Keywords: bypass valve, flow coefficient, CFD, CFX, thermal stresses, ABAQUS.

Flow Analysis and thermal stress analysis of a High Pressure bypass valve are computed using commercially available software. The flow analysis is performed using the commercially available software CFX while stress analysis was done using ABAQUS. The valve is modeled in Prof-E and the mesh generation package used is ICEM. The fluid considered is high temperature steam. The volume of water in the flow is small and hence is neglected in the analysis. Thus the problem becomes single phase flow unlike the two phase flow problems considered by Soo [1] and Gerber [2].

The input to the CFD analysis is the pressure at the inlet side of the valve which is varied from 150 to 200 bar. For the flow analysis, turbulence model was used. The number of elements is around 600,000 and recourse is made to a high computing environment. For the inlet pressure case of 150 bar, CFX solution for the flow path analysis predicted a temperature of steam at the tip outlet to be 540oC giving a sonic speed of 570 m/s. The velocity at the tip outlet is around 624.8 m/s. Thus the exit steam is supersonic, resulting in shock stresses. From the pressure data the flow coefficient (non-dimensional) is calculated and compared with the experimental value. The results agree very closely within 2% accuracy. For the other cases also the results from the CFD analysis compared very well with the actual test conditions.

The stress analysis is then made using ABAQUS software. The input for the stress analysis is the internal pressure estimated using CFD analysis. Two materials SA 182 F 22 Class 3 and SA 182 F 91 were considered separately for their suitability. The results for SA 182 F22 class3 shows that the maximum stresses developed near the region of upstream side where there is transition of both geometry and pressure. Even though the maximum stresses crossed the allowable limit slightly, here the effect is localized and valve is safe. Similarly, the plot results for SA 182 F91 also shows that the maximum stresses have developed in the same region as said in the previous case, near the region of upstream side where there is transition of both geometry and pressure is taking place but since the effect of high stresses is localized, both the materials considered were found to be suitable.

1

L. Soo, "Multiphase mechanics of single component two-phase flow", Physics of Fluids, Volume 20, Issue 4, pp. 568-570, April 1977. doi:10.1063/1.861913

2

A.G. Gerber, "Two-Phase Eulerian/Lagrangian Model for Nucleating Steam Flow", Journal of Fluids Engineering, Volume 124, Issue 2, pp. 465-475, June 2002. doi:10.1115/1.1454109

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