by Dr. Lev Nelik, P.E. APICS
December 17, 2011

Pump Challenge #1 - an impeller cut problem - is an application of the Affinity Laws to reduce the motor amps and solve a motor tripping problem. The full details of the challenge can be seen by clicking here.

Thanks to everyone who submitted answers to Pump Challenge #1 (P&S, July 2009). We will discuss this problem by reviewing the Pump Affinity Laws (PAL). PAL shows that when the impeller diameter trimmed, every point on the head-capacity curves changes. Flow, head and power are reduced, but in what proportion? See Figure 1.

x = ODnew / ODold

Q ~ x

H ~ x2

BHP ~ x3

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Figure 1

Here is a practical method for solving the first challenge. Pick several points on the original performance curve, and tabulate flow, head, power and efficiency. Multiply each flow point by the ratio of the new impeller diameter by the old. The head gets multiplied by the square of that ratio, and the power as the cube of the ratio.

First, pick a new diameter. For example, try the cut from the original 19.625 in to 19.000 in (see Figure 2).

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Figure 2

Note that the power decreased from 460 hp to 417 hp at the worst case flow, which still exceeds the 400 hp motor rating. Try another cut, until you get below the 400 hp mark, but not too far below. After several tries, you should get something like Figure 3.

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Figure 3

The highest power on the curve is just below 400 (399 hp). You got it! Next, plot the new performance curves (see Figure 4).

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Figure 4

Congratulations! You successfully applied the Pump Affinity Laws.

 

Click here for Pump Challenge #2: Moving a Pump from the Bottom of a Dry Pit