Design Quality Improvement – Bulletin 02 4/29/19 Prepared by – Todd Sisti, PE
Design of Pipe Expansion Loops
Quality Concern:
What is the practical range of axial pipe line growth that a designer should plan for in the design of an expansion loop?
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In Case 1 below the expansion loop is absorbing up to 20” (10” per side) of pipe line growth without a resulting pipe stress problem.
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In Case 2 multiple expansion loops are designed which absorb only 1” to 2” of axial line growth.
Discussion:
A balance must be achieved between how many expansion loops are needed and/or how large an expansion loop may need be. While pipe stress needs to be considered, it is usually not a limiting issue in the design of a loop. If pipe stress is a problem, this usually means you have undersized the loop(s) or you are binding the system up somewhere.
Recommended Practice:
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Design expansion loops for a moderate amount of expansion to be absorbed. Recommended practice would be 3” to 8” of total axial pipe expansion into the loop. For small expansion (less than 3”), try to absorb the expansion in the natural bends of the piping system. Ie. Anchor the center of the runs allowing growth toward the ends. When very large expansion is present, add pipe loops. Ie. Anchor the ends of the rack forcing growth to the center of the run where loops are placed to absorb the movement.
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With pipe rack piping, place at least one anchor or axial limit stop support in each straight run of pipe to help control the line from jumping in a start-up or surge scenarios. This will also make the expansion direction more predictable. Additionally, pipe line guides should be placed on every third supports and always just prior to an expansion loop.
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Use the rack width to accommodate the width of expansion loop. Some overhang is permissible but as mentioned above, too much overhang will require extra supports off the rack. When supporting the pipe loops the supports should be dead weight supports only. Do not try to control the piping system on the loop. No guides, anchors, or limit stops on the loop.
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Width of the pipe rack. Good piping layout for a high temperature line in a pipe rack would consider placing the run of pipe along the outside of the rack. This puts the loading of the line closer to the support columns and then allows the width of the rack to be used for an expansion loop. The back side of the expansion loop can be supported from the opposite side of the rack. Some overhang of the loop beyond the rack is permissible, but too much overhang will require additional supports to carry the pipe loop. Extra supports potentially add extra cost to the project. Hence, the width of the rack may limit the amount of expansion in a loop.
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Narrow pipe rack as in Case 2 below. With a narrow rack the decision of whether more loops should be added or if extra support steel should be added beyond the rack to accommodate a larger loop. An evaluation of cost between extra steel or extra pipe loops should be considered. In general, the cost of the extra loops would normally be greater than the extra pipe support.
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Existing pipe rack. Existing pipe racks need to be evaluated carefully for new guide and axial pipe line loads. Depending on the size of the loops the load on the rack can vary widely. In general, the larger the pipe loop (within reason) the lesser the axial support load will be on the rack.
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Length and type of Pipe Support Shoes. The amount of axial displacement of pipe, particularly just prior to the pipe loop, needs to be considered. For large movements, the design and placement of the shoe must allow for contact between the shoe and the supporting steel. Lines occasionally slug or jump in the rack potentially causing the support shoe to fall off the support steel. This can be devastating and must be avoided at all costs. Extra-long pipe shoes can be made to allow for excessive shoe travel. Caution must used with large travel to avoid pipe shoes from binding with support steel. Rust, weld slag, of burs in the steel can result in a pipe shoe snagging the support steel and potentially pushing the rack steel over.
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Nested lines in the pipe loop. Often several pipe lines are routed in the same pipe loop called, nested lines. Spacing between lines can be a limiting factor in the amount of expansion for any particular line.
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Case 1 – HP Steam Line – large expansion, extra support steel to accommodate the size the of loop.
Case 2 – Main Steam Line – multiple loops, small expansion into loop, existing pipe rack
Recommended Practice:
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Design expansion loops for a moderate amount of expansion to be absorbed. Recommended practice would be 3” to 8” of total axial pipe expansion into the loop. For small expansion (less than 3”), try to absorb the expansion in the natural bends of the piping system. Ie. Anchor the center of the runs allowing growth toward the ends. When very large expansion is present, add pipe loops. Ie. Anchor the ends of the rack forcing growth to the center of the run where loops are placed to absorb the movement.
-
With pipe rack piping, place at least one anchor or axial limit stop support in each straight run of pipe to help control the line from jumping in a start-up or surge scenarios. This will also make the expansion direction more predictable. Additionally, pipe line guides should be placed on every third supports and always just prior to an expansion loop.
-
Use the rack width to accommodate the width of expansion loop. Some overhang is permissible but as mentioned above, too much overhang will require extra supports off the rack. When supporting the pipe loops the supports should be dead weight supports only. Do not try to control the piping system on the loop. No guides, anchors, or limit stops on the loop.
How to calculate Axial line growth:
General equation for expansion
∆L = α * L* ∆T
∆L = Change in length, growth (in inches)
α = 7 x10-6 for Carbon Steel (approximate)
α = 10 x 10-6 for Stainless Steel (approximate)
L = Length of Pipe (in Inches, multiply feet of pipe by 12 to get inches)
∆T = Change in Temperature (Design Temp (°F) – Ambient Temp (usually 70°F)) °F