Gas valve unit (GVU)
The certified valve skid between an LNG fuel tank and the engine, carrying the master gas valve, the double block and bleed arrangement and the vent to a safe location.
The gas valve unit is the certified skid that forms the last controlled barrier between the fuel gas supply and the engine on a gas-fuelled ship. It carries the master gas valve, a double block and bleed valve arrangement, a pressure regulating valve, gas flow measurement and a vent connection to a safe location on the open deck.
The double block and bleed arrangement is what makes the barrier testable in service. Two block valves sit in series with a bleed valve between them, so leakage past the upstream block raises pressure in the bleed space and is detected there rather than reaching the engine. That same architecture allows each block valve to be tightness tested without gas freeing the whole supply train.
The full article will cover the valve train and its instrumentation, the fail-closed logic and required closing time, the gas valve room and its ventilation, detection and access rules, the in-service tightness test, and the maintenance consequences of cryogenic thermal cycling on stem seals. See LNG fuel system and IGF Code .