Challenges in bolted flange connection performance evaluation

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Bolted flange connections are probably the most important separable connections in pressure equipment. Their design is mostly driven by two requirements: strength and tightness. The strength is quite accurately covered by ASME, EN and other standards. Although tightness is also considered in some calculations, these are usually very simplified and discrepancies between design and actual performance can be expected. Connection tightness can be quantified by leak rate in milligrams per year and gasket diameter. According to leak rates, connections can be divided into several tightness classes. When designing bolted flange connection, tightness requirements are usually specified in terms of specific tightness class. In ideal case with uniform contact pressures, the leak rate of the connection depends on gasket contact pressure at the moment and the highest gasket contact pressure in the history. Other significant factors influencing the leak rate include type of fluid, fluid pressure and temperature, and temperature distribution in the bolted flange connection. Two main challenges are estimation of gasket contact pressures and deriving overall connection tightness from these pressures. Estimating gasket contact pressures can be in theory done very accurately using finite element analysis, however, in practice, simplifications are often inevitable. Gasket behavior is usually highly nonlinear and parameters describing these nonlinearities may not be available. Temperature distribution may also severely affect contact pressures, however it is difficult to estimate it accurately. It is also important to note, that the whole history of the connection starting with assembly must be simulated, because important gasket contact pressures are not only at the end of the history, but also maximum values in time at every location. The presentation will be focused on discussing some of the issues, especially using gasket contact pressure histories to estimate connection tightness. Although it is not feasible to precisely estimate the exact leak rate, estimation of its lower and upper bounds may be possible. Consequently, conservative evaluation of bolted flange connection tightness class can be accomplished.

Bolted flange connections are probably the most important separable connections in pressure equipment. Their design is mostly driven by two requirements: strength and tightness. The strength is quite accurately covered by ASME, EN and other standards. Although tightness is also considered in some calculations, these are usually very simplified and discrepancies between design and actual performance can be expected. Connection tightness can be quantified by leak rate in milligrams per year and gasket diameter. According to leak rates, connections can be divided into several tightness classes. When designing bolted flange connection, tightness requirements are usually specified in terms of specific tightness class. In ideal case with uniform contact pressures, the leak rate of the connection depends on gasket contact pressure at the moment and the highest gasket contact pressure in the history. Other significant factors influencing the leak rate include type of fluid, fluid pressure and temperature, and temperature distribution in the bolted flange connection. Two main challenges are estimation of gasket contact pressures and deriving overall connection tightness from these pressures. Estimating gasket contact pressures can be in theory done very accurately using finite element analysis, however, in practice, simplifications are often inevitable. Gasket behavior is usually highly nonlinear and parameters describing these nonlinearities may not be available. Temperature distribution may also severely affect contact pressures, however it is difficult to estimate it accurately. It is also important to note, that the whole history of the connection starting with assembly must be simulated, because important gasket contact pressures are not only at the end of the history, but also maximum values in time at every location. The presentation will be focused on discussing some of the issues, especially using gasket contact pressure histories to estimate connection tightness. Although it is not feasible to precisely estimate the exact leak rate, estimation of its lower and upper bounds may be possible. Consequently, conservative evaluation of bolted flange connection tightness class can be accomplished.

Bolted flange connection; gasket; tightness; finite element analysis

26.08.2020

2653-8911

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