All piping engineers are well acquainted with expansion loops (Fig 1). These are mainly required in piping system design to
- Reduce system stress and
- Limit thermal displacements
Fig 1 shows typical loops used in piping system.
- 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 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 are reduced.
Expansion loops are categorized in different styles:
Symmetric loop vs Nonsymmetric loop (Fig. 2):
Ideally loops shall be located centered between anchors with equal legs on either side of anchor. Symmetrical loops are advantageous to absorb the equal amount of expansion from both directions.
When this isn’t practical make legs on either side of anchor as equal as possible.
Friction Forces are determined by the number of pipe supports a line crosser. By making these legs equal, the forces at the anchor should remain nearly balanced.
2-D vs 3-D Loops (Fig. 3): Loops may be 2-D or 3-D types. Normally for steam lines, flare lines, condensate lines etc 2-D loops are preferred. Otherwise 3-D loop can be provided.
Multiple Loops (Fig. 4):
More than one 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 loop becomes too large to support or fit into space available.
- Anchor forces become too unbalanced and steel cannot be economically braced.
- More than one loop may be required when, the forces required to bend the loop are too great, and the anchors cannot be economically reinforced.
Placing Expansion Loops/ Expansion Loop Placements:
- Loop width should always be based on utilizing existing supports.
- Thermal expansion must be allowed for 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 secondary effect on results.
- Minimum loop height depends on the berthing of the line with respect to the location of the loop support.
- Loops cannot extend too far beyond an existing support or the overhang will cause the loop to “loose 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 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 Loops:
- Anchor lines near their center to determine which lines require loops by checking the allowable expansion at each end of the run. If each end will absorb the resulting expansion, no loop is required, usually. If the line spacing cannot be adjusted to take the movement, a loop is required.
- 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 loops between two pipe supports using 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 stress engineer for height of loop.
- Send finished pipeway to stress for accurate calculation of anchor forces for transmittal to Structural and accurate evaluation of stresses in the piping.