So now what do you do? As you may know, this is a fairly common occurrence, particularly with pumps that have been around a long time; possibly since the days that maintenance records weren’t kept as well as they are now.
If you need to know the head the pump is supposed to be developing, the obvious solution is to fit a pressure gauge on the suction and discharge side of the pump at the same elevation, identify the difference and convert to feet of head. Simple!
However if you are in one of these plants where gauges appear to be in short supply, there is a Rule of Thumb that will help you approximate the total head (H) being supplied by the pump when you know the impeller diameter (D).
H = D2
There are, of course, some conditions.
1. The pump must be running at 1800 rpm.
2. The impeller must be measured in Inches.
3. The resulting Head will be in Feet.
For example, a 12 inch diameter impeller will provide approximately 144 feet of head when run at 1800 rpm.
For those of you who will check this, you will note that the result will represent the head somewhere on the performance curve, but necessarily in the same area. You may even notice that, as the impeller diameter gets larger, i.e. over 10 or 12 inches the resultant head will move gradually towards the Best Efficiency Point from shutoff.
Obviously, not every pump runs at 1800 rpm. That’s when the affinity laws help us out. For example......
At 3600 rpm, H = 4 x D2
At 1200 rpm, H = 0.45 x D2
This is not just a magic trick that will impress your associates (or maybe even your boss!), it can also be extremely helpful when troubleshooting an operational problem. Frequently, I’ve been able to question a discharge pressure quoted to me when I know the impeller diameter and pump speed, and thereby come to a faster and more accurate solution to the problem.
Author of “The Practical Pumping Handbook” and a specialist in Pumping Reliability, Ross Mackay can be reached at www.practicalpumping.com or at 1-800-465-6260