SUBJECT: Some common misconceptions about mechanical seals 11-3

• Two hard faces are a sensible choice if there are dirt or solids in the product you are pumping.
  • ans. Seal faces are lapped to less than three light bands (less than one micron) of flatness. Dirt and solids cannot penetrate these faces unless they open. The trick to sealing solids and slurry is to keep the lapped seal faces together.
• Dual seals are a good choice for a slurry application.
  • ans. Putting a clean liquid between two seals is not going to stop solids from clogging the inner seal. Since the barrier fluid is at a higher pressure than the stuffing box pressure you will probably end up diluting your product.
• Putting the seal outside the stuffing box can keep the springs and other parts from clogging in an abrasive slurry.
  • ans. As the seal faces wear the seal is going to have to move into the slurry that will restrict its movement. It is the same problem you face with many of the dual seal applications used to seal dirt and solids..
• You should not use ceramic seal faces in a mechanical seal. They will crack when subjected to temperature transients.
  • ans. Space vehicles are covered with ceramic so they can take temperature transients, its just a matter of which ceramic you are using.
• Modern seal designs are made to fit A.N.S.I. and I.S.O. pump designs without having to make any modifications to the seal or pump.
  • ans. The seals will fit, but they will not have enough outside diameter clearance for proper operation. The stuffing box bore should be enlarged.
• Seal faces have to be lubricated.
  • ans. Not necessarily. Carbon graphite is a natural lubricant. Electric motors have used carbon/graphite brushes for years that do not use any external lubricating source.
• Vibration analysis is a good technique for predicting seal failure.
  • ans. Vibration analysis requires that you know the frequency of the piece of hardware you are analyzing. This is easy for bearings that are always made out of the same material, always in the same basic medium and vary little in shape. Seals come in a variety of shapes and materials and run in all sorts of mediums.
• Oil is a good barrier fluid to use between dual mechanical seals.
  • ans. Actually it is one of the worse. It has too low a specific heat number and it is not a very good conductor of heat compared to other liquids.
• Teflon is a universal elastomer. It makes sense to use it in mechanical seals.
  • ans. Teflon is not an elastomer because it does not have a memory. To use it in a mechanical seal you must spring load it to the shaft and that is never a good idea because you will end up with expensive shaft damage (fretting). O.E.M. suppliers use Teflon because they are not sure where the pump is going to be used.
• Shrinking a carbon seal face into a metal holder is an acceptable manufacturing technique.
  • ans. It really is a bad one. The out of roundness tolerance of the metal holder will clash with the out of roundness tolerance of the carbon, causing high loading at several points on the carbon outside diameter. The carbon should be pressed into the metal holder allowing it to shear and conform to the metal out of roundness.
• It is a good engineering practice to glue the O-rings in a split mechanical seal design.
  • ans. The glue will create a hard spot that will give you a leakage problem.
• You should connect the flush connection to the top of gland.
  • ans. It should be connected to the bottom of the gland or stuffing box. This will allow the flushing fluid to fill the box prior to spilling over the end restriction in the stuffing box. American prints show the top half of the drawing, that is why this error is so frequently made.
• The elastomer Viton is acceptable in water.
  • ans. It is a worse choice. The proper material for water is ethylene propylene. Some specific grades of Viton can be used in cold water , but none of them are good for hot water. Viton is cured in sulfur and what ever attacks the cure attacks the compound. Needless to say sulfur and water are not a good combination.
• Split seals leak
  • ans. It all depends upon your definition of leakage. If you are talking "fugitive emissions" that are measured at parts per million you can build a case for leakage, but if you mean "no visible leakage" then split seals should be as leak free as any other mechanical seal manufactured from the same materials.
• You should put a lubricant on seal faces when you install them.
  • ans. It's not a good idea to put anything on the lapped faces. The trick is to keep the lapped faces together.
• No one can predict seal life.
  • ans. That is a fact, but we know how long seals should last. They should run leak free until the sacrificial carbon wears down. (90% of mechanical seals fail long before that happens).
• In most seal applications the carbon is running on a hard face.
  • ans. The graphite comes out of the carbon /graphite face and deposits on the hard face. You can easily see the black mark made by the graphite. The seal face you are actually running is carbon on graphite. The hard face is just some place to put the graphite. This is the reason the seal faces can run dry.
• It is good engineering practice to put a stationary seal ( the type where the springs do not rotate with the shaft) on a cartridge.
  • ans. Tightening the cartridge sleeve set screws will pull the cartridge sleeve to one side, causing the rotating face to no longer be perpendicular or square to the rotating shaft. This squareness to the shaft is essential to the performance of any stationary seal design.
• If you are installing a mechanical seal in a vertical centrifugal pump, you use the same procedure as installing a seal in the horizontal version.
  • ans. Vertical pumps trap air in the stuffing box. You will have to install some type of vent above the seal faces and dynamic elastomer to avoid "dry running" in these locations.
• PV factors are a legitimate way of predicting seal performance.
  • ans. Carbon/graphite seal faces are sensitive to pressure(P), but not to velocity(V) so PV has limited value.
• The three hundred series of stainless steel is a good choice for seal metal components.
  • ans. That is true for most of the metal components, but not for springs or metal bellows. The three hundred series is sensitive to chloride corrosion problems in these locations.
• You must not use ceramic as a seal hard face because it will crack with a rapid temperature change.
  • ans. Some ceramics have this problem, and you should not use them in your designs. The ceramic called "silicone carbide" is a good choice as a hard face and does not have the "cold shocking" problem.
• The metal bellows seal should be your first choice for a hot application.
  • ans. the metal bellows seal is always a good choice in hot fluid to eliminate the temperature sensitive elastomer or O-ring, but it is not effective in hot petroleum applications because of "coking problems. In these applications you have to cool the stuffing box area to prevent the oil from forming coke solids on the seal moving parts and faces.
• The mechanical seal should be positioned against a shaft or sleeve shoulder to insure the correct face load and then set screwed to the shaft or sleeve..
  • ans. To compensate for shaft axial growth or open impeller adjustment, the seal must be positioned on an adjustable cartridge sleeve.

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