Why do we need Piping Expansion Loops?
All piping engineers are well acquainted with expansion loops in the piping systems. Whenever thermal displacements are more than a certain value these expansion loops are added to absorb the displacement inside the expansion loop. These are mainly required in any piping system design to
- Reduce system stress and
- Limit thermal displacements
Piping expansion loops are used to increase the flexibility of the piping system. To reduce the generated expansion stress and displacement caused by thermal expansion or contraction, legs perpendicular to the main piping system are provided. This perpendicular length is known as the length of the expansion loop. The more this expansion loop leg length, the better for the piping system. But this leg length is limited by support feasibility, vibration tendency, and cost. That is the reason the length of the absorbing leg in an expansion loop is decided only to meet the requirement of stress qualification.
Piping expansion loops are widely used in long runs of pipes running over the pipe racks or sleepers. Pipeline expansion loops are usually provided at every 500m length as the design temperature of pipelines are normally less as compared to piping systems.
Fig 1 shows typical loops used in the piping system.
Functions of Piping Expansion Loops
Expansion loops serve various purposes as listed below:
- Piping Expansion Loops provide the necessary leg of piping in a perpendicular direction to absorb the thermal expansion. They are safe when compared with expansion joints but take up more space.
- Load due to axial expansion causes bending stresses to be developed, increasing upwards in the vertical pipes and becoming a maximum at the loop elbows.
- That bending moment stays at that maximum bending moment level for the entire length of the top horizontal pipe until it gets to the next elbow and starts’ reducing until it reaches the bottom pipe on the other side of the loop.
- As the loop gets higher, both axial resultant stress in the horizontal pipes and the bending moments in the loop is reduced.
Types of Piping Expansion Loops
Piping Expansion loops are categorized into different styles:
Symmetric loop vs Nonsymmetric loop (Fig. 2):
Ideally, loops shall be located centered between pipe anchors with equal legs on either side of the anchor. Symmetrical loops are advantageous to absorb an equal amount of expansion from both directions.
When this isn’t practical make legs on either side of the anchor as equal as possible.
Friction Forces are determined by the number of pipes supporting a line crosser. By making these legs equal, the forces at the anchor should remain nearly balanced.
2-D vs 3-D Expansion Loops (Fig. 3)
Expansion Loops may be 2-D (Two-dimensional loop) or 3-D (three-dimensional loop) types. Normally for steam lines, flare lines, condensate lines, and sloped lines, where is the possibility of two-phase flow, 2-D expansion loops are preferred. Otherwise, the 3-D loop can be provided.
Requirements of Multiple Expansion Loops
More than one expansion loop may be required when:
- It is impossible to make branch connections flexible enough.
- Spacing between branches and neighboring lines or steel is limited.
- When the loop becomes too large to support or fit into the space available.
- Anchor forces become too unbalanced and steel cannot be economically braced.
- More than one expansion loop may be required when, the forces required to bend the loop are too great, and the anchors cannot be economically reinforced.
- When the thermal displacements are exceeding the project’s specifically allowed displacements.
- When the length of shoe supports due to high thermal displacement is becoming too high that it looks odd.
Placing Expansion Loops/ Expansion Loop Placements
- Loop width should always be based on utilizing existing supports.
- Thermal expansion must be allowed when spacing adjacent loops.
- Loop width does not have to be near 20 feet just because the loop nomographs happen to use that number. Loop width has only a secondary effect on results.
- Minimum loop height depends on the berthing of the line with respect to the location of the loop support.
- Piping Expansion Loops cannot extend too far beyond existing support or the overhang will cause the loop to “lose its balance.” This sets the maximum allowable loop height.
- The first two points have more influence on loop design than stress formulas, from the piping point of view.
- Three-dimensional expansion loops are widely used because this arrangement does not block the routing of low-temperature lines under the loop.
- Vertical loops are placed at road crossings and sometimes are nonsymmetrically located due to the location of the road.
Method for Sizing Pipeway Expansion Loops
Anchor lines near their center to determine which lines require loops by checking the allowable expansion at each end of the run. If the thermal displacement at each end is within the project-specific limit and it will not clash with other lines, no expansion loop will be required. However, if the line spacing cannot be adjusted to take the movement, expansion loops need to be added.
Determine which of the lines requiring loops need the largest loop, second largest, etc., by the following:
- Multiply the total expansion of each line between its proposed anchors by the pipe’s moment of inertia (E). (The stiffness of a line is measured by its “Moment of inertia.”).
- The line with the largest of these calculated numbers will require the largest loop, the next smaller number, the next smaller loop, etc.
- The above rule does not check stress. This is checked after the loops are roughly dimensioned.
Fit the expansion loops between two pipes supports using the minimum spacing plus allowance for line expansion and bowing. Make the loops as wide as possible, but keep the height to a minimum. If stress or force is extremely high, check with the stress engineer for the height of the loop.
Send finished pipe way to stress for accurate calculation of anchor forces for transferring to Structural and accurate evaluation of stresses in the piping.
Estimating Expansion Loop Leg Requirement / Expansion Loop Calculator
A preliminary estimate of the expansion loop absorbing leg requirement can be made from the equations derived from the guided cantilever method. As per that formula, the required absorbing leg, L (Refer to Fig. 5) for the piping expansion loop is given by
where E=Elasticity modulus, D=Pipe OD, S=Allowable Stress at the maximum temperature, and ∆=thermal expansion.
Providing an expansion loop in a piping system needs additional space and additional elbows (additional cost) which may not be possible in some instances. In such a scenario piping expansion joints are used to absorb the thermal growth.
I have created an expansion loop calculator for preliminary loop sizing using the above-mentioned formula which can be downloaded by clicking here.
Video Tutorial on Piping Expansion Loop
For further doubts kindly refer to the following video tutorial
Few more resources for you…