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.
- 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.