Engenheiro quimico

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Rigorously Size Relief Valves for Supercritical Fluids
Ryan Ouderkirk, Fluor Corp.

Previously published methods can be tricky to apply, and may lead to improperly sized valves. Here is a stepwise, detailed method that more-accurately determines the orifice area.

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ideal-gas behavior. Relief venting of supercritical fluids upercritical fluids exhibit characteristics typihas been studied previously, resulting in guidelines for cal of both liquids and vapors. Transport calculating the relief rate (3, 4) and the relief-valve oriproperties, such as viscosity and diffusion fice area (5, 6, 7). However, if one is not careful in readrate, are closer to those of typical vapors, ing these guidelines, he or she may size the relief valve while solvent strength resembles that of typical liquids. based on ideal-gas simplifications or merely for the Further, the properties can be strong functions of presmaximum mass relief-rate. The reader can inadvertently sure and temperature, allowing the solvent strength to be use the vapor or liquid sizing-equations (for an ideal or easily manipulated. This led to the use of supercritical compressible gas), for example. Also, Ref. 4 tells the fluids in industrial processes such as extraction of cafreader how to find the maximum relief rate, as opposed feine from coffee with carbon dioxide, and the Residuto the maximum orifice area. The maximum required um Oil Supercritical Extraction (ROSE) process with either butane or pentane (1). However, these same deviaorifice area may not occur at the maximum mass relief tions from both ideal gas and incompressible fluid berate or the maximum volumetric relief-rate (Figure 1). havior present distinct challenges for relief valve sizing. Here is a step-by-step procedure to rigorously size Here is a rigorous procedure to calculate the relief the relief valve for supercritical fluids. In presenting rate and size the relief valve for supercritical fluids. The relief rate is modeled over time for a