SAFETY SHARE - Explosive Conditions: Pre-Heating on Confined Space Prior to Welding

Posted on: Monday, 9 September 2019




A number of incidents have occurred globally involving cutting, pre-heating or welding on a confined space have resulted in violent explosions.  


A customer of CablePrice had an experienced boilermaker weld a cover on a hatch following repairs. A build-up of Liquid Petroleum Gas (LPG) and oxygen O₂ was ignited by the welding activity and the resulting explosion threw the hatch in to the boilermakers chest and caused burns on his face. 


The investigation identified that the basic cause of the incident was a mixture of LPG and oxygen.  After tack welding and preheating, he began to weld the hatch in to place.  This risk was not identified in the Job Hazard Analysis undertaken. The  LPG pressure gauge was not operational. Improper combustion due to faulty equipment can increase the levels of unburnt fuel. 


For the prevention of harm to worker from explosion or fire burst due to the ignition of combustible gases and other materials while installing hatches or covers onto enclosed (or partly enclosed compartments). The following assumptions apply; all repairs, testing and hatch portal preparation has been completed prior to hatch installation and it is not covered under the scope of this Safety Share 

Summary: During the installation of hatches (cover/lid) it is possible that an explosion or flame burst will occur during or after preheating a hatch in-situ with a naked flame (e.g. LPG torch). Unburnt hydrocarbons can readily enter the compartment through weld prep areas from a naked torch flame. It is also possible for dust and other introduced combustible contaminates to be introduced during repairs which must also be controlled to prevent similar explosive conditions. By carefully cleaning and then fully purging the compartment with Argon gas to expel oxygen within the compartment to levels of 2% or less the risk of fire and explosion is eliminated as combustion cannot occur.  

Required Competencies:
 Trade qualified boilermaker/fabricator/welder or equivalent
 Training in the correct use of a gas detector
 Safe use and operation of LPG heating equipment. 

Equipment Required:
 Purge level monitor to read O₂ concentrations
 Argon bottle with flow meter (10-40 litres per minute range)
 Purging tube
 Hatch prepared with 3/8 purging ports.
 Opening pre-installed with backing bar
 Oven or second person to pre-heat hatch
 Mechanical fasteners to hold hatch
 Soapy water for leak testing
 Infrared temperature gun or similar

Control Strategy:
 Correct use, handling and storage of flammable
gasses and materials.
 Hot-work permit system and strategies shall apply
at all times.
 Test correct operation of all equipment before use
Personnel Required:
 Second person to preheat hatch away from compartment opening.

PPE Required:
 Standard PPE and all PPE required for hot work operations,
welding helmet, visors, fume control, welding gloves etc.

 Explosion, fire, burns (Hot work)
 Manual handling
 Inert gas (Argon, simple asphyxiant)
 Soapy water to conduct leak testing on gas appliances
 Follow manual handling procedures/ use mechanical
 lifting aids as required.
 Adequate ventilation and gas monitoring of
 immediate work area (O2 levels)
 Argon (or similar inert gas) purge tube vented
to an area well outside workers breathing zone