The design of a pressure vessel and testing procedures for the determination of the effect of high temperature pressurised helim on valve contact welding / Hein Schmidt
The PBMR (Pebble Bed Modular Reactor) is one of the current developments in the field of nuclear power generators. The control philosophy of the PBMR system relies heavily on the controlling of valves. The current control valves are subjected to a maximum temperature of 350°C with a pressure difference of 90 bar. Control optimisation can be obtained by including 'hot valves" into the system. The biggest improvement is possible with a bypass-valve after the low-pressure turbine outlet. This valve will be subjected to a temperature of 720°C with a pressure difference of 52 bar. PBMR personnel raised the concern that the components of these valves (valve seat and sealing surfaces) in contact with the hot helium gas could tend to weld to each other when they are in contact. An investigation was done to establish whether these surfaces tend to weld together. As no literature was found on testing for prevention of welding of materials under high temperature pressurised helium conditions (Chapter 2), a testing facility was designed to test the hot bypass-valve material (AISI H10) under operating conditions. This included the design of a pressure vessel according to ASME Vlll Division 1 (Chapter 3) to be able to simulate the helium operating conditions and a bolted connection (Chapter 4) to simulate the valve contact conditions. A finite element analysis was done, using ALGOR FEMPRO software (Chapter 5), to verify the internal stresses of the pressure vessel based on the maximum allowable stresses for material UNS NO6230 (Haynes® 230© Alloy), from Appendix 4 of ASME Vlll Division 2. Firstly, a steady state heat transfer analysis was done to calculate the pressure vessel temperature distribution. During a static stress analysis, these results were used to assign the temperature dependent material properties to the various finite element elements. The helium pressure and external pressure were simulated as uniform surface pressures. Based on the Tresca effective stress results the maximum allowable 0.2% yield strength of Haynes 230 was exceeded. According to this analysis, the pressure vessel will yield when subjected to the specified operating conditions. The calculated stresses also exceeded the ASME Vlll Division 2 - Appendix 4 maximum allowable material stresses. It is recommended that the same analysis be done with another FEM analysis software package, to verify the calculated material stresses. This analysis should be incorporated into the follow-up study, where the water-cooling system must also be designed. Before the manufacturing of the pressure vessel can commence, a third party inspector must approve the design. Any design updates necessary from the inspector's report should also be included in the follow-up study.
- ETD@PUK