Part 1 of this two part series presented the remarkable accuracy of pilot-operated regulators and demonstrated how loading-style pilot-operated regulators operate via an animated graphic. Part 2 will explain why pilot-operated regulators are so accurate, the operation of unloading-style pilot-operated regulators, and why a minimum pressure differential is required when using pilot-ops.
Why pilot-ops are so accurate
How can pilot-operated regulators hold outlet pressure so much closer to setpoint than a direct-operated regulator? The secret is in harnessing the power of the pressure differential across the regulator.
Direct-operated regulators’ movement is directly tied to outlet pressure. The more outlet pressure decreases, the more open the direct-op will move, increasing flow. Boost can be used in product design to increase the accuracy at low set pressure but overall direct-operated regulators are limited, requiring substantial droop off of setpoint in order to open.
The pilot on a pilot-operated regulator is simply a direct-operated regulator so it is constrained by the same droop issue; however, pilots don’t have to travel very far. When downstream demand is zero, the pilot is closed, making loading pressure equal to outlet pressure. Then, when downstream demand increases, the pilot opens allowing high pressure to flood into the loading chamber. The pilot responds to the small decrease in outlet pressure and sends the main valve a greatly amplified signal via the loading pressure change. This pressure amplification from the pilot is called gain.
In the below example, loading pressure equals 10.1 psi, the same as outlet, while there is no flow demand. When flow demand increases, the pilot opens and loading pressure increases. The amount it increases depends on how much the pilot travels. As a hypothetical, suppose the loading pressure goes up to 30 psig. So the downstream equipment sees a 0.2 psi change in pressure while the main valve sees a 20 psi increase. The gain from the pilot causes the main valve to respond to small outlet pressure changes maintaining outlet pressure very close to setpoint.
Pilot-operated regulators require a pressure differential in order to operate the main valve. The main spring holds the main valve in a closed position until loading pressure increases high enough to overcome the spring force. The downstream pressure and main spring force push down on the main valve diaphragm while the loading pressure force pushes upward.
A force balance on the diaphragm yields the below equation showing the pressure difference between the loading pressure and outlet pressure must be greater than the force of the spring divided by the diaphragm area in order to move the main valve open.
The maximum possible loading pressure equals inlet pressure; therefore, it is the differential pressure between the application’s inlet and outlet pressure that is important. Pilot-operated regulator data sheets state the minimum differential pressure to fully stroke the main valve open.
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 dumps loading pressure downstream, decreasing the loading pressure (green) which allows inlet pressure to force the main valve open. The restrictor continually bleeds inlet pressure to loading so when the pilot closes, the loading pressure becomes equal to inlet 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 an unloading-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 movement by the pilot dumps loading pressure downstream, decreasing pressure in the loading pressure chamber low enough that its downward force on the main valve diaphragm is overpowered by the upward force from the inlet pressure. The main valve opens, matching the increased flow demand while holding outlet pressure slightly below setpoint.
The above animation demonstrates how an unloading-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 allows inlet pressure bleeding through the restrictor to increase the pressure in the loading pressure chamber until it is the same pressure as inlet. The increase in loading pressure force downward on the main valve diaphragm plus the main spring overpowers the upward force from the inlet pressure. The main valve closes, matching the zero downstream demand while holding outlet pressure just above setpoint.