Fire in a refinery

A pipe carrying synthesis gas ruptured in the ammonia synthesis unit of an ammonia and urea production plant. The affected pipe was a 24" stainless steel syngas supply line from the CO shift converter to the carbon dioxide absorber. The rupture point was where water was injected into the synthesis gas to cool it from 230 C to 179 C at 28 bar.

About 60 kg of hydrogen contained in the initially released gas formed an unconfined vapour cloud and exploded. Additional 220 kg of hydrogen continued to leak from the upstream section of the failed pipe, burning as a jet-flame, about 70 m long.
Pipes on either side of the rupture were displaced because of the depressurisation.
An 8 m length pipe downstream rotated upon itself of about 380° while a 12 m length pipe upstream rotated of about 90°. This 12 m section damaged the pipes on an adjacent pipe-rack rupturing the vent header of the desulphuriser unit and causing the release of approximately 600 kg of methane.
The released methane initially burned as a cloud (60 m in diameter) and then as a jet-fire from the ruptured vent header.

The plant operator shut-off the supply of natural gas to the plant and shut-down the individual units. After about 5 minutes the fire extinguished.

The INITIATING CAUSE was the rupture of a steel pipe due to cyclic fatigue corrosion, induced by thermal cycling.
According to the eMARS report, The ROOT CAUSE were: an inadequate plant design, insufficient maintenance procedures and an inadequate management problem-solving approach, because it failed to clarify the causes of previous accidents.

pipe, pipe-rack, carbon dioxide absorber, CO shift converter

DESCRIPTION OF THE PROCESS
For Ammonia/Urea production, synthesis gas (SYNGAS) is carried from the CO shift converter to the carbon dioxide absorber of the ammonia synthesis unit. Water is injected into the gas to cool it from 230 to 179 degrees Celsius at 28 bar.

The explosion damaged the pipes on an adjacent pipe-rack and a heat exchanger. It also caused the glasses breakage and superficial damages to the buildings. A small amount of ammonia vapours which had been released from a fitting on a vapour return line dispersed harmlessly.The cost of the damage has been estimated at about 0.65 M Euros.

The off-site overpressure was small. The nearest residence was 200 metres away. The estimated overpressure there was approximately 2 kPa, enough to cause breaking some windows and damaged roof tiles. Nevertheless, the explosion was audible up to 2.5 km away and caused considerable alarm to residents. 193 people lived within a 0.5 km radius from the plant.

Metallurgical examination of the 24" pipe showed that a fatigue failure occurred, induced by thermal cycling. Examination of the fracture faces of the ruptured seam weld showed a significant amount of fatigue cracking initiating at the bore of the pipe along a length of about 3 metres of the weld downstream the water injection point.
At the upstream end, the rupture propagated circumferentially around the pipe as a shear failure. At the other end, the rupture arrested at a circumferential weld and then propagated as a circumferential tear in the pipe material. The spray pattern from the nozzle was such that water at 80°C was intermittently hitting the inner surface of the pipe and quickly cooling the surface. The process gas stream at a temperature of 230°C quickly heated up the pipe wall again only for it to be quickly cooled by water impingement at the start of the next cycle. This phenomenon which affected several areas in the pipe wall downstream of the water injection nozzle caused a set of small fatigue cracks which ultimately developed so that the material near the longitudinal weld failed.
Subsequent examination of the spray nozzle showed that this may have been distorted for some time prior to the accident, causing preferential impingement possibly enhancing the failure mechanism although it is not concluded that this distortion was the main cause of the failure.

There was a two-year long history of leaks in welds downstream of the pipe section affected in this event. At the time the plant operator wrongly diagnosed the cause as external stress corrosion cracking.

The plant was modified, by replacing the water injection system with a reboiler system. This will eliminate the probable cause of the pipeline rupture (i.e. the presence of water droplets within a much hotter gas stream in the pipe).
Parts of the plant where a similar phenomenon was possible (high-pressure steam superheaters used during start-up only) were examined, but no degradation was detected. An assessment of plant preventive maintenance procedures and expertise.