Mitigating the impact of an ammonia release

Given the health hazards associated with ammonia, special codes and requirements exist to ensure safety in large industrial NH₃ systems. One important requirement involves a means to mitigate ammonia that may be released through a pressure valve, thereby preventing the ammonia from leaving the facility. In their paper “Comparison of Various Methods of Mitigating Over Pressure Induced Release Events Involving Ammonia Refrigeration Using Quantitative Risk Analysis (QRA),” Donald Fenton and Tyler Hodges from the Department of Mechanical Engineering at Kansas State University compared the effectiveness of five selected methods of mitigating ammonia releases through a pressure relief device in an NH₃ system:

• Discharge into a tank containing standing water
• Discharge into the atmosphere
• Discharge into a flare
• Discharge into a wet scrubber
• An emergency pressure control system (EPCS system)

Methodology & QRA Analysis

The study involved a “reference release” from a typical industrial refrigeration facility using ammonia, where the refrigeration load was 362 tons (1270 kW) and the ammonia charge was 11,590 lbm (5,300 kg), rather than an actual ammonia refrigeration facility. The impact of the ammonia plume was assessed determining the consequent medical costs associated with the exposure to the population. The reference ammonia release was defined as 35.3 lb/min (16 kg/min) for a duration of one hour. The water diffusion tank served as the method to which all other systems were compared.

Using quantitative risk analysis, also involving the use of the dispersion model SLAB (2012), which predicted downwind concentrations in the ammonia vapour plume.

Direct discharge of ammonia release results in lowest relative impact

Direct discharge of ammonia into the atmosphere, in which the ammonia is released from the facility and diluted by the atmosphere to lower concentrations, is very different from the other four methods in this study, with the discharge released at a height of 30 ft (9.14m) above ground, compared to a height of 20 ft (6.1m) above ground, for the other methods.

As a result of the lower downwind concentrations, the downwind health impact of the NH₃ was significantly reduced. The analysis in Fenton’s and Hodges’ study indicated that, under the assumed conditions, the relative impact (cost) of a release at 30 ft (9.14 m) would be less than 2% of the impact from the other methods of mitigation examined in the study.

However, surrounding population density, topography and other factors might cause the direct discharge method to be unworkable if used alone.

Mitigation systems used in conjunction with direct discharge decreases impact of ammonia release

Discharge into a diffusion tank, scrubber or flare can all be used in conjunction with discharge into the atmosphere, thereby decreasing the impact of the released ammonia. An EPCS has a similar performance to those three methods and would also produce the same reduction. Therefore, EPCS combined with direct discharge into the atmosphere was found to be an effective approach to mitigating the impact of an ammonia release.