A 90-kg butane leak occurred at 5:30 am on a liquefied gas (LPG) storage area located in an asphalt plant. The originating tank had been equipped with a pressure control valve placed on a pipe segment leading to a burner. The excess gas transited via the valve and returned into the tank. This segment had undergone repairs during July 1997 and the valve was installed at that time. Valve operations remained independent of the pressure sensor. The calibration pressure for this (bellow) valve was set using a manual flywheel compressing a spring. The envelope surrounding the adjustment spring was exposed to outside air, and the outdoor temperature was -5°C.

The alarm was sounded by the owner of a neighbouring firm, upon noticing the odour; he called police around 6:45 am. First responders stopped traffic on the nearby road and cut electricity supply. The valve was isolated by a subcontractor specialised in handling LPG, then the gas still present in the tank was burned with a flare. A crack in the valve bellow, subsequent to ice formation in the lower part of the bellow, had caused the leak. According to the inspection, the system was inappropriate (absence of a 2nd barrier, a burst seal due to frost action). To avoid repeat incidents, regional authorities inspected all other installations equipped with this same system.