Release of light hydrocarbons in a refinery

The operator detected a leak of aerosol hydrocarbons (diesel, hydrogen and hydrogen sulphide) at the top of the diesel desulphurisation unit. The leak was located at the outlet of the desulfurization reactor, in the cooling towers. 

The aerosols cloud was released at a 10 m height due to the fan of the cooling tower at the origin of the leak. it did not ignited and 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 flare, an purged by nitrogen injection.

The INITIATING CAUSE was 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 piercing was the effect of corrosion by ammonium disulphide, formed by the combination of H2S coming from the desulfurization process and of nitrogen coming from the denitrification process in the reactor. 

The following factors created the conditions for the failure:  

1. Previous unit modification: as normal practice, water was continuously injected upstream to dissolve solid substance and avoid deposits. However, a retrofit of the unit five years before had increased the rate of denitrification in the reactor.
2. Weld manufacturing: 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. 
3. Inspection shortcoming: 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. The plant procedure for these cases prescribed the replacing the tube when reaching a residual thickness below 2 mm. However, the radiography operator and service employees did not note this data, due to the difficulties to spot it on the radiographic device. The uniform nature of the loss of thickness in this part of the tube was more difficult to detect than a punctual loss of thickness on the radiographic films. 
4. Inadequate learning from the past: 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. 

Desulphurisation reactor

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).

The plant firefighters team was alerted, which installed a water curtain around the unit.

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.