Types of pressure relief valves
A direct loaded relief valve is one in which the loading due to the fluid pressure is opposed by direct mechanical loading such as spring; weight loading is rarely found in compressed air systems. Direct loaded valves can be either poppet valves in which the sealing member is directly loaded onto a seat by the spring and the system pressure acts directly on the seating member (Figure 1), or diaphragm valves in which the spring acts against a diaphragm exposed to the system pressure (Figure 2).
Poppet valves suffer from serious drawbacks. Their performance is inconsistent due to “stiction”, they are insensitive because of the small seat areas and they have a limited flow capacity. Diaphragm operated valves eliminate some of the drawbacks of the simple poppet valves: because pressure is sensed over a larger area (that of the diaphragm) they are more sensitive and have a larger flow capacity. However direct operated diaphragm valves still have performance limitations, due to the size of the diaphragm in relation to the spring loading.
Most modern pressure relief valves, at least in the larger sizes, are pilot valves, of which there are two varieties. A ‘pilot controlled relief valve’ is a valve in which the loading spring is replaced by fluid pressure from a control pilot; the pilot pressure comes from a pressure regulator which is remote from the relief valve and has its own supply which may be independent of the main supply, see Figure 3. A ‘pilot operated relief valve’, usually a diaphragm type, is one in which a small integral spring-operated valve controls the opening of the main valve; system pressure normally keeps the main valve closed and no external supply is needed.
Pilot controlled relief valves have superior characteristics to those of direct spring systems. Pressure is applied to the control side of the diaphragm from a pilot regulator. A small bleed is necessary to prevent control variations due to temperature fluctuations. Relief valves controlled by pilot regulators do have shortcomings when used for system relief, but they are useful when the control has to come from a separate supply or when the relief pressure needs to be altered from time to time; a pressure gauge can be incorporated in the pilot line for pressure setting purposes. Unfortunately regulators are not fail-safe and if one has to incorporate another relief valve in the pilot line, the whole circuit becomes very complicated.