Transient Analysis of Safety Valves Using Fluid Structure Interaction Techniques

Pressure relief valves (PRV’s) and their reliable performance is crucial for the safety of pressurised components across a variety of engineering industries. In this research so far, two different types of PRV’s conforming to ASME 8 standards has been modelled using computational fluid dynamics (CFD) using an HPC cluster on ARCHIE WEST to account for the transient fluid-structure interaction processes. The main objective is to assess its accuracy against experimental measurements and assess the validity of commonly used quasi-steady analysis approaches. The commercial CFD code FLUENT was used to investigate the flow behaviour for air at various pressures by measuring the flow rate and aerodynamic forces acting on the valve disc for both steady state and transient conditions. To capture the dynamic behaviour of the PRV during transient simulation, a hybrid dynamic mesh methodology was developed alongside a user defined function (UDF) to provide a robust solution to modelling PRV dynamics using CFD. Once modelled, the transient dynamic mesh simulation would be able to provide a history of piston movement with time to highlight overpressure and blowdown characteristics. Experimental testing was performed within the pressure testing facility at the University of Strathclyde and Broady Flow Control Ltd. This testing program combined both steady state and transient measuring techniques to enable validation of the computational model for both conditions. An opportunity was also available in this study to compare the accuracy of steady state CFD simulation and the assumption of quasi steady conditions to direct transient simulation on capturing the dynamics of the PRV. Such a study has not yet been performed within available literature therefore it seemed essential to build upon previous validation work using steady state approaches. It was found that steady state simulation using a modified Langtry and Menter Transition SST turbulence model was capable of predicting steady state aerodynamic forces to within 1% average accuracy and flow rate to within 2% average accuracy of experimentally measured values across the full lift range. The transient simulation was capable of predicting overpressure to within 1.5% and blowdown to 0.3% as well as accurately modelling the dynamic motion of the PRV. Significant differences in flow structure between transient dynamic mesh and steady state approaches were also observed, questioning the validity of quasi-steady analysis approaches. It is therefore recommended to accurately capture the dynamics of PRV motion using CFD; a transient dynamic mesh approach should be adopted, if feasible, at the cost of increased computation compared to steady state modelling methods. Work has also mostly been completed for the Henry Technologies PRV with research to be published within a paper at the IMECE 2019 conference in Salt Lake City, USA and work is currently being performed on ARCHIE to test the viability of utilising multiphase physics within transient models. Such models are also being validated with experimental techniques developed in house.

For more information about the project contact Dr William Dempster (william.dempsterstrath.ac.uk), Senior Lecturer at the Department of Mechanical and Aerospace Engineering at the University of Strathclyde.

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