The contact surface with gasket placed between flanges and compressed to play a sealing role is called flange sealing surface or compression surface. The selection of sealing surface type is related to operating conditions, consequences of leakage and the nature of gasket. The most common structural types are as follows.
① Plane sealing surface: its structure is shown in figure (a) below. The sealing surface is not a smooth plane. There are often 2 ~ 4 grooves with triangular cross-section distributed in concentric circles (i.e. flange waterline).
② Concave convex sealing surface: the structure of the sealing surface is shown in figure (b) below. It is equivalent to a pair of flat sealing surface flanges. One of them is made into a pressing surface with a raised platform, and this flange is called a convex flange, and the other is made into a concave surface, which is called a concave flange. The gasket with the same size as the concave surface is embedded in it, which is convenient for alignment.
③ Tenon and groove sealing surface: in the middle of the width direction of a pair of plane sealing surfaces, one of them is made into a section such as tenon, and the other section is used as the pressing surface of the groove. As shown in figure (c), the former is called tenon flange and the latter is called groove flange.
In addition, there are trapezoidal groove sealing surface and conical sealing surface. The former uses an oval section annular metal gasket, and the latter is equipped with a lens annular metal gasket. The above two structures are forced seals, which are commonly used on high-pressure pipelines.
The form and surface performance of flange sealing surface play an important role in the sealing effect. The flatness of flange sealing surface and the perpendicularity of sealing surface and flange centerline directly affect the stress uniformity of gasket and the good contact between gasket and flange.
The roughness of flange sealing surface shall match the requirements of gasket. Radial knife marks or scratches are not allowed on the surface, and surface cracks are not allowed.