Releases that result in both liquid and vapor flow simultaneously through the pressure relief valve are classified as two-phase. Two-phase releases typically result from either (1) a liquid release that flashes (i.e., reaches saturation) at a pressure above the valve’s constant back pressure or (2) inadequate free-board volume in the overpressured equipment to allow vapor-liquid disengagement upstream of the relief valve – the two-phase flow in the equipment then continues into the valve. In order to determine the potential for two-phase flow from an initially all-liquid release, a process simulation package should be used to flash the relief stream adiabatically from relieving conditions to the constant back pressure. In the event that the stream remains all-liquid, the methods summarized above (item 1, “Non-Flashing Liquid” on page 1200-39) should be used. If the liquid begins to vaporize, two-phase flow calculation methods are required. To determine the potential for two-phase relief flow due to a lack of vapor-liquid disengagement, consideration must be given to the fluid properties, available disengagement space, and vessel geometry. Consult a two-phase relief expert whenever adequate disengagement is an issue.
Given the variation in the physical properties across the relief valve, two-phase flow capacity evaluations are not amenable to simple hand calculation. In addition, different situations may require application of specific methods to achieve an accurate result. Refer to Guidelines for Pressure Relief and Effluent Handling Systems for further information on vapor disengagement and twophase relief sizing.