A Pressure Balanced Elbow Assembly is designed to absorb axial movement and/or lateral deflection, while absorbing pressure thrust. This is achieved by means of tie rod restraints that connect a line bellows with an opposed balanced bellows also subjected to line pressure. This type of expansion joint can only be used at a change in direction of the piping system. Principle of Operation: Reference to figure 1 shows that during the movement cycle, internal pressure acting on the bellows element (A), which is in the flow line, is balanced by the same pressure in the balancing bellows element (B). The force exerted by the internal pressure against the line elbow is balanced by an equal and opposite force transmitted to the line through the tie rods (D) from the blank end (C) of the balancing section.

This type of expansion joint is usually seen at a turbine casing or other piece of rotating equipment where minimum forces and moments are required. It is also used in installations where the application of a main anchor would not be practical. The only loads seen by the turbine are the sum of the axial force required to compress or extend the line bellows and balancing bellows in the expansion joint.

Example: In a pipeline with unrestrained expansion joints, the maximum load on the anchors always occurs at the change of direction in the piping system. Such an anchor is always a main anchor. The load exerted at this point is composed of the internal pressure acting over the effective area of the bellows plus the force required to flex the expansion joint. In a large expansion joint, or one operating under extremely high pressure, the resultant pressure thrust is considerable. To eliminate the thrust, a Pressure Balanced Elbow Assembly is the most practical solution.