Hydrogen explosion at a refinery

The accident occurred at a distillate dewaxing Unit (MDDW) of the refinery, during the operation of regeneration of the catalyst. To perform this operation, both temperature and pressure have to be increased. After approximately six hours after having reached the correct regeneration conditions, the 10 inches pipe at the bottom to one of the two reactor experienced a guillotine break, and hot gas was released from both ends. The gas released consisted of 81% of hydrogen, 9.5% of methane and some light hydrocarbons. It rapidly ignited (0.7 sec), causing an explosion. A shock wave from the resulting explosion expanded through the adjacent neighbourhood, causing varied degrees of blast damage to residential homes. The ensuing flash fire spread up to 230 feet west of the tank farm.

The INITIATING cause of the accident was the corrosion of pipe, inducing thinning of the pipe wall. The corrosion went uninspected for years.

The metallurgical analysis (see references) identified the same kind of suffixation corrosion already found in accidents in other plants; sulphur compounds in the process stream corroded a steel piping segment, causing the pipe walls to become severely thin. While sulfidation is a well-known damage mechanism at refineries that requires regular inspection and monitoring, the segment that failed has no record of ever being inspected.
This suggests a ROOT CAUSE related to lack of effective maintenance and inspection procedures and to management shortcoming.

distillate dewaxing unit, catalytic bead

DESCRIPTION OF THE PROCESS
Catalytic dewaxing is a particular hydrocracking process used to improve cold flow properties of middle distillates and lubricants by cracking normal paraffin. The pipe failure occurred at the exit of the one of the two catalyst bed reactors of the distillate dewaxing Unit, on the line bringing the gas to the heat exchanger.

The unit was undergoing hot hydrogen regeneration of the catalyst at the time of the explosion. Catalyst regeneration is a procedure performed periodically, in which the normal liquid hydrocarbon feed stops and a hydrogen-rich gas mixture is fed through the catalyst bed. The process occurs at a temperature (800 F, 370 C), higher than the operative temperature (700 F, 430 C).

The vapour cloud found an ignition source and the ensuing flash fire spread up to 230 feet west of the tank farm. A shock wave from the resulting explosion expanded through the adjacent neighbourhood, causing varied degrees of blast damage to residential homes.

The accident was due to pipe walls being thinned due to corrosion that went undetected for years (no inspection record were found). Regular and adequate inspection is hence key to prevent such event.