Gas Combustion Systems on Liquefied Gas Carriers

Dive into the heart of LNG carrier technology with an in-depth exploration of the Gas Combustion Systems. From main boilers to emergency operations, discover how this system optimizes efficiency and safety while powering the world’s LNG transport fleet.

Reference: SIGTTO “LNG Shipping Suggested Competency Standards”, Sections:

1 Have an awareness of system types:

• plant parameters;
• plant limitations;
• safety arrangements.

2 Have an awareness of the purging and starting sequence for gas burning:

• fuel oil operation established;
• nitrogen purge.

3 Know and understand the pre-conditions for fuel gas burning. Conditions and reasons before gas can be supplied into engine room:

• gas temperature;
• engine room ventilation status;
• gas burning equipment space ventilation;
• gas detection.

4 Know and understand the pre and post gas burning purge and venting sequences:

• pre purge sequence;
• post purge sequence;
• interlocks and trip functions.

5 Know and understand the procedures to supply vapour to the machinery:

• The procedures to supply vapour to machinery at correct temperature and pressure using the low duty (LD) compressor and gas heater for pressure control purposes only:
  • relationship between LD compressor and LD heater.
• The procedures to supply 100 % vapour to machinery at correct temperature and pressure using:
  • spray pump;
  • forcing vaporiser;
  • fuel gas pump;
  • LD compressor and gas heater.

6 Know and understand the maintenance and testing requirements:

• calibration;
• function testing;
• shut down.

7 Have an awareness of the non-standard operating conditions:

• manual firing;
• loss of nitrogen gas supply;
• loss of fuel gas pressure;
• leaks within gas hoods or ducts;
• inability to operate steam dump facility or gas combustion unit where applicable;
• loss of LD gas compressors, heaters, vaporisers etc.;
• Key equipment not available.

Moss ships are controlled by gauge pressure while on ballast passage and absolute pressure when loaded. Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. The ballast condition control circuit takes into account the spraying operations necessary to maintain the cargo tank equatorial ring temperature.

Gas (LNG vapour) combustion on an LNGC falls into one of two categories:

• fuel gas burning, where BOG is used as fuel in the ships’ main boilers, dual/tri fuel engines, ME-GI engines, X-DF engines or a gas turbine;
• control of excess BOG by utilising a gas combustion unit (GCU) (see Liquefied Natural Gas Reliquefaction Plant“Gas Combustion Unit (Oxidiser)”). The purpose of the GCU is to control the cargo tank pressure by safely burning the BOG.

Each propulsion option has differing requirements, including:

Main boilers

• each boiler must have a separate exhaust uptake and a dedicated forced draught system;
• a crossover between boiler force draught systems may be fitted for emergency use, providing safety functions are maintained;
• combustion chambers and uptakes of boilers must be designed to prevent any accumulation of gaseous fuel;
• burner systems should be suitable to burn either fuel oil or gas alone or fuel oil and gas simultaneously. The burners should be designed to maintain stable combustion under all firing conditions;
• gas nozzles (spuds) are to be suitably located to obtain ignition from the oil flame;
• an inter-locking device is fitted to prevent the gas supply being opened until the oil and air controls are in the firing position;
• an automatic system is fitted to change over from gas operation to fuel oil operation without interruption of the boiler firing in the event of loss of gas supply;
• fuel oil alone is to be used for start-up;
• unless automatic transfer to fuel oil is provided for manoeuvring and port operations, it should be possible to change over easily and quickly from gas to fuel oil operation;
• unless satisfactory ignition has been established and maintained, gas flow to the burner must be automatically cut off;
• a manually operated shut-off valve must be fitted on the supply to each gas burner and, in addition, each burner supply pipe must be fitted with a gas shut-off valve and a flame arrester;
• an IG or steam purging connection must be provided on the burner side of the gas shut-off arrangements so that the pipework to the gas nozzles are immediately and automatically purged before and after gas firing;
• the automatic fuel changeover facility must be monitored with alarms to ensure continuous availability;
• if there is flame failure on all operating burners, the combustion chambers of the boilers must be automatically purged before relighting;
• the boilers must be capable of being manually purged;
• low water level trips must be fitted when operating on either fuel oil or gas;
• a notice must be posted at the firing platform, stating: “if ignition is lost from both oil and gas burners, the combustion spaces are to be thoroughly purged of all combustible gases before re-lighting the oil burners”.

Dual fuel internal combustion engines

• When gas is supplied in a mixture with air through a common manifold, flame arrestors are to be installed before each cylinder head;
• an isolating valve must be fitted at the inlet to the gas supply manifold. This must close automatically in the event of low gas pressure or failure of cylinder to fire;
• a flame arrester must be fitted at the inlet to the gas supply manifold for each engine;
• the gas supply to each engine must be capable of being operated manually from the control position;
• each engine shall have its own separate exhaust uptake configured to prevent any accumulation of unburnt gaseous fuel;
• unless sufficiently strengthened to withstand overpressure from ignited gas leaks, air inlet manifolds, scavenge spaces, exhaust system and crankcases must be fitted with suitable pressure relief systems;
• each engine must be fitted with independent vent systems for crankcases, sumps and cooling systems;
• each cylinder must be provided with an individual gas inlet valve admitting gas either to the cylinder or to the air inlet port. The timing of this valve should be such that no gas can pass to the exhaust during the scavenge period nor to the inlet port after closure of the air inlet valve;
• prior to admission of gas fuel, the operation of the pilot oil injection system on each unit shall be confirmed;
• for dual fuel engines fitted with a pilot oil injection system, an automatic system shall be fitted to change over from gas fuel operation to oil fuel operation, with minimum fluctuation of the engine power;
• in the case of unstable operation on engines. when gas firing, the engine shall automatically revert to oil fuel mode;
• dual fuel type engines must be capable of immediate changeover to oil fuel only;
• all “starting” shall be done on oil fuel alone;
• when stopping the engine, the gas fuel supply must be automatically shut off before the ignition source;
• arrangements must be provided to ensure that there is no unburnt gas in the exhaust gas system prior to ignition;
• crankcases, sumps, scavenge spaces and cooling system vents must be provided with gas detection equipment;
• where trunk piston type engines are used, a means of injecting IG into the crankcase must be provided;
• a system allowing continuous monitoring of possible sources of ignition within the crankcase must be fitted;
• combustion must be monitored to detect anomalies that may lead to unburnt gas fuel in the exhaust system during operation and, if detected, the gas fuel supply immediately shut down.

Note: Natural gas has a very low ignition quality and requires a high temperature to ignite. This allows the gas (LNG vapour) to be mixed with air during induction into a diesel engine prior to the gas/air mixture being compressed. The compressed gas/air mixture requires a separate ignition source. Ignition of the gas/air is achieved by use of a pilot flame. The pilot flame injection system typically provides 7 % of the final input energy, with the main energy release being due to the combustion of the gas.

Gas Turbine

• Each turbine must have its own separate exhaust with the exhausts configured to prevent any accumulation of unburnt gas fuel;
• unless sufficiently strengthened to withstand overpressure due to ignited gas leaks, air inlet manifolds and exhaust system must be fitted with suitable pressure relief systems;
• pressure relief systems within the exhaust uptakes should be led to a non-hazardous location, away from personnel;
• an automatic system must be fitted to change over easily and quickly from gas operation to fuel oil operation with a minimum fluctuation of engine power;
• combustion must be monitored to detect anomalies that may lead to unburnt gas in the exhaust system during operation and, if detected, the gas supply is to be immediately shut down;
• each turbine must be fitted with an automatic shutdown device for high exhaust temperatures.

Although each of the gas consumers will have a different gas system, equipment and layouts. The same basic procedures apply, with a “fail safe” methodology incorporated into the IAS control system.

Prior to, and on completion of, gas burning in any of the consumers, the pipework from the main gas valve (MGV) to the fuel gas consumer must be purged with N₂. This is an automated sequence initiated from, and controlled by, the IAS. Incorporated safety features, including interlocks and trip functions, ensure that there will be no combustion of BOG in any consumer in the event that the N₂ purge is interrupted or a trip function activated.

The N₂ purge is achieved by passing N₂ through the system, or part of it, for a sufficient time period to expel any trapped gas or air. The N₂ is supplied from the on board N₂ plant.

In addition, prior to any maintenance, the fuel gas system must be purged with N₂ to ensure no trapped gas remains and thereafter isolated. On completion of maintenance, the fuel gas system must be purged with N₂ to ensure any air ingress is removed.

During slow streaming, manoeuvring or periods when the ship is stopped, the steam dump system reliquefaction plant or the GCU should be capable of managing the BOG before the vapour vent valve automatically opens.

MGV safety chain

This will vary depending on the Types of propulsion systems on ships carrying LNGpropulsion type and the system architecture. On a steam ship, trips would normally initiate MGV closure:

• fuel gas supply temperature low-low;
• gas detected in the boiler gas hood room;
• gas detected in the E/R vent fan rooms;
• BOG heater drain level high;
• gas hood room ventilation failure;
• fuel gas pressure high-high or low-low;
• manual request for closure of the MGV;
• loss of authorisation to burn gas from the wheelhouse or CCR;
• failure of E/R exhaust vent fans;
• release offixed fire-fighting medium into the E/R, the emergency generator room, gas compressor motor room, gas compressor house or cargo switchboard room;
• blackout.

On steam propelled ships, the MGV may take 30 seconds to travel from “full open” to “full closed”. When the valve begins to move away from the “full open” position it activates a “normally open” proximity switch, which is designed to allow the BMS to start the FO burners before the gas fires are extinguished.

On a modern ME-GI propelled ship, with DF diesel generators, the fuel gas system architecture is more complex. This includes separate MGVs for each of the main engines, another for the DF diesel generators and one more for the GCU (assuming there are two main engines – four MGVs in total).

The following would initiate MGV closure:

• ESD manual push buttons – all MGVs close;
• ESD fusible plugs – all MGVs close;
• hydraulic oil pressure low-low – all MGVs close;
• pneumatic ESD link pressure low – all MGVs close;
• air control board pressure low – all MGVs close;
• blackout – all MGVs close;
• ESD SF failure – all MGVs close;
• vapour header high-high pressure – all MGVs close;
• vapour header low-low pressure – all MGVs close;
• IS ESD solenoid valve failure – all MGVs close;
• gas supply trip push button – all MGVs close;
• cargo tank level 99 % (high-high) – all MGVs close;
• cargo tank/primary insulation space differential low – all MGVs close;
• fire detection in E/R – all MGVs close;
• No. 1 ME common gas alarm – No. 1 ME-GI MGV closes;
• No. 2 ME common gas alarm – No. 2 ME-GI MGV closes;
• fire detection in cargo compressor room – all MGVs close;
• gas detection for DF diesel engine – DF diesel engine MGV closes;
• GCU fuel gas vent outlet pipe gas high-high – DF diesel engine MGV closes;
• cargo compressor room gas high-high – all MGVs close;
• GCU room loss of ventilation – DF diesel engine MGV closes;
• GCU ventilation outlet gas high-high – GCU MGV closes;
• gas detection in E/R – all MGVs close;
• gas detection system communication fault – all MGVs close;
• fuel gas compressor discharge temperature low – all MGVs close;

MGV safety systems differ – ensure familiarity with the system as fitted on your ship.

An ESD may not initiate closure of the MGV on all LNGCs. This must be verified on each specific ship.

Fuel gas burning initiation

As there are now a number of different propulsion options available for LNGCs using BOG as the primary or supplementary fuel, it is essential that manufacturer, flag State, Class, company recommendations, rules and regulations are followed. Each propulsion option has differing requirements.

Generally:

• with fuel gas available to the system and all parameters healthy, select gas burning option on the IAS;
• N₂ pre-purge will operate to ensure that there are no pockets of gas in the fuel gas system;
• on completion of the purge sequence, select the required gas consumer ensuring automatic operation is selected – gas compressors, etc. will now start;
• it is imperative that the gas consumer is monitored throughout to ensure there are no problems with the gas supply or combustion;
• when gas burning commences, the burning of liquid fuel is reduced accordingly and, depending on the propulsion system, it may be completely shut off if sufficient gas is available;
• in the event of the loss of fuel gas firing, there is an automatic change over to liquid fuel firing and for this reason, the liquid fuel system is maintained in operational condition – if necessary by controlled recirculation, depending on fuel type;
• on completion of fuel gas burning and changing back to liquid fuel (stopping of gas compressors, etc.), the nitrogen post-purge system operates. This replaces the combustible gas in the pipework, from the MGV to the burners, with an IG.

Due to the numerous propulsion options, it is essential that type-specific training and instructions, for the safe implementation of gas firing, are documented and provided to the ship’s crew.

The cargo tank NBOG, and FBOG where necessary, enter the vapour main via the cargo tank gas domes where it is directed via the mist separator to the LD or FG compressors. The LD or FG compressor outlet then passes through a heater or, depending on the system, an after cooler before entering the fuel gas pipework.

On ships with dual fuel propulsion, the primary objective of the LD or FG compressors fitted in the cargo machinery room is to maintain the cargo tank pressures and temperatures at a pre-set level and to deliver the BOG to the E/R at a constant pressure.

On an Low Duty Compressor(s) on the Liquefied Natural Gas CarriersLD compressor, the inlet guide vane position is governed by the cargo tank pressure in relation to the required pressure, while the flow of gas through the operating compressor is controlled by adjusting the variable guide vane position. As the compressors are used to control the tank pressures in this way, the compressor output can vary and will correspond directly to the conditions in the tank.

On ships fitted with DFDE/TFDE generators, where only one main generator is in operation, the fuel gas demand may be lower than the minimum discharge capacity of one compressor. During these conditions, and to prevent the compressor from surging, optimum gas flow is maintained by returning the excess gas from the compressor, either ashore via the vapour main or to the GCU.

The cargo and gas burning piping system is arranged so that excess BOG can be vented should there be any inadvertent stopping of gas burning in the E/R or GCU plant. The automatic control valve on No. 1 vent mast can be used for this purpose, although this should only be used as a last resort.

If the gas main pressure falls to a pre-determined level an alarm will sound.

In the event of automatic or manual shutdown of the gas burning system, or (on a membrane ship) if the tank pressure falls to 1 mbar above the insulation space’s pressure, the automatic valves will close and the gas burning supply line to the E/R will be purged with nitrogen.

If the NBOG is insufficient, additional gas (FBOG) can be obtained by utilising the forcing vaporiser. This is fed by fuel gas pumps, supplying the LNG liquid to the vaporiser via the fuel gas main. The gas is then delivered from the forcing vaporiser via the mist separator, to the inlet of the compressors. The outlet temperature of the gas from the vaporiser is automatically controlled by a bypass valve that passes the FBOG to the mist separator.

The forcing vaporiser operates automatically and will be started and controlled from the ECR if the NBOG from the cargo tanks is insufficient to maintain the system pressure and meet the E/R demand.

Read also: Dual-fuel diesel electric/Tri-fuel diesel electric (DFDE/TFDE)

When a fuel gas pump is in operation, it will maintain a pressure set by the operator in the fuel gas main. If the pressure exceeds the setpoint pressure, the fuel gas return valves will open and return the excess LNG to the tank.

If a compressor is only running to supply gas to the E/R, its load is controlled by engine demand. If the forcing vaporiser is also in use, the compressor is controlled by the tank pressure and the vaporiser, by engine demand.

For steam powered LNGCs, provision should be made for free flow of BOG to the boilers when the LD compressors are shut down and isolated.

Maintenance

To ensure all elements of the fuel gas burning system are covered, the planned maintenance system (PMS) must be suitably structured with the following considered:

• maintenance guidelines, intervals and specifications given by the manufacturers;
• company specific requirements;
• Class and flag State requirements;
• history of equipment, including failures, etc.;
• ISM Code and statutory requirements;
• age of the ship;
• third party inspections;
• consequences of failure of equipment on safe operation of the ship;
• critical equipment and systems;
• redundancy and obsolescence.

Other specific requirements may include:

• calibration of all instrumentation;
• alarm test procedures for all equipment in the fuel gas burning safety chain, including gas detection;
• trip simulation to test the fuel gas burning shutdown facility for each trip function.

Emergency operation

There is no one-off protocol or procedure that will apply to all propulsion options. Each procedure will vary depending on the type of propulsion system fitted, the builder and the associated IAS. It is important that the procedures established by the manufacturers are adhered to completely and that the emergency operation procedures are documented, readily available and understood.

Training and instruction on the safe implementation of emergency procedures should be provided to the ship’s crew.

It is important that any emergency operation procedure is clear and precise, allowing for no ambiguity.

Essential equipment – reliquefaction plant, LD compressor, FG compressor, oil mist separator, GCU, steam dump system, BOG heaters, FBOG or spray pumps, gas detection system and nitrogen plant.

Note: all fuel gas consumers on an LNGC have a dual fuel capability. If any problem is encountered with the fuel gas supply, or if the NBOG/FBOG is insufficient to meet demands of the fuel gas consumers, the fuel gas and fuel oil systems are designed to automatically changeover to dual fuel. This includes the ability to automatically change over to 100 % fuel oil consumption, in a “bumpless” manner, should the need arise.