Explosion at mine site when pre-heating an inspection hatch on box section prior to welding

An incident occurred recently when a contract tradesman was installing an inspection plate after welding internal cracks in a structural hollow section on the track frame of a large excavator.

What happened?

After pre-heating the hatch, the tradesman started to weld the inspection hatch back into position. This resulted in an explosion ejecting the hatch into the tradesman’s chest with force, along with flames and hot gas.

The tradesman was attended by emergency response personnel and taken to hospital with minor injuries. Had the hatch struck the head region the consequences could have been much worse.

How did it happen?

Weld joints are required to be pre-heated to specified temperatures before welding; in this case at least 40°C. To get the inspection plate up to this temperature the plate was tack-welded into position and a gas torch was used to heat the plate. An explosive mixture built up within the structural box section. The explosion occurred when the welder attempted to weld the hatch.

The original equipment manufacturer (OEM) repair manual did not address this hazard.

[image] Oval shaped inspection window on outside of gray steel box section
Figure 1: Box section showing inspection window
[image] View beneath excavator showing steel box section with inspection window and a hammer next to it
Figure 2: Box section under excavator

Comments

When using an LPG torch to heat a plate tacked onto the structural box section, a fuel-rich mixture may occur in the box section. This is due to unburnt LPG and/or combustible products of partial combustion of the LPG entering the box section around the gaps between the plate and the box section. After heating, air will be sucked into the box section as it cools, and the LPG fuel mixes with the air and becomes explosive. When the fitter attempted to weld the plate into position, this provided an ignition source for this fuel rich mixture.

What can be learnt from the incident?

Conduct a risk assessment to determine controls to be implemented to prevent explosions of gas in relation to welding repair work where accumulation of explosive mixtures of gas may occur.

Controls may include:

  • cleaning and decontaminating the box section compartment
  • using mechanical fastener for hatch retention
  • preheating the plate in an area away from the hollow section
  • preheating the area on the box section with a fully functioning flame torch
  • placing the inspection hatch onto the box section (using welding gloves and onto the mechanical fasteners)
  • purging compartment with a non-flammable gas
  • ensuring the work area is not contaminated from the non-flammable gas
  • establishing safe work procedures for the person within his zone
  • using trained and certified welders using PPE.

More information

Explosive Conditions: Pre-Heating on confined space prior to welding (NSW Energy and Resources) (PDF 103 KB)

Health and safety in welding (PDF 603 KB)

Authorised by Mark Pizey, Chief Inspector Extractives
Issued By Worksafe New Zealand 
Mines Safety  Alert 1 – Version 1