Perth truck HC explosion: how it happened

Blown TX valve

Blown TX valve

EXACTLY how two truck occupants came to be seriously injured by hydrocarbon refrigerant escaping into their cabin and igniting as they drove down a suburban street in Perth, has been revealed in a report produced during an independent investigation of the incident.

The explosion happened on April 28 this year, exactly five months after the 14 year-old Mitsubishi Canter FK-FM 600 Series truck was retrofitted with hydrocarbon refrigerant as part of a major air conditioning system overhaul.

READ MORE: Perth truck blast probe recommends temporary HC refrigerant ban

With the charge of hydrocarbon refrigerant, the truck’s refurbished air conditioning system operated well on the day of installation, when temperatures reached 31.6 degrees, and it continued functioning for the duration of a typically hot Perth summer.

When the failure happened it was a relatively cool day in Perth with a peak temperature of 19.4 degrees, so the air conditioning system would not have been straining to keep the cabin temperatures comfortable.

The report says the driver and passenger were travelling in a 50km/h suburban zone when they saw “smoke” moments before the blast. Investigators say the “rapid discharge of liquid/saturated [refrigerant] vapour would have looked like a mist/fog or smoke and caused turbulent mixing with the air”.

“The rapid combustion that followed might be described as an explosion. It could have had fatal consequences.”

By now suffering injuries – including some serious second- and third-degree burns – the occupants “exited the cabin without turning off the engine”.

“The truck was not in gear and the hand brake was not applied. Once the driver and passenger were outside, others present assisted in putting objects in front of the wheels to prevent the vehicle rolling away,” says the report.

“Had the vehicle been travelling at speed the consequences could have been far worse … the driver was dealing with an explosion in his cabin and this could easily have resulted in a collision with further dire consequences.”

Investigators found the diaphragm of the truck’s TX valve had separated from the main body, a rare occurrence that the manufacturer of the failed valve, Fujikoki, told investigators would have required pressure exceeding 4.45MPa.

The TX valve’s location behind the truck’s dashboard allowed refrigerant to enter the cabin at high pressure. The report says a propane/air mixture requires just 0.2 millijoules to ignite, less than the spark energy from a person’s fingertips when getting out of a vehicle on a cold day.

Other ignition sources listed include electrical components “including relays, brushes on a commutator and other electrical contacts”. The report includes an expert opinion demonstrating that common circuits in a motor vehicle are sources of ignition under normal operating conditions.

A test of three new and unused valves revealed that they failed at a pressure of 4.6MPa when subject to a temperature of 14 degrees, but when the investigators raised temperatures to 80 degrees they failed at 3.8 and 3.9MPa due to the effect of heat on the “soft solder fixing the diaphragms”.

Given the mild temperatures on the day of the incident, normal operating pressures would have been “in the range from 0.8 to 1.2MPa”, and even in summer with the vehicle parked in full sun the pressure would have only been “close to 2.0MPa”.

It was not until the third inspection of the truck that investigators found the reason why the system’s pressure got high enough to rupture the valve and cause an explosion – a non-operational condenser fan.

When removed from the truck the condenser fan was found to be working but when the explosion investigators reassembled the system they found the electrical plug connecting the condenser fan to be “extremely dirty”.

“The reason for that level of contamination comes from the fact that the condenser is mounted near the left hand front wheel well and the electrical plug assembly is not fitted with a rubber boot,” reads the report.

“Only after a number of times making and breaking of the electrical connection could the fan be made to show some degree of rotation.”

Although the report says the consequences of this incident could have been far worse if the vehicle had been travelling at speed, they note that the increased airflow to the condenser might also have reduced the chances of it happening.

A test of the system’s “generic aftermarket” pressure switch found the contact to be open at 0MPa (zero pressure), then closed from 0.2MPa all the way up to the test limit of 9MPa, leading investigators to conclude it to be “most likely of the wrong type” and designed only as a low pressure cut-out switch.

“From the outside it cannot be established who the maker was, what type it is or whether it is faulty or not,” says the report.

Investigators contacted the parts distributor that supplied the Sanden compressor fitted to the truck as part of the air conditioning refurbishment, where they found the same model of compressor as fitted to the truck on display and “inspection confirmed there was no relief valve fitted on that compressor”.

The parts distributor told the investigators that “relief valves on compressors started to disappear when R-12 was phased out in the 1990’s”.

Resulting from the lack of suitable pressure switch or relief valve, the truck’s air conditioning compressor was uncontrolled once the condenser fan stopped working and with the runaway build-up of pressure ultimately reached the point where the TX valve became the weakest link and failed.

It appears the air conditioning system of the truck was an aftermarket installation as investigators could not match the layout to those shown in official Mitsubishi repair manuals for identical models sold in Australia and the USA.

“The USA truck version would have been fitted with a pressure switch that would cut out at 2.94 MPa and a compressor incorporating a safety relief valve that discharged at 3.5 to 4.1 MPa would have been installed,” says the report.

“If the fitout of the system had been in accordance with this manual then there would have been two independent safety devices that should each have prevented this rupture of the TX valve from occurring.”

Consulting two Australian manuals, both of which described different air conditioning systems to the USA version, the investigators found “no reference to the pressure cut-out switch nor the safety relief valve” in the document most closely resembling the system installed in the truck.

“However the wiring harness on the truck incorporates a pressure switch connection that does interrupt the current to operate the clutch of the compressor when required.”

The second Australian manual described “an entirely different system that makes reference to a high/low pressure switch but not to a compressor with integrated safety valve”.

Overall the report points to a lack of failsafes in the air conditioning system fitted to the Mitsubishi truck and the presence of the TX valve in the cabin. With these failsafes in place and a TX valve on the other side of the firewall, you would probably not be reading about this incident.

More to the point, if the system had been charged with the industry standard refrigerant, R134a, the two occupants would certainly not have suffered burns.

“The TX valve would have also ruptured had refrigerant R-134a been used rather than a (hydrocarbon) blend of R-290 & R-600 and the circumstances would have been similar in as much as there would have been a rapid release of gas into the cabin. However, there would not have been a deflagration and the consequent burn injuries,” the report says.

“Mitsubishi does not approve the use of HC refrigerants and were very specific about this.”

READ MORE: Perth truck blast probe recommends temporary HC refrigerant ban

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