IGF Code checklist for LNG fuel gas supply systems (FGSS), gas valve unit (GVU) double block-and-bleed valves, tightness testing, nitrogen purging and BOG.
← All technical guides · Category: LNG FGSS & GVU · Updated: · 9 min read
| System / Safety Barrier | Requirement or Typical Sequence | Failure Consequence | Operational & Survey Verification |
|---|---|---|---|
| Pre-Entry GVU Tightness Test | Valve-proving sequence set by the GVU maker: the space between the two shut-off valves is pressurised or vented and the pressure change monitored over a set time | Gas mode is blocked if the pressure change exceeds the maker’s limit, indicating a leaking shut-off or vent valve seat | Review tightness test pass/fail logs and include the GVU valves in the PMS and class survey programme |
| Post-Stop Venting & Nitrogen (N2) Purge | When the master gas fuel valve shuts automatically, the gas branch downstream of the double block-and-bleed valves is vented automatically (IGF Code 9.4.7); where fitted, nitrogen then purges the line to the vent mast | Gas left in engine-room piping is a hazard during maintenance and can upset the next gas start | Check nitrogen supply pressure, non-return valves and purge-complete confirmation before any gas pipe flange is opened |
| Double-Wall Gas Pipe / Duct Ventilation | Mechanical underpressure (extraction) ventilation of at least 30 air changes per hour; may be reduced to 10 per hour if the duct is filled with nitrogen automatically on gas detection (IGF Code 9.6.1.2 and 13.8.1) | Loss of ventilation in a duct inside a machinery space shuts off the gas supply to that space (IGF Code Chapter 15, Table 1) | Test duty/standby fan changeover and the loss-of-ventilation alarm at the intervals in your PMS |
| Gas Detection Loop | Alarm at 20% LEL and safety action at 40% LEL on two detectors (IGF Code 15.8.6); in ventilated ducts around gas pipes in machinery spaces, alarm at 30% LEL and shutdown at 60% LEL (15.8.7 and Table 1) | Poorly calibrated detectors risk late leak detection or nuisance gas trips | Calibrate with certified span gas as the detector maker requires and record results in the gas detection log |
The gas valve unit (GVU), either in a gas-tight ventilated enclosure in the engine room or in a dedicated GVU space, is the safety and pressure-control interface between the ship’s LNG fuel gas supply system (FGSS) and each dual-fuel or spark-gas engine. Gas arrives from the FGSS vaporiser and heater at the pressure and temperature the system is designed for, and the GVU pressure control valve sets the gas pressure to the engine in step with the engine’s charge-air pressure.
Before an engine may start on gas, many GVU control systems run a valve-proving sequence. Typically, the space between the two shut-off valves is pressurised and isolated to show that the downstream shut-off valve and the vent valve hold pressure, then vented and isolated to show that the upstream shut-off valve does not let gas through from the supply. The exact steps, times and limits are set by the GVU maker.
On LNG-fuelled ships, fuel gas reaches the GVU either as boil-off gas (BOG) from the tank vapour space or as LNG vaporised in a heat exchanger, often heated by a glycol-water circuit. Methane boils at a lower temperature than ethane and propane, so BOG is mostly methane and has a high methane number, while LNG that has aged or is force-vaporised carries more ethane and propane and has a lower methane number.
If the supply switches quickly from BOG to forced vaporisation without the engine control allowing for the change in methane number, or if heater fouling lets gas that is too cold reach the GVU, the engine may knock or GVU components may be exposed to temperatures below their rating.
Whether you are preparing an LNG carrier, a dual-fuel container ship or an onshore LNG-to-power facility, combining hands-on GVU fault drills with structured inspection checklists reduces surprises during class surveys and vetting.
A tightness test fails when the pressure trapped between the double block-and-bleed valves rises or falls by more than the maker’s limit during the test time. Common causes are debris or pipe scale on a valve seat, a slow solenoid pilot, or low actuator air pressure.
The IGF Code (IMO Resolution MSC.391(95), 9.6.1.2 and 13.8.1) requires the space around gas fuel piping in a ventilated pipe or duct to have mechanical underpressure ventilation of at least 30 air changes per hour. This can be reduced to 10 air changes per hour if the duct is filled with nitrogen automatically when gas is detected. The alternative in 9.6.1.1 is a double-wall pipe whose annular space is kept with inert gas at a higher pressure than the gas fuel.
IGF Code 9.4.7 requires the gas branch downstream of the double block-and-bleed valves to be ventilated automatically when the master gas fuel valve shuts down automatically. Where the system has a nitrogen purge, the maker’s procedure sets when it runs, and gas piping must always be made gas-free before it is opened for maintenance.
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