The incident occurred in a hydrocracker unit of the refinery. A control valve on the liquid line between the high- and the low-pressure separators was opened in error and the liquid allowed to drain. High pressure hydrogen passed uncontrolled into the closed LP Separator which had limited pressure relief capacity. It over-pressurized rupturing at an estimated pressure of 50 bar. The explosion disintegrated the separator and also damaged other vessels and pipes. Released flammable substances were ignited resulting in jet-fires. The explosion occurred in the hydrocracker unit in which waxy oils reacted with hydrogen at high temperature and pressure in presence of a catalyst to form light petroleum spirits and gases (approximately 23 m3/h). The following substances were released from the LP separator after the explosion: hydrogen (1 tone)). Petroleum Gases: (3 tones)), Hydrocarbon Liquids in released aerosol (10 tones)). The following substances were released from the nearby pipework and vessels damaged by the explosion: Light Petroleum Spirits ((190 tones). -Heavy Hydrocarbons Liquids (220 tones). 8 people were directly involved in the explosion and the subsequent fire (1 person was killed by explosion). The smoke from the fire did not affected the locality. The effects of the LP separator burst were equivalent to those of an explosion of 90 kg of TNT. The accident occurred because of an operator error (due to both the insufficient training of the operator and to the inappropriate procedures) in opening the control valve on the liquid line from the HP to the LP separator without noticing that the liquid contents in the HP vessel was draining away. Alarms did not operate because they had been previously disconnected without taking into account the decrease in safety. The main reason for the disconnection of the alarms was due to avoid operational difficulties because of the insufficient design capacity of the plant.