Metal Expansion Joints

By: Anna Woodward

Piping; lots of piping, maybe miles of it. Look at the schematic. Straight lines; bends; right angles; every twist and turn requires expansion joints. It makes the Los Angeles freeway system looks like child's play. Whether the piping is one inch diameter, or 200; whether the system operates at full vacuum, or 2000 psig, at full capacity, there is considerable pressure. Depending on the medium, the system may have to accommodate 4000 degree heat or -325 degree cold. Pressure and temperature cause expansion and contraction. The result is stress on the system and the system engineer. The mission? Control the flow and keep the operation running at optimal capacity and efficiency.

The answer? High quality Metal Bellows Expansion Joints. Expansion joints allow movement of a medium through a piping system while containing the medium, and controlling system pressure. Thermal growth or diminution, equipment movement, vibration or pressure anomalies can cause movement in a piping system. Metal Expansion Joints are an excellent, and often essential, solution, to piping system movement.

The expansion joints may experience one or a combination of axial, (straight line); angular, (longitudinal rotation); lateral, (shear); or torsional, (twisting), movement. Insertion of these metal expansion joints into the system, of course, requires end connections. End connections may require flanges, or specialized flanges, including "slip-on, "angle," or perhaps "Vanstone" modified flanged ends with the flexibility of resolving bolt-hole misalignment. Weld end connections can also be used to mate metal bellows expansion joints to any pipe or duct in a system. A variety of unrestrained and restrained metal bellows expansion joints are available to increase system integrity.

Unrestrained metal expansion joint assemblies include: – Single Joint Assemblies, the simplest type, consisting of a single metal bellows welded to either a flange or pipe end. – Universal Expansion Joint Assemblies allow more flexibility with regard to axial, lateral and angular movement, by connecting two bellows with flange or pipe ends with a center spool piece. – Externally Pressurized Expansion Joints allow line pressure to act externally on the bellows component through inclusion of a pressure chamber. This configuration provides more convolutions to be used to control significant axial movements, and negates concerns of bellows instability. Standing media concerns are allayed by the benefit of self-draining convolutions. Proper installation requires appropriate anchors and guides.

With respect to restrained Metal Expansion Joint Assemblies, four clear choices emerge: – Tied Single Bellows Assemblies combine the Single Bellows Assembly with tie rods, thereby increasing piping system design flexibility. Main anchors are unnecessary as the system thrust pressure is carried by tie rods attached to the flange or pipe with lugs. – Tied Universal Assemblies, similar in manufacture to Universal Assemblies, limit lateral offset and angular movement by utilizing tie rods to absorb pressure thrust. – Use of Hinged Bellows Assemblies limits angular movement to one plane. The configuration of two or three Hinged Bellows Assemblies is typically employed in high pressure piping situations to handle large amounts of expansion. – Finally, Gimbal Bellows Assemblies are engineered to allow angular movement in all planes, while absorbing pressure thrust and torsional movement. Gimbal Assemblies, when paired, or used in conjunction with a Single Hinged unit, are particularly suited to maximize absorption in multi-planer piping systems.

Complimenting and enhancing the various Metal Bellows Expansion Joint assemblies are numerous options and accessories. Ties rods, referred to above, typically devised as rods or bars, attach to an expansion joint assembly for the purpose of mitigating both the inevitable pressure loads, as well as ancillary forces such as dead weight. Limit rods are a "fail-safe" methodology designed to address the situation of occasional anchor overload or system malfunction which, otherwise, could cause metal bellows joint expansion beyond design parameters. Limit Rods are a precautionary measure that does not come into play under normal operating conditions. Liners are internal sleeves which are used to protect the internal surface of the metal bellows from media which may compromise its integrity. In high temperature media conditions, such as oil industry applications, Purge Connections are used in tandem with Liners to keep the bellows within acceptable tolerances. Covers, otherwise known as "shrouds," are used to protect the Metal Bellows Expansion Joints from any external elements which might cause concern.

In sum, the appropriate use of Metal Bellows Expansion Joints is critical to the operating "up-time" of any significant piping system. Operations, production and, ultimately, profit are tied to the smooth and seamless flow of media through the intricate, and often unseen, "highway" of fluid transmission, whether liquid, gaseous, super-heated or super-cooled. "Forewarned is forearmed" is no cliche; either prepare or repair.

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