GRADER MACHINERY FIRE AT MINE SITE
A fire occurred on a grader being operated within a mine site. A similar but undamaged grader was examined for comparison purposes. Each grader was fitted with an in-line six cylinder diesel engine, with the cooling fan and radiator mounted at the rear of the machine. The left side of the engine was the ‘hot’ side with the exhaust and turbocharger assemblies, while the right was the ‘cold’ side with fuel and air supplies.
The operator described how the power dropped and at around the same time the fire suppression alarm operated. He was able to continue driving to a safe place to park and leave the cabin. He did not operate the emergency stop and the engine was still running as he left the area. Although the fire suppression system had activated, the fire continued to burn until water carts arrived and extinguished it.
During the post-fire inspection by site personnel, one of the fuel filter housings was found detached and lying low down on the right side of the engine, in contact with the starter motor assembly and cables. This was preserved for later inspection.
Burning was effectively confined to the engine compartment and was more severe on the right side. The right side of the engine and ancillary equipment was more damaged by fire towards the lower rear, with directional damage patterns showing fire development upwards and forwards from there.
Below and to the rear of the air filter remains there was a steel bracket extending from the side of the cylinder head. Mounted on the rear of this bracket was the fuel water separator assembly, comprising the separator/filter cartridge screwed onto a cast aluminium alloy housing, which was bolted to the bracket through lugs in the casting.
The front of the bracket should have carried a second fuel filter housing and cartridge assembly, bolted to the bracket in the same way, but this was missing. There was a small amount of melted alloy from the lugs remaining in place between the bolt heads and the bracket. Fuel hose fittings which would have been connected to the housing were hanging free, below and forward of the housing location.
Almost directly beneath but slightly inboard of the bracket, attached to the side of the engine block, was the starter motor and solenoid assembly. Located under the machine was a quantity of heat damaged electrical conductors, including heavy gauge cable from the starter motor supply which had some melted areas and beads indicative of electrical arcing. This had fallen away completely after the machine came to a stop.
The mechanical fuel pump of the engine was located at the rear on the right side, with hoses connecting it between the forward fuel tank and the separator-filter complex above the starter motor. The hose leading from the pump to the filter was detached from the pump outlet and there was intense heat damage to the surrounding area.
The heat damage patterns on the rear hatch corresponded closely with the location of the fuel pump and separated hose inside the engine compartment. The layout of the fire suppression system was such that this area, low down in the compartment and partially beneath overhanging components, was largely protected from the fire suppression system discharge. This indicated that, although the automatic fire detection and suppression had operated correctly, it was unable to extinguish the burning completely and when the system was depleted the fire was able to grow again unchecked.
The detached alloy filter housing recovered by the site personnel had two fracture surfaces. One of these was relatively clean and bright, while the other was heavily sooted and had a more weathered appearance. There was an area of melting to the casting which had a screw thread imprint, consistent with resting on the starter cable terminal while being heated.
The filter housing was compared with an intact one in place on the undamaged machine, and the relative positions of the incoming and outgoing fuel hoses was noted. The intact alloy housing was mounted to its steel bracket by bolts and washers through two cast lugs. These lugs were missing from the damaged housing and corresponded to the two fracture surfaces.
The fuel filter housing was subjected to specialist mechanical and metallurgical examination. This confirmed that the casting had suffered fractures adjacent to each of the mounting lugs. The first fractured lug on the inboard side of the housing occurred cold and therefore pre-dated the fire, while the second on the outboard side occurred during heating.
On the unburnt grader a protective shroud, designed to cover the starter solenoid terminal and exposed cable end, had moved away and slipped down the cable sheath. Information from a fitter indicated that this was a common occurrence, suggesting that the burnt grader was probably in a similar condition with the corresponding terminal and cable end exposed.
Taking into account the damage to the fuel filter housing, the most likely series of events leading to the fire was considered to begin with a mechanical or stress-related failure of the fuel filter mount inboard lug. Movement of the assembly then caused excessive tension on the fuel line connection to the pump, partly disconnecting the coupling and releasing pressurised diesel fuel which ignited.
Enough fuel continued to flow in the line to run the engine at reduced power, while the developing fire heated the filter mount to the point where the remaining outboard lug failed under combined thermal and tensile stress. At this point the fuel line pulled completely free from its fitting and the engine powered down due to lack of fuel supply.
Two possible ignition sources for escaping pressurised diesel fuel were present. The first was a hot surface such as the exhaust manifold or turbocharger housing. This was considered less likely as the hot surfaces were on the opposite (left) side of the engine from the point of escape and the suppression alarm followed almost immediately after the power loss.
The second was that part of the lug which broke off the mounting fell into contact with the solenoid terminal or associated positive cable and a grounded surface, producing an electrical arc contemporaneous with the emission of the pressurised fuel. This ignition source would be much closer to the fuel escape and therefore minimise delay between power loss and fire suppression activation.
The report presented to assist the ICAM concluded that:
The fire originated low down on the rear right side of the engine.
It was due to escape of diesel fuel from a compromised fuel line, following a mechanical failure of the fuel filter housing inboard lug.
This resulted either from incorrect torque on the bolt, causing excessive vibration when the engine was running, or from a weakness in the casting there.
Ignition of the escaping diesel fuel probably occurred when one or more pieces of casting fell into contact with exposed electrical conductors around the starter motor.
These conductors were intended to be covered by a shroud.
The fire suppression system discharge operated correctly but the area of origin was largely shielded from its effects.
Suggestions for preventive maintenance and design changes were made including:
Checking and securing shrouds over starter motors and cables.
Checking torque settings on fuel filter mounting bolts.
Checking alloy filter housings for incipient cracking.
Considering re-design of fuel line couplings to prevent partial disconnection under tension.
Considering re-routing fuel lines away from potential ignition sources.
Considering automatic engine shut-down when unexpected fuel flow decrease detected.
Liaising with fire suppression suppliers to consider increasing discharge system coverage and duration.