Why is a welded grease duct bowing when it heats up?

Hot welded duct can move. If supports are locking it rigid with no movement path, stop and get an approved support/expansion detail before hangers, welds, or structure get damaged.

['Grease duct near heavy kitchen equipment can expand as temperature rises. If the run is locked tight with rigid clamps or hard anchors in the wrong places, the metal may bow, push, crack supports, or stress welds.', 'This is not a casual apprentice adjustment. Check the engineered support layout, expansion provisions, vertical risers, shaft constraints, fire wrap clearance, and hanger type. Likely recovery involves sliding supports, expansion joint/detail, or revised anchoring by the foreman/detailer/engineer.']

Stop if

• Use this as training guidance. The foreman, approved drawings, project specs, manufacturer installation instructions, employer safety policy, and AHJ/code requirements always control the final answer.

Check

• Thermal movement can damage supports, welds, fire wrap, penetrations, and nearby structure if not allowed for correctly.

Steps

• Confirm where the duct is bowing, binding, rubbing, or cracking hangers/supports.
• Check if the run has rigid clamps at multiple points with no movement path.
• Look at hangers, shaft penetrations, wrap, cleanouts, and nearby combustibles/structure.
• Verify the support/expansion detail in the approved drawings and submittals.
• Stop startup/cover if movement is damaging supports or rated/fire-wrap conditions.

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