Mechanical seals, rules of thumb
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SUBJECT : A few rules of thumb for mechanical seals 2-5
Before selecting your mechanical seal design there are three things you want to remember:
All of the seal materials must be chemically compa tible with any fluids that will be pumped through the system and that includes solvents, cleaners o r steam that might be introduced into the system to flush or clean the lines. It also includes any barrier fluids that are used to circulate be tween dual mechanical seals. The seal faces must stay together. If they open the seal will leak and an d allow solids to penetrate between the faces where the solids will eventually de stroy the lapped surfaces. Good seal life is defined as running the mechanical seal until the carbon face is worn away. Any other condition is called a seal failure and is always correctable
The following is offered as a guide when dealing with mechanical seals in general. If possible you should contact the manufacturer for specific recommendations and limits. I have spent the past twenty seven years lecturing about seals and pumps and during that time have picked up a number of rules that are worth remembering. Here are some of the most important:
Selecting materials - The elastomer ( the rubber part) There are two temperature limits for a mechanical seal: You must not exceed the temperature of the seal components. As an example Ethylene Propylene rubber cannot seal hot fluids in excess of 300° degrees Fahrenheit ( 150° C) without taking a compression set and eventually leaking. You must not exceed the temperature limit of the fluid you are pumping. Many fluids will change from a liquid to a gas, solid or crystal at elevated temperature. tempe rature. In almost every case this will cause a seal failure. As an example, petroleum lubricating oil cokes between 250 and 300 degrees Fahrenheit (120° C. to 150° C.) and restricts the movement of the seal components. A Viton® O-ring, in this application would not have been subjected to its temperature limit, but we had the seal failure because we exceeded the temperature limit of petroleum products. Halogens will attack Teflon® coated elastomers . Halogens are easily identified because they end in the letters " INE". The list would include Bromine, Chlorine. Astatine, Fluorine, and Iodine. The se Halogens will penetrate the Teflon® coating c oating and attack the base rubber material causing it to swell and split the Teflon sleeve or coating. Most Viton® compounds are attacked by water. Be sure to check if you have the correct one. Remember that steam is another name for water and the steam cleaning of o f lines is very common in the process industry. Caustic is another common cleaner and caustic contains a high percentage of water also. Buna "N" (Nitrile) is an elastomer that has a short shelf life. This is the elastomer that is most often used in Rubber Bellows Seals. The problem is Ozone attack. Ozone is produced by the sparking from electric motors, so it is a very common problem. A typical shelf life for most Buna compoun ds would be one year. If a round O-Ring becomes square in operation (compression set) it is almost always caused by excessive heat. Chemical attack is usually recognized by a swollen and soft elastomer while high heat will produce a shrunken, hard one. Chemical attack of the elastomer will usually cause a seal failure within five to ten days. day s. The swollen elastomer will "lock up" the mechanical seal and in some instances, open the lapped seal faces.
Determine the correct O-Ring by one of the following methods:
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Mechanical seals, rules of thumb
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Look up the chemical in published O-Ring charts provided by all reputable seal companies. You will find a chart in the chart ch art section of this web site Check to see if the plant has any experience with O-Rings, in this fluid, in another seal application. ORings can also be found in filters, strainers, valves, flanges, expansion joints etc.. Test the O-Ring by immersing it into the sealing fluid for one week. If the O-Ring changes weight, shape, or appearance, it is not compatible with the fluid. Use a universal O-Ring compound such as Green Tweed's Chemraz, Dupont's Kalrez® or a similar product. When choosing an O-Ring, or any other elastomer, be sure to consider c onsider any cleaners or solvents that might be flushed through the lines or that could come into contact contac t with the seal. The elastomer must be compatible with these fluids also. Never use " glued together" elastomers in a split seal or any "dynamic" application. A hard spot will be created that will interfere with the movement of the dynamic elastomer.
Selecting Materials - The Faces.
Carbon and most hard face materials have an expansion rate of o f about one third that of stainless steel. Use two hard faces if the product has a tendency to solidify between the seal faces. Never use plated or coated hard faces in these applications. Hard faces are recommended if you find that it is impossible to keep the seal faces together and solids are present in the sealing liquid. Two hard faces are also recommended in the sealing of hydrocarbons that have to pass a "fugitive emissions" test. Coke particles forming between the faces will pull pieces of carbon out of the carbon/graphite face presenting a leak path for fugitive emissions. Although many carbon graphite compounds are available unfilled carbons are the best because they are corrosion resistant to almost all chemicals except oxidizing a gents and some de ionized water applications. These oxidizing agents will combine with the carbon to form Carbon Monoxide and Carbon Dioxide. The most common oxidizers are oleum, sulfur trioxide, strong bleaches and nitric Acid. You cannot use any form of carbon in these applications. Keep in mind that black elastomers will also be attacked by oxidizing agents because of their carbon content. Ceramic vs. ceramic is a good choice for oxidizing chemicals. If you are going to select plated Tungsten Carbide as a face material, use only the nickel base Tungsten Carbide. Cobalt base is too hard and can crack with normal seal face differential temperatures. Nickel base, because of its superior corrosion resistance is the preferred material for solid Tungsten Carbide faces also. Reaction bonded Silicone Carbide has excellent wear characteristics, but contains up to 17% free silica which can be attacked by many chemicals including caustic. Alpha sintered Silicone Carbide is also available and is Silica free. 85% ceramic should never be recommended as a hard seal face as it can break with as little as a 100 degree Fahrenheit (55 C) temperature difference. 99.5% would be a much better choice. Plating or coating a seal face will not give it corrosion resistance. Coatings are used for wear resistance and low friction. To get corrosion resistance the outer coating must be at least 1/8" (3 mm) thick. If the base material is not corrosion resistant to the pumping fluid and any cleaners clea ners or solvents used in the lines the corrosive will go through the coating and attack the base, causing the plating to come off in sheets.
Selecting Materials - The Metal Parts. Pa rts.
Be sure to use low expansion expa nsion metal such as Carpenter 42 or Invar 36 in your metal bellows seal face holder if the product temperature can exceed 400° Fahrenheit (205°C). These low expansion steels
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will prevent the carbon or hard seal faces from leaking between the face and the metal holder. Needless to say glue or epoxy is not a sensible solution to differential expansion problems. If your pump is manufactured from Iron, steel, stainless steel, or bronze, you can probably use a seal manufactured from 316 stainless steel components. The springs or bellows, however, must be manufactured from Hastelloy "C" to avoid problems p roblems with Chloride Stress Corrosion.
Sealing Limits
Use only stationary mechanical seals (the springs do not rotate with the shaft) if the face surface speed exceeds 5000 feet per minute ( 25 M/sec.), but never in a cartridge design unless some method has been provided to insure that the cartridge sleeve is square to the shaft. Use O-Ring balanced seals in vacuum applications down to 10-2 inches or one millimeter of mercury (1 Torr.). The O-Ring is the only elastomer that can seal both vacuum vacu um and pressure. Split seals will work in these applications, but they must be turned around for best operation. ope ration. Any good quality, balanced, O-Ring seal can seal stuffing box pressures to 400 psi (28 bar) and temperatures to 400 degrees Fahrenheit (205° C). There is a compound of Dupont's Kalrez® that is satisfactory to 600 degrees Fahrenheit (370 ° C), but it is not acceptable at ambient temperatures (it gets too hard).
Application
A Balanced O-Ring seal will not vaporize va porize the product at the seal face if the stuffing box pressure is at least one atmosphere above the products vapor point. The easiest product to seal is a cool, clean, lubricating liquid. All problem chemicals can be placed into several categories. If you know how to seal these categories you should have no trouble making seals work in your applications : Products that crystallize (caustic or sugar solutions) Viscous products (asphalt or molasses) Products that solidify (polymers or chocolate) Products that vaporize (hot water or benzene) Film building liquids (hot petroleum or plating solutions) High temperature fluids (heat transfer oil or liquid sulfur) Dangerous products (fire hazard, explosive, radioactive, bacteria) Non lubricating liquids (solvents or hot water) Gases and dry running applications (hydrogen) Dry solids (cake mix or pharmaceuticals) Corrosive fluids (acids or strong bases) Cryogenics (liquid nitrogen) Slurries (river water, sewage, most raw products) In addition to these chemical categories ca tegories there are other sealing problems that include: High pressure Hard vacuum High speed Excessive motion Dual seals should be balanced in both directions to prevent failure when barrier fluid pressure changes. The practice of using "one direction" seal balance is commonly employed by most seal companies and should be avoided for both safety and reliability. Use motion seals on mixers, agitators, sleeve bearing eq uipment and any rotating device that has motion greater than 0.005" (0,15 mm.) in a radial or axial direction. Pump seals do not work well in these applications because the hard faces are too narrow and the internal seal clearances are too tight.
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Mechanical seals, rules of thumb
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Do not use flushing fluid as a coolant in stationary mechanical seals. The coolant will be directed to only one side of the seal and since a stationary seal does not rotate the sliding components the differential temperature can cause the faces to go out of flat. In the case c ase of stationary bellows seals it could cause a bellows rupture. The best way to cool a seal is to use the jacketed stuffing box that came as a part of the pump. This jacket will not only cool down the seal area, but will provide the necessary cooling to the shaft so that it will not transmit stuffing box heat back to the bearings. The use of steam in a Quench gland is another solution, but not as good as the jacketed stuffing box. It is all right to dead end fluid in a stuffing box if a jacketed stuffing box is being used. Do not attempt to recirculate back to the suction side and cool the stuffing box at the same time. When using a jacketed stuffing box it is best to install a carbon bushing in the bottom to act as a thermal barrier the pumping fluid and the seal. Do not use rotating, "Back to Back" double seals in dirt or slurry service. The solids will prevent the inner seal from moving forward as the faces wear and if the barrier fluid pressure is lost, solids will penetrate the inner seal faces. Be sure to vent vertical pumps back b ack to the suction side of the pump. Air trapped in the stuffing box can cause the seal faces face s to run hot and in some instances destroy the elastomer. Cyclone type separators or "in line filters" are not a good method of cleaning up the fluid in the stuffing box. Heat affects a seal several ways: The faces can be attacked. Plated faces can have the hard coating crack off and filled carbons can have the binder melted out in high heat. The elastomer (rubber part) has a temperature limit determined by the compound used. The corrosion rate of all liquids increases with temperature. Thermal expansion can cause seal face loads to alter and seal face flatness to change. Many products will change from a liquid to a solid or gas in the presence of high temperature. If this should occur between the seal faces, they can be blown open. Do not be tempted to put the mechanical seal outside of the stuffing box to keep the springs out of the fluid. As the face wears the seal must move into the slurry where it will eventually "ha ng up" and leak. In these applications centrifugal force is throwing solids into the lappe d faces and if there is excessive exce ssive pressure in the system the seal faces will be blown open. When choosing the pressure range of o f a mechanical seal be sure to consider the stuffing box pressure not the pump discharge pressure. Very few seals will ever see discharge pressure.
Technical
Seals lapped to less than three helium light bands ( 0.000034") inches or 1,0 microns) should not show visible leakage. Visible leakage occurs at about 5 light bands. A typical mechanical seal face load would be 30 psi. (0,2 N/mm2) when the carbon is new and 10 psi. (0,07 N/mm2) when the carbon is fully worn away. You must never guess as to how much to compress a mechanical seal. Either take the information from the seal print or calculate the correct length from the above information. Both rotating and stationary metal bellows seals require vibration damping. Elastomer seals do not experience this vibration problem because the elastomer touching the shaft is a natural vibration damper. Vibration can be either harmonic or caused by poor lubricating fluids (slip stick) Use only non fretting seal designs. Shafts and sleeves cost too much to ignore this severe problem. Carbon throat bushings should have a shaft clearance of 0.002 inches/inch (0,002 mm/ millimeter) of shaft diameter. If they are to be used as a support bearing you should cut the clearance down to 0.001 inches/ inch (0,001 mm/millimeter) of shaft diameter. It is not necessary to lubricate seal faces at installation. If the product you are sealing c an vaporize
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Mechanical seals, rules of thumb
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between the faces and cause freezing then you must remove any lubricant that might have been placed there by the manufacturer. Balanced mechanical seals consume about one sixth the horsepower of packing. Packing a pump would be like running your automobile with the emergency brake engaged. The car would run, but the fuel consumption would be high. Single spring seals are wound in either e ither a right or left handed direction. Check Che ck to see if your seal has ha s a problem in keeping the faces together because of the spring winding. Open impeller pumps require impeller adjustment. Use only c artridge or split seals in these applications. Do not use seals that locate against a shoulder or set screw to the shaft, as the face load will change when the impeller is adjusted. Do not relap the carbon face unless it is an emergency. Seal face opening is a common seal failure. When the faces open solid particles imbed them selves into the carbon face and will be driven in even further during the lapping process. If you must relap in an emergency never use lapping powder, as the abrasive particles will imbed into the soft carbon. You cannot balance an inside seal by removing material from the carbon face. To get seal balance you must do one of the following: Use a stepped sleeve with rotating seals. Let the carbon slide in a case that is sealed to the shaft. Use a metal bellows. The balance is not perfect, but good enough. Use a stationary seal design, they require no stepped sleeves. Seal face hardness is a confusing subject because of the various measuring scales employed. The two most common are Rockwell "C" and a nd Brinnell. If you divide the Brinnell scale by ten (10) it is almost equal to the Rockwell "C" scale. Avoid oil as a barrier or buffer fluid between two mechanical seals. Most petroleum base and other oils have a low specific heat (0.2 - 0.4) and combined with poor conductivity (0.5 of water) makes them a poor choice compared to fresh water. If oil is mandatory, a clean heat transfer oil would be your best choice. A convection tank can often be used between two balanced O-Ring seals. If you use unbalanced seals the heat generated by this type of seal is usually excessive for convection cooling. Contact the seal manufacturer for his recommendations concerning speed, diameter, face combination and pressure limits for convection cooling. If convection is not satisfactory, a pumping ring or forced lubrication is another option. If you decide to repair your you r mechanical seals in house, be sure to purchase the parts from the original manufacturer. If you decide to have them repaired send them back to the original manufacturer. It is important that the seal be rebuilt with the original materials and it must meet the original tolerances. This information is not available from the manufacturer because of product liability problems. O-ring seal designs can tolerate three to four times the "run out" capability of sliding or pusher seals incorporating wedges, chevrons, U- cups etc.. Oil on the seal faces can cause the faces to stick together during long periods of non running. If you do not intend to run the equipment soon remove any oil that might be on the seal faces during the assembly procedure.
® DuPont Dow elastomer
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