Inside a vegetable transformation / conservation facility, a 50-kg ammonia (NH3) refrigeration leak occurred at 4 pm during maintenance work on a deep-freeze installation.
The previous week, the operator reported a malfunction during the de-icing phase on a deep-freeze tunnel with temperatures recorded differing from the standard guideline by more than 2°C, thereby causing an energy loss deemed abnormal. A set of vertical valves (GPX) downstream of each of the 6 tunnel batteries were suspected to be at the origin of this malfunction. A replacement step was decided after observing their opening and upon the unit designer’s advice.
A subcontractor performed the works on 2nd June. According to the operator, the tunnel insulation procedure was applied, consisting of: closing of 2 sectional valves (liquid NH3 / “hot gas”); and draining of the NH3 entrapped between the two valves through the low-pressure bottle. The GPX valves were then replaced; upon reassembling one of them, a bolt fell into the pipe.
The technician requested authorisation to disassemble the “hot gas” shut-off valve in order to recover the bolt that had fallen nearby. The gaseous NH3 leaked during disassembly of the valve, which was ejected, triggering the detection systems and alarms.
Feeling ill, the 4 experienced technicians supervising the works were washed by the safety team and hospitalised for observation. The placement of installations underneath the building had hindered their evacuation, as did the presence of only one emergency exit. Arriving on the scene in 10 min, responders gathered the 30-some employees at the designated meeting point, and another 25 individuals were confined at a neighbouring site. Two technicians wearing self-breathing apparatuses reinstalled the shut-off valve, then the premises were ventilated. The intervention was completed by 10 pm.
The NH3 discharge was due to a slight pressure surge in the low-pressure bottle (0.3 bar) following shutdown of the cold storage rooms over the previous 4 to 5 hours and the rise in installation temperature / pressure. Under normal operating conditions, the pressure drop would have enabled containing the NH3 within the bottle from the outset; in contrast, the bottle could have reached a pressure of 2 bar, according to the operator, had the cold storage rooms been shut down for a longer period (e.g. 4 days during the Klauss storm), with the eventual formation of a toxic cloud and more dire consequences for both the employees and the environment.
The municipality was duly informed. The Prefecture issued a press release.
Remedial measures were adopted: revision of installation layout, emergency exits and minimum required protective gear (glasses and gloves) for personnel intervening on NH3 pipes; shut-off procedures upon the advice of 2 technicians instead of just 1; an added gas return shut-off valve on Tunnel 2 to facilitate its isolation (already the case for tunnel 1), revalidation of the installation by a third party, revision of procedures, use of certain service valves (manometers) to check the absence of gas next to the work to be done.
