Report Detail:

Louisville, KY, March 12, 2004 – Investigators from the U.S. Chemical Safety and Hazard Investigation Board (CSB) said the April 11, 2003, explosion and resulting ammonia release at the D.D. Williamson and Co. plant in Louisville were caused by over-pressurization of an eight-foot-tall food additive processing tank. The CSB said the accident could have been prevented had the company installed an emergency pressure relief valve on the tank. The CSB also noted that the tank that exploded had a history of prior damage.

The explosion took the life of an employee who had worked for five years at the plant. It caused extensive damage to the facility, which makes caramel coloring for use in food products such as soft drinks. The explosion, which occurred around 2:10 a.m., blew the top of the tank some 100 yards to the west. The tank shell struck a nearby ammonia tank, knocking it off its foundation. This resulted in the release of an estimated 26,000 pounds of aqua ammonia (ammonia gas in a water solution) over a five-hour period, forcing the evacuation of 26 residents and requiring 1500 others to remain sheltered in their homes.

Investigators presented the findings at a public meeting today in Louisville, during which Board members were scheduled to vote on the report and its safety recommendations. The Lead CSB investigator said, "The accident was avoidable. In the 1980s, the company shipped two used tanks, including the one that exploded, from out of state to the Louisville facility. The tanks had not been inspected, certified, or registered as pressure vessels prior to bringing them into Kentucky — a requirement of the state's Boiler and Pressure Vessel Safety Act."

The Lead CSB investigator also said the company routinely heated liquid caramel in the vessels to 160°F and then used compressed air to help push the caramel out to a dryer. He said, "Since the vessels were operated above pressures of 15 pounds per square inch, the company should have classified these tanks as pressure vessels as required by law. The tanks should have been equipped with emergency pressure relief valves, pressure and temperature alarms, and automatic systems to shut down the process in case of over-pressurization. In the absence of these safety measures, operators had to rely on visual inspection of temperature and pressure gauges to keep the process under control."

Investigators determined that on the night of the incident, two workers, who were brothers, filled the tank with liquid caramel and turned on the heating steam to the vessel. Meanwhile, they were occupied in another room re-labeling some product boxes that had been mislabeled. Returning later to the tank room, the second operator noticed that the caramel was leaking from the top of the vessel and called in the lead operator. A metal insulation band snapped in two as the tank expanded under the increasing temperature and pressure inside. The lead operator then sent his brother to locate a mechanic. Moments later the vessel exploded, killing the lead operator.

Investigators said the lead operator likely had attempted to open the tank's air vent to release the excess pressure. But the vent was not designed for emergency pressure relief and was not adequately sized for the vessel. In any event, investigators later found that the vent pipe had clogged with solidified caramel product.

The CSB concluded that it was "improbable" — based on the temperature of the heating steam — that the pressure inside the tank ever exceeded 130 pounds per square inch (psi). Drawings show that the tank was built with a maximum working pressure of 40 psi, and CSB investigators estimated that the tank, as originally designed, was probably capable of withstanding pressure up to 180 psi. Therefore, the CSB said, the "more likely cause of failure" was that the tank had been weakened sometime earlier. The report noted that the tank had been deformed on two occasions prior to being installed in Louisville when it was subjected to excessive vacuum, and was subsequently repaired. The repairs were not inspected or certified.

The CSB Board Chairman said, "The tragedy that befell this worker is another example of why plant owners and managers must have effective engineering oversight and hazard analysis systems in place. They should be regularly analyzing various scenarios that could lead to accidents and put into place safety systems that result in extra layers of protection."

Investigators cited several root causes, noting that the feed tanks were installed without a review of their design or fitness for service. Investigators concluded that D.D. Williamson and Co. did not have effective programs to determine if equipment and processes met basic engineering requirements. The company also lacked effective systems for assessing the hazards of its processes.

Finally, the company did not instruct workers on the hazards of overheating or over-pressurizing the caramel vessels. CSB investigators proposed several recommendations be issued to the D.D. Williamson and Co. examine all vessels at company facilities and ensure that each pressure vessel has adequate pressure relief systems and alarms. The CSB also recommended the company upgrade operating procedures, train its operators, and implement a hazard evaluation procedure to determine the potential for catastrophic accidents. The CSB recommended that the Kentucky state government inform pressure vessel owners, mechanical contractors, engineering companies, and insurers that used pressure vessels must be inspected and registered before being placed in service in Kentucky.