Page 221 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

When a seal is installed into a pump, and the motor started, an
imaginary line is drawn, and the seal begins a journey toward the day
when the seal will come out of service, either from premature failure, or
from obtaining its maximum service life. On one side of the imaginary
line are the events in the manufacture of the seal, its handling, storage,
and installation. On the other side of the imaginary line are the events
that occurred after the pump and seal were started the first time.

Maybe you'vf 1 heard your auto mechanic say on installin! 3 a new radiator water pump
or alternator onto your car ... that if it runs for 5 min utes, then it will run for 15
.., . .,
r. . r
years. If it's going TO Tail, ir wiii ao so wirnin rne Tirsr Tive minutes.
. . e.,..
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Mechanical seal problems originating in the factory, storage, handling,
and installation will be evident within the first few moments or hours of
operation. Consider fractured faces (from poor handling), or a missing
O-ring (from poor assembly), or installing a 50 mm seal onto a 48 mm
shaft (poor installation).
These failures and the leaks will be immediately evident.
If there is a chemical incompatibility between the liquid and an O-ring
rubber compound, or if heat is generated from too much spring
tension, this will be evident within a few hours or days.. Galvanic
corrosion or inadequate spring tension will reveal itself in a few weeks.
Certainly, at the moment of starting a pump with a new seal installed,
the events prior to the installation begin to disappear as a cause or
origin of failure, and the factors of operation, process, and design in the
system begin to appear as possible reasons for any premature failure.
Let's begin the analysis, or autopsy with the physical evidence on the
component seal parts.

O-ring (The elastomer) failure

About half of all pumps in the shop today were pulled out of service
because they were leaking or wouldn't hold pressure or pump. This is
most likely a leaking O-ring. The O-ring is the rubber component of
most mechanical seals. The O-ring controls the temperature, pressure,
and chemical resistance of the mechanical seal (Figure 14-1).
The difference between a mechanical seal in a pump in alcohol service
and a pump in steam service is the O-ring. It is not the stainless steel, or
the ceramic face of the seal. The difference between a mechanical seal in