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Getting into the Groove

Larry Thau, making a case for grooved mechanical systems, highlights the design benefits and advantages of the grooved piping method when used to accommodate thermal expansion and contraction

| | Mar 12, 2012 | 11:41 pm
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Excess stress on the piping system can lead to problems like ruptures and leaks, as well as stress on boilers, chillers and other equipment and components. Larry Thau, making a case for grooved mechanical systems, highlights the design benefits and advantages of the grooved piping method when used to accommodate thermal expansion and contraction.

The key to effectively accommodating thermal expansion and contraction in a building is to allow the predictable, controlled movement of piping itself. This can be done in a variety of ways, and the selection of a specific method depends upon the engineer, the type of piping system and the project parameters.

Thermal transients may impose stress on a piping system, as the pipe grows when heated and contracts when cooled. All materials, including pipe, experience dimension changes as a result of varying temperatures and their coefficient of expansion. This often occurs at directional changes in the pipework or causes ‘bowing’ at the mid points of long straight pipe runs, resulting in stress on the piping system and equipment.

Flexible grooved mechanical couplings are designed to allow controlled linear and angular movement at each joint that can accommodate pipeline deflection

When a system is subjected to changes in temperatures, it may experience horizontal movement, vertical movement and angular deflection simultaneously. Additional strains on the piping system vary based on whether the piping is vertical or horizontal. For horizontal piping, the major obstacle is typically the space constraints around the length and turns of the pipe. For vertical piping, considerations are different and should involve dynamic, static and elevation head calculations of the pressures and loads that are exerted on the bottom portion of the pipe.

Carbon steel pipe will experience thermal expansion or contraction at a rate of 1.905cm for every 38°C change in temperature per every 30.48m of pipe. Piping subject to temperature changes is put in a condition of stress, with potentially damaging reactive forces on components or equipment. The forces generated during this thermal dimension change are often significant and the movement must be accommodated and controlled, to prevent transmission of these stresses throughout the piping system.

Inadequate accommodation of this movement can result in business risks caused by excess stress on the piping system, including increased incidence of ruptures and leaks, increased stress on boilers, chillers, valves and other equipment and components, and increased downtime and labour expenses. This can negatively impact the owners of the building by resulting in increased maintenance costs and potential business shutdowns.

GROOVED ACCOMMODATION

When accommodating thermal expansion and contraction, the grooved pipe joining system conforms to industry practices. Simultaneously, it provides design flexibility, reduces stress on the piping system and provides a more compact, easy-to-inspect and productive method of installation over other pipe-joining methods, such as welding or flanging. Additionally, with the grooved method, all sealing elements are combined within a metallic housing.

Grooved mechanical couplings allow for movement in the pipe due to the design of the components. The dimensions of the coupling key are narrower than the groove in the pipe, allowing room for the coupling key to move in the pipe groove. Additionally, the width of the coupling housing allows for pipe end separation. This leaves room for controlled linear and angular movement. The mechanical coupling remains a self-restrained joint, and the unique pressure-responsive design provides sealing even under deflection and pipe movement.

Grooved mechanical couplings are a great alternative to welded U-shaped expansion loops, welded offsets, expansion joints and rubber bellows. These couplings are easier and faster to install and accommodate movement within the design capability of the coupling. Nonetheless, this takes place within the product`s ‘free range of motion’. Consequently, piping system movement caused by thermal expansion and contraction can be accommodated in smaller spaces, with low stress on the components.

There are four common methods for accommodating thermal pipe movement in a grooved system:

  1. Providing an expansion joint utilising grooved mechanical pipe components
  2. Allowing the system to ‘free-float’
  3. Utilising the linear movement/deflection capabilities of flexible grooved couplings
  4. Providing an expansion loop utilising grooved mechanical components

The selection of one of these methods is dependent on the system type, the scope of the project and the engineer’s preference. Since it is impossible to predict all system designs, this article will call attention to the design benefits and mechanical advantages of the grooved piping method when used to accommodate thermal expansion and contraction.

Utilising expansion joints:
Grooved mechanical couplings are available with two distinct performance features. One class is designed as ‘rigid’ and the other as ‘flexible’. Rigid grooved mechanical couplings are designed to ‘fix’ the joint in its installed position, permitting neither linear or angular nor rotational movement at the joints. Flexible grooved mechanical couplings, on the other hand, are designed to allow controlled linear and angular movement at each joint that can accommodate pipeline deflection.

Expansion joints are devices that can be compressed or expanded axially and are generally the most costly alternative for accommodating thermal movement. A welded expansion joint is typically an expensive specialty joint, flanged into the system and requiring regular maintenance. More cost-effective expansion joints utilise grooved mechanical couplings and specially grooved, short pipe nipples with flexible couplings placed in long straight runs of pipe and pre-set to allow the desired amount of contraction and/or expansion. Axial movement can be adjusted by simply adding or removing couplings. When a series of flexible couplings are installed, the resulting grooved expansion joint will further protect equipment by reducing vibrations and stresses in the system.

Whether using specialty expansion joints or a grooved expansion joint, the adjacent piping must be properly guided to ensure the movement is directed into the device and no lateral movement is experienced.

Grooved expansion joints may be used as flexible connectors. However, they will not simultaneously provide full expansion and full deflection. Expansion joints installed horizontally require independent support to prevent deflection, which will reduce the available expansion.

FREE-FLOATING SYSTEMS

Flexible grooved couplings for linear movement and deflection:

Grooved mechanical couplings are an alternative to welded U-shaped expansion loops, welded offsets, expansion joints and rubber bellows. Associated with a free floating system, flexible couplings are used in piping systems to accommodate piping thermal growth – without any additional components or piping configuration required. In this example, maximum linear movement values available at flexible grooved pipe joints are published under performance data for each coupling style. For design and illustration purposes, the figures in Diagram B should be reduced by the following factors to allow for pipe groove tolerances.

  • Y = Misalignment (inches)
  • G = Maximum Allowable Pipe End Movement (inches) as shown under performance data (reduce published value by Design Tolerance).
  • Θ = Maximum Deflection (degrees) from Centre Line as shown under performance data (reduce published value by Design Tolerance).
  • D= Pipe Outside Diametre (inches)
  • L= Pipe Length (inches)

NOTE: Joints which are fully deflected can no longer provide linear movement. Partially deflected joints will provide some portion of linear movement.

Flexible grooved type couplings allow angular flexibility and rotational movement to take place at joints. These features provide advantages in installing and engineering piping systems, but must be considered when determining hanger and support spacing.

As illustrated in Diagram C, it is obvious that this system would require further hangers to eliminate the drooping of the pipes that would occur. Hanger positions must be considered in relation to the angular and rotational movement that will occur at joints.

Flexible couplings allow linear movement. Therefore pressure thrusts must be taken into account, which would allow the pipe ends to move to the maximum extent allowed by the coupling, and, which would accumulate at the end of the system if the joints had been installed butted or only partially opened when pressurised. By using flexible couplings at changes of direction and directing the movement towards the directional change with properly placed anchors and guides, movement is accommodated by the joining method itself. This method also produces little or no additional stresses in the system, unlike a welded expansion loop.

Flexible couplings also can be used strictly for their axial movement capabilities. In this case, straight runs are anchored on each end and the piping is guided at every other length. Each flexible joint is pre-gapped (either fully gapped or fully closed/butted) at installation to ensure that there are enough couplings to accommodate the expected expansion and/or contraction.

Flexible grooved type couplings allow angular flexibility and rotational movement to take place at joints. In order to determine the appropriate number of couplings to use, the designer must compute the change in the linear length of the piping system by taking into account the length and size of the system and maximum and minimum operating temperatures.

Where full linear movement is required, a grooved expansion joint can be used, but joints which are fully deflected can no longer provide linear movement. Partially deflected joints will provide some portion of linear movement. Furthermore, standard cut-grooved pipe will provide double the expansion and contraction or deflection capabilities of the same size standard roll-grooved pipe.

When considering offsets utilising grooved mechanical joints, the latter must be capable of deflecting sufficiently to prevent harmful bending movements at the joints. If the pipes were to expand due to thermal changes, then further pipe growth would occur at the ends.

Flexible couplings do not automatically provide for expansion or contraction of piping. The designer should always consider the best setting for pipe end gaps. In anchored systems, gaps must be set to handle combinations of expansion and contraction. In free floating systems, offsets of sufficient length must be used to accommodate movement without over-deflecting joints.

EXPANSION LOOPS UTILISING FLEXIBLE COUPLINGS AND FITTINGS

In vertical straight runs of pipe, expansion loops utilising a U-shaped pipe configuration can also be designed into the piping system to accommodate expansion and contraction. These can be designed as welded or grooved. Welded expansion loops require eight welded joints and fittings to assemble. In a welded expansion loop, the piping bends or flexes to accommodate the straight run movement. Although this method works, the forces to bend and flex the pipe are much greater than in a grooved loop, and the forces generate a larger magnitude of stress, which requires larger anchors and guides to direct the movement and protect components and structures.

The flexible mechanical joint can be used in expansion loops without inducing stresses in the pipes, elbows or joints. Also, it is important to note that expansion loops which utilise rigid couplings are not designed to accommodate angular deflection. However, an expansion loop which utilises rigid grooved copper couplings is designed to conform to industry standards.

The deflection capability of flexible couplings allows for thermal growth/contraction to be absorbed within the couplings at the elbows as the thermal forces induce deflection. A total of eight flexible grooved mechanical couplings, four grooved end 90-degree elbows and three pipe spools are required to complete each expansion loop. As system temperatures fall and the pipe run contracts, the loop expands and the deflection capability of the couplings accommodates this movement. As system temperatures increase, the opposite effect occurs as the pipe run expands and the loop contracts with the couplings accommodating the deflection in the opposite direction (see Diagrams A to C).

Using flexible couplings in an expansion loop configuration reduces the amount of force needed to flex the loop, and the loop itself is much smaller. A loop constructed in this manner will be 1/2 to 1/3 the size of a welded loop with the same capacity.

The space constraints of today’s buildings also make this a more attractive option in HVAC piping, though welded expansion loops are still required in some system applications.

MAKING THE BEST CHOICE

Grooved mechanical systems offer four different methods to provide flexible, controlled movement of a piping system. The selection of expansion joints, free-floating systems, flexible couplings or expansion loops will be based on the type of piping system, the amount of anticipated movement and the mechanical engineer’s preference.

In conclusion, choosing the grooved mechanical method is an efficient way to accommodate excess stress on any piping system. It eliminates incidents of ruptures and leaks due to thermal expansion, decreases maintenance needs of equipment, and simplifies the commissioning process.

The writer is the Executive Vice President and Chief Technology Officer at Victaulic. Practising mechanical engineer for 35 years, he holds over 35 patents and lectures on piping technology around the world. He can be contacted at: LThau@victaulic.com


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