Reverse Flow in Control Valves

Certain types of solenoid valves and regulators may close against flow in one direction, and still allow flow in the reverse direction. In cases like this, a check valve may also be needed.

In refrigeration, for instance, evaporators normally run at low pressure. During defrost, however, they may be at higher pressure than the liquid line that normally feeds them. Thus the liquid solenoid valve, which is often of the mentioned type, will be followed by a check valve in the line.