TST Module 40 • LNG fuel systems

LNG Fuel Systems, Fuel Gas Handling & IGF Code

High-level visual crib sheet for marine engineers: LNG storage, bunkering, vaporisation, gas conditioning, safety systems, engine gas admission and oral-exam fault logic. Scope deliberately excludes Tier III emissions detail.

Storage

−162°C

Vapour expansion

≈600:1

Flammable range

5–15%

Safety basis

IGF

What Module 40 is really testing

LNG is not just a cold fuel. The examiner is testing whether you can follow the complete fuel route, explain why each safety barrier exists, control tank pressure without casually venting, and respond correctly to gas alarms, ventilation faults, bunkering abnormal conditions and maintenance isolations.

Tank-to-consumer fuel route.

IGF Code equivalent safety.

Type C and membrane tank concepts.

Inerting, purging and gas freeing.

GVU, MGV and double block and bleed.

Gas engine Otto-cycle operation.

Section 1

Complete LNG fuel system route

1. Bunker stationESD link, drip tray, exclusion zone, communication and purge readiness.

2. Type C tankCryogenic independent tank with pressure, level, temperature and relief protection.

3. LNG pumpSubmerged pump supplies liquid to vaporiser or transfer route.

4. VaporiserGlycol/water heat converts LNG liquid to gas.

5. Gas heaterRaises gas to consumer inlet temperature.

6. GVURegulates and isolates final gas supply to engine or boiler.

Protected piping

Gas fuel pipework through machinery spaces is normally enclosed by double-wall piping or ducts with ventilation or inerting and gas detection.

Safety logic

Ventilation failure, gas detection, ESD, fire detection or consumer fault can close master gas valves and stop gas operation.

Oral warning

Do not describe LNG like a diesel system. Always include cryogenic conditioning, gas detection and shutdown barriers.

Section 2

IGF Code and class-rule thinking

Equivalent safety

Gas and low-flashpoint fuel systems must achieve a safety level comparable with conventional oil fuel systems.

Prevention

Prevent releases by approved containment, materials, pipe design, valve arrangements, pressure control and maintenance.

Detection

Detect abnormal gas, temperature, pressure, ventilation and valve states early enough for automatic action.

Mitigation

ESD, isolation, pressure relief, ventilation, fire protection, inerting and emergency procedures reduce consequences.

Area	What the rules are controlling	Engineer’s oral explanation
Hazardous areas	Ignition-source control where gas may occur.	Zone classification drives Ex equipment selection, inspection and maintenance.
Bunkering	Ship-shore compatibility, ESD, communications, transfer control and emergency response.	It is a planned operation, not just connecting a hose.
Fuel supply	Safe pipe routing, double-wall containment, ventilation, isolation and monitoring.	A leak must be contained, detected and isolated before it becomes an engine-room hazard.
Crew competence	Training, drills and fuel-handling procedures.	People are part of the safety barrier.

Section 3

Tank designs, membrane concepts and pressure control

Type A

Independent tank concept requiring full secondary barrier due to crack/leak philosophy. Not the usual compact LNG fuel tank answer.

Type B

Independent tank with advanced stress analysis and partial secondary barrier based on crack-propagation assumptions.

Type C

Independent pressure vessel used widely for LNG fuel. Designed for cryogenic temperature, pressure control and relief protection.

Membrane

Thin primary/secondary barriers supported by insulation and hull structure, common in LNG cargo containment but conceptually important.

Tank pressure management = heat ingress + BOG generation − designed consumption / cooling / compression route

BOGBoil-off gas from heat ingress raises tank pressure.

PBEPressure build-up evaporator raises tank pressure when needed.

Top fillSpray cooling can condense vapour and reduce pressure.

PRVProtective last barrier, not routine pressure control.

Section 4

LNG bunkering sequence

PlanCompatibility, transfer plan, SIMOPS and exclusion zone.

ConnectHoses/arms, dry-break, drip tray, bonding/earthing if required.

TestCommunication, ship-shore ESD, gas/fire detection.

PurgeNitrogen purge, leak test and oxygen/moisture control.

CooldownControlled cool-down to prevent thermal shock and flash gas.

TransferRamp rate, monitor pressure, temperature, level and alarms.

If gas detection alarms during bunkering, the answer is not “investigate at the leak”. Stop/ESD as required, isolate, keep personnel clear, control ignition, ventilate/gas test and only resume after a controlled review.

Section 5

Gas admission, pilot fuel and Otto-cycle explanation

Low-pressure Otto-cycle gas engine

Gas is admitted to the air path to form a lean gas-air charge. The piston compresses this mixture. A small pilot-fuel injection near the end of compression provides ignition, then the burning charge expands to produce work.

Why pilot fuel?

Methane has high auto-ignition temperature and will not reliably ignite like diesel in a premixed low-pressure gas engine. Pilot fuel gives a reliable ignition source.

Methane number

Indicates knock resistance. Low values reduce knock margin.

Wobbe Index

Indicates energy delivery through a nozzle at a given pressure drop.

High-pressure GI

Gas is injected late directly into the cylinder and behaves closer to Diesel-cycle combustion.

Section 6

Abnormal conditions and TST fault logic

Abnormal condition	Likely causes	Immediate engineering response
High tank pressure	Heat ingress, high transfer rate, poor vapour return, BOG route unavailable.	Use designed pressure control, reduce/ramp transfer as required, avoid relief-valve operation.
Low gas temperature	Vaporiser duty low, heating-medium fault, excessive LNG flow, sensor fault.	Check heater/vaporiser, flow, alarms and consumer limits.
Gas detection alarm	Leak, detector fault, ventilation issue, purge failure.	Alarm, isolate/ESD as required, ventilate, gas test and restart only after cause is proven.
Unexpected icing	Cryogenic leak, insulation failure, cold vapour/liquid contact.	Do not chip mechanically; isolate and investigate safely.
Knock / unstable combustion	Gas quality, methane number, high charge temperature, timing or load issue.	Follow engine logic, reduce load if required, check gas quality and control parameters.

Best final sentence

If gas quality, containment or safety-barrier status is uncertain, stop or reduce gas operation according to the approved procedure, isolate where required, prove the atmosphere safe and only then restore operation.