First determine the potential overpressure scenarios. These are usually liquid overfilling from the source - a pump on a truck, rail car or barge, for example. Or back-flow from a user of the ammonia - a compressor or pump leading to the refrigeration system or process. A smaller fraction of the time, a fire-size case is the determining scenario. This means taking the external surface area of the tank, below 25 ft elevation, and typically and applying a radiant heat flux. The heat input into the tank, divided by the latent, which may be 500 BTU/lb for ammonia, for example provides a vapor relief rate. The fluid latent heat is different for aqueous ammonia than dry ammonia. These mass flow rates are compared, to find the maximum case. The mass flow can be used in a relief valve vendor software, or formula, to provide a preliminary relief valve size.
