Pump Power Platitudes

In PrP 18’s Did You Know, where we often bust common misbeliefs, our technical staff asserted the following: Restricting the output of a centrifugal pump by partially closing a valve ? which causes the pump’s output pressure to rise substantially ? will not make the pump work harder; it makes the pump work less. Most folks believe the former, since it seems logical that forcing a pump to “push” into a greater and greater restriction will make the pump motor “work harder”, therefore draw more current. Since work is loosely defined as “moving a mass through a distance”, the opposite is true as less mass (water) is moved. 
Dennis Berkshire, engineer for Aquatic Design Group in Seaside, California, sent us some pump curves and detail substantiating our assertion. Look at the descriptions and the table below, taken from the data of a typical pump…
At design flows, dictated by the Total Dynamic Head (flow resistance) values created by the plumbing system’s design, a 6.5 horsepower pump will produce “design flow” in the plumbing ? in this case, about 336 gpm. The motor operates warm, at an appropriate operating temperature. 
On the other hand, imagine the filter is loading up, the heater bypass is closed, or valve settings for some reason have reduced the system’s ability to pass water freely. The 80%, 60% and 40% flow figures below show clearly that when the flow is substantially reduced, the efficiency goes way down, and the pump draws much less power. The motor actually runs cool.
Even more important, what if the pump were looking at LESS resistance: A 6.5 HP pump, designed for 336 gpm at 65 feet of head, was operated at 120% of that output by reducing the head to 57 feet TDH (less resistance). The result is a draw of 7.3 HP and a flow of 403 gpm. That’s 60 GPM more than we bargained for! This could occur with a freshly backwashed filter, the heater bypass wide open, and/or flow-trimming valves fully opened… any or all in a generously sized plumbing system. The extra flow is not for free, however: the electric bill went up with the horsepower and the pump’s motor is running plenty hot. Plan on replacing the motor; you will eventually burn it up.

	Flow	TDH	Efficiency	HP
120%	403 gpm	57 ft.	80%	7.3
100%	336 gpm	65 ft.	83%	6.5
80%	268 gpm	70 ft	77%	6.2
60%	201 gpm	75 ft.	70%	5.3
40%	134 gpm	78 ft.	57%	4.6

It makes sense to operate your pump at design values, only. ~ kw

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