Knowledge Of Sea

Dual Fuel Engines

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The future of shipping industry rests with cleaner and more efficient fuels. Use of cleaner source
like LNG/ LPG as an engine fuel is fast growing globally. Main engines designed for dual-fuel
engine are going to be more dominant in future as more stringent air pollution and Emissions
regulation are schedule to come into force.

Engines can use LNG, LPG, Ammonia, Methanol or mix of these as the main fuel source, with a
small amount of compliant pilot fuel. Flexibility is therefore retained by using these cleaner and
compliant fuels to take advantage of optimal fuel market prices.

Dual Fuel engines, when on low-load operation (starting, stopping, reducing speed) run on Fuel
oil (Primary Fuel). After the engine attains minimum recommended Power (MCR), then engine’s
fuel system changes to the Alternate fuel (Secondary Fuel). In case of any malfunction in engine,
as a fail-safe arrangement, the fuel system automatically changes over to Fuel oil (Primary Fuel)

Basic Principle of Dual Fuel Engine
Various types of fuel like LPG, LNG, Methanol, Ammonia etc., are used in dual fuel engines in different forms.
a. LPG is generally injected into the main engine fuel oil system as a liquid. It is supplied to the engine at about 51~55 bar supply pressure to the engine which is boosted in pressure (FBIV) by the Fuel Booster Injection Valve, with similar combustion cycle as conventional fuel.
b. LNG or methane is injected as a gas.
c. Ammonia gas burning principle is similar to LPG.

Key Consideration for Fuel Management during voyage

  1. Prior start of voyage, the Charterers needs to declare type of voyage. All voyages are to be conducted primarily basis gas burning as default, unless otherwise advised owing to operational safety concerns and/ or a clear commercial advantage to the contrary.
  2. The Master should ensure that the use of Fuel Oil (i.e., other than LPG) is avoided or always minimised. If the Master considers that this requirement may prejudice vessels ETA or adversely affect operational flexibility, the Master should contact charterer / operator well in advance.
  3. Unless otherwise advised, the Master should maximise the quantity of cargo delivered to the Buyer.
  4. Unless otherwise advised, only sufficient cargo (Heel) should be retained on board in the designated tank, to provide sufficient heel to carry out the ballast voyage at the speed required by the delivery program and to maintain the required load port target arrival temperature.
  5. Where the Master feels a greater than normal heel on departure discharge port is warranted, due to forecast weather on passage or other operational reasons, permission in advance should be sought from Owner/ Charterer.
  6. While calculating heel for the passage master has to consider below
  1. It is recognised that forecast weather enroute will have a bearing on the heel quantity retained and Masters are required to consider this in calculating the departure heel required. It is also recognised that good heel management may mean an occasional requirement for a partial cool down. While this should be avoided, if possible, it is preferable to continual excessive arrival heel quantities.
  2. During the voyage, for any technical trouble faced in gas burning equipment, considering safety of the vessel master may change to fuel oil however Master must promptly inform to all parties citing the reasons.

For dual-fuel engines fitted with a pilot oil injection system, an automatic system shall be fitted to change over seamlessly from gas fuel operation to oil fuel operation with minimum fluctuation of the engine power.

In the case of unstable operation on engines with the arrangement as above when gas firing, the engine shall automatically change to FO mode and same time it will purge the gas line with Nitrogen.

Ship’s staff will require to top up LPG Service tank from Cargo tank through (LPSS) depending on length of voyage.

Common Safeties & arrangement in vessels with Dual fuel engine:

The safety of the ship depends on the care and attention of all on board. Most safety precautions are a matter of common sense and good housekeeping and are detailed in the various manuals available. Nonetheless, records show that even experienced operators sometimes neglect safety precautions through over-familiarity / complacency. Following basic rules must be remembered all times.

The purpose of probability and consequences of fuel-related hazards limited to a minimum through arrangement and system design, such as ventilation, detection, and safety actions.
Suitable control, alarm, monitoring, and shutdown systems are provided to ensure safe and reliable operation to be fully operational.

Ship’s staff must refer to the IGC & IGF code for regulations & to familiarize as required.

Properties/ Fuel Injection Pressures/ Supply Pressures for Different Fuels:

Vessels use LPG as Fuel:

Design philosophy:

The overall focus in the development of the dual fuel (HFO/ MDO + LPG) system concept has been process & personal safety. Secondly, it has been important to sustain engine reliability with automatic changeover to fuel mode in case of gas failure. This is achieved by:

  1. Gas safe engine room (with double walled gas pipelines + Hydrocarbon detection + Nitrogen Purging)
  2. A single failure results in gas shut down or gas stop will not hinder the engine operation on MDO/ HFO.
  3. The LPG system is an add on to the ME engine and must not affect the engine running on MDO/ HFO.
  4. Control and safety functionalities are separated on different hardware units like SACU (Second fuel Auxiliary Control Unit ), SPCU (Second fuel Plant Control Unit), SPSU (Second fuel Plant Safety Unit), SCSU (Second Fuel Cylinder Safety Unit).
  5. ELWI (Electrical Window Valve (pilot valve)) interlock with ELGI (Electrical Gas Injection Valve) to Fuel Booster Injection Valves which ensures safety of injection.

ME-LGIP engine safety principle

  1. ME-LGIP engines are designed according to the rules for ‘Gas Safe Machinery Space’.
  2. In case of a problem in the gas injection system, the use of gas is stopped and the engine is changed-over to fuel oil operation.
  3. The stop sequence of the gas injection and –supply system is:
    a. Gas supply to engine and machinery space is stopped
    b. The LPG pressure in the engine system is reduced followed by LPG liquid recovery procedure in which LPG is returned to the LPG tank
    c. Gas return line is closed, and the engine system is depressurized and nitrogen purged.
  4. If the LPG Tank System does not allow to send LPG back to the tank, The LPG is depressurized directly to the Knockout drum followed by a nitrogen purging sequence.
  5. Fail-safe state is the same as item no. ‘4’ above.

The Main components in the LPG supply system (MAN B&W 6G60ME-C10.5-LGIP-HPSCR), are listed below:

  1. LPG Service Tank: During engine operation, the recirculated LPG will be heated in the engine, and it may contain traces of oil from the injection valves. In order to prevent oil contamination of cargo or fuel tanks, the recirculated LPG must be returned to a dedicated service tank of a certain size. During purging of the engine, the same tank can be used for nitrogen separation and bleed off from the recovered LPG. The tank capacity and design pressure are functions of the overall system setup. From the service tank, a built-in or external low-pressure pump will supply the pressure needed for the high-pressure pump in the Low-flashpoint fuel supply system (LFSS).

Purpose is to:

  1. Low -flashpoint fuel supply system (LFSS): The LFSS will contain the equipment needed to ensure the required temperature, pressure and fuel quality on the engine, i.e., a highpressure pump, a heater and filters. Furthermore, the LFSS contains the valves and control systems to maintain the pressure and temperature at varying engine consumptions.

Purpose is to:

  1. Fuel Valve Train (FVT): The FVT represents the interface between the engine and the auxiliary systems. The FVT is intended for safe isolation of the engine during shutdown and maintenance and provides nitrogen purging functionalities. The purging functionality ensures a safe environment on the engine after shutdown.

The FVT has the quality standard necessary for reliable safety functions, and ultimately it ensures a safe and reliable operation of the engine.

  1. Nitrogen system: Nitrogen needs to be available for purging after normal engine operation and for the purpose of gas freeing prior to maintenance and tightness testing after maintenance. Therefore, the nitrogen system must be able to deliver a certain flow at a pressure higher than the service tank pressure.
    The required nitrogen setup can be achieved by a nitrogen booster and bottle bank if the vessel already has a nitrogen generator on board. Alternatively, a skid containing nitrogen generation,
    booster and storage facilities can be made available from various suppliers.

Purpose is to supply Nitrogen for:

  1. Vent system and Knock-out drums: The vent system consists of a number of vent masts with knock-out drums, which in the event of a system leakage and shut down of LPG operation, ensures that no liquid is released via the vent system. Furthermore, in the event that the return line is blocked during engine stop, the engine must be able to release the on-engine LPG volume to a knock-out drum, which must be sized for this purpose. The vent systems must be separated to ensure that safe isolation of the engine is not bypassed by the vent system.

Purpose of the Knock Out drums are to:

  1. Double-walled ventilation system:
    In order to detect leakages from the engine room systems and direct these to a safe location, the LPG systems and piping inside the engine room are double walled. A constant flow of ventilation air is kept in the outer pipe in accordance with IMO requirements. A constant supply of dry air ensures the corrosion resistance of the system

The functions of SACU (Second fuel Auxiliary Control Unit ) and SPCU (Second fuel Plant Control Unit) are:

The SPCU monitors:

The functions Second fuel Plant Safety Unit (SPSU) monitors the essential gas process values are:

The second fuel cylinder safety unit SCSU monitor scavenge air pressure Pscav and the cylinder specific pressure listed below and evaluate the sensor values at each injection:

The main sequences and the corresponding functions of the LPG fuel system are described in below figures:

a. When the engine is not in LPG operation, the LPG fuel systems inside the engine room are depressurized and completely isolated from the supply and return systems by means of the double block and bleed arrangements in the FVT. Prior to every start, as shown in the figure, the systems will be pressurized by nitrogen in order verify the tightness of the system.

b. The below figure shows the fuel system during LPG operation. LPG is supplied from the LPG tank via the fuel supply system to the engine. A small portion is continuously recirculated to the LPG fuel tank to constantly maintain the required fuel condition on the engine.

c. When LPG operation is stopped, as shown in the Fig. 3, the LPG on the engine is transferred to the LPG tank by means of nitrogen pressure, which will push back the LPG. When purging is complete, the FVT will once again ensure that the engine room systems are isolated from the supply and return systems.

d. When LPG operation is stopped, as shown in the Fig. 4, the LPG on the engine is transferred to the LPG tank by means of nitrogen pressure, which will push back the LPG.
When purging is complete, the FVT will once again ensure that the engine room systems are isolated from the supply and return systems.

Vessels use Methanol as Fuel:
System works same as ME-LGIP, the difference is in the Supply pressures.

Vessels use LNG as Fuel:

LNG burning dual fuel System (ME-GI Engine, ME-GA, WinGD-XDF, 4 stroke DF engines)

The Main components of LNG fuel gas unit are listed below:

  1. Main engine fuel gas system (Fuel Gas Pump)- The pump creates the pressure required by the engines. With a dual fuel engine, this is 6 to 8 bar, while gas engines require a pressure of 2 to 3 bar. A lower tank pressure will benefit the tank’s holding time. The choice of pump is determined by the fuel consumption of the engines
  2. Low Duty Fuel Gas Compressor – Low duty (LD) compressors are installed in the compressor room on deck and are routinely used for compressing the LNG vapour produced by natural boil-off to a sufficient pressure to be used in the boilers as fuel.
  3. Heavy Duty Fuel Gas Compressor – Heavy duty (HD) compressors are installed in the compressor room, used to pressurize the gas fuel train on 300 Bar.
  4. LNG Vaporiser – The LNG vaporiser is a shell and tube type heat exchanger that is used for vaporising LNG liquid for the following operations:
    a. when discharging cargo at the design rate without the availability of a vapour return from the shore.
    b. Purging of cargo tanks with vapour after inerting with inert gas and prior to cool down
    c. Emergency forcing by manual operation.
  5. Forcing Vaporiser – The forcing vaporiser is used for vaporising LNG liquid to provide gas for burning in the boilers to supplement the natural boil-off.
  6. Mist Separator – The mist separator prevents liquid from entering the compressors. It receives natural boil off from the cargo tanks and forced boil off gas from the forcing vaporiser.
  7. Gas Valve Unit (GVU) – The main functions of the Gas Valve Unit are to regulate the gas feeding pressure to the engine, and to ensure a fast and reliable shut down of the gas supply.

Procedure for Supplying Fuel Gas to the Main Engines:

Natural Boil-Off Gas (BOG)
a. Burning Heat transfer to the LNG in the cargo tanks during cargo operations or during a voyage, results in the generation of BOG. As BOG is generated, it would produce excessive pressure inside each of the tanks if it were allowed to accumulate, so it has to be continually removed to allow the tanks to be maintained just above atmospheric pressure.
b. The primary objective of the Low Duty (LD) /HD or BOG compressors fitted in the cargo machinery room is to deliver any boil-off gas produced to the engine room at a constant pressure as per the type of engine considering the cargo tank pressure.
c. Under normal operating conditions when vessel is at sea, one LD/HD or BOG compressor will be required to supply fuel gas to the engine room to ensure that a stable gas supply to the main engines and generator engines is achieved.

Forced Boil-Off Gas Burning
a. If the fuel consumption of the main engines and auxiliary engines cannot be met by the gas supplied by natural boil-off from the tanks, additional gas can be obtained by utilising the forcing vaporiser or cargo tank spraying.
b. Cargo tank pressures should be monitored during gas burning operations. If boil-off gas is removed at too fast a rate, the tank pressures could be reduced below a safe working limit.
The cargo tanks should always be maintained within their designed operating pressure range either by forcing vaporiser or cargo tank spraying.

c. Engine Room Systems: Once the boil-off gas has left the deck area at the correct working pressure and temperature, it can be burnt in the main engines/ generator engines/Boiler /Gas combustion unit.

Gas Safety
a. Gas is led to the engine in a double-walled pipe. The inner pipe is certified to 1.5 times the operation pressure, and the thickness of the outer pipe is sized to take 1.5 times the rupture pressure of the inner pipe.
b. The pipes are located so heavy objects cannot be dropped on the piping and cause damage.
c. In case of a leak in the inner pipe, on-line hydrocarbon detectors cause the system to immediately revert to fuel operation, and the system lines purged of gas by nitrogen.
d. If the system shuts the gas down, the switch-over to fuel is immediate with no loss of power.

ME-GI:

WinGD-XDF

Dual fuel operation modes:
a. Gas mode “Minimum fuel”

b. Mixed mode “Specified gas”

Vessels use Ammonia (NH3) as Fuel:

Challenges and advantages of an ammonia fuel:

There are challenges but also advantages associated with storage, transport and combustion of ammonia governed by the physical and chemical properties

The following sections highlight the main principles of the fuel supply system for the ammonia engine and dual-fuel operation.

Principles of dual-fuel operation During dual-fuel operation, the ammonia fuel supply to the engine comes from the storage tanks via the fuel supply system.

To maintain the required fuel conditions at the engine, a small portion of the ammonia fuel continuously recirculates to the Fuel Supply System (FSS) via the recirculation system. When the engine is not in dual-fuel mode, the double block-and-bleed arrangements of the Fuel Valve Train (FVT) depressurise and completely isolate the ammonia fuel systems inside the engine room from the ammonia fuel supply and return systems.

Before every start, the systems are pressurised with nitrogen to verify the tightness of the system. When dual-fuel operation stops, the nitrogen pressure pushes back the ammonia fuel from the engine to the recirculation system. When the purging sequence is complete, the FVT will once again ensure the isolation of engine room systems from the supply and return systems. Throughout the entire operation, the double-walled ventilation system from existing MAN ES dual-fuel engines detects any ammonia fuel leakage and directs it away from the engine room to a separate ammonia trapping system.

Recirculation system:
The recirculated ammonia fuel will heat up in the engine during operation. To avoid two-phase conditions, a certain amount of the ammonia fuel is recirculated to a dedicated recirculation line. The same recirculation line recovers the ammonia fuel from the engine whenever dual-fuel operation is stopped. The recirculated fuel may contain traces of sealing oil from the injection valves. The recirculation line eliminates the risk of contaminating fuel storage tanks with oil. The recirculation line also separates and bleeds off nitrogen from the recovered ammonia fuel.

Fuel supply system
The FSS contains the equipment necessary to ensure that ammonia fuel is delivered to the engine at the required temperature, pressure and quality. In most cases, the FSS has a high pressure pump, a heater, filters, valves and control systems to maintain the ammonia fuel pressure and temperature at varying engine consumptions.

Fuel valve train
The FVT is the interface between the engine and the auxiliary systems. The purpose of the FVT is to ensure a safe isolation of the engine during shutdown and maintenance, and to provide a nitrogen-purging functionality. This functionality ensures a safe environment on the engine after shutdown.

Nitrogen system
Nitrogen must be available for purging the engine after dual-fuel operation, for gas freeing prior to maintenance and for tightness testing after maintenance. The capacity of the nitrogen system must be large enough to deliver a certain flow at a pressure higher than the service tank pressure

Double-walled ventilation system
To maintain a safe engine room, it is vital to detect any leakages from the ammonia fuel system and direct these to a safe location. This has led to the double-walled design of ammonia fuel systems and piping inside the engine room. A constant flow of ventilation air is kept in the outer pipe in accordance with IMO requirements. The system is already part of other MAN B&W dualfuel engine designs.

Ammonia capture system
The ammonia systems must be designed with an ammonia capture system to prevent release of
ammonia to the surroundings

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