The ANSI flange is a three-part flange used for low to moderate pressure applications. It consists of a flat flange, a flange with a rubber O-ring, and the O-ring. The flange is welded to the ends of the pipe and secured with bolts. The gasket must be pushed into place after some of the bottom bolts have been seated. The smaller gaskets require more skill to install and are subject to leaking or blowing out. Problems with leaks are caused by incorrect assembly, grease buildup, debris, and incorrect alignment of the gaskets.
The American National Standards Institute (ANSI) requires the use of an ANSI flange for use on specific applications. The ANSI flange is a three-part flange consisting of a flat flange, a flange machined to accommodate a rubber O-ring, and the rubber O-ring. The rubber O-ring is placed in the groove machined into one side of the flange and the assembly is tightened against the flat side of the ANSI flange and secured with bolts tightened to a specified amount of pressure or holding force. This flange is welded to the ends of the pipe to create a connection that is easy to separate. Other variations use a larger flat gasket that fits inside the bolt circle of the flange.
While not rated for excessive pressure, the ANSI flange is commonly used on piping that carries low to moderate pressure air, water, and steam. While some flange types use a gasket that not only covers the face of the flange, the ANSI flange also covers the bolt circle. These flange bolts ride through the gasket and aid in gasket alignment. The ANSI flange does not use this type of gasket, so the gasket must be pushed into place after some of the bottom bolts have been seated. The bolts help support the gasket which floats in the center of the flange, sealing only the flange opening and creating a leak-proof passage for pipe products to pass through when the flange is tightened.
The lower pressure rating of the ANSI flange allows the gasket to remain floating in the center of the two flanges instead of being held in place by the flange bolts. Unfortunately, this also creates a seal which, while sufficient for its intended purpose, is subject to completely leaking or blowing out much easier than the larger seals which are trapped in place by the flange bolts. The smaller gaskets also require more skill to install as they are not a simple drop type of gasket.
Most problems with an ANSI flange start at the time of assembly. Some installers use a screwdriver to push the gasket into place while tightening the flange bolts. This can create a small scratch that eventually develops into a crack over time. Other problems that contribute to leaks are grease buildup on the gasket, debris trapped between the flanges, and incorrect alignment of the gaskets upon installation.
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