Control Valve

Types of pressure relief valves

Pressure relief valves:

Pressure relief valves are safety valves that are designed to prevent internal pressure from rising above a predetermined maximum pressure in a pressure vessel exposed to abnormal or emergency conditions.


Spring Loaded Design:











The valve consists of an inlet valve or nozzle mounted in the pressurized system, a disk held against the nozzle to prevent flow under normal operating conditions, a spring to keep the disk closed and a body/bonnet to hold the operating elements. The spring load is adjustable to vary the pressure at which the valve will open.

Pilot-operated relief valves:












It consists of the main valve with a piston or a disk operated by the diaphragm and a pilot. Under normal operating conditions, the pilot allows system pressure in the piston chamber. Since the area of the piston is larger than the area of the disk seat, the disk remains closed. When the set pressure is reached, the pilot actuates to close the system fluid to the piston chamber and at the same time ventilates the piston chamber. This causes the disk to open.

Safety Relief Valve:

The safety valve is a PRV actuated by static inlet pressure and characterized by rapid opening or ‘pop’ action. Typically used for steam and air service.

Other pressure relief vents

Emergency Relief Vent:


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Emergency pressure vents are designed to provide an emergency relief capability beyond that provided by the operating ventilation in the tanks. Under normal operating conditions, the ventilation platform assembly is closed providing an effective vapor seal. In the event of an emergency (involving the tank in the fire), the platform rises in response to the increased pressure in the tank’s vapor space. The steam is expelled, thus protecting the tank from a dangerous overpressure. The pallet assembly closes automatically and reseals when pressure is reduced. Emergency vents do not provide vacuum relief. Vacuum relief should be provided by independent conservation vents.

Conservation Vent:

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It is designed for use where both pressure and vacuum relief are required. The vanes in the ventilation housing allow air to enter and escape from the vapours, since the tank normally inhales and exhales. The pallets are opened and closed to allow only the relief of entry or exit necessary to remain within the permissible working pressures and avoid damage to the tank.

Flame Arrester:

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Installed where it is not necessary to conserve vapours but where low flash point liquids must be protected against fire and explosion from exterior sources of ignition. Flame arresters are mounted on the end of a vent pipe from
the tank. Vapours are allowed to escape into the atmosphere and air can be drawn into the tank through the specially designed flame arrester grid assembly.


Instrumentation Engineer

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