Chemical Engineering December 2016 - 73

Engineering Practice
Pump Safety: Flirting with Disaster
Be vigilant about parallel pumping conditions that can lead to catastrophic failures
Robert X. Perez
Machinery Engineer
D
uring my career, I have seen
several massive centrifugal
pump failures caused by
pump users attempting to
operate pumps in parallel without the
proper equipment review, procedural
review or instrumentation. Parallel
pump operation happens when two
centrifugal pumps are installed side
by side with common suction and
discharge headers (Figures 1 and 2).
In an ideal scenario, if both pumps
have essentially identical head flow
characteristics, then each pump
should contribute equally to the total
flow going into the discharge header,
barring any unexpected line restrictions.
However, problems often arise
when operators assume that both
that the pumps are identical in head,
and that flow characteristics and
flows will remain similar throughout
the pumps' lifetimes.
Operating parallel pumps
Let's look at a case (Figure 2) where,
due to manufacturing and assembly
differences, the first pump delivers
300 gal/min and the second pump
delivers 315 gal/min. The total flow
is 615 gal/min, which means that the
first pump is delivering 48.8% of the
total flow and the second pump is
delivering 51.2% of the total flow.
If these pumps have best efficiency
point (BEP) flows of 400
gal/min, then the first pump is running
at 75% of its BEP flow, and
the second pump is operating at
78.8% of its BEP flow. This example
probably represents a typical situation
in which pumps provide similar
head flow characteristics to the
job at hand.
When centrifugal pumps are operated
in parallel, individual pump
flows are additive, while pump heads
are not. After studying Figure 3, the
reader will surmise that, when operated
in parallel, individual pump
FIGURE 1. When parallel pumps are installed and operated in parallel, individual pump flows are additive,
but pump heads are not. Problems often arise when operators assume the pumps are identical in head
and flow characteristics and that flows will remain similar throughout the pumps' lifetimes
flows add to the right of the headflow
curves while the head that each
pumps experiences remains the
same. For example, as shown in Figure
3, at a value of about 288 ft of
head, each pump delivers about 400
gal/min, yielding a total combined
flow of 800 gal/min. Remember
that this example assumes identical
pumps are piped in parallel - which
is rarely the case in the real world.
Now, let's look at a situation where
one of the pumps operating in parallel
has some wear. For example,
Figure 4 shows the pump curves
for a healthy and a worn pump on
a single plot. For simplification, the
pump curves have been linearized.
As seen in Figure 4, the worn pump
produces less head at all flows. As
the wear becomes more and more
severe, there will be a flow value
where the healthy pump will completely
overcome the worn pump
and cause its flow to drop to zero.
A zero-flow condition occurs because,
even at shut-in flow conditions,
the weaker pump can no
longer generate sufficient head presCHEMICAL
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DECEMBER 2016
sure to overcome the discharge
header pressure and open its discharge
check valve. It goes without
saying that zero flow is very bad for
centrifugal pumps.
It has been well-documented in
the literature that operating centrifugal
pumps at low-flow conditions relative
to their BEP can lead to excessive
vibration, pressure pulsations,
accelerated bearing and seal wear,
Differential
head
Pump A
Inlet flow
Pump B
Inlet manifold
Discharge manifold
Both pumps see the same differential head
Total flow = pump A + pump B
(at the same differential head)
FIGURE 2. Diagram of two centrifugal pumps installed
in a parallel pumping arrangement
67
Total flow

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Chemical Engineering December 2016

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