The Investigation On Sealing Performance Of Bolted Flange Connection At High Temperature

Temperature and pressure have a significant effect on the sealing performance of flange connection system, especially under high temperature, variable temperature and pressure condition. It is easy to leak for flange connection at elevated temperature after survey. Less related research work has been found at home, therefore there is a great engineering application value to carry out a research on this subject. This dissertation carried out the research with experimental and numerical methods on the following aspects:how to obtain homogeneous bolt preload while pre-tightening bolts, the causes for the leakage of high temperature flange and the affects of gasket performance changes on the the flange sealing performance. Sealing performance improvement schemes were proposed. The main contents and conclusions are summarized as follows:(1) High temperature sealing performance testing device of DN200flange was established. The bolt preload methods at room temperature were studied by using it and the existing DN100flange testing device. And the current authoritative pre-tightening methods ASME PCC-1(LEGACY) and JIS B2251were compared. Results indicated that uniform bolt preload could be obtained by using the both methods. But JIS B2251method was much more efficient and errorless when the number of bolts was more. At the same time, it was also found that clockwise tightening method of increasing bolt load stepwise could effectively reduce dispersion degree of bolt forces and homogeneous bolt forces were obtained.(2) It was found by the experimental studies that the bolt preload in DN200flange with flexible graphite spiral wound gasket would decrease when internal pressure was applied at room temperature and the greater the pressure within1.5MPa, the smaller the bolt force. The bolt force was almost back to its original level after pressure relief. The reason for the reduction in bolt force is flange rotation and serious bolt bending caused internal pressure.(3) It was found by the experimental studies that the bolt preload in DN200flange with flexible graphite spiral wound gasket would decrease at elevated temperature; Applying pressure before elevating temperature, the internal pressure would grow as a linear relationship with the temperature, and the bolt force would first increased and then decreased.(4) The finite element analysis model of high temperature flange connection was established and ANSYS numerical simulation results are in good agreement with the results of above-mentioned experimental results. The effects of heating rate on the gasket stress changes were investigated using the numerical simulation method. Results indicated that the requirement of substituting steady state thermal-structure coupling field analysis method for the transient method within a certain error range is the heating rate should be under6.8K/min.(5) The causes for the leakage of high temperature flange were analysied by the numerical simulation method and sealing performance improvement schemes were proposed. It was also indicated that the width of gasket was one of the important factors affecting the gasket stress changing trend and whether need hot bolting after heating up.(6) The sealing performance of flange connection with metal, semimetal and nonmetal gasket was analysied respectively by the numerical simulation method, and the effects of gasket compression-resilience properties and thermal expansion coefficient on the sealing performance of flange connection have been discussed emphatically. The results indicated that metal gaskets generally did not need hot bolting operation while the semimetal gaskets needed it in most cases, but particular case was particular analysis. The stress of asbestos rubber gaskets (nonmetallic gaskets) reduced lager under elevated temperature and was not suitable for the application in high temperature conditions. The Young’s modulus of non asbestos fiber gasket increased after raising temperature, and its high temperature sealing performance was better than that of asbestos rubber gaskets. Non asbestos fiber gasket did not need hot bolting operation.Gasket compression-resilience curve shifting to the left (Young’s modulus of gasket become larger at elevated temperature) and having a similar thermal expansion coefficient with flange and bolt were the two important factors improving the sealing performance of flange connection at elevated temperature.