Solenoid valves are electrically operated shutoff valves. Probably the most common is the normally closed (NC) valve, but normally open (NO) valves are also available4. With both types, system pressure works to keep the valve closed when that position is desired. Solenoid valves thus can hold against high upstream pressures, but will not restrain much pressure in the reverse direction. Two classifications of solenoid valves are:
2. pilot-operated (to be discussed in the next section)
In the direct-acting solenoid valve, as shown in Fig. 11.4, the magnetic force developed by the electric coil draws the stem and connected plunger off the valve port when the coil is energized. Some solenoids are designed to allow the stem to start its motion before engaging the plunger which is seated against the system pressure. The momentum of the stem thereby helps open the valve. When the coil is de-energized, the plunger either drops into the closed position by gravity, and/or a light spring assists the closing.
A solenoid valve must be selected to be able to open against the maximum operating pressure differential (MOPD), a characteristic of the valve listed in the manufacturer’s catalog. Because direct-acting valves must generate sufficient force in the coil to open the valve against the system pressure, they are limited to line sizes of perhaps 6 to 25 mm (1/4 to 1 in).