Following an internal leak, phosphorus trichloride and methane passed through a heat exchanger in the steam heat circulation system (steam) and flowed into a condensate collecting tank. Here the phosphorus trichloride hydrolysed into phosphoric acid (H3PO3). This disproportionated slightly on heating above approx. 100° C and phosphine was formed. This escaped from the collecting tank via the air vent pipe and immediately combusted. Because of the methane that had likewise entered the condensate collecting tank, the fire spread to the plastic ducts of the air extractor fan. Owing to the increasing pressure in the collecting tank, this could no longer be removed via the air vent pipe, and the syphon of the collecting tank emptied into a reception pit in the production building. This resulted in phosphine and the contained methane being released from the syphon pipe, which ignited and set on fire the building’s waste air duct, made of highly flammable polypropylene, close to the reception pit. The fire then spread along the entire waste air duct. Procedures were followed in accordance with the company’s alarm and hazard prevention plan (BAGAP) for the plant protection products facility and the corresponding plan of the chemical park.
– Alert issued to the plant’s fire service, and other fire services were also contacted.
– Removal of production waste water and fire-fighting and surface water to a water-treatment plant;
– Central Command notified, D3 notification, broadcasting in the chemical park, alerting with the use of sirens, press releases, environmental measurements, etc.

Firefighting; removal of production waste water and fire-fighting and surface water to a water-treatment plant; alerting of staff at the chemical plant and neighbouring sites (broadcasting, siren alarm, notification via radio and press); measurement of phosphine at nearby points around the facility (outside the chemical park). External services: Help with fire-fighting; measurements in the surrounding area.

Technical failure of a heat exchanger owing to a manufacturing defect.

The causes were investigated in the context of an expert report under Article 29a of the Federal Immission Control Act, and measures were proposed. The following measures are envisaged in the facility:
1. Installation of additional PLT equipment for the early detection of internal leaks in the heat exchangers affected by the incident and comparable heat exchangers in the facility and to prevent substance release by locking the feed and drain units.
2. Locking of the feed unit for phosphorus trichloride upstream of the heat exchanger in the event of the overfill warning alarm going off at the existing flow measuring point.
3. Enhanced quality assurance in future procurement of comparable heat exchangers.
4. Increase in the diameter of the vent line at the condensate collecting tank in order to prevent the syphon from emptying there and thereby to avoid any intake via the building’s air extraction system and prevent the possibility of fire breaking out and spreading in the exhaust duct.
5. Additional monitoring of the filling level in the syphon of the condensate collecting tank for improved detection of the syphon emptying.
6. Additional monitoring of methane in the vent line of the condensate collecting tank.
7. Installation of additional fire-protection equipment in the building’s air extraction system.