Traditionally, relief valve is a safety valve in which the degree of opening is proportional to pressure. It is used on liquid service only.
The function of a relief valve is to set the maximum pressure in a hydraulic system. Although there are many designs and varieties, they can all be denoted by the general symbol (a) in Figure 2. It is a normally closed valve which partially opens permitting flow to the tank port when the pressure at the inlet port overcomes the spring force. Symbol (a) does however more accurately represent a direct acting valve. If there is no arrow through the spring the valve is pre-set, i.e. non-adjustable. A two-stage relief valve may be shown as Figure 2(b) as this more closely illustrates its operation which will be described later.