Question

With reference to LNG diesel engine installations:

(a) Describe, with the aid of a sketch, a Gas Valve Unit, explaining its purpose and indicating where it is located in the gas train (8)

(b) Explain why ventilation and inert gas systems must be installed with the engine fuel gas system (4)

(c) State why pilot injection must be provided when burning fuel gas, explaining how a pilot injection system operates. (4)

Answer 1

Part (a)

The gas valve unit (GVU) controls the gas feed pressure according to the engine load. Throughout the engine operation, the load conditions are dynamic and change which in turn requires changing gas pressure along with ensuring safe operation of engine with a timely response to changing load conditions. This task is achieved by a series and parallel combination of shuttle and vent valves which form a GVU (gas valve unit). A schematic diagram of the GVU process system is shown in the figure. To achieve the best performance of the engine in response to transient conditions, the GVU must be placed as close as possible to the engine. Recommended fuel gas pipe length between GVU and engine should be less than 10 m

Part (b)

Ventilation is provided in hazardous zones which are the engine room itself and the annual space of the double skin pipeline. This is required to prevent the accumulation of gas in the protected zone if a leak occurs. The ventilation system is installed with detectors to find if there is any trace of gas, this will serve as an early indication should a leak occur.

The inert gas system is provided to substitute any remaining natural gas in the pipeline or system with inert gas (nitrogen). This is required when any maintenance work is carried out in the system. This is a safety process that ensures that the natural gas cannot leak into the surrounding areas with potential risks.

Part (c)

The natural gas will not ignite until its temperature is raised to the minimum ignition temperature, which is 600°C. The temperature in the cylinder cannot be raised to that high temperature during compression, so auto-ignition of natural gas will not take place. Pilot injection is provided in a dual-fuel engine to start the ignition of the natural gas mixture in the combustion chamber. The pilot injector is controlled electronically which injects fuel at proper timing. About 5% of total fuel consumption is injected as pilot fuel. In some cases, the spark plug is used instead of the pilot injector to ignite the air-fuel mixture in the combustion chamber.

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Gas Valve unit for your reference: