Lost the pump curve?
So now what do you do? As many of you 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. Not overly difficult.
However, if you are in one of these plants where gauges appear to be in short supply, or you're simply in a rush, there is a Rule of Thumb that will help you approximate the total head being supplied by the pump.
Head = the impeller diameter squared
This only applies when the pump is running at 1800 rpm and the impeller diameter is measured in inches.
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 not necessarily in the same area. You may even notice that, as the impeller diameter gets larger, the calculated 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.
At 3600 rpm, the Head = 4 x the impeller diameter squared.
At 1500 rpm, the Head = 0.7 x the impeller diameter squared.
This is not just a cute trick that might 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.