When the normal function of the control valve is to control the level or flow of a liquid, and the high-pressure side contains vapor, consideration must be given to all credible combinations of the fluid phase flowing through the control valve and the phase flowing through the pressure relief valve (PRV). The fluid passing through the control valve will vary from liquid immediately following the failure, to two-phase when the liquid has drained from the upstream vessel (assuming liquid is present in the normal upstream feed), to 100% vapor when the feed is blocked in response to the overpressure scenario or if the feed contains negligible liquid fraction. The fluid entering the PRV may also be liquid, two-phase, or vapor, depending on the initial liquid inventory in the upstream vessel, the initial vapor volume in the relieving vessel, and the capacity available for outflow from the relieving vessel. At a minimum, the evaluation should consider and document the following potential combinations of control valve and PRV fluids:
- Vapor flow through both the CV and the PRV.
- Two-phase (liquid/vapor) flow through both the CV and the PRV.
- Vapor flow through the CV with liquid flow through the PRV. This combination may be deemed not credible if the free-board volume or outflow capacity available downstream are sufficient to prevent the downstream equipment from becoming liquid full.