARPENTER 20CB-3 STAINLESS (ACTIVE) ALUMINUM BRONZE (CA 687) HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE) LEAD-TIN SOLDERS LEAD TIN INCONEL 600 (ACTIVE) NICKEL (ACTIVE) 60 NI-15 CR (ACTIVE) 80 NI-20 CR (ACTIVE) HASTELLOY B (ACTIVE) BRASSES
COPPER (CA102) MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905) SILICONE BRONZE NICKEL SILVER COPPER - NICKEL ALLOY 90-10 COPPER - NICKEL ALLOY 80-20 430 STAINLESS STEEL NICKEL, ALUMINUM, BRONZE (CA 630, 632) MONEL 400, K500 SILVER SOLDER NICKEL (PASSIVE) 60 NI- 15 CR (PASSIVE) INCONEL 600 (PASSIVE) 80 NI- 20 CR (PASSIVE) CHROME IRON (PASSIVE) 302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE) 316, 317, STAINLESS STEEL (PASSIVE) CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825 NICKEL - MOLYBDEUM - CHROMIUM - IRON ALLOY (PASSIVE) SILVER TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.) GRAPHITE ZIRCONIUM GOLD PLATINUM PROTECTED END (CATHODIC OR MOST NOBLE)
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SUBJECT: Mechanical Seal design, operation, and maintenance problems 8-11 In my seminars I teach that mechanical seals fail prematurely because: ● ●
The lapped faces open. A seal component becomes damaged.
In the following paragraphs we will learn how these failures can be separated into: ● ● ●
Design problems. Operation problems. Maintenance problems.
These individual subjects have been discussed in other sections of this Technical Series. The purpose of this paper is to give you an overview of the subject, and assist you in your troubleshooting function. MECHANICAL SEAL DESIGN PROBLEMS Problems with the Seal Faces: ●
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Wrong carbon or hard face selected. The material is not compatible with the fluid you are sealing, and the cleaner or solvent used to clean or flush the system. Face flatness problems: The face cross section is too narrow causing temperature or pressure distortion problems. The material modulus of elasticity is too low. The face is not hard enough. All clamping forces must be "equal and opposite" to prevent face distortion. In many designs they are not. The differential expansion between the seal face and its holder can cause the face to go out of flat. The faces were not lapped at a cryogenic temperature and the seal is being specified for cryogenic service. Bad packaging. Poor heat conductivity: Carbon is a poor conductor of heat compared to most hard faces. Many ceramics are not good conductors of heat . Plated or coated faces can "heat check" due to a differential expansion rate between the coating and the base material. The seal face is sometimes insulated by a gasket or elastomer. Low expansion steel face holders are not usually corrosion resistant. No vibration damping has been provided to prevent "slip stick" vibration problems. This is a major problem with metal bellows seals. Unbalanced seal designs require excessive flushing or cooling to remove unwanted heat. The carbon must be dense enough to prevent entrained air pockets from expanding and causing pits in the carbon face. An "unfilled carbon" with four impregnates is the best.
The Springs or bellows. ● ● ● ● ● ● ●
Springs in the fluid can clog easily, especially the small springs. Stainless steel springs and bellows are sensitive to chloride stress corrosion problems. A single spring can be wound in the wrong direction. Thin bellows plates and small cross section springs are sensitive to abrasive wear. Rubber bellows experience a catastrophic failure mode when the bellows ruptures. Stressed metal corrodes faster. Springs and metal bellows are subjected to high stress. Too much spring or bellows movement will cause an early fatigue of the metal.
The Dynamic Elastomer (the one that moves) ● ●
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Some elastomers do not move to a clean surface as the face wears. Spring loaded elastomers stick to the shaft or sleeve and are sensitive to the shaft diameter and finish. Elastomers positioned in the seal face are subject to the heat generated between the seal faces. Dynamic elastomers are very sensitive to the shaft tolerance and finish.
Operating conditions too severe for the design. ● ● ● ● ● ● ●
Elastomers and some seal faces are sensitive to temperature extremes. Excessive pressure can distort seal faces causing them to go out of flat. Excessive pressure can cause elastomer extrusion. High speed can separate the seal faces in rotating seal designs. High speed can cause excessive heat at the seal faces. Excessive shaft movement separates faces also. Hard vacuum can "out gas" an elastomer causing it to leak.
Dual seals ● ● ● ●
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Rotating "back to back" designs: Centrifugal force throws solids into the inner faces. Inner seal blows open if barrier fluid pressure is lost. Inner stationary face is not positively retained to prevent movement if the pressure is lost between the faces. When the outboard seal fails the inboard will fail also due to the pressure drop between the faces. The inner seal has to move into the sealing fluid as the face wears. This is a major problem if the fluid contains solids. Failure to use "two way" hydraulic balance causes the inner faces to open with a reversal in barrier fluid pressure.
Design problems that cause excessive shaft movement ● ●
An elbow is installed too close to the pump suction inlet. The mass of the foundation is not five times the mass of the pump and its driver.
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Wrong size pump was specified because of safety factors and, as a result, the pump is operating off the B.E.P. The pump was selected oversize in anticipation of a future need. A "centerline" design should have been selected when the operating temperate exceeded 200°F (100°C). The shaft L3/D4 is too high.
The pump is cavitating due to a design problem. ● ● ● ● ● ● ●
Too high a N.P.S.H. is required. You need a double suction pump. The suction specific speed number is too high. You are using too low a specific speed impeller. A reducer has been installed up side down, letting an air pocket into the suction. The impeller to cutwater clearance is too low. There is too much suction resistance due to excessive piping. Too much suction lift for the fluid temperature.
Other design problems ●
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Some seal designs cannot compensate for thermal shaft growth or impeller adjustment. Cartridge versions are needed for this feature. The pumping fluid is located at the inside diameter of the seal faces. Solids will be thrown into the lapped faces destroying some face materials. Solids will pile up in front of the movable faces, preventing them from compensating for wear. Most seal faces are weak in tension. Hysteresis (delay) problems caused by the seal mass and sliding elastomers. Poor packaging that allows face damage during shipment and storage. Designs that frett (damage or groove) the shaft or sleeve. High speed requires the use of stationary seal designs. Centrifugal force can open rotating designs above 5000 fpm. (25 m/sec.) The seal is positioned too far from the bearing housing. Lack of a self-aligning feature is causing excessive face movement. A tapered stuffing box can cause face damage. No vent has been provided to vent the stuffing box in a vertical application. Hardened shafts and sleeves can cause the seal set screws to slip. A discharge recirculation line is aimed at the lapped faces, causing them to wear, and interfering with the seal movement.
Problems caused by the product you are sealing. ● ● ● ● ● ●
The fluid can flash or vaporize between the faces. Viscous fluids open seal faces as they restrict seal movement. Products that solidify will open and damage seal faces. Crystallizing products restrict seal movement and open the faces. Film building products cause the faces to open. Hot oil is typical. The fluid can attack one of the seal components, especially the elastomer.
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All chemicals have the potential for corroding a seal component. It is just a mater of time. Some fluids are poor lubricants: This can cause excessive wear. Color contamination problems as the carbon wears. "Slip stick" vibration problems. Slurries clog up the sliding seal components and open the faces. Cryogenic fluids can attack some carbon faces and most elastomers. High temperature fluids attack elastomers and change the state of the fluid you are sealing. Some fluids can cause the formation of ice outboard the seal, restricting seal movement as the face wears. Agitation can cause some fluids to change their viscosity. Cleaners or solvents are attacking a seal component.
OPERATION PROBLEMS Operations that cause excessive shaft movement that will open or damage the seal faces ●
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Opening and closing valves in the suction and/or discharge causing the pump to operate off the B. E.P, and the shaft to deflect. Pumping the supply tank dry, causing excessive vibration and heat. Series or parallel pump operation can cause shaft deflection. Running at a critical speed will cause the shaft to defect. Cavitation problems: Low N.P.S.H. Air getting into the system through packing. A stuffing box, suction recirculation line is heating the incoming fluid. A discharge bypass line is heating the suction fluid A discharge recirculation line is aimed at the seal face restricting its movement. Water hammer is opening or damaging the lapped faces. The piping system has been altered since the pump was installed. The pump is being started with the discharge valve shut or severely throttled. Starting a pump with the discharge valve open is just as bad.
Operations that cause excessive heat and corrosion problems ● ● ● ●
Cleaners or solvents used in the lines can attack a seal component, especially the elastomer. A product concentration change will affect corrosion. A change of product. Either a temperature or pressure change in the system will affect both.
Operations that cause the seal faces to open ● ● ● ●
The seal is seeing frequent reversing pressures. Loss or lack of an environmental control. Flush not working. Quench is shut off.
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Barrier fluid not circulating. Loss of heating or cooling. Heating jacket clogged. Pressure drop in the stuffing box. Flushing with a dirty product. Quenching with shop water leaves solids outboard Of the seal that will cause a hang-up as the seal moves forward to compensate for wear. The quenching steam pressure is too high. It is getting into the bearings.
MAINTENANCE PROBLEMS ● ● ● ● ● ● ● ● ● ●
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The pump and driver are not aligned&emdash;causing excessive seal movement. Pipe strain. Thermal growth. Bad installation techniques that can injure a seal component. Wrong lubricant put on the dynamic elastomer. The impeller clearance was set after the seal installation. The face is inserted backwards, only one side is lapped. The seal is set at the wrong installation length. The sleeve moved when the impeller was tightened to the shaft. A lubricant was put on the seal face that froze when the product evaporated across the lapped faces. The rotating assembly is not dynamically balanced. The shaft is bent. The sleeve is not concentric to the shaft. Impeller clearance is not being maintained, causing vibration problems. The impeller is positioned too close to the cutwater. The seal has been set screwed to a hardened shaft. No seal or gasket between the shaft sleeve and the solid shaft. This is a big problem with double ended pumps. The seal environmental control is not being maintained. Flushing fluid is being restricted or shut off. Quenching steam is shut off. The barrier fluid tank level is too low The convection tank is running backwards. The cooling jacket is restricted due to a calcium build up. You are running both a discharge recirculation line and a cooling jacket. Out of tolerance shaft dimensions will restrict seal movement. The impeller clearance was made without re-adjusting the seal face load. The shaft sleeve was removed to accommodate a smaller diameter seal. The sleeve was providing corrosion resistance. A gasket is protruding into the stuffing box restricting the seal movement.
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SUBJECT : A quick reference guide for mechanical seal failure 4-11 Of all the seal related activities, analyzing mechanical seal failure continues to be the single greatest problem for both the consumer and the seal company representative. I have addressed this problem in several of my other technical papers. If you will take a little bit of time to familiarize yourself with the following outline you should feel a lot more comfortable the next time you are called upon to do some seal troubleshooting. I should mention here in the beginning that as you look over the failed seal components keep in mind that a rebuilt seal may have some marks that occurred during a previous failure, making them especially difficult to analyze, but regardless of the design mechanical seals fail for only two reasons: ● ●
Damage to one of the components The seal faces open prematurely.
We will start with damage. This damage is almost always visible. Look for : Corrosion - The elastomer swells or the other seal parts become "sponge like" or pitted. ● ●
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The product you are sealing is attacking one of the seal components. The attack is coming from the cleaner or solvent used to clean the lines between batches or at the end of a "run". The attack is coming from lubricants put on the elastomers or seal faces. Petroleum grease on Ethylene Propylene O-rings will cause them to "swell up". Galvanic corrosion - Happens with dissimilar materials in physical contact and connected by an electrolyte. As an example: stainless steel can attack the nickel binder in a tungsten carbide face. Oxidizers and Halogens attack all forms of carbon including black O-rings.
KEEP IN MIND THE CORROSION INCREASES WITH TEMPERATURE Physical damage. ● ● ●
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Wear or rubbing of a flexible component. Thermal shock of some seal face materials. Especially those that are hard coated or plated. Thermal expansion of the shaft or sleeve can break a stationary seal face or interfere with the free movement of a dynamic elastomer. The rotating seal hits something because of shaft deflection. Temperature extremes (both high and cryogenic) will destroy elastomers and some seal face materials. Erosion from solids in the product you are pumping. Fretting caused by the dynamic elastomer removing the passivated layer from the corrosion resistant shaft or sleeve. Fluid abrasion that can weaken materials and destroy critical tolerances. A discharge recirculation line circulates high velocity liquid with entrained solids that can break a metal bellows and injure lapped seal faces, as well as interfere with the free movement of the seal.
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The elastomer or rubber part can swell and breaks the face. Problems at installation. This includes mishandling, setting at the wrong compression, putting the wrong lubricant on the elastomer etc. Fatigue of the springs caused by misalignment.
The seal faces opening prematurely is the second cause Scoring or wear of the hard face is the most common symptom of this failure. The scoring occurs because the solids imbed into the softer carbon face after they open. The seal faces must stay in contact, but there are all kinds of conditions that are trying to force or pull them open. Physical causes ● ● ● ● ● ● ● ● ●
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Axial shaft movement (end play or thrust). This is normal at start up. Radial shaft movement (run out or misalignment) Operating off of the pump's best efficiency point. Hysteresis caused by a viscous (thick) product. Centrifugal force tries to separate the faces in a rotating seal application. Hydrodynamic forces generated between the lapped faces. Pressure distortion caused during pressure peaks such as water hammer and cavitation. Thermal distortion that can cause the seal face to separate from its holder or "go out of flat". A failure to provide equal and opposite clamping across the stationary seal face will cause distortion. A hardened sleeve can cause the seal set screws to slip. A wrong initial setting of the face load. Springs can clog if they are located in the product. Loose set screws. If the sleeve is too soft they can vibrate out. Shaft tolerance and finish is out of specifications. The rotating shaft or seal hits something. The discharge recirculation line can force open the faces. Outside springs painted by maintenance people. A cartridge seal installation method can compress one set of faces and open the other. Vibration. Fretting hang up. Cartridge mounted stationary seals move excessively unless they have some type of "built in" self aligning feature.
Product problems . With a loss of an environmental control the fluid can: ●
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Vaporize between the lapped faces forcing them open and causing a "chipping" of the carbon outside diameter as well as leaving solids between the lapped faces. Become viscous preventing the faces from following normal "run out". Solidify between the lapped faces or around the faces. Crystallize between the faces or around the dynamic portions of the seal. Build a film on the sliding components or between the faces causing them to separate. Be a slurry and/ or abrasive
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Operate in a vacuum causing the ingestion of air between the faces of some unbalanced seal designs. Swell up the dynamic elastomer, locking up the seal . Cause slipstick between the faces if the sealed fluid is a non, or poor lubricant
The common causes of shaft displacement. ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
Operating off the pump's best efficiency point (B.E.P.). Misalignment between the pump and its driver. The rotating assembly is out of balance. A bent shaft. A non concentric sleeve or seal. Vibration Slip stick Harmonic Induced Passing through, or operating at a critical speed. Water hammer in the lines. The stuffing box is not square to the shaft, causing misalignment problems. Pipe strain. An impeller adjustment is made to compensate for normal impeller wear. Thermal growth of the shaft in both a radial and axial direction. Bad bearings or a poor bearing fit. Two direction axial thrust at start up is normal. The motor is finding its magnetic center. Cavitation - there are five separate types of damage that can be observed. The sleeve moved when the impeller was tightened. The unit is pulley driven causing excessive side thrust The impeller is positioned too far from the bearings. This is a severe problem in mixer or agitator applications.
How to preventing product problems that cause premature seal failure. Control the environment in the stuffing box. ● ● ● ● ● ● ● ● ● ● ●
Control the temperature in the seal area Use the correct spring or bellows compression. Use only hydraullically balanced seals. Select a low friction face combination. Avoid "dead ending" the stuffing box. Jacket the stuffing box Quench behind the seal with the correct temperture steam or fluid Use a gland jacket Utilize two seals with a barrier fluid between them Use heat tape around the stuffing box Use a heat pipe to remove heat from the stuffing box.
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Vent the stuffing box, especially in a vertical application Flush in a cool compatible liquid. Control the pressure in the seal area ❍ Be sure to use only hydraulically balanced seals. ❍ Discharge recirculation will raise the pressure if you put a restrictive bushing into the bottom of the stuffing box. ❍ Suction recirculation will lower the pressure in the stuffing box. ❍ Use two seals and let the barrier fluid control the pressure between the seals. ❍ Cross connect the stuffing boxes to equalize the stuffing box pressures in a multi stage pump. ❍ Stage the stuffing box pressure with tandem seals. ❍ Impeller pump out vanes will lower stuffing box pressure. Give the seal more radial space ❍ Bore out the existing stuffing box if it is possible. ❍ Make or buy a new back plate with the large stuffing box cast into it. ❍ Make or buy a large bore stuffing box and attach it to the back plate after you have machined the old one off. Flush the product away if you are unable to control it. ❍ Suction recirculation will bring fluid into the stuffing box from behind the impeller, where it is usually cleaner. This works on most closed impeller pump applications and those open impeller pump applications where the impeller adjusts to the volute rather than the back plate. ❍ Flush with a clean liquid from an outside source. ❍ A pressurized barrier fluid between two seals can keep solids from penetrating between the faces if the faces should open. This application will also work if the solid particles are less than one micron in diameter (Kaoline is such a product).
Build the seal to compensate for operating extremes. Slurry features that can be part of seal design. ● ● ● ●
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Springs out of the fluid Teflon coating the metal parts so particles will not stick to sliding components. The elastomer moves to a clean surface as the face wears. Keep the sealing fluid on the outside diameter of the seal to take advantage of centrifugal force that will throw solids away from the lapped faces. Rotate the fluid with the seal to prevent erosion of the seal components. A simple vane arrangement can accomplish this. Use two hard faces if you find it impossible to keep the lapped seal faces together. Use a pumping ring to keep solids away from the faces. Mount the seal closer to the bearings to diminish the affect of shaft deflection.
Design for higher temperature capability ● ●
Eliminate elastomers when ever possible. If you cannot eliminate elastomers, the O-ring location becomes important. Try to move the
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elastomer away from the faces. Hydraulically balanced seals generate less heat. Select low friction faces. Fool proof, correct installation dimensions are necessary. A cartridge design is your best choice. Keep a good product circulation around the components. A good lapping technique will keep the faces flat at high and cryogenic temperatures. Pumping rings will keep fluid circulating between two seals. If you are using balanced seals a simple convection tank is usually more than adequate. An air operated diaphragm pump can be used in the line to increase the circulation. Try to avoid the use of petroleum based fluids as the barrier or buffer fluid between the seals. Petroleum based fluids have a very low specific heat that will increase the temperature between the seals, Gland features such as quenching, recirculation, venting and flushing help. Choose well designed faces that will resist thermal distortion. The closer you get to a "square block" design, the better off you are going to be. Do not insulate the faces with an elastomer.
Design for pressure resistance ● ●
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Limit the number of diameters in any single seal component Laminated bellows will allow you to keep a low spring rate while maintaining pressure capability, if you are using a welded metal bellows design. Finite element analysis of the seal components will prevent pressure and temperature distortion. Use more mass to resist hoop stresses. Higher modulus materials will resist bending and deformation. Use a tandem seal design for pressure break down between two seals.
Design for corrosion resistance ● ● ● ●
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Choose good materials, clearly identified by type and grade. Eliminate elastomers when possible. Elastomers are the most corrosion sensitive part of the seal. Design non stressed parts when ever possible Try not to weld any of the metal components. If it is necessary, monitor the temperature to prevent inter granular corrosion Control the temperature. Corrosion increases with temperature. Use non metallic materials for non metallic equipment. Watch out for galvanic corrosion when using dissimilar materials. Do not use stainless steel springs. Stick with Hastelloy "C" if the metal parts of the seal are manufactured from iron, steel, stainless steel, or bronze. If the seal is manufactured from a different metal, use springs manufactured from that material. Do not depend upon flushing to provide corrosion resistance. Use the correct materials, keeping in mind that solvents and steam are sometimes used to flush the lines. Any materials that you select must be compatible with these flushing or cleaning fluids also.
If you need cryogenic capability ●
Go to a welded metal bellows configuration to eliminate all elastomers.
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You will need a special carbon/ graphite face that has an organic material impregnated to assist in the release of the graphite. Avoid plated or coated hard faces. Differential expansion will cause them to crack. Always lap the faces at a cryogenic temperature. Do not coat the faces with grease or oil. It will freeze at cryogenic temperatures.
Link to Mc Nally home page
SUBJECT : An overview of seal troubleshooting 3-1 Seal problems are almost always associated with face leakage, but as we will soon learn there are other leak paths in addition to the obvious one between the lapped seal faces. In the following paragraphs, we will be looking at all of these leak paths. Keep in mind that seals are classified into many categories : stationary, rotary, balanced, unbalanced, inside, outside, metallic, nonMetallic, single, dual, elastomer, metal bellows, rubber bellows, cartridge, split, solid, etc.. These classifications were described in another paper in this series Try to keep these classifications in mind as we investigate the cause of seal failure. As with former papers, I will be presenting the troubleshooting hints in an outline form. You should not find these terms confusing because I have assumed you have a pretty good knowledge of mechanical seals or otherwise you would not be attempting to trouble shoot them. In the event you do have trouble with some of the terms or techniques any representative of a reputable seal company should be able to explain them to you. LEAKAGE AT THE SEAL FACES. The seal face is not flat. (Flatness should be measured within three helium light bands, (0,000033" or 1 micron) ● ●
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The face was damaged by mishandling. Poor packaging. The seal should be able to survive a 39" (1 meter.) drop. To insure this, the seal must be shipped in a reusable box insulated with plenty of foam or any other adequate insulation. The face was distorted by high pressure or surges in pressure. "Water hammer" would be an example. It was distorted when you tightened the stationary face against an uneven surface. The clamping is not "equal and opposite" across the stationary face. This is a common problem with "L" shaped and "T" shaped stationary faces. The "hard" seal face has been installed backwards. You are running on a non lapped seal face. It is common practice to lap only one side of a hard face. The face is being distorted by a change in temperature. This happens when you forget to vent a vertical pump. The face never was flat. You have a bad part. The carbon metal composite was not stress relieved after the carbon was "pressed in".
The face has been chemically attacked. ● ● ●
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Oxidizing agents attack all forms and grades of carbon graphite. Some de ionized water will attack any form of carbon. Corrosion increases with any temperature increase. A 10 ° Centigrade (18°F.) rise in temperature will double the corrosion rate of most corrosives. A cleaner or solvent is being flushed through the lines and it is attacking the carbon.
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You are using a poor grade of Carbon. Go to an unfilled grade such as Pure Carbon Company grade 658 RC. This is a common occurrence if the seal is being repaired by some one other than the original manufacturer.
The plating or hard coating is coming off of the hard face. ●
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All coatings are porous. The chemical is penetrating this porous coating and attacking the bond between the coating and the base material, or the base material its self. An inferior plating was originally put on the base material. Differential expansion of the dissimilar materials is causing them to separate.
The seal face is cracked, pitted or damaged. ●
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High temperature is heat checking (cracking) the plated face. This is a common problem with cobalt based tungsten carbide. The nickel base version is less likely to crack. The product is solidifying between the faces and they are breaking at start up. Most face materials have high compressive strength, but tend to be weak in tension. Excessive vibration is causing the drive pins to crack the face. Low cost seals experience this problem quite often. There is a high temperature differential across the ceramic. 7 to 10 cycles can break even good ceramics in hot water or hot petroleum products. Air is trapped in the Carbon. Heat is causing it to expand and blow out pieces of the carbon face. The carbon usually blisters prior to blowing out. The solution is to go to a more dense carbon. The product is vaporizing and allowing solid material to blow across the lapped face. This is a common occurrence in boiler feed water applications. The seal faces have opened, solids penetrated and imbedded into the soft carbon causing rapid wear in the hard face. The same problem occurs if the carbon was relapped using lapping powder. Lubricant, on the faces is freezing in cryogenic (cold) applications. The elastomer is being chemically attacked and swelling up. This can break the face in those seal applications where the elastomer is positioned in the seal inside diameter. In some instances the swelling elastomer will open up the two faces, allowing the solids to penetrate. This can be a problem with boot mounted faces The rotating shaft, or sleeve, is hitting the stationary face. This can happen if the pump is running off of its B.E.P., which almost always occurs at start up. The seal is being mishandled during installation. Good packaging and proper training can solve many of these problems. The crack may have occurred during disassembly. Check to see if there is discoloration deep in the crack. Discoloration means that it occurred during, or before, operation. Petroleum products can "coke" at the face causing pieces of carbon to be pulled out as the face rotates. You will have to select two hard faces for this application. The rotating face is not centered in the stationary face and is running off the edge of the stationary face. Look for rubbing marks around the O.D. of the rotary unit. A bent shaft or out of balance rotating assembly is the most common cause You will notice a much wider wear track if you are experiencing this problem. The seal will appear to "spit" as lubricant is dragged across the face and off the seal outside diameter.
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Dirt can be dragged across the faces as they separate.
The movable face is not free to follow whip, wobble or run out. ● ●
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The rotating face is hitting the I.D. of the stuffing box. The recirculation line from the pump discharge is aimed at the seal faces and interfering with their free movement. Dirt or solids are clogging the movable components. Magnetite is a very big problem in most hot water applications. The product is interfering with the free movement of the components. It is: Crystallizing ( like sugar) Solidifying (like glue) Viscous (molasses) Building a film on the sliding components ( hard water or paint) Coking (oil or any other petroleum product) The elastomer has been chemically attacked causing it to swell up and interfere with free movement of the face. Temperature growth of the shaft is interfering with the free movement of the movable face. The shaft or sleeve is the problem. It is over size - + 0.00" - 0.002" ( 0,00-0,05 mm.) is ideal. It is too rough; it should be at least 32 R.M.S. (0,8 microns) It is fretted, corroded or damaged in some way. Solids have attached themselves to that portion of the shaft where the dynamic elastomer is located. A gasket or fitting is protruding into the stuffing box. Solids from outside the stuffing box are getting under the faces. This is a common problem with vertical pumps. The elastomer is spring loaded and the interference on the shaft is restricting the face movement. The elastomer has extruded because of high pressure or excessive clearance. A foreign object has passed into the seal chamber and is interfering with the free movement of the seal.
The product has plated, or formed on the face and a piece of it has broken off. ●
This problem occurs with products that are sensitive to temperature and/ or pressure changes.
The set screws have come loose. ● ● ● ●
The shaft has been hardened. They have worked loose in a sleeve that is too soft. The hardened set screws have corroded. They were not replaced when the seal was rebuilt and as a result are not "digging" into the shaft.
The face has lost its spring load. ●
The initial setting was wrong.
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Temperature growth of the shaft has altered the original setting. The impeller has been adjusted towards the wet end of the pump. The sleeve moved when the impeller was tightened to the shaft. The cartridge seal was pushed on the shaft by pushing on the gland and the seal is now over compressed. In a dual seal application this will over compress the inner seal and open up, or unload the outer seal.
The product is vaporizing and blowing the faces open. This happens in hot applications if there is water in the product. ●
It can also occur if the pump/seal was hydrostatically tested with a water base fluid.
The inner seal, of a dual seal application was not balanced in both directions and is opening up with reversing pressure. This is a common problem in unbalanced seals that are subject to both vacuum and pressure or if the barrier fluid pressure varies. The single spring, found in some seal designs, was wound in the wrong direction for the shaft rotation. The Bellows seal has lost cooling and the anti vibration lugs are engaging the shaft. Shaft movement will cause the faces to open. LEAKAGE AT THE ELASTOMER LOCATION. Compression set ( the elastomer has changed shape). ●
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Either the product is too hot or there is too much heat being generated at the seal faces. You must vent vertical pumps to prevent this problem. This is a common problem with most grades of Dupont's Kalrez® material, preventing it from being free to flex and roll.
The elastomer is cracked. ●
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The shelf life has been exceeded. Buna N (Nitrile) has a shelf life of only twelve months because of its sensitivity to ozone attack. High heat is the main cause. Chemical attack. In most cases the elastomer swells but cracking and shrinking does occur in isolated cases. Cryogenic (cold) temperatures freeze the elastomer and it will crack when hit.. The rubber bellows did not stick to the shaft because the wrong lubricant was used. The shaft turned inside the bellows causing high heat. The seal faces stuck together. The shaft was turning inside the rubber bellows causing excessive heat.
The elastomer is cut or damaged.
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Mishandling. The elastomer was slid over a rough spot on the shaft or sleeve. Be careful of old set screw marks, splined shafts, key ways, etc. It was extruded by high pressure. You may need a backup ring. The product is penetrating into the elastomer and blowing out the other side. This problem is a common occurrence when you are trying to seal ethylene oxide. Teflon jacketed O-Rings can split in the presence of halogenated fluids. The halogen will cause the elastomer to swell up, inside of the Teflon jacket. Halogens can be recognized because most of them end in the letters "ine", such as bromine, astintine, chlorine, fluorine, iodine, etc..
The elastomer is not seated properly. ● ● ● ● ● ●
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It was twisted during installation. High pressure can cause the elastomer to extrude or twist in the O-Ring groove. Solids have "built up" or penetrated between the elastomer and the shaft. The shaft is corroded, damaged, or fretted. The shaft is oversized. Excessive travel can cause the elastomer to "snake". Most o-rings can roll up to one half of their diameter. The O-ring groove is damaged or coated with a solid material.
The elastomer has swollen or changed color. ● ●
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Product attack. This is the most common cause and usually occurs within five to ten days The wrong lubricant was used at installation. As an example, you should never put petroleum grease on EPR O-rings. Solvents or chemicals used to clean the lines are not compatible with the elastomer. Steam can harm many elastomers including most grades of Viton®. Oxidizers can attack the carbon black in O-Rings and other elastomers.
The elastomer leaks when pressurized in the opposite direction. ●
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A common problem with unbalanced, dual seal applications. Two way balanced seals are recommended for these applications. Remember that O-Rings are the only common elastomers that seal in both directions. Wedges, U cups, and chevrons do not have this ability.
OTHER LEAK PATHS TO CONSIDER Between the carbon and its metal holder. ●
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Some seal companies, and most seal repair facilities, glue the carbon in place. The glue may not be compatible with the product you are sealing. "Pressed in" carbons can leak in a high temperature application because of the differential expansion between carbon and metal. Low expansion metal is available for these applications
Between the shaft and the sleeve. ● ●
Damaged gasket or gasket surface. Distorted sleeve or shaft.Many packed, double ended pumps have this problem because there is no gasket between the impeller and the sleeve that is holding it in place.
Stationary face gasket or elastomer leaking. ●
This leak path is not always visible. It often looks like face leakage.
Gland gasket or gasket surface leakage. ●
This leak path should always be visible.
Pipe flange leaking above the seal and dripping into the seal area. ●
I found this one after every other avenue was exhausted.
At the weld location if a seal face holder is welded to the cartridge sleeve. At the pipe connections, ancillary hardware, A.P.I. Gland fittings, and recirculation lines. A scratch or nick in the o-ring groove. Remember that up to 100 p.s.i. (6 bar) o-rings seal on the O.D. and the I.D. not the sides. Seal faces will not leak visibly if they are lapped flat and we keep them in total contact. Shaft movement is the main contributor to the opening of the seal faces and allowing solids to penetrate. Shaft movement is caused by many factors. In the following paragraphs we will be looking at most of them. CAUSES OF EXCESSIVE SHAFT MOVEMENT, INCLUDING VIBRATION. Cavitation ● ●
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Vaporization caused by too high a product temperature or too low a suction head. Air is entering the stuffing box. A common problem with pumps that run in a vacuum or taking a suction from an evaporator or condenser. Internal recirculation. Occurs when the Suction Specific Speed is too high, or when either the impeller or wear ring clearance becomes excessive. The vane passing syndrome occurs if the O.D. of the impeller is too close to the pump cutwater. This clearance should be at least 4% of the impeller diameter in the smaller size impellers and at least 6% in the larger diameter impellers (greater than 14 inch or 355 mm.) Turbulence. Occurs if there is not laminar flow in the lines.
The bearings are worn excessively.
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Contamination of the lubricant is the biggest cause. Grease or lip seals have a useful life of only 2000 hours (84 days). Poor fit or installation. Serious misalignment. The misalignment can be the result of pipe strain or misalignment between the pump and its driver.
The shaft is bent. ● ● ●
Usually occurs during sleeve removal or if the bearing was installed with an arbor press. Improper storage with the long shaft supported only on the ends. Heating the shat to remove the sleeve is another common cause
The impeller is out of balance. ● ● ● ●
The impeller was damaged by either wear, corrosion or cavitation. Product has built up on the vanes or in the balance holes. The impeller diameter was reduced and the impeller was not re balanced The impeller never was balanced.
An unbalanced rotating assembly. Pressure surges or water hammer. Worn coupling. The pump is operating off of its best efficiency point. Rubbing of a rotating component. ● ● ● ●
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The shaft is hitting the wear ring, or a stationary wear ring is contacting a rotating wear ring. The shaft is hitting the seal gland or stationary face. A seal rotating component is hitting the stuffing box I.D.. A recirculation line aimed at the seal faces is causing a pulse each time the impeller vane passes the fitting. A gasket or fitting is protruding into the stuffing box.
The stationary seal face is not perpendicular to the rotating shaft. This causes the spring loaded, rotating face to move back and forth twice per revolution. ● ● ● ● ● ●
The stuffing box face is not square to the shaft. The stuffing box face is often a rough casting. Tightening the gland bolts through a gasket is cocking the stationary face. Pipe strain. Temperature growth. A convection tank, or some other heavy device is hanging off of the gland. Bearing fit or wear.
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Coupling alignment. Shaft deflection. The deflection can be caused by operating the pump off of its best efficiency point, the rotating assembly is out of balance, or the shaft is bent. Poor installation technique.
VIBRATION AT THE SEAL FACES. Harmonic vibration. ●
The seal is vibrating in harmony with some rotating component. The same thing that causes a rear view mirror to vibrate in an automobile. Most harmonic vibration can be stopped by changing the speed of the equipment or "damping" the vibrating component.
Slipstick--caused by: ● ● ● ● ● ● ● ● ● ● ●
Poor lubricating fluids. Hot water. Solvents. Some detergents. Gases Dry running applications. Too high a face load. Using unbalanced seals. Poor installation technique. Face load has changed because of temperature growth or impeller adjustment. You are using a high friction face combination. Often occurs if you use two hard faces.
A discharge recirculation line aimed at the seal faces. ●
Each time the impeller passes the recirculation connection it causes a pulse of fluid at the seal face.
Vaporization of the product at the seal face. ● ●
Happens frequently with products that contain water, and are operated at elevated temperature. Can occur at the seal face because of high face load or if you use unbalanced seals.
EXCESSIVE AXIAL MOVEMENT OF THE SEAL ● ● ● ● ● ●
Temperature growth. The impeller was adjusted, after the seal was installed, to compensate for wear. The rotor motor, moved to its magnetic center at start up. The equipment is equipped with sleeve or babbitted bearings and has excessive end play. Shaft thrust. There is a thrust towards the bearings caused by the combination of the fluid changing direction
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in the impeller and acting on the shaft and/or impeller surfaces. This thrust is offset by a thrust towards the wet end caused by the impeller shape. In centrifugal pumps the resulting force can be in either direction, depending upon how close the pump is operating to its best efficiency point. Above 65% of its best efficiency, the thrust is towards the wet end. Below 65% of the best efficiency the thrust is towards the power or bearing end. There is little to no movement at 65% of the pumps best efficiency. This means that at start up the shaft moves in both directions accounting for a higher percentage of seal failure at start up. Vertical mixer shafts often lift vertically when solids are mixed with liquid.
SHAFT NOT CONCENTRIC WITH THE STUFFING BOX, this will cause a wiping action in stationary seals. ● ● ● ● ● ● ● ● ● ● ●
The shaft is bending as you move away from the pump B.E.P. It bends at 240 degrees, from the cutwater, at low flow and high head. It bends at 60 degrees, from the cutwater, at high flow and low head. Coupling misalignment. Poor bearing fit. Pipe strain. Temperature growth causes the stuffing box to move relative to the shaft. The sleeve is not concentric with the shaft. The seal is not concentric with the sleeve/ shaft. A bolted on stuffing box has slipped. The back plate is not machined concentric to the stuffing box.
Heat is always an indication of wasted energy, but it can also have a disastrous affect on seal life and performance. Let's take a look at what is causing this heat CAUSES OF HIGH HEAT AT THE SEAL FACES. Too much spring compression. ● ● ● ● ● ● ●
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Installation error. No print was used or the mechanic cannot read the print he was given. The shaft was marked in the wrong location. The mechanic used the wrong marking tool. The mark is too wide. The sleeve moved when the impeller was tightened. The impeller was adjusted after the seal was installed. A cartridge seal was installed on the shaft, by pushing on the gland. Interference from the sleeve elastomer has caused an over compression of the seal. In some dual seal applications the outer seal will become under compressed. The shaft moved because of thrust. Thermal growth of the shat.
Problems with some seal designs. ●
Unbalanced seals are supplied by original equipment companies. They generate more heat than
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balanced seals. The elastomer is located too close to the seal faces. The heat generated at the faces is affecting both the elastomer and the seal face. The carbon face is insulated by an elastomer. The face is too wide causing the hydraulic force to generate excessive heat. The Carbon seal face is too narrow causing excessive heat from the spring pressure. A vertical seal installation is not being vented. The faces are running dry in a bubble. Speeds above 5000 F.P.M. (25 m/sec) require a special balance and less spring load. A 60/40 balance and a face load of 8 psi. to 15 psi. ( 0,07 to 0,2 n/mm2) would be normal. An outside metal or elastomer bellows seal is almost impossible to vent. Spring loaded elastomers cause varying seal face loads. The actual load depends upon shaft tolerance and installation dimension. Some seal faces are glued in. The glue acts as an insulator preventing the face heat from conducting to the metal holder. Many single spring designs are uni-directional requiring both right handed and left handed seals on a double ended pump. Many metal bellows designs lack effective vibration damping. Stationary seal designs require clean flushing if solids are present. centrifugal force does not throw the solids away from the moveable (spring loaded) components.
Problems with face materials. ● ● ●
Heat conductivity is low in some materials. (ceramic, carbon, Teflon) The coefficient of friction varies with face combinations and various sealing products. Carbon/ metal composite faces conduct heat better than plain carbon/ graphite, as long as there is a true interference fit and they are not glued together to hold them in place.
Problems with the pump operation that causes high heat at the faces. ● ● ● ● ● ● ● ● ● ● ● ● ●
Operating off of the B.E.P The degree of the problem is determined by the L3/D4 ratio. Operating at vapor point, causing cavitation. Running dry. Gases. Dry solids. Pumping a tank dry. Losing barrier fluid in a dual seal application. Shutting off the flushing water. Vacuum applications. Vertical pumps not vented in the stuffing box. The liquid is not a lubricant. Pump out rings on the back of the impeller running too close to the pump back plate.
Other causes of high heat. ●
The shaft, or sleeve is rubbing a stationary component.
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The gland. The bushing in the bottom of the stuffing box. The bushing in the A.P.I. gland. A pump wear ring. A protruding gasket. A fitting. The stationary portion of a mechanical seal. The shaft, or sleeve, is not straight. It is bending, because the pump is operating off of its best efficiency point. It is bent. This often happens when the sleeve is removed. The rotating assembly is not balanced. The shaft never was straight. There is not enough circulation around the seal. Install a large diameter stuffing box. You should be able to get at least 1" (25 mm.) all around the rotating unit. Connect a recirculation line from the bottom of the stuffing box to the suction side of the pump. You can do this in almost every case except when you are pumping a product at its vapor point or if the solids have a specific gravity lower than the fluid. The cooling jacket is clogged. There is no carbon restriction bushing in the bottom of the stuffing box and you are using the cooling jacket. The restriction bushing slows down the heat transfer. Loss of an environmental control. The flush is not constant. The pressure is changing. Quenching steam or water has been shut off during pump shut down. The double seal barrier fluid is not circulating. The cooling jacket has become clogged by the calcium in the hard water. Try condensate instead. The filter, or separator, is clogged. Either the suction or discharge recirculation line is clogged. If you are using double seals, remember that two seals generate twice as much heat and conventional cooling may not be sufficient. Contact the manufacturer for the rules when using convection tanks and dual seals. You may need a "built in" pumping ring. Solids in the stuffing box are interfering with a rotating component.
® DuPont Dow elastomer
Link to Mc Nally home page
SUBJECT : Troubleshooting mechanical seals at equipment disassembly 3-9 All seals fail for the same reasons: ● ●
The faces open up and allow dirt or solids to penetrate. One of the seal components has been damaged by either the product, heat, or a cleaner used to flush the system.
After the failure has occurred you will frequently get a chance to analyze the failed components. You are going to be looking for several things: ● ● ● ● ● ● ●
Evidence of corrosion. Wear patterns on those parts that should be rubbing. Evidence of rubbing or wear on those components that should not be in contact. Discoloration of any of the seal components, especially the metal parts. Parts that are missing. Springs, set screws and drive lugs as an example. Loose hardware. Either a seal component or a foreign object. Product attaching to a rotating component. Carefully inspect the impeller and rotating part of the seal.
In the following paragraphs we will be inspecting the individual components and looking for evidence of the above. THE CARBON FACE Chipping on the O.D. of the carbon. Indicating vibration. ● ● ●
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This can be caused by harmonic vibration, or if the rotating equipment hits a critical speed. Slipstick can occur if you are pumping a fluid with poor lubricating qualities. Mishandling is a common problem. Look for evidence of drive lug wear to eliminate this as a possibility. Vaporization of the liquid causing the faces to rapidly open and then close as the leaking fluid cools the faces. A discharge recirculation line is aimed at the carbon seal face. The pump is cavitating. Remember there are five types of cavitation. Water hammer is a another possibility.
Pits in the carbon face. This problem is usually associated with poor grades of carbon/ graphite. ●
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Exploded carbon. Air trapped in the pores of the carbon expands and expels pieces of the carbon when the seal faces get hot. Prior to ejection polished patches will be visible, usually with small cracks visible in the center. If the product solidifies between the faces it will tear out pieces of the carbon at start up. This is a common occurrence with ammonia compressor seals because petroleum oil is mixed with the
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ammonia and it can coke at the elevated temperature. Most petroleum products will "coke" because of the higher face temperature, and pull out small pieces of the carbon as the faces rotate. You will see evidence of these small pits if you inspect the carbon face under a magnifying glass.
Chips at the I.D. of the carbon ●
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Solids, or a foreign object of some type from outside of the pump are getting under the gland and are being thrown into the seal faces. This can occur if the seal leaked at some time and the product solidified on the outboard side of the seal. It can also occur if liquid, containing solids, is used in the quench connection of an A.P.I. type gland. If the seal was installed outside of the stuffing box, as is the case with non metallic seals, solid particles in the fluid can be centrifuged into the rotating carbon face. If the stationary face is manufactured from carbon it can be chipped if it comes into contact with the rotating shaft. This is a common problem at pump start up, or if the pump is operating off of its B.E.P.
Phonograph finish on the carbon face. ●
A solid product was blown across the seal face. This happens in boiler feed water applications.
Chemical attack of the carbon. ●
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You are using the wrong carbon. Something in the product or the flush is attacking the carbon filler. Switch to an unfilled carbon such as Pure grade 658 RC or C.T.I. grade CNFJ. You are trying to seal an oxidizing agent. Oxidizers attack all forms of carbon including the unfilled type. The carbon combines with the oxygen to form either carbon monoxide or carbon dioxide. Some forms of de ionized water will pit and corrode carbon faces
Cracked or damaged carbon face. ●
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The product is solidifying between the faces. Carbons are strong in compression but weak in tension or shear. This problem is common with intermittent pumps each time they start up. Excessive vibration can bang the carbon against a metal drive lug. A cryogenic fluid is freezing a lubricant that was put on the face. The elastomer is swelling up under a carbon or hard face. The shaft is hitting the stationary face or the rotating seal face is hitting a stationary object. Mishandling. Poor packaging. The lapped seal faces should be able to survive a 39" (one meter) drop. Ice is forming on the outboard side of the seal and preventing the seal from moving to compensate for face wear.
A coating is forming on the carbon face: ●
A change in temperature. Many products solidify at temperature extremes.
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The product is taking a pressure drop across the seal faces and solidifying. Selective leaching is picking up an element from the system and depositing it on the seal face. The stuffing box is running under a vacuum because the impeller was adjusted backwards and the impeller "pump out vanes" are causing the vacuum. The system protective oxide is depositing at the faces. In hot water systems we experience this problem with magnetite (Fe3O4) until the system stabilizes.
Coking ● ●
This is a problem with all oils, and petroleum products in particular. Coking is caused by the combination of high temperature and time. Contrary to popular belief the presence of air or oxygen is not necessary.
Shiny spots, cracks and raised portions of carbon. ●
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The carbon is not dense enough, causing the expanding gases trapped beneath the surface of the carbon to explode through the face. Product is solidifying between the faces and pulling out pieces of the carbon as the seal revolves.
Excessive carbon wear in a short period of time. Evidence of excessive heat is usually present. ●
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Heat checking of the hard face. It shows up as a cracking of the hard face. This is a problem with coated or plated hard faces. Cobalt base tungsten carbide is a typical example. The shaft is moving in an axial direction because of thrust. This can cause an over compression and heating of the seal faces The impeller is being adjusted towards the back plate. This is problem with seals installed in Duriron pumps or any other pump that adjusts the open impeller against the back plate. Any installation problem: The inner face of a "back to back" double seal application is not positively locked in position. A snap ring must be installed to prevent the inboard stationary face from moving towards the rotating face when the high pressure barrier fluid pressure is lost or overcome by system pressure. The seal was installed at the wrong dimension. A cartridge double seal was installed by pushing on the gland. Friction, between the shaft and the sleeve O-Ring is compressing the inner seal. A vertical pump was not vented. Solids have penetrated between the faces. The faces are not flat. The movable face is sluggish. The product is vaporizing between the faces because of either high temperature or low stuffing box pressure . Non lubricants will cause rapid face wear. A non lubricant is any fluid with a film thickness less than one micron at its load and operating temperature..
The carbon has a concave or convex wear pattern
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High pressure distortion. The stationary face is not perpendicular to the shaft. Some companies lap a concave pattern as standard. Check with your manufacturer. The shaft is bending because the pump is running off of its B.E.P.
The carbon is not flat. ● ● ● ● ● ● ● ●
Mishandling. Poor packaging. The hard face has been installed backwards and you are running on a non lapped surface. The seal was shipped out of flat. The metal/ carbon composite has not been stress relieved and it is distorting the carbon. When the carbon was lapped the lapping plate was too hot and as a result, not flat. The carbon was lapped at room temperature and the seal is running at cryogenic temperatures. Solids are imbedded in the carbon. The faces have opened. ❍ The seal was set screwed to a hard shaft. ❍ The elastomer (rubber part) is spring loaded to the shaft causing the faces to open as the shaft moves due to end play, vibration or carbon wear. The shaft/ sleeve is over sized causing an excessive interference between the elastomer and the shaft/ sleeve. ❍ The sleeve finish is too rough. ❍ The product has changed from a liquid to a solid. ❍ Dirt or solids are interfering with the seal movement. ❍ Some one put the wrong compression on the faces. ❍ Shaft fretting is hanging up the face. ❍ The face has been distorted for some reason allowing solid particles to enter. ❍ The sliding elastomer has swollen up causing too much interference on the shaft/ sleeve. ❍ Poor centering is causing the rotating face to run off the stationary face. Keep in mind the gland bolts are not always concentric with the shaft. ❍ The single spring was wound in the wrong direction. ❍ An out of balance rotating assembly or bent shaft is causing the rotating face to "run off" of the stationary face.
THE HARD FACE. Chemical attack. ●
Some ceramics and silicone carbides are attacked by caustic. Check to see if your seal face contains silica. As an example: both reaction bonded silicone carbide and 85% ceramic have this high silica content.
Cracked or broken. ● ● ● ●
The product is solidifying between the faces. Most hard faces have poor tensile or shear strength. Excessive vibration will cause cracking at the drive lug location.. A cryogenic fluid is freezing a lubricant that was put on the face. The elastomer is swelling up under an outside seal face. This problem can also occur if the seal
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design allows a spring to contact the I.D. of the hard face. The shaft is hitting the stationary face or the rotating seal face is hitting a stationary object. Mishandling. Poor packaging.
Heat check (a common problem with coated or plated faces) ●
Caused by a high heat differential across the face. Most hard coating have only one third the expansion rate of the stainless steel base material.
Hard coating coming off of the face. ●
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The base material not compatible with the sealed product. These coating are very porous so if the product attacks the base material the coating will come off in sheets. The plating process was not applied correctly.
Analysis of the wear track on the hard face. Deep grooves&emdash;excessive wear. Solids imbedded in the carbon are causing the problem. The solids were trapped between the faces when the seal faces opened. ● ● ●
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The seal was set screwed to a hard shaft. The elastomer is spring loaded to the shaft preventing it from flexing as the shaft vibrates.. The shaft/ sleeve is over sized causing the dynamic elastomer or bellows vibration damper to hang up.. The shaft/ sleeve finish is too rough The product has solidified in the seal components. Dirt or solids are interfering with seal movement. Not enough spring compression on the faces. Fretting of the shaft/ sleeve is hanging up the face. The face has been distorted by either excessive temperature or pressure. The sliding elastomer has swollen up due to chemical attack of the product or a cleaner that was flushed through the lines. The wrong choice of rubber lubricant, at installation, can also cause the problem Poor centering is causing the rotating face to run off of the stationary face.. The single spring was wound in the wrong direction.
The wear track is wider than the carbon. ● ● ● ● ● ●
Worn bearings. Bent shaft. Unbalanced impeller. Sleeve not concentric with the shaft. Seal not concentric with the sleeve. In a stationary seal, the stationary carbon is often not centered to the shaft, causing a wiping action.
The wear track is narrower than the carbon. ● ● ● ●
The soft face (carbon) was distorted by pressure. The hard face was over tightened against an uneven surface. The hard face clamping forces are not "equal and opposite". The face never was flat, or it was damaged during shipment.
Non Concentric pattern. The wear track is not in the center of the hard face. ● ● ● ● ● ●
The shaft is bending because the pump is running off of its best efficiency point. Poor bearing fit. Pipe strain. Temperature growth is distorting the stuffing box. The stationary face is not centered to the shaft. Misalignment between the pump[ and its driver.
Uneven face wear. The hard face is distorted: ● ● ● ● ● ● ●
High pressure. Excessive temperature. Over tightening of the stationary face against the stuffing box. The clamping forces are not equal and opposite. The hard face is not wide enough. You are using a two bolt gland and the gland is too thin causing it to distort. You are using a pump seal in a motion seal application.
The product is sticking to the seal face. The product is changing state and becoming a solid. Most products solidify for the following reasons: ● ● ● ●
A change in temperature. A change in pressure. Dilatants will solidify with agitation. As an example: cream becomes butter. Some products solidify when two or more chemicals are mixed together.
The hard face is not flat. ● ● ● ●
Mishandling. Poor packaging. The hard face has been installed backwards and you are running on a non lapped surface. It was shipped out of flat.
THE ELASTOMER. Compression set. The O-ring has changed shape.
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… High heat is almost always the cause unless you are dealing with Kalrez, Chemraz, or a similar material where a certain amount of compression set is normal.
Shrinking, hardening or cracking. ● ●
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High heat. The shelf life was exceeded. This is a big problem with "Buna N" that has a shelf life of only twelve months. Cryogenics will freeze just about any elastomer. Chemical attack normally causes swelling, but in rare cases can harden an elastomer. Oxidizing liquids can attack the carbon that is used to color most elastomers black.
Torn nibbled, or extruded. ● ● ● ●
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Mishandling. Sliding over a rough surface. Forced out of the O-Ring groove by high pressure. The liquid has penetrated the elastomer, vaporizing inside and blowing out pieces. This is a problem with Ethylene Oxide. Halogenated fluids can penetrate the Teflon coating on an elastomer and cause the base material to swell up, splitting the Teflon jacket.
Swelling, changing color, weight or size. Almost always caused by: ● ● ● ● ●
Chemical attack. Be careful of the lubricant used to install the elastomer. Solvents or cleaners used in the system may not be compatible with the elastomer. Some compounds are sensitive to steam. Most Vitons are a good example of this problem. The elastomer is not compatible with something in the fluid you are sealing.
Torn rubber bellows. ● ● ● ●
The bellows did not vulcanize to the shaft because you used the wrong lubricant. The shelf life was exceeded. The seal faces stuck together and the shaft spun inside the bellows. The pump discharge recirculation line was aimed at the rubber bellows. Solids entrained in the high velocity liquid are abrading the bellows.
THE METAL CASE OR BODY OF THE SEAL. Corrosion. ●
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General or overall. This is the easiest to see and predict. The metal has a "sponge like" appearance. It always increases with temperature. Concentrated cell or crevice corrosion. Caused by a difference in concentration of ions, or
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oxygen in stagnant areas causing an electric current to flow. Common around gaskets, set screws, threads, and small crevices. Pitting corrosion. Found in other than stagnant areas. Extremely localized. Chlorides are a common cause. Can be recognized by pits and holes in the metal. Stress corrosion cracking. Threshold values are not known. A combination of chloride, tensile stress, and heat are necessary. Chloride stress corrosion is a serious problem with the 300 series of stainless steels used in industry. This is the reason you should never use stainless steel springs or stainless metal bellows in mechanical seals. Inter granular corrosion. Forms at the grain boundaries. Occurs in stainless steel at 800-1600 F. (412-825 C.), unless it has been stress relieved. A common problem with welded pieces. Stabilizers such as columbium are added to the stainless steel to prevent this. Rapid cooling of the welds, the use of 316L and stress relieving after the welding are the common solutions. Galvanic corrosion. Occurs with dissimilar materials in contact with and connected by an electrical current. Common in brine, caustic, and salt water applications. Erosion / Corrosion. An accelerated attack caused by a combination of corrosion and mechanical wear. Vaporization, liquid turbulence, vane passing syndrome, and suction recirculation are special cases often called cavitation. Solids in the liquid and high velocity increase the problem. Selective leaching. Involves the removal of one or more elements from an alloy. Common with demineralized or de ionized water applications. Micro organisms, that will attack the carbon in active stainless steel.
Rubbing--All around the metal body. ● ● ● ● ● ●
A gasket or fitting is protruding into the stuffing box and rubbing against the seal. The pump discharge recirculation line is aimed at the seal body. The shaft is bending due to the pump operating off of its best efficiency point. Pipe strain. Misalignment between the pump and its driver. A bolted on stuffing box has slipped.
Partial rubbing -- On the metal body. ● ● ● ● ● ● ●
Bent shaft. An unbalanced impeller or rotating assembly. Excessively worn or damaged by corrosion or solids in the product. The product has attached its self to the impeller. The impeller never was balanced. The impeller was trimmed, and not re balanced. The seal is not concentric with the shaft, and is hitting the stuffing box I.D..
Discoloration. Caused by high heat. Stainless steel changes color at various temperatures. FAHRENHEIT
COLOR OF THE METAL
CENTIGRADE
700 - 800
Straw Yellow
370 - 425
900 - 1000
Brown
480 - 540
1100 - 1200
Blue
600 -650
> 1200
Black
> 650
NOTE: To tell the difference between discoloration caused high heat and product attaching to the metal part, try to erase the color with a common pencil eraser. Discoloration will not erase off. Product sticking to the metal surfaces. ● ● ● ● ● ● ● ● ● ●
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Heat is the main cause. The product pressure has dropped. Air or oxygen is getting into the system. Valves above the water line. Through the stuffing box. The product was not deaerated. The pump suction is not completely submerged. The bypass return is too close to the pump suction. The liquid is vortexing in the suction line. A non O-Ring elastomer is being used in the seal allowing air to enter the stuffing box when you are sealing a vacuum application. The system protective oxide coating is depositing on the sliding metal components.
The following applications cause a vacuum to be present in the pump stuffing box. ● ● ● ● ● ●
Pumps that lift liquid. Heater drain pumps. Pumping from an evaporator. Pumping from the hot well of a condenser. Pumps that prime other pumps. The open impeller was adjusted in the wrong direction and the impeller pump out vanes are causing the vacuum.
The Teflon coating is coming off some of the metal parts. ●
Coatings are very porous. They do not provide corrosion resistance. The base material is being attacked by the product.
DRIVE LUGS, PINS, SLOTS, etc. Broken. ● ● ●
Chemical attack. Excessive side load. The seal faces are glued together because the product has solidified.
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A cryogenic fluid is sticking the faces together.
Wear on one side of the drive lug or slot. ● ● ●
Vibration. Slipstick. The stationary is not perpendicular to the shaft.
The drive pins are falling out of the holder. ● ● ● ●
Corrosion. Improper fit. Bad part. Excessive vibration.
THE SPRINGS. Broken or cracked. ●
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The stationary face is not perpendicular to the shaft causing excessive spring flexing in the metal "plastic range". The spring material has "work hardened" and fatigued. Chloride stress corrosion problems with 300 series stainless steel.
Corroded. ●
Stressed material corrodes much faster than unstressed material. The springs are always under severe stress.
Clogged. ●
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Be sure to distinguish between "cause and effect". If the springs are located outside the liquid, it happened after the failure. If the product solidifies or crystallizes it can clog springs exposed to the pumped fluid. Dirt or solids in the fluid can clog exposed springs.
Twisted. ●
Almost always an assembly problem. The lugs were not engaged in the slots. This is a problem with many seal designs. Check to see if your seals can come apart easily or if the drive lugs can change position when the seal is not compressed.
The drive lugs or slots are worn on both sides. ● ●
Excessive vibration. The single spring, rubber bellows seal, was not vulcanized to the shaft.
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The stationary is not perpendicular to the shaft, causing excessive spring movement.
Broken Metal Bellows. ●
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Fatigue caused by over flexing in the plastic range of the metal ❍ Harmonic vibration. ❍ Slipstick. The discharge recirculation line is aimed at the thin bellows plates. Excessive wear from solids in the stuffing box. Faces sticking together as the product solidifies. Chloride stress corrosion with 300 series stainless steel.
Because these seals do not have a dynamic elastomer to provide vibration damping some other means must be provided or vibration will always be a problem. THE SLEEVE, OR SHAFT. Grooves or pits at the seal dynamic elastomer location. ● ● ● ●
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Fretting. Concentrated cell corrosion. The rubber bellows did not vulcanize to the shaft/ sleeve. The set screws slipped on a hardened shaft or were not tightened properly. The seal faces stuck together causing the shaft to rotate inside the static elastomer. Salt water applications are particularly troublesome when a static elastomer or clamp is attached to the shaft. Pitting caused by the chlorides and the low PH of salt water are the main problems.
Rubbing at the wear ring location. ● ● ● ● ● ● ● ● ● ●
The pump is running off of its best efficiency point. The shaft is bending. Bad bearings. Excessive temperature. Sleeve is not concentric with the shaft, or the seal with the sleeve. Bent shaft. Unbalanced impeller or rotating assembly. Pipe strain. Misalignment between the pump and its driver High temperature applications require a "center line: pump design.
Corrosion. See above description under metal corrosion .THE SET SCREWS. ●
Stripped from over tightening.
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Corroded. Check to see if you are using hardened set screws. This type is normally supplied with most cartridge seals and can corrode easily. Rounded Allen Head. Alan wrenches wear rapidly. They are an expendable tool. Loose. ❍ Sleeve too hard. They are not biting in. ❍ Sleeve too soft. They are vibrating loose.
THE GLAND. Rubbing at the I.D. ● ● ● ● ● ● ● ● ● ● ● ● ●
Partial rubbing. The gland has slipped. Improper installation. It was not centered to the shaft. The shaft is bending. Pipe strain. Rubbing all around. The shaft is not concentric with the sleeve. The seal is not concentric with the sleeve. Bad bearings. Bent shaft. Unbalanced impeller or rotating assembly. Solids attached to the shaft, or caught between the shaft, and the gland. Cavitation.
Corrosion. ●
If there is evidence of rubbing the corrosion will be accelerated.
Passages clogged or not connected properly. ●
A.P.I Gland. ❍ Hooked up wrong. ❍ Flushing connection clogged. ❍ Quench connection clogged.
BUSHINGS Rubbing at the I. D. ● ● ● ● ● ●
Partial rubbing. The A.P.I. gland has slipped. Improper installation. It was not centered to the shaft. The shaft is bending. The gland bolt holes are often not concentric with the shaft/ sleeve. Misalignment between the pump and its driver.
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Excessive pipe strain. Rubbing all around. The shaft is not concentric with the sleeve. The seal is not concentric with the sleeve. Bad bearings. Bent shaft. Unbalanced impeller. Cavitation
Erosion. ●
Dirt and solids are present in the discharge or suction recirculating fluid.
Link to Mc Nally home page ●
SUBJECT : Troubleshooting mechanical seals at the pump site 4-2 Leakage can occur at any time throughout the life of the mechanical seal. To troubleshoot seals effectively it is helpful to know just when the leakage starts. This is the advantage of being able to troubleshoot a running pump or one that is still hooked up to its piping. By noting the type of leakage and when the leakage occurs we can do a more thorough job of analyzing any seal failure. In addition to leakage we will be looking for other symptoms that are visible to the trained troubleshooter. We will start with the different types of leakage. Please look at the following diagram.
The leakage occurs while the pump is both running and stopped. The leakage can be detectable visually, by odor, or by instrumentation. A strobe light can sometimes be used to determine its location. As you can see in the above diagram there are several leak paths possible. You must determine which ones you have. The seal can leak : At the lapped faces. Since they are a wearable surface the leak will probably get either better or worse. It should never remain constant. The leak started because: ● ● ● ●
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The outside springs in a dual cartridge seal were painted during routine maintenance. The spring load has been reduced because of thermal growth, axial thrust, or impeller adjustment. The seal was set screwed to a hardened shaft and has vibrated loose. One or both of the seal faces is not flat. Solid tungsten carbide and silicone carbide faces are often lapped flat on only one side. Check to see if the face has been installed backwards. The dynamic elastomer has swollen up and seized the spring loaded face, preventing it from remaining in contact with the stationary face. The product prevented the lapped seal faces from remaining in contact. Dirt has gotten into the sliding components. The product has crystallized.
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The product solidified or became very viscous. The product is vaporizing across the seal faces expanding and blowing them open.
At the static and dynamic elastomer locations. ●
This type of leak tends to remain constant and will often stop when the small opening clogs up with solids. The leak can be caused by a damaged elastomer or damage on the surface where the elastomer seals. In some instances the elastomer is not seated properly. It is twisted because of either poor installation, excessive shaft movement, or high pressure extrusion.
At the gland gasket. ●
This is the easiest leak to detect because it is very visible and does not change with shaft rotation.
Between the shaft sleeve and the shaft. ●
This is a common problem with double ended pumps, where the sleeve is used to position the impeller and there is no method of sealing the sleeve against the impeller.
Between the seal face and its metal holder. ●
The leakage frequently increases, as the product temperature increases, because the metal face holder has an expansion rate three times that of the carbon or hard faces.
Through fretting damage ● ●
The damage is caused by spring loaded dynamic O-rings, Teflon wedges, chevrons, U- cups etc. You can't miss the frett marks. They will be located on the pump shaft, pump sleeve, or inner sleeve of the mechanical seal.
The seal leaks only when the pump is running. ●
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The stationary face has been over tightened against the stuffing box face causing it to go out of flat. Statically the carbon will readjust to the distorted hard face. The clamping is not equal and opposite across the static seal face. Look for different width gaskets at the front and rear of the static face. Again, the carbon will readjust when the shaft is not turning. Between face and holder. The holder heats up and expands faster than the pressed in face. The leak will begin when the metal holder comes up to temperature. Remember that metal expands three times faster than a seal face. Cryogenic (cold) service will harden the elastomer. Be sure to check the lower temperature limit of the elastomer that was selected. Misalignment between the pump and the driver. The shaft is bending and not allowing the seal to move freely. This occurs if the pump is operating off of its best efficiency point and the shaft L3/D4 is not small enough to resist the
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bending. The product is vaporizing across the seal faces. Cavitation, slip stick, harmonic, or some other type of vibration is bouncing the faces open, check the lugs or drive pins for sign of excessive wear. The seal was installed without enough compression or the impeller was adjusted after the seal was installed and thermal expansion of the shaft is opening the faces. A discharge recirculation line is aimed at the seal faces or some other critical point and the faces are being forced open. A non- concentric seal, bad sleeve installation, or an out of balance rotating assembly, is causing the rotating portion of the seal to run off the stationary face. A bent shaft can cause the rotating portion of the seal to run off the stationary face. The rotating portion of the seal is hitting a stationary object. Look for: A protruding gasket or fitting. A foreign object that has worked its way into the stuffing box area. A stationary portion of the rotating equipment, such as a close fitting bushing. At elevated temperature the product thins out (the viscosity decreases) and is leaking through an elastomer. It will not leak at the cooler temperature when the product viscosity is higher. High temperature is causing the lapped seal face to go out of flat.
The seal leaks only when the pump is not running. ●
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The seal is also leaking while running, but the leak is vaporizing and not visible. Hold a piece of white paper over the seal area and see if the paper becomes damp. A meniscus caused by centrifugal force and liquid surface tension had formed at the inside diameter of the seal faces. This prevented a leaking seal from dripping while the shaft was turning. You are using high temperature grade Kalrez. It is too hard at ambient temperature and will soften at operating temperature. The pump is running under vacuum and while it is running air is being pulled into the system. The fluid leaks out when the shaft is static. This can occur if an open impeller that was designed to be adjusted against the volute has accidentally been adjusted backwards against the back plate. The impeller "pump out vanes" can then pull a vacuum in the stuffing box. This is a common problem if you use a lot of Duriron pumps and then bring in a few of another brand.
The leak occurs only at start up and then stops after a short time. ●
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Face distortion. Caused by a high pressure surge that was created when the pump was started with the discharge valve shut. The shaft is bending, and interfering with the seal movement. Occurs because the pump is running with the discharge valve throttled or shut. Operators shut the discharge valve at start up to save electricity and prevent cavitation. The same problem can occur if the pump is started with the discharge valve wide open and because of the lack of discharge resistance the pump will run to the right hand side of its curve. In some cases you could also burn out the electric motor. The product has changed state, and becomes a liquid again when the pump comes up to operating temperature. The product had :
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Crystallized Solidified Became viscous Excessive axial shaft movement at start up. This is a common problem with sleeve bearing equipment.
The seal leaks intermittently or after the pump has run for a fixed period of time Look for reoccurring events that initiate the leakage. They can include: ● ● ● ● ● ●
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Flushing the lines at the end of a batch or season. Alternating pumps in a multiple pump arrangement. An additive is being put into the product. Batch operations are beginning or ending. The cooling water is passing through temperature cycles. The outside ambient temperature has changed dramatically. I ran into a situation where a supplier was oiling the bed of his truck to prevent solids from sticking in the winter and this oil attacked the elastomer in the seal. Hard water is being used as a flush and it is gradually restricting the flush lines or cooling jacket. A filter or strainer is clogged in a flush line. The flushing water pressure drops at certain times of the day because of demand. The boiler or cooling tower is being blown down. There is a control valve in the pump discharge that is causing the pump to occasionally operate too far off of the B.E.P. The stuffing box is cycling between a positive and negative pressure. Vortexing can occur if the pump suction falls too low. This also occurs in mixers and agitators.. You are quenching a high temperature application with water. As the quench water vaporizes it leaves dissolved solids outboard of the seal restricting axial movement as the seal faces wear. The pump is cavitating on a regular or intermittent basis. Here are a few possibilities: The suction level falls too low The tank vent freezes. The velocity is too high on the suction side of the pump. A suction strainer is plugged up. A stuck or broken check valve in the pump suction piping. A temporary loss of discharge head. A booster pump has shut off. A suction eccentric reducer was installed up side down allowing slugs of air into the suction of the pump. The fluid is vortexing in the supply tank. The level is too low for the pump capacity. The pump is lifting liquid and the foot valve is sticking.. The impeller is too close to the cutwater. Air is entering the system through the pump packing. A lower "specific speed" impeller as been substituted. The pump was specified with too low a "suction specific speed" number. The pump is running at a higher speed or a larger impeller was installed after the system heads were calculated.
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In some parallel pump installations, a stronger pump can throttle the weaker one causing shaft deflection. The wrong lubricant was used on the dynamic elastomer, causing it to swell up. Reaction bonded type, Silicone Carbide can crack if the lines were flushed with Caustic solution.
TYPES OF LEAKAGE. The leak rate is changing, It gets better or worse. ●
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This type of leak is usually associated with seal face leakage because the seal face is a wearable surface. The carbon seal face is not flat. The seal face was damaged at the time of assembly. Dirt or solids are imbedded into one of the faces Coke (over heated oil) or some other solid has formed on the seal faces causing them to separate. The rotating face is hung up on the shaft. Outside seal springs have been painted during routine maintenance.
The faces spit liquid. ●
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The product is vaporizing at the faces - check the fluid vapor point. When using balanced seals the stuffing box pressure must be at least one atmosphere higher than the product vapor point. Unbalanced seals require a much higher differential pressure. The rotating face is running off of the stationary face. The stationary was not centered to the shaft - a common problem. The seal is not concentric with the shaft. The rotating assembly is out of balance. The shaft is bent.
Fire hose type leakage. The leak is following shaft rotation. ●
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Product has solidified on the seal face and a piece has broken off. This is usually initiated by a high temperature between the faces. The rotating face is cracked. The hard surfacing, or coating, is lifting off of the rotating face.
Intermittent leakage. ●
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Temperature changes or pressure surges are altering the face flatness within the elastic range of the material. The stuffing box is alternating between vacuum and pressure The movable face is sluggish and not able to follow run out. The product is viscous. The product has started to solidify. The shaft/ sleeve is too large in diameter restricting movement of the seal. Spring loaded, dynamic elastomers such as Teflon wedges, U- cups, Chevrons and spring loaded O-ring designs
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are very sensitive to this problem Dirt or solids are clogging the seal and preventing it from following shaft run out. In a non O-Ring version, the spring load is too high causing the elastomer or Teflon to stick to the shaft. The product is occasionally vaporizing between the faces. There is a leak between the face and the holder that becomes visible only when the unit comes up to operating temperature. A bending, or bent shaft is causing the seal outside diameter to contact the inside diameter of the stuffing box, or some other stationary object. The pump is running with too high or too low a head. Check the pump curve against actual operating conditions. The application is cycling between ambient and cryogenic temperatures causing the elastomer to harden on the cold cycle and the faces to go out of flat.
The seal area is damp. There is no visible leakage. ● ●
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There is a leaking flange or fitting above the seal that is dripping close to the seal location. The product is vaporizing. Hold a clean piece of white paper over the running seal, and check for leakage. The paper will become damp. Any condition that could cause intermittent leakage will cause this problem.
Constant dripping. It gets neither better nor worse. This cannot be a damaged seal face leak because seal faces are a wearable surface and the leak rate would have to change. ● ● ●
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The elastomer is cut or nicked. The shaft/sleeve is damaged at the elastomer location. There is damage in the O-Ring groove. Maybe the O-ring was removed with a sharp metal instrument and this has caused a scratch in the O-ring groove. There is a leak path between the carbon and the holder. Leaking at the cartridge sleeve location. Leaking between the sleeve and the shaft. Leaking between the gland and the stuffing box. Leaking between the stationary face and the seal gland. Seal faces are stuck open. The elastomer has swollen up due to chemical attack by either the product, the flush, what ever is being used to clean the lines, or by the lubricant that was put on the elastomer to help the installation. This attack usually takes place within one week of exposure to the non compatible lubricating fluid.
THE STUFFING BOX AREA IS GETTING HOT. Heat is being generated at the seal faces. Unbalanced seals generate more heat than balanced seals. ● ●
The carbon is being insulated by an elastomer and cannot dissipate the heat. High friction face materials. Two hard faces usually generate more heat than carbon vs. a hard face.
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The faces are running dry. The stuffing box has not been vented. This is especially important in vertical applications. You do not have a barrier fluid between the seals in a dual seal application. You have lost an environmental control. Flushing. Quenching. The cooling jacket is clogged or not functioning for some reason. The discharge or suction recirculation line is clogged. The barrier fluid has stopped circulating in a dual seal application or you are using oil as a barrier fluid. Oil has a low specific heat and poor conductivity, making it a poor choice as a heat transfer medium. If you must use oil as the barrier fluid you may have to forsake convection and go to a forced circulation system or a pumping ring. An A.P.I. type gland has been piped incorrectly Poor conductivity of the hard face. Silicone carbide is better than 99.5 ceramic. There is too much spring load on the seal faces: A wrong installation measurement. The impeller was adjusted after the seal was installed. Any pump impeller that adjusts against the back plate has this problem. Durco pumps are a good example. Excessive axial movement of the shaft. Thermal expansion
A seal component is rubbing the inside diameter of the stuffing box, or against a product that has attached its self to the inside of the stuffing box. ● ● ●
The seal is not concentric with the shaft. The shaft is out of balance. The shaft is bent.
The sleeve, shaft or rotating seal is hitting a stationary component. ● ●
A protruding gasket or fitting. A bushing in the bottom of the stuffing box.
A foreign object is loose in the stuffing box. A suction recirculation line was used to lower stuffing box pressure. The high velocity recirculation is heating up the return line. NOISE IN THE STUFFING BOX. ●
The product is cavitating in the pump. There are five types of cavitation: ❍ Vaporization. ❍ Internal recirculation ❍ The Vane Passing Syndrome ❍ Turbulence ❍ Air ingestion
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A component is rubbing. The bearings are bad. The seal has come loose from the shaft. A foreign object has entered into the stuffing box. The sleeve is hitting an A.P.I. disaster bushing. The seal faces are running dry. They will make a whistling noise. You have hit a critical speed. Coupling misalignment. The noise is coming from the motor or some near by equipment. "Slip stick" at the seal faces.
AUXILIARY EQUIPMENT FAILURE. The convection tank ● ● ● ● ● ● ● ● ●
It is running backwards. The seal faces are off center causing a pumping action across the faces. The inlet and outlet ports are not drilled properly. A cartridge double seal has not been centered properly The pressure or level in the tank changes. One of the seals is leaking. The pressure or level change should tell you which one. Temperature change. No air pocket in the tank. Not convecting. It was installed incorrectly. The minimum and maximum dimensions were ignored.
Flow meter not indicating. ● ● ● ●
Meter broke. Line clogged. The flow is not high enough. The gage graduations are too large.
No flow through the quench and drain connections. ●
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You are piped to the wrong connection. Most glands that have been drilled for a quench connection, have a flush connection also. Valve not open Line clogged
Loss of jacket cooling. The incoming and out going lines are at the same temperature. ● ●
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A layer of calcium has built up on the inside of the cooling jacket. A discharge recirculation line is connected to the stuffing box (it may be hidden inside the insulation). Some one has shut off the cooling water or steam.
VIBRATION. ● ● ● ● ● ● ● ● ● ● ● ● ● ●
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Cavitation. Remember there are five types. The pump is operating off of its best efficiency point. Unbalanced impeller or rotating assembly. Look for wear or product is attached. Bent shaft. Bad bearings. Misalignment between the pump and driver. Pipe strain. Maybe you need a center line design pump Rotating component hitting a stationary component. The pump is running at a critical speed Harmonic vibration induced by nearby equipment. Loose hold down bolts. Pipe hangers are spaced improperly. The mass of the pump base is not five times the mass of the pump, and motor. The base is too narrow. Imaginary lines extended downward thirty degrees to either side of a vertical through the pump shaft must pass through the bottom of the foundation, not the sides. Seal "slip stick" that can occur when pumping non lubricants such as hot water and most solvents.
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SUBJECT : Why Mechanical Seals fail 2-2 A mechanical seal can either wear out or fail. To determine which your seals are doing, look at the wearable face. In most instances this will be the face manufactured from some grade of carbon/ graphite. Since the seal face is the only sacrificial part of the mechanical seal, a worn out seal is identified as one that has no carbon nose piece left at the time it started to leak. A failed seal is identified by the fact that it has substantial carbon remaining at the time it started to leak.
The above illustrations show the difference between a worn out and a new mechanical seal. Most consumers experience seal failure rates in excess of 85%, and for the most part these seal failures are easily correctable. Seal failures fall into only two broad categories, either the seal faces opened, or one of the seal components was damaged by contact, heat or corrosion. Whenever we try to troubleshoot any mechanical seal it is wise to remember that only three things are visible: ● ●
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Evidence of rubbing. Evidence of damage including corrosion, physical damage, or discoloration of one of the seal component materials. Most mechanical seals are constructed of three materials: ❍ Metal parts ❍ A face combination ❍ Some rubber like parts (called elastomers) The product is attaching to a sliding component causing sticking, or coating on the face causing face separation.
Here are some reasons why a mechanical seal face would open: The dynamic elastomer is not free to slide or move on the rotating shaft or sleeve. ● ●
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The shaft is oversize. A tolerance of + 0.000 - 0.002 inches (+ 0,00 - 0,05 mm) would be typical. The shaft finish is too rough. Most seal companies want at least a 32 R.M.S. (0,8 micro meters) surface finish in the area of the dynamic (sliding) elastomer. The fluid we are pumping is causing the elastomer to stick to the shaft. The dynamic O-ring can generate a lot of heat if there is misalignment between the shaft and the stuffing box face. The rapid movement of the elastomer will generate localized heat causing the following to occur at a faster rate:
The product is solidifying (glue and paint will do this) ❍ It is crystallizing (sugar syrup and caustic are good examples) ❍ It is building a coating on the shaft (petroleum products will form varnish or coke at elevated temperatures, or hard water will form a layer of calcium. etc.) Dirt or solids are restricting the elastomer from moving. Chemicals added to treat water or impurities in the water can collect on the seal sliding surfaces A chemical has attacked the elastomer causing it to swell up and restrict the movement of the seal. In some instances a swollen elastomer has been known to open seal faces while the pump was not running in a standby mode. The shaft or sleeve has been hardened and the set screws have slipped. Many sleeves were hard coated to resist packing wear. Stock rooms are full of these sleeves. The seal has lost its compression. ❍ It was installed with the wrong compression. ❍ The impeller was adjusted after the seal was attached to the shaft. This is a very common problem with A.N.S.I. or other back pull out pumps. ❍ A temperature change has altered the location of the seal. Remember that each inch of stainless steel shaft will grow one thousandth of an inch for each one hundred degree Fahrenheit rise in temperature or 0.001"/1"/100°F . Metric grows 0,001 mm/1 mm of shaft for each 50°C rise in temperature. ❍ The open impeller was adjusted to compensate for normal wear. Typical pump specifications allow the impeller and the casing each to wear as much as 0.125 inch (3 mm) and still be adjusted back to the correct pump efficiency. This is important when you realize that the average mechanical seal has a carbon nose that extends only 0.125 inch (3 mm). The springs, spring or bellows are not operating properly. ❍ A single spring has been installed backwards allowing the faces to stay in contact while the shaft or sleeve rotates within the dynamic elastomer or end fitting. ❍ Excessive misalignment is causing rapid flexing of the spring or bellows causing them to fatigue. ❍ The drive lugs have failed and the multiple springs are twisted in their holder. ❍ The product has clogged the springs. ❍ Many times the outside springs of a dual seal have been painted either at the pump company or as part of a normal maintenance routine. ❍
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Something is restricting the free movement of the seal. ●
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The product is viscous. Remember that some products become more viscous with agitation. These products are called dilatants (cream becomes butter with agitation) A recirculation line from the discharge of the pump is aimed at the seal and interfering with its movement. A foreign object is in the stuffing box. A protruding gasket is touching the movable part of the seal
The shaft is being displaced causing the seal to hit something as it rotates or to cause the rotating face to run off of the stationary face.
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The pump is operating off of its best efficiency point (B.E.P.) causing the shaft to bend. The rotating assembly is out of dynamic balance. The shaft is bent. There is misalignment between the motor and the pump. Pipe Strain is twisting the pump stuffing box. Heat causes expansion and that always opens the possibility for rubbing or wear. Cavitation, slip stick, harmonic vibration, bad bearings or some other form of vibration is causing excessive movement of the shaft. The shaft sleeve is not concentric with the shaft causing it to run "off center". The pump designed with sleeve or babbitted bearings and shaft movement is excessive.
The seal face is being distorted by either temperature or pressure. The product is vaporizing between the seal faces causing the faces to blow apart. ●
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If boiler feed water vaporizes it leaves behind all of the chemicals that were added to the water to prevent hardness, adjust PH, soften boiler scale etc.... In cryogenic (cold) applications the vaporizing fluid can freeze any lubricant that might have been placed on the seal faces. This frozen lubricant can damage the carbon/ graphite seal face.
An environmental control has failed. There are many types used with Mechanical Seals, here are a few of the common environmental controls: ● ● ● ● ●
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Flushing is used for cooling and to wash away solids. Quenching is used for temperature control and vapor removal. Barrier fluids are used to keep air away from a fluid and to provide temperature control. Cooling/ heating jackets are used to keep products in a liquid state and at the proper temperature. A suction recirculation line is installed from the bottom of the stuffing box to the suction side of the pump. This is done to remove stuffing box solids in the pumping fluid and to provide cooling to the seal components. A line can be installed from the discharge of the pump to the stuffing box to increase stuffing box pressure whenever you pump a fluid close to its vapor point. It is also wise to install a carbon restriction bushing in the bottom of the stuffing box with a clearance of approximately 0.005" to 0.007" (0,13 mm to 0,018 mm) on the inside diameter. Dual seals can be installed to prevent a pressure drop across the inside seal face and to control the temperature at the seal face.
Unbalanced seals and some split seals can open their lapped faces in vacuum applications. ●
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Those pumps that run under vacuum include: condensate pumps, heater drain pumps, pumps that lift liquid and any pump that takes its suction from a condenser or evaporator. Remember to use O-ring elastomers in vacuum applications as this shape elastomer will seal either vacuum or pressure. The product has built up on one of the seal faces causing the faces to separate. This is a common problem with petroleum products or any product that can build a film on a surface. Since this coating is not dense enough to provide good sealing, it can cause the faces to leak at shutdown.
When a seal face opens it allows solids to penetrate between the lapped surfaces. The solids imbed themselves into the softer carbon/graphite face causing it to act like a grinding wheel. This grinding action will cause severe wear in the hard face. It should be noted that seal face opening accounts for the largest majority of mechanical seal failures. The second major cause of seal failure is when one of the seal components is attacked by the sealing fluid or a chemical being used to clean or flush the lines. Chemical attack is easy to see: ● ●
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The Carbon will appear to have a sponge like appearance Plated materials will have their hard coating peel off when the base material is attacked. This same thing happens when you allow rust to penetrate behind automobile paint and you then notice that the paint is peeling off in sheets. The elastomer will usually swell up and get soft. When an elastomer shrinks and gets hard it is almost always evidence of excessive heat. Prior to failure caused by excessive heat, most elastomers will take a compression set ( the round O-Ring becomes square) Metal components will develop pits and an overall dull appearance. The color of the metal is often an indication of the amount of heat it was subjected to: FAHRENHEIT
COLOR OF THE METAL
CENTIGRADE
700 - 800
Straw Yellow
370 - 425
900 - 1000
Brown
480 - 540
1100 - 1200
Blue
600 -650
> 1200
Black
> 650
Here are a few things to consider when you suspect corrosion is the problem : The corrosion rate of almost all chemicals doubles with each 18 degree Fahrenheit (10 C.) rise in temperature. ● ●
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Be sure to vent vertical pumps. Air trapped in the stuffing box is a good insulator. See if the operator is running the pump with a restricted discharge. In addition to deflecting the shaft it can cause a severe heat rise in the pump. The control valve may be stuck in the throttled position. Try to use a recirculating line from the bottom of the stuffing box to the suction side of the pump. This is practical in almost any application other than when we are pumping a product close to its vapor point and there would be a danger of vaporizing the product in the stuffing box. When ever possible bore out the packing stuffing box or install a large seal chamber in place of the packing stuffing box. This extra room will allow centrifugal force to centrifuge and clean the fluid in the seal chamber as well as provide extra cooling in the seal area. It is normal to dead end the fluid in the stuffing box when a cooling or heating jacket is being used. If a recirculation line is installed in the stuffing box along with the cooling jacket, the jacket will become inoperative because the circulating hot fluid will not be in the stuffing box long enough to be cooled by the jacket.
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Be sure to check that the cooling jacket is functioning. A layer of calcium inside the jacket, can just about stop heat transfer. If the water is too hard in your area, consider condensate as an alternative cooling fluid. More than one stuffing box jacket has frozen in cold weather, be sure to use non freezing cooling fluids at lower temperatures If a convection tank is being used with dual seals make sure it is operating. Every design has limits, make sure you are not exceeding them. Also check that the fluid is flowing from the top of the stuffing box to the convection tank and returning to the bottom of the stuffing box. I have seen many of these applications running backwards. Use only balanced seals. They generate less heat than unbalanced seals. If there is a bypass line installed from the discharge piping to the suction side of the pump, it may be heating up the incoming fluid. Check to see if the cooling jacket has been isolated and drained. This often occurs when a metal bellows seal is used in hot oil applications. An empty cooling jacket will act as an insulation to the stuffing box fluid. Remember that the cooling jacket is also there to cool down the shaft and protect the bearings. Do not disconnect it.
When you look for corrosion be sure to check out any cleaners or solvents that are used to flush out the system or clean the lines. Many grades of Viton® can be attacked by cleaning the lines with steam or caustic. It is important to identify all of the materials used in the seal components. ● ● ● ● ●
Carbon fillers can be attacked by heat and chemicals Plated materials can crack due to differential expansion. Stainless Steel springs can break due to Chloride Stress Corrosion. Hardened set screws can corrode and vibrate loose. Some elastomers can be attacked by steam. Be careful of using petroleum grease on elastomers as some compounds can be attacked by any petroleum product.
Some hard coatings have very little flexibility and will crack with a small differential temperature. Be careful of tungsten carbide with a cobalt binder; nickel binder would be a much better choice. ®DuPont Dow elastomer
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SUBJECT: Why do not good seals wear out? 9-9 We know that a mechanical seal is supposed to run until the carbon wears down, but our experience shows us this never happens with the original equipment seal that came installed in the pump. We buy an expensive new mechanical seal and that one doesn't wear out either. What is wrong? Was the new seal a waste of money? Not really. You are doing something that appears logical, you are trying to solve the seal problem by purchasing a different seal, but that is like trying to get a good paint job on an automobile by buying a good brand of paint. If you wanted to get a good paint job on an automobile you would have to do four things and purchasing a good brand of paint is only one of them. Here are the things you would have to do in no particular order: ●
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Prepare the body. This is the most costly part involving metal repair, rust removal, sanding, masking etc. Buy a good brand of paint. All paint is not the same, and like anything good it will cost more money than many other brands. Apply the paint correctly. This means exactly the right amount of air pressure and a technique that guarantees no drips or runs. It also means a super clean paint room and frequent sanding between primer and finish coats. Needless to say the paint job can be ruined in this step. Take care of the paint after it has been applied. This means that you have to keep the car washed and waxed and garaged in bad weather. It also means frequent touch ups and paying attention to small details.
If you did those four things correctly, how long can a paint job last on an automobile? Obviously for years. Step outside and watch the cars go by and you will see evidence of people that are not doing those four things. In fact it is so rare that when we see an older car that looks good, we stare at it. Getting good seal life involves four steps also. They should be obvious, but let's look at them any way.: ● ● ● ●
Prepare the pump for the seal, that's the body work Purchase a good seal, the good paint. Install the seal correctly, apply the paint correctly Apply the correct environmental control if necessary (and it probably is),washing and waxing.
We will look at each of these subjects in detail and hopefully begin to increase the life of our mechanical seals to the point where most of them wear out. We will be discussing seals for centrifugal pumps in this paper, but the information applies to just about any kind of rotating equipment including mixers and agitators. Prepare the pump for the seal ●
Do an alignment between the pump and driver. Use a laser aligner. A "C or D" frame adapter is
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an even better choice. Dynamically balance the rotating assembly. You can use most vibration analysis equipment to do this. Check with your supplier if you do not have the program. Make sure the shaft is not bent. Rotate it between centers. Avoid shaft sleeves. A solid shaft is less likely to deflect and is much better for a mechanical seal. Reduce pipe strain where ever possible. Use a "center line" design pump if the product temperature is greater than 200°F (100°C). This will reduce some pipe strain problems at the pump. Use pumps with a low ratio. This is extremely important with intermittent service pumps. Use an oversize stuffing box. Avoid tapered designs. Give the seal lots of room. Try to get the stuffing box face as square to the shaft as possible. There are facing tools available to do this. Reduce vibration by any techniques you know or can learn. Do not let the pump cavitate. The seal faces will bounce open and possibly become damaged. Water hammer can occur if power is lost to the pump while it is running. Maybe you can take some preventative action to avoid water hammer problems. Be sure the mass of the pump/motor pedestal is at least five times the mass of the hardware sitting on it. Be sure there are ten diameters of pipe between the pump suction and the first elbow. Be sure the base plate is level and grouted in place. Keep the open impeller adjusted to lessen vibration and internal recirculation problems. Make sure the bearings have the proper amount of lubrication and that water and solids are not penetrating into the bearing cavity. Replace the grease or lip seals with labyrinth or face seals. Avoid discharge recirculation lines connected to the stuffing box. In most instances suction recirculation will be better. If the pump has wear rings, check their clearance. Make sure the wetted parts of the pump are manufactured from corrosion resistant materials. Cleaners and solvents in the lines sometimes cause problems that the designer never anticipated. Seal off any air that might be leaking into the suction side of the pump and remove any that might be trapped in the volute.
Purchase a good seal ● ●
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Use hydraulically balanced designs that seal both pressure and vacuum. If you are going to use an elastomer in the seal try to use an o-ring. They are the best shape for lots of reasons, but don't let any one spring load the o-ring or it will not flex or roll as it should. Use non fretting seal designs. Shaft fretting is a major cause of premature seal failure. Stationary seals (the springs do not rotate with the shaft) are better than rotating seals (the springs rotate) for sealing fugitive emissions and any other fluids. If the seal has small springs keep them out of the fluid or they will clog easily. There are plenty of seal designs that have this non-clogging feature. A wide hard face is excellent for the radial movement we see in mixer applications and those seals that are physically positioned a long way from the bearings. You will need some sort of vibration damping for high temperature metal bellows seals. They lack the elastomer that normally performs that function. Use designs that keep the sealing fluid at the seal outside diameter, or centrifugal force will throw solids into the lapped faces and restrict their movement when the carbon wears.
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Use unfilled carbons for the seal faces. They are the best kinds and the cost is not excessive. Be sure you can identify all of the seal materials. It is impossible to troubleshoot a "mystery material". Do not let the supplier tell you that his material is proprietary. If that is his attitude find another supplier or manufacturer, otherwise you deserve all of the problems you are going to have. Try to keep elastomers away from the seal face. The elastomer is the one part of the seal that is the most sensitive to heat, and the temperature is hottest at the faces. Any dangerous or expensive product should be sealed with dual seals. Be sure the hydraulic balance is in both directions or you are gambling that one of the faces might open in a pressure reversal or surge. If the design has a carbon pressed into a metal holder, be sure the carbon was pressed and not "shrunk in". Pressed carbon will shear to conform to irregularities in the metal holder&emdash; helping to keep the lapped faces flat.
Install the seal correctly ●
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Cartridge seals are the only design that makes sense if you want to make impeller adjustments and they are a lot easier to install because you do not need a print, or take any measurements to get the correct face load. Cartridge dual seals should have a pumping ring built in. Use buffer fluid (lower pressure) between the seals when ever possible to avoid product dilution problems. Avoid any type of oil as a buffer fluid because of oil's low specific heat and poor conductivity. Keep the seal as close to the bearings as possible. There is usually room to move the seal out of the stuffing box and then use the stuffing box area for a support bushing to help stabilize the rotating shaft. Depending upon the application you will have to decide if this support bushing has to be retained axially. Split seals make sense in just about any application that does not require dual seals or fugitive emission sealing (leakage measured in parts per million) Split seals are the only design to use on double ended pumps or otherwise you will have to replace both seals when only one seal has failed. They also allow you to change seals without having to do a re-alignment with the pump driver. Do not lubricate seal faces at installation. Keep solids off the lapped faces. If there is a protective coating on the seal faces be sure to remove it prior to installation Rubber bellows seals require a special lubricant that will cause the bellows to stick to the shaft. It is normally a petroleum based fluid, but you can check with your supplier to be sure. Rubber bellows seals require a shaft finish of no better than 40 RMS, or the rubber will have difficulty sticking to the shaft. In a vertical application, be sure to vent the stuffing box at the seal faces. You may have to install this vent because the pump manufacturer never provided it. Many cartridge seals have a vent built in that you can connect to the pump suction or some other low pressure point in the system.
Take care of the seal ●
The seal would prefer to be sealing a cool, clean, lubricating liquid. We seldom have one of those to seal so maybe you can apply an environmental control in the stuffing box area to change your product into a cool, clean, lubricating liquid:
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●
If you are using a jacketed stuffing box, be sure the jacket is clean. Condensate or steam are the best fluids to circulate through the jacket. Install a carbon bushing in the end of the stuffing box to act as a thermal barrier that will help to stabilize the stuffing box temperature. Flushing is the ultimate environmental control. It causes product dilution, but if you are using the correct seal you won't need much flush. Four or five gallons per hour (notice I said hour not minute) should be enough for that type of seal. Keep the fluid moving in the stuffing box to prevent a build up of heat. Suction recirculation will remove solids that are heavier than the product you are sealing. Since that is the most common slurry condition, use suction recirculation as your standard. Learn where not to use it also. Discharge recirculation will allow you to raise the pressure in the stuffing box to prevent a fluid from vaporizing between the lapped faces. Try not to aim the recirculation line at the lapped faces, it could injure them. If you are using a metal bellows the recirculation line can act as a sand blaster and cut the thin bellows plates. If the product is too hot, cool the stuffing box area, There are lots of ways to do this. Check other sections of the Technical Series for ideas. It is important to remember that these environmental controls are often more important when the pump is stopped because soak temperatures and shut down cooling can change the stuffing box temperature drastically, causing the product to change state. Dangerous products will need an A.P.I. type gland if you elect not to use dual seals. The disaster bushing that is part of the A.P.I. configuration should protect the seal from physical damage if you should lose a bearing when the pump is running. Do not put too much steam or water through the quench connection or it will get into the bearing case. Leakage out the drain connection is often perceived as a seal failure by operators. Be sure they know the difference. Be sure the connections are made correctly. It is easy to mix up the four ports and get the flush or recirculation line into the quench port.
Does any one ever do all of these four things? Unfortunately not. If we did eighty five or ninety percent of our seals would be wearing out rather than the ten or fifteen percent that wear out now. The prematurely failed seal with plenty of carbon face left continues to be the rule. The most common excuse we hear to explain our lack of good seal life is that there is never time to do it right, followed by the cliché, "but there is always time to fix it". Most of us do one or two of the necessary steps and experience an increase in our seal life. There is nothing wrong with an increase in seal life, but that is a long way from wearing out seals. Think about it for a minute. If the seal is lasting a year, how big can the problem be? The temperature cannot be too high or the pressure too severe. If that were true it wouldn't take a year to fail the seal. The product can't be too dirty for the same reason. We often find the problem is as simple as a seal design that is fretting the shaft, causing a leak path through the damaged sleeve or shaft. Other times we find that the flush that is used to clean the lines once a year is the culprit, and no one is changing the seal materials to reflect this threat to the seal components.
CHARTS AND GRAPHS YOU WILL NEED
● ● ● ● ●
●
● ● ● ● ● ● ● ● ● ● ● ● ●
● ● ●
● ●
ANSI pump dimensions, inch sizes ANSI pump dimensions, metric sizes Bearing fit tolerances C/R lip seal life chart Calculating the friction loss in inch size fittings and valves ❍ Fittings, inch size ❍ Valves, inch size Calculating the friction loss in inch size piping ❍ 1.250" and 1.500" pipe ❍ 2.000" and 2.500" pip ❍ 3" pipe ❍ 6" pipe Calculating the friction loss in metric pipe and fittings Carbon physicals Carcinogens list Classifying chemicals for sealing Cross section of a volute pump showing shaft deflection English to metric conversion tables 8-5 Galvanic series of metals Glossary of seal and pump terms Mechanical seal hard face materials NPSH reducton chart O-ring compatibility guide OSHA 1910 regulation Pump types ❍ The A.N.S.I. pump ❍ The close coupled pump ❍ The closed impeller, end suction, centrifugal pump Pump term relationships Seal face flatness readings Vapor Pressure Charts ❍ Vapor pressure chart -60 to 240°F. ❍ Vapor Pressure chart -180 F. to 60 F. Viscosity corrections for centrifugal pumps Water properties
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ANSI PUMP DIMENSIONS INCH SIZE
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ANSI PUMP DIMENSIONS METRIC SIZES
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The Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767
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The Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767
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The Mc Nally Institute
1637 Sand Key Estates Ct. Clearwater, Fl 33767 Link to the Mc Nally home page
TThe Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767 Link to the Mc Nally home page
The Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767
Link to the Mc Nally home page
The Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767
Link to the Mc Nally home page
The Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767 Link to the Mc Nally home page
CALCULATING THE FRICTION LOSS IN METRIC SIZE PIPING SOME NOTES FOR THE METRIC PIPE FRICTION CHART SHOWN BELOW ● ● ● ● ● ●
The chart is calculated for fresh water at 15°C. Use actual bores rather than nominal pipe size. For stainless steel pipe multiply the numbers by 1.1. For steel pipe multiply the numbers by 1.3 For cast iron pipe multiply the numbers by 1.7 The losses are calculated for a fluid viscosity similar to fresh water
THE FOLLOWING CHART WILL LET YOU CALCULATE THE LOSES THROUGH VARIOUS TYPE VALVES AND FITTINGS.
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PURE CARBON COMPANY CARBON GRADE PHYSICALS The following chart will assist you in selecting the correct grade of carbon/graphite for your mechanical seal. With the exception of sealing cryogenics and dry running applications, Pure Carbon company's grade 658RC will probably be your first choice
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A LIST OF KNOWN CARCINOGENS Known carcinogens are defined as "those substances for which the evidence from human studies indicates that there is a casual relationship between the exposure to the substance and human cancer." In the following list I have noted some commercial uses for each of the carcinogens shown. As you would expect, this is a developing subject. Be sure to use dual seals any time you have to pump these chemicals. 4-Aminobiphenyl...... No commercial use in the United States. Was used as a rubber antioxidant and as a reagent for detecting sulfates. Analgesic mixtures containing Phenacetin...... Prescription and over the counter drugs. Arsenic and certain Arsenic compounds...... Pesticides, wood preservatives, alloying additive, glass and nonferrous alloys. Asbestos...... Insulation, gasketing, packing, coatings, plastics, textiles, friction materials. Azathioprine...... Medical use. Benzene ... Solvent, gasoline additive. Benzidine ..... Dyes in textile and paper Bis(chloromethyl)ether and technical grade Chloromethyl Methyl Ether ..... Synthesis of plastic and ion exchanger resins 1,4-Butanediol Dimethylsulfonate (Myleran) ..... Medical use. Chlorambucil ..... Medical use.0Chromium and certain Chromium compounds ..... Stainless steel, pigment, medical, plating, wood treatment, paint. Conjugated Estrogens ..... Medical uses. Cyclophosphamide ..... Medical uses. Diethylstilbestrol ..... Medical uses. Melphalan ..... Medical uses. Methoxsalen with Ultra-violet A Therapy (PUVA) ..... Medical uses.
Mustard Gas ..... Biological studies, weapons. 2-Naphthylamine ..... Dyes, rubber, used only for research purposes. Thorium Dioxide ..... Nuclear, flame spraying, welding electrodes, high temperature ceramics. Vinyl Chloride ..... Plastics, wrapping film, phonograph records, credit cards, floor tiles. In addition to the above chemicals there are substances which may reasonably be anticipated to be carcinogens. Defined as "those for which there is a limited evidence of carcinogenicity in humans or sufficient evidence of carcinogenicity in experimental animals". 2-Acetylaminofluorene ..... Medical uses. Acrylonitrile ..... Synthetic fibers resins, plastics, elastomers . Adriamycin ..... Medical uses. Aflatoxins ..... Research. 2-Aminoanthraquinone ..... Dyes, paints plastics, rubber, printing inks. o-Aminoazotoluene ..... Pigments, coloring oils, wax polishes. 1-Amino-2-methyllanthraquinine ..... Dye for synthetic fibers as well as animal furs. Amitrole ..... Herbicide, now limited to non-crop applications. o-Anisidine Hydrochloride ..... Dyes. Benzotrichloride ..... Plastics, dyes and pigments. Beryllium and certain Beryllium compounds ..... Alloys for aerospace applications, ceramic additive to glass and plastic. Bischloroethyl Nitrousourea ..... Medical uses 1,3-Butadiene ..... Synthetic rubber, tires, nylon carpet backing, latex adhesives. Cadmium and certain Cadmium compounds ..... Coating and plating. Carbon Tetrachloride ..... Production of Freon 11 & 12, degreasing, plastic and resin production. Chlorendic Acid ... Flame retardant, foams.
Chlorinated Parraffins (C12, 60% Chlorine) ..... Lubricant additive, flame retardant, rubber production 1-(2-Chloroethy)-3-cyclohexyl-1-nitrosourea (CCNU) ... Medical uses. Chloroform ..... Production of fluorocarbon, refrigerant, heat transfer medium in fire extinguishers. 3-Chloro-2-methylpropene ..... Fumigant, textile additive, plastics. 4-Chloro-o-phenylenediamine ..... Hair dye, photographic chemicals. C.I. Basic Red 9 Monohydrochloride ..... Dye for textiles, leather printing inks, china clay. p-Cresidine ..... Dyes. Cupferron ..... A reagent to separate tin from zinc and copper and iron from other metals. Dacarbazine ... Medical uses. DDT ..... Insecticide. In the US. it used only under Public Health Service supervision. 2,4-Diaminoanisole Sulfate ..... Fur, acrylic fiber, polyester, wool , cotton and hair dye. 2,4Diaminotoluene ..... Polyurethane, dye. 1,2-Dibromo-3-ch loropropane ..... Soil fumigant. 1,2-Dibromoethane (EDB) ..... Gasoline antiknock additive, pesticide. 1,4-Dichlorobenzene ..... Space deodorant (toilets, rooms) germicide. 3,3'-Dichlorobenzidine and 3,3'-Dichlorobenzidine Dithydrochloride ..... Pigments. 1,2-Dichlorethane ..... Component of leaded fuel, production of vinyl chloride. Dichloromethane (Methylene Chloride) ..... Solvent in paint removers, manufacture of vitamins, degreasing agent. 1,3-Dichloropropene (Technical Grade) ..... Pesticides. Diepoxybutane ..... Curing agent for polymers. Di(2-ethylhexyl)phthalate ..... Used to make poly vinyl chloride.
Diethyl Sulfate ..... Surfactants, dyes, agricultural chemicals. Diglycidyl Resorcinol Ether ..... Liquid epoxy resin. 3,3'-Dimethoxybenzidine ..... Production of azo dyes. 4-Dimethylaminoazobenzene ..... To color polishes and other wax products. 3,3'-Dimethylbenzidine ..... Dye, chlorine test kits. Dimethylcarbamoyl Chloride ..... Dyes, pesticide. 1,1-Dimethylhydrazine ..... Propellant for liquid fuel rockets. Dimethyl Sulfate ..... Used to manufacture other chemicals. Dimethylvinyl Chloride ..... Organic synthesis. 1,4-Dioxane ..... Stabilizer in chlorinated solvents. Direct Black 38 ..... Dye. Direct Blue 6 ..... Dye. Epichlorohydrin ..... Epoxy resins. Estrogens (Not Conjugated): Estradiol-17ß ..... Medical uses. Estrogens (Not Conjugated): Estrone ..... Medical uses. Estrogens (Not Conjugated): Ethinylestradiol ..... Medical uses. Estrogens (Not Conjugated): Mestranol ..... Medical uses. Ethyl Acrylate ..... Paper coatings, emulsion based polymers. Ethylene Oxide ..... Manufacture of ethylene glycol and polyester. Ethylene Thiourea .... Rubber, O-rings, electroplating. Formaldehyde (Gas) ..... Adhesives, chemical production, Medical uses. Hexachlorobenzene ..... Pesticide.
Hexamethylphossphoramide ..... Solvent for polymers, de-icing additive for jet fuels. Hydrazine and Hydrazine Sulfate ..... Agricultural chemicals, rocket fuel, oxygen scavenger in boiler feed water. Hydrazobenzene ..... Dye, additive to motor oil. Iron Dextran Complex ..... Medical uses. Kepone® (Chlordecone) ..... Insecticide, no longer used in the US. Lead Acetate and Lead Phosphate ..... Drier in paints and varnish, colorant in hair dyes. Lindane and other Hexachlorocyclohexane Isomers ..... Insecticidal treatment for wood, grain and live stock. 2-Methylaziridine (Proplyleneimine) ..... Paper, textile, rubber. 4,4'-Methylenebis(2-chloroaniline) (MBOCA) ..... Curing agent. 4,4'-Methylenebis(N,N-dimethyl)benzenamine ..... Dye. 4,4'-Methylenedianiline and its Dihydrochloride ..... Manufacture of polyisocynates and isocyanates. Metronidazole ..... Medical uses. Michler's Ketone ..... Dyes and pigments. Mirex ..... Pesticide, fire retardant. Nickel and certain Nickel compounds ..... Stainless and alloy steel. Nitrilotriacetic Acid ..... Detergent, water treatment. 5-Nitro-o-Anisidine ..... Dye. Nitofen ..... No present commercial use. Was a herbicide. Nitrogen Mustard Hydrochloride ..... Medical uses. 2-Nitropropane ..... Solvent, inks, paints polymers. N-Nitrosodi-n-butylamine ..... Medical uses.
N-Nitrosodiethanolamine ..... No commercial use. N-Nitrosodiethylamine ..... Stabilizer in plastics, gasoline and lubricant additive. N-Nitrosodimethylamine ..... Liquid rocket fuel, solvent. p-Nitrosodiphenylamine ..... Rubber, dye. N-Nitrosodi-n-propylamine ..... No commercial use. Used in cancer research. N-Nitroso-N-ethylurea ..... No commercial use. N-Nitroso-N-methylurea ..... Medical uses N-Nitrosomethylvinylamine ..... Research chemical. N-Nitrosomorpholine ..... No commercial use. N-Nitrosonornicotine ..... Research chemical. N-Nitrosopiperidine ..... Epoxy resin. N-Nitrosopyrrolidine ..... No commercial use. N-Nitrososarcosine .... No commercial use Norethisterone ..... Medical uses 4,4'-Oxydianiline ..... Production of polyimide and poly(ester)mide resins. Oxymetholone ..... Medical uses. Phenacetin ..... Medical uses. PhenazopyridineHydrochloride .. .. Medical uses. Phenoxybenzamine Hydrochloride ..... Medical uses. Phenytoin ..... Medical uses. Polybrominated Biphenyls ..... Flame retardant, plastics. Polychlorinated Biphenyls .... Heat transfer and hydraulic fluids.
Polycyclic Aromatic Hydrocarbones, 15 listings .... Coal tar, roofing, creosote, asphalt ● ● ● ● ● ● ● ● ● ● ● ● ● ●
Benza(a)anthracene Benzo(b)fluoranthene Benzo(j)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Dibenz(a,h)acridine Dibenz(a,j)acridine Dibenz(a,h)anthracene 7H-Dibenzo(c,g)carbazole Dibenzo(a,e)pyrene Dibenzo(a,h)pyrene Dibenzo(a,l)pyrene Indeno(1,2,3-cd)pyrene 5-Methylchrysene
Procarbazine Hydrochloride ..... Medical uses. Progesterone ..... Medical uses. 1,3-Propane Sultone ..... Detergents lathering agents. Propiolactone ..... Medical uses. Propylene Oxide ..... Coatings and adhesives. Propylthiouracil ..... Medical uses. Reserpine ..... Medical uses. Saccharin ..... Sweetening agent. Safrole ..... Flavoring agent. Selenium Sulfide ..... Shampoos. Streptozotocin ..... Medical uses. Sulfallate ..... Herbicide. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) ..... Research chemical. Tetrachloroethylene (Perchloroethylene) ..... Dry cleaning and textile production.
Thioacetamide ..... Replacement for hydrogen sulfide in qualitative analysis. Thiourea ..... Animal glue. Toluene Diisocyanate ..... Polyurethane foam. o-Toluidine and o-Toluidine Hydrochloride ..... Dyes and pigments. Toxaphene ..... Insecticide. 2,4,6-Trichlorophenol ..... Wood preservative, anti mildew. Tris(1-aziridinyl)phosphine Sulfide ..... Medical uses. Tris(2,3-dibromopropyl)phosphate ..... No longer used in the US. Was a flame retardant. Urethane ..... No commercial use because of its toxicity
Occupational exposures associated with a technical process that are known to be carcinogenic ● ●
Coke oven emissions Soots, tars and mineral oils
Delisted Chemicals ● ● ● ●
Aramite® ..... No US. residents exposed. N,N-Bis(2-chloroethy)-2-naphthylamine (Chlornaphazine) ..... No US. residents exposed. Cycasin ..... No US. residents exposed. Methyl Iodine ..... Re-evaluated by IARC. Now considered to be in group 2: no evidence for humans, limited evidence for animals.
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t SUBJECT: Classifying chemicals for mechanical sealing. I. Fluids sensitive to small changes in temperature and or pressure. The temperature change can cause the fluid to: ● ● ● ● ● ●
Vaporize Crystallize Become viscous Solidify Build a film on the seal sliding surfaces Increase the corrosion rate of the fluid. A 10°C. increase can double the corrosion rate of most fluids.
II. Fluids sensitive to agitation (non Newtonian fluids) ● ● ●
Dilatants increase their viscosity with agitation Thixothrophic fluids decrease in viscosity with agitation. Plastic fluids release their viscosity suddenly. Catsup is an example.
III. Fluids that need two seals ● ● ● ● ● ●
Dangerous fluids Pollutants Carcinogens Fluids that emit fugitive emissions Expensive fluids Any time a premature seal failure would cause an expensive down time.
IV. Slurries ●
● ●
You must pump the fluid at a velocity of 4 to 7 ft/sec. (1,2 to 2,3 meters / sec. to prevent a bed of solids in the pipe. Slurries are defined as solids in liquid. The number and size of the solids is not important. Abrasion becomes a problem when you pump at a velocity greater than 10 Ft./sec. (3 Meters/ sec.). The abrasive fluids can: ❍ Clog internal seal parts ❍ Throw the impeller out of balance ❍ Cause frequent open impeller adjustments ❍ Can cause excessive wear of the metal bellows plates
V. Non Lubricating fluids (in order of difficulty to seal) ●
Liquids
● ●
Gases Dry solids
VI. Fluids that react together to form a solid. Often a mixer or blender application ● ●
Polymers Epoxies
VII. Clean lubricating liquids ●
Most finished products fall into this category
VIII. Extreme operating conditions. These are non-chemical, but present seal problems. ●
●
●
●
● ●
●
The fluid is too hot for a seal component ❍ The O-ring or any elastomer is the most sensitive to high temperature. ❍ Some filled carbon have a low temperature limit. The fluid is a cryogenic. It is too cold for a seal component ❍ The O-ring is sensitive to temperature changes ❍ The carbon must be lapped at cryogenic temperature. The pressure in the stuffing box (not the discharge pressure) is too high. It is greater than 400 psi (28 bar) ❍ The load on the seal faces can be too high causing excessive heat. ❍ The elastomer (O-ring) can extrude and become damaged. ❍ The lapped seal faces can deform and go out of flat. ❍ The seal can push off the set screws. Hard vacuum. Less than 1 torr ❍ The elastomer (O-ring) will shrink and leak. High shaft speed. Greater than 5000 fpm. (25 m/sec.) Excessive shaft axial & radial motion ❍ Axial movement causes changes in the seal face loading. Radial movement can separate the lapped faces. Excessive vibration
IX. Not listed in the above categories. ●
●
Polishing liquid ❍ Antimony Peroxide Anaerobic fluids ❍ Super glue
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The Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767
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SUBJECT: CONVERSION TABLES VELOCITY m/s 1
ft/s
m/min ft/min
3.281 60
0.305 1
km/h
196.85 3.6
18.288 60
mile/h 2.2369
1.0973 0.6818
0.017 0.055 1
3.281
0.06
0.0373
0.005 0.017 0.305
1
0.0183 0.01136
0.278 0.911 16.667 54.68
1
0.6214
0.447 1.467 26.822 88
1.6093 1
MASS FLOW RATE kg/s
lb/s
kg/h
lb/h
ton/h
tonne/h
1
2.205
3600 7936.64 3.5431
3.6
0.454
1
1633 3600
1.633
2.78 x10-4
6.12 x10-4 1
2.205
1.607
9.84 x 10-4 0.001
1.026 x10-4 2.78 x10-4 0.454 1
4.46 x 10-4 4.54 x 10-4
0.282
0.622
1016 2240
1
1.016
0.278
0.612
1000 2204.6
0.9842
1
VOLUMETRIC RATE OF FLOW
U.S. L/s
1
L/min
60
m3/h
ft3/h
ft3/m imperial g/min
gal/min
U.S. barrell/ day petroleum
3.6
127.133 2.1189 13.2
15.85
543.439
0.06
2.1189
0.264
9.057
4.403
150.955
0.0167 0.104
0.125
4.275
0.472 28.317 1.6990 60
1
7.480
256.475
0.076 4.546
0.2728 9.6326
0.1605 1
1.201
41.175
0.063 3.785
0.2271 8.0209
0.1337 0.833
1
34.286
0.002 0.110
0.0066 0.2339
0.0039 0.024
0.029
1
0.017 1
0.278 16.667 1 0.008 0.472
0.0353 0.22
35.3147 0.5886 3.666
0.0283 1
6.229
FORCE kilogram force Newton N kilonewton kN
kgf (kilopond kp)
pound force lbf
1
0.001
0.102
0.225
1000
1
101.97
224.81
9.807
0.0098
1
2.205
4.448
0.0044
0.454
1
PRESSURE AND LIQUID HEAD
Newton per square meter N/M2
kilo pascal
or
bar
kPa
pound kilogram force force per per square square centi-meter inch kgf/cm2
Pascal
foot of water
meter of water
millimeter of mercury
inch of mercury
mm Hg
in Mg
lbf/in2
(Pa)
1
0.001
1x105
1.02x105
1.045 x10-4
3.35x10-4
1.02x10-4
0.0075
2.95x10-4
1000
1
0.01
1.02x10-2
0.145
0.335
0.102
7.5
0.295
100000
100
1
1.02
14.5
33.52
10.2
750.1
29.53
98067
98.07
0.981
1
14.22
32.81
10
735.6
28.96
6895
6.895
0.069
0.0703
1
2.31
0.703
51.72
2.036
2984
2.984
0.03
0.0305
0.433
1
0.305
22.42
0.882
9789
9.789
0.0980. 0.1
1.42
3.28
1
73.42
2.891
133.3
0.133
0.0013 0.0014
0.019
0.045
0.014
1
0.039
3386
3.386
0.0338 0.0345
0.491
1.133
0.345
25.4
1
1 Pascal equals 1 newton per square meter (1Pa = 1N/M2) 1 mm Hg is also called 1 "torr" The international standard atmosphere (1 atm) = 101325 pascals or 1.01325 bar. This is equal to 1.03323 kgf/cm2 or 14.6959 lbf/in2 1 millibar = 100 pascal (1mb = 100 Pa ENERGY AND WORK
joule
kilojoule
megajoule
J
kJ
MJ
foot pound force
British thermal unit
kilowatt hour therm
ft lbf
B.t.u.
kWh
1
0.001
1 X 10-6
0.737
9.48 x 10-4
9.48 x 10-9 2.78 x 10-7
1000
1
0.001
737.56
0.9478
9.48 x 10-6 2.78 x 10-4
1 x 106
1000
1
737562
947.82
9.48 x 10-3 0.2778
1.356
1.36 x 10-3 1.36 x 10-6 1
1.28 x 10-3
1.28 x 10-8 3.77 x 10-7
1055.1
1.0551
1
1 x 10-5
2.931 X 10-4
1.05 x 10-3 778.17
1.0551 x 108 105510
105.51
7.78 x 107
100000
1
29.307
3.6 x 106
3.6
2.65 x 106
3412.1
0.03412
1
3600
1 joule = 1 newton meter POWER kilogram force meter per sec.
Watt
foot pound force per second
horsepower
metric horsepower
W
kgf m/s
hp
ft lbf/s
1
0.102
0.00136
0.738
0.0013
9.806
1
0.133
7.233
0.0131
735.5
75
1
542.476
0.9863
1.356
0.138
1.84 x 10-3
1
1.82 x 10-3
1.0139
550.0
1
745.70 76.04
1 watt = 1 joule per sec = 1 newton meter per second VOLUME cubic millimeter
cubic centimeter
mm3
cm3
cubic inch
cubic meter m3
cubic foot
cubic yard
ft3
yd3
in3
1
0.001
1 x 10-9
6.1 x 10-5 3.531 x 10-8 1.3068 x 10-9
1000
1
1 x 10-6
0.061
3.531 x 10-5 1.308 x 10-6
1 x 109
1 x 106
1
61024
35.31
16387
16.39
1.639 x 10-5 1
5.787 x 10-4 2.143 x 10-5
2.832 x 107
2.832 x 104
0.0283
1728
1
0.0370
7.646 X 108
7.646 x 105
0.7646
46656
27
1
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1.308
GALVANIC SERIES OF METALS AND ALLOYS CORRODED END ( ANODIC OR LEAST NOBLE) MAGNESIUM MAGNESIUM ALLOYS ZINC ALUMINUM 5052, 3004, 3003, 1100, 6053 CADMIUM ALUMINUM 2117, 2017, 2024 MILD STEEL (1018), WROUGHT IRON CAST IRON, LOW ALLOY HIGH STRENGTH STEEL CHROME IRON (ACTIVE) STAINLESS STEEL, 430 SERIES (ACTIVE) 302, 303, 321, 347, 410,416, STAINLESS STEEL (ACTIVE) NI - RESIST 316, 317, STAINLESS STEEL (ACTIVE) CARPENTER 20CB-3 STAINLESS (ACTIVE) ALUMINUM BRONZE (CA 687) HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE) LEAD - TIN SOLDERS LEAD TIN INCONEL 600 (ACTIVE) NICKEL (ACTIVE) 60 NI-15 CR (ACTIVE) 80 NI-20 CR (ACTIVE) HASTELLOY B (ACTIVE) BRASSES COPPER (CA102) MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905) SILICONE BRONZE NICKEL SILVER COPPER - NICKEL ALLOY 90-10 COPPER - NICKEL ALLOY 80-20 430 STAINLESS STEEL NICKEL, ALUMINUM, BRONZE (CA 630, 632) MONEL 400, K500 SILVER SOLDER NICKEL (PASSIVE) 60 NI- 15 CR (PASSIVE) INCONEL 600 (PASSIVE) 80 NI- 20 CR (PASSIVE) CHROME IRON (PASSIVE)
302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE) 316, 317, STAINLESS STEEL (PASSIVE) CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825NICKEL - MOLYBDEUM CHROMIUM - IRON ALLOY (PASSIVE) SILVER TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.) GRAPHITE ZIRCONIUM GOLD PLATINUM PROTECTED END (CATHODIC OR MOST NOBLE)
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GLOSSARY OF PUMP AND SEAL TERMS A
B C D
E
F
J
K
L
M N O P
Q
R
G H
S
I
T U V W X-Z CLICK ON THE APPROPRIATE BOX TO FIND THE DEFINITION OF YOUR WORD.
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MECHANICAL SEAL HARD FACE MATERIALS Hard Face Material
Elastic Coeff. Tensile Thermal Thermal Temperature Hardness Modulus Density of Strength Expansion Conductivity Limit E Friction
GN/m2
MN/m2 µm/mK
Watts/m°K
mg/ mm3
°C(a)
vs. Carbon
Gray cast iron 5
100
200
10
45
7.2
200
-
Hastelloy "B"
6
230
1300
18
45
8.9
800
-
M-2 Tool steel 7
200
2000
11
25
8.2
500
-
Niresist
4
100
400
18
15
7.4
500
-
316 Stainless
4
200
600
16
16
8
600
-
440C Stainless 5
200
800
10
25
7.8
600
-
Stellite
7
220
1000
14
15
8.4
1000
-
T/C - Cobalt
8
600
1400
4
100
15
400
0.07
T/C - Nickle
8
600
600
5
90
15
250
0.07
Ceramic 85%
8
200
150
5
12
3.4
1400
0.07
Ceramic 99.5% 8
350
250
7
25
3.9
1700
0.07
SiC Alpha Sintered
9.7
400
250
4
130
3.1
1000
0.02
Sic Reaction Bonded
9.7
400
250
4
150
3.1
1000
0.02
Mohs
(a) Severe oxidation in air, or significant loss of hardness, or changed microstructure.
Hard Face Material Vickers
N/mm2
Watts/m°C
Gm/cc. °C(a)
2000
16
50
1.95
232
2400
365
145
3.08
1372
152
153
2.8
538
Siliconized graphite PE-8148 Reaction Bonded PR9242 Reaction Bonded plus graphite PG9723 Alpha sintered PS-10070
3000
400
130
3.1
1649
PS-10138
3000
407
130
3.1
1649
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NPSH REDUCTION CHART
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CHEMICAL LISTING GUIDE FOR O-RINGS This is an alphabetical listing of most of the common chemicals we find in industry. The chart was made to be used with my technical paper, " Selecting the correct elastomer for your application". You can learn several things from this listing: ● ● ●
The preferred O-ring material for the application. Column #2 If the fluid is soluble in water. Column #3 If the fluid is hazardous. Column #4
Click any of the following headings to go to the right spot on the list: A
B
C
D
E-H
I-L M-O P Q-S T-Z Please remember that your chemical is often a mixture of several chemicals that can react with the Oring differently than any of the single components of the mixture. You should additionally be aware that is very common to clean and flush process lines with a solvent or steam. The O-ring you select must be chemically compatible with these cleaners and solvents. The last thing to remember is that each O-ring has an upper and lower temperature limit. Do not exceed these numbers.
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SUBJECT: O.S.H.A. 1910 REGULATION The regulation is predictably vague, and presently only applies to pressure vessels, storage tanks, processing piping, relief and vent systems, fire protection system components, emergency shut down systems, alarms, interlocks and the part that is important to you, pumps . For the first time Washington is telling the pump user that he has to now document the training he provides to those people (including contractors) that will be operating or repairing his pumps. Be sure to pick up a copy of this regulation for your library. Here are some of the ingredients you will find in the regulation : ●
●
●
●
● ● ●
●
●
●
●
●
●
The chemicals in the O.S.H.A. # 1910 specification are different than those chemicals identified by the E.P.A. for fugitive emission consideration. The O.S.H.A. list identifies those chemicals that are considered "extremely hazardous" chemicals. O.S.H.A. feels that the general industry standards are not sufficient for these chemicals Your employer is going to have to create a Process Safety Management audit team (PSM) that will audit company training programs along with insuring that present and future engineering practices conform to accepted standards and codes. The employer is going to have to identify the codes and standards he relied upon to establish his engineering practices. If he departs from these codes and standards, he must document that the design and construction are suitable for the intended purpose. The written training programs must be reviewed for adequacy of content, frequency of training, and the effectiveness of the training in terms of goals and objectives. These training programs must be revised if after the training the employee is not at the level of skill or knowledge that was expected. Contract employees must also receive updated and current training. If an accident occurs, the plant is going to have to prove that their training program was adequate. Any mechanical changes made by the maintenance department have to be evaluated to determine whether operating procedures and practices also need to be changed. The term "Change" includes all modifications to equipment. For existing processes that have been shut down for turnaround or modification, the employer must ensure that any changes other than "replacement in kind" made to the process during shutdown go through the management of change procedures. Equipment installation jobs need to be properly inspected in the field for use of proper materials and procedures to insure that qualified workers do the job. The employer must ensure that the contractor has the appropriate job skills, knowledge, and certification. The regulations require detailed records of every action taken in maintaining or rebuilding a pump. The employer must identify which procedures were followed and why he elected to use those procedures. He must also identify the training that maintenance personnel had on repairing pumps in that service. Equipment used to process, store or handle hazardous chemicals has to be designed, constructed, installed and maintained to minimize the risk of release of such chemicals. The employer must prepare three lines of defense to prevent hazardous chemical from injuring personnel: ❍ Contain the chemical in the equipment. The use of two mechanical seals and a convection
❍
❍
tank is a good example of containing the chemical. Control the release of the chemicals through venting with a seal quench and vent connection to a scrubber or flare, or to surge or overflow tanks designed to receive such chemicals. Dikes or designed drainage systems would be another alternative. A sensible evacuation system is the third line of defense.
If an accident happens and any of the listed chemicals are released to the environment, the employer is going to have to prove he did every thing he could have to prevent the accident and contain the spill. If O.S.H.A. does not agree with his assessment, the employer is likely to suffer stiff penalties. Since you have knowledge that 90% of mechanical seals are failing prematurely (the carbon sacrificial face is not wearing out) I expect this new regulation should encourage your employer to send more people to seal and pump schools and enroll his engineering, maintenance, and supervisory people in an appropriate certification training program.
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The Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767
The Mc Nally Institute 1637 Sand Key Estates Ct. Clearwater, Fl 33767 Link to the Mc Nally home page
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THE RELATIONSHIP BETWEEN HEAD CAPACITY, HORSEPOWER AND EFFICIENCY For information on how to use this chart, click here
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SEAL FACE FLATNESS READINGS The following chart describes the patterns you could observe if you looked through an optical flat at a lapped mechanical seal face that was illuminated by a monochromatic light source. For an explanation of how these patterns were formed you can read the paper Reading seal face flatness 6-3 on another page in this web site.
I have left the white background to assist you in seeing the patterns a bit easier. 1 Helium light band 2 Helium light bands 3 Helium light bands 4 Helium light bands 0.0000116"
0.0000232"
0.0000348"
0.0000464"
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PROPERTIES OF WATER
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SOME HANDY LINKS TO OTHER WEB PAGES
BEARINGS ● ● ● ● ● ● ● ●
Bearing fits Bearing tolerances Carborundum SKF Bearings Timken Bearings NASA non-lubricated foil bearings NASA Self lubricating composite coatings Foil Air Bearings, Jennifer Hooper paper
CHEMICALS AND ELEMENTS ● ● ● ● ●
Abbreviations of chemical compounds Chemical resistant chart Metals and powdered metals Periodic table of the elements, with details University of Akron Hazardous chemical database
CHEMICAL AND ENGINEERING BASICS ● ● ● ● ● ● ● ● ● ●
Cavitation University of Texas Density and specific gravity NPSHA tutorial Specific Speed of Pumps Specific Speed calculator Suction Specific Speed calculator Affinity Law Calculator Hydraulic Power Calculator Calculate the tip speed of an impeller Horsepower vs. Torque
●
COUPLINGS ● ●
Frontline Lovejoy
●
Rexnord
ENGINEERING SOCIETIES ● ● ● ● ● ● ● ● ●
National Academy of Engineering National Society Of Professional Engineers Association For Computing Machinery American Institute Of Chemical Engineers American Society Of Mechanical Engineers American Society For Testing And Materials American Nuclear Society ASM International Society Of Women Engineers
ELASTOMERS AND O-RINGS ● ● ● ● ●
Green Tweed elastomers O-Ring information including sizes O-ring dimensions Static O-ring gland design Kalrez o-ring technical information
GOVERNMENT AGENCIES ● ● ● ● ● ● ● ●
BSI (British Standard Institute) EPA (Environmental protection agency home page) NSF National Science Foundation NIST Nat'l Institute of Standards and Technology NTIS National Technical Information Service OSHA home page Smithsonian Institution US Federal Government Agencies Comprehensive list
INSTRUCTION ● ● ● ● ● ● ●
Applets to make pump calculations Centrifugal pumping system tutorial Cooling towers. All you need to know. Fluid design centrifugal pump tutorial Irrigation pumping systems. The basics PD pump instruction Pump system glossary from Fluid Design
● ●
Pump types Different types explained Specific Speed primer
LEGISLATION THAT AFFECTS PUMPS AND SEALS ●
EPA clean air act
METAL ● ● ●
Hafnium Metals index Stainless steel data base
MISCELLANEOUS SUBJECTS ● ● ● ● ● ● ● ●
Currency converter Demonstrated life of pump component from pump practices Dial indicator alignment procedures Dictionaries 281 available Government statistics Maintenance information from MAXIMO Pump system formulas in Metric ZIP codes
PUMP COMPANIES ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
Pump Web a link to all pump company home pages An explanation of pump types British Pump makers Association Members Bell and Gossett Blackmer Carver Cat Discflo Eddy Pump Flowserve Fybroc Gorman-Rupp Goulds pump Grundfos Hayward Gordon Jabsco
● ● ● ● ● ● ● ●
LaBour/Taber Moyno Peerless Roper Seepex Sims Viking Wilfley
PUMP AND SEAL ANCILLARY EQUIPMENT ● ●
Presay Pneumatic seal Seal Master Pneumatic seal
SEAL AND PUMP STANDARDS ● ● ● ● ● ●
A.P.I (American petroleum Institute) AIP (Australian Institute Of Petroleum) ISO standards ISO 9000 in plain english Pump Station Standards Hawaii well construction and pump installation standards
SEAL COMPANIES ● ● ● ●
Chesterton USA Flowserve (Durametallic seals) Inpro John Crane USA
SEAL FACE MATERIALS ● ●
Carborundum Morgan advanced materials
TECHNOLOGY APPLICABLE TO PUMPS AND SEALS ● ● ●
Heat pipe (NASA) technology What is a heat pipe? Piezoelectric ceramics f.a.q.
TROUBLESHOOTING
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BEARING FIT TOLERANCE CHART Shaft Fits (k5) Below
Go To Housing Fits (H6)
Shaft Fits (k5) BASIC NUMBER
NOM. BORE (mm)
SHAFT ROTATING DIAMETER MAXIMUM MINIMIM (inches) (inches)
03
17
0.6697
0.6693
04
20
0.7878
0.7875
05
25
0.9847
0.9844
06
30
1.1815
1.1812
07
35
1.3785
1.3781
08
40
1.5753
1.5749
09
45
1.7722
1.7718
10
50
1.9690
1.9686
11
55
2.1660
2.1655
12
60
2.3628
2.3623
13
65
2.5597
2.5592
14
70
2.7565
2.7560
15
75
2.9534
2.9529
16
80
3.1502
3.1497
17
85
3.3472
3.3466
18
90
3.5440
3.5434
19
95
3.7409
3.7403
20
100
3.9377
3.9371
21
105
4.1346
4.1340
22
110
4.3314
4.3308
. HOUSING FITS (H6) BASIC NUMBER
NOM. STATIONARY HOUSING BORE OD MIN. (in.) MAX. (in.) 200 Series (mm)
NOM. STATIONARY HOUSING BORE OD MIN. (in.) MAX. (in.) 300 Series (mm)
03
40
1.5748
1.5754
47
1.8504
1.8510
04
47
1.8504
1.8510
52
2.0472
2.0479
05
52
2.0472
2.0479
62
2.4409
2.4416
06
62
2.4409
2.4416
72
2.8346
2.8353
07
72
2.8346
2.8353
80
3.1496
3.1503
08
80
3.1496
3.1503
90
3.5433
3.5442
09
85
3.3465
3.3474
100
3.9370
3.9379
10
90
3.5433
3.5442
110
4.3307
4.3316
11
100
3.9370
3.9379
120
4.7244
4.7253
12
110
4.3307
4.3316
130
5.1181
5.1191
13
120
4.7244
4.7253
140
5.5118
5.5128
14
125
4.9213
4.9223
150
5.9055
5.9065
15
130
5.1181
5.1191
160
6.2992
6.3002
16
140
5.5118
5.5128
170
6.6929
6.6939
17
150
5.9055
5.9065
180
7.0866
7.0876
18
160
6.2992
6.3002
190
7.4803
7.4814
19
170
6.6929
6.6939
200
7.8740
7.8751
20
180
7.0866
7.0876
215
8.4646
8.4657
21
190
7.4803
7.4814
225
8.8583
8.8594
22
200
7.8740
7.8751
240
9.4488
9.4499
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Web Site Technical Tables by Copyright 1999, all rights reserved. Necessary Disclaimer Last Revision: 10/15/99
TECHNICAL DATA TABLE OF CONTENTS SUFFIXES TORRINGTON SPHERICAL ROLLER BEARING SUFFIXES INTERNAL CLEARANCES - DEFINITION RADIAL INTERNAL CLEARANCES Single Row Deep Groove Ball Bearings - Cylindrical Bore Single Row Deep Groove Ball Bearings - Tapered Bore (K) Self-Aligning Ball Bearings - Cylindrical Bore Self-Aligning Ball Bearings - Tapered Bore (K) Cylindrical Roller Bearings - Cylindrical Bore Cylindrical Roller Bearings -Tapered Bore (K) Spherical Roller Bearings - Cylindrical Bore Spherical Roller Bearings - Tapered Bore (K) Spherical Roller Bearings - Cylindrical Bore -F80 Spherical Roller Bearings - Tapered Bore (K) -F80 AXIAL INTERNAL CLEARANCES Four-Point Ball Bearings Angular Contact Ball Bearings - Double Row (5200 & 5300 Series) Angular Contact Ball Bearings - Double Row - Split Inner Race BEARING TOLERANCES (METRIC AND INCH) - DEFINITION Oil Seals (Metric) Radial Bearings (Metric - Except Tapered Roller Bearings) Inner Ring Radial Bearings (Metric - Except Tapered Roller Bearings) Outer Ring Tapered Roller Bearings (Metric) Inner Ring Tapered Roller Bearings (Metric) Outer Ring
Thrust Ball and Roller Bearings (Metric) Radial Bearings (Inch- Except Tapered Roller Bearings) 1600 Series Radial Bearings - Inch Thrust Ball Bearings - Inch Roller Thrust Bearings - Inch INCH - MILLIMETER CONVERSION TABLES Millimeters to Inches Fractions to Decimals General Conversions
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SiC Ceramic Materials for Design of HighPerformance Applications Saint-Gobain Ceramics offers a broad range of ceramic materials to meet ● English your exact needs for a wide variety of applications. With ● •体中文 decades of experience, we can supply these materials Navigation as finished components, ❍ unfinished, machined History of SiC blanks, pressed near net ❍ shape blanks, high volume Hexoloy Difference pressed net shape ❍ components, and complex, Product highly machined parts. Applications Hexoloy® SA Silicon ❍ Carbide – high hardness, Hexoloy Grades compressive strength and ❍ Physical Properties light weight ❍
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Creating a Turbomachinery Revolution Research at Glenn Enables an Oil-Free Turbine Engine Image right: View of foil air bearing showing the nonrotating bearing housing, top foil, compliant support structure (bump foil) and rotating journal shaft. Credit: NASA Everyone assumes that engines need oil for lubrication. But that has not stopped researchers at the NASA Glenn Research Center from defying traditional concepts of lubrication. At NASA's lead center for turbomachinery, work is progressing on a revolution in the use of oilfree bearings that will eliminate the need for an oil-lubrication system in high-temperature, high-speed rotating machinery. The foremost challenge for this technology is the design of an oil-free turbine engine to power 21st century aircraft. Bearings are used between the rotating and stationary parts of turbomachinery. Ball and roller bearings are commonly used in turbine engines. These bearings require an oil lubrication system that provides a thin film of oil between the moving parts of the bearing. Without oil, the metal-to-metal contact would cause the engine to grind to a halt. Use of oil-free bearings removes the need for the oil system thus reducing weight, maintenance, and complexity of the engine. Studies have shown a potential reduction of 15 percent in engine weight. Oil-free operation is attained through the use of foil air bearings. These bearings support a rotating shaft with two main elements: a top foil and a bump foil. The top foil provides a scuff-resistant start/stop surface and captures air to support the bearing. The bump foil provides support for the top foil and can be tailored for the required stiffness of the bearing. This technology has been used for a number of years in low-temperature applications from magnetic disk drive read/write heads in PC's to the air cycle machines that maintain the pressure in commercial aircraft cabins. Recent advances in bearing design, high-temperature surface coatings and computer modeling are enabling new applications of these bearings. Riding on Air Foil air bearings are self-acting hydrodynamic bearings that use ambient air as the lubricant instead of oil. Both air and oil are fluids that can perform the job of separating moving metal parts. The very high rotational speeds of turbine engine shafts (>10000 rpm) allow the replacement of oil with air. At these speeds, foil air bearings maintain the air film between moving parts by actually pumping air between the rotating and stationary surfaces. Air is drawn in and sticks to the bearing surface, no matter how fast it is moving. Image right: Diagram showing load-carrying capacity of early and current designs of the bump foil. Credit: NASA The pumping of air between the moving surfaces creates an air pressure that generates a load-carrying capacity. The trapped air and its cushioning effect act just like the air in a car shock absorber. These bearings provide a stiff, shock-tolerant support for rotating machinery. At
high speeds, a very thin layer of air, just under 0.001-in. thick, can support hundreds of pounds. The weight that a bearing can support is represented by its load capacity. For a foil air bearing, the maximum load capacity is dependent on the design of the top and bump foils, diameter, and length and speed of the rotating shaft. A faster shaft speed increases the load capacity by pumping more air between the top foil and the shaft. Larger bearing size also increases load capacity by providing a more bearing supporting area. The foil design has a different effect. It determines how quickly the load increases with speed for a particular bearing. Early designs used a uniform corrugated bump foil (GEN I). Air leakage at the edges of the bearing caused it to have a relatively low maximum load capacity. Current designs (GEN III) are more intricate with multiple slits in the foil and changes in the number or spacing of bumps across the foil. These design features are better at trapping and maintaining the air film between the shaft and top foil. These refinements have resulted in great improvements in the weight that can be supported by foil air bearings. Current designs are capable of supporting a pound of load for each inch of bearing diameter and square inch of bearing projected area for every 1000-shaft rpm. Designers must tailor features of the bearing for a specific application. The stiffness of the backing bump foil is varied to maintain the thin compressed film of air and reduce air leakage out the sides of the foil. This will set the load-carrying capacity of the bearing. Also, high rotational speeds can make the shaft vibrate much like having an out of balance tire. These vibrating modes can be predicted with a rotordynamic model. This computer model is used to predict the forces on the bearing and the motion of the overall shaft-bearing system. With changes in the shaft-bearing diameter, length, and thickness and the location of the bearing on the shaft, these vibration modes can be eliminated. Computer models let designers try out the design without going to the expense of building and testing actual hardware. Traditional ball and roller bearings have rotational speed limits. At high speeds, the balls and rollers are forced away from the shaft and place large stresses on the bearing. Foil bearings do not have this limitation as there are no rotating parts of the bearing except the shaft. Image left: The PS304 coating is plasma sprayed on the bearing shaft. Credit: NASA Hot and Fast
limit from about 400 °F to over 1200 °F.
Until recently, foil air bearings were relegated to applications under approximately 400 °F. One obstacle was the need for a high-temperature coating on the shaft. During startup and shutdown, the bearing and shaft make rubbing contact until sufficient rotational speeds pump air into the bearings. To reduce wear, a dry lubricant coating is applied to the foil and shaft. Most current foil-bearing applications use a polymer coating such as Teflon® that is limited to about 400 °F. NASA Glenn Research Center pioneered new surface coatings that have pushed this upper temperature
These new coatings are applied by plasma spraying the shaft and then finish grinding it to specification. This results in a hard, wear-resistant coating that meets the life and temperature requirements of the bearing. With the advent of new bearing designs, foil air bearings are now ready for higher temperature environments. To validate these new foil-bearing capabilities, NASA set out to demonstrate the use of foil air bearings. A 150-hp turbocharger was modified to operate using foil air bearings. The turbocharger was run with a 1200 ° F turbine inlet temperature and shaft rotational speeds in excess of 60 000 rpm. This oil-free turbocharger was able to perform over 100 000 start/stop cycles without a failure.
Foil bearings now excel where high temperature and high rotational speed bearings are needed. The turbocharger test showed the bearings are candidates for operation in the hot section of turbine engines without the need for cooling. Image right: High-temperature, high-speed foil bearing test rig at 1400 °F. Credit: NASA Spinning It Up NASA Glenn Research Center has developed a patented selflubricating bearing material (U.S. Patent #5866518) capable of a wide temperature range of -250 to 1650 °F. The PS300 coatings and PM300 powder metallurgy system is a series of self-lubricating composites with a duplex microstructure consisting of a hard nickel-chrome/chrome oxide phase with a soft noble metal and stable fluoride phases. The composition of the bearing material acts as a system in providing lubrication over a wide temperature range. The ratio of hard to soft phases can be tailored depending on design requirements such as conformability and desired thermal expansion coefficient and hardness. Glenn has also licensed the coatings for use in powder metallurgy bearing products. Leveraging NASA/Industry Successes NASA Glenn Research Center is working with Williams International to leverage the success of the recently completed NASA General Aviation Propulsion (GAP) Project. This project resulted in the development of the Williams FJX-2 turbofan engine. The FJX-2 is a 700-lb thrust engine that weighs under 100 lb with a 14in. inlet and only 41-in. long. A derivative of this revolutionary engine, the EJ-22, is being FAA certified and will be used on the twin engine Eclipse 500 jet. NASA is currently working with Williams and Mohawk Innovative Technology (a foil bearing manufacturer) to make a future derivative of the EJ-22 the first oil-free turbine engine by using foil air bearings. Removing the oil system components will reduce weight and maintenance, resulting in a lower cost engine that will enable the production of lower cost aircraft. Image right: Cross-section of PS304 coating. Credit: NASA Spinning It Off Turbocharger companies are currently considering using foil bearings, with the NASA Glenn high-temperature coatings, for possible use in automobiles, trucks, and aircraft engines. This new foil-bearing technology is also having an effect on the power-generation industry. Many onsite gaspowered generators use turbine engines to run the electrical generator. Recent events have generated and increased demand for these independent power sources. While large turbine-based systems rely on traditional oil-lubricated bearing technology, two companies have developed small oil-free turboalternators. These small gas-powered turbine generators are already using foil air bearings in their turbines. Research at NASA Glenn Research Center in developing high-temperature coatings can have a positive effect on these systems and is under consideration. The electrical power output will get a boost from the higher turbine operating temperature of the foil bearings. NASA Glenn Research Center is a world leader in propulsion and power technology. Research is focused on aerospace industry with spinoffs into other areas. Glenn's leadership role is pushing the frontier of new technology and its application to products that have a positive effect on the world. This work on oil-free turbomachinery typifies Glenn's efforts and will enable a turbomachinery revolution.
Image above: Oil-free foil air bearing shock tolerance demonstrator rig. Credit: NASA
FS-2001-07-014-GRC [Printer-friendly PDF version] For more information, visit NASA Glenn's home page at: http://www.grc.nasa.gov/WWW/oilfree/ Or contact Information and Publications Office NASA Glenn Research Center 21000 Brookpark Road, MS 8-1 Cleveland, Ohio 44135 (216) 433-5573
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Editor: David DeFelice NASA Official: Brian Dunbar Last Updated: March 17, 2006 + Contact Glenn + SiteMap
