SEAL MISCONCEPTIONS S023
Here is a listing of some of the most common misconceptions I find that people have about mechanical seals.
Two hard faces are a sensible choice if there is 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; it’s just a matter of which ceramic you are using.
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.
Oil is a good buffer or 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. Not really, the glue will create a hard spot that will give you a leakage problem.
You should connect a clean 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 most split seals are 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 clean and 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 that seal faces can run dry.
It is good engineering practice to put a stationary seal 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 mechanical seal; you are going to need some type of self-aligning feature. See stationary cartridge seals.