The incident occurred on 10 October last year. At first, EVC told the Environment Agency that about two tonnes of VCM had been released from one of the three PVC production streams at the Sully works in Barry. A month later, the company revised the figure to 5.7-8.5 tonnes.
The Agency served a prohibition notice barring EVC from using the process stream until it had identified the cause of the incident and measures to reduce the risk of a recurrence. This was replaced a few days later with an enforcement notice requiring manufacturing and control systems to be brought up to the required standard by the end of January.
The incident was caused by overheating of a reaction to polymerise VCM to PVC. Following the failure of a pump in a mixer vessel, poor mixing of a catalyst suspension meant that the active ingredient was added too rapidly to the batch.
EVC had several options to cool down such "fast batches". Its preferred approach was "top recovery", in which the reactor is depressurised by allowing some of its unreacted contents to pass into a blowdown vessel. Other options include adding a moderator to slow the reaction, or adding a-methylstyrene to stop it altogether.
The latter approach was specified in EVC's integrated pollution control (IPC) application. However, the Agency found that the company had later introduced written instructions saying that a-methylstyrene should not be added without written approval from senior management. Addition of the chemical leads to loss of product by stopping the reaction - and hinders future productivity because it can contaminate recovered VCM.
On 10 October, the process operators first relied on the top recovery approach - thus preventing an automatic injection of a-methylstyrene which would have been triggered by pressure build-up in the reactor. They then tried to add moderator to the vessel, but encountered difficulties in doing so.
The system's last remaining line of defence was that if pressure in the blowdown vessel exceeded 3 bar, the valve connecting it to the reactor was designed to close. However, this did not occur because lines in a pressure gauge on the blowdown vessel had been blocked by product.
Pressure in the blowdown vessel increased to at least 6.9 bar, leading to the rupture of four bursting discs. However, the Agency says that environmental monitoring by EVC showed that the explosive vinyl chloride gas - also a human carcinogen - was blown towards the sea and that there was no risk to health from the release.
Appearing before Barry magistrates on 7 May, EVC pleaded guilty to three charges of failing to comply with IPC authorisation conditions, contrary to section 6(1) of the Environmental Protection Act 1990. The charges were: failure to maintain plant and equipment; failure to provide operators with appropriate written instructions; and failure to carry out the process in accordance with the IPC application.
The company was fined £6,000 on each count, and ordered to meet the Agency's costs of £13,000.
The Agency says that, after a series of incidents in 1994 - including relatively minor VCM releases - EVC promised to review its use of top recovery and carry out regular flushing of the lines on the blowdown vessel's pressure gauge. However, it appeared not to have followed through on this commitment. Earlier this year, the Agency issued a variation notice requiring EVC no longer to use top recovery.