According to ASME B31.3, Process Piping, root gap and hi-lo (internal misalignment) are critical parameters during tack welding and fit-up of piping spools. The standard specifies tolerances: for wall thickness up to 5 mm, hi-lo must not exceed 1.5 mm; for thicker walls, the limit is 25% of the thinner wall thickness.
Excessive hi-lo creates stress concentrations at the weld root, increasing the risk of lack-of-fusion and root cracking. During hydrostatic testing, these defects may propagate, leading to leaks or failure. Auditors and inspectors routinely check fit-up dimensions before welding is allowed to proceed.
Common fit-up issues include:
• Root gap too wide, causing excessive shrinkage and distortion.
• Root gap too narrow, preventing full penetration.
• Hi-lo misalignment from ovality or out-of-round pipes.
To control these, shops use internal alignment clamps, line-up tools, and careful measurement with calibrated gap gauges. Tack welds must be strong enough to hold alignment but not so large that they interfere with the final weld.
What methods do you use to ensure consistent root gap and alignment on large-diameter spools?
Source: ASME B31.3, 2022