Travis Irrigation Plans and Supply


How to design a sprinkler system which uses a pump

We have an extensive list of FAQs (Frequently Asked Questions). If you have questions you don't see answered below, look for the answer in our FAQ section.

Designing a sprinkler system to operate using an existing pump and pressure tank takes care to make sure the system isn't designed to overload or UNDERLOAD the pump. Yes, you can "underload" a pump.

You can't go over and you can't go under...
If the station flow rates are too high the pump will be overloaded and will not be able to build the required pressure to operate the sprinklers efficiently. Conversely, if the stations are designed with a flow rate that is too low, the pump will be "underloaded" and run ahead of the demand for water. The pressure tank will fill up while the station is watering and the pump shut-off switch will activate. As the sprinklers continue to operate, the tank pressure will drop below the minimum setting and the pump will cycle on again. This repeated cycling on and off will wear out your pump relay before you can say "I screwed up".

Design between max and minimum flow...
To prevent "underloading" the pump, you want the sprinkler stations to be sized large enough that the pump can never reach the shut-off pressure setting on the tank. The flow rate at the shut-off (maximum) tank pressure is a lower limit for station design flow rate. The maximum flow rate for any design will be the flow that results in such a low pressure that you can barely make adequate pressure at the sprinklers. Of course, at all times you must observe the maximum 5 psi pressure drop due to pipe friction losses as described in our layout section. Ideally, the sprinklers will operate at the ideal pressure for that particular type of sprinkler and the station flow rate will be designed to match the flow your pump can deliver with that pressure at the sprinkler head. By calculating the pressure drop in the pipe between the pump and sprinkler, you can determine the pressure at the pump outlet during operation.

Defining the operating curve...
To size the station flow rates you need to determine what flow and pressure your pump can deliver. If you can't find the curves for it, and don't want to pay someone to check the flows for you, you can do it yourself. Measure the flow rate of the pump at the max and min pressure switch settings for your tank. To do this, install a tee and a manual valve where you want to feed the sprinkler system, then open the valve. Adjust the manual valve so the tank can't quite reach the shut-off pressure, then read the flow rate by measuring the water in a large bucket and timing how long to fill it up. Make sure the system is at steady-state when you read the flow. Don't be fooled by the tank dumping a huge load of accumulated water, leading you to think the pump can keep that up forever! Make sure to wait long enough to take each reading so that the tank will not be fooling you about how much water the tank can deliver. Next, adjust the manual valve so the pressure tank runs near the low end of the range, and measure the flow again at steady-state. Do this at several pressures between the on and off switch settings for your system. By doing this you will establish the operating curve for your pump in it's existing setting.

Determine optimum pump operating pressure...
Determine the ideal operating pressure for the sprinklers you plan to use. For Hunter PGP rotor nozzles, for example, the ideal pressure is 50 to 60 psi. For most spray sprinklers, it's 30 to 40 psi at the sprinkler. If you have a system with some rotor stations and some spray stations, you may have much larger flow rates for spray stations than for rotors, or you'll have to install flow control valves to drop pressure for the spray stations. Calculate the pressure drop through the pipe from the pump to the sprinklers. (This should be less than 5 psi, if not, you screwed up the pipe sizes!) Add the desired sprinkler head pressure and the pressure drop through the pipe, and about 2 psi extra for valve pressure losses. This total is the pressure you want to operate your pump at.

Determine optimum flow rate...
Look at the operating curve you defined above, and read the flow rate that coincides with the total pressure calculated above. This is the optimum flow rate your sprinklers want to operate at.

Hedge your bet...
Design the station flow rates to be 10% to 20% lower than the flow rate you just determined above. This will allow room for the pump output to reduce, yet still provide adequate flow. If this results in a flow rate and pressure that is close to the minimum flow just before the the pump shuts off, you can add a small valve between the pump outlet and the pressure tank. By adjusting this valve partially closed, the pump flow rate will be reduced at a given tank pressure, giving you some room to make adjustments to balance the pump flow and tank pressure.

Time to get to work...
With a careful analysis of the flow characteristics of your pump, the sprinklers you plan to use, and the pressure drop through your pipes, you can design your stations to operate within the design window of your pump, as well as the sprinklers you have chosen. If you try to cut corners, you can end up with a real mess, and create a lot of ongoing maintenance... or worse! By following these instructions, you should be able to determine whether your pump is adequate for the sprinkler system you have planned.

Pump Relays
Any irrigation system fed by a pump will require a relay such as the Hunter PSR Pump Start Relay listed in the controller section of our catalog to provide power for the pump. The controller simply can't provide nearly enough power for the pump, so a large power line is run directly to the pump from the breaker box, and a pump start relay located at the pump is activated by the controller when it's time to water. The pump start relay requires about the same amount of power to activate as a standard station valve, so the controller must be designed to support the additional power required. Most of the cheap controllers at the retail stores don't have this feature, and even when they do, their transformers are often severely taxed by the additional load. Nearly all of the controllers we offer in our catalog can comfortably run a pump relay in addition to a station valve, and many can run two valves and a pump simultaneously. If your controller advertises a "master valve" or "pump start" feature, this indicates it can support the power required to activate the pump start relay. Some people who do not have a pump choose to use this feature to power a Master valve.

You'll also find some information relevant to pump installations on our well page.

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