Method and assembly of a pair of pipe flanges, a sealing ring and tensioners for sealing between the pair of flanges. At least one flange face has a circular groove to at least partly receive a sealing ring which has its ring wall rectilinear as seen in a cross-section when front faces of the pipe flanges are spaced apart. The ring fits at least partly into the flange groove. The ring in side view exhibits a truncated cone shape. The pipe flanges are attached to each other such that the flanges in operative state are forcibly held in abutting and facial engagement, and that upon tensioning of the tensionersto create said operative state, compressive forces act onto the sealing ring to yield that its wall, viewed in cross-section, is bent radially to exhibit a curved configuration. The groove may in an embodiment exhibit an isosceles trapezoid cross-section.