In extreme cases, expansion joints may be added to a system in order to increase flexibility when there is insufficient room for a loop. Expansion joints resemble bellows and provide very flexible pipe legs within very short leg lengths.
Use of expansion joints is usually a last resort solution since expansion joints present maintenance problems due to their fatigue failure mode. (It should be noted that expansion joints may on occasion actually offer an economical solution in extreme cases, such as when the alternative is expansion loops of very large diameter pipe of expensive material such as alloy or stainless steel.) Expansion joints also present problems due to the fact that they cannot transmit tension through their wall. This problem is discussed in Section 2.1.5.
Since the failure mode of expansion joints is fatigue, the relative expansion displacements between the start and end of the expansion joint must be checked against the manufacturer's allowables. Note that the allowables provided will not be absolute values, but will be based upon a specific number of cyclic applications. The manufacturer must always provide a fatigue curve or some other type of adjustment factor in order to determine the allowable displacement for a different number of cycles. For example, a manufacturer may require that the allowables be divided by a factor based upon the number of load cycles:
If the manufacturer provides allowable displacements in the axial, bending, and lateral directions, all three movements should be evaluated using a linear interaction formula:
In the event that the manufacturer only provides allowable axial movements, the other two displacements may be converted to equivalent axial displacements. In that case, the following requirement must be met:
Expansion joints are especially weak in torsion, so this type of loading should be kept to a minimum. Means of modeling various expansion joint configurations is discussed in Section 3 of this document.