Relief Valve (RV): automatic system that relief by static pressure on a liquid. It specifically opens proportionally with pressure increasing. Safety Valve (SV): automatic system that relief by static pressure on a gas. It specifically opens almost straight to full lift after a pop sound. Relief valve always position in the suction line in order to relief the extra pressure, while safety valve is always placed at discharge line to release excess pressure. I would take that one step further by stating that a RELIEF VALVE" is one that is "meant" to be lifted off its seat, as opposed to a "SAFETY RELIEF VALVE" which is not! Safety valves are always mounted on vessels to protect them against pressure build up (even if discharge valves are closed accidently) whereas relief valves are installed on pipe lines to relieve excess pressure and are normally installed across pumps. A good example can be seen in pressure cookers, the center valve is a relief valve whereas it has a safety valve in case relief valve fails.

What is the difference between a safety valve and a relief valve?

A safety relief valve, or pop-off valve, is set to pre-determined pressure using a spring force to counteract the air pressure force. When the force created from the air pressure becomes greater than the spring force, the valve 'pops' fully open and vents air to the atmosphere. Safety relief valves are sized in such a manner that the valve will always have a greater flow capacity than the source creating the air pressure (i.e. an air compressor). The valve will remain open until a reseat pressure is reached. This reseat pressure varies with valve type and is usually in the range of 35% to 75% of the set pressure. A relief valve is set to a pre-determined pressure with a spring force counteracting the force created by the pressure from the air. When the force created from the pressure of the air becomes greater than the force of the spring, the valve will start to open and flow to atmosphere. As the pressure continues to increase, the valve will open further, allowing more flow. As the pressure

decreases, the valve will close near the pressure at which it opened.

The ASME / ANSI PTC25.3 standards applicable to the USA define the following generic terms:

Pressure relief valve - A spring-loaded pressure relief valve which is designed to open to relieve excess pressure and to reclose and prevent the further flow of fluid after normal conditions have been restored. It is characterized by a rapid-opening 'pop' action or by opening in a manner generally proportional to the increase in pressure over the opening pressure. It may be used for either compressible or incompressible fluids, depending on design, adjustment, or application. This is a general term, which includes safety valves, relief valves and safety relief valves. Safety valve - A pressure relief valve actuated by inlet static pressure and characterized by rapid opening or pop action. Safety valves are primarily used with compressible gases and in particular for steam and air services. However, they can also be used for process type applications where they may be needed to protect the plant or to prevent spoilage of the product being processed. Relief valve - A pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure. Relief valves are commonly used in liquid systems, especially for lower capacities and thermal expansion duty. They can also be used on pumped systems as pressure overspill devices. Safety relief valve - A pressure relief valve characterized by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure, depending on the application, and which may be used either for liquid or compressible fluid.