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Case 12
Deflagration in Sewage Tank System

Narrative

The passenger vessel Island Princess was originally fitted with a holding tank sewage system. With extended port stays, its limited tank capacity proved to be insufficient and an Omnipure sewage treatment plant was added. It is designed to oxidise and disinfect raw sewage by means of an electrochemical reaction. It macerates raw sewage, mixes it with sea water and then passes it through two electrolytic processing cells. The resultant electrochemical reaction produces clean water suitable for discharge overboard together with hydrogen, chlorine and other gases. About 0.2 ft³/min of hydrogen gas per cell is produced and discharged overboard using a sea water eductor.

Island Princess was on passage towards Sydney, Australia, when a loud bang was heard and a bump felt. An inspection of the machinery space revealed that the walls of two of the tanks forming part of the Omnipure sewage system were bowed outwards. The system, which had been in use for about 13¹/₂ hours, was immediately shut down. Further inspection showed that a small number of toilet and deck scupper seals in the crew accommodation had been blown out and that a crack had developed in the sewage vent main riser pipe.

Further investigation found:

• the inlet filter on the sea water eductor was choked and was preventing the flow of sea water through the eductor;

   

• the vacuum breaker fitted between the treatment tank vent line and the hydrogen eductor suction manifold had been fitted the wrong way round;

   

• a small bore drain line had been fitted in the eductor manifold;

   

• a small hole was found in the water trap in the vent line between the treatment tanks.

The cause of the over-pressurisation was deflagration* resulting from ignition of a build-up of an explosive gas mixture within the last two treatment tanks of the Omnipure sewage plant. There was visual evidence of over-pressurisation but no tell-tale indications of where ignition took place. Since hydrogen mixed with air requires very low energy levels to ignite, all potential low energy sources were examined but without any positive result.

The high pressure alarm fitted to the system did sound but only as a result of the deflagration and the subsequent over-pressurisation. It was not triggered by the initial gas build-up.

The hydrogen gas build-up in the last two sewage treatment tanks was due to a choked sea water inlet filter to the gas discharge eductor and the incorrectly fitted vacuum breaker. Additionally, the small bore drain line allowed hydrogen gas to enter the second treatment tank while the hole in the water trap allowed air to be drawn via the vent line into the last two tanks.

Following the investigation:

• The vacuum breaker was refitted correctly;

   

• The small drain connection fitted on the eductor manifold was removed;

   

• The eductor, sea water pump and pipe system was renewed with a larger pump, pipes and inlet filter;

   

• The damaged tanks were repaired.

The Lessons

1. When new equipment is being installed, either by contractors or suppliers, always check at each stage of the installation, that items are correctly fitted.

2. Sewage treatment plants are not the most interesting part of a vessel but this incident illustrates the importance of monitoring all machinery systems. Gases released during sewage treatment can be explosive under certain conditions and toxic under others. Always make yourself familiar with the operating principals of the system and know what potential problems can occur.

Footnote

*Deflagration can be defined as:

“A very rapid oxidation (or combustion) with the evolution of heat and light as well as the generation of a very low intensity pressure wave of moving gases.”

Note that this is a low-pressure wave and not a shock wave.