Page 231 - Intro Predictive Maintenance

222 An Introduction to Predictive Maintenance

which either increases or decreases depending on the amount of work the pump must
perform.

Flowrate. The volume of liquid delivered by the pump varies with changes in TSH.
An increase in the total system back-pressure results in decreased flow, whereas a
back-pressure reduction increases the pump’s output.

Correcting Problems
The best solution to problems caused by TSH variations is to prevent the variations.
Although it is not possible to completely eliminate them, the operating practices for
centrifugal pumps should limit operation to an acceptable range of system demand for
flow and pressure. If system demand exceeds the pump’s capabilities, it may be nec-
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 applications where the TSH is too low and the pump is operating in run-out 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, called false
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 it’s 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
system’s back-pressure by eliminating line resistance caused by elbows, extra valves,
and so on.

10.1.2 Positive-Displacement Pumps
Positive-displacement pumps are more tolerant to variations in system demands
and pressures than are centrifugal pumps; however, they are still subject to a variety
of common failure modes caused directly or indirectly by the process.

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

Table 10–2 lists common failure modes for rotary-type positive-displacement pumps.
The most common failure modes of these pumps are generally attributed to problems
with the suction supply. They must have a constant volume of clean liquid in order to
function properly.