1. Introduction to Safety Relief Systems

2. Relief Valve
A relief valve (RV) is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire. The pressure is relieved by allowing the pressurized fluid to flow from an auxiliary passage out of the system.

4. Operation: The safety valve will start to open; reach full lift and close over a range of pressures. There are three pressures that are important when specifying a relief valve.
Set pressure – This is the pressure at which the safety valve will begin to open.
Relieving pressure – This is the pressure at which the safety valve is fully open and working at full capacity.
Re-seat pressure – The pressure at which the safety valve closes after a relieving event.

5. Pressure relief valve (PRV) or pressure safety valve (PSV): The difference between PRV & PSV is that PSVs have a manual lever to activate the valve in case of emergency, while most PRVs are spring operated. At lower pressures some use a diaphragm in place of a spring. The oldest PRV designs use a weight to seal the valve.
Relief valve (RV): A valve used on a liquid service, which opens proportionally as the increasing pressure overcomes the spring pressure.
Safety valve (SV): Used in vapor/gas service. Most SVs are full lift or snap acting, in that they pop completely open.
Safety relief valve (SRV): A relief valve that can be used for gas or liquid service. However, the set pressure will usually only be accurate for one type of fluid at a time.

6. Low-pressure safety valve (LPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small and near the atmospheric pressure.
Vacuum pressure safety valve (VPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small, negative and near the atmospheric pressure.
Low and vacuum pressure safety valve (LVPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small, negative or positive, and near the atmospheric pressure.
Pressure vacuum release valve (PVRV): A combination of vacuum & pressure relief valve in one housing. Used on storage tanks for liquids to prevent implosion or over pressure.

7. Modulating valve: Opens in proportion to the overpressure.
Snap acting valve: The opposite of modulating, refers to a valve that "pops" open. It snaps into full lift in milliseconds. Usually accomplished with a skirt on the disc so that the fluid passing the seat suddenly affects a larger area and creates more lifting force.
Pilot-operated relief valve (POSRV, PORV, POPRV): A device that relieves by remote command from a pilot valve which is connected to the upstream system pressure.
Temperature and Pressure Safety Relief Valve: This device is typically used on potable water heaters. In addition to its pressure-relief function, it also includes a temperature-sensing element which causes the device to open at a predetermined temperature regardless of pressure. The set temperature on these devices is usually 210°F.

8. Rupture Disc
A rupture disc (also known as a burst disc, bursting disc, or burst diaphragm) is a non-reclosing pressure relief device that, in most uses, protects a pressure vessel, equipment or system from over-pressurization or potentially damaging vacuum conditions.

9. Safety Integrity Level (SIL)
Safety Integrity Level (SIL) is defined as a relative level of risk- reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a Safety Instrumented Function (SIF). In the European Functional Safety standards based on the IEC standard four SILs are defined, with SIL 4 being the most dependable and SIL 1 being the least. A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management.