An ammonia storage tank was taken out of service for its 10-year non-destructive examination. After the tank was purged and opened, two (2) contractors were removing two (2) relief valves for their 5-year inspection and tests. The two (2) relief valves are 8 bolt flanged connected to a wye piping fitting equipped with a selector valve (as shown in the picture to the left). The selector valve is used to select one of the relief valves in-service. In addition, the wye is fitted with a 1/2” bleed valve between the relief valve(s) and the selector valve, which vents to atmosphere.
The two contract employees had removed one of the safety relief valves and were in the process of breaking the flange on the second relief valve by using a cut off wheel to remove the flange bolts. The contract employee was lying down on the platform while cutting the 5th flange bolt when trapped ammonia gas escaped through the cracked flange. The ammonia gas contacted the contractor’s face shield, which was approximately two feet from the flange. The contract employee inhaled ammonia gas and had exposure to his face and eyes, which caused severe irritation. The second contract employee was exposed to the ammonia gas as he was located downwind from the escaping ammonia. However, the second contract employee did not experience any irritation or difficulty breathing as a result.
Causal Factors
- The purged and opened ammonia tank created a false sense of security that all ammonia liquid and gas was removed from the ammonia tank and system. With this false sense of security full use of PPE that is specific to ammonia was not utilized
- Five other safety relief valves on three other ammonia tanks (two safety relief valves per ammonia tank) were removed using the same procedure adding to the confidence that ammonia gas was removed from the ammonia tank and piping
- Ammonia transfer and vapor piping to and from the ammonia tank was LOTO’ed, disconnected, and blanked
- The bleed valve was not part of the tank isolation procedure or used to verify zero stored energy or ammonia gas
Recommended Actions
- Ensure all work is performed by trained persons in line break processes
- Perform pre-job briefs with meaningful discussion and specific hazards identified of task
- Review ammonia system procedures and LOTO’s to include all bleed valves are identified, labeled, and utilized during servicing and maintenance work activities
- Ensure bleed valves operate correctly
- Follow safe work practices for activities being performed
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