LET’S TALK ABOUT MECHANICAL SEAL LIFE ST001
How do you tell the difference between normal seal life and premature seal failure? It’s easier than you think. You are experiencing normal seal wear when sacrificial parts of the mechanical seal are wearing at an acceptable rate.
Just what are the sacrificial parts of a mechanical seal? Please refer to the following illustration of a stationary seal design so that we can discuss individual components:
(D) The gland is not sacrificial.
- If you have chosen a corrosion resistant material there is no reason for this part to be considered sacrificial.
- Nothing is supposed to be rubbing against this part so there is nothing to wear out.
(1) The dynamic O-ring is not sacrificial
- This elastomer should have been selected for chemical compatibility with the product you are sealing and any cleaners or solvents that might be flushed through the lines.
- All elastomers have a high and low temperature limit. This O-ring should have been selected to operate well within that range.
(2) The static O-ring is not sacrificial.
- Same rules as the dynamic O-ring, but not as critical. A small amount of swelling might not fail the seal but a great deal of swelling could break the hard face because the O-ring is mounted in the hard face inside diameter and any swelling would put this face into tensile stress where most hard materials are weaker.
(A) The stationary face holder is not sacrificial.
- This part should be constructed from corrosion resistant metal.
- There is the possibility of some fretting damage where the elastomer contacts the outside diameter of the part, but since the manufacturer has control of all of the tolerances and squeezes, the part should not be labeled sacrificial
The springs are not sacrificial.
- Springs should be manufactured from hastelloy C material to prevent chloride stress corrosion and there is no reason why they should not operate within the elastic range of the metal.
- I prefer to have the springs isolated from the pumpage so that they will not clog.
(4) The rotating hard face is not sacrificial.
- The softer carbon-graphite face is running against this hard face so there is no reason for it to wear.
(5) The gland gasket is not sacrificial.
- This is a static component chosen for chemical compatibility with the product you are sealing.
The metal component holding the rotating face (B) is not sacrificial.
- Like all metal components in the seal, this part is selected to be chemically compatible with the product you are sealing and any cleaners or solvents that might be in the system.
The carbon-graphite face pressed into the holder (A) is sacrifical
- The carbon-graphite face is the only sacrificial part of a mechanical seal. Its life is determined by the lubricating properties of the fluids you are trying to seal.
This means that a mechanical seal should run leak free until the carbon is completely worn away (just like the tread on an automobile tire). More than 85% of the mechanical seals in use today fail prematurely.
When leaking seals are removed from the pump there is almost always plenty of carbon left, so someone is going to have to get pretty good at troubleshooting if you ever hope to get acceptable seal life.