Doyle Online Pump Training 3

• Centrifugal Pump Theory

• Pump Terminology

• Reading Centrifugal
  Pump Curves

• Centrifugal pump
  Operating Range

• Pump Enquiry Information

• System Curves

• Pump Parallel / Series
  Operation

• Cavitation

• NPSH - Nett Positive
  Suction Head

• Affinity Laws

• Troubleshooting

• Disclaimer

Reading Centrifugal Pump Curves

Centrifugal pump performance is represented by multiple curves indicating either:

Various impeller diameters at a constant speed.
Various speeds with a constant impeller diameter.

The curve consists of a line starting at "shut head" (zero flow on bottom scale / maximum head on left scale). The line continues to the right, with head reducing and flow increasing until the "end of curve" is reached, (this is often outside the recommended operating range of the pump).

Flow and head are linked, one can not be changed without varying the other. The relationship between them is locked until wear or blockages change the pump characteristics.

The pump can not develop pressure unless the system creates backpressure (ie: Static (vertical height), and /or friction loss). Therefore the performance of a pump can not be estimated without knowing full details of the system in which it will be operating.

Refer to fig.2 below for a sample curve showing (text colours relate to colours used in fig.2):
Three performance curves ( various impellers or speed).
Curves showing power absorbed by pump (read power at operating point, see note 1).
Best efficiency point (BEP).
Recommended operating range(operation outside this range reduces pump life).
Nett positive suction head required by the pump (NPSH [R] ).
The circled numbers indicate the following for bottom curve (ie: smallest diameter impeller or slowest speed curve shown):

Maximum recommended head.
Minimum recommended head.
Minimum recommended flow.
Maximum recommended flow.

The points refered to as "shut head: and "end of curve".

Note 1: Power absorbed by pump is read at point where power curve crosses pump curve at operating point. However this does not indicate motor / engine size required. Various methods are used to determine driver size.
Select motor or engine to suit specific engine speed or operating range - most cost effective method where operating conditions will not vary greatly. (very risky - as a pump is often not used for the purpose it was originally intended)
Read power at end of curve - most common way that ensures adequate power at most operating conditions.
Read power at operating point plus 10% - usually only used in refinery or other applications where there is no variation in system characteristics.
By using system curves all operating conditions can be considered - best method where filling of long pipelines, large variations in static head, or syphon effect exist. -use orifice plates or valves to control power usage.

fig. 2