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What Is an ANSI Flange?

By Lori Kilchermann
Updated May 17, 2024
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The American National Standards Institute (ANSI) calls for the use of an ANSI flange to be used on specific applications. The ANSI flange is a three-part flange consisting of one flat flange, one flange machined to take a rubber O-ring and the rubber O-ring. The rubber O-ring is placed into the machined groove in one side of the flange and the assembly is tightened against the flat-faced side of the ANSI flange and secured with bolts which are torqued to a specific amount of pressure or tightness. This flange is welded to the ends of pipe to create an easy-to-separate connection. Other variations use a larger flat gasket that fits inside of the bolt circle of the flange.

While not rated for excessive pressure, the ANSI flange is commonly used on pipelines transporting air, water and steam under low to moderate pressure. While some flange types use a gasket that not only covers the flange face, the ANSI flange covers the bolt circle as well. These flange bolts run through the gasket and aid in the aligning of the gasket. The ANSI flange does not use this type of gasket, thus the gasket must be slipped into position after some of the bottom bolts have been put into place. The bolts aid in supporting the gasket that floats in the center of the flange, sealing only the flange opening and creating a leak-proof passage for the pipeline 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 middle of the two flanges instead of being held in place by the flange bolts. Unfortunately, this also creates a seal that, while sufficient for its intended purpose, is prone to leaking or blowing out completely much easier than the larger gaskets that are trapped in place by the flange bolts. The smaller gaskets also require more skill to install as they are not a simple drop-into-position type of gasket.

The majority of problems with an ANSI flange originate at the time of assembly. Some installers use a screwdriver to push the gasket into position while tightening the flange bolts. This can create a small nick that ultimately develops into a crack over time. Other problems that contribute to defects in sealing are getting grease on the gasket, trapping debris between the flanges and improperly lining up gaskets at the time of installation.

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