The design of fire relief systems for petroleum refineries, petrochemical, chemical and natural gas plants requires two accurate system design parameters: the latent heat of vaporization and sonic velocity. These design parameters further require accurate thermodynamic properties. At present, various approximate methods and thermodynamic models are utilized to estimate the latent heat of vaporization of fluid mixtures with varying degrees of success. Similarly, approximate methods have been used in industry to estimate the sonic velocity at which compressible fluid mixtures can flow through relief valve orifices and discharge piping. This paper presents the following: 1. Thermodynamically rigorous methods for the calculation of the latent heat of vaporization and the sonic velocity. These methods are applied to typical fluid mixtures using industry-standard property models and compare the calculated results with data published in scientific literature. 2. The American Petroleum Institute (API) recommendation of the minimum value of latent heat of vaporization of hydrocarbon mixtures is too large to accept for safe fire relief system design when no accurate latent-heat value is available near the critical point. Note: recommendation by API for minimum approximate heat of vaporization of multicomponent hydrocarbon mixtures in the absence of latent-heat data at the critical point is 50 Btu/lb (116 kJ/kg) - see Section 3.15.3.1 of API Recommended Practice 521: "Guide for Pressure-Relieving and Depressuring Systems, 1997"). 3. The need of accurate thermodynamic property data of fluid mixtures necessary to validate the thermodynamic model at relief system operating conditions. Currently, these types of data are scarce in the open literature because data measurements have been difficult at the temperatures and pressure of interest. (c) 2006 Elsevier B.V. All rights reserved.