Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Failure Analysis
Mechanical Seal Reliability 557
The purpose of a mechanical seal is to prevent, or at least limit, leakage.
Normally, the leakage from a mechanical seal is so slight that it is
difficult to measure. For example, a 2-inch seal containing water at 100
psig should leak less than 2 ml/hr. Current federal regulations require
that the concentration of certain volatile organics be less than 10,000
ppm near the seal leakage point. In some areas, this limit is 1000 ppm. It
is likely that none of these examples would show any visible leakage. Because
most seals are wearing devices, they will fail, that is, leak excessively
t at some point in time. The machinery engineer must therefore be
prepared to learn from each seal failure and use that information to improve
future designs and applications so that seal life is improved. In order
to do this, detailed records of each failure should be maintained.
Failure analysis is simply allowing the seal to tell why it did not perform
as expected. This means acting the part of a detective. The following
simple guidelines will make failure analysis easier and more consistent:
• Know what a new seal looks like.
» Know what a successfully applied seal looks like.
» Examine the failed seal carefully.
• Write down the differences between the new, successful, and failed
seals.
• Formulate a consistent explanation about the differences.
Although manufacturers and mechanics are familiar with new seals,
sometimes a machinery engineer may not see the unused item. He only
sees the failures. Similarly, he may not see many successfully applied
seals because they are not called to his attention. He is too busy looking
at failures. On the other hand, some failure modes are so common that a
mechanic may come to accept them as "normal wear." For failure analysis
to be useM, all differences in the new and failed seal should be noted.
These differences must be written down in a consistent format. The
form shown as Table 21-1 is a good starting point and can be modified to
suit particular needs. There is a great temptation to take mental notes and
write them later. This temptation must be overcome, and written notes
must be made while examining the seal. If these notes become soiled,
they can be rewritten later.
For many refineries and chemical plants, the statistics in Table 21-2
have been developed from analysis of many pump seal failures.
Statistics show that most seals fail prematurely, that is, prior to wearing
out. They also show that these early failures can be prevented by sim-