PV valves alarm settings
Pressure/Vacuum valves are designed to provide protection of all cargo tanks against over/under pressure and provide for the flow of small volumes of tank atmosphere resulting from temperature variations in the cargo tank(s) and should operate in advance of the pressure/vacuum breaker, where IG system is in use.
The alarm settings for the pressure sensors must be set to activate when the tank pressure or vacuum reaches a reasonable margin of safety above the normal actuation settings of the Pressure/Vacuum valves themselves.
Petroleum tankers should set the high pressure alarms at 10% above the design opening settings of the pressure valve.
With regards to the low pressure alarms, this will vary depending on the vessel’s inerted state:-
a) For inert tanks the pressure in the tank should never be permitted to fall below zero and so the pressure sensors within the IG system required by the Inert Gas Code and SOLAS should be set to alarm at positive 200 mmWg (LOW) and 100 mmWG (LOW -LOW). If the low pressure alarms sound, discharging operations must be suspended immediately.
b) With regards to the individual tank pressure sensors, these should be set to positive 50 mmWg in order to give warning in the event that the main IG alarms have failed.
c) For non-inert tanks, the sensor should be set at a vacuum 10% greater than the normal actuation settings of the vacuum valves.
Ref: Chemical tanker guide website
