DOSSIER

 

 

            Reducing the risk of fire in the engine room(II)

 

 

Gaseous fuels
A relatively small number of ships utilise gas as a fuel for propulsion. Gaseous fuels typically encountered in an engine room are acetylene and propane. Oxygen, although not a fuel, will also be present for oxy-gas welding and flame cutting operations. These gases are supplied in colour coded cylinders with gas specific regulators and flashback arrestors. Gas cylinders should not be stored in the engine room. Oxygen and acetylene cylinders should be stored upright in separate ventilated steel compartments above the weather deck, separated from other compartments. From there, the gases at low pressure are distributed via flashback arresters and steel pipes to outlet stations in the engine room fitted with stop valves which should be kept closed when not in use. Alternatively, gases can be distributed at high pressure to outlet stations fitted with flashback arrestors, regulators and stop valves. Where only portable oxy-gas welding or cutting equipment is available this should be secured upright when not in use in designated ventilated compartments on or above the weather deck.

Flexible hoses designated for use with oxy-gas equipment, colour coded blue for oxygen, orange for propane and red for acetylene must be used. When laid out in an engine room the hoses should not be kinked or pass over sharp surfaces that could cause damage. When a cutting or welding torch is not in use the gas supplies must be isolated at the shut off valves. Under no circumstances should hoses be folded over to temporarily isolate a gas supply to the torch. Hoses should be subjected to frequent inspection and damaged hoses should be replaced in accordance with manufacturers’ recommendations.

All equipment must be properly maintained and leak tight. The leakage of acetylene into enclosed spaces causes an explosion and fire risk. Acetylene is an extremely reactive gas and, when mixed with air in certain proportions can detonate. In a previous accident a pressurised acetylene flexible hose was leaking close to the air intake of the starting air compressor, resulting inflammable mixture being created in the air receiver. Subsequently, when the air was used to start a generator, a series of serious explosions occurred, fracturing pipe work and other equipment. Similarly, oxygen leaks must be prevented and special care must be taken to exclude even traces of grease in oxygen handling equipment, such as regulators. Combustible materials that may ordinarily not be easily ignited will ignite readily and burn violently in oxygen enriched atmospheres.


Electrical fires


Electrical circuits are distributed from the main electrical switchboard to all parts of the ship via sub-distribution boards. Cables are protected against overload by using fuses or circuit breakers. Fuses and circuit breakers are rated specifically for the size of the cable and the load they are protecting and it is dangerous to replace these with protection devices of a higher rating. All circuits should be correctly labelled at the main switchboard and the sub­distribution boards. Where cable routings have been altered it is essential to make permanent changes to circuit labelling at the fuse or circuit breaker board and to updated electrical drawings. Temporary labels marked on adhesive tape or written on adhesive paper to cover over the original label can deteriorate and detach, leading to confusion over which circuit is energised.

Spaces behind switchboards should be clear of packaging materials and the floor area should not be used for storage purposes. Such practices increase the risk of a serious fire developing there. The inside of switchboard casings should also be kept clear of dust, dirt and other flammable materials.

In a normal circuit there should be no added resistance introduced at junction points, such as where cables are screwed to terminal connectors or where plugs are inserted into sockets, or as a result of thinning of conductors in a cable caused by mechanical damage. Unfortunately this is not always the case. A failure to ensure that terminal connections are correctly made and tight can cause a point of local resistance, unwanted heating and a fire hazard (‘resistance heating’). Such defects are usually self­worsening as a result of thermal cycling and an accelerated formation of surface oxide which increases the resistance thus further reducing the effectiveness of the terminal contact. It is important, therefore, to not only ensure that terminal connections are correctly made, but also to inspect these whenever the opportunity arises.

Routine inspections of busbar connections on the main electrical switchboard can be made by using an Infra-red temperature gun of the type to which reference was made earlier. However, such inspections will not always be a useful indicator of incipient resistance heating faults at sub-circuit or equipment terminal connections. Whereas the incipient fault may not be apparent at the time of the Infra-red temperature measurement, it could self-worsen exponentially with time. Nevertheless, such measurements are to be encouraged generally, even though the results should be interpreted with caution. There are also companies that specialise in undertaking such surveys.


Although the table in Appendix 1 provides a useful and quick source of reference, it is helpful to illustrate how a failure to comply with SOLAS Regulations and to provide for effective maintenance and tidiness in an engine room can lead to a serious fire with the potential for loss of life and injury, major financial consequences and unnecessary litigation.


Oil fires


Oil fires are invariably the most serious category of engine room fires. Two ships entered with the Club recently suffered significant engine room fires with remarkable similarities. Both fires originated in the region of the generators when leaking oil sprayed onto hot exhaust surfaces and the subsequent efforts to extinguish the fires were hindered because of a failure to maintain the fire smothering systems correctly and/or a lack of understanding by the crew of the correct method of deploying the systems. In one case, two crew members suffered smoke inhalation injuries and in the other, one died while trying to fight the fire. In both cases significant damage to the engine room occurred resulting in towage and expensive repairs.
Fires can result from a failure to attend to small persistent leaks that can, for example, spread across machinery surfaces to reach parts operating at a high temperature, and from larger leaks that develop suddenly. For example those caused by:
- Loose joints
- Fractured pipes and mechanically damaged (perforated) pipes on both high and low pressure fuel lines
- Bleed cocks on generator fuel filters working loose
- Pipe unions that are over or under tightened
- The fracture of flange bolts if over tightened
- The fracture of cyclically stressed bolts or studs that are under-tightened, such as those securing fuel injector pumps
- The use of unsuitable seals or gaskets which deteriorate due to the effects of heat
- The rupture of high pressure oil and hydraulic fluid hoses due to mechanical damage or aging
Correct maintenance procedures should be strictly adhered to. High pressure pipes should be sheathed and flange joints enclosed where they are in proximity to hot surfaces in order to comply with SOLAS Regulations. Any hot surface shielding should also be effectively maintained.


Hot surface ignition and preventative measures


Oil fires usually occur when oil from a large leak or a smaller but persistent leak comes into contact with a nearby hot surface at a temperature that exceeds the ‘minimum auto ignition temperature’ (MAIT) of the oil. MAITs of diesel and fuel oil are typically about 250°C, but MAITs as low as 225°C have been reported. Lube oils and hydraulic oils have somewhat higher MAITs. High pressure sprays comprising fine droplets of oil can ignite immediately on contact with the hot surface, and liquid leaks can ignite after a short period of time sufficient to evaporate the oil and generate a flammable concentration of fuel vapour. Under certain circumstances, such as where flammable concentrations of vapour form in confined spaces, the fire may be preceded by an explosion. Clearly, all oils should remain contained within their intended systems. Oil fires often develop and spread quickly compromising the safety of engine room personnel and, in the case of generators, damaging associated main electrical cabling feeding the switchboard which can lead to a loss of electrical power and, as a result, motive power.

Spray shields should be fitted around flanged joints, flanged bonnets and any other threaded connections in fuel oil piping systems under pressure exceeding 0.18 N/mm2 which are located above or near units of high temperature in accordance with SOLAS II-2 Reg. 4.2.2.5.3 and MSC.1/Circ1321. Furthermore, high pressure fuel delivery pipes should be sheathed within jackets capable of containing leaks from pipe failures, the annular spaces of which must be equipped with suitable drainage arrangements to facilitate the rapid drainage of oil to a safe location, such as a drain tank.
It is essential to employ good maintenance systems and engineering principles in order to reduce the risk of oil leaks. This includes, for example:
- attending to minor leaks without delay
-tightening connections to fuel injectors and fuel injection pumps to the correct torque to prevent leakage and/or fatigue fractures caused by cyclical stresses induced by operation of the pump
-maintaining oil leak detection and alarm equipment that can warn of the presence of oil leaks in concealed areas such as a‘hot box’ enclosing fuel pumps on some types of generator .

The maintenance of leak detection/alarm equipment is especially important where oil vapour from a leak of hot oil at a temperature above its flashpoint can, for example, migrate from the hot box of a generator, across the engine entablature to exhaust system enclosures where the vapour can auto-ignite

The insulation on cable conductors and motor windings can deteriorate over time. A breakdown in cable insulation can lead to stray electrical currents and ultimate short-circuit arcing in a cable. This can be a highly energetic event that can readily melt plastics and may completely evaporate metal contacts and cable conductors resulting in the explosive ejection of molten metal providing a source of ignition. A breakdown in the insulation of motor windings can be a source of localised heating and fire. It is essential, therefore, that a programme of routine insulation resistance testing of cables and other equipment is maintained, and be aware that cable insulation can deteriorate from exposure to UV light.

Arc flash incidents can occur where engine room personnel work carelessly on live equipment and cause a short circuit with a tool. No matter how well a person may be trained, distractions, weariness, pressure to restore power, or over-confidence can cause an electrical worker to bypass safety procedures, work unprotected, drop a tool or make contact between energised conductors. This may not only lead to serious injury or death, but also provide a source of ignition for a fire.

The contacts of switching mechanisms, such as in contactors, can become eroded and this may cause contacts to s

tick closed or provide a source of resistance heating. Routine inspections of such equipment may not be practicable, especially in respect of small compact devices. However, the equipment should be repaired or replaced if there is evidence that the contacts fail to open and close correctly or if signs of localised heating are discovered.

The confluence of electrical cables in distribution boards necessitates a large number of terminal connections to both cable conductors and overload protection devices such as circuit breakers. Inspections of terminal connections and, ideally temperature measurements of the same made by using an Infra-red temperature gun, should form part of shipboard inspection and maintenance programmes. Where multi-stranded electrical conductors are connected to a terminal care should be taken to ensure that there are no stray strands that could inadvertently make contact with another part of the installation. It should be established that all switchgear is clean and circuit breakers are in good condition. Fire stopping around cable glands should be in good conditionto minimise the risk of a fire spreading from the distribution board to surrounding areas.

Larger electrical cables are often steel braided or steel wire armoured so that combustible insulation is not exposed and the risk of flame spread is minimal. Even though cable insulation is invariably flame retarded to lessen the risk of ignition and flame spread, groups of cables fixed to a cable tray can spread flame, especially when exposed to an external source of fire, such as a fire on an auxiliary generator.

 

 

 

 

 

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