Three ways to use grooved couplings to accommodate thermal movement in riser piping

Grooved couplings serve double-duty by offering a safe, efficient means for joining riser piping while also permitting thermal expansion and contraction.

By Dave Hudson, Victaulic, Easton, Pa. May 5, 2016

Learning Objectives

  • Identify the two distinct classes of grooved mechanical couplings and describe the movement characteristics of each.
  • Understand the three methods for accommodating thermal movement in riser piping using grooved couplings.
  • Explain the advantages of grooved techniques for thermal-movement accommodation.

When subjected to variations in temperature, piping will experience linear expansion or contraction. The key to effectively accommodating this thermal movement in a building’s riser piping is to allow the predictable, controlled movement of the piping.

There are other methods for accommodating this type of movement. However, grooved piping components offer three distinct ways to accommodate this pipe thermal movement, allowing the system designer to choose the method that is best for each specific application.

Grooved for riser piping

When installed with design considerations in mind, grooved mechanical couplings are capable of accommodating piping thermal movement. This means that to achieve this added benefit, system designers must recognize this critical characteristic when specifying grooved pipe-joining systems. Grooved mechanical couplings are available in rigid and flexible designs:

  • Rigid pipe couplings mechanically resist linear and angular movement of the pipe at the joint.
  • Flexible pipe couplings permit a controlled amount of linear and angular pipe movement.

In a flexible pipe coupling, the dimensions of the coupling key are narrower than the groove in the pipe, providing room for the coupling key to move within the pipe groove. Additionally, the width of the flexible pipe-coupling housing allows for pipe-end separation, leaving room for controlled linear and angular movement. The flexible pipe coupling remains a self-restrained joint, and the pressure-responsive gasket design provides positive sealing even during piping system movement.

There are three standard methods for employing grooved mechanical couplings to accommodate piping thermal movement (expansion and contraction) in a building riser:

Method 1: Use the angular-deflection capability of flexible pipe couplings to accommodate the movement at the top of the riser.

Method 2: Use the linear-movement capability of flexible pipe couplings to accommodate the movement at each joint.

Method 3: Use the linear-movement capability to accommodate the movement in a grooved in-line expansion compensator.

The selection of a specific methodology will depend on the project parameters as well as the designer’s preference.

Method 1: Free-floating

The first method for accommodating thermal expansion or contraction in a riser with grooved couplings is to build a free-floating system in which the movement is directed to the top of the riser with a base anchor and guides. The grooved pipe couplings joining the riser piping sections are rigid, while those at the top on the first horizontal pipe are flexible, as shown in Figure 2. This rigid and flexible coupling arrangement makes use of the angular-deflection capabilities of the flexible pipe couplings at the top of the riser. To compute the required horizontal pipe length, divide the anticipated amount of thermal movement by the coupling’s deflection-from-centerline capability, taking into account any required design factors.

In addition to the flexible grooved couplings at the top of the riser, flexible couplings must also be used on the branch piping to allow deflection of the branch connections as the riser expands or contracts. At least two flexible, grooved pipe couplings are required on each branch line to take up the vertical displacement. The length of pipe between the flexible pipe couplings at branch connections must be sufficiently long so that the maximum angular deflection of the couplings is never exceeded and that they will accommodate the anticipated movement of the riser. With this method, the amount of movement of the branch piping increases from the first floor to the top floor.

Example: A 200-ft riser constructed of 6-in. carbon steel pipe (expansion value: 0.75 in. per 100 ft per 100°F ΔT). The riser is installed at 60°F, which is also the lowest temperature, and has a maximum operating temperature of 180°F.

0.172 in. per feet of pipe        Coupling deflection from centerline on roll-grooved pipe including design-reduction factor

1.8 in.                                 Pipe thermal movement based upon 120°F ΔT

10.5 ft                                 Minimum horizontal pipe length at top of riser.

Method 2: Movement accommodated at each flexible, grooved coupling

The second method using grooved mechanical couplings for thermal-movement accommodation in a riser is to use all flexible pipe couplings to join the riser piping sections. Using the linear-movement capability of the flexible, grooved couplings, expansion or contraction is accommodated within each coupling.

During installation, pipe-end gaps must be set within the grooved couplings proportional to movement requirements (fully gapped for expansion only, fully butted for contraction only) to accommodate the expected riser movement. The riser should be anchored at the top and bottom and the pipe guided to prevent deflection at the joints ("snaking" of the line) and direct the movement into the grooved, flexible pipe couplings.

To compute the number of flexible pipe couplings required to accommodate the expected expansion or contraction, divide the anticipated change in length of the piping by the linear-movement capability (including any design factor) of the type and size of coupling to be installed.

Although guiding requirements are increased, this method minimizes branch movement in comparison with the free-floating method.

Example: A 200-ft riser constructed of 6-in. carbon steel pipe. The riser is installed at 60°F, also the lowest temperature, and has a maximum operating temperature of 180°F.

0.097 in.     Coupling linear movement capability on roll-grooved pipe including reduction factor

1.8 in.         Pipe thermal movement based upon 120°F ΔT

19              Required number of couplings.

In this example, the use of a grooved reducing tee for branch connections on each floor would provide the required number of flexible couplings. If the calculated required number of couplings is greater than the number of couplings needed to install the riser piping, then additional flexible couplings would need to be evenly spaced out along the riser.

Method 3: Expansion compensator made of grooved mechanical couplings

The third method for using grooved couplings to accommodate thermal expansion and contraction is to employ a grooved in-line expansion compensator constructed from flexible mechanical couplings. This method entails the use of all rigid couplings on the riser pipe joints with movement directed into the grooved expansion compensator.

Grooved expansion joints, or compensators, accommodate up to several inches of axial movement. They consist of a series of flexible pipe couplings and specially grooved pipe nipples to accommodate a range of movement requirements, depending on the number of couplings in the device. A grooved expansion compensator must be sized and preset to the designer’s requirements to accommodate the anticipated movement: fully expanded, fully compressed, or in a position proportional to installation temperature and system temperature extremes.

For proper operation of the grooved expansion compensator, the riser must be properly anchored and guided. The grooved expansion compensator is installed between opposing anchors, which directs movement into the device and prevents the joint from opening to its maximum expanded length upon system pressurization from the pressure thrust force.

The pipe must be guided on both sides of the grooved expansion compensator to prevent deflection of the unit. Deflection will reduce the available axial movement and may cause damage, depending on the type of device used. When system conditions permit, the guides on one side of the expansion compensator can be eliminated by locating the unit adjacent to an anchor.

The riser may require multiple expansion compensators based upon the height of the riser, amount of piping thermal movement, and allowable deflection of branch lines. In this case, the riser would require main anchors at the bottom and above the uppermost expansion compensator. The use of intermediate anchors would be determined based upon the riser support method used.

Grooved expansion compensators require no maintenance. They are typically constructed of the same pipe material and wall thickness as the riser pipe itself. This results in a more reliable and durable expansion joint, thus avoiding the long-term inspection and maintenance issues required by other expansion-joint designs.

Example: A 200-ft riser constructed of 6-in. carbon steel pipe. The riser is installed at 60°F, also the lowest temperature, and has a maximum operating temperature of 180°F.

1.8 in.      Pipe thermal movement based upon 120°F ΔT

3 in.        Maximum linear movement accommodated by grooved expansion compensator

1             Number of expansion compensators required.

Regardless of the method selected, the specification of grooved mechanical couplings for a building’s riser piping offers several additional advantages.

Tradition dictates that separate methods are required for joining the pipe and accommodating thermal movement, but this is not the case. Grooved mechanical couplings both join the pipe and accommodate thermal movement within the design capability of the coupling.

Grooved couplings also are much faster to install than other joining methods, which can help fast-tracked projects finish on time or even ahead of schedule. Standard grooved couplings can be installed up to five times faster than welded joints while "ready-to-install" couplings can double that time savings.

Grooved mechanical couplings eliminate space-consuming welded expansion loops, which, in the tight confines of a riser shaft, generally aren’t realistic. Grooved mechanical couplings enable piping movement to be accommodated in smaller spaces with lower overall system stresses.

The bottom line is that engineers need a reliable method for accommodating thermal movement in a building’s riser piping. The successful, long-term use of grooved piping in building-services piping systems around the globe supports the method’s viability in these applications. Engineers and building owners can have confidence in specifying grooved mechanical piping as an efficient and safer way to join pipe in addition to accommodating thermal expansion and contraction.

Dave Hudson is a senior engineer with Victaulic, a manufacturer of mechanical pipe-joining and fire protection systems. Hudson is a practicing mechanical engineer with more than 34 years of experience.