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This expansion joint is a powerful solution to a difficult design problem, when axial deflections exist, and anchoring is impractical for structural or economic reasons, such as high in the air or short straight pipe runs between two large vessels. The principle of this type of unit is essentially the same as the pressure balanced elbow type, in that the axial pressure thrust is reacted by the pressure acting on a cross-sectional area equal to the area of the working or primary bellows. Since this unit is entirely axial, and there are no directional changes in the pipe, the cross-sectional area needed to balance the pressure is placed around the outside of the unit. Since the pressure forces are generated by the pressure acting on the annular surface between the primary and outer, or balancing bellows, the arrangement of the tie rods transfers and balance the pressure thrust created in the pipe on each end. Now the forces needed to compress or extend the unit are only the result of the spring resistance of the bellows, and main anchoring of the pipe or vessels is not required.
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