What happens when we change the flow going through a piping system? 16-06

The affinity laws tell us what is going to happen to the fluid flow, head and power requirement when we change the diameter of the pump impeller or change the speed of a centrifugal pump, but what happens to the system when we change the amount of fluid going through the piping?

If the pipe size is not going to change, the friction loss in the piping will vary as the square of the capacity ratio. That sounds kind of complicated so let’s look at an example.

You have been pumping 300 gallons per minute through your piping system and now the production people want to increase the capacity to 500 gpm. You had pulled out your pipe friction charts and calculated that the combined piping, valve and friction losses totaled 20 feet when you were pumping 300 gpm. What is the loss going to be now that the capacity has increased to 500 gpm? Here is the formula:

If we enter the numbers we get:

= 56 feet of head loss

Look what happened! We increased our flow by 1.67 times and our piping resistance increased 2.8 times. Why do you need to know this? Because you need this new friction head number to add to the static and pressure heads:

- To determine how close you are going to be to the pump’s BEP. If you are too far off the BEP you are going to have mechanical seal and bearing problems as the internal hydraulic forces try to deflect the shaft from its designed centerline.
- To calculate the horsepower you will need to pump this increased amount of fluid through the piping.
- To determine if the pump might go into cavitation. Higher capacities require more NPSH available

Suppose we reduced the flow through the piping from 300 to 250 gpm? What happens then? Let’s put those number into our formula

= 14.2 feet of head loss