Introduction to the Design and Maintenance of Cargo System Pressure Relief Valves on Board Gas Carriers
: Witherby Seamanship International Ltd
The book advises on PRV maintenance and testing, emergency closure, and operating problems and faults. It should be remembered that this publication is a general guide to the ship’s staff and is not designed to replace the manufacturers’ instruction manuals. It is recommended that staff who are responsible for the maintenance of these valves should attend the relevant manufacturer’s training course.
This book was written following an incident in 1992 involving failure of a relief valve when a semi-pressurised LPG carrier was being loaded. The cargo being loaded was propane, and the pressure relief valves (PRV) were set to operate at 11 bar gauge. During the final stages of loading at a pressure of 7 bar gauge, one of the pressure relief valves fitted to a cargo tank failed.
There were no guidelines available on board or at the terminal to provide any assistance in the emergency methods for closing the valve. The valve was eventually blanked off with the tank pressure reduced to 0.02 bar gauge, but by this time a large amount of the product had been lost into the atmosphere. The investigation that followed discovered that there was a failure of the pilot valve diaphragm, possibly due to the diaphragms being in service for too long.
If gas or nitrogen at cargo tank pressure had been applied to the top of the main valve dome, the valve would have closed and limited the amount of propane that was released into the atmosphere. Unfortunately, the crew were unaware of this procedure.
As a result of these and other incidents, SIGTTO produced “Guidelines on the Maintenance of Pressure Relief on Board Gas Carriers”. In 1998 this booklet was expanded and updated to become this title.