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:
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
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