Retrograde and Phase Change Flow Considerations for Relief and Depressuring Systems

Numerous scenarios can lead to retrograde and phase change (RPC) flow in relief and depressuring systems. Potential hazard scenarios considered often include, but are not limited to, depressuring during process upsets (cold depressuring), relief or depressuring under fire exposure, and relief or depressuring under runaway reactions. These scenarios are evaluated to determine relief requirements, the potential for equipment failure due to metal weakening because of increased metal temperatures during fire exposure and/or metal brittle fracture due to the formation of cold liquids in the equipment while depressuring. Time to failure, associated safety and environmental consequences or impacts of relief effluents discharged directly to the atmosphere or vent containment and flare systems, and the effectiveness of any proposed or existing pre or post release mitigation measures represent additional important and required information.

Phase change has to be considered for the vessel contents as well as relief flow conditions. For example, depending on the vessel contents composition and the initial starting conditions of temperature and pressure relative to the contents phase envelope, depressuring might lead to the formation of substantial amounts of cold liquids and/or hydrates. All vapor venting where the vessel contents contain a mixture of vapor and liquid can lead to the formation of two-phase mixture at the discharge of a relief device and as a result the associated choke points and flow rates will be in error if RPC conditions are not considered. Liquid carryover to downstream equipment may need to be considered as a result.

Depending on the scenarios being evaluated and the complexity of the relief/depressuring system, equipment pressures/temperatures may increase or decrease depending on the equipment connectivity and whether or not the equipment is exposed to fire or has ongoing chemical reactions. Chemical compositions can also change over time due to chemical reactions, preferential depletion of light ends, and/or already different starting compositions present in different interconnected equipments. As a result, RPC conditions will dynamically change during the relief/depressuring process. Expected RPC conditions are illustrated and discussed using several case studies. Some of the case studies are simulations of selected actual large scale test data where RPC flow was observed and measured.