USING BOTH COMPRESSORS DURING SHIP UNLOADING CAUSES AN INCIDENT

At 6:55 am, a propane ship unloaded its cargo into 2 mounded spherical storage tanks at a Seveso plant. At 8:50 that evening, the liquid phase had been completely unloaded and the vessel’s pumps were turned off. Unloading of the gaseous phase via the ship’s compressors began a few minutes later. At 9:35 pm, the 2 relief valves on one of the tanks opened at their calibration level (10.9 bar) for 30 seconds. The on-duty pump operator stopped the transfer and connected the 2 spheres in order to lower the pressure, steadying it at 9.8 bar. The plant manager and ship captain jointly decided to halt the unloading operation and monitor pressure of both tanks every 30 minutes. According to the site operator, the sphere’s pressure rise from 9.2 to 10.9 bar in 35 min was due to the simultaneous use of both propane ship compressors to accelerate unloading. The installation inspection revealed that pressure alarm thresholds on the sphere had been set at a higher value than the valve calibration pressure. Subsequent to the incident, the pre-alarm levels (visual and sound) and sphere alarm were calibrated at 10.4 and 10.7 bar, respectively, i.e. below the valve tripping values. The effective closure of the sphere filling valve and opening of the spraying valve were both prominently displayed on the control room displays

Source: Aria ACCIDENT ANALYSIS OF INDUSTRIAL AUTOMATION