At a chemical plant, probes detected monomer vinyl chloride (MVC) in the atmosphere when transferring PVC slurry containing MVC between an autoclave and a degasser. The first probe activated displayed a value of 40 ppm. Afterwards, out of the 20 probes, 5 had readings of more than 10 ppm, with a maximum of 230 ppm. The measured values remained considerably below the lower flammable limit.
The discharge, estimated at 244 kg (theoretical value obtained from a materials evaluation), took place subsequent to a bottom valve fault on the degasser, which was left partially open. Since this malfunction had occurred during the weekend, plant personnel did not inform their supervisors, who only learned of the situation on Monday morning.
The incident was caused by a combination of 2 conditions. The degasser bottom valve locked in a non-closed position due to the presence of PVC crust, which was difficult to avoid with the ongoing process and a deficient end-of-stroke signal on the degasser bottom valve, indicating valve closure despite it still being open and thus authorising the transfer of unstripped slurry into the degasser. The sensor or its associated relay remained in the ‘closed valve’ position and transmitted this information to the programmable controller following transmission of a closure request to the valve. This fault however could not be replicated.
After this incident, the operator replaced both the end-of-stroke relay at the bottom valve closure under the degasser and the associated electronic relay. When these devices were not present, the operator had to plan on installing an end-of-stroke at bottom valve openings of the installation’s other degassers and then modifying controller programming to continuously ensure no disagreement between the transmitted end-of-stroke signals. In the event of disagreement, the programmable controller was to interrupt the progression of sequences and prohibit the transfer of unstripped slurry from the autoclave to the degasser. Moreover, on similar site installations, the operator was required to identify facilities where hazardous products had been transferred sequentially with the risk of valves being blocked by solid matter; after this identification step, corrective measures were defined and applied as needed.
