Page 255 - Root Cause Failure Analysis

essary to change the pump, the system requirements, or both. In many applications,
the pump is either too small or too large. In these instances, it is necessary to replace
the pump with one that is properly sized.
For the application where the TSH is too low and the pump is operating in runout con-
dition (i.e., maximum flow and minimum discharge pressure), the system demand can
be corrected by restricting the discharge flow of the pump. This approach, calledfulsv
head, changes the system’s head by partially closing a discharge valve to increase the
back pressure on the pump. Because the pump must follow its hydraulic curve, this
forces the pump’s performance back toward its BEP.

When the TSH is too great, there are two options: replace the pump or lower the sys-
tem’s back pressure by eliminating line resistance due to elbows, extra valves, and the
like.

POSITIVE DISPLACEMENT
Positive-displacement pumps are more tolerant to variations in system demands and
pressures than centrifugal pumps. However, they still are subject to a variety of com-
mon failure modes caused directly or indirectly by the process.

ROt8V TVpe
Rotary-type, positive-displacement pumps share many failure modes with centrifugal
pumps. Both types of pumps are subject to process-induced failure caused by
demands that exceed the pump’s capabilities. Process-induced failure also is caused
by operating methods that either result in radical changes in their operating envelope
or instability in the process system.

Table 19-2 lists common failure modes for rotary-type, positive-displacement pumps.
The most common failure modes of these pumps generally are attributed to problems
with the suction supply. The pumps must have a constant volume of clean liquid to
function properly.

Reciprocating
Table 19-3 lists the common failure modes for reciprocating-type, positive-displace-
ment pumps. Reciprocating pumps generally can withstand more abuse and variations
in system demand than any other type. However, they must have a consistent supply
of relatively clean liquid to function properly.

The weak links in the reciprocating pump’s design are the inlet and discharge valves
used to control pumping action. These valves are the most frequent source of failure. In
most cases, valve failure is due to fatigue. The only positive way to prevent or minimize