Restrained Pipe Run After a Guide Failure
A 6.0 m long, 316L stainless steel pipe (OD 114.3 mm, wall 6.02 mm) carries water at 180 °C and was installed at 20 °C. Its design includes an expansion loop, but a walkdown finds that the loop guide has failed and a temporary clamp has locked one end, so the run is effectively restrained between two anchors. The pipe is being heated for a test and is at 120 °C and rising. A paint crack and a 3 mm gap at one anchor bolt are visible; system pressure is normal. Material data: alpha = 16.5 × 10^-6 /K, E = 193 GPa, yield strength at 180 °C = 205 MPa. The operator wants to continue to 180 °C because pressure and flow are within limits. The test procedure has no hold point for support inspection. A junior engineer suggests that the expansion loop will take up the movement even though the guide is failed.
1. Calculate the free thermal expansion from 20 °C to 180 °C and the fully restrained thermal stress. Compare the stress with yield strength.
Read the worked answer
Free expansion: ΔL = alpha × L × ΔT = 16.5×10^-6 × 6.0 m × 160 K = 0.01584 m = 15.8 mm. If fully restrained, thermal stress = E × alpha × ΔT = 193×10^9 × 16.5×10^-6 × 160 = 509×10^6 Pa = 509 MPa. At 120 °C, ΔT = 100 K, so stress = 318 MPa. Both values exceed the 180 °C yield strength of 205 MPa. The pipe cannot remain fully elastic; it will yield, buckle, or load the anchors. The calculation is an upper bound because real supports allow some movement, but the margin is not small.
2. Explain why the observed anchor gap matters and whether continuing to 180 °C is acceptable.
Read the worked answer
The 3 mm gap and paint crack indicate that the anchor or pipe has already moved or yielded; the run is not behaving as the design assumed. The expansion loop cannot take up movement if its guide has failed and a clamp locks one end. Continuing to 180 °C would increase the thermal load and could cause a sudden release of stored strain energy, anchor failure, or pipe deformation. Pressure and flow being normal do not demonstrate structural adequacy; they are not the governing limit. The operator should hold at the current temperature or cool down, not proceed to 180 °C.
3. Recommend immediate actions and procedure changes.
Read the worked answer
Stop heating and reduce temperature/pressure under the test procedure; barricade the area; inspect supports, anchors, and the expansion loop; perform NDT or dimensional checks on the cracked paint area and anchor; restore the guide and remove the temporary clamp only under an approved procedure. A qualified piping engineer should assess whether the run can be heated with the current support configuration. Procedure changes should add a hold point for support and guide verification before heat-up, require walkdown after maintenance, and prohibit temporary restraints without engineering approval.