The International Institute of Ammonia Refrigeration recently completed a research study using quantitative risk analysis, or QRA, to evaluate the effectiveness of five methods for mitigating an ammonia release during an overpressure event.

Under the direction of the IIAR’s board of directors, the intent of the study, called “Using Quantitative Risk Analysis to Evaluate Various Methods for Mitigating an Ammonia Release During an Overpressure Event,” was to use QRA to calculate the relative risk of each method and to assign a cost to that risk.

“Prior to our research, this had never been done before,” said Bruce Nelson, president of Colmac Coil Manufacturing, Inc. “The project provided a clear indication of the risk from each method, with the QRA serving as a template for members to use to evaluate the relative risk at their facility.”

The situations studied included: direct ammonia charge into the atmosphere; ammonia absorption into a water tank; the discharge of ammonia through a scrubber system; burning discharged ammonia using a flare system; and the effects of an emergency pressure control system.

The study determined the impact of an ammonia release to a surrounding population and the associated damage to health cost. This health cost, in conjunction with the expected failure rate of a particular mitigation system, provided the cost impact of that specific system.

For the purposes of the study, a conservative assumption was made concerning the pressure relief of the vessel, so that the flow of ammonia through the pressure relief valve would continue at the rated condition for one hour, Nelson said.

The analysis involved use of the dispersion model SLAB (2012) that predicted downwind concentrations in the ammonia vapor plume. A typical industrial refrigeration facility using ammonia where the refrigeration load was 362 tons and the ammonia charge was 11,590 lbm was used as a model. The impact of an ammonia plume was assessed by determining the medical costs associated with exposure to the population.

The mitigation systems analyzed all reduced the consequences of an ammonia release to the surrounding population, but each method can now be more fully analyzed for risk and costs by individual facilities using the QRA, said Nelson.

“This is not one-size fits all,” he added. “The study did not point to one method as being appropriate to use in all circumstances.”

The study confirmed that each method carries different levels of risk, and the choice of method for a facility depends on a variety of factors, including their location. “For example, if a processing plant is in the middle of nowhere far away from a population center, the risk of ammonia by dispersion is tolerable,” Nelson said.

A discharge to atmosphere, the simplest system, proved to be the most reliable because there are fewer components with the potential to fail. Conversely, a water diffusion tank has eight components that could fail and cause a release, while a scrubber system has 15 components and a flare system, the most complex of all, has 18.

“These mitigation methods are all still valid, and the project reinforced that,” Nelson said. “But the most valuable result of this project was that it provided us with a scientific method to calculate relative risk in a way that gives guidance to end users regarding each mitigation method they might consider.”