How to Calculate Pressure Drop Due to Friction in a Piping System – Part 2
This month, we will use this information to calculate the pressure drop due to friction loss in a typical residential system. The system is broken into two parts — from the pump to the pressure tank and from the pressure tank to the highest or farthest fixture in the house. In the first part, the pump must be able to produce enough pressure to get the water to the pressure tank, to overcome the pressure loss in the piping system between the two and to charge the pressure tank to its operating pressure. For the second part, the operating pressure range of the pressure tank must be high enough to provide sufficient pressure at the most demanding point in the house taking friction loss and vertical head into consideration. This is why it is necessary to calculate the pressure drop due to friction for both sections of the system — pump to tank and tank to the most demanding fixture in the house.
1. Make a sketch of the piping system like Figure 1. Note the flow rate through each section of pipe starting at the pump and working to the end of the system on your sketch. It will not be necessary to calculate the pressure drop through each and every branch in the system, just through the most demanding flow path in terms of pressure loss. If your pump can handle the most demanding branch, it will be able to handle the less demanding branches. Also on your sketch, note the diameter, type and length of the pipe in each section.
2. Make a chart like Table 1 showing the type of pipe, pipe diameter, pipe length and flow rate for each section.
3. From a pressure drop table like Tables 2 and 3, find the pressure loss per 100 feet of pipe and calculate the loss for each section of your system.
4. Using Table 4, add the equivalent lengths for the valves and fittings to get the total apparent length of each section.
5. Finally, add up the pressure losses for the sections in each part of the system.
Let’s try an example using the system in Figure 1. We will plumb the pump to tank section in 11⁄4-inch Schedule 80 PVC and the tank to house in copper. The pump flow rate is 20 gpm. For simplicity sake, we show only three fixtures in the house, each drawing 5 gpm. The valves and fittings are not drawn but are described below.
Table 4 shows the loss for the sections on the house side of the pressure tank in our example. Figure 1 does not show the valves and fittings, but you should assume a 11⁄4-inch swing check valve and elbow in Sections 1 and 2, a 1-inch gate valve in Section 3, a 1-inch straight flow tee and a 3⁄4-inch branch flow tee in Section 4, a 3⁄4-inch straight flow tee and a 90-degree 1⁄2-inch elbow in Section 5 and branch flow 1-inch tee in Section 6. For practice, cover the answers and see if you can come up with the same results.
As mentioned at the beginning of this article, it is not necessary to include all sections in your calculations, only those sections in the most demanding flow path. In our example, the flow path through Sections 4 and 5 is more demanding than Section 6, so Section 6 is ignored in terms of frictions loss. Section 6, being a second-story fixture, would be taken into consideration when figuring the static head required from the tank to the highest fixture. Remember that the pressure setting at the tank takes care of the static, dynamic and fixture pressure requirements of the system, and the pump takes care of getting the water from the well to the pressure tank and pressurizing the tank to the shut-off pressure setting of the pressure switch.
1. The lift pressure — the pressure required to get the water from down in the well to the pressure tank. In this example, we have a total lift pressure requirement of 100 feet.
2. The household pressure — 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, another 15 psi to get it from the pressure tank up to that fixture and 7.3 psi to overcome the friction in the pipes (16.4’ X .433 = 7.3 psi). You’ll need a total of 37.3 psi or a 40/60 pressure switch. (Remember to use the higher number of the pressure switch to determine the pumping pressure required to fill the tank. For a 40/60 pressure switch, you’ll need to allow for 60 psi of pump pressure plus what it takes to get the water from the pumping level to the surface and to overcome friction loss from the pump to the pressure tank.)
3. The friction loss in the piping system as we did this month.
Remember not to mix pressure terms when doing your math. In the preceding 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 on the pump side, multiply the pressure switch shut-off of 60 psi by 2.31 to get 139 feet of head. Add that to the 80 feet of lift pressure (assuming a pumping level of 80 feet) and 10.5 feet of friction loss gives you a total pump pressure requirement of 229.5 feet of head. You can see that the pipe and fittings do not add too much pressure loss to the total (about 5%) as long as they are adequately sized and not too long.