DOUBLE SKIN FUEL LINES

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The main cause of engine room fires for many years is the ignition of oil on hot surfaces escaping from pressurised lines.

In order to reduce this risk;

1.         All hot surfaces should be protected. Although this should be easily carried out, repetitive maintenance on machinery can often reduce the quality of the fit of the protective devices, or shields used.

2.         Pressurised oil lines should be encased. It is difficult to protect all lines, as that would mean all transfer lines would need protecting which is uneconomical. However an outer protective sheath should cover vulnerable lines that are exposed to vibration, and are close to the hot surfaces. In fact it is Class regulations that ALL fuel injection lines are protected in this way for UMS operated ships.

Although the Class or Flag requirements do not state that an alarm system has to be fitted, it is a prudent option, as if a fuel injection line fails, the quantity injected into the cylinder will fall, and there will be an imbalance of cylinder developed powers. Various methods have been used to indicate failure of the main fuel line:

A.        Detection of the pressure between the fuel line and the outer sheath. This pressure should obviously be zero when no leakage occurs. A pressure sensor is fitted to monitor this pressure, and when it rises the alarm system will either lift the fuel pump concerned, or activate the leakage alarm. There is a small vent on this void space to prevent any small leakage at the connections or joints falsely activated this alarm. Note that the sheath can not accept high pressures, so that a vent valve is often fitted which will prevent the sheath pressure rising above 1 bar. This method was fitted to many MAN B&W GF series engines.

B.        Monitoring the quantity of liquid leaking from the void space between the sheaths. Under normal conditions no leakage occurs, but should a fault exist then the leak off fluid is collected in a small tank. Once the quantity of liquid exceeds a pre-determined limit, an alarm will be sounded via the activation of a float switch. To prevent this alarm occurring due to a build-up of liquid, rather than a definite leak, there is often a small leak-off hole fitted to the tank. This system is shown on the diagram overleaf.

C.        Monitoring of the cylinder exhaust temperatures will also indicate when a fuel line has failed, although this is not this system's primary function. It may occur that even though a fuel line has partially failed, the sensitivity of the exhaust temperature monitor may not be sufficient to provide an alarm.

The MCA have published MIN 25 on the Failure of Engine Low Pressure Fuel Systems. This investigation was in response to the frequent failures and consequent fires on-board due to fuel leakage, often from low pressure rather than high pressure fuel lines.

The main contributing factors of this leakage was the effects of high pressure pulses (up to 60-80 bar) caused by the spill operation at the end of the fuel injection, coupled with the effects of frequent dismantling and re-assembly for maintenance.

Some of the recommendations are listed:

·         More stringent controls on the installation, inspection and maintenance of low pressure fuel systems

·         ISM assessors should consider whether the ship’s SMS should contain procedures to identify possible causes of pipe failure, and protection of hot surfaces

·         Pipe work supports and retaining devices to be checked every 6 months

·         Flexible pipes to be closely examined, and care taken when tightening to avoid twisting. Recommends that these pipes are pressure tested every five years or even renewed.

·         Engine builders to limit the pressure pulses to under 16 bar.  Only pressure accumulators fitted to the fuel pump have proved effective in limiting the pressure pulses.

·         All copper and aluminium-brass piping should be heat treated to reduce work hardening effects

·         Spray and defector plates to be correctly fitted after dis-assembly

·         Change in pipe design to eliminate compression fittings and copper/brass pipework

·         Remote isolation valves should be fitted in the event of fuel leakage as suggested in M Notice 1456

·         Knowledge of the fuel systems and the problems of pressure pulses should be included in the training and examination of Engineering Officers.