On 18 September 2010 at 05:17 a section of a chlorine production plant was damaged by a series of explosions, reports the SA Process Safety Forum in a circular that does not name the operators, to encourage chemicals operators to report their incidents for the benefit of process safety and sheq practitioners.
The plant had two sets of membrane electrolysers, one set 20 years old, the other two years old. The new set of electrolysers, where the explosion occurred, is an upgraded version of the older set.
Severe damage was caused to fiberglass vessels and piping. Electrical cables were damaged by debris. No injuries or emissions occurred.
Hydrogen is produced as a co-product in the chlor-alkali process. Hydrogen and chlorine react explosively at a certain ratio. A number of contributing factors were revealed;
• Hydrogen had mixed with chlorine due to torn membranes.
• The mixture could rise to an explosive ratio.
• Operators did not detect, or did not react, to torn membranes, despite a number of signs being evident.
• Pressure fluctuations, causing ‘tears’, were not detected or not acted on.
• Seal tank to emergency scrubber.
• Empty seal, and faulty level transmitter.
• Explosion limit meter could not work properly due to lines under vacuum.
• Operator discipline was lacking.
• Poor communication from supplier of new technology.
• Operational and maintenance differences were insufficiently understood or planned for.
Management and operators had assumed that the old and new technology risk was low, due to operating a chlor alkali plant for 17 years. The recent modification was in replacing outdated electrolysers with modern electrolysers, incorporating activated nickel cathodes, two years ago.
Operators treated the new plant the same as the old plant. Membrane tears on older technology did not cause explosions.
Both types of electrolysers look and operate similarly and their P&I diagrams look nearly the same. A technology study had concluded on inherent safety of the older technology, and it was noted merely that the new technology performed significantly better.
Incident damage and loss
Damage occurred to piping, tanks, and filter unit. Electrical cables were shorted due to debris. There was damage to electrical gear, and a pressure wave in lines. There was damage to piping and nozzles in units downstream of the explosion, and production losses.
An incident investigation team recommended these preventive measures;
• Study implications of the new technology.
• Retrain all disciplines on site.
• Prevent and reduce membrane tears from pressure fluctuation.
• Prevent trips from various sources.
• Revise procedures.
• Revise ramp down sequences.
• Install stronger membranes.
• Detect pressure fluctuations and tears.
• Monitor pressures continuously, linked to specific alarms.
• Add hydrogen monitors closer to electrolysers.
• Continuously measure online voltage.
• Install an extra layer of protection on seal tank.
• Change seal tank layout.
Operators were advised to study major and minor technology changes, and not assume assurance by suppliers or assumptions of ‘standard’ installation or operation.
Plant hazard operability studies (Hazops), assessments, and Hazan re-assessments should be done at regular intervals.
Chemicals operators should maintain discussion groups on their major products and hazards.
PHOTO; Hydrogen and chlorine react explosively at a certain ratio. Severe damage was caused to fiberglass vessels and piping. Electrical cables were damaged by debris.