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'Pressure cooker': Expert says heaters of machine at centre of fatal Tuas industrial blast may have topped 1,000ºC

SINGAPORE — Heaters of a mixer machine at the centre of an explosion at a Tuas industrial worksite that claimed three lives may have reached temperatures of up to 1,095ºC at some point, although the machine was mainly used to heat up water with a boiling point of 100ºC.

'Pressure cooker': Expert says heaters of machine at centre of fatal Tuas industrial blast may have topped 1,000ºC

The scene at Stars Engrg's workshop in Tuas after an explosion and fire on Feb 24, 2021.

  • A forensic report prepared offered a theory on why the mixer machine at a Tuas worksite exploded in February
  • Low levels of thermic oil in its oil jacket made air – a poor conductor of heat – the medium of heat transfer
  • The heaters therefore generated heat, possibly reaching 1,095ºC at some point, to transfer enough heat to the water in its mixing chamber
  • It did not help that all of the oil jacket’s openings were closed under this temperature, generating high pressure as oil boils and evaporates

 

SINGAPORE — Heaters of a mixer machine at the centre of an explosion at a Tuas industrial worksite that claimed three lives may have reached temperatures of up to 1,095ºC at some point, although the machine was mainly used to heat up water with a boiling point of 100ºC.

This emerged as a specialist in failure analysis testified on Monday (Oct 4), the 11th day of public hearings by an inquiry committee looking into the causes and circumstances leading to the blast on the site run by Stars Engrg, which supplies fire protection systems, on Feb 24.

Mr Robert Shandro, a principal consultant with Matcor Technology and Services, said that the temperatures could have gotten so high because air, which is a bad conductor of heat, took over as the medium of heat transfer when the oil jacket was filled with too little heat transfer fluid.

Matcor, a Singapore firm owned by a French parent company, was engaged by the Ministry of Manpower (MOM) to conduct forensic examination of the machine, which eventually ruptured in the blast, and it submitted a 186-page report of its findings.

Mr Shandro shared the findings through video link from France, while a Singapore representative of the firm, Mr Ashley Ng, took the stand in person, both on Monday.

The committee also heard that the heat-transfer fluid — thermic oil — did not hug any surface of the machine’s mixing chambers, where water would be poured in as one of the ingredients to make fire clay, a key component of a fire-rated insulation wrap that the workshop produced.

To make matters worse, all openings of the oil jacket, where the heater and thermic oil were located, were closed during operation. The committee had previously heard that Stars Engrg boss Chua Xing Da ordered for it to be shut because he did not want the oil to escape by evaporation.

With this, the machine turned into a “pressure cooker”, Mr Shandro told the committee, adding that the pressure contained within could not ease off easily since Stars had added insulation on the exterior surfaces of the oil jacket after their workers complained of hot surfaces.

The Matcor report stated that high pressure — generated by the boiling and evaporation of thermic oil — caused cracks to form along the welds in the oil jacket. And while Stars had tried to repair hairline cracks found on the machine through welding, these only stopped the leakages temporarily.

“With poor weld quality and unresolved high internal pressure, a failure at the oil jacket was bound to occur,” the report stated.

During the hearing, Mr Shandro was shown a video that was taken by one of Stars Engrg’s employees in January. It showed white smoke coming out from the machine.

Upon watching this, he remarked: “I am terrified to see this video because I cannot imagine leaving this machine running in this condition… Oil can support 250ºC, I think, before reaching boiling point. This means that the oil inside was boiling all the time.

“This is a clear indication that the pressure inside is very high. It is like a cooking machine… a pressure cooker,” he added.

The committee also heard that the machine did not cut off power to its heaters as it should, to ensure that the oil jacket’s operating temperature stays within a certain limit. Stars Engrg did not make it a practice for their workers to measure the temperature in the oil jacket using the temperature sensor provided.

Mr Shandro affirmed that the temperature sensor was designed with a thermostat attached to it, adding that the heaters would continue heating if the temperature was not monitored.

Asked if the machine was designed with an adequate safety mechanism, Mr Shandro said that it does, adding that the type of mixer machine used at the workshop — a Sigma mixer machine — had existed for years.

“This type of machine is used in chemical processes to mix liquids and it has been used for a very long time… If used properly, it should not have problems because (its thermocouples) can stop the high temperature,” he added.

The committee also heard that Stars Engrg’s weld repairs, which were of poor quality and had voids, were made near or over the manufacturer’s original welding, and this reduced the integrity and strength of the oil jacket. The repairs thus acted as “stress raisers and promote cracking”, the report stated.

Mr Shandro estimates that the pressure limit in the oil jacket was halved by poor welding.

The hearing continues on Tuesday.

Diagrams showing the mixer machine used by Stars Engrg. Source: Court documents

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