VISAKHAPATNAM, INDIA (REUTERS) - A clogged cooling system was the likely cause of a temperature surge in a storage tank which led to a gas leak at a chemical plant in India, killing 12 people, three state government investigators told Reuters.

They said workers at LG Polymers, a unit of South Korea's LG Chem near the southern city of Visakhapatnam, did not use a warning system to alert local residents but informed company management after the leak in the early hours of May 7.

The three investigators asked not to be named as they are not authorised to publicly discuss the investigation, launched by the government of Andhra Pradesh state.

LG Polymers said the situation at the plant was now under control but it, and LG Chem, did not respond to detailed questions from Reuters for this article.

About 800 people taken to hospital after the leak, and some were asleep when gas drifted over their homes. Police have filed a culpable homicide complaint against LG Polymers.

When a Reuters witness visited the Venkatapuram village adjoining the plant on Tuesday (May 12), leaves on trees exposed to the gas looked shrivelled and had turned brown.

The Andhra Pradesh state government declined to give any details of its investigation.

The three investigators said the temperature inside the oldest of the three storage tanks holding styrene monomer, a raw material used in polystyrene products, rose to over 150 deg C, more than six times the permitted level.

"The chiller unit was clogged because of a polymerisation process, which occurs when temperature exceeds certain levels,"one of the officials said.

To slow the polymerisation process, a chemical reaction that generates heat, styrene monomer must be stored at 20-30 de C with a cooling system and chemical compounds, or inhibitors, typically used to keep temperatures under control.

If unchecked, the heat generated from the polymerisation process causes the chemical to vaporise.

But the cooling system was clogged and the likely cause of the blockage was a chemical reaction inside the tank holding the styrene monomer, the three investigators said.

The skeletal workforce at the facility overnight was unable to bring the situation under control, they said.

"When temperatures started rising, the staff at the plant were not able to use the cooling system," one said.

LG Polymers said in a statement on Thursday that it planned to ship remaining styrene monomer inventories to South Korea.

"The team of production, environment, and safety experts are currently investigating the cause of the incident," it said.

The team would work with authorities "to analyse the cause of the incident, prevent a recurrence, and support damage recovery."

RAPID ESCALATION

Company officials told investigators the temperature within the storage tank was in the allowed range till temperature rapidly rose early on May 7, the three officials said.

Experts say it generally takes weeks for the temperature in a Styrene monomer storage to rise to up to 65 degrees, before a runaway polymerisation process occurs, leading to the liquid chemical reaching its boiling point of 145 degrees in minutes.

But conditions in parts of a storage tank - such as corners or the surface - could provide an environment or "localised hotspots" where the process can accelerate, and subsequently lead to a rapid escalation in temperature.

One of the officials said they were investigating the possibility of a localised hotspot at the surface.

Control operations were hampered by a lack of inhibitors to slow a runaway polymerisation process in the event of a cooling system failure, two other investigators said.

The factory had been in the process of re-opening after a weeks-long nationwide lockdown imposed to curb the spread of the coronavirus.

Workers said they first noticed a dense vapour near the base of one of the plant's three storage units and sprinkled water on it, but minutes later spotted thick smoke coming out of the top of the same storage unit.

One of the investigators said the gas leak was a result of a lack of safety precautions.

"It was clearly a crime of omission," he said.