Accommodating piping movement in plumbing systems

BY DAVID L. HUDSON

Piping movement within a structure is often a concern for engineers designing and specifying for pipe installation. If not adequately accommodated, repeated stress on a piping system can cause damage to equipment and threaten the structural integrity of the building itself. The key to effectively accommodating movement in a building is to allow the predictable, controlled movement of the piping itself.

A piping system 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, spatial constraints around the length and turns of the pipe should be considered during the design phase. 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.

Depending on the type of material used for the plumbing system, there are several methods for accommodating pipe movement. The selection of one of these methods is dependent on the type of system, the project requirements and the engineer's preference. Not all system designs can be predicted and described all at once. Since it is impossible to predict all system designs, this article will discuss the design benefits and mechanical advantages of the grooved piping method when used to accommodate movement in a building’s plumbing system.

Deflection and linear movement

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, angular nor rotational movement at the joints. On the other hand, flexible grooved mechanical couplings are designed to allow controlled linear and angular movement at each joint, which can accommodate pipeline deflection, building creep and settlement.

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 that coupling key to move in the pipe groove. Additionally, the width of the coupling housing allows for pipe end separation, therefore leaving 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 deflection and linear movement within the design capability of the coupling, all the while doing this within the product’s “free range of motion.” This means that imparted deflections can be accommodated in smaller spaces, with low stress on the components.

Settlement

Unanticipated pipeline deflection can damage a building’s equipment or even compromise the structural integrity of the building itself. The piping system designs must work in concert with the building design. Deflection imposed on a piping system may occur due to uneven settlement, particularly when considering new additions to existing structures.

In a stainless steel or galvanized system, piping misalignment due to uneven building settlement is addressed by using an even number of flexible couplings and permitting the intermediate pipe to “toggle” as the movement occurs. To determine the number of couplings required, define the amount of lateral misalignment on a particular pipe run and the length of that pipe run. The objective is to achieve the required displacement using the minimum number of couplings. Due to symmetry around a transition point, the point of inflection is a pipe spool and not a coupling. The number of couplings and the length of the pipe spools are two variables that can be altered to obtain the desired misalignment. Other factors, such as the maximum angle of deflection at each coupling and the maximum pipe end separation, are a function of the size and style coupling being used.

Thermal transients

Thermal transients may impose deflection on a piping system as the pipe grows when heated and contracts when cooled. All materials, including pipe, machinery, structures and buildings, experience dimension changes as a result of changes in temperatures. This will often occur at directional changes, or cause “bowing” at the mid points of long straight pipe runs, resulting in stress on the piping system and equipment.

Expansion joints are devices that can be compressed or expanded axially. Cost-effective expansion joints utilize grooved mechanical couplings and specially grooved, short pipe nipples with flexible couplings for galvanized carbon steel or stainless steel systems, 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.

Perhaps a more desirable way to accommodate for thermal movement is to build a free-floating system. These piping systems allow thermal expansion and contraction without the use of expansion joints. As long as this movement does not cause bending moment stresses at branch connections, it is not harmful to joints and changes in direction or to parts of structures and other equipment. Typically galvanized carbon steel and stainless steel systems utilize a free-floating system since it requires the use of flexible couplings.

A free-floating system can be achieved by installing additional grooved mechanicals joints or by installing guides to control the direction of movement. The affects of pressure thrusts must be taken into account when utilizing flexible grooved couplings, as the pipe will be moved to the full extent of the available pipe end gaps when allowed to free-float. To make sure the added stress on the system is properly supported, a free-floating system may also require more supports and anchors along the pipe length.
In a stainless steel or galvanized system, a flexible mechanical joint can also be used in expansion loops. Also, it is important to note that expansion loops utilizing rigid couplings are not designed to accommodate angular deflection, however an expansion loop utilizing rigid grooved copper couplings is designed to conform to industry standards based on the Copper Development Association’s Copper/Brass/Bronze Product Handbook, and the American Society for Metals’, Source book on Copper and Copper Alloys.

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 and four grooved end 90° elbows and three pipe spools are required to complete each expansion loop. (Figure A, below) As system temperatures lower and the pipe run contracts, the loop expands and the deflection capability of couplings accommodates this movement. (Figure B, below) 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. (Figure C, below) A significant benefit to using this configuration is that a loop constructed in this manner will be 1/2 to 1/3 the size of a welded loop with the same capacity, and will accommodate the movement without inducing stress into the pipe.

Building creep or subsidence

Similar to thermal transients, deflection or linear movement imposed on a piping system may occur due to building creep. Building creep is the common term for the amount of actual building shrinkage that will occur over time. This is an important consideration for high rise construction.

Depending on the pipe material, accommodating building creep can be addressed three different ways using mechanical piping systems: flexible system, rigid system or a combination of both.

In a flexible grooved system utilizing only flexible grooved mechanical pipe joints, risers are installed with anchors at the top and bottom with the piping guided every other pipe length to prevent “snaking” of the line. A sufficient number of flexible couplings must be utilized to accommodate the anticipated movement. Proper gapping of the pipe ends within the coupling is required in order to allow the riser to compress with the building.

In a rigid system utilizing only rigid grooved mechanical pipe joints, risers can be treated similar to a welded system, and where movement is required expansion joints or offsets are designed into the riser to accommodate movement and prevent damage to components.

By designing risers with a combination of both rigid and flexible grooved joints, engineers can utilize rigid couplings to reduce guiding requirements and the flexible grooved joints to accommodate the movement required.

The bottom line

When specifying and designing a plumbing system, grooved mechanical systems offer several different methods to provide flexible, controlled movement. The choice of expansion joints, free-floating systems, flexible couplings or expansion loops will be based on the type of piping system, especially depending on whether a stainless steel, galvanized carbon steel or a copper system will be installed. It will also depend on the amount of anticipated movement, and finally, the plumbing engineer’s preference.

Overall, choosing the grooved mechanical method is an efficient way to accommodate excess stress on any piping system, eliminate incidents of ruptures and leaks due to piping movement, decrease maintenance needs of equipment, and simplify the commissioning process.

A senior product engineer for Victaulic Company Inc., David L. Hudson is a practicing mechanical engineer with almost 30 years of experience. He can be reached at dhudson@victaulic.com.