Effects of Operating Pumps
Away from the Best Efficiency Point
The Best Efficiency
Point, or BEP, is defined as the flow at which the pump operates at the
highest or optimum efficiency for a given impeller diameter and identifies
an operating region or point along the pump performance curve.
a pump away from the Best Efficiency Point occurs when the pump
operates at flows greater than or less than the flow designated by the
BEP. Operating a pump at flows higher or greater than the flow at the
BEP is called operating to the right of the BEP, while operating
a pump at flows lower or less than the flow at the BEP is called operating
to the left of the BEP.
Under ideal circumstances,
a pump will not operate at flows greater than BEP plus 10% or flows less
than BEP minus 10%. While we try not to stray too far from the BEP, in
general, most pumps operate away from the BEP to one degree or another,
and this is acceptable for intermittent duty.
a Pump Away from BEP
There are many consequences, however, to
operating your pump too far to the left or right of its Best Efficiency
Point for a sustained period of time. Some of these effects can include:
is caused by the formation of vapor bubbles which violently collapse,
eroding impeller surfaces and resulting in reduced mean-time-between-repair.
Cavitation can occur when operating the pump to the far right of the
BEP. For most centrifugal pumps, as the flow increases beyond the BEP,
the Net Positive Suction Head required (NPSHr) also increases; and when
the NPSHr exceeds the Net Positive Suction Head available (NPSHa), cavitation
occurs. Remedies are limited to increasing the NPSHa, which is not always
possible, reducing the flow to values resulting in lower NPSHr, or installing
special impellers which are designed to operate under low NPSHr conditions.
can be caused by many factors and can create bending moments in the
shaft, resulting in poor pump performance and risk of shaft failure.
Excessive vibration can occur when pumps operate too far to the right
of BEP, due in part to cavitation which causes hydraulic imbalances
within the impeller as voids are formed by the vaporization of the liquid.
Excessive vibration can also occur due to higher bearing loads associated
with pump operation closer to run-out or shut-off conditions.
Damage can be caused by cavitation, and excessive vibration
could potentially cause the rotor to make contact with the casing. As
the vapor bubbles, formed during the onset of cavitation, migrate to
the higher pressure regions of the impeller, they implode with enough
force to send shock waves to the surrounding area which in turn breaks
molecules from the parent metal, leaving behind the telltale signs of
cavitation - pitting and erosion.
& Discharge Recirculation, which can occur depending on
the hydraulic design of the pump, happens when the fluid does not flow
through the pump properly. This phenomenon can cause significant instability
and can reduce flow. The damage caused by suction or discharge recirculation
resembles cavitation and can lead to catastrophic failure of the pump
when portions of the impeller inlet or discharge vanes fatigue and fail
by breaking off.
Bearing and Seal Life can occur as a result of recirculation
and cavitation and will increase the maintenance costs as these components
will need to be frequently replaced. The rotor instability that occurs
at off-BEP operation can lead to shaft failures, premature packing wear,
mechanical seal failures, or simply higher bearing temperatures leading
to premature lubrication breakdown.
In our quest for higher
efficiencies and increased reliability resulting in longer mean-time-between-failure,
we often make modifications to the existing pumps so we can get their
BEP to coincide with the duty point of the pumping systems. If you are
not aware of your pump’s Best Efficiency Point for your specific
application, consider testing your pump.
If you would like ot learn more about pumps, R.F. MacDonald Co.
offers a complete schedule of Basic and Hands On Workshops. Or better
yet, call one of our offices and have our team set up a training for your
entire staff at your facility. http://www.rfmacdonald.com/training
As seen in Empowering Pumps http://www.empoweringpumps.com
Based on an Article Written by: Bob Jennings, Hydro Inc.