Exam Question

### (a) (i) The purpose of the pressure vacuum valve
Pressure/Vacuum (P/V) valves are critical safety devices fitted to the vent lines of all cargo oil tanks on tankers. Their primary purpose is to protect the structural integrity of the cargo tanks and to prevent the formation of ignitable atmospheres within the tanks, thereby mitigating the risk of explosion or implosion. This is achieved by regulating the pressure within the tank relative to the ambient atmospheric pressure.
* **Over-Pressure Protection**: During cargo loading, ballasting of segregated ballast tanks (SBTs), or due to thermal expansion of cargo (e.g., in tropical waters), hydrocarbon vapours are generated or compressed within the tank. If the internal tank pressure exceeds a pre-set safe working limit, the P/V valve opens automatically, allowing vapours to be safely vented to the atmosphere through the mast riser or vent line. This prevents the tank structure from being overstressed and potentially rupturing.
* **Under-Pressure (Vacuum) Protection**: During cargo discharge, de-ballasting, or due to thermal contraction of cargo (e.g., transiting from tropical to colder waters), a vacuum can be created inside the tank. If the internal tank pressure falls below a pre-set safe working limit, the P/V valve opens automatically, allowing atmospheric air (or inert gas from the inert gas system, if applicable) to enter the tank. This prevents the tank structure from being subjected to excessive external pressure, which could lead to an implosion or structural collapse.