Normally pressure in a pipeline is absorbed by tension in the pipe walls:
Due to the axial flexibility of expansion joints, they are incapable of carrying this large axial load. Due to the larger internal diameter of the convolutions of the expansion joint, the pressure thrust force is calculated from an effective diameter, which is:
The pressure force is actually developed at the point where the pressure encounters the first metal area perpendicular to the axis of the expansion joint — for example, a capped end or a change in direction. The distribution of the pressure thrust loads in a number of configurations is shown in Figures 3-38 through 3-42.
Pressure thrust loads in untied, unanchored expansion joints:
Pressure thrust loads in anchored expansion joint installation:
Pressure thrust in tied expansion joint:
Pressure thrust loads in rotating equipment without tie bars (ends anchored):
Pressure thrust loads on rotating equipment with tie bars:
Note that there are no reaction loads (T4) when tie bars are used.
Most pipe stress programs such as CAESAR II automatically calculate the unbalanced pressure thrust load and simplify the model by applying the entire magnitude at either end of the bellows. In most cases, this is an adequate approximation of the actual situation. Greater modeling accuracy can be achieved by disabling the application of pressure load at the bellows (by defining an effective diameter as 0.0), and calculating and applying the thrust load manually to the model as so:
Apply the force T - T1 at the ends of the bellows. Apply the force T2 - T1 at the locations identified by the T2 arrows. Expansion joint, tie rod, and reaction loads T, T3, and T4 will be calculated correctly by the program.
In any event, even though the tension/compression in the pipe wall may not be completely accurate in the default computer model, the load tending to open the bellows will be; this is usually a much more critical detail to consider when designing a system to absorb the pressure thrust. It is left to the user to confirm that this is normally not a major design issue.
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