Release of light hydrocarbons in a refinery

The operator detected a leak of aerosol hydrocarbons (diesel, hydrogen and hydrogen sulphide) in the diesel desulphurisation unit, causing a release of hydrogen and hydrogen sulphide at the top of the unit.
The leak was located at the outlet of the desulfurization reactor, in the cooling towers.
The cloud of hydrocarbon aerosols was released at a 10 m height due to the fan of the cooling tower at the origin of the leak. The cloud quickly dispersed into the air.

The operator initiated the automatic shut-down of the unit and the cooling of the walls of the furnace. The unit was depressurised by sending the diesel content to the torch, an purged by
nitrogen injection.

The INITIATING cause is related to corrosion / erosion of a the cooling pipe of one of the cooling towers.
The investigation showed that the leak was due a piercing on a section of ordinary carbon steel pipe in one of the four air coolers units. The boring was caused by ammonium disulphide, formed by the combination of H2S coming from the desulfurization process and the nitrogen coming from the denitrification process in the reactor.
As normal practice, an injection of water upstream was continuously provided, to dissolve solid substance and avoid deposits, but a retrofit of the unit five years ago had increased the rate of denitrification in the reactor.
An additional degradation mechanism was the erosion derived from the excess of penetration of the weld beads on an elbow of the tube, thereby creating a turbulent flow erosion after a pipe elbow. An examination of the radiographic controls made three months before by the plant inspection service had captured a residual thickness of 1.5 mm on the tube; nevertheless, the radiography operator and service employees did not note this date, due to the difficulties to spot it on the radiographic device.

The normal plant procedure in these cases involved replacing the tube when reaching a residual thickness below 2 mm. The uniform nature of the loss of thickness in this part of the tube is more difficult to detect than a punctual loss of thickness on the radiographic films. Similar cases related to corrosion / erosion had already occurred on some of the 92 tubes and 368 elbows belonging to the 2 oldest cooling towers, bringing to replacements without releases.

diesel desulfurization unit, cooling towers

No casualties were registered. Residents complained concerning odours. The company suffered production losses worth several million Euros due to the slowdown of the units of the platform,
the closure of the desulfurization unit for several days and the non-desulfurized diesel which is more difficult to sell.

The company restarted the unit at a reduced production rate, to be able to use only the two most recent cooling towers (15 years old) which were not affected by degradation problems.
The two oldest towers were dismantled and all pipes elbows inspected: the minimal thickness measured was 1.7 mm (thus below the acceptable value of 2 mm).

Following the detailed inspection of the affected cooling installation, the operator deduced the following lessons learnt (see ARIA report in the references):
1) It was difficult to perform adequate inspections on this type of installations; one single out-of-specification elbow out of a total of 368 elbows could cause an accident;
2) The radiography inspections regularly were designed to detect of localized degradation causing very irregular surfaces, not for the corrosion really occurring, on a straight part of the pipe after the weld, characterised be a rather smooth surface.
The plant operator decided to replace the old cooling installation with a new one using steel with higher resistance to the corrosion phenomena occurring in the unit, and an improved design of the elbow and the welds.