Tuas blast inquiry: Expert says machine could have exploded even if turned off

High pressure could have built up in the machine and led to a mechanical failure of the welded seams in the heating jacket.
High pressure could have built up in the machine and led to a mechanical failure of the welded seams in the heating jacket.PHOTO: ST FILE

SINGAPORE - About three hours before an explosion ripped through a workshop in Tuas, workers spotted a small fire on a mixer machine used to make an insulation material.

But even if the machine was turned off before the blast, the explosion could have still occurred, a chemical engineering expert on Tuesday (Oct 5) told the inquiry committee looking into the Feb 24 incident.

Dr Shaik Mohamed Salim from the Agency for Science, Technology and Research's (A*Star) Institute of Chemical and Engineering Sciences said high pressure could have built up in the machine.

This could have led to a mechanical failure of the welded seams in the heating jacket, which in turn caused a sudden release of the thermic oil from the oil jacket which then formed an aerosol.

Dr Shaik said the aerosol could have found an ignition source, which may have been a hot surface in the unit, and caused the explosion.

The mixer machine was used by Stars Engrg to make an insulation material called fire wrap.

The machine heated up oil in a jacket, which then heated ingredients, including potato starch, in the mixer component.

The inquiry committee, chaired by Senior District Judge Ong Hian Sun, heard from other experts on Monday that the heaters in the mixer machine could have been overheated and therefore over-pressured because there was insufficient thermic oil in the jacket.

Agreeing with the observation, Dr Shaik said: "This could also result in phase changes and potential decomposition of the (thermic oil), with gases and vapours accumulating in the void space above the liquid level."

The explosion occurred at around 11.20am at the workshop unit at 32E Tuas Avenue 11 and injured 10 workers.

Three of them - Mr Subbaiyan Marimuthu, 38, Mr Anisuzzaman Md, 29, and Mr Shohel Md, 23 - later died in hospital of severe burns.

The inquiry committee had earlier heard that a small fire broke out on the mixer machine at around 8.40am, hours before the explosion, and witnesses were divided on whether its heaters were turned on again.

Project engineer of the company, Mr Lwin Moe Tun, was questioned on the stand last week on whether he had told the workers to turn off the machine and stop work after the fire.

Closed-circuit television (CCTV) footage taken from the workshop and neighbouring units, which was played in court, showed flashes minutes after the initial blast.

But even if the workers had turned off the machine, Dr Shaik said the blast could have still occurred because of the high temperature of the thermic oil, which meant its molecules were decomposing.

This process causes more heat to be emitted, meaning that even without a heat source, the pressure in the oil jacket would have remained high and could have eventually caused the explosion.

"Decomposition is net exothermic, which means that it has the ability to generate heat... even without a heat source," said Dr Shaik, who agreed with State Counsel Kristy Tan that the process was "like a loop".

Dr Shaik, whose research interests include dust explosion studies, suggested that potato starch could have caused multiple flash fires following the initial blast.

This was supported by potato starch residue he found in the factory unit after the blast.

The hearing continues on Wednesday with more experts providing evidence.