Explosion at the cracking unit of a refinery

The accident occurred in the hydro-cracking unit, specifically in its sulphur recuperating system. The location was on a 8" piping, which cracked at a 90° bend. Due to the loss of confinment, a mixture of flammable gases was released from the pipe, which ignited in contact with air and exploded. This was followed by a fire.
The released gas was recycled process gas consisting of 80 vol% hydrogen, 14 vol% methane, 1.8 vol% hydrogen-sulphide , rest C2, C3 and C4 gases and water vapour.

A large part of the plant was destroyed. The fire was mainly sustained by the petroleum derived. The smoke cloud was not toxic.

The INITIATING CAUSE was the release and ignition of process gas through a crack in a high-temperature pipe.

The pipe had been the object of regular inspection, when its wall thickness was regularly measured. Because of abnormal corrosion, a part of the bend had been modified. Other bends in the same piping were not corroded.
An inadequate inspection can be excluded as root cause.

The ROOT CAUSE could be identified in some design shortcoming. It is known that sharp bends in gas pipes operating under harsh pressure and temperature conditions are weak elements, because bending causes microstructural phenomena acting as stress intensifiers of the base pipe metals, and because bends causes additional stresses due to changes in gas flow direction.

pipe

The operational values were 50 bar at 300°C.
The plant was built in 1974. The last shut down for a complete revision was in 1989. During those maintenance works it was changed a curve of the gas recycle system due to its corrosion. Other curves in the same piping were not corroded. In the period between the years 1989 and 1992 the thickness of the tube was regularly measured and never was found an abnormal corrosion

A large part of the plant was destroyed

The gas was allowed to burn out and its flow was slowed by using helium in the gas phase.

The installation was rebuild up, the use of 90° curves restricted and the gas speed decreased by increasing the tube diameter.