With the recent fatal accident involving methyl mercaptan, I thought it would be a good time to remind us of another significant methyl mercaptan accident that claimed the lives of three workers. The recent accident claimed four workers via inhalation/asphyxiation and the one in 2001 involved ignition and the workers died as a result of the flash fire. So obviously methyl mercaptan’s toxic and flammable properties are SIGNIFICANT and must be respected. It is still too soon to know all the facts of the recent methyl mercaptan accident, but we have a LOT TO LEARN from the 2001 accident. One such lesson is the IMPROPER RELIANCE on EXCESS FLOW VALVES as a “safeguard” during unloading of hazardous materials from transport containers. About 10 years ago I began restricting clients from claiming these devices as “safeguards” in PHAs based on the 2001 incident in MI and personal experience with these devices not functioning as we had hoped. It turns out, excess flow valves are designed to close and stop the release of product from the tank car in the event a tank car valve or fitting is broken or sheared off during transit. Attaching cargo transfer apparatus to a tank car can change product release rates and flow rate characteristics and can prevent the excess flow valve from closing in the event of an emergency. As noted by the Chlorine Institute in its Chlorine Manual and by the Safety Board in its investigation of a July 30, 1983, accident at the Formosa Plastics plant in Baton Rouge, Louisiana,tank car excess flow valves are NOT designed to act as an emergency shutoff device during cargo transfer.
About 3:45 a.m. on July 14, 2001, at a plant in Riverview, Michigan, a pipe attached to a fitting on the unloading line of a railroad tank car fractured and separated, causing the release of methyl mercaptan, a poisonous and flammable gas. About 4:09 a.m., shortly after the Riverview Fire Department chief arrived on the scene, the methyl mercaptan ignited, engulfing the tank car in flames and sending a fireball about 200 feet into the air. Fire damage to cargo transfer hoses on an adjacent tank car resulted in the release of chlorine, a poisonous gas that is also an oxidizer. The fire was extinguished about 9:30 a.m. Three plant employees were killed in the accident. There were several other injuries; most of the injured were treated for respiratory symptoms and released. About 2,000 residents were evacuated from their homes for about 10 hours. Two cars, railroad track, and plant equipment (including hoses and fittings) were damaged in the fire.
The Safety Board determined that the probable cause of the accident was a fractured cargo transfer pipe that resulted from
- the failure of the plant to adequately inspect and maintain its cargo transfer equipment, and
- inadequate Federal oversight of unloading operations involving hazardous materials.
Contributing to the accident was the plant’s reliance on a tank car excess flow valve to close in the event of a leak from cargo transfer equipment and the company’s failure to require appropriate safety equipment for employees involved in tank car loading and unloading operations.
Mechanical Integrity program for Piping
Metallurgic examination of the failed transfer pipe revealed evidence of erosion-corrosion resulting in a significant thinning of the pipe wall. The flow of the liquefied methyl mercaptan through the pipe caused a gradual erosion of the metal. The erosion was accelerated during each exposure of the interior of the pipe to the weather when atmospheric corrosion converted small amounts of the steel to iron oxides (rust). Subsequent liquid flow during unloading eroded, or swept away, the iron oxide on the interior of the pipe, revealing clean steel that readily corroded during its next exposure to the atmosphere. The consequence of such cyclic action is the gradual wearing away of the interior surface of the pipe wall, which reduces the strength of the pipe.
During use, the failed pipe was subjected to bending forces by the attachment of an unloading apparatus that weighed about 53 pounds. The Safety Board’s metallurgy staff estimated that about 175 pounds of additional downward force applied on the outer end of the apparatus would have resulted in the failure of the pipe; however, this is only an estimate, and the actual force required may have been different. The Safety Board concluded that erosion and corrosion had weakened the transfer pipe such that application of a force such as an individuals falling, leaning, or stepping on the pipe or dropping an object on it, in combination with the weight of the unloading apparatus, could have caused the pipe to break and release the methyl mercaptan.
The plant’s Process 46 general operating instructions specified that operators perform a visual, external inspection of transfer piping each time to unload connections were made to a methyl mercaptan tank car. However, external visual inspections would not have detected the reduction in wall thickness caused by the erosion-corrosion that led to the transfer pipe failure in this accident.
The plant’s mechanical integrity program included written procedures that covered the inspection and maintenance of all plant equipment used in the handling of hazardous materials, including the transfer pipes used to unload hazardous materials from tank cars. But the inspection procedures under the mechanical integrity program did not establish specific inspection cycles. Instead, the procedures set out subjective and vague inspection standards such as “often” when deterioration is “extreme” and “seldom” when deterioration is “minimal.” While the plant could not provide data or records to confirm whether or when the transfer pipe that failed in this accident had last been inspected under the mechanical integrity program, the erosion-corrosion that was found within the failed pipe indicated that the program had clearly not been effective. The Safety Board concluded that the plant’s failure to implement effective procedures for inspection and maintenance of its unloading pipes and fittings allowed the transfer pipe in this accident to gradually deteriorate and ultimately fail.
SOP & PPE (SCBA)
The plant’s procedures at the time of this accident did NOT require that employees wear self-contained breathing apparatuses (SCBAs) while performing cargo transfer operations on methyl mercaptan tank cars. In fact, The plant’s procedure for leak-testing the unloading apparatus by having the operator pressurize it with the methyl mercaptan and attempt to detect the odor of this poisonous gas actually required the operator to be unprotected to perform the test. This procedure subjected employees to the risk of injury. Because methyl mercaptan, like chlorine gas, is toxic by inhalation, the use of approved respiratory protection equipment is appropriate to prevent inhalation exposure that could lead to incapacitation and death. An operator wearing such equipment when the release occurred would not have been incapacitated and would have had time to escape the area and/or respond successfully to the emergency. Even an escape hood with an emergency air supply that can be donned in the event of a sudden and unexpected release of poisonous gas would have provided sufficient oxygen to permit an individual to escape the area in the case of such an emergency. Therefore, the Safety Board concluded that the use of proper personal protective equipment, such as SCBAs or escape hoods, would likely have allowed the employees in this accident to survive the initial release of methyl mercaptan and either safely evacuate the area or close the unloading valve and stop the leak.
Excess Flow Valve and RMP/PSM Process Safeguard
Both the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) required the plant to develop and document safety plans for the Riverview facility that included safeguards intended to reduce the risk and consequences of catastrophic releases of hazardous materials. The plant’s risk management plan (mandated by the EPA) and process hazard analysis (mandated by EPA and OSHA) included an accident scenario that involved the failure of a flexible hose on the unloading apparatus for a methyl mercaptan tank car- a scenario similar to this accident. Under both plans, the plant indicated that the release of methyl mercaptan would be stopped by the automatic closure of the tank car’s excess flow valve. Further, the plants’ risk management plans explicitly noted that excess flow valves on the tank car would activate in the event of a pipeline or unloading hose rupture. However, when the transfer pipe failed on July 14, 2001, the excess flow valve on the tank car did not close and stop the release of the methyl mercaptan.
Calculations made by Safety Board engineers and parties to the investigation indicated that the flow rate of methyl mercaptan through the broken transfer piping was insufficient to cause the excess flow valve to close. Excess flow valves are designed to close and stop the release of product from the tank car in the event a tank car valve or fitting is broken or sheared off during transit. Attaching cargo transfer apparatus to a tank car CAN CHANGE product release rates and flow rate characteristics and can PREVENT the excess flow valve from closing in the event of an emergency. As noted by the Chlorine Institute in its Chlorine Manual and by the Safety Board in its investigation of a July 30, 1983, accident at the Formosa Plastics plant in Baton Rouge, Louisiana,’ tank car excess flow valves are not designed to act as an emergency shutoff device during cargo transfer.
To determine whether reliance upon tank car excess flow valves as safety mechanisms during transfer operations is restricted to the Riverview plant or is a broader problem, Safety Board investigators interviewed a sampling of domestic chemical companies. Interviews with personnel responsible for company safety plans revealed that six of nine companies surveyed rely on tank car excess flow valves as a method of stopping or limiting a leak in the transfer equipment. Only one company reported having remotely operated shutoff valves on the unloading piping just outside the tank car dome. (The other two companies did not respond to the Safety Board’s inquiry.) Although the Safety Board’s sampling was limited, the results suggest that the inappropriate use of tank car excess flow valves may be a widespread practice in the chemical industry.
Over a number of years, the Safety Board has attempted to address the safety problems associated with hazardous materials loading and unloading operations from bulk containers by making individual recommendations to the various Federal agencies including the EPA, OSHA, the FRA, the Research and Special Programs Administration (RSPA), and the DOT- that share elements of the responsibility for providing safety oversight of these operations.
For example, many of the deficiencies with Federal oversight identified during this investigation are almost identical to the safety issues the Safety Board noted in its report on the previously referenced 1983 accident at the Formosa Plastics plant in Baton Rouge, Louisiana. That accident involved the release and ignition of a toxic gas from a tank car during transfer operations at a petrochemical facility. As a result of its investigation, the Safety Board concluded that contributing to the accident was the fact that Federal regulations concerning working conditions and the equipment used at the transfer station were not being enforced at the plant because neither the FRA nor OSHA had inspected the plant between 1977 and 1983.
CLICK HERE for the NTSB memo