Agricultural irrigation systems generally require adequate and constant pressure in order to operate efficiently while minimizing problems. In areas that have unavoidable pressure drops and spikes, the system may require certain control valves, regulators or compensators to protect and enable the system to perform as efficiently and uniformly as possible.
These controls have functions such as pressure regulation, pressure sustainment, pressure relief and pressure compensation. Knowing which function is necessary and where to use it takes knowledge and experience and can make or break a system design. At the same time, these products must be sized and rated for the given flow rates and pressures in order to function properly.
Knowing which function is necessary and where to use it takes knowledge and experience and can make or break a system design.
In order to fully grasp the design concepts of when and how to use these products, it’s important to understand their different functions and terminology. As mentioned, the main hydraulic controls to improve uniformity and distribution include pressure regulation, pressure sustainment, pressure relief and pressure compensation.
Pressure regulation typically refers to any device that reduces higher and sometimes varying upstream pressures to lower constant downstream pressures. Regardless of the incoming pressure and flow rate within a certain range, the regulator must be able to maintain the set pressure downstream. More often than not, it also must be able to respond quickly to changing upstream conditions with minimal variations in performance.
The two most common regulators are pressure regulating control valves and inline hose regulators. They are most often installed at the head of a hose, head of a manifold or head of a submain. Hose regulators are typically nonadjustable whereas most regulating valves are adjustable. They are both used in order to protect downstream systems from over-pressurization. Sizing pressure regulators is very important because they all work within a given flow range. The valves need a certain velocity and pressure loss across them in order to function properly. If the system flow rate is too high or too low, the valve will most likely fail and inevitably cause problems or breaks down the line.
Opposite to pressure regulating valves, pressure sustaining valves control and maintain a desired upstream pressure, regardless of fluctuating flow or downstream pressure variations. The valve will gradually open or close in order to maintain the minimum preset upstream pressure. If the pressure upstream of the valve is less than the setpoint on the valve, it will remain closed. As the pressure increases it will start to open the valve, and once the line pressure exceeds the setpoint on the valve, the valve will fully open.
The most common application for pressure sustaining valves finds them downstream of any type of self-cleaning filter. All self-cleaning filters have a minimum operating pressure requirement that can most easily be met using a properly sized pressure sustaining valve. Other common uses for sustaining valves include controlling pipeline fill-up and preventing pipeline emptying, pump overload and pump cavitation.
In low-pressure drip systems, a combination pressure reducing and sustaining control valve is often used. This is a hydraulically operated, diaphragm-actuated control valve that sustains a minimum backpressure for a filter while reducing and never exceeding downstream pressure requirements. Just like pressure regulating valves, these valves must be sized accordingly in order to function properly.
One of the most important safety features in an irrigation system is a pressure relief valve. Although there are a few types of pressure relief valves, they all function by exhausting water out of the system when pressure exceeds a desired preset value. They are typically hydraulically operated and spring- or pilot-actuated control valves. If sized correctly, they respond to a system pressure spike quickly, accurately and repeatedly. When placed in the correct locations, relief valves will protect an irrigation system from accidental pressure bursts as well as provide a visual indication of high pressures or system failures.
The most common causes of pressure spikes include sudden closing of valves, excessive water velocity and clogging filters. The most common locations for pressure relief valves include low points, pump stations and anywhere in the system where certain products need protection from over-pressurization. On pump stations without variable frequency drives, a pressure relief valve can act as a reservoir or tank return valve when the system demand is significantly lower than the pump capacity. In high-pressure situations, it is often recommended to use both a spring pressure relief as well as a pilot-controlled hydraulic relief valve in the same location. Adding a relief valve to an irrigation system is a low-cost preventative that should be considered by all growers.
With low pressures and relatively flat terrain, a properly designed system can be fairly efficient using conventional nonpressure compensating emitters. As systems have evolved and demands have changed, designers looked for new ways to improve uniformity and help farmers save water. Arguably one of the most important improvements for irrigation system efficiency and uniformity is the pressure compensating emitter. Opposite of a pressure regulator, a pressure compensator will allow a dripper or microsprinkler to run at the same or very close to the same flow rate over a wide range of pressures. They have allowed irrigation designers to provide systems with nearly perfect distribution uniformity, while helping farmers save up to 40% of their water.
The emitter has a rubber diaphragm that is designed to open or close the emitter orifice, contingent on incoming pressure, while maintaining a constant output flow rate. This allows designers to stretch hose runs, add more emitters on one line and enable systems to operate uniformly on undulating terrain. Replacing old nonpressure compensating systems with new pressure compensating systems is usually the first step growers take to improve their system uniformity and efficiency.
There are many different types of hydraulic control products that can be used in even more applications. All have been designed and implemented in order to protect irrigation systems as well as improve efficiency and uniformity. When choosing a control valve or pressure compensating emitter, consider factors including operating pressure ranges, flow requirements and installation locations. Consult with an irrigation designer or technician to ensure proper sizing and setting of all types of control valves.