Plant Protection Equipment
Types of Pressure Relief Valves
Many processes in the oil and gas industry operate at high pressure and temperature. The changes in pressure and temperature due to upsets can lead to build up of liquid and gas pressures in process vessels as well as in pipes and storage facilities. Also there are occasions to develop low pressures inside vessels and tanks, which may collapse or buckle them. High pressure can build up very rapidly and rupture vessel valves and pipes quickly. This high pressure should be relieved quickly by means of pressure relief valves/devices.
There are five types of pressure relief devices.
- a) Relief Valves
- b) Safety valves, Pilot operated safety valve.
- c) Pressure Vacuum Safety relief valves (PVV):
- d) Rupture disc.
- e) Safety hatch. (Blow off Hatches).
- a) Relief valves:
Relief valves are used in systems where system pressure during processing generally builds up slowly OR even fast. These are used both on liquid and gas systems. When pressure increases above the normal level, the valve opens to bleed (release) off some of the fluid into the drain & vents systems.
Spring loaded safety relief valve
Relief valves are typically set at maximum allowable working pressure. If the system pressure continues to build more liquid must flow out of the system. When the pressure is back to normal the valve closes.
- b) Safety valves:
Safety valves are used in systems where the pressure builds up rapidly and needs to be relieved quickly to protect the equipment.
A safety valve also known as a pop valve or pressure safety valve, quickly open all the way to relieve the high pressure condition. The valve quickly closes all the way when pressure is back to normal.
Spring loaded safety relief valves and pilot operated relief valves are the two types of safety valves used in PDO.
Pilot operated safety valve
This type of safety valve uses the flowing medium itself, through a pilot valve, to apply the closing force on the safety valve disc. The pilot valve itself is a small safety valve.
The piston and seating arrangement incorporated in the main valve is designed so that the bottom area of the piston, exposed to the inlet fluid, is less than the area of the top of the piston. As both ends of the piston are exposed to the fluid at the same pressure, this means that under normal system operating conditions, the closing force, resulting from the larger top area, is greater than the inlet force. The resultant downward force therefore holds the piston firmly on its seat.
If the inlet pressure was to raise, the net closing force on the piston also increases, ensuring that a tight shut-off is continually maintained. However, when the inlet pressure reaches the set pressure, the pilot valve will pop open to release the fluid pressure above the piston. With much less fluid pressure acting on the upper surface of the piston, the inlet pressure generates a net upwards force and the piston will lifted from its seat. This causes the main valve to pop open, allowing the process fluid to be discharged to the flare system.
When the inlet pressure has been sufficiently reduced, the pilot valve will reclose, preventing the further release of fluid from the top of the piston, thereby re-establishing the net downward force, and causing the piston to reseat.
- c) Pressure Vacuum Safety relief valves (PVV):
In a fixed roof tank the space between the oil and the roof is filled with vapor. Owing to presence of this vapor space, evaporation losses from breathing and filling occur during use. Breathing losses are caused by the difference in night and day temperatures
In the case of fixed roof tanks, blanketing gas with pressure control is used to reduce breathing losses. If this system fails the tanks were safeguarded by installing pressure /vacuum relief valves on the roof. These valves open at a certain overpressure (usually 20 mbar or 50 mbar) to relieve pressure to atmosphere and at certain under pressure (usually 7.5 mbar) to admit air into the tank.
Filling losses occur during the filling of a fixed roof tank. The in flowing quantity of oil causes an equal volume of vapor to be expelled from the tank. Another measure to eliminate vapor-filling losses is to install vapor-equalizing lines between vents of various tanks and draw vapor from header to maintain required pressure inside the tanks. These valves are used in low-pressure applications, usually storage tanks, where inflow and outflow of oil changes the vapor volume in the tank. Operation is similar to pilot operated safety relief valves.
Pressure vacuum valves are used in fixed roof tanks to avoid damage to the tanks on both over and under pressures. PVV protects the tank from both over pressure and vacuum. The PVV is capable of handling the maximum calculated in and out breathing requirements of the tank. The set pressure of PVV is 2 kPa pressure and 0.6 kPa vacuum
- d) Rupture disc:
Rupture disc is used as a failsafe pressure relief by itself, or in addition to any relief valve or safety valve. Rupture discs are typically set at 25% over the setting of the relief valve or safety valve. When system pressure reaches the rating of the disc, the disc bursts open to provide pressure relief. Once it opens to relieve the overpressure, it stays open and the system must be shutdown to replace the disc.
A bursting disc assembly consists of a circular membrane which may be made of metal, plastic or graphite and which is sandwiched between two plates or holders. This assembly is usually installed between a pair of flanges. Bursting discs operate by simply ‘bursting’ the membrane and thus allowing instantaneous relief. Bursting discs are sensitive to temperature variations.
- e) Safety Hatch (Blow off Hatch):
Safety Hatch (emergency vent) is the portion of the tank roof where a thin metal plate is welded to protect the tank in case of a sudden rise in pressure.
When the pressure increases on a sudden, the safety Hatch comes out and releases the pressure and prevents the damage to the whole tank.