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While standard “U” shaped convolutions are suitable for the majority of expansion joint applications, there are limits to their applications for higher pressures and temperatures.
Even at medium pressure and temperatures, reinforcement of the bellows root may be necessary.
Reinforcing rings are used to control the root deformation and allow for the use of thin wall bellows material.
When “U” shaped convolutions are subjected to internal pressures their sidewalls naturally tend to deflect and assume a circular cross section. Thin wall material provides for lower deflection stresses than a thicker wall material, a substantial consideration in the service life of an expansion joint.
In addition, with standard “U” shaped bellows convolutions, hoop stresses due to pressure increase proportionately with increasing bellows diameters even when the thickness of the material and convolution shape does not change. The toroidal bellows convolution has pressure stress characteristics which are practically independent of the expansion joint diameter.
The toroidal convolution circular cross section is formed like a curved tube wrapped around the circumference of the expansion joint. This style of bellows convolution is primarily used in heat exchanger applications.
The pressure stresses are directly proportional to the torus radius and wall thickness of the torus cross section which results in lower operating stresses than in the “U” shaped convolution for any given pressure.
Since the pressure stresses are lower, the use of thin bellows material for the toroidal convolutions is practical even in large diameters.
This results in lower deflection stresses since deflection stresses are less in a thin wall convolution than in a thick wall. The toroidal style convolution has advantages in high temperature applications where materials operate with reduced physical properties.
Toroidal shaped bellows have no circumferential welds around the outside periphery of the convolution which complies with most expansion joint technical specification and therefore are not subject to the resultant stress concentration of such welds.
The convolutions are hydraulically formed and have a minimum number of longitudinal weld seams which are the same thickness as the parent material and cannot act as a focal point for stresses.
Through an extensive research and development program, we have developed a unique hydraulic forming process that allows a toroidal bellows to be formed from 0.188 inch stainless steel material.
This design provides a bellows convolution pressure capacity in excess of 2150 psig.
Bellows having a toroidal shaped convolution are available in any diameter expansion joint ranging from 14 NPS to 72 NPS.