Steam Main Expansion Between Anchors
A 150-ft straight run of 6-inch Schedule 40 carbon steel steam main is installed between two anchors. The line operates at 50 psig saturated steam, about 298°F, and was erected at 60°F. The run has a single 90° elbow at one end and no expansion loop or expansion joint. The supports are adjustable rod hangers, and the line is insulated. The owner wants to add a branch 20 ft from one anchor and asks whether the existing run can absorb the movement. You calculate the free expansion using a coefficient of 6.5 × 10⁻⁶ in/in·°F. The branch will be tied in with a weld-o-let and a 2-inch line. The anchors are designed for axial load only, and the line has not yet been hydrotested. You must decide whether to add an expansion loop, where to put guides, and how to cold-spring the line if required.
1. Calculate the free expansion of the 150-ft run and state whether the existing straight run can absorb it.
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Convert length to inches: 150 ft × 12 in/ft = 1,800 in. Temperature change = 298°F − 60°F = 238°F. Expansion = 1,800 in × 6.5×10⁻⁶ in/in·°F × 238°F = 2.78 in, about 2.8 in. A straight run anchored at both ends has essentially no flexibility over 150 ft; forcing it to absorb 2.8 in would produce very high axial load and could buckle the line or damage anchors. The elbow at one end helps only if the line is free to move in the perpendicular direction and is guided. With the information given, the run cannot safely absorb the movement; add a loop or an expansion joint, or relayout with changes in direction.
2. Where would you place an expansion loop and guides?
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Put the loop near the middle of the run or at a point that divides the expansion roughly equally, so each anchor sees about half the movement. The loop should be in the same plane as the movement and sized by the flexibility calculation or a standard table. Put guides on both sides of the loop to keep the pipe from buckling sideways and to force movement into the loop; place the first guide close to the loop, typically a few pipe diameters away, and follow the engineering drawing. Do not clamp the pipe at intermediate supports; use sliding or roller supports that allow axial movement. The branch 20 ft from the anchor will move about 20/150 × 2.8 = 0.37 in; the branch connection and the 2-inch line must be flexible enough or have an offset to accept that movement.
3. How would you cold-spring the line, and what limits apply?
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Cold spring means installing the line short by half the calculated expansion and pulling it to the anchor at ambient temperature. Here half of 2.8 in is 1.4 in. The pipe is cut 1.4 in short, the anchors are set, and the joint is pulled with an approved come-along or chain fall until the flange or weld gap closes; then the joint is made and the pull is released. The pull must be applied at a point that does not damage the pipe or anchors, and the amount must be recorded. Cold spring reduces the initial anchor load but does not replace proper flexibility; the loop must still be designed for the full expansion. Do not cold-spring a line that is already restrained or use it to correct a misalignment.