Pilot-operated regulators vs Direct-operated regulators
As shown in How does a Direct-Operated Regulator Work?, direct-operated regulators are simple mechanical devices which automate pressure control. Direct-operated regulators are well suited for many applications, but not all. They are only moderately accurate and the size of the actuator becomes prohibitive above many 2” line size applications especially at higher pressure ratings. Pilot-operated regulators offer superior accuracy and larger flow capability, which are both critical for applications such as natural gas distribution, gas turbine feeds, and nitrogen blanketing large tanks.
The below direct-op performance curve demonstrates that in order to open further to supply an increasing flow demand, a direct-operated regulator can’t maintain downstream pressure at setpoint. Instead, downstream pressure must droop below setpoint because the only way the internal parts of a regulator will travel open is if the diaphragm senses a decrease in that outlet pressure. The pilot-operated regulator must also sense a decrease in downstream pressure to open but to a much smaller extent as reflected on the chart.
All applications require some level of accuracy so even though both products in this example reach the same maximum flow, the direct-operated regulator has much less useful flow capability. Accuracy constraints for a potential application are plotted below showing that for the same accuracy, the flow capability of the pilot-operated regulator is several times more than the direct-operated regulator.
Pilot-operated regulators have two major components: the pilot and the main valve. The pilot is simply a self-operated regulator with external registration. It is the brains of the regulator and it controls the opening and closing of the main valve. The main valve is an actuator connected to a valve through which essentially all of the flow passes.
The pilot and main valve both have an orifice, valve plug, and spring. The pilot spring is adjusted to establish the pressure setting while the main valve spring’s purpose is to provide shutoff force therefore it comes preset from the factory with the correct compression. The restrictor is a small hole that allows pressure to equalize after the pilot closes, closing the main valve.
Two styles of pilot-operated regulators exist:
In this example application, 100 psig inlet pressure (red) is reduced to 10 psi outlet pressure (blue). When the pilot opens, it allows some of the high inlet pressure to enter the loading pressure chamber, increasing the loading pressure (green) which forces the main valve open. The restrictor continually bleeds loading pressure downstream so when the pilot closes, the loading pressure escapes downstream through the restrictor allowing the main valve to close.
All pilot-operated regulators follow the same sequence of events: 1) The downstream pressure changes. 2) The pilot senses the pressure change and moves in response. 3) The pilot movement alters the loading pressure. 4) The change in loading pressure forces the main valve to reposition.
The above animation demonstrates how a loading-style pilot-op reacts when the downstream flow demand increases. The regulator must open in order to supply the increased flow demand. First, the downstream pressure decreases because the regulator is not meeting the increased flow demand. The pilot then detects this decrease in pressure below its 10 psig setpoint. The force from the decreased outlet pressure is now less than from the spring, moving the diaphragm and valve plug downward. This small movement by the pilot allows inlet pressure to flood into the loading pressure chamber, increasing the loading pressure high enough that its upward force on the main valve diaphragm is sufficient to exceed the downward force from the downstream pressure and main spring. The main valve opens, matching the increased flow demand while holding outlet pressure slightly below setpoint.
The above animation demonstrates how a loading-style pilot-operated regulator responds when downstream demand decreases to zero. The regulator must close in order to meet the zero flow demand. First, the downstream pressure increases because the regulator is still open, exceeding the flow demand. The pilot then senses this increase in pressure above its 10 psig setpoint. The force from the increased outlet pressure is now higher than the spring, moving the diaphragm and valve plug upward and closed. The pilot closing stops inlet pressure from entering the loading pressure chamber and the restrictor bleeds the loading pressure down until it is the same pressure as outlet. The decrease in loading pressure force upward on the main valve diaphragm is overpowered by the downward force from the downstream pressure and main spring. The main valve closes, matching the zero downstream demand while holding outlet pressure just above setpoint.
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