Workers overcome by poison gas discharge

Lookout! Issue 35, June 2016

Employees and contractors working on board a vessel were overcome by chlorine gas fumes originating from the engine room during cleaning.

The toxic discharge occurred when chemicals used to clean out the main engine room heat exchanger reacted with each other to produce chlorine gas.

The 45-metre fishing trawler was alongside the wharf while routine ‘turn-around’ activities were underway to prepare it to return to sea. One of the activities involved the second engineer cleaning or ‘descaling’ the main engine heat exchanger.

A heat exchanger transfers heat from the vessel engine’s coolant to raw water pumped from the sea. Because marine growth and salt can build up and reduce the efficiency of the cooling process, heat exchangers need to be periodically cleaned.

Chemical cleaning is a common method, carried out at about two-month intervals for the type of heat exchanger on this vessel. Cleaning involves connecting a domestic wheelie bin to the heat exchanger and pumping cleaning chemicals from the bin through the cooling system.

The boat, floating upside down.
Cleaning involves connecting a domestic wheelie bin to the heat exchanger and pumping cleaning chemicals from the bin through the cooling system.
Maritime New Zealand ©2020

During the evening, the second engineer added 8–10 litres of descaling liquid (hydrochloric acid) to the bin, along with 40–60 litres of fresh water. He then started the pump to circulate the solution through the heat exchanger. The following morning, he stopped the pump and dumped the solution into the engine room bilges, from where it was removed by suction to a truck on the wharf for disposal.

The second engineer then flushed the hoses, pump and wheelie bin with hot fresh water, and ran the main engine cooling pump to flush salt water through the system for several minutes.

He added 8–10 litres of sodium hypochlorite to 40–60 litres of fresh water in the wheelie bin, stopped the salt water flush, and pumped the new chemical solution into the heat exchanger.

About two minutes later, he noticed a strange smell and tried to dilute the chemical by adding water to the wheelie bin solution. However, the smell grew stronger and he realised that it was gas and he was becoming affected by it. He stopped the pump, but – thinking the event was isolated and under control – didn’t set off an emergency alarm on the bridge. When he moved from the engine room into the locker room some time later, he found two contractors being treated by ambulance staff.

Four crew and two contractors were treated with oxygen in hospital and discharged later the same day. The Fire Service HAZMAT team supervised ventilation of the vessel, and continued monitoring the atmosphere until it was safe to clear the vessel for re-entry the next day.


  • Several people were treated for gas inhalation, and it is fortunate that more were not affected. This incident shows that raising a general alarm is vital to the safety and welfare of everyone on the boat.
  • Chlorine gas and salt water are created when hydrochloric acid (the ‘descaler’) and sodium hypochlorite (factory sanitiser) are mixed. The reaction and gas discharge was likely caused by residual traces of the original chemical mixing with the second introduced chemical.
    • The normal process for cleaning the heat exchangers is to flush the system with hydrochloric acid solution, followed by a salt water flush. However, the chief engineer, instructed the second engineer to use the sodium hypochlorite after the salt water flush to help in the cleaning process. This was a departure from the usual method and, though the engineer had conducted some small-scale tests using undiluted sodium hypochlorite during the previous voyage, neither he nor the second engineer fully understood the risks involved.
  • This event was completely avoidable. It happened because key staff had insufficient knowledge about how to use and mix chemicals. However, both engineers had completed on-board safe operating procedures for chemical usage, which advised them to read the material safety data sheets before using chemicals, and not to mix acids and alkalines.
    • Had the engineers followed the company’s documented health and safety processes and risk management processes, the risk of contamination from mixing chemicals would have been identified. Referring to the material safety data sheets and risk assessment tools would also have provided them with the confidence and controls to vary the normal process. But they did not consult them.
  • There was no documented safe operating practice in place for cleaning any of the 10 heat exchangers on board the vessel, as the company considered it a routine undertaking that didn’t pose any hazard. The company has introduced a range of corrective actions aimed at reducing the likelihood of the event happening again. These include developing a safe operating practice, involving the single descaling liquid as previously used.
  • The company also now requires engineers to raise the alarm to the bridge during any hazardous event, even if they believe it is isolated.

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