An explosion occurred within the hydrogen processing system of a chemical plant that produces sodium chlorate for bleaching pulp and paper. The chemical process utilizes electrolytic cells and is pH-dependent. Hydrogen is produced as a byproduct and is utilized as a fuel.
At the time of the incident, the plant was at an abnormal operating level of 25% capacity. A non-routine maintenance operation to repair high-pH liquid piping was in progress. To assist, operations personnel rerouted the high-pH liquid stream to the plant sump. However, in doing this, the liquid eventually made its way back into the electrolytic process by design. Ultimately this created the root cause of the explosive condition in that the pH of the electrolytic process increased faster than the computer-controlled acid-injection system could compensate for in this abnormal setup. (The acid system is designed to maintain proper process pH by lowering pH in the electrolytic process.) As the amount of high-pH electrolyte circulating through the system continued to increase, so did the amount of oxygen generated by the electrolytic process until the hydrogen concentration fell below the upper flammability limit (UFL). In spite of existing standard operating procedures (SOPs) and training, operations personnel on duty failed to realize and act according to the seriousness of the condition, and also questioned instrument readings.
After operating with the increased oxygen level and the hydrogen concentration below the UFL for several hours, the hydrogen ignited and an explosion occurred in the system. The explosion caused extensive damage to piping, process vessels and the containment building, but did not start a fire or cause any injuries. This event shut down the manufacturing process for over a week for emergency repairs.
- Provide additional retraining of operators. Curriculum should emphasize the proper response to high oxygen in the hydrogen gas.
- Evaluate current knowledge of operators. Operators should know proper response, such as contacting engineering or management support as needed, to evaluate any potentially dangerous process conditions they might observe.
- Revise interlock strategy. The plant control system was reviewed and enhanced with additional interlocks that automatically shut down the process and safely secure the hydrogen systems on certain pH levels and certain oxygen levels.