Page 102 - Know and Understand Centrifugal Pumps

Understanding Pump Curves

you could meet the requirements of flow, but not the requirements of
head or pressure. The pump is prone to suffering cavitation, high flow,
high BHp consumption, high vibrations, and radial loading (about
240” from the cutwater), resulting in shaft deflection. To counteract
these results, the operator should restrict the control valve on the
pump’s discharge to reduce the flow.
Operating the pump in zone ‘D’ is very damaging to the pump. Now
the pump is severely over-designed for the system, too far to the left of
the BEP. The pump is very inefficient with excessive re-circulation of
the fluid inside the pump. This low flow condition causes the fluid to
overheat. The pump is suffering high head and pressure, and radial
loading (about 60” from the cutwater), shaft deflection and high
vibrations. To deal with or alleviate these results, you need to modify or
change the system on the pump’s discharge (ex. reduce friction and
resistance losses on the discharge piping), or change the pump (look for
a pump whose BEP coincides with the head and flow requirements of
the system).
In the final analysis, pumps should be operated at or near their BEP.
These pumps will run for years without giving problems. The pump
curve is the pump’s control panel, and it should be in the hands of the
personnel who operate the pumps and understood by them.

Special design pumps
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The majority of centrifugal pumps have performance curves with the
aforementioned profiles. Of course, special design pumps have curves
with variations. For example, positive displacement pumps, multi-stage
pumps, regenerative turbine type pumps, and pumps with a high
specific speed (Ns) fall outside the norm. But you’ll find that the
standard pump curve profiles are applicable to about 95% of all pumps
in the majority of industrial plants. The important thing is to become
familiar with pump curves and know how to interpret the information.

Family curves

At times you’ll find that the information is the same, but the
presentation of the curves is different. Almost all pump companies
publish what are called the ‘family of curves’. The pump family curves
are probably the most usehl for the maintenance engineer and
mechanic, the design engineer and purchasing agent. The family curves
present the entire performance picture of a pump.