Two-Phase and Flashing Flow

Let s first define exactly what we are talking about Two-phase flow describes a condition whereby a flow stream contains fluid in the liquid phase and at the same time in the gas or vapour phase. Flashing flow occurs as a result of a decrease in pressure, and all or a portion of a liquid flow changes into vapour. It is possible for both flowing conditions, two-phase and flashing, to occur simultaneously within the same application. The complexity of the issue results from the fact that this condition is never stable and constantly changes during a relief cycle. 

Therefore, it is necessary that those responsible for selecting SRVs for two-phase and flashing applications be knowledgeable and up to date on current two-phase flow technology, as well as familiar with the total system on which the valve will be used. 

Flashing flow and condensing flow are two examples of multiphase flow with phase change. Flashing flow occurs when pressure drops below the bubble point pressure of a flowing liquid. A frequently... 

Darby, R. On two-phase frozen and flashing flows in safety relief valves. J. Loss Prev. Process Ind. 2004, 17 (4), 255-259. 

Phase changes during the transportation of crude oil or natural gas results in two-phase oil/gas flows. When the hot fluid from the reservoir rises to the sea floor, the extreme temperature changes can cause liquefaction of the gas. Also, due to the long distances used in multiphase flowlines, pressure drop can cause flashing and a change of phase. 

Discharge Flow Regimes Upon developing a puncture in either the vessel or a line attached to the vessel, as in Fig. 26-62, the subsequent depressurization can cause a volatile liqmd to flash and develop bubbles in the liquid. These bubbles cause an expansion, or. 9well, which raises the two-phase, or frothy, level. If the puncture is in the vapor space of a vessel or on a line from the vapor space, the discharge will be at least initially all vapor. This is the simplest discharge case and is treated here as a special case. 

Figures 26-63 and 26-64 illustrate the significant differences between subcooled and saturated-liquid discharge rates. Discharge rate decreases with increasing pipe length in both cases, but the drop in discharge rate is much more pronounced with saturated liquids. This is because the flashed vapor effectively chokes the flow and decreases the two-phase density.

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