Chilled-Water Thermal Expansion and Anchor Failure
A 300-ft run of 8-inch Schedule 40 chilled-water pipe is installed in a tunnel between a central plant and a terminal building. The pipe operates between 40°F and 95°F. The installation was made at 60°F. The contractor installed two anchors at each end and used 8-inch V-groove couplings every 20 ft, but no expansion loops or expansion joints. After six months, the pipe has pulled out of a coupling near the middle, and the anchors show cracked welds. The pipe is supported on rollers every 12 ft. The engineer asks you to calculate the thermal movement and recommend a repair. The coefficient of linear expansion for carbon steel is 6.5 × 10⁻⁶ in/in/°F.
1. Calculate the thermal expansion and contraction of the 300-ft run and the restrained force if the anchors are rigid.
Read the worked answer
Use ΔL = L × α × ΔT. For expansion from the 60°F installation temperature to the 95°F maximum: ΔT = 35°F. L = 300 ft × 12 in/ft = 3,600 in. ΔL = 3,600 × 6.5×10⁻⁶ × 35 = 0.819 in. For contraction from 60°F to 40°F: ΔT = 20°F, ΔL = 3,600 × 6.5×10⁻⁶ × 20 = 0.468 in. Total travel range is about 1.29 in. If fully restrained, force F = E × A × α × ΔT. For 8-in Schedule 40, metal area A is about 8.4 in² and E is about 29×10⁶ psi. For 35°F: F = 29×10⁶ × 8.4 × 6.5×10⁻⁶ × 35 ≈ 55,000 lb, or 55 kips. For 20°F contraction: F ≈ 31,700 lb. These forces are far beyond the capacity of the anchors and couplings as installed.
2. Evaluate the installed support and anchor arrangement. What caused the coupling pullout and anchor cracking?
Read the worked answer
The installation has no expansion loops or joints, so the approximately 1.29 in of movement is restrained by the end anchors. The rollers support weight but do not guide lateral movement, and V-groove couplings are not expansion joints. The resulting thrust cracked the anchor welds and pulled the pipe out of a coupling. The pipe likely buckled or walked until a joint separated. The problem is a design and installation error: thermal movement was not accommodated, the anchors were not designed for 55 kips, and the pipe was not guided to force movement into a loop or joint.
3. Recommend a repair that will accommodate the movement and specify anchor and guide requirements.
Read the worked answer
Cut in two expansion loops, or approved bellows expansion joints, at calculated locations, typically midway between anchors, sized for the 1.29-in total travel. Replace the cracked anchors with anchors designed for the calculated thrust. Install pipe guides on each side of the loops, about four diameters from the loop and then every 10 to 20 ft, to prevent buckling. Keep rollers for weight support but add guides. If bellows joints are used, install them between two anchors with proper guides and set the cold position. After repair, recheck alignment and allow the line to expand freely; do not lock the couplings. Consider cold spring only if specified by the engineer.