Horsepower of a Centrifugal Pump

The Horsepower of a Centrifugal pump….. (And more reasons to qualify when and where to use VFD’s on them)

Too many people equate the horsepower of a pump being able to produce more flow and / or pressure using the SAME pump with the SAME impeller diameter by simply increasing the HP of the motor.
A centrifugal pump tells the motor how much power is needed to perform a specified flow at a specified head and will continue to deliver as much flow as possible until the curve of the pump crosses the “System Resistance Curve” of the system in which it will be used.

Here’s the controlling factor. The system (assumed to be in place) has a particular curve associated with it. It involves the pipe sizes, run lengths, elbows, valves, difference in elevation, specific gravity, viscosity (of the fluid) to name but a few.

It starts at 0 flow and 0 system resistance. As flow increases, so does the systems resistance (it takes more pressure to force the liquid through the system as flow increases).
A fixed system will require more pressure from ANY pump if……. the flow is increased. This cannot be done with increasing the horsepower. Neither can the pressure (head). It is done with centrifugal pumps by increasing the impeller diameter or the number of stages in the pump. The quantity and pressure can be attained by increasing its speed but, this is limited using VFD’s which can only provide a maximum of 10% overspeed.

Thus, the specifying engineer will build in a safety factor to his estimates and often arrives at a flow and pressure for any given system with a safety margin of 20-25% above the required values. The beauty of this is that, should the pump (when operational) be way too much for the system, it’s a simple fix by modulating the discharge valve so that the flow is restricted to what the engineer originally designed for. In doing so (throttling [partial closing] the discharge valve) the HP absorbed by the pump is automatically reduced because the pump told it so. Not because a VFD had to sense and make so slight a speed adjustment of the pump).

In other words, a slight adjustment of the discharge valve told the pump, hey, I have another system resistance curve and I command you to deliver what I ask for and I’ll tell you how much HP I need to do it..
Obediently, the motor falls into line and voila……you get what you asked for and you didn’t have to switch out the 7-1/2HP motor for a 10HP motor after all (which would not have made a hoot of difference anyway).
I’ll remind those whom are interested, and can learn from the following affinity laws for centrifugal pumps:

  • FLOW…is directionally proportional to speed.
  • PRESSURE proportional to the square of the speed.
  • HORSEPOWER is proportional to the cube of the speed.

Thus, a pump rated 300GPM at 250ft operating at 3500RPM utilizing 25HP by the hydraulic affinity laws will deliver 150USGPM at 62.5FT operating at 1750RPM utilizing 5HP(1/2 flow, ¼ pressure. 1/6th HP).

It’s a very quick and easy way to determine if in fact, increasing the HP of any given pump will produce more than you originally asked for it to do. You simply cannot. A change in impeller diameter, operational speed or reducing the system resistance (larger pipe sizes) is the only way you can change ‘your pump’ and tell it what you want it to do.

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