Understanding pressure

Irrigation systems are designed to distribute a predetermined amount of water over a specific area. Applicators operate within a specific range of flows and pressures. Because pressure affects flow, pressure regulators are used, so applicators run at the designed operating pressure to help assure delivery of the designed application rate. All systems experience pressure fluctuations. Some of the causes include elevation changes within the irrigated area; pressure loss through pipes and fittings; fluctuations when zones cycle on or off; system demand change on large projects with multiple wells providing water; and activation of end guns and corner arms on mechanized systems.

Pressure regulators control excessive and varying inlet pressures to a constant outlet pressure and significantly improve overall irrigation system efficiency. They can help lower energy costs and save water by reducing wind drift, evaporation, and runoff.This presentation provides the basics of pressure regulators, how to install pressure regulatorsin different types of irrigation systems, how to recommend the correct model for different installations, and how to identify problems and troubleshoot issues that can occur.

 

PRESSURE REGULATOR INSTALLATION RECOMMENDATIONS

Direction of flow. Each pressure regulator is marked to indicate the direction of the flow. If installed improperly, pressure regulators will behave more like a check valve, allowing little to no downstream pressure. This improper installation will also damage internal components.

Flow range. Each pressure regulator is designed to handle a specific flow range printed on the outside. Flows that greatly exceed this range could shorten the life of the pressure regulator. Flows that fall beneath that range will prevent the regulator from functioning properly and can produce a high-pitched squealing sound.

Maximum pressures. Each pressure regulator is designed with a maximum pressure rating, typically 80 psi or 5.51 bar above the designed pressure rating for that model. Operating outside these recommen­dations will damage the regulator and may cause vibration that manifests itself in a loud chattering sound. Check the manufacturer’s specifications.

Shut-off valves. Pressure regulators should be installed after the shut-off valve to avoid damaging the regulator from prolonged exposure to back pressure.

Water hammer. If systems fill with water quickly at start-up, it can create high pressure shock waves or a water hammer which can damage system components. If this hammer exceeds the regulators maximum inlet pressure, it can damage regulators. This damage is often evidenced by leaking between the upper and lower housings.

Mechanized systems. Pressure regulators are usually installed imme­diately preceding the sprinkler. Some prefer to install the pressure regu­lator on the outlet or inlet side of the gooseneck. Increased pressure or head between the regulator and the sprinkler should be a design consideration.

Solid Set field installations. Regulators are usually installed at the beginning of the lateral.  Although, based on the design, one regulator can be used to manage pressure for several laterals.  Certain installations may require a pressure regulator for each sprinkler. In high flow scenarios, a high flow model should be used. Although, multiple regulators in manifold can be used to handle the specific flow requirements. Timer-control installations employ regulators after the control valve, whether for multiple units or inside a valve box.

 

WHAT IS THE DIFFERENCE BETWEEN A PRESSURE REGULATOR AND A PRESSURE REGULATING LIMIT VALVE?

“Pressure regulating limit valves are used where there is a shut-off valve downstream. When this shut-off valve is closed, the limit valve’s [throttling] stem flow-passage closes and seals to limit the outlet pressure to only 10 to 15 psi or 0.69 to 1.03 bar above its normal regulating pressure. This helps protect downstream components from potential damage due to high static upstream water pressure. With a standard regulator, when the downstream shut-off valve is closed, the [throttling] stem is unable to seal completely against the harder seat. The high inlet pressure eventually equalizes across the regulator and up to the valve. Upon opening the shut-off valve, a high-pressure surge could damage downstream meters, sprinklers, or other plumbing components” (Senninger Irrigation, 2018, p. 15).

 

WHAT IS THE DIFFERENCE BETWEEN A PRESSURE REGULATOR AND A PRESSURE CONTROL VALVE?

Pressure control valves are sized by flow rate to control pressure in large higher flow piping systems. Most open and close slowly to help prevent water hammer and surge (Nelson Irrigation, 2019).

Pressure control valves react through a pressure control regulator. Depending on the needed functionality, you can choose pressure reducing valves, pressure sustaining valves, or both, with many models available with manual or electric closing control.

 

WHAT ABOUT FLOW CONTROL NOZZLES?

Flow control nozzles can sometimes eliminate the need for pressure regulators. In some cases, they may pass debris more easily than conventional nozzles and provide a more economical system. Operating pressure alters the nozzle orifice to change the flow rate. The flow remains “nearly constant” (Kranz, Irmak, Martin, & Yonts, 2007). Be sure your system has pressure sufficient to activate flow control nozzles and yet not so high as to negatively affect sprinkler performance.

 

SUMMARY

Investing in new pressure regulators is worth the investment when compared with the time and money lost in wasted input costs and potential yield loss. Pressure regulators help make the most of the water applied to improve crop yield. They are an important tool to help provide food, fiber, and fuel for a growing population.

 

Author
by Nelson Irrigation Corporation, Senninger