Relief valves are the most commonly used pressure control valve as they are required in all systems to prevent the generation of excessive pressures. In many systems they are used in combination with a pump to provide a source of flow at constant pressure.

Single stage relief valve
Single stage relief valves, as with all valves, can use either a piston, or a spool, that is opened by a pressure force against a preloaded spring as shown in Figure 1. On opening, the valve allows some of the supply flow to be passed back to the tank thus limiting the maximum pressure in the supply. The valve needs to be sized such that all of the supply flow can be returned to tank at a supply pressure that does not exceed the maximum desired level.

Single stage relief valves are the simplest and lowest cost valve. Considering the diagram of a poppet valve in Figure 1, the valve will start to open at its 'cracking pressure' when the force on its face due to the inlet pressure is equal to the spring preload.

Figure 1. Poppet Type Valve  

As the pressure increases above this value the valve opens progressively thus allowing flow to pass through the valve. This feature is referred to as the 'pressure over-ride'.

The rate of the spring will determine the relationship between the valve dis­placement and the inlet pressure. In order to minimise the free length of the spring requires the spring to have a high stiffness. However, the higher the stiffness the greater will be the pressure over-ride. There will be some hysteresis due to fric­tion between the components that will result in the pressure being slightly higher when the valve is being opened than when it is being closed.

As is discussed in earlier pagevthere is an additional force that arises from the increase in the momentum of the fluid as it passes through the valve opening which acts in the direction to close the valve. This force acts as a spring the rate of which is added to that of the mechanical spring and its effect is to increase the pressure override.

Two Stage Relief Valves

The two stage valve shown in Figure 2 uses a spring loaded pilot poppet (pilot relief valve) to sense the pressure level in the supply at A. When this pressure causes the pilot relief valve to open the flow through the balancing orifice creates a pressure drop across the main valve poppet that has a spring preload in the region of 2 bar. This causes flow from the supply to be returned to tank at a controlled level of the supply pressure.
Two stage valves have a much reduced pressure over-ride as compared to single stage valves because the main spring is not required to be preloaded to the controlled pressure level. This allows a reduced spring rate to be used that re­duces the pressure over-ride. This can be of advantage where it is required to control the supply pressure within close limits.
The pilot relief valve can be isolated from the main valve. There can be more than one pilot valve which can be set at different pressures so that the con­nection of any one will operate the relief at the respective pilot set pressure.
Figure 3 shows a typical cartridge type of two-stage relief valve. Cartridge valves are available for installation in individual housings, sandwich mounting (stacking) and in special manifold blocks.
Relief valves are also available with electric control using proportional sole­noids to provide the necessary force in place of a mechanical spring. A major

 Figure 3. Cartridge Relief Valve

advantage of this type of valve is the ability to adjust the pressure setting from an appropriate voltage controlled amplifier.