Exam Question

### (a) (i) The purpose of the Pressure Vacuum (PV) Valve
A Pressure Vacuum (P/V) valve is a critical safety device fitted to the vent mast of a sealed cargo tank on an oil tanker. Its primary purposes are:
1. **Over-Pressure Protection**: During cargo loading operations or due to thermal expansion of the cargo/vapors from solar radiation, the pressure inside the tank increases. The P/V valve is set to open at a predetermined positive pressure (e.g., +1400 mm Water Gauge). It opens automatically to vent the excess pressure, preventing structural damage or rupture of the tank.
2. **Vacuum Protection**: During cargo discharge operations or due to thermal contraction of vapors (e.g., cooling at night or rain), a negative pressure (vacuum) can develop inside the tank. This can lead to the collapse or buckling of the tank structure. The vacuum side of the valve is set to open at a predetermined negative pressure (e.g., -350 mm Water Gauge), allowing air or inert gas to enter and relieve the vacuum.