In a plastics plant, upon restarting a PVC workshop that had been down since December 19th, alarms sounded on the vinyl chloride monomer (VCM) analysers at the time of activating the semi-hydro circuit (SHP-circuit) on a reactor, and a total of 8 kg of VCM were discharged over a 40-minute period.

This SHP circuit constitutes a system for injecting de-mineralised water into the reactor as a step planned to compensate for the drop in volume subsequent to the exothermic polymerisation of VCM, thereby making it possible to maintain an optimal exchange surface with the double envelope that provides for reactor cooling.

The injection pump was stopped and 2 employees, each wearing a self-breathing apparatus, worked to isolate the SHP circuits. The internal emergency plan was activated at 9:10 pm. The VCM concentration inside the building, which had reached 3,500 ppm according to explosimeter readings adjacent to the SHP pumps, began to drop 15 minutes later to settle at a level of less than 1 ppm by 9:45 pm. The installation was secured and preventive resources were deployed between 9:30 and 10:10 pm.

The SHP injection lines on each reactor would be verified during the next 2 days. Many leaks were observed on the various circuit flanges and sealed prior to restarting the workshop on January 5th. The check valves on the lines were also inspected, with only the one of the reactor involved in the incident remaining locked open.

The outside temperature had plummeted to -18°C during the period preceding unit restart. This low temperature on the building’s ground floor during the down period caused freezing to the pipes connected to the SHP circuit pumps, followed by a breach on the flange seal when placing the installation back into service.

Upon restarting the SHP circuit, the low injection flow rate was inhibited for a few seconds in order to allow for the pump head to rise; the gas had been emitted during this lapse of time at the level of the leaking flanges upon opening the valves. Subsequent to the discharge leak on the reactor’s SHP pump, MVC returned from the reactor located on the 2nd floor to the building ground floor.

Several measures were adopted, namely: installation of a heating system on the building’s ground floor, a 5-bar pressure test on the SHP line using de-mineralised water as a complement to the reactor restart protocol as well as during any line intervention, and a series of procedural modifications for shutting down units during cold weather periods in anticipation of installing a heating system.