In the attic of a slaughterhouse, 2.2 tonnes of ammonia (NH3) intended for recycling back to the unit leaked onto a solenoid valve on the low-pressure return circuit of a ground beef freezer. Employees were evacuated for 24 hrs, 40 local residents remained confined to a 500-m safety perimeter, and 20 fire-fighters (including a chemical emergency squad) installed a water curtain. The leak was stopped in 2 hrs by closing a series of valves; premises were ventilated for 30 hrs. NH3 odours were noticeable up to 1 km away. A technician had to be hospitalised for exposure; property damage and operating losses were assessed at 3.9 and 0.6 million francs, respectively.

An investigation was commissioned. The refrigeration unit, placed in service 1 month prior, was using 8.5 tonnes of NH3. The DN150 solenoid valve, fastened with 8 non-interlocking bolts of a new type featuring a flat joint (part of phasing out asbestos joints), broke under NH3 pressure. A series of dynamometric measurements revealed looser clamping on 2 bolts (insufficient initial clamping, gradual loosening by a water hammer or temperature/pressure variations?), leading to a stripped joint cross-section. Lock nuts were recommended to maintain clamping. Positive safety electric valves both upstream and downstream of the leaky valve, capable of closing upon NH3 detection, might have been manually forced into an open or closed position. Had they been operating normally, only the 450 kg of NH3 from the freezer would have leaked. Experts backed the hypothesis of a sustained leak, with the upstream valve manually opened and not completely closed before the accident, thus limiting the positive safety effect. This valve, whose use was restricted to service start-up, came online in May 1997 and was apparently handled despite being strictly forbidden by the operator. Extraction hatches for evacuating NH3 to the outside were inoperable subsequent to an anomaly found in the electrical connection. Final testing was not performed due to scheduling conflict between the project owner and engineer.

The inspection addressed several violations: a draft internal plan not validated by emergency services and lacking written instructions for implementing response measures; residential evacuation and call to public rescue personnel; all-purpose sound alarm not servo-controlled by NH3 detectors, whose planned number and layout did not provide for a detection system guaranteeing personal safety; and inadequate individual protective gear and personnel training in NH3 safety.

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