Abstract: Employees #1 through #7 were working between one and four blocks away from an alkylation unit, which was operating as usual. An worker in that unit had been instructed to block in the flow transmitter 83 FC 102, which was connected to a 2 in. internal hydrogen fluoride regeneration line at an orifice plate. He was to close the two 90 degree block valves, which were stubbed off the high and low sides of the flanged orifice plate, and open the two 90 degree bleed valves on both the high and low sides of the differential pressure instrument. According to the operators, there were no valve handles on the two 90 degree block valves because of physical interference from adjacent equipment. The worker that was to block in the instrument apparently failed to close one or both of the block valves before he opened the bleed valves. A vapor cloud of hydrogen fluoride developed. The leak continued for about five minutes, with the release of approximately 100 lb of hydrogen fluoride. A water deluge was used to minimize the vapor cloud. No injuries were reported in the alkylation unit, but Employees #1 through #7, who were working downwind, sustained minor inhalation injuries. On May 26, 1996, Citgo reported that all but two of the seven employees would be released from Memorial Hospital. One of those hospitalized was a gate guard, of Murray Security, who was stationed at the north entrance to the Citgo Refinery.