PLAYING WITH A FEW PUMP TERMS 12-11

In any discussion of centrifugal pumps you will find that there are several terms that are interrelated:

- Head
- Capacity
- Horsepower consumption
- Efficiency

In past papers I showed you formulas that calculated some of these relationships.

As an example, here is the formula for measuring the water horsepower, or the horsepower out of the pump:

Efficiency is defined as the horsepower (water horsepower) out of the pump divided by the horsepower (brake horsepower) into the pump. The formula to calculate it with head and capacity numbers is:

- TDH = the total discharge head measured in feet
- GPM = gallons per minute.
- HP = horsepower required. This number is shown on the pump print.
- 3960 = a conversion number we get by dividing 8.333 (the weight, in pounds, of one gallon of water) into 33,000 ( foot pounds in one horsepower).

Like all mathematical formulas you can change the order of the formula to calculate a different term. As an example if you want total discharge head :

Or you can convertr it to read gallons per minute instead:

Horsepower required might be another choice:

If you are not comfortable using formulas, another way to do this is to use a chart like tthe following one. If you know any of the following three numbers, the chart will give you the fourth

- Head (TDH)
- Capacity (GPM)
- Efficiency
- Horsepower in (HP)

Lets do an example. The following numbers were supplied. What head that will be produced by the pump?

- Brake horsepower in = 17.6
- Efficiency = 70%
- Capacity = 350 gpm

To determine the head you would enter the graph at 17.6 horsepower, go up to the 70% efficiency point and then run parallel to the existing lines until you reach the “break line”.

From the break line you would go up the chart to 350 gallons per minute and then over to the head of 140 feet. That wasn’t too bad was it?

Here is another example: How many gallons per minute will this pump put out?

- Brake horsepower in = 20
- Efficiency = 50%
- Capacity = 150 gpm

Did you get about 265 gpm? I know the numbers are hard to read, but if you do not want to work with formulas and you need the information, outside of asking someone else, what are your choices?