Retrograde and Phase Change Flow Considerations for Relief and Flare Systems

Retrograde and phase change (RPC) flow considerations are important for relief and flare systems design and evaluations. RPC flow can occur in high pressure systems, including subcooled and/or supercritical flow, or at lower pressures where the starting relief conditions are close to a phase boundary. RPC flow is also encountered in other flow systems applications such as gas transmission pipelines for example.

This type of flow can present technical challenges in relief and flare systems design and evaluations for pure components and mixtures alike. Proper consideration of RPC flow is essential for the evaluation of:

• Proper relief device sizing, selection, and installation,
• Pressure Relief Valve (PRV) stability,
• Depressuring systems,
• Piping vibration risk,
• Temperature estimates in downstream piping, i.e. metallurgy considerations, and 
• Effluent handling and flare systems design.

This paper discusses best practices for the identification of RPC flow and for the numeric evaluation of associated choking pressures, temperatures, flow rates, and compositions. Two examples dealing with pure ethylene and an ethylene-butyl acrylate mixture are used to illustrate a variety of concepts dealing with RPC flow.

Versions 8.20 and higher of Process Safety Office^® SuperChems™ include options for solving flow applications where RPC flow is possible.

Figure 1: RPC Flow Categories for pure Ethylene