How to Determine the Pressure Requirements of a Pump
There are three factors that go into the determination of the pressure required from a pump in a residential water system.
Lift Pressure - pressure required to get the water from down in the well to the pressure tank.
Household Pressure - pressure required in the house to properly feed the fixtures.
Friction Loss - pressure required to overcome the friction in the pipes and fittings.
This month we will examine the first two factors and tackle friction loss next time.
Lift PressureThe first factor to consider is the pressure required to lift the water from the well to the pressure tank. The total lift required is the distance from the top of the water at the pumping level in the well to the highest fixture in the system, typically a second-story showerhead. We'll let the pressure tank and pressure switch take care of the in-home lifting/pressure part as will be explained in a few moments. For now, let's only consider getting the water from the well to the pressure tank.
The pumping level in the well is the level that the water drops to after the pump has been running long enough for the water level to stabilize. When the pump has been off long enough for the water level to rise to its natural height, this level is called the standing water level or the static level.
The difference between static level and pumping level can be as little as a few inches in an underutilized high-yield well to several hundred feet in a slow producing well. The amount of lifting work the pump has to do is determined by the pumping level, not the static level or position of the pump in the well. The pumping level can change from season to season and year to year, depending on ground water availability. Make sure you size your system to take into account the lowest probable pumping level.
Household PressureIt is important to provide enough water pressure at the fixtures in the home to satisfy the comfort needs of the family. The Uniform Plumbing Code requires a minimum of 15 psi at each fixture. The easiest way to deal with the household pressure component of the total system pressure requirements is to choose a pressure tank operating range which will provide at least 15 psi at the highest fixture. More on this in a moment.
Lift pressure is measured in terms of feet of head, i.e. the pumping level is 60 feet below the pressure tank. The household pressure is measured in terms of pounds per square inch (psi), i.e. the pressure tank operates through a range of 30 psi to 50 psi, for example. At this point, we need to re-introduce the relationship between psi and feet of head. In case you missed our previous article on this relationship, here is a repeat of the summation paragraph from that article:
"Pressure is measured in terms of psi (pounds per square inch) or feet of head. You probably are familiar with psi because you can see it on a pressure gage, but how does feet of head relate? Picture this: take an empty piece of pipe, any diameter, about three feet long, plug the bottom and put a pressure gage in the plug. Stand it up straight and fill the pipe until the pressure gage reads 1 psi. This is a hypothetical exercise, but if you had an accurate enough pressure gage, you would see that 2.3 feet of water standing in the pipe produces 1 psi on the gage. If you lowered the water level to 1 foot, you would see 0.433 psi on the gauge. Both terms are used in the pump industry and you can convert from one to the other by using the factors, 1 psi = 2.3 feet of head or conversely 1 foot of head = 0.433 psi."
The Pressure SwitchThe function of a pressure switch is to turn the pump on when the system pressure drops to the minimum set point and to turn it off when the system pressure reaches the maximum set point. The most commonly used pressure switches have a 20-psi spread between the turn-on point and the turn-off point, and they are available in a number of on/off presets - 20/40, 30/50, 40/60, etc. With a 20/40-pressure switch, the pump will be turned on when the pressure drops to 20 psi, and turned off at 40 psi. With a 30/50-pressure switch, the pump will be turned on when the pressure drops to 30 psi and off at 50 psi, and so on.
It is the pressure switch turn-on pressure (30 psi for a 30/50 switch) that determines the minimum system pressure at the highest fixture in the house. And it is the pressure switch turn-off pressure (50 psi) that is the household pressure portion of your system pressure calculation. Let's say you have a 30/50-pressure switch. That means the minimum system pressure at the tank will be 30 psi. For each foot in elevation above the tank, you loose 0.433 pounds. If you have a single story house and the pressure tank is in the garage, 3 feet below the living level, that 7-foot high showerhead in the bathroom will be 10 feet above the pressure tank. Therefore, the pressure at this point will be 4.33 psi less than the pressure at the tank (10 feet X .433). The minimum system pressure at the showerhead will be 25.7 psi (30 - 4.33 = 25.67), assuming no friction loss in the pipes.
If you have a two-story house with a basement, that showerhead could be 25 feet above the pressure tank. In this case, the available pressure at the shower would be 30 - (25 X 0.433) = 19.2 psi, sufficient to meet the minimum of 15-psi requirement.
Calculating the total pressure required of the pump - The static water level in the well is at 50 feet below the pressure tank, and the well draws down10 feet, making the pumping level 60 feet. The household pressure requirement would be 50 psi because you are using a 30/50-pressure switch. The total pressure required without friction loss, in terms of feet of head, would be as follows:
Lift pressure: 60 feet
Household pressure: 50 psi X 2.31 = 116 feet
Total: 176 feet
Converting from feet of head back to psi you get, 176 feet X 0.433 = 76 psi. (Mathematically speaking, it doesn't matter which terms you use, just be consistent. You cannot add feet of head to psi in an equation. All factors in the equation must be in the same terms. Once you've finished your math, you can convert back and forth.)"
Next month we will finish the pressure part of our pump sizing exercise by examining pressure drop in pipes.