Manufacturers specify the amount of compression for the installed gasket to ensure that the gasket is correctly stressed and exhibits the desired resiliency. The resultant gasket operating thickness must be controlled by controlled bolt loading, or the depth of a recess for the gasket in the flange, or by inner and/or outer compression rings. The inner compression ring has the additional duty of protecting the gasket from erosion by the fluid, while the outer compression ring locates the gasket within the bolt diameter.
The load-carrying capacity of the gasket at the operating thickness is controlled by the number of strip windings per unit width, referred to as gasket density. Thus, spiral wound gaskets are tailor-made for the pressure range for which they are intended.
The diametrical clearance for unconfined spiral wound gaskets between pipe bore and inner gasket diameter, and between outer gasket diameter and diameter of the raised flange face, should be at least 6mm (1/4 in). If the gasket is wrongly installed and protrudes into the pipe bore or over the raised flange face, the sealing action of the gasket is severely impaired. The diametrical clearance recommended for confined gaskets is 1.5mm(1/16 in).
The metal windings are commonly made of stainless steel or nickel based alloys, which are the inventory materials of most manufacturers. The winding may be made also of special materials such as mild steel, copper, or even glod or platinum. In selecting materials for corrosive fluids or high temperatures, the resistance of the material to stress corrosion or intergranular corrosion must be considered. Manufacturers might be able to advice on the selection of the material.
The gasket filler material must be selected for fluid compatibility and temperature resistance. Typical filler materials are PTFE(polytetrafluoro-ethylene), pure graphite, mica with rubber or graphite binder, and ceramic fiber paper. Manufacturers will advise on the field of application of each filler material.
The filler material also affects the sealability of the gasket. Gaskets with asbestos and ceramic paper filler materials require higher seating stresses than gaskets with softer and more impervious filler materials to achieve comparable fluid tightness. They also need more care in the selection of the flange surface finish.
In most practical applications, the user must be content with flange face finishes that are commercially available. For otherwise identical geometry of the flange-sealing surface, however, the surface roughness may vary widely, typically between 3.2 and 12.5µm Ra (125 and 500µin. Ra). Optimum sealing has been achieved with a finish described in ANSI B16.5, with the resultant surface finish limited to the 3.2 to 6.3µm Ra (125 to 250µin. Ra)range. Surface roughness higher than 6.3µm Ra(250µin. Ra)may require unusually high seating stresses to produce the desired flange seal. On the other hand, surface finishes significantly smoother than 3.2µm Ra(125µin. Ra) may result in poor sealing performance, probably because of insufficient friction between gasket and flange faces to prevent lateral displacement of the gasket.
没有评论:
发表评论