The Stanlow Refinery in Cheshire, north-west England had been shut down for eight weeks in autumn 2013 for planned maintenance work. The explosion occurred in the fractional distillation unit when the plant was being recommissioned.
Stanlow was previously owned by Royal Dutch Shell but Mumbai-based Essar Oil acquired it in 2011.
During normal operation, crude oil is preheated in furnaces before it is pumped to the distillation unit's atmospheric distillation column where liquefied petroleum gas, naphtha and kerosene are taken off as fractions for further processing.
The remaining heavy crude oil is reheated in another furnace and passes to the vacuum distillation column for fractionation (splitting) into gasoil of different weights.
The first failure that led to the explosion involved a three-way diverting valve that had been in situ for more than 40 years and was replaced during the 2013 shutdown.
Residue gas from the vacuum distillation columns enters the unit sour gas system. Here, the valve diverts it back to the furnace that feeds the vacuum column when the furnace is operating or directs it to the atmosphere when it is not.
In 1970, the original three-way diverting valve was incorrectly designed to divert gases into the furnace even when it was shut down. The fault was identified early and rectified. However, the valve retained a serial number which, when Essar used it to order the replacement from the manufacturer, triggered the resupply of the original defective design. The valve was not checked before it was installed.
Health and Safety Executive (HSE) inspector Mark Burton told IOSH Magazine that, due to Essar's inadequate inspection and testing, it did not identify that the valve was operating in the wrong direction. "That was one of the fundamental failings," he said.
The second failing related to a high-level trip device that had been fitted on a secondary vacuum distillation column to prevent it overfilling.
Essar's management of change had not identified that this column had a bypass line on the main line into the column. It fitted the trip valve after the bypass line, so, although the trip device prevented further inflow through the main supply route, the bypass line defeated its purpose and the column could continue to fill.
Burton said: "Essar had a suitable procedure for the management of change of process. However, in the case of the installation of the column high-level trip system procedure, it failed to apply the requirements of its own procedure."
Essar had isolated the main fuel lines to the furnace but had not closed the secondary fuel line. This failure to isolate any of the manual valves from the secondary sour gas line feeding into the furnace left it to rely solely on the compromised three-way diverting valve. The result was that nothing was in place to prevent flammable hydrocarbons entering the furnace when it was shut down.
The explosion happened just after 2am on 14 November 2013 when the fractional distillation unit was being restarted. The furnaces and paired convection bank heaters were operational; the vacuum distillation unit furnace had yet to be started. The bypass line on the secondary distillation column was open, which enabled naphtha to overfill it and subsequently flow into the sour gas system, where it was sent to the vacuum furnace through the incorrectly operating three-way diverting valve.
Naphtha vapours flowed to the top of the furnace, entered the flue line to the convection bank and were ignited when they came into contact with the heat from the atmospheric furnace, which was running at about 900°C. The flame front flowed back into the vacuum furnace where it rapidly expanded in the hydrocarbon-rich furnace and triggered the explosion.
No one was injured, but the incident, which was reported to the HSE as a major accident under Sch 7 of the Control of Major Accident Hazards (COMAH) Regulations 1999, caused £20m worth of damage. The vacuum furnace was destroyed and had to be partly demolished and re-built before it was put back in to operation in August 2014.
Essar admitted the series of failings, all of which were causative of the explosion. It pleaded guilty to failing to prevent a major accident under reg 4 of the Control of Major Hazards Regulations 1999 and was fined at Liverpool Crown Court on 3 April.
Mr Justice William Davis said: "The issue is whether the defendant company fell far short of the appropriate standard. It does not seem to me that it can be said that there was serious failure within the organisation to address risks to health and safety. This is not a company where neglect of health and safety was systemic.
"In an industry where safety issues are paramount, it is of concern that there were [a number of] separate failings in the part of the defendant company, each of which contributed to a significant explosion creating a very high risk to life."
How the judge applied the sentencing guidelines
| Culpability: |
Medium |
| Seriousness of harm risked: |
Level A |
| Likelihood of harm: |
High |
| Harm category: |
1 |
| Number of workers exposed to the risk: |
4 |
| Significant cause of actual harm: |
No |
| Size of the organisation: |
Very large adjusted to large to reflect low profit margin |
| Turnover: |
£3.8bn |
| Starting point for fine: |
£3m |
| Mitigation: |
Cooperation with the HSE, acceptance of responsibility, remedial action since the accident, early guilty plea |
| Fine: |
£1.65m plus £57,645 costs |
