Ammonia Release | Relief Valve Termination (High Pressure Receiver)

Summary

In July 2017, the termination of a relief valve to the atmosphere resulted in a release of approximately 1,500 lbs of ammonia, and four personnel injuries. Proper operating procedures were not followed during a seasonal startup of the ammonia system. Several mistakes were made which resulted in the high pressure receiver overpressurizing, and the relief valve lifting. 

Incident Description

The ammonia refrigeration system had been previously shut down for the off-season at this cold storage facility. In preparation for the upcoming season, the refrigeration system was turned back on, but operating procedures were not followed during the start-up process.

During start-up, the condenser fans were not turned on which allowed the discharge pressure to rise. When the discharge pressure reached the high pressure cutout setpoint, the compressor did not shut down as intended. The compressor controls had been left in “local” mode which allowed the compressor to bypass the high pressure cutout shutdown. Consequently, the discharge pressure continued to rise until the high pressure receiver relief valve lifted at 250 psig. 

The relief valve terminated to the atmosphere above the facility roof. The termination point exceeded the requirement of being located at least 7.25 feet above the roof, However, due to the close proximity of public receptors (a busy freeway, businesses, etc.), the following recommendation was made following the release:

“Deliberate consideration should be given to installing an ammonia diffusion tank because of facility location to the freeway and local businesses.”

Although the facility was not required to install a diffusion tank per the applicable RAGAGEP, it would have significantly reduced the chance of employee injury and exposure to nearby receptors. In some instances, diffusion tanks can be a prudent defensive strategy in containing ammonia vapor during a release.

Release Calculation

The release occurred in two (2) phases: Phase 1 is the period when the relief valve was relieving at full capacity; Phase 2 is the period when the relief valve had mostly closed and was “seeping” ammonia. During Phase 2, it was assumed that the relief valve relieved 10% of its full capacity. The amount of ammonia released during each phase is calculated below:

Phase 1 Analysis

Ammonia Released (lb) = 90.5 lb/min × 13 min = 1,176.5 lb

Phase 2 Analysis

Ammonia Released (lb) = 90.5 lb/min × 10% × 33 min = 298.7 lb

Total Ammonia Released

Ammonia (lb) = Phase1+Phase2 = 1,176.5 lb+298.7 lb = 1,475.2 lb ≈ 1,500 lb

The calculations indicated that approximately 1,500 lbs of ammonia was released from the relief valve.

Significance

This release illustrates the importance of training personnel on the operating procedures for start-up, shutdown, and other phases of operation. The following recommendation was made regarding the release, and training of personnel:

“Current employees have not been adequately trained in an overview of the refrigeration system or its operation. Ensure applicable employees are trained in: (1) Overview of the process, including its operating limits and consequences of deviation (2) Operating Procedures (3) Emergency Response.”

If the proper procedures had been followed, the condenser fans would have been operating, and the compressor switched out of “local” mode. Had this been the case, it is likely that the discharge pressure would not have climbed as high as it did and the relief valve would not have lifted. 

Human error is an inescapable reality of the world we live in. Everyone makes mistakes, but proper training and operating procedures can help reduce the frequency, and severity of a mistake when it happens.