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What is a PV breaker? Why is it fitted?

This is a U tube manometer filled with an ethylene glycol solution (as a measure against 
freezing). The height of the manometer is manipulated such that the pressure on either side 
may displace the solution, into tanks incase of vacuum inside cargo tanks and outside on to  the deck incase of over pressurization inside cargo tanks. Usual set values of PV breaker are  +1800mmHg on high pressure and -700mmHg on vacuum side.

Pressure-Vacuum breaker is fitted to the IG main line on the deck to protect cargo tanks 
from the following: 
a. Abnormal rise of pressure in the cargo tanks which occurs when cargo is loaded with 
specified rate and that all gas outlets are closed 
b. Abnormal drop of pressure in cargo tanks which occurs when cargo is unloaded with 
the rated capacity of the cargo pumps and the inert gas blower fails 
c. Abnormal rise or drop of pressure in cargo tanks which occurs, when the breather 
valves fails to operate properly for the fluctuation of the pressure in cargo tanks, due 
to variation in atmospheric and sea water temperatures, altering the vapor pressure 
inside tanks

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