At a chemical facility, the pressure in a refrigeration compressor rose after refrigeration had been cut. The safety relief valve protecting the installation opened by pressure switch before the compressor shut down; it then closed without forming a complete seal. 300 kg of ammonia (NH3) leaked out over 1 hour; 10 ppm of NH3 were measured 10 m away.
The investigation held first cited insufficient modification monitoring. Given the high temperatures, a temporary assembly served to supply the compressor with water at 17°C via a mobile pump connected to a ‘worksite’ electrical socket. This type of socket could be off-loaded in the event of an alarm in the zone. One such alarm tripped as a preventive measure around 3 am after a minor problem was encountered at another site. The water pump stopped, but employees could not establish a correlation between the zone alarm and pump operations. Since the target installation was down and its needs very limited, compressor pressure slowly increased until reaching the maximum safety activation point near 5 am.
The 2nd flaw stemmed from erroneous safety settings: the initial safety level, i.e. a pressure switch shutting down the compressor, should have logically tripped immediately, with the relief valve only opening in case of a defective pressure switch. In the present situation, both the pressure switch and relief valve had been set at the same pressure. The valve opened first, as the compressor must have been manually turned off. This unsealed valve then exacerbated the consequences of the unfolding incident. No anomaly had previously been observed on the valve, which had been regularly verified, disassembled and calibrated. The 17°C water supply was steadied (normal electrical supply), and the pressure switch activation threshold was set at a value below the relief valve calibration pressure. A facility-wide memo was disseminated and a compliance verification performed of the safety features comprising several activation stages.
