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.

Pressure 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 small 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 of venting can cause an accumulation of flammable vapours at deck level. Tanks should be vented while filling, through mast head vents or through special high velocity vents.

What is the normal IG deck pressure maintained in the cargo tanks ?

700 to 800 mm of Water Column

What is the low pressure setting for IG and what happens if there is a low pressure ?

Low pressure alarm is at 200 mm of Water Column. Low low pressure trip is at 100 mm of Water Column. At this pressure cargo pumps are tripped.

What is high pressure setting ?

When IG pressure reaches 1200 to 1300 mm of Water Column, recirculation valve for IG opens .

What is pv valve lift setting ?

Pressure vacuum valve operates at 1400 mm of Water Column and at a vacuum of -250 mm of Water Column.

What is 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.

How much pressure a cargo tank can withstand ?

Cargo tank is normally tested to a positive pressure of 2500 mm of Water Column and negative pressure of -750 mm of Water Column.

How to convert mm of Water Column to bar ?

1000 mm of Water Column = 0.0980665 bar

For practical purposes we can take 1000 mm of water column = 0.1 bar

 

References

“General Engineering Knowledge” by H.D. McGeorge