Article 17: What Happens When a Pump Operates Off-Peak?
Last week I got a
call from a friend
asking for help with a pump at a
Paper Mill. The problem: piping vibrates
violently, pump is noisy, and failures
are constant. Besides, pulsations
started to affect the quality of the
paper, obviously an unacceptable
thing to the mills’ customers. The
question was - what can be done?
As we looked
closely at the details of operation,
it became clear that this double
suction pump, initially specified and sized to operate
at the 3000 gpm (which is where its
BEP is), actually runs at about 500
gpm. Several options were presented
to the mill in the past, such as by-passing the excess flow, or buying a smaller
pump – although would solve the problem
with vibrations – are an expensive way to solve problems.
Getting a new pump is “easy”, - as long as you do not pay for it from
your own pocket. Piping
modifications alone could exceed the cost of a new pump, and sometimes even
that is not possible, and prohibitively
disruptive.
The question was – could something
be done to pump internals, while
keeping the same casing, and not having to change the piping? Fortunately, the answer
was yes. A new impeller, with modified hydraulics
was designed to fit the existing casing, and shift the pump best efficiency
point to 500 gpm.
When a centrifugal
pump operates way to the left of its
BEP, many nasty things happen. First,
a radial thrust
grows exponentially, resulting in significant shaft deflections – thus
quick seal failures, reduced bearings
life, worn out bushings and rings. Also,
a hydraulic phenomenon called “rotating stall” sets-in, which is essentially a
back-flow, leaving the impeller eye,
and progressing
backwards, resulting
in violent piping vibrations, pressure
pulsations, and wear-out of the
components. The problem becomes worth especially when a hydraulic
parameter
called “suction specific speed” (NSS)
is high. Suction specific speed is an
indirect indication of the impeller eye being too large,
but also depends on several other factors,
related to design, installation and
application. There are certain
engineering rules
and principles related to minimum allowable flow - as function of
pump energy, specific speed (NS),
suction specific speed (NSS), and other
factors, which – when violated – can
cause trouble.

A properly redesigned impeller
can thus be an effective way to solve such hydraulic
problems, and is only a fraction of a cost of a complete pump replacement. And,
these days, when funds are tight,
and maintenance and purchasing departments are
struggling to find better ways to extend the equipment life, - equipment upgrades through
hydraulics optimization come to the rescue. Paper
mills, chemical plants, refineries, municipalities, - all have the same goal:
maintain production, while reducing expenditures
by extending equipment life. Working
with the end users, such improvements in equipment reliability
are possible, and require
sound engineering approach, where
attention to details, and appreciation
of hydraulics and mechanical interactions, can be realized
effectively, and economically.
.
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to:
DrPump@Pump-Magazine.com
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