Last month, we talked about friction loss due to
pressure drop, with a look at the factors that contribute to friction loss –
flow velocity, pipe size, length of pipe, viscosity of the fluid, smoothness of
the inside of the pipe, and the number and types of valves and fittings in the
system. We presented charts showing the pressure drop for water in various
sizes of pipes, as well as a chart showing the pressure loss through valves and
fittings.

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 away fixture in the house.

In the first part, from the pump to the pressure tank, 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 (as
controlled by the pressure switch) 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 most demanding fixture in the house.

There are just five steps to determine the pressure
drop due to friction in a piping system:

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 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’s sake, we
show only three fixtures in the house, each drawing 5 gpm. The valves and
fittings are not drawn, but are described below.

The first two sections, from the pump to the tank, total 110 feet of 11⁄4-inch
schedule 80 PVC. It has a 11⁄4-inch spring check valve and a 90-degree elbow.
Table 3 shows the check valve to have the same friction loss as 9 feet of
11⁄4-inch of PVC pipe, and the elbow to be equal to 7 feet of PVC. Therefore,
the apparent total length of the first section is 126 feet. From the Table 1
above, we see that the pressure loss in terms of feet of head for 11⁄4-inch of
PVC pipe at 20 gpm is 8.3 feet per 100 feet. Dividing 8.3 feet by 100, and
multiplying by 126, we get the total loss for the 110-foot section with one
check valve and one elbow. The answer is 10.5 feet of head.

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 assume the
following: A 11⁄4-inch spring 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; 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.

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 the
pumping level in the well to the pressure tank. In this example, we have a
total lift pressure requirement of 100 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,
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 ft. x 0.433 = 7.3 psi). You’ll need
a total of 37.3 psi or a 40/60 pressure switch. (Remember to use the lower
number of the pressure switch to satisfy the household pressure requirements,
and the higher number of the pressure switch to determine the pumping pressure
required to fill the tank. So, for a 40/60 pressure switch, the 40-psi turn-on
pressure satisfies the 37.3-psi requirement of the household, and the 60-psi
turn-off number means 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 tank and to overcome
friction loss from the pump to the pressure tank.)

And 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.

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, and see if you can find a pump that would give you 20 gpm in your
favorite pump supplier’s catalog. ’Til next month … .

ND

# Tech Topics: How to Calculate Pressure Drop Due to Friction in a Piping System - Part 2

February 1, 2011

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