The basic spring loaded pressure Relief Valve has been developed to meet the need for a simple, reliable, system actuated device to provide overpressure protection.
The image on the right shows the construction of a spring loaded pressure Relief Valve.
The Valve consists of a Valve inlet or nozzle mounted on the pressurized system, a disc held against the nozzle to prevent flow under normal system operating conditions, a spring to hold the disc closed, and a body/Bonnet to contain the operating elements. The spring load is adjustable to vary the pressure at which the Valve will open.
When a pressure Relief Valve begins to lift, the spring force increases. Thus system pressure must increase if lift is to continue. For this reason pressure Relief Valves are allowed an overpressure allowance to reach full lift. This allowable overpressure is generally 10% for Valves on unfired systems. This margin is relatively small and some means must be provided to assist in the lift effort.
Most pressure Relief Valves, therefore, have a secondary control chamber or huddling chamber to enhance lift. As the disc begins to lift, fluid enters the control chamber exposing a larger area of the disc to system pressure.
This causes an incremental change in force which overcompensates for the increase in spring force and causes the Valve to open at a rapid rate. At the same time, the direction of the fluid flow is reversed and the momentum effect resulting from the change in flow direction further enhances lift. These effects combine to allow the Valve to achieve maximum lift and maximum flow within the allowable overpressure limits. Because of the larger disc area exposed to system pressure after the Valve achieves lift, the Valve will not close until system pressure has been reduced to some level below the set pressure. The design of the control chamber determines where the closing point will occur.
The difference between the set pressure and the closing point pressure is called blowdown and is usually expressed as a percentage of set pressure.
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