FLOWTREND
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(281) 990 8582 (888) 271 FLOW (281) 990 8644
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FLOWTREND Mechanical Seals Troubleshooting Procedures
FLOWTREND mechanical seals are precision engineered and manufactured “ FLOWTREND components, yet one of the most common causes of pump failure are due to the malfunction of a mechanical seal”.
By design, mechanical seals are friction contact devices and can be subjected to a very wide (typically harsh) range of operating environments.
WHY DO MECHANICAL SEALS FAIL? There are several reasons why mechanical seals fail. Here are just a few: 1) Pump/system problems a. Pump runs dry b. Vibration (inappropriate TIR of stub shaft, etc) c. Cavitation d. Misalignment of piping e. Excessive temperatures f. Pressure spikes g. Etc. 2) Mechanical Seal problems a. Damage to seal faces prior to installation b. Incorrect installation c. Insufficient or no attachment of water flush component d. Excessive temperature e. Excessive pressure
In the “Troubleshooting Procedure” section, we attempt to help you find the primary cause of why the mechanical seal has failed. We discuss how to use the evidence of the failed seal to establish what was, or still is, happening inside the pump or seal area. The analysis of the seal failure will enable us to put forward our recommendations for successfully sealing the pump. TROUBLESHOOTING PROCEDURES
FIRST: -
Collect the entire seal Examine the wear track (compare to sketches below) Examine the faces for damage Examine for signs of heat (bluing of stainless, etc.) Examine the seal drive component for damage Examine the springs for damage Examine the elastomere’s for damage
THEN, Compare:
Proper Wear Track. The pattern on the wide face will be equal to the width of the narrow seal face. When this occurs, the cause of leakage is unlikely to be seal related. The wear pattern should be concentric with the inner diameters.
Narrow Wear Track. When the wear track is narrower than the thinnest width face, this means the seal has been over pressurized and has bowed away from the pressure. This bowing causes the seal to seal only on a portion of the face width. This is from improper design and the seal must be changed to a higher pressure, more rugged design.
No Wear Track. This usually means that both of the seal faces are rotating with each other. Check that the drive mechanism or anti rotation devices are working correctly.
Wide Wear Track. The wear pattern is wider than the narrow face. This pattern indicates that opening of the faces has allowed excessive film between the faces. This can be caused by a cocked stationary face, pump misalignment, pipe strain, or pump cavitation. The pump misalignment or vibration can cause the rotary seal to move back and hang up on the shaft. The most practical solution to this is realignment of the pump. A wide wear pattern is also symptomatic of excessive shaft motion.
Intermittent Wear Track. This wear pattern can be caused by cocking of the stationary face and/or an excessive surface roughness of the stationary or rotating seal face. Insure that all faces are not cocked and that they are flat at the time of installation. Flowtrend seal faces are warranted to have a face flatness of less that 3 Helium Light Bands.
Uneven Wear Track. This is an uneven wear pattern that rotates off of the entire wide face at two points. The most probable cause of this is poor pump shaft-drive motor alignment.
Cracked Hard Face. There are two main causes for cracking. Ceramic seals can crack due to thermal shock (changing temperatures rapidly across the ceramic seal face. If as stream of water hits a face, such as ceramic, that is running hot, it will cause it to fracture.). The second cause of cracking is mechanical shock, which can occur if the face is distorted by dropping or hitting of the seal.
Chipped Edges. Chipping is caused by a large separation of the faces and consequential breaking when they slam back into each other. It is most often associated with Flashing (which is common in hot water systems or in fluids that may have water condense in them). When water changes from a liquid to a gas, it expands thousands of times in volume and can cause a large face separation. This is also known as chattering. This can also be caused by severe pressure spikes in the pump associated with cavitation. Cavitation is cured by proper liquid volume entering the pump (refer to NPSHr of pump). Usually, small vibrations, misalignments and the like can not cause the breakage because they do not separate the faces enough. The cure in the case of flashing is to reduce the face heat. This is done by insuring that the seal spring tension was not excessive due to the installing it wrong. Using of a single or double flush seal arrangement will provide proper cooling fluid so that this does not occur.
Flaking and Peeling. This occurs primarily when a hard face is plated with a material. A good example is the Chrome Oxide plated Stainless rotating seals used by Fristam. Other OEM’s also plate stainless steel faces with a hard planning of satellite, ceramic, tungsten carbide, or a variety of other materials. Flaking is usually a sign of either a defective coating, or chemical attack at the bond between the base metal and the plating material. The attack may be aggravated by heat generated at the seal face. Switching to a solid face material will solve this problem. FLOWTREND only uses solid face materials.
Pitting, Blistering, & Corrosion. The carbon used in FLOWTREND mechanical seals is selected for the particular application and should not be subject to these problems. The problems occur when the wrong carbon is being used. Most seal carbons use an impregnated face ( FLOWTREND uses a triple impregnated carbon) and this is not obtainable when a carbon is machined from tube stock by indiscriminate producers. Corrosive attack of carbon can be stopped by selecting carbons which are relatively binder free. In the few fluids which attack a pure carbon or carbon graphite, such as nitric acid, oleum, chlorosulphonic acid and some exotic highly oxidizing acids, the alternative is to use a PTFE or filled PTFE face. Faces made from PTFE are a poor substitute for carbon, but are appropriate for the few fluids where a pure carbon will not withstand the fluid.
Deep Wear. This is typically associated with externally mounted seals in misaligned pumps, seals in severe abrasive service, and/or products that under go a phase changes to form crystals on the seal face. The particles embed in the carbon face and grind the hard face. The problem is often compounded by the reuse of the carbon face, because it shows little wear. The common cure for this type of seal wear, is to adapt a single or double flush seal arrangement to the pump and by adapting the use of hard faces such as Silicon Carbide.
Heat Checking. This is often caused by the inability of the face to conduct heat away. The heat can come from the product temperature, friction, vaporization of the product, coupled with brief cooling of the faces from the product being pumped. If any discolorations of the seal parts are observed, then high friction between the seal faces has been created. Further evidence in O-ring hardening and “setting” may support this. The most probable cause is dry running, flashing, or very poor face lubrication.The common cure is to reduce face temperature by adapting a single or double flush seal arrangement and/or changing the seal face materials.
Other: Seal Drive Problems : Most seal designs use some way to transmit torque from the shaft to the rotary seal face. Quite often it is done with pins, set screw, o-rings, or some other form. To check for this clue, you must first determine for your particular seal where the drive junction is located. Seals are usually loose in torsion, that is, outside the pump, you can twist them slightly before they engage. You are looking for signs of wear at the pin, screw, etc. Slip Stick: If the two faces stick together the drive ring will lodad up with high stress. This is then transferred back to the face causing it to accelerate and then stick again. Instead of a smooth rotary motion, the face is being beaten around in its circular path. Slip stick is caused by a lack of face lubrication. This can be caused by a variety of problems. You must look at the other clues to determine the most likely. Lack of face lubrication can be caused by: - Installing the seal at the wrong working length. - Too much pressure acting on the face, i.e. using an unbalanced seal where a balanced seal should be used. - The fluid being sealed has poor lubricating properties. - The face combination is bad. Using faces for their chemical resistance without regard for their ability run as a seal face. - Pump cavitation.
- On vertical pumps air trapped in the rotor case chamber. This is a very important clue because it tells you about the nature of your product. Double seal arrangements are necessary when a product is not a good lubricant. This clue will tell you about your products lubricating properties. Spring Breakage: Springs usually break because of chemical attack at the same time the device is being stressed. The phenomen of stress corrosion cracking is explained by many different theoretical methods. It is commonly seen in seals when stainless steel springs are used in certain fluids. When the fluid being sealed contains chlorine, bromine, iodine, fluorine and ions or compounds of these elements, they often will attack the chrome oxide layer that protects most grades of stainless steel. While the oxide layer is being attacked, the flexing will open up small cracks. If the oxide particles wedge into these cracks, a sudden failure can occur. For this reason, most spring materials should be high grade stainless steel or alloys such as Hastelloy or Monel. Spring breakage accompanies flexing of the device, but repeated axial compression of a spring will not cause fatigue failure. This happens when a portion of the spring is extended too much or flexed in torsion. When the seal is mounted externally, the spring is not actually in the pumped media, and can only be attacked if primary seal failure has occurred. Some springs are meant to rotate in one direction. When a deformed spring is notices, it was most likely that the pump was rotated in the wrong direction, or the wrong spring was used. Elastomere Issues: Swollen, sticky, or disintegrating elastomeres. This is a sign of chemical incompatibility. It is solved by using a different material. Charts should be consulted, or if none are available, product immersion testing can be performed. If the product is a mixed solvent and no elastomere is suitable, then a PTFE encapsulated or Kalrez O-ring should be selected. Hardening, Charring, Cracking, of the elastomere. All of these are signs of excessive heat. Usually the source of the heat is the face or a metal to metal contact of two parts. Excessive face heat is caused by lack of lubrication and subsequent friction. It could also be a sign that the pump has run dry. Look for signs of metal to metal contact. This is very common, yet often overlooked because the marks look like they may have been machined onto the seal originally. Rubbing: Some easily overlooked causes for rubbing are: - Flushing lines coming into the seal chamber housing and extending into the seal chamber itself. - Gaskets slip into the seal cavity - Rotary or stationary rings come in contact with the rotating shaft.
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Build up of scale in the seal area. Seal are not concentric with the shaft. Excessive shaft deflection caused by throttling the discharge or otherwise operating the pump at its wrong capacity or pressure.
