Preparing for a burner shroud and lance change on kiln
Contractors were asked to prepare for a burner shroud and lance change on Kiln 2, Shaft 2 (see diagram – additional pdf). They had been signed in and issued a confined space entry PTW in order to carry out the task prior to starting.
Following kiln shut down – the process closes 2 of 3 gas valves automatically and stops the combustion air blower. The filter ID fan is turned into manual mode and dampers set to seal shaft 2 and exhaust up shaft 1. The 3rd gas valve is closed via a separate action by the kiln control room operator.
From the control screen it was noticed that the 2 automatic valves had closed but the 3rd safety valve was in alarm mode. This was not acted upon and the work continued.
To confirm that the stone level had lowered sufficiently, a visual inspection was necessary through the part 5 door by the contractors. The initial inspection by the contractors confirmed that further lowering would be necessary and this was carried out by the kiln control room operator. When the contractors opened the part 5 door for the second time there was an ignition within the shaft causing burn injuries to the contractors.
It appears that the incident was caused by auto ignition of leaking gas: ignition taking place once the inspection door was open providing oxygen. It is possible the rear two shut-off valves had been leaking for some time, but a failure of the main valve allowed the incident to happen. In addition, failure to identify and act on the valve alarm from the control room is seen as a significant causal factor. The failure to routinely close the manual valve at table level indicates the reliance placed upon the hydraulic shut-off valves and this hadn’t been considered that the valves could pass gas.