A process area alarm activated. The alarm was caused by an instrument channel located above a reaction vessel off-gas system final HEPA filter canister, which indicated 25% of the lower explosive limit (LEL) for hydrogen. Since the only source of hydrogen is from the reaction vessel during the reaction of sodium with concentrated sodium hydroxide, the immediate actions were to shutdown the reaction process and place the facility in a safe condition.
The root cause was inadequate or defective design. Had the pre-filter drains been vented to outside the building, no hydrogen could accumulate in the process area. The corrective action for this is to complete an Engineering Task Authorization (ETA) to install a sample/drain collection system with loop seals to prevent any release of hydrogen to the process area. The direct cause was the pre-filter canister drain valve being left open after the fluid stopped draining, which allowed hydrogen from the reaction vessel off-gas system to accumulate, causing the alarm at 1% hydrogen in air. A contributing cause may have been that the operator was not properly instructed to shut the valve immediately when the fluid stopped draining. Corrective action for this is to issue improved instructions for sampling/draining the off gas system and training the operators on these improved procedures. An ETA was also initiated to provide interlocks and administrative controls to ensure the process area roof exhaust fans are running prior to beginning reaction vessel sodium injection and stop sodium injection should either fan stop running.
The source of the hydrogen released to the process area was traced to the routine operation of draining accumulated fluid from the one of two off-gas system pre- filter canisters. This is done to the off line filter canister every six hours when the filters are shifted. This has been done routinely since the filter drains were installed. Approximately 188 hours of sodium reaction at .75 gpm sodium flow has been completed with no other hydrogen alarms being received. The sodium reaction process was restarted a few hours later with concurrence of facility management. Hydrogen leak checks were completed on the off-gas system with none found.
Hazard assessment is critical during the design, fabrication, and installation of system modifications to ensure hazards and potential hazards are addressed prior to system start-up and operation.