Taking the mystery out of irrigation pump sizes and selection
This well pump system features a check valve, tank and pressure switch. The size of pump you need depends on many variables.
Photos courtesy of Brian Vinchesi.
Many irrigations systems that utilize potable water have less water pressure than needed to properly operate the sprinklers. In those cases a booster pump is needed. Additionally, in today’s “sustainability” environment, using potable drinking water for irrigation systems is discouraged. As a result, alternative water sources such as rainwater, stormwater, lakes, ponds and wells are becoming more and more popular as the water source for irrigation systems.
When using alternative water sources for irrigation, pumping is a must, and on larger systems, several different pumps may be involved. So how do you decide what size pump to install for an irrigation system?
Flow and Pressure: Although there are many different parameters that need to be considered when selecting a pump, the two most important are flow (GPM) and pressure (TDH). This is also the minimum amount of information that a supplier will need to pick an appropriate pump for your application.
Flow is based on the largest irrigation zone that is to be supplied by the pump system or the largest combination of zones if the system is envisioned to operate more than one zone at a time. Flow can also be determined by how much water needs to be applied divided by how fast you want to irrigate.
For example, a 1-acre site that needs to apply 0.15 inches of water over a period of six hours would require a minimum pump flow of 12 GPM.
TDH or “Total Dynamic Head” is a bit more complicated. It consists of three components: elevation change, friction loss and the operating pressure of the sprinklers. Elevation change is the distance in feet from the lowest point that the water reaches to the highest point on the irrigation system.
Friction loss is all of the pressure losses through the system including: service line, water meter, backflow prevention device, pipe, fittings, electric valves and swing joints/sprinkler supports.
Operating pressure is the pressure required at the base of the sprinklers. If the system has a mixture of different type sprinklers, use the highest pressure requirement.
These three parameters are calculated/measured in PSI or feet so some factors will need to be converted so you can add them up.
Pumps are in “feet” not “PSI” by definition total dynamic head, so it is best to convert all the units to feet. For example an irrigation system might have: 20 feet of elevation change; 15 PSI in friction losses; and a sprinkler operating pressure of 45 PSI with a flow requirement of 30 GPM. The TDH is then 20 feet + 34.65 feet (15 PSI x 2.31 feet/PSI) + 103.95 feet (45 PSI x 2.31 feet/PSI) for a total of 158.60 feet or 159 feet. The pump requirement is then 20 GPM @ 159 feet.
Pump Type:Now that the pump size has been determined, you will need to decide what type of pump you would like to install. The type will be determined by the application, horsepower required and water source. If the pump is going to be used to boost the pressure on a potable water supply then an end suction centrifugal pump is also needed.
If pulling from a pond, lake or stream, end suction centrifugal pumps will also work within limits or a submersible pump can be used. If a shallow well then an end suction centrifugal well or a jet pump are good choices, but if over 25 or so feet deep, a submersible would be needed.
Remember atmospheric pressure (14.7 PSI) limits the ability of a pump on the suction side so whenever a suction lift is being used consult with the supplier and keep the pump as close to the water as possible.
On larger commercial systems that utilize high flows and have a lake, pond or stream as the water source you might consider a turbine-type pump. Turbine pumps are more efficient and require less maintenance then other types of pumps. They also have no suction side, which minimizes issues as 99 percent of pump problems are on the suction side. Turbine pumps require a wet well for their installation, which keeps the motor high and dry, so the upfront costs are higher, but the long-term operating costs are less.
As mentioned, alternative water supplies require lots of pumping and the applications can be quite different depending on the supply source. For example, if you are collecting rainwater into a tank, you could use a submersible pump in the tank or in a wet well beside the tank, flooded suction centrifugal in a chamber to the side of the tank in a manhole. The application of the pump will also be dependent on the type of storage tank or cistern that is storing the water.
More Than Just A Pump: No matter what the pump application there are certain features that every pump system needs to have to properly operate, troubleshoot and service it.
First, make sure there is some type of shutoff valve on the discharge of the pump. This can be a gate valve or on larger systems a butterfly valve. Ball valves are not suggested. These isolation valves are used to provide back pressure on the pump when filling an empty pipeline and to isolate the pump so the system would not have to be drained.
The discharge side of the pump should also have a check valve to prevent water from flowing backward through the pump causing it to spin backwards and to also take pressure off the foot valve. On the suction side of the pump you want to have as little stuff as possible so that you do not impede the pumps ability to receive enough water.
There should be some sort of strainer to prevent debris from entering the pump when pumping out of a stream or lake. If in a well, the well screen will serve this purpose. In a booster situation there is just piping.
This end suction centrifugal pump has a pressure transducer and flow meter. Every installation must have some way to control pressure.
In a lift situation you will also need a foot valve to keep the water from running out of the suction side piping when the system is off. In most cases you would purchase a combination foot valve and strainer to be installed in the water source.
Once you have a pump, you will also need to decide how the pump is going to start. There are several options: pressure, flow or controller/relay start. Small pumps, less than 2 hp, can be easily started from the irrigation controller using a pump start relay. Once larger than 2 hp, there will need to also be a starter as the relay alone will not have enough power to turn on the pump. Remember that with a controller start, however, the pump will only come on if an irrigation zone also comes on so manual watering is not feasible.
A flow start is possible under the right circumstances. The pump needs to be in a location where the flow switch can sense that the water has moved. As such, the site would need to have no upward elevation change. Flow switches have very limited applications.
The most common type of pump start is with pressure, utilizing a pressure switch or pressure transducer. A pressure start works on the premise that when the irrigation system goes on and water starts to flow, the pressure will drop, and that will be a signal to turn the pump on. Conversely the pressure switch or transducer will sense that the pressure goes up when the water stops flowing and the pump will then turn off. Pressure switches are mechanical devices and transducers are electronic devices that do basically the same thing.
Pressure Regulation: Based on the pump’s capacity and the irrigation system zone sizes, a pressure tank may be needed to keep the pump from cycling. Frequent cycling of the pump on and off wears out the motor quickly. Ideally, when using a pump you should try and keep all the zones approximately the same size so that the pump does not cycle. If not, a pressure tank or tanks of the appropriate size will act as a buffer and when the water stored in the tank is exhausted the pump comes back on.
In lieu of the tank a variable frequency drive (VFD) can be used for systems with varying flows. A VFD moves the design point around so that the discharge pressure stays the same at variable flows.
With any pump system installation, there has to be some way to control the pressure for varying flows, otherwise, best case scenrio the sprinklers do not operate at the proper pressure, and worst case scenario, things break.
Selecting pumps for irrigation systems can be daunting as there are thousands of pumps manufactured. The easiest way is to figure the required flow and pressure and know the application. With that knowledge, go to your pump supplier and let them pick the most efficient pump for your unique application. All pump systems are different so each pump needs to be picked for its specific application.
Brian Vinchesi, the 2009 EPA WaterSense Irrigation Partner of the Year, is president of Irrigation Consulting, Inc., an irrigation design and consulting firm with offices in Pepperell, Mass., and Huntersville, N.C., that designs water conserving irrigation systems throughout the world. Contact him at [email protected].