At a nitroglycerine manufacturing plant, a second explosion killed eight investigators while responding a day after an explosion in a pump that had killed one worker in 1992.
Eventual independent incident investigation results were discussed during a presentation by process safety specialist Rod Prior at a Process Safety Forum Gauteng meeting in April 2011.
A large commercial explosives factory processed nitro glycerine, a highly sensitive explosives component, used in mixtures. After nitration of glycerine, a centrifuge separates H2SO4 (70%) and HNO3 (9%) from nitroglycerine (2.9%).
Operators had started pumping waste acid from a 30 ton tank to a de-nitration plant, via a 1km line, when an explosion killed a worker at a pump station.
On the next day, as a technical investigation team gathered at the spent acid tank farm, a second explosion in spent acid tanks killed eight people.
Explosives manufacturing process
The process flow includes an NAB nitrator, centrifuge, nitroglycerine going to a washing and emulsification site, while spent acid goes to a denitration plant. The spent acid tank farm contains upper and lower tanks.
A spent acid dispatch pump moves acid 1.1km to a chemicals storage site.
The centrifuge separates nitroglycerine and waste acid, into separate outlets. The nitroglycerine outlet line was blocked off by a delaminated PVC pipe and nitroglycerine exited with spent acid to waste tanks.
An acid despatch pump probably started on pure nitroglycerine, which had separated out form the mixture. The pump initiated an explosion by material impact.
On the day after the pump explosion, a technical team was taking samples when a tank exploded, presumably due to heat or some action. Eight people were killed and the tank farm was destroyed.
Incident root causes found
The incidents and fatalities are attributed to failure of change management. A centrifuge outlet pipe was changed from solid PVC, to laminated PVC. The change had required a compulsory hazard and operability (HAZOP) study, which was not done.
Other plants, when checked, had pipes closing off in similar manner. Piping design was defective, with many pockets and dead legs where nitroglycerine could collect.
Acid sampling was unrepresentative and could not identify nitroglycerine droplets. The sampling process was not subjected to a HAZOP. Sampling results were only available after 24 hours.
The manufacturing plant had no system to deal with large volumes of nitroglycerine entering and gathering in the tank farm. A process HAZOP may have identified a high likelihood of this possibility, and would have designed procedures for identifying and responding to similar incidents.
Plant control was inadequate to detect missing explosive material, even if 1.8 tons of nitroglycerine went ‘missing’ from the proper process flow.
Investigation methods in this hazardous plant, was not subjected to a Hazop and risk analysis, breaking the first principle in explosives manufacturing, handling and use.
PHOTO; Centrifuge design specifies a PVC pipe, but a laminated PVC pipe was later installed, without conducting a Hazop.