At 7:40 p.m., an employee of a pharmaceutical plant classified as high-threshold Seveso, alerted by slight wisps of white smoke, reported a leak of hydrogen chloride (HCl gas) on a transportable sphere. This sphere, in use, was located at the production building’s unloading station. It was under a pressure of 40 bar.
The facility operator triggered its pre-internal contingency plan, activated the emergency cell, confined the area concerned and neighbouring areas. The water curtain on the unloading station was activated and set up 2 water nozzles. The transfer was stopped, and the line was decompressed. An employee wearing a protective suit inspected the sphere and discovered that the leak was from the blind flange bolted to the lower part. The leak, with a very low flow rate, went undetected by the load cell.
The tank still contained 1.1 t of anhydrous HCl. After consultation with the sphere supplier, the facility operator drained the tank into a reactor filled with water and connected to a water-type deposition tower. During the emptying process, the leakage rate increased, and there was a risk of a rupture at the sphere. The draining ended at 5 a.m., the day after the leak had started.
The emergency services, the Inspection authorities for classified facilities, and the 3 municipalities concerned by the site’s special intervention plan were informed the day after the leak. During the entire pre-internal contingency plan, HCl measurements in the atmosphere performed every 15 minutes were below the detection limits.
The economic consequences of the event were estimated at 0.2M Euros, including 77K Euros in direct costs (property damage) and 176K Euros in indirect costs (operating losses, etc.).
The sphere was sent back to the supplier for expert assessment. The report indicated a high level of corrosion on the bottom flange and blind flange, but the inner surface was in good condition, excluding the presence of water in the sphere. One stud exposed to HCl leakage was severely corroded, and another had several of its threads mechanically damaged.
The supplier favoured the hypothesis of external corrosion caused by the operating conditions of the spheres. When in use (transfer of HCl from the spheres to reactors), the spheres are sprayed with a ring of hot water at 45 °C. Watering is stopped between 2 transfers. Heating with hot water increases the internal pressure of the sphere, which increases the risk of micro-leaks. The use of water, combined with fugitive acid emissions, may have initiated localised and progressive corrosion of the flange. The alternation of operating phases with watering and storage phases (without watering) would have allowed the corrosion phenomenon to worsen.
Five HCl spheres of the same type were present on site and also needed to be returned to the supplier. Three had exceeded their periodic requalification dates by more than 3 months and therefore could not be transported full under the ADR (Accord for Dangerous goods by Road) regulation. Before being returned, they were therefore used on site according to the usual provisions, supplemented by a measurement (every 2 hours) of the HCl content in the air at the lower part of the sphere.
The supplier introduced the use of a washer to reduce metal wear around the stud holes and to optimise the distribution of clamping. It was also recommended to stop spraying the spheres with hot water. After 2 months of testing, this solution was abandoned by the pharmaceutical plant operator due to the constraints: the appearance of frost on the transfer pipe at the gas expansion level and the negative impact on the transfer rates. Reduction of the water temperature used during the spraying operation (30 °C) was tested. Another possibility would be the use of a hot air blower or a heated covering.
Neither the operator nor the supplier managed the periodic requalification of the spheres (transportable pressure equipment). Although the regulatory monitoring of the spheres is the responsibility of the supplier, the site operator undertook several actions:
- raising awareness in the procurement department of the regulatory obligations associated with transportable pressure equipment,
- implementation of follow-up by the logistics department of the containers’ test dates upon their arrival to the site,
- an exhaustive inventory of transportable pressure equipment on site and request for information from suppliers regarding the dates of the last test and the next regulatory test.
Several lessons from crisis management were integrated into the operational training module for the staff.
An HCl leak on the bottom flange of a sphere had already occurred at the same site in 1993.