The two men, who were employed by Matrix Services Inc., died of nitrogen asphyxiation while working in the refinery's hydrocracker unit, which produces light distillates from heavy gas oils.
CSB sent an investigative team to the scene of the incident shortly after it occurred. CSB interviews with Valero and Matrix Services employees indicate that the men who perished were assigned to re-attach piping to a vessel that was being prepared for a return to service, according to the agency.
The vessel was filled with nitrogen to prevent oxygen and moisture from reaching the catalyst inside the reactor. Nitrogen is an odorless and invisible gas that can cause asphyxiation.
The ongoing CSB investigation will focus on several issues, including confined space hazards, rescue issues, work permitting and the extent to which the company oversaw its contracting workers.
Investigators are determining how Valero communicated information to workers about the nitrogen hazards of working on or near the vessel. The work permit issued to the two contractors who perished did not mention a nitrogen hazard in or around the vessel, nor did it require that special breathing apparatus be used, according to CSB.
"We will continue probing into how Valero and its contractors managed the hazards in and around confined spaces," CSB lead investigator John Vorderbrueggen, PE, said. "Company practices for hazard communication and issuing work permits are some other aspects of this case that we will study closely."
CSB: Nitrogen is a 'Silent Killer'
One of the two contract employees who died was working near a 24-inch opening on top of the reactor. He likely became disoriented, passed out and fell into the vessel after he inhaled nitrogen around the opening, CSB investigators believe.
Witnesses report that the contract employee had been reaching into the vessel with a long wire hook in an attempt to retrieve some debris that had fallen inside. Witnesses also state that the second contract employee likely entered the vessel in an attempt to rescue the first victim. He also was asphyxiated.
In 2003, the CSB produced a Safety Bulletin, "Hazards of Nitrogen Asphyxiation." The bulletin explains that nitrogen-enriched environments may present a hazard for workers, especially in or around confined spaces. Nitrogen is not a poison in the traditional sense, according to the safety bulletin. Instead, it presents a hazard when it displaces oxygen in the air.
"Nitrogen is a silent killer," CSB Board Member John Bresland said. "It is especially dangerous because the exposed person cannot detect that the oxygen level of the air they are breathing is too low. Only one or two breaths in an oxygen-deficient atmosphere can have very serious immediate effects, including loss of consciousness."