To make the solenoid valve practical for large pipe diameters, concepts must be applied other than direct force on the valve stem from the magnetic coil. Two approaches are pilot-operated and gas-pressure-operated valves. In the pilot operated valve, a small solenoid opens to apply high upstream pressure on a power piston. The power piston has a greater area than that of the valve plug, which is also subjected to the upstream pressure, so the force of the power piston opens the valve. One design of a pilot-operated solenoid is shown in Fig. 11.5 where the opening of the pilot solenoid permits upstream pressure at M to pass through the passage N to the power piston.
The pilot-operated solenoid valve requires a minimum pressure drop across it, even when completely open. There are situations where the pressure drop through the open valve is to be kept as low as possible, such as in low-pressure vapor lines, or in the liquid leg or the return line of a flooded evaporator. In other situations, viscous oil may be coating the moving parts of the valve, which requires more positive action to open and close the valve. The gas-pressureoperated valve5, such as ahown in Fig. 11.6, may be considered in such cases. The construction is similar to the pilot-operated solenoid, except that pressure from the condenser or another high-pressure source is applied to the power piston. Usually a pressure of the high-pressure gas of 69 kPa (10 psi) higher then the inlet pressure to the valve is adequate to open the main valve. When the valve is to be opened, the pressure pilot solenoid opens to allow the high pressure gas access to the power piston. To close the valve, the pressure pilot solenoid valve closes and the bleed pilot solenoid valve opens. With this valve status, the pressure above the power piston vents to the low-pressure side, permitting the spring to close the main valve.