Both a pressure regulator and a pressure regulating limit valve maintain a constant outlet pressure when handling rated flows. The difference comes when there is no flow through the device.
Pressure regulating limit valves are designed to control pressure when there is NO water flow through the device. In contrast, a standard pressure regulator will not regulate without flow.
Controlling pressure when there is no water flowing through a system might seem strange, but consider that some irrigation systems have their shut-off valves downstream, as shown in the image above.
When the downstream shut-off valve is closed, there is still pressure in a system, but there is no water flow. The high inlet pressure eventually equalizes across the regulator and down to the shut-off valve. When this shut-off valve is opened again, a high-pressure surge can damage downstream meters, sprinklers and other components.
A pressure regulating limit valve can protect these components.
How Does a Limit Valve Work?
If we were to place a standard pressure regulator upstream, the regulator’s T-stem will be unable to completely seal against the hard seat in the device to stop the flow. A pressure regulating limit valve is designed so that its t-stem closes the flow-passage and seals on the rubber portion of the seat.
This holds the pressure 10 to 15 psi (0.69 to 1.03 bar) above the design pressure and helps protects downstream components from a high pressure surge.
Remember: a pressure regulator MUST be installed downstream (after) from all shut-off valves. A pressure regulating limit valve, with its different internal construction, CAN BE installed upstream (before) of a shut-off valve.
With either device, it's recommended to use a filter upstream is (100 mesh or finer, capable of flowing up to 18 gpm with no more than 5 psi internal friction loss).
Go in-depth into pressure regulator fundamentals
Learn the fundamentals of pressure regulation with Senninger's free on-demand Pressure Regulation course on Hunter University. Learn more about the difference between a pressure regulator and a limit valve, how to install pressure regulators in different irrigation systems, identify wear issues, causes of pressure fluctuations and more.