A hydrogen explosion and fire occurred in the benzene unit of a styrene plant in a large petrochemical complex. The unit was being restarted following a scheduled maintenance shutdown. The explosion followed the release of about 30 kilograms of 700-psig hydrogen gas from a burst flange into a compressor shed. Two men were killed and two others were injured. If it had not been a holiday, the death toll and injuries would probably have been much worse.
The operators were bringing the plant online and increasing the hydrogen circulation pressure. About 10-15 seconds before the explosion, they heard a pop and then a loud hiss of pressure being released within the compressor shed. Witnesses reported seeing a white flash and then a large fireball. The fires burned out in 2-3 minutes, except for benzene leaking from a ruptured flange that continued to burn for 8 hours because there was no emergency shutoff valve to isolate the vessel.
The metal roof and heavy metal door of the compressor shed were blown several hundred feet away. Piping, ladders, and other steel equipment were severely deformed by the blast. Ceilings fell and instruments were damaged in the control house, about 75 feet away from the compressor shed. Office buildings about 1200 feet away sustained damages to windows, doors, and masonry.
The investigation team concluded that hydrogen gas was released through a failed 19-inch diameter gasket and ignited under the roof of the compressor shed where it was partially confined. Some gas escaped from the shed prior to the explosion, but it was confined beneath the deck of an adjacent structure and overhead piping. The compressor shed was originally just a roof over the compressors, but over time, walls were added to aid winter operation and maintenance. These walls resulted in confinement of the hydrogen and contributed to the violent explosion.
Unauthorized modifications played a major role in this incident. The team discovered that the original design specifications called for a spiral-wound gasket, but for the previous 7 or 8 years, only compressed asbestos fiber (CAF) gaskets had been used. It appears that the risk of the gasket disintegrating or blowing out during a high-pressure leak had not been identified.
Actions taken as a result of the incident included the following:
- Checklists for startup and shutdown procedures, and design and engineering safety were updated and became mandatory.
- Pressure testing at operating pressure became mandatory prior to process startup.
- Process hazard analysis was introduced. - Gasket material specifications were revised.
- Separation standards were developed and implemented for all site buildings and facilities.
- Shatter-resistant windows and doors were installed.
- Process enclosures were minimized in new designs and existing enclosures were opened up and/or forced ventilation was upgraded.
- A formal modification procedure was instituted (i.e., management of change).
- Additional combustible gas detectors were installed, with frequent calibration and maintenance required. - Emergency shutoff valves were installed on vessels with critical hydrocarbon inventories.
- Awareness of chemical processing hazards was increased among all employees through better communication and training.