2 KILLED DUE TO IMPROPER CLEARANCE

In a crude oil distillation unit, tests showed that the flare system valve was not providing effective isolation and would require eventual removal for overhaul at a scheduled shutdown of the flare. A ‘cold work’ permit to work was issued two days prior to the incident. Alternate flange bolts were removed and the other bolts lubricated as a standard practice to save time. Sufficient bolts remained at all times to retain the flange seals. There was at that time no need to verify line conditions. Two contractors wearing breathing apparatus completed the work. When almost all the bolts were undone, liquid leaked from the gap between the flanges and gas escaped from the top of the joint.
The men stopped work, came down to ground level and sought advice. The supervisor checked the
platform and saw gas issuing from the top and liquid leaking from the bottom of the flange. He concluded that neither was under pressure and that the quantity of liquid was small. Without any further tests assured the contractors that it was safe for work to continue. The fitters remained concerned, thus asked and received 'spark proof' tools. Liquid continued to leak as more bolts were removed then, as the last bolt was undone and the crane took the strain and started to lift the valve, the spacer suddenly sprang upwards. A large quantity of liquid was released, a flammable vapour cloud formed and ignited by the nearby compressor. Two workers died in the incident.
Key learning points
A tower scaffold with a working platform and access ladder had been erected for work on the valve but due to access restrictions, it was necessary to climb over or under it. This seriously limited the route of escape. Work on the valve should not started prior to verification of the isolation and should not have continued after the first leak occurred until all doubt about the safety of the situation had been resolved. The absence of the spark arrester on the compressor was not known of until after the incident.

Source:IChemE 

 

EXPLOSION IN FLARE LINE

A series of severe electrical storms caused refinery wide unit shutdowns, including the fluidised catalytic cracking unit (FCCU). The crude distillation unit was shut down as a result of a fire, which had been started by a lightning strike. A process upset in the FCCU’s gas recovery section ultimately led to a high liquid level in the flare drum and several shutdowns of the wet gas compressor, together with other process anomalies. As a result of the wet gas compressor shutdown, there was a large vapor load on the FCC flare system, which led to a high liquid level in the flare drum. When the hydrocarbon liquid overflowed into the outlet line of this drum, the line ruptured due to mechanical shock and approximately 20 tonnes of flammable hydrocarbons escaped. This mixture found a source of ignition 110 m from the flare drum and subsequently exploded. This caused a major hydrocarbon fire at the flare drum outlet itself and a number of secondary fires.
Key learning points
The explosion was caused by flammable hydrocarbon liquid being continuously pumped into a process vessel that, due to a valve malfunction, had its outlet closed. The only means of escape for this hydrocarbon once the vessel was full was through the pressure relief system and then to the flare line. The flare system was not designed to cope with this excursion from normal operation and due to liquid breakthrough at the FCCU flare knock out drum, a failure occurred in the outlet pipe. The investigation revealed, that internal acidic corrosion also contributed to the pipeline rupture. Apparently, the company did not inspect the weakest points of the pipeline which were exposed to corrosion

Source:IChemE 

EXPLOSION IN SPENT ACID TANK DURING HOT WORK

A massive explosion destroyed a large storage tank containing a mixture of sulphuric acid and flammable hydrocarbons at an oil refinery. One contract worker was killed, eight others were injured, and sulphuric acid from collapsed and damage tanks polluted the local river. The explosion occurred during welding operations to repair a catwalk above the sulphuric acid tank, when flammable hydrocarbon vapour was ignited by welding sparks. This resulted in a powerful explosion inside the tank.
Key learning points
On the day of the accident, flammable gas testing was performed only at the start of the hot work, but monitoring was not conducted for the duration of the repair activities. It is also important as to where the explosive atmosphere measurements were made. Through the five hours between the last gas test and the explosion, the ambient temperature raised significantly. This warming caused the hydrocarbons inside the tank to vapourise. The resulting flammable vapour leaked out from corrosion holes in the tank into the work area. The company had a hot work program that included written permits, but the program was inadequate. Hot work was allowed near tanks that contained flammable materials without continuous atmospheric monitoring and the control of welding sparks was not required.

Source:IChemE