Page 220 - Know and Understand Centrifugal Pumps

Failure Analysis of Mechanical Seals

before the start-up. This failure is probably in the installation, or
handling or manufacture of the seal. If the seal fails in a few days, the
failure might be an incorrect specification of a component like an O-ring
seal. But if the mechanical seal fails after three weeks, or 2 or 7 months
of service, now we must consider the operation and/or design of the
system.
On starting the pump and motor, the operators control the service of
the mechanical seal. The operators and the process engineers have a
tremendous influence on the optimal life of the mechanical seal, just as
the operator of a car has the most influence over the optimal life of his
automobile. The pump must be operated at, or close to it’s best
efficiency point (BEP) on the pump curve.
If the pump is operated away (to the left or right) from it’s BEP on the
curve, the pump will vibrate. This damages the bearings and the seal
faces leading to premature failure. Also, operation to the left of the BEP
on the pump curve adds more heat to the fluid, which can damage the
O-rings within the seal. In severe cases the fluid can vaporize leaving the
seal to run dry without cooling or lubrication. This damages the seal.
And if the pump is operating to the right of the BEP on its curve,
besides the vibrations, the poor pump can go into cavitation, and this
certainly will kill the seal. If a person insists on mistreating his car,
driving his car like bumper cars at the circus or fair, it is not the fault of
the auto mechanic if he can’t maintain the car in good operating
condition.

u U) Llldl LIIC IIICCIldlllC>, ur the manufacturer, are
blamed when a mechanical seal fails after 3 months of service. If the seal fails on
start-up, maybe you could point to the mechanic or the seal, but not after 3 weeks or
4 months of operation. This WOL ild most likely be an operational failure (a failure in
operations), or a design failure (a failure in the system’s ( jesign). And what is really
amazing is that t his statement and these words have nevi er been recognized or said
before.)

There’s a need to introduce some logic to mechanical seal failure, and
all pump failures. When an operator sees a pump leaking and dripping
through the seal, he blames the mechanic, and the mechanic blames
the seal manufacturer. The seal manufacturer blames the pump
manufacturer. The pump manufacturer blames the plant’s purchasing
agent. The purchasing agent blames the engineer, and the engineer
blames the operator. Now we’ve gone all around the block just to get
next door. I suppose we could say that when an orchestra gives a bad
concert, then no one is to blame and everyone is to blame.

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