Safety Incident Topic : Release Of Hydrocarbon
Location Of Incident : Hull, UK
Date Of Incident : 20th April, 2006
Brief Account Of Incident
During the start-up of the dehydration section of the DF2 plant, approximately 30tes of DIPE (Di-isopropylether) and 53tes of Acetic Acid were released in to the plant’s open channel trade effluent system from the base of steam stripper C205. This release began when C205 was started up with valve ‘A’ in the open position. This valve is normally closed during start-up and normal operation of the unit as referenced in the local procedures. With this valve open, C205 was gradually over-loaded with hydrocarbons and experienced high operating pressures. To maintain the start-up and prevent C205 tripping on high-pressure, the control temperature was deliberately kept low, leading to the loss of hydrocarbons from C205 base to effluent. Short duration pressure spikes had been experienced during start-ups on C205 in the past and the duty shift team felt that the column would recover (as previously) if left operating in this manner. Limited plant reviews took place to determine the cause of this sustained poor control.
DIPE is used in this part of the process to break the Acetic Acid/water azeotrope. Normally, with valve ‘A’ closed, water (with residual hydrocarbons) flows from the base of liquid-liquid separator C213 to the E204 condensers before undergoing further separation in decanter D204. With valve ‘A’ open the system hydraulics allow back-flow of condensed hydrocarbons from the overheads of the Azeotrope column C204, directly in to C205. This was initially DIPE, but later became rich in Acetic Acid when the reflux back to C204 was lost.
High TOC levels were first recorded by a local effluent on-line analyser at 19:55hrs on the 20th April and were further confirmed by spot sample effluent results (that were being taken every four hours) and continued until the unit was shutdown and isolated at 08:09hrs on the 21st April. Effluent had already been diverted to on-site storage before this event, so operating teams were confident there would be no breaches of consent.
DIPE is a highly flammable immiscible hydrocarbon solvent with a low specific gravity, so readily floats on water. C205 base drops in to an open channel that passes through the plant before reaching the local effluent pit. With such a large quantity of highly flammable material in an open channel, there was the potential for combustion with resulting escalation.
What Went Wrong (Critical Factors)
· Engineering/Design. Valve ‘A’ should not have been open at start-up. The valve was not part of a locked closed valve register. Potential back-flow from E204 had not been recognised in any previous process safety studies.
· Inattention/Lack of Awareness. Failure to adequately respond to high TOC levels from both on-line analysis and spot sample results and failure to appreciate volumes of hydrocarbons involved. Limited plant investigation took place.
· Communication. Local Shift Site Manager (SSM) was not informed. Duty shift teams did not use local procedures for guidance on the actions to take on activation of high TOC alarm.
· Poor judgement. Mistrust of local TOC analyser. Persistent operation of C205 at low temperatures to avoid high pressure spikes/trips, believing column control would recover naturally.
What Went Well
· There were no breaches of consent, as effluent was already diverted to on-site storage as part of the plant re-start.
· The DIPE was recovered in the DF2 local effluent system and removed for off-site disposal.
· To immediately investigate and respond to any unexpected sustained rises in local effluent TOC results from on-line or spot sample analysis.
· To ensure process bypass valves are included in the plant’s locked valve register
· To improve communication between shift operating teams and the local SSM.
· To actively investigate any abnormal effluent condition and seek further support as necessary.
· Reinforce the expectation that operating teams shut the plant down rather than trying to recover from a persistent process upset condition.