Relief valve

A relief valve is a special type of safety valve used to control the pressure in the hydraulic system.

A pressure in hydraulic systems might otherwise build up and create a process upset, instrument or equipment failure, or even fire. Pressure is relieved by letting the pressurized fluid to flow from an auxiliary passage out of the hydraulic system. A relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits and to prevent unnecessary accidents. When the set pressure is exceeded, the relief valve becomes the "path of least resistance" as the valve is set to open and a portion of the fluid is diverted through the auxiliary route. The diverted fluid is usually routed through a piping system known as a "flare header" or "relief header" to a central, elevated gas flare where it is burned most of the time and the resulting combustion gases are released. The pressure inside the vessel will stop growing as the fluid is diverted. Valve will close when it will reach reseating pressure point. 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.

It is recommended in high-pressure gas systems 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 in the piping system downstream of the relief valve. This means that when the set pressure is reached a relief valve will not re-seat. For these systems relief valves are used and often called "differential". This means that the pressure is only working in an area that is much smaller than the openings area of the valve. If the valve is opened the pressure has to decrease hugely before the valve closes and also the outlet pressure of the valve can easily keep the valve open. If other relief valves are connected to the outlet pipe system, they may open as the pressure in exhaust pipe system increases.

In some cases, a so-called "bypass valve" acts as a relief valve by being used to return back to a storage reservoir part of the fluid discharged by a pump or gas compressor. It is important to protect the pump or gas compressor and any associated equipment from too much pressure. 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).

Many fire engines have relief valves to prevent the overpressurization of fire hoses. In other cases, equipment must be protected against being subjected to an internal vacuum. In such cases, "vacuum relief valves" are used to open at prearranged low-pressure limit and to admit air or gas into the equipment to control the amount of vacuum.