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
(a) Blocking or stopping of flow
(b) Direction of flow
(c) Pressure of flow media
(d) Flow quantity