Like all materials, pipe experiences dimensional changes as a result of changes in temperature, expanding when heated and contracting when cooled. The key to accommodating thermal expansion and contraction in a piping system is to allow predictable and controlled piping-system movement.

Inadequate movement accommodation can cause excess stress on a piping system, resulting in ruptures and leaks; increased stress on boilers, chillers, valves, and other equipment/components; and escalating downtime and labor expenses. This can impact building owners negatively, increasing maintenance costs and the likelihood of business shutdowns.

This article discusses four methods of allowing flexible, controlled, piping-system movement provided by grooved mechanical pipe-joining systems.

THERMAL MOVEMENT

When a piping system is subjected to varying temperatures, it may experience horizontal movement, vertical movement, and angular deflection simultaneously. Vertical and horizontal piping systems can experience additional strains. For example, space constraints can be a major obstacle around a horizontal piping system's length and turns. Vertical-piping considerations should involve dynamic, static, and elevation-head calculations of the pressures and loads exerted on the lower portion of the system.

Carbon-steel pipe experiences thermal expansion or contraction at a rate of 0.75 in. for every 100°F change in temperature per 100 ft of pipe. The piping typically is placed in stressful conditions, under which reactive forces can damage components or equipment. The forces generated are often significant; movement must be accommodated and controlled to prevent transmission of stresses throughout the piping system.

GROOVED MECHANICAL PIPE-JOINING SYSTEMS

A grooved mechanical system can accommodate thermal pipe movement in four ways:

• By utilizing the linear movement/deflection capabilities of flexible grooved couplings.

• By providing an expansion joint that utilizes grooved-mechanical-pipe components.

• By providing an expansion loop that utilizes grooved-mechanical-pipe components.

• By allowing a piping system to “free-float.”

Flexible grooved couplings for linear movement and deflection

FIGURE 1. A grooved mechanical coupling.

The design of a grooved mechanical coupling's components permit pipe movement (Figure 1). The coupling key's dimensions are narrower than the groove in the pipe, allowing the coupling key to move in the pipe groove. Additionally, the coupling housing's width allows the pipe end to separate, leaving room for controlled linear and angular movement. The mechanical coupling remains a self-restrained joint, and its pressure-responsive design provides sealing even under deflection and pipe movement.

Grooved mechanical couplings are an alternative to welded U-shaped expansion loops, welded offsets, expansion joints, and rubber bellows. The couplings accommodate movement within a piping system's “free range of motion,” meaning piping-system movement caused by thermal expansion and contraction can be accommodated in smaller spaces with little stress on components.