A relief valve is a type of valve used to control or limit the pressure in a system or vessel which can build up, leading to an instrument or equipment failure, or in extreme cases, even a fire.
The concept behind a relief valve is to relieve pressure by allowing the pressurised fluid to flow from an auxiliary passage out of the system. The relief valve is designed to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their limitations. 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. The diverted fluid (liquid, gas or liquid–gas mixture) is usually routed through a piping system to a central, elevated gas flare where it is usually burned and the resulting gases are released to the atmosphere.
As the fluid is diverted, the pressure inside the vessel will drop. Once it reaches the valve’s reseating pressure, the valve will close. The blowdown is usually stated as a percentage of set pressure and refers to how much the pressure needs to drop before the valve reseats. The blowdown can vary from roughly 2–20%, and some valves have adjustable blowdowns.
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