Pressure Regulating Valve Pilot Characteristics - 2-Way vs. 3-Way Pilots

Pressure regulation valves, with 3” – 6” diameters, are commonly found at the entrance to agricultural irrigation blocks and manifolds. These valves are typical of a “hydraulic” design, in which a chamber above a flexible disc/diaphragm is filled or emptied. As the chamber is filled or emptied, the irrigation water passes through a smaller or larger orifice, and the pressure downstream of the valve is lowered, increased, or maintained at a constant target pressure. The flow in/out of the chamber is controlled by a small “pilot valve”. The characteristics of these pilot valves are discussed here, in general terms. Of course, there are always variations and some pilots are sold as some type of combination 3-way/2-way design. Knowing something about pilot valve operation is important because the pressure losses across pressure regulation valves are often different with and without a pilot valve attached. Manufacturer pressure loss curves, if developed without a pilot valve attached, may grossly under-represent the actual pressure loss across a valve in the field. The design of a pilot valve is also very significant in determining if the valve will display hysteresis, and how well the valve will maintain a set pressure if the flow rate changes, or if the upstream pressure changes. ITRC provides these notes based on discussions with valve representatives and upon results seen in hydraulic valve testing at ITRC laboratories