The API 520 recommends to do a preliminary sizing of safety valves with the proposed flow-coefficients 0.975 for gas/vapour, 0.65 for liquids and the effective orifice areas written in API 526 (D to T). So, the API uses effective discharge-coefficients and effective areas. At the end this standarizes the flow capacity and allows the engineers to select valve sizes at early stages. API Valves (range 1″ x D x 2″ to 8″ x T x 10″) which are approved by the various National Boards must prove that the capacities are achieved.
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