Pressure control valves are used in hydraulic circuits to maintain desired pressure levels in various parts of the circuits. A pressure-control valve maintains the desired pressure level by (1) diverting higher-pressure fluid to a lower-pressurareae area, thereby limiting the pressure in the higher pressure area, or (2) restricting flow into another area. Valve that divertfluid can be safety, relief, counter-balance, sequence, and unloading types. Valves that restrict flow into another area can be of the reducing type.
A pressure-control valve may also be defined as either a normally closed or normally open two-way valve. Relief, sequence, unloading types and counter-balance valves are normally closed, two-way valves that are partially or fully opened while performimh their design function. A reducing valve is normally open valve that restricts and finally blocks fluid flow into a secondary area. With either type of operation, the valve can be said to create automatically an orifice to provide the desired pressure control. An orifice is not always created when the valve is piloted from an external source. one valve of this type is the unloading valve ---it is not self-operating; it depends on a signal from an external source. Relief, reducing, counterbalance, and sequence valves can be fully automatic in operation, with the operating signal taken from within the envelop.
