The pressure vacuum breaker is fitted to a branch line off the inert gas deck main line. The purpose of the pressure vacuum breaker is to safeguard against over-pressurisation as well as under-pressurisation of the cargo tanks. The pressure vacuum breaker operates normally at a line pressure of 1800 mmWG and a vacuum of 700 mmWG.
The pressure vacuum breaker is the final backup for any of the pressure vacuum valves and consists essentially of two concentric tubes. The inner tube acts as a weir over which the water is displaced by excess backpressure. By excess vacuum, the inert gas main connecting pipe acts as a weir over which the water is displaced. After installation onboard the breaker should be kept filled with fresh water.
In cold sea water, 20% of the water should be replaced by glycol (CH2OH) or other antifreeze additive.
NOTE: Total liquid quantity required is given on design specification and p/v breaker drawing.
Workings: The air vent on the PVB valve is designed to allow air to enter the system when the pressure drops, preventing a vacuum from forming that could suck water back into the cargo tanks. This is important because a vacuum can occur if the pressure in the system drops due to a water main break or other issue, which could cause contaminated water to be sucked back into the cargo tanks.
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