Chemical Engineering October 2021 - 36

FIGURE 2. The Barringer curve relates pump performance to efficiency and
reliability. Pump problems are highlighted in areas when operating outside the
preferred operating region (POR) of 80 to 110% of the pump's best efficiency
point (BEP)
pumping efficiency was below 40%.
In the evaluation, 10% of the pumps
operated below 10% efficiency [2].
This is alarming, but probably not surprising.
Major contributing factors to
minimal efficiency included throttled
valves and pump over-sizing. That
said, the efficiency value of the pump
itself is not the most critical parameter.
The best efficiency point (BEP)
is of much more importance and is
directly related to reliability.
The BEP for a pump is the specific
flowrate at which the pump operates
where it is most efficient. The primary
goal of operation should be to operate
as close to this flowrate as possible.
When operating at flowrates
away from the BEP flowrate, not only
will the pump become less efficient,
but it will also become less reliable
due to various problems that will
begin to present themselves.
Figure 2 presents a graph known
as the Barringer curve, which relates
a pump's efficiency to its reliability.
A typical example of a pump curve
is plotted with its efficiency curve,
which comes directly from the pump
manufacturer. The BEP flowrate is
identified with a vertical dotted line
that crosses the efficiency curve at
its maximum value. The reliability
curve represents a qualitative measure
of a pump's mean time between
failure (MTBF). The MTBF is equal to
one at the BEP and represents the
most amount of time that a pump will
be able to operate until a failure of
some sort. There are operating envelopes
known as the preferred operat36
ing
region (POR)
and the allowable
operating region
(AOR). The
POR is usually
dictated by various
standards
such as ANSI/HI
Standard 9.6.32017
or ANSI/
API 610 11th
Edition (2010).
The POR brackets
the flowrates
at which it is desired
to operate
for as much as
possible. Typically,
the POR
will range from 80% of BEP to 110%
of BEP. Note that operating at the
boundaries of the POR may reduce
the MTBF of the pump by half.
The AOR tends to be a wider envelope
where operation is allowed but
should be minimized. Typical AOR
envelopes might range from 70% of
BEP to 120% of BEP.
As you can see on the Barringer
curve, significant problems start to
occur when operating outside the
POR. Issues like higher temperatures,
cavitation, recirculation and
so on will very quickly take a heavy
toll on the pump, seals, bearings and
more. Parts start to wear out and repair
and maintenance costs become
very expensive.
Finding root causes
The problem is that simply replacing
these parts or fixing them will
only address the symptoms and
will never evaluate the root cause
that brings these issues about. Why
is the pump operating so far away
from its BEP? So much is lost when
only examining issues on a component-by-component
basis. This is
where system-level analysis is critical
to determining root cause so that
these issues can be resolved on a
more permanent basis.
How do pumping systems become
unreliable and inefficient? Standards
for designing pump systems can
seem lacking. This tends to cause an
over-sizing of pumps, control valves
and other pieces of equipment to ensure
individual components will work.
The engineer adds in a safety factor.
Then the reviewer or manager adds
on a safety factor, followed by the
client, then the pump manufacturer
and so on. Now you have an oversized
pump with unnecessary safety
factors and valves will need to be
throttled to meet current demands.
Another cause of a lack of reliability
and efficiency is that systems are
sometimes designed to meet future
requirements, but are operated to
meet current market needs. When
the pumps are sized for larger flow
demands, the operation needs to be
modified to meet current demands
and this can cause pumps to operate
away from their BEP.
System aging and equipment wear
also take their toll on reliability and efficiency.
Process changes and modifications
will have a similar effect that
can introduce reliability issues.
Applying a system approach
When applying a system approach
of evaluation to pumps and the piping
system together, a better understanding
is available for complex
relationships across all system components.
Cause and effect is easier
to define. An example would be determining
where there are significant
pressure drops in the system that
affect pump performance. The best
part is the ability to consider all parts
of the system together to consider
all system interactions. This is especially
beneficial when evaluating
other operating scenarios.
A system approach is greatly
streamlined with flow-analysis software.
There are many advantages
to flow-analysis software to provide
better understanding of complex
system interactions, including the
evaluation of the following:
* Multiple load cases
* Seasonal considerations
* Different operating configurations
and requirements
* Waterhammer and cavitation
issues
* Choked flow considerations in
gas flows
Flow-analysis software also helps
with system sizing for pumps, pipes,
valves, orifice plates, heat exchangers
and so on. Other tasks you can
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