In this installment of “Pumping Prescriptions,” I have a quiz for our readers. A typical pump system is shown in Figure 1. A centrifugal pump delivers 200 gallons per minute of water from an open tank. Using the information from Figure 1, answer the following questions:
Figure 1 (right). Typical centrifugal pump system
1. What is the available NPSH?
2. What pressure does a suction gage read?
3. What differential head is this centrifugal pump producing? What differential pressure is it generating?
Here are a few rules and pointers to consider when solving this quiz. The first is the relationship between pressure and head. Figure 1 shows a mix of “apples and oranges”—units of pressure and units of head. You will need to convert them all to a consistent set expressed in feet.
The next hint is to review the basic definition of pump head. It is the difference between the total discharge head and the total suction head. But what is total head? It has three components: pressure, elevation and velocity (dynamic). You need to estimate each (there is enough data provided to do that). Then calculate the difference.
With NPSHA, you need to consider the absolute pressure versus gage pressures (or absolute/gauge head expressed in feet). Then you will need to look up a vapor pressure of the fluid given (to make it easy, this system uses cold water).
Figure 2 (right). The basics needed for the quiz
One thing you are not given is the length of suction pipe from the tank to the pump. So do not worry about calculating friction losses to adjust the NPSHA (perhaps next time!). However to make it a lot more real, the suction losses are nevertheless calculated for you, as 10 feet of head loss from the tank to the pump. Therefore, those of you who are super adventurous, may want to (for extra credit) back-calculate what the length of this 3-inch suction pipe needs to be to produce 10 feet of head loss.
With these hints and pointers, you are ready to start!
Those who e-mail the correct answers will be acknowledged and will receive an admission ticket to the next Pump School!