PV valve and PV breaker

PV VALVE
                          Moderate pressures of 0.24 bar acting on large surfaces in liquid cargo tanks are sufficient to cause damage and rupture. The pressure on each unit of area multiplied by the total area gives a large loading on the underside of the top of a tank or other surface, which may then buckle or the metal plate may be torn. Similarly, pressure drop within a tank can cause damage due to greater atmospheric pressure on the outside.
PV valvePressure vacuum valve or PV valve in the ventilation system will prevent either over or under pressure. They are set usually so that tank pressure of about 0.14 bar will lift the main valve (The smaller valve will lift along with it) and release excess pressure. The vapour passes to atmosphere through a gauze flame trap. Drop in tank pressure compared with that of the outside atmosphere will make the s mall valve open downwards to equalize internal pressure with that outside. 
Pressure vacuum valve or pv valve can relieve moderate changes in tank pressure due to variations in temperature and vapour quantity. A drop towards vacuum conditions as the result of the condensation of steam will also be handled by the valve. Rapid pressure rise due to an explosion would not be relieved The fast rate at which a tank is filled while loading produces a very rapid expulsion of the previous contents (vapour and inert gas). The pressure vacuum valve is not designed as a filling vent and neither should the tank hatch be left open. The latter method.
PV breaker
Pressure Vacuum Breaker or usually known as PV Breaker is a safety measure used in the IG line on deck. The major functions of a PV breaker are:-
1)Abnormal rise of Pressure in Cargo tanks when loaded specified rate of gas outlets.PV breaker
2)Abnormal rise of Pressure in Cargo tanks when cargo is unloaded beyond specified rate of the inert gas blower.
3)Abnormal rise or drop of pressure in cargo tanks when the breather valve doesnot operate properly for the fluctuation of the pressure in cargo tanks due to variation in atmospheric and sea water temperatures.
Operation
Working of PV breaker1) When Pressure Rises
When the pressure in the cargo oil tanks rise, the seal liquid rises in the inner pipe. At this time , if the pressure beyond the specific capacity of the breaker, the seal liquid will push out of the pipe to let the pressure inside the be out
2) When Pressure drops
When the pressure in the cargo oil tanks fall, the seal liquid rises in the outer pipe. If the pressure beyond the specific capacity of the breaker, the seal liquid is drown into the cargo oil tanks, and atmospheric air will be inhaled in the tank
PV breaker lift setting : PV breaker operates at 1600 to 1800 mm of Water Column and at a vacuum of -400 mm of Water Column






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