The rate of discharge required for a given sized tank is determined by a calculation of the surface area of the tank. Should there be a question of the pressure rating the manufacturer of the vessel should be consulted for additional information. The data plate on the vessel will provide the maximum allowable working pressure for the container. These vapor safety relief valves will need to be sized accordingly to meet the maximum working pressure of the vessel. Refrigeration systems are equipped with intermediate process vessels that may be equipped with a lower rated pressure relief valve. The vapor safety relief valves will need to be sized accordingly to meet the specific criteria as set forth by the specific state and local agencies. State and local codes should be verified as to the specific pressure rating required as some states may require a higher maximum working pressure vessel. Be aware that once a valve does discharge and pressures subside, it may not fully reseat as designed. Once the excess pressure has been relieved within the container, the valve will reseat. The safety relief valve will be fully open at the 250 PSIG rating. As the pressure continues to raise the valve will continue to open to relieve pressures. As the pressure rises in the tank nearing the set pressure point, the valve will start to seep. These valves will start to open prior to this pressure. Stationary storage tanks are equipped with valves that are rated for 250 PSIG. Vapor safety relief valves are set and sealed by the manufacturer to function at a specific 'start to discharge' pressure in accordance with regulations. Operation of Vapor Pressure Relief Valves: These valves also need to be re-certified or replaced on a five year interval. There are safety relief valves in the market place that are equipped with other seating surfaces that are compatible with anhydrous ammonia. They are equipped with a manufacturer’s date stamped onto the body of the valve and have a service life of five (5) years these valves must be replaced or re-certified prior to the five (5) year service life. Because of the seat material, these types of relief valves have a finite life. Vapor safety relief valves typically have a spring loaded, rubber surfaced valve seat.

Pressure safety valve sizing