7 Flexible Connections

In Chapter 3, we discussed thermal expansion and piping flexibility. A piping system has to be flexible enough to absorb the thermal expansion displacement, without creating unacceptable stresses in the pipe or excessive reaction loads in the connecting equipment. The piping may have enough flexibility just from the turns and offsets created by the natural layout. Additional loops and offsets can be provided if the flexibility from the natural layout is not enough. There are circumstances, however, when flexible connections are needed for economic or practical reasons.

Aside from several concerns, which will be discussed later, the flexible joint is an easy solution to the piping flexibility problem. This can be demonstrated by the simple system shown in Fig. 7.1. The system is an L-shaped 8-in. (200 mm), schedule-40 (8.18 mm thick) piping operating at 600°F (316°C) from an ambient installation condition of 70°F (21°C). The system, with its natural layout, generates an anchor force of 1135 lb, an anchor moment of 13,381 lb-ft, and an elbow stress of 14,300 psi. The stress is well within the code allowable range, but the anchor loading, especially the moment, is too high for most connecting equipment. A complex loop with cleverly placed restraints would be required to reduce the anchor load to an acceptable level. The problem, on the other hand, can be easily solved with flexible joints.