Experimental Determination of Two-Phase Flow Rates of Hydrocarbons Through Restrictions

Accurate prediction of flow rate through a given restriction is important in the design of pressure relief and blowdown systems. While this prediction is well understood for single-phase gases and liquids, it is much less well understood for two-phase gas-liquid flows. We report here a large number of measurements conducted on highly volatile mixtures of hydrocarbons (mainly C1 to C10) at pressures up to 100 bar and flow rates of up to 4 kg s−1. For mixtures in which the liquid mass fraction is below about 0.8, we find that the homogeneous equilibrium model (HEM) provides a good approximation: the discharge coefficient varies from 0.90 for pure single-phase gas flow to about 0.98 when the upstream liquid fraction is 0.8. For flows of compressed volatile liquid, we find that the incompressible-flow model provides a good approximation, with a discharge coefficient of 0.60: however, preliminary experiments indicate that this simple model progressively breaks down as the volatile gas content increases. We also find that, in a number of circumstances, the widely-used recommendations of the American Petroleum Institute (API) can significantly over- or under-predict flow rates through restrictions if misapplied.