4.3 Tack Welding, Line-Up Clamps & Weather Restrictions

4.3 Tack Welding, Line-Up Clamps & Weather Restrictions

Field execution of pipeline girth welds depends on disciplined control of fit-up retention, pass timing, environment, and electrical grounding. Section 7 of API 1104:2021 sets the operational rules; the qualified WPS carries the numeric values.

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1. Tack Welds and Hinge Tacks

Tack welds hold root alignment and gap until the root bead replaces them. API 1104's direct rules are short but strict:

• Hinge tacks are prohibited as a means of forcing the root gap (7.2): hinging the pipe joint with tacks to achieve proper root gap is not permitted, and hammering to obtain lineup is kept to a minimum.
• Tack welds that will be incorporated into the joint are made under the qualified WPS by qualified welders; standard field practice is to grind tack ends to a feather edge so the root pass fuses through them without lack of fusion, and to fully remove cracked or oxidized tacks before welding.

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2. Line-Up Clamp Release (7.3 + 5.3.2.11)

The standard's clamp rule has two parts:

1. Use and removal of lineup clamps shall be in accordance with the WPS (7.3). The WPS states whether the clamp is internal or external — or that no clamp is required — and specifies the minimum percentage of the root bead that must be completed before the clamp is released (5.3.2.11).
2. When a clamp is removed before the root bead is complete, the completed part of the bead must be in approximately equal segments spaced approximately equally around the circumference (7.3).

Exam trap: API 1104 prints no fixed release percentage. Numbers like "100% for internal, 50% for external clamps" come from individual company specifications and WPS values, not from the standard. Pipe movement, lowering-in, or sideboom release before the joint has adequate strength is prohibited by good practice and company rules for the same reason: an incomplete, hot root bead cracks under bending load.

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3. Time Between Passes (5.3.2.10)

When EXX10 or EXX11 cellulosic electrodes are used, the WPS must specify the maximum time from completion of the root bead to the start of the second (hot) pass, and from the second to the third pass. Cellulosic root beads carry high diffusible hydrogen and heavy restraint; a promptly deposited hot pass tempers the root bead and drives hydrogen out. Mainline crews therefore treat the root-to-hot-pass interval as a hard production parameter — if a joint sits too long, it must be brought back to at least the minimum preheat temperature before the hot pass.

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4. Weather Conditions (7.5)

Welding shall not be done when the quality of the completed weld would be impaired by prevailing weather — the standard lists low temperatures, airborne moisture, blowing sands, and high winds. Key points:

• Windshields or temporary shelters may be used when practical.
• The company decides whether conditions are suitable for welding.
• The standard sets no numeric wind-speed limits; as a practical matter, gas-shielded processes (GMAW/FCAW-G/GTAW) lose shielding in light wind and always need shelter outdoors, while SMAW tolerates more. Rain, snow, or standing moisture on the joint stops work until the joint is dried and returned to preheat temperature.

Section 7.6 adds a workspace rule: above-ground welding should have at least 16 in. (400 mm) of clearance around the pipe, and trench bell holes must give welders ready joint access. Section 7.7 requires scale and slag removal between beads (power tools when the WPS calls for them) and grinding out surface porosity clusters, bead starts, and high points before depositing over them with semiautomatic or mechanized processes.

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5. Grounding and Arc Burns

An arc burn is localized melting of the pipe surface from accidental arcing — a dragging electrode, a loose ground clamp, or a tack-welded ground bar. The surrounding cold steel quenches the spot into brittle untempered martensite with micro-cracks: a fracture initiation point on a pressurized line.

• Ground clamps attach directly to the pipe with positive mechanical contact; wrapping loose cable or tack-welding ground bars to the pipe is prohibited practice.
• Suspected arc burns are ground to sound metal and verified by MT or PT.
• Unrepaired arc burns may be left only where an Annex A engineering critical assessment accepts them — Table A.6 gives acceptable limits for unrepaired arc burns. An arc burn containing a crack visible to the eye or on a radiograph is outside that allowance.