Published on November 7, 2007
Incident Investigation Case Study 2: Incident Investigation Case Study 2 Caustic Dilution Tank Eruption Dennis C. Hendershot Rohm and Haas Company, retired SACHE Workshop September 2005 Bristol, PA Incident 2 – Caustic Dilution Tank Eruption: Incident 2 – Caustic Dilution Tank Eruption Note – This incident description is derived from a real incident (actually contains aspects of several), but the detailed information in this exercise is not historically accurate. Aspects of the incident, including procedure descriptions and personnel “testimony”, have been created for purposes of this exercise. It should not be considered to be a description of an actual incident. General description of operation: General description of operation Batch polymer plant Dilute caustic solution preparation from water and solid (flake) sodium hydroxide Small (a couple hundred gallons) agitated tank with loose fitting top and hatch to charge solids Tank cooled with cooling tower water through a coil. No flow meter on cooling water, no sight glass or other way to confirm flow. On third floor of plant Facility layout: Facility layout Other ingredients in small containers Other ingredients in small containers Note: There are several other similar tanks on the third floor used for blending various ingredients from small containers. Written and EHS Reviewed procedure summary: Written and EHS Reviewed procedure summary Charge specified amount of water to the dilution tank through water meter Wearing specified Personal Protective Equipment (PPE), weigh the appropriate amount of solid NaOH in a 5 gallon pail PPE includes long rubber gloves, boots, jacket, pants, goggles, face shield, and hard hat Solid NaOH and weigh scale are located on 3rd floor near the dilution tank The product of this operation is a 10% aqueous NaOH solution. Procedure summary, continued: Procedure summary, continued Put cooling water on the charging tank coil. Take the bucket of solid NaOH to the dilution tank and charge it slowly through the charging hatch. Watch the tank temperature during charging (local thermometer on top of tank right next to the charging hatch). If it exceeds 60 Deg C, stop adding NaOH until the temperature goes below 40 Deg C. When the NaOH is all added, close the hatch and mix for 30 minutes before using in the process. What happened: What happened During charging, the batch erupted through the open charging hatch, spraying on the operator, who was not wearing his face shield and rubber suit. The operator had difficulty finding the safety shower, slipped on the slippery floor, and fell in a puddle of hot diluted NaOH ejected from the tank. He was able to get up and get to the safety shower, but suffered chemical burns and had to be hospitalized. Some information from the operator: Some information from the operator “I opened the valve on the cooling water to the dilution tank at the vessel, and also the cooling water return valve.” A check of the system after the incident showed that the cooling water valves at the dilution tank were open. “The reactor operator called me on the radio and told me that he needed the dilute caustic solution immediately.” More information from the operator: More information from the operator “My rubber suit was in the locker room on the ground floor in another building, and I was in a hurry to get the batch made because it was needed in the process.” “I’ve done this a million times with no problem.” Actually, the operation has been done about 400 times over a several year period by any one of the building shift operators, without incident. More information from the operator: More information from the operator “The caustic was really sticky. I was trying to charge it slowly, but a big pile of it stuck together and fell in the tank all at once.” “The hatch was open on this tank, and on several nearby tanks. There was a bunch of other stuff in pails and small drums in the area being used for other blends by other people.” “I remembered where the safety shower was, but the floor was slippery and I fell down on the way to the shower.” Some other information: Some other information The plant has done a chemical interaction matrix for the materials handled on the 3rd floor. These materials include acids (acetic, aqueous HCl, dilute sulfuric acid), bases (ammonia) and various solid additives which are relatively non-reactive. Incompatible materials are stored in separated areas. The 3rd floor has 4 similar looking mix tanks, lined up in a row against the wall. They are identified by number with labels (~ 6 inch letters) – Tanks 301, 302, 303, 304. The incident occurred in Tank 304, which is only used for the caustic dilution operation. Parts of the plant cooling water system had been shut down for maintenance the night before the incident. Your job: Your job Construct a root cause incident investigation tree for this incident Identify potential root causes for the incident Determine what additional information or evidence you need to identify the most likely true root causes Identify any laboratory experiments or calculations you recommend to help identify the root causes Develop recommendations for the plant Consider process changes and opportunities for inherently safer design.