What is a Pressure Regulator?

By name and definition, a pressure regulator is simply a valve designed to automatically regulate water pressure in pipes or tubing downstream of its placement, to prevent downstream pressure from exceeding a predetermined set point. Irrigation system designers use pressure regulators to control and tailor a system's hydraulics to meet specific design objectives.

A pressure regulator maintains downstream pressure by automatically modulating the area of opening through the device. By changing the area of opening as upstream pressures vary, pressure loss through the valve changes proportionally to maintain the downstream pressure at a relative constant.

There are various models of pressure regulators available to meet specific flow and pressure requirements. To ensure proper regulator function, the designer should always observe the operating parameters stated by the pressure regulator manufacturer. The manufacturer's parameters usually include a maximum and minimum upstream pressure, downstream pressure to be maintained and an acceptable flow range. Understanding that all pressure regulators are designed to accurately reduce system pressure, it is important to always design for an upstream pressure greater than the desired downstream pressure.

Why Regulate Pressure?

Distribution Uniformity

A standard design objective is to take a predetermined amount of water and apply it uniformly over a predetermined area. Uncontrolled pressure fluctuations into applicators or emitters result in unwanted flow deviations. Consequently, it is important to keep system pressures somewhat constant in order to expect uniform distribution of water throughout an irrigation system.

System pressures will vary throughout the system due to friction loss through pipe and fittings and elevation changes. Once the system is installed, pipe diameters are constant and topography does not change. The predictability of these pressure variations makes it possible for a designer to adjust for them on stationary systems. However, with mechanical move systems have the potential to experience elevation and pressure changes as they operate. This scenario could easily cause flow fluctuations in excess of plus or minus 5%, in which case, pressure regulators are recommended. 

More difficult to forecast and design for on all systems, are some of the factors that cause system pressures to change during operation. A classic example in low volume or solid set is blocks or zones cycling on and off. For mechanical move, it might be a center pivot system's end gun or corner arm valves turning on and off. These changes in system flow demand may also cause significant fluctuations in pressure.

Proper placement of pressure regulators will prevent any of the above-mentioned causes for system pressure variations from affecting the uniformity of water distribution. The designer need only provide adequate pressure up to the point of regulation.

System Component Performance

Some system components may fail or exhibit unacceptable performance characteristics if pressures exceed a specific point. Most low-volume tubing or tape products have a maximum pressure rating to prevent product damage or failure. Pressure compensating products have a pressure range which, if exceeded, will fall out of a tolerable flow variation.

Pressure regulators may be required if the pressure produced by the pump is too large or if zones vary greatly in size. A pump that was sized for a previously existing sprinkler system would likely operate at pressures which are excessive for components of a new system. A system might consist of different sized zones. The pump must deliver the amount of water required in the largest zone at the pressure required. If some zones are significantly smaller, the pump will produce higher pressure at the smaller discharges required by these zones. This pressure must be reduced by pressure regulators to the pressure required by the drip tape used for the lateral lines.

Mechanical move system components have a range of pressures that provide optimal performance. For example, when pressures are allowed to exceed 15 to 20 psi, spray nozzles can produce water droplets that are very susceptible to evaporation and wind drift. Product longevity can be enhanced for many other applicators by heeding the manufacturer's maximum pressure recommendations.

Energy and Water Conservation

An increasing awareness of energy savings and pumping costs are resulting in efforts to conserve energy through improve irrigation system design. This objective can be achieved by lowering the operating pressure of a system. However, the lower the system's design pressure, the more critical it is to control pressure fluctuation. If a system designed for 60 psi experiences a 5 psi drop, the result is a 4% flow deviation. If the same system were designed at 20 psi, the same pressure 5 psi drop would result in a 12% flow deviation. That's three times greater.  

With current water availability and quality concerns, placing ever-increasing demand for greater efficiencies in irrigation systems, the use of pressure regulation is critical. Good distribution uniformity is dependent on proper system design. Proper system design hinges on a designer's ability to control the hydraulics of the system through pressure regulation.

The Pressure vs. Flow Relationship

One key formula to calculate the change in flow resulting from a change in pressure is:

New Flow Rate = Old Flow Rate x the square root of (New Pressure divided by Old Pressure)

A common design practice is to regulate any system that, any combination of variables would result in a flow fluctuation of plus or minus 5% on a specific area of the system (10% maximum pressure variation throughout the entire system).

Article submitted by Senninger Irrigation, Inc. www.senninger.com