CFD-based Design Basis for Avoidance of Hydraulic Shock in A

CFD-based Design Basis for Avoidance of Hydraulic Shock in Ammonia Pipework Systems (2022)-Incidents of ammonia releases due to hydraulic shock in the refrigeration pipework have been reported in the past and could occur in the future. Such shock events carry significant commercial risks and are a concern for the health and safety of humans. Two locations where hydraulic shock occurs frequently are, (a) in the evaporator coil headers at the beginning of a hot gas defrost sequence, and (b) in the wet suction piping from the evaporator at the end of the hot gas defrost sequence. Research on experimental and numerical characterization of hydraulic shocks in ammonia systems was carried out through two impactful projects funded by ASHRAE. In the latter project a validated CFD model was developed using the experimental data as a basis. This CFD model was subsequently applied to a real incident where a shock amplitude of 4,000 psia was predicted as expected from forensic analysis. The validated CFD model was used in this study to simulate a large set of conditions related to generic hot gas defrost piping, to establish a design basis prevent hydraulic shock. The parametric study consisted of 164 three-dimensional, unsteady simulations for a generic horizontal test pipe configuration and a set of initial conditions. Simulations were run for 2”, 4”, and 6” pipes with different lengths, liquid levels, evaporation temperatures and hot gas flow rates. A correlation for the shock amplitude for fast acting valves was developed as a function of operating conditions. This correlation can be directly used to assess the hydraulic shock risk in current refrigeration systems with single or two step fast-acting valves. The critical mass flow rate correlation which gives an upper limit on the mass flow rate for fast acting valves if hydraulic shocks are to be prevented is presented. Simulations of motorized valves for different opening times have shown that valve opening times greater than the system length divided by the average slug velocity will significantly reduce the shock pressures. This time scale can be directly estimated from the correlation developed in this study for the shock amplitude as a function of operating conditions. Based on these results, this study can lay the foundation for new design guidelines in the future.
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