Why would spiral duct bulge or unzip at the lockseam under pressure?

Bulging or unzipping spiral pipe means the pipe/joint is not handling the pressure or system condition. Stop the test/startup and verify gauge, pressure class, joint, and replacement/reinforcement options.

['Spiral pipe is strong when it is the right material, gauge, diameter, seam, and pressure class for the system. If the lockseam starts bulging or opening, that is not a normal leak; it is a mechanical failure warning.', 'The field recovery is usually replacement with correctly rated factory pipe or an approved structural repair sleeve/detail if allowed. Also check whether a damper, cap, or control issue caused an abnormal pressure spike.']

Round, Spiral & Oval Duct 4th Year YELLOW

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

A failing spiral seam can dump air, fail testing, damage nearby work, or become unsafe if the pressure condition continues.

Steps

Stop or secure the system/test if the seam is actively opening.
Confirm duct diameter, pressure class, pipe type, gauge/material, and shop markings if available.
Check if pressure spiked due to closed dampers, blocked outlets, VFD ramp, or wrong test setup.
Inspect hangers and joints near the bulged section.
Ask before adding any sleeve or clamp as a permanent fix.