Severity
Incident
Leak
Yes
Ignition
Yes
Ignition Source
self-ignition

A hydrogen leak at the flange of a 6-inch synthesis turbocharger valve in an ammonia production plant ignited and exploded. Hydrogen detectors and the fire alarm alerted the control room, which immediately shut down the plant, and the fire was then extinguished rapidly. There were no injuries caused by the accident, since the operator heard a wheezing sound and was able to run away just before the explosion occurred. The leaking gas was composed of 70% hydrogen at a flow rate of 15,000 cubic meters per hour. Property damages in the turbocharger included electrical cabling, melted siding, and heavily damaged pipes. The ammonia plant was shut down for more than a month.Five days before the incident, a problem with the CO2 absorber column led operators to open the vent downstream of the column. In retrospect, this excessive venting was an operational error. It caused a reduction in the suction pressure of the ammonia synthesis turbocharger and the activation of the plant emergency stop. The relief valve on the line between the turbocharger and the methanation reactor was then exposed to high pressure, causing it to open without the operator noticing. Production resumed the next day, but abnormal consumption of syngas led the operator to conduct further investigations. He discovered that the valve was no longer leak-proof and was allowing the gas to escape through a 47-meter chimney. The plant was shut down again to replace the relief valve.When the plant was restarted, the methanation reaction was initiated at 10:00 PM, the synthesis turbocharger started operating at 1:30 AM, and the incident occurred at 3:14 AM on the flange of the newly installed 6-inch-diameter valve. The incident was caused by vibrations in the relief valve, resulting in the quick release of the flange screws, which were probably not tightened sufficiently. In addition, when the relief valve was replaced, it was probably under-calibrated.

Incident Date
Nov 29, 2006
Setting
Equipment
  • Motive Power Systems
  • Compressor
  • Piping/Fittings/Valves
  • Piping
  • Piping/Fittings/Valves
  • Valve
  • Piping/Fittings/Valves
  • Flange
Probable Cause
Characteristics
When Incident Discovered
Lessons Learned

The following actions were proposed as a result of this incident: The company in charge of valve calibration and maintenance will be subject to approval of the plant service inspection team. Plant operating procedures will be improved. The specifications concerning the mounting and revision of valves will be strengthened. An additional pressure sensor will be installed.