There are just six steps to determine the pressure drop in a piping system, Robert Pelikan describes.

There are just six steps to determine the pressure drop in a piping system:

1) Make a sketch of the piping system like Figure 1.
2) Determine the peak flow rate.
3) Determine the flow rate through each section of pipe, working from the farthest fixture back to the pump, and note it on your sketch. Also note the diameter, type and length of the pipe in each section.
4) Add the equivalent lengths for the valves and fittings to get the total apparent length of each section.
5) From a pressure drop table like Table 1, find the pressure loss per 100 feet, and calculate the loss for each section of your system.
6) Finally, add up the pressure losses of all of the sections, and you have the total dynamic pressure loss.

Let's try an example using the system in Figure 1. We will plumb it all in copper and ignore the valves, tank fittings and fixtures for simplicity's sake. In an actual pressure loss calculation, the loss through these items should be considered.

The first section, starting at the pump, is 80 feet of 1-inch copper. It has a 1-inch copper elbow, which the table in last month's article shows to have the same friction loss as 3 feet of 1-inch copper pipe. Therefore, the apparent total length of the first section is 83 feet. From Table 1, we see that the pressure loss in terms of feet of head for 1-inch copper pipe at 15 gpm is 16.5 per 100 feet. Divide 16.5 feet by 100 and multiply by 83 to get the loss for an 80-foot section with a 1-inch elbow. The answer is 13.7 feet of head. Table 2 shows the loss for all of the sections in our example. For practice, cover the answers, and see if you can come up with the same results.

To review, there are three elements that must be considered in determining the pressure requirements of pumps:

  • The lift pressure, which is the pressure required to get the water from down in the well to the pressure tank. In our November example, we had a total lift pressure requirement of 60 feet.

  • The household pressure, which is the pressure required to feed the highest point in the system with at least 15 psi. This household pressure is controlled by the pressure switch. If you need 15 psi at the highest fixture in the house and another 15 psi to get it from the pressure tank up to that fixture, you'll need a 30/50-pressure switch. Remember to add the pressure switch differential to determine the pumping pressure required. For a 30/50-pressure switch, you'll need to allow for 50 psi of pump capability to have a minimum of 15 psi at the highest point.

  • The friction loss in the piping system, as we did this month when we came up with 27.5 feet.

And remember not to mix pressure terms when doing your math. In the preceeding summary, the discharge pressure was expressed in terms of psi and the lift pressure and friction loss in terms of feet of head. To add them up, you will have to convert to the same term. To use feet of head, multiply the household pressure of 50 psi by 2.31 to get 115 feet of head. Add that to the 60 feet of lift pressure and 27.5 feet of friction loss gives you a total pump pressure requirement of 202.5 feet. You can see that the pipe and fittings do not add too much pressure loss to the total (less than 10%) as long as they are adequately sized and not too long.

Next month we will talk about the pump selection process, how to read pump curves and how to select the right pump. If you want to get a head start on pump selection, use the pressure requirements we came up in these last three articles on pumped water systems, and see if you can find a pump that would give you 15 gpm in your favorite pump supplier's catalog.