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PART 2 – Flexible grooved mechanical couplings key to accommodating thermal piping movement

In Part One of this series, Marcel Ley, Victaulic Regional Sales Manager looked at why flexible grooved mechanical couplings are essential for successful thermal pipe movement mitigation. Part Two will now examine the four methods of flexible grooved systems in more detail and will explain the benefits associated with each.

Thermal pipe movement is an area of construction that engineers cannot eliminate and, while many quick-fixes are available, the only true manner to negate extensive thermal pipe movement is through flexible grooved mechanic couplings.

As mentioned in the first part of this article, there are four common methods that these joints offer when considering thermal pipe movement, including:

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  • Providing an expansion joint utilising flexible grooved mechanical pipe components,
  • Allowing the system to free-float,
  • Utilising the linear movement/deflection capabilities of flexible grooved couplings,
  • Providing an expansion loop utilising flexible grooved mechanical components.

Providing an expansion joint utilising flexible grooved mechanical pipe components

There are two distinctive performance features to grooved mechanical couplings, which are classified either as rigid or flexible. The rigid variant is designed to fix the joint in its installed position, and as such permits neither linear, angular, or rotational movement at the joint. On the other hand, flexible couplings are designed to allow for controlled linear and angular movement at the joint, which results in pipeline deflection.

Traditional expansion joints allow for compression or expansion axially, and are a costly alternative for accommodating thermal movement. And while a welded expansion joint is generally an expensive speciality joint, it is flanged directly into the system and requires regular maintenance to ensure its effectiveness.

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More cost-effective expansion joints utilise flexible 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.

In application, pipe alignment guides are essential to ensure axial movement of the expansion joint, and permitting, the expansion joint should be adjacent to an anchor within four pipe diameters. The first and second alignment guides on the opposite side of the expansion joint should be located a maximum distance of four and 14 pipe diameters, respectively.

It is imperative to remember that although grooved expansion joints may be used as flexible connectors, 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.

Accommodating thermal movement utilising a free-floating system

While free-floating systems allow for thermal reflection without the use of expansion joints, this method is beneficial as long as the movement does not result in bending moment stresses at branch connection, and does not pose additional stresses on components such as joints and other equipment.

As such, a free-floating system can be achieved by installing additional grooved mechanical joints or by installing guides to control the direction of movement. It’s crucial that engineers take the effects of pressure thrusts into account when utilising flexible grooved couplings, as the pipe will be moved to the full extent of the available pipe end gaps when allowed to float.

Finally, engineers must ensure that branch connections and offsets are long enough so that the maximum angular deflection of the coupling is never exceeded and it can accommodate the anticipated total movement of the pipes. Alternatively, it is advised to anchor the system and to direct movements.

Utilising the linear movement/deflection capabilities of flexible grooved couplings

Grooved mechanical joints provide an alternative to welded U-shaped extension looks, welded offsets, expansion joints, and rubber bellows. Closely related to free floating systems, flexible couplings are used to accommodate for piping thermal growth void of any unnecessary components or piping configuration.

By utilising flexible couplings at changes of direction and guiding movement toward 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 welded expansion loops.

These flexible couplings also feature optimal axial movement performance, and are ideal as they provide enhanced angular movement tolerance. For example, for a system that features straight runs anchored at each end with the piping guided at each other length, flexible joints are installed and pre-gapped so as to ensure that there are adequate couplings to accommodate for the anticipated deflection.

When determining the number of couplings to make use of, engineers compute the change in linear length of the piping system by taking into account the length and size of the piping system, as well as determining maximum and minimum operating temperatures.

In sections where full linear movement is required, a grooved expansion joint can be installed. Engineers will also consider the fact that standard cut-grooved pipe will provide double the expansion and contraction or deflection capabilities of the same size standard roll-grooved pipe.

It’s important to note that flexible couplings do not automatically provide for expansion or contraction of piping, and that setting and other factors are integral to the success of the joint. This includes pipe end gaps, anchors, support, and offsets.

Expansion loops utilising grooved mechanical components

In applications featuring straight pipe runs, expansion loops can be used to accommodate deflection, with these loops either being welded or incorporating grooved technology. While welded expansion loops require eight welded joints to assemble, the forces exerted on the joint are far greater than those applied on a grooved example, and generate greater stress, which ultimately requires larger anchors and guides in order to direct the movement.

A flexible mechanical joint can be utilised in expansion loops without inducing stresses in the pipes, elbows or joints. Further, it’s important to note that expansion loops using rigid couplings are not designed to accommodate angular deflection; however an expansion loop employing rigid grooved copper couplings is designed to conform to industry standards.

The deflection capabilities of flexible couplings allow for thermal growth and contraction to be absorbed within the couplings at the elbows, as 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 lower 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. 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.

Regardless of the method used, grooved mechanical couplings are an efficient way to accommodate excess stress on any piping system, eliminate incidents of ruptures and leaks due to thermal expansion, decrease maintenance needs of equipment, and simplify the commissioning process.

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