Page 58 - Know and Understand Centrifugal Pumps

The Affinity Laws

2 x velocity = 22 = 4 x Head
= 22 = 4 x Pressure (psi)
= 22 = 4 x NPSHr
= 22 = 4 x Misalignment in the bearings.
2 x velocity = 23 = 8 x (BHp) requirements
= 23 = 8 x maintenance costs

= 23 = 8 x downtime
= 23 = 8 x erosion in pipes and elbows
= 23 = 8 x impeller wear
= 23 = 8 x wear in wear rings

= 23 = 8 x other close tolerance wear
= 23 = 8 x friction losses (Hf) in pipes
= 23 = 8 x Hf in fittings, valves, etc.
Consequently, if the pump and motor speed were reduced by 50%:
The flow would be divided by 2.

The head would be divided by 4 (22x)
The BHP would be divided by 8 (23~)
The relationship between velocity and horsepower requirement (BHp)
presents some good arguments in favor of variable speed motors, VFDs.
When normal manufacturing plant operations don’t depend on time
(for example, if you have all night to drain a tank) the pump operator
can perform this function at 50% speed, while consuming one-eighth
the BHp. VFDs are practical in manufacturing plants because they
permit the pumps and other equipment to work at their best efficiency.
Controlling pump flow with a VFD is better than using a constant
speed motor and controlling the flow by opening and closing valves.
VFDs work well on most PD pumps, and also centrifugal pumps
performing a flow service. Remember that some centrifugal pumps are
required to comply with head, pressure and elevation applications. An
example of this would be a boiler feed water pump, or a pump pushing
a fluid through a filter. In these applications, the VFD may only be
effective at 85% to 100% maximum speed. Incorrect use of the VFD or
lack of understanding of the affinity laws could prejudice these
applications because running the VFD and pump at 50% speed would
only generate one-fourth the head, pressure, or elevation.

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