A Relief Valve or Pressure Relief Valves (PRV) is a type of safety valve used to control or limit the pressure in a system. Otherwise the pressure might build up and create 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.
The relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels. it is also designed to protect other equipment from being subjected to pressures that exceed 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.
The diverted fluid (liquid, gas or liquid–gas mixture) is usually routed through a piping system known as a flare header or relief header to a central, elevated gas flare. From that point it is where it is usually burned and the resulting combustion gases are released to the atmosphere. As the fluid is diverted, the pressure inside the vessel will stop rising.
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.
High Pressure Relief
In high-pressure systems, it is recommended that the outlet of the relief valve is in the open air. In systems where the outlet is connected to piping, the opening of a relief valve will give a pressure build up. The Pressure Relief Valves will also be in the piping system downstream of the relief valve.
This often means that the relief valve will not re-seat once the set pressure is reached. For these systems often so called “differential” relief valves are used.
This means that the Pressure Relief Valves is only working on an area that is much smaller than the openings area of the valve. If the valve is opened the pressure has to decrease enormously before the valve closes. Also the outlet pressure of the valve can easily keep the valve open.
Another consideration is that if other relief valves are connected to the outlet pipe system, they may open as the pressure in exhaust pipe system increases. This may cause undesired operation.
In some cases, a so-called bypass valve acts as a relief valve by being used to return all or part of the fluid discharged by a pump back to either a storage reservoir or the inlet of the pump or gas compressor.
This is done to protect the pump or gas compressor and any associated equipment from excessive pressure. The bypass valve and bypass path can be internal (an integral part of the pump or compressor) or external (installed as a component in the fluid path).
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