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SEAL FAILURES
1.0 Mechanical seals
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The failure mechanisms of mechanical seals are covered extensively by B J Woodley in "Mechanical Seal Practice for Improved Performance" (J D Summers-Smith (ed.) 2nd edition 1992). It is not proposed to cover the same ground here, though it is perhaps worth mentioning that failures of mechanical seals are frequently catastrophic with the resulting damage making it difficult to distinguish the original cause of failure. Most failures are the result of operating faults, e.g. failure of external cooling, flush or quench resulting in breakdown of the film between the seal faces. With the seal damaged, wear continues, even with the re-establishment of the coolant supply, so that it is hard to identify that the real problem was earlier, temporary loss of coolant.
In such circumstances the nature of the face materials plays an important role. With two hard faces (e.g. silicon carbide, tungsten carbide), failure is almost immediate; whereas the seal with one soft, self-lubricating face (e.g. carbon-graphite, ptfe) can continue to operate some time before failure while the soft face wears out. If failure does not manifest itself for some weeks after the event that prompted it, identification of the cause of failure is not straightforward.