Hydraulic
valves control and direct the oil from the time it leaves , the pump
until it starts on its return to the reservoir in a hydraulic circuit.
The relief valve is almost always the first one encountered by the oil
after it leaves the pump. From that point on throughout the circuit, the
type of valve employed depends upon the design of the circuit. In a very simple circuit the second and only other valve used will probably be the master control valve. In a complex circuit the oil may flow through a sequence valve, master control valve, reducing valve, flow control valve, check valve and several others before it finally returns to the reservoir.
Hydraulic valves as used in industrial applications are built in several pressure ranges. The majority of present applications are in the range of 0 to 1500 lb per sq in., but the trend is toward higher pressures. Various types of valves are used in hydraulic systems to control or regulate the flow medium. Basically valves are expected to control:
(a) Blocking or stopping of flow
(b) Direction of flow
(c) Pressure of flow media
(d) Flow quantity
