This paper is to review the methods currently used in engineering design calculations for predicting the relieving capacity of a safety relief valve under various entering flow conditions. The methods considered include the Recommended Practice (RP) 520 of the American Petroleum Institute (API), the Homogeneous Equilibrium Model (HEM) and various published empirical Slip Models.
Recent research conducted by the Design Institute for Emergency Relief System (DIERS) has indicated that the API method leads to undersized relief valves in comparison with HEM under certain conditions. Researchers have found that the experimentally observed relief discharge rates are a factor of three times higher than discharge rates predicted by HEM, especially for low pressure fluids. The Slip Models give results close to experimental data, however there are several correlations from which the slip ratio must be carefully selected to obtain appropriately conservative results.