Safety Relief Valves
A relief valve or pressure relief valve (PRV) refers to the type of safety valve that is used to control or limit the pressure in a system which otherwise might build up and create disruption in process, instrument or equipment failure, or fire. The main objective of a safety valve is the protection of life, property and environment. A safety valve is designed to open and relieve excess pressure from vessels or equipment and to reclose and prevent the further release of fluid after normal conditions have been restored.
The pressure is relieved by allowing the pressurised fluid to flow from an auxiliary passage out of the system. The relief valve is designed or set to open at a predetermined set pressure to prevent pressure vessels and other equipment from being subjected to pressures exceeding their design limits. When the set pressure is exceeded, the relief valve becomes the "path of least resistance" as the valve is forced open and a portion of the fluid is diverted through the auxiliary route.
Generally, the diverted fluid is routed through a piping system known as a flare header or relief header to a central, elevated gas flare where it’s usually burned and the resulting combustion gases are released in the atmosphere. With the diversion of fluid, the pressure inside the vessel stops rising. Once it reaches the valve's reseating pressure, the valve shuts. The blowdown is usually stated as a percentage of set pressure and refers to how much the pressure needs to drop before the valve resets. The blowdown can vary from roughly 2–20%, and some valves have adjustable blowdowns.
Moreover, there can be various reasons behind the pressure in a vessel or system exceeding a predetermined limit. A detailed guideline is provided by API Standard 521/ISO 23251 Sec. 4 about causes of overpressure; of which the most common are:
- Blocked discharge
- Exposure to external fire, generally known as “fire case”
- Thermal expansion
- Chemical reaction
- Heat exchanger tube rupture
- Cooling system failure