Chemical Engineering December 2016 - 75

to the point where it
is producing
only 95% of its original head. After
doing some research, you determine
that the ratio of the head at BEP
and shutoff head (HBEP/Hso) is 0.85.
If we plug this information into the
Qso/QBEP equation we get the following:
(6)
In
this example, we see that we can
drop the flow on the " healthy " pump
down to 33.3% of the BEP flow before
a shutoff flow conditions is experienced.
Example
3. Finally, let's assume that
one of the pumps is 100% healthy
and the sister pump has degraded
to the point where it
is producing
only 95% of its original head. After
doing some research, you find that
the ratio of the head at BEP and
shutoff head (HBEP/HSO) is 0.7. The
reader will notice that the QSO/QBEP
ratio is much lower in this example,
which means that this pump curve
has a higher head rise to shutoff.
If we plug this information into the
Qso/QBEP equation we get:
(7)
Notice that when there is a larger
head to
rise
to
shutoff, a lower
" healthy " pump flow can be tolerated
before a shutoff condition is
reached. The converse to this statement
is that pumps with flat headflow
curves are much more sensitive
to flow imbalance when internal
wear occurs.
Recommendations
When multiple pumps are run in
parallel, there is a very real likelihood
of experiencing imbalanced
flows during the course of their operation.
Pump manufacturing differences,
internal pump wear, large
swings in process conditions, internal
pluggage and asymmetrical
piping designs can all contribute to
unbalanced pump flows. No matter
how hard you try, there will always
be a possibility that a laggard
pump could one day crash if performance
degradation goes unnoticed
before a shut-in condition occurs.
FIGURE 6. Centrifugal pump failures are often caused by pump users attempting to operate pumps in
parallel without the proper equipment review, procedural review or instrumentation
The safest philosophy is to assume
there will always be some level of
pump flow imbalance, so operators
should continuously monitor
individual pump flows as a means
of avoiding catastrophic low-flow
conditions.
Here are some guidelines for designing
parallel pump installations:
* Always try to install parallel pumps
with similar head-flow characteristics.
Running dissimilar pumps in
parallel is asking for trouble
450
400
350
300
250
200
150
100
50
Hso = shutoff of healthy pump
Qso
H*so = shutoff of worn pump
Slope = m = (HBEP-Hso)/QBEP
BEP flow
* Ensure that the associated pump
piping is symmetrical so that flows
are better balanced
* Never operate pumps in parallel
that were not designed for such
operation
* Parallel pumps should have dedicated
flowmeters installed on
each pump. Flowmeters installed
to measure the total pump flow in
parallel applications are useful for
process control, but can mislead
you into thinking that everything is
Linearized pump curves
100
200
300
400
Pump flow, gal/min
FIGURE 5. The " pump-shutoff triangle " is constructed using the linearized curve for the " healthy " centrifugal
pump, the linearized pump curve for the " worn " pump and the ordinate
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
DECEMBER 2016
69
500
Healthy pump
600
Worn pump
700
800
Pump head, feet

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Chemical Engineering December 2016

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