Abstract: On April 6, 1994, a unit operator with ARCO Products was conducting rounds of the coker unit when he observed a leak coming from the mechanical seal of the heavy gas oil pump of coker unit #1. The operator decided to seek assistance; the head unit operator and six or seven unit operators responded. The operators placed water and steam on the leak to suppress the vapor from the seal. The head unit operator decided to shut down the pump and transfer the product to the secondary pump. As the operator shut down the primary pump, the mechanical seal blew, causing a vapor cloud to generate from the seal. The operators continued to put steam and water on the seal and isolated the pump from the pipe line. The remaining product in the pipe line leading to the primary heavy gas oil pump vaporized, leading to the dispersion of the vapor cloud. The operators who responded were wearing bunker gear and several wore emergency respirators. Those with respirators isolated the pump from the pipe line by closing the suction and discharge valves. The operators who were not wearing emergency respirators stationed themselves upwind of the vapor cloud and put water on the cloud; however, the wind changed direction several times, exposing unprotected operators to vapors. Employees #1 and #2, two unprotected operators who responded to incident, were brought to Long Beach Memorial Hospital to be treated for inhalation of hydrocarbons. Employee #1 was hospitalized.