Two-Phase Flow Onset and Disengagement Methods

Two-phase flow is often considered in system hydraulics as well as the evaluation and design of pressure relief and effluent handling systems. A variety of scenarios can lead to two-phase flow under relief conditions.

In general, two-phase flow during relief can occur because of flow hydrodynamics and poor vapor/ liquid disengagement where (a) the liquid swells due to generation of vapor bubbles in the liquid 1, (b) fluid expansion occurs due to heating, and/or (c) the superficial vapor velocity is high enough through the pressure relief device. Oversized relief devices can induce two-phase flow because a large relief flow area yields a higher superficial vapor velocity. Runaway chemical reactions and/or chemical systems that are viscous and/or foamy almost always lead to homogeneous two-phase flow.

Two-phase flow can also occur by entrainment, for example, where gas is sparged at a high enough rate in the liquid. In some systems, condensation leading to two-phase flow in the discharge piping can also occur due to expansion cooling caused by pressure reduction through a control valve or a pressure relief device.

Numerous two-phase flow models have appeared in the literature. These models represent broad ranges of theory. Some are based on single-phase critical flow, others on homogeneous equilibrium flow, frozen flow, separated flow, slip flow, and/or non-equilibrium flow.

Homogeneous equilibrium flow models assume equal vapor and liquid velocities and calculate the change of quality with pressure using an isenthalpic or isentropic thermodynamic path. Homogeneous frozen models assume equal vapor and liquid flow velocities and that the quality is frozen along the flow path, i.e., no change with respect to pressure or temperature. The separated flow models assume different vapor and liquid flow velocities and account for mass, momentum and heat transfer between the separate phases.

Two-phase Flow Implications

It is preferred to eliminate or significantly reduce the potential for two-phase flow. This can be accomplished by either (a) reducing the risk/likelihood of the scenarios that can lead to two-phase flow to a tolerable level and/or (b) specific relief and effluent handling systems design considerations and implementations.

More mass is vented from a vessel during two-phase flow than during all vapor flow. During all vapor flow, the liquid has to make up the lost vapor and beneficial energy tempering occurs. This helps to reduce the relief requirements for fire exposure scenarios for example.

As a result of more mass being discharged due to two-phase flow, potential dispersion, fire, and explosion hazard footprints can become significantly larger. Vent containment and/or flow separation are often required to reduce the risks of two-phase flow. When homogeneous two-phase flow occurs, the specific ratio of vapor to liquid does not change in the vessel during venting and as result beneficial energy tempering does not occur. When more vapor is vented relative to liquid, beneficial energy tempering occurs because the liquid has to make up the lost vapor. This is one of the primary reasons why homogeneous two-phase flow results in large relief requirements for vessels exposed to external fire, external or internal heating, and/or where chemical runaway reactions are the cause of the homogeneous two-phase flow.

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