A flange assembly and related methods of connecting two exhaust pipes together with a flange assembly are provided. The flange assembly includes a first flange having a first mating face and a second flange connected to the first flange, wherein the second flange has a second mating face. A sealing structure is formed with the mating faces of the first and second flanges, wherein the sealing structure comprises a groove positioned on the first mating face and a ridge positioned in the second mating face, wherein the ridge occupies at least a portion of the groove with the first flange connected to the second flange.

Provided are a seal structure in which sealing performance is ensured over a long period of time even when it is used for low-temperature fluids, a sealing method, and a coupling including the seal structure. The seal structure includes an annular gasket interposed between a first member and a second member. The first member is provided on its surface facing the gasket with: a first surface located on the inner diameter side; a first annular protrusion located on the outer diameter side of the first surface; a second surface located on the outer diameter side of the first protrusion; a second annular protrusion located on the outer diameter side of the second surface and having a greater protruding amount with respect to the second surface than that of the first protrusion; and a third surface located on the outer diameter side of the second protrusion.

Provided are a seal structure in which sealing performance is ensured over a long period of time even when it is used for low-temperature fluids, a sealing method, and a coupling including the seal structure. The seal structure includes an annular gasket interposed between a first member and a second member. The first member is provided on its surface facing the gasket with: a first surface located on the inner diameter side; a first annular protrusion located on the outer diameter side of the first surface; a second surface located on the outer diameter side of the first protrusion; a second annular protrusion located on the outer diameter side of the second surface and having a greater protruding amount with respect to the second surface than that of the first protrusion; and a third surface located on the outer diameter side of the second protrusion.