Research On Dynamic Instability Of Pressure Relief Valve Based On Fluid Structure Interaction

As the backstop of a pressure system, pressure relief valve (PRV) is negatively affected by disk fluttering and chattering on its service life and reliability. Previous studies on the dynamic instability phenomenon were limited by the inevitable defects in their methodologies, which commonly results in a doubtful explanation of the instability forming cause and the lack of universality in its solution.A complete PRV system is multiple-scaled both in space and time. To handle this intrac-table feature, the mathematical modeling method is applied to the pressure vessel, while the pipe and PRV itself are modeled using computational fluid dynamic(CFD) model, and these two models are coupled by the User Defined Function(UDF) compiled based on the mathe-matic fluid structure interaction model eventually. Distortion that happens in converting the three-dimension PRV model to a two-dimension one can be reduced by the outlet equivalent area method, and the Fluent Dynamic Layering technique promotes the robustness of the dy-namic mesh.Based on the complete PRV system model, this paper explores the parameters of three components in a PRV system including the pressure vessel, the inlet pipe and the PRV that may have influence on PRV dynamic instability. Five parameters was investigated including pressure rising rate, vessel volume, inlet pipe length, adjust ring height and rod friction. Ac-cording to the pressure wave spread phenomenon captured by the application of ultrashort time step, the main cause of disk chattering and fluttering can be explained accurately and so it becomes more visualized to verifying the two different instability determining criterion. Findings from this study indicate that:a. The vessel volume and the pressure rising rate have no influence on dynamic perfor-mance or characteristic pressure when confined to a certain extent. Instability slightly increases when the vessel is too small or pressure rising rate becomes rapid because the vibration energy cannot be absorbed by the PRV system in time.b. The adjust ring position determines where the curtain area is. The Laval Nozzle theory can be applied for a reasonable analysis of the flow distribution near the curtain area as well as the disk force. In general, a higher adjust ring position leads to more fluid fore and detrimen-tally, a larger blowdown.c. The rod friction is found to play a positive role in the robustness of PRV system. But the uncertainty of PRV performance rises while the sensitivity comes down with the increase of rod friction, which indicates a proper friction may bring about a most balanced and stable PRV performance.d. Pressure loss in inlet pipe becomes significant when the disk lift is relatively big, fluid total pressure around the nozzle decreased afterwards, which will increase the possibility of chattering and fluttering. Since the threshold value of chattering is much bigger than 3% set pressure, the pressure loss criterion is not accurate.e. Pressure wave actually generates when the disk acts and the way it spreads and reflects matches the description in pressure surge criterion. Fluttering and chattering happens in the PRV opening and reclosing process as a result of the resonance between disk and pressure wave on the premise of not too much pressure loss. And the proper pipe length that avoids resonation fluctuates cyclically.