Dependence of the seal property of ConFlat-type flanges on the fine dimensions of the knife edge

Abstract

The shape of the sealing face on ConFlat-type flanges has not been addressed at all in the International Standards Organization standards, yet it is the distinct shape of the sealing face on the flange, generally referred to as the “knife edge” that primarily distinguishes the ConFlat sealing system. Results of profiling on the sealing face of 70 mm diameter ConFlat-type flanges, provided by ten individual manufacturers, reveal that while all the sealing faces share a common 20° angle on the taper face, there are variations in the tip radius of the knife edge, and in the angle of the counterslope against the 20° taper face. In flange tightening tests using the ConFlat flanges, the tip radius was observed to influence flange penetration into the gasket. It was also determined that the knife edges of all the test flanges were plastically deformed to some extent in the process of tightening twice at a torque of 120 kgf cm. The knife-edge shape was neither found to be dependent on the magnitude of deformation, nor did the deformation have an observable influence on the seal property. In order to closely investigate the dependence of the ConFlat-type flange seal property on the fine shape of its sealing face, four types of 152 mm diameter flanges, each with a different knife-edge shape, were subjected to flange tightening tests. The results indicated that flanges with a small tip radius and a small counterslope angle maintained a satisfactory seal ability, even when applied tightening torque was lower, but had difficulty retaining sealing stability under an elevated temperature. On the other hand, flanges with a large tip radius or large counterslope angle on the knife edge had an advantage in terms of seal stability in baking conditions, but required relatively higher tightening torque to attain secure sealing. To evaluate the tip radius and the counterslope angle at which the flange seal capability and the sealing stability at an elevated temperature are in good balance, the stress condition in the tightened gasket and the flange penetration into the gasket was computer simulated at various tip radii and counterslope angles, using the finite-element method. Based on the results of the computer simulation, a reasonable range for the tip radius and the counterslope angle was estimated to be 0.1 mm to 0.2 mm, and 20° to 40°, respectively.