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Flexible mechanical joints can be used in expansion loops without inducing stress in pipes, elbows, or joints. It is important to note that expansion loops utilizing rigid couplings are not designed to accommodate angular deflection; however, an expansion loop utilizing rigid grooved copper couplings is designed to conform to industry standards.

Flexible couplings' deflection capability allows thermal expansion/contraction to be absorbed at elbows as thermal forces induce deflection. Eight flexible grooved mechanical couplings, four grooved-end 90-degree elbows, and three pipe spools are required to complete an expansion loop. As system temperatures decrease and the pipe run contracts, the loop expands. The couplings' deflection capability accommodates the movement. As system temperatures increase, the opposite effect occurs as the pipe run expands and the loop contracts. The couplings then accommodate deflection in the opposite direction (Figure 6).

Using flexible couplings in an expansion-loop configuration reduces the amount of force needed to flex the loop. This type of loop will be half to one-third the size of a welded loop with the same capacity. The space constraints of modern buildings make this an attractive HVAC piping option, although welded expansion loops still are required in some system applications.

FIGURE 6. Left: An expansion loop. Middle: During thermal contraction, the pipe shrinks and the loop expands. Right: During thermal expansion, the pipe grows into the loop and the loop contracts.

Free-floating systems

Free-floating piping systems allow thermal expansion and contraction without the use of expansion joints. As long as movement does not cause bending-moment stresses at branch connections, it is not harmful to joints, changes in direction, parts of structures, or other equipment. A free-floating system can be achieved by installing additional grooved mechanical joints or guides to control the direction of movement. Engineers must consider the effects of pressure thrusts when utilizing flexible grooved couplings because a pipe that is allowed to float will move as far as its end gaps permit.

Branch connections and offsets must be long enough so that a coupling's maximum angular deflection is never exceeded and can accommodate the anticipated total movement of a pipe. Otherwise, the system should be anchored and movement directed.

CONCLUSION

Grooved mechanical pipe-joining systems offer four methods of providing flexible, controlled piping-system movement. The selection of flexible couplings, expansion joints, expansion loops, or free-floating systems should be based on the type of piping system and the amount of anticipated movement.

In addition to accommodating thermal expansion and contraction, benefits of the grooved method include a simplified assembly process that is readily inspectable. Also, mechanical couplings reduce the need for welding and reduce man-hours and material handling on site, making job sites safer and reducing the risk of injury. During operation, the simple disassembly of a coupling reduces chances of deferred maintenance and lengthy downtime for routine or unscheduled maintenance. The grooved-mechanical method is an efficient way of accommodating excess stress on a piping system, eliminating ruptures and leaks caused by thermal expansion, decreasing maintenance needs, and simplifying the commissioning process.

For past HPAC Engineering feature articles, visit www.hpac.com.

Larry Thau is executive vice president and chief technology officer for Victualic Company Inc. A practicing mechanical engineer for more than 35 years, he holds more than 35 patents and lectures on piping technology around the world.