API 1104 Welding: Acceptance Criteria & Inspection

Long-distance pipelines move energy under thousands of pressure cycles per year, and the girth welds that join their joints are where the welding quality of the whole line is decided. API 1104 — the American Petroleum Institute’s “Welding of Pipelines and Related Facilities” — is the standard that defines how those welds must be made, qualified and inspected. Unlike ASME Section IX, which only qualifies procedures and welders, API 1104 also sets the acceptance standards for production welds, and it offers an alternative fracture-mechanics route (Annex A) that lets the company replace the workmanship limits with limits based on imperfection height as well as length. This guide walks through the inspection methods that API 1104 recognises, how the acceptance limits that drive most pass/fail calls in the field are built, and the practical evidence package an inspector should demand before signing a girth-weld report.

API 1104 at a glance: what the pipeline welding standard requires

The short answers, checked against the 22nd edition (July 2021), the current edition on API’s standards list in October 2026, with Errata 1 and Addendum 1 (the addendum published September 2026). Clause numbers below are those of the 22nd edition; a copy of the 21st edition, which US federal pipeline rules still cite, may number some clauses differently.

The rest of this guide expands the scope, the methods, the acceptance criteria, Annex A and repairs, then covers the part API 1104 does not control: what happens while each weld is being made.

Scope: what API 1104 covers and what it does not

API 1104 covers gas and arc welding of butt, fillet and branch welds in carbon and low-alloy steel piping used to compress, pump and transmit crude oil, petroleum products, fuel gases, carbon dioxide and nitrogen, both in new construction and in in-service welding. It is not a pressure vessel code or a plant piping code: those welds fall under ASME Section VIII and ASME B31.3. Special services such as sour service or hydrogen are normally handled by project specifications that add requirements on top of API 1104.

Two acceptance regimes co-exist in the standard:

• Workmanship criteria (Section 9) — empirical limits that give primary importance to imperfection length: individual length, aggregate length in any continuous 12 in. (300 mm) of weld, and width. Used by default and on most cross-country jobs.
• Alternative criteria (Annex A) — fitness-for-purpose limits from fracture mechanics, which weigh imperfection height as well as length. Used only at the company’s option and only when the additional procedure qualification tests, stress analysis and inspection required by Annex A are in place.

A girth weld inspector must know which regime applies before opening a film envelope or AUT scan, because the limits are built on different quantities and are not interchangeable.

Inspection methods recognised by API 1104

Section 9 of API 1104 sets acceptance standards for radiographic testing (RT), magnetic particle testing (MT), liquid penetrant testing (PT) and ultrasonic testing (UT), plus visual and mechanical limits for undercut. Section 11 gives the test procedures, including automated ultrasonic testing (AUT) with phased-array or time-of-flight diffraction techniques. Each method has a defined acceptance scope and qualification path.

Visual examination

Every welder qualification test weld is examined visually first: it must be free of cracks, inadequate penetration and burn-through, present a neat workmanlike appearance, and keep undercut within the limits the standard sets. On production welds, API 1104 gives visual and mechanical depth limits for undercut adjacent to the cover or root bead; when both mechanical and radiographic measurements are available, the mechanical one governs. Alignment of abutting pipe ends (Section 7) caps the offset for pipe of the same nominal wall thickness, and allows a larger offset only when it comes from pipe-end tolerances spread evenly around the circumference.

Radiographic testing (RT)

RT is the historical workhorse for pipeline girth welds and is still the default acceptance medium on many onshore projects. Section 11.1 sets the radiographic procedure, image quality indicators and both film and digital imaging. Acceptance limits in 9.3 are length- and width-driven: for example, on pipe of 2.375 in. (60.3 mm) outside diameter or more, more than four isolated slag inclusions up to 1/8 in. (3 mm) wide in any continuous 12 in. (300 mm) of weld is a defect (9.3.8.2). Porosity is judged on individual pore size against wall thickness and on distribution charts (9.3.9). The 22nd edition dropped the separate cluster-porosity criterion that earlier editions had, and 9.3.9.3 now covers hollow-bead porosity.

Automated ultrasonic testing (AUT)

AUT — phased array, often combined with TOFD — is the usual choice when running ECA per Annex A, because Annex A needs a technique that measures imperfection height with known accuracy. AUT advantages over RT include height sizing of planar flaws (incomplete fusion at the bevel, sidewall lack of fusion), results available during production, and no radiation exclusion zone. Disadvantages: AUT requires a procedure qualified on reference reflectors of known geometry and depth, and operators trained on that procedure.

Magnetic particle and penetrant testing

MT and PT are surface methods used as supplements, typically for branch connections, repairs, and the visible portion of root welds before cap deposition. They are not substitutes for volumetric inspection.

Workmanship acceptance criteria — the table that drives the field

Section 9 of API 1104 contains the workmanship acceptance standards. The table below shows what each limit is measured on, not the limit values: read the values in the edition your contract cites, because some of them changed between editions.

The table follows the radiographic criteria of 9.3; magnetic particle and liquid penetrant indications have their own criteria in 9.4 and 9.5. Ultrasonic indications are classified as linear, transverse or volumetric and judged mainly on length; limits based on imperfection height belong to Annex A.

Annex A and the engineering critical assessment route

Annex A gives alternative acceptance standards for girth welds. The Section 9 limits are empirical and put primary weight on imperfection length; Annex A uses fracture mechanics and fitness-for-purpose criteria that weigh imperfection height as well, and can allow larger imperfections. Its use is entirely at the company’s option, and it never prevents Section 9 from being applied to any weld. The prerequisites are non-trivial:

1. Welding procedure toughness. The welding procedure is qualified with additional fracture-toughness tests — CTOD and Charpy impact tests — on welds representative of production.
2. Stress analysis. The company determines the axial design stresses and strains acting on the girth welds, including the residual stress from welding, and documents them.
3. Height-sizing inspection. Imperfection length, height and depth below the surface must be measured by a technique whose accuracy has been established, with any inaccuracy counted conservatively. Conventional radiography measures length but not height, which is why AUT is the usual partner of Annex A.

Even under Annex A, transverse planar imperfections — those running across the girth weld — must be repaired or removed (A.5.1.6). Annex A tends to pay off where the extra testing and analysis are cheaper than the repairs the workmanship limits would trigger, and where AUT is already on the spread.

Welder and procedure qualification

API 1104 separates welding procedure specification (WPS) qualification (Section 5) from welder qualification (Section 6). A WPS is qualified by welding a test joint and testing specimens cut from it — tensile, nick-break and bend tests — and it stays valid only within the essential variables listed in Table 1; Annex A adds fracture-toughness tests when the alternative acceptance route is used. A welder is qualified by making a test weld with a qualified procedure, which is examined visually and then tested destructively (tensile, nick-break and bend specimens) or, at the company’s option for butt welds, by nondestructive testing — radiography or automated ultrasonic testing (6.6). Repair welders are qualified separately, by destructive testing on a repair made with the repair procedure.

Modern operators tie WPS, PQR, welder qualification certificates, NDT reports and consumable lot traceability into a single digital record per girth weld. Therness’s welding QMS software and digital welding quality records workflows show how an audit-ready package looks for an API 1104 spread. The pipeline monitoring hardware that feeds those records is what makes the capture automatic rather than manual.

Repairs and rework

Repairs are tightly controlled by Section 10. Cracks are repaired only with company authorization, a cracked weld that is not repaired is cut out, and the company decides how other defects are repaired. A qualified repair procedure is required in the cases Section 10 lists — for example when the repair uses a different process or filler metal from the original weld, and for every repair in a previously repaired area (a double repair). Repair procedures are qualified by repair type: full-thickness, internal or external partial-thickness, cover-pass and back-weld repairs, within the essential variables of Tables 1 and 9 (10.3.4). The repaired area is inspected again and must meet the acceptance standards of Section 9.

What an audit-ready acceptance package looks like

Beyond the welds themselves, an API 1104 audit asks for an evidence trail per girth weld. A clean package contains:

• WPS and PQR covering the joint geometry, position, consumable and heat input range used.
• Welder qualification record valid for the position and direction of progression.
• Consumable certificates with lot traceability to the actual rod or wire used on the weld.
• Preheat and interpass temperature record (see ISO 13916 monitoring for the equivalent ISO method).
• NDT report (RT or AUT) with operator qualification certificates, calibration record and the acceptance regime applied (Section 9 vs Annex A).
• For Annex A: the ECA report, the AUT procedure qualification record and the project specification clause authorising Annex A use.

Organisations running large pipeline programs typically replace paper folders with a digital NCR and traceability layer. See the welding NCR management flow for how an NCR raised on a girth weld feeds the repair workflow without losing the inspector–welder–procedure linkage.

How real-time monitoring complements API 1104

API 1104 is an acceptance standard, not a process-control standard. It tells the inspector what an acceptable weld looks like after it has been made; it does not, by itself, prevent a girth weld from being deposited outside its qualified parameter window. A real-time weld monitoring system tracking arc current, voltage, travel speed, heat input and interpass temperature closes that loop: girth welds outside the qualified envelope can be flagged before NDT, root cause is captured at the time of deviation, and AUT scan time is concentrated on the welds that need it. A welding camera system extends that same loop to the weld itself: thermal imaging reads heat distribution through the arc while a high-speed visual camera resolves the molten pool, so undercut, burn-through and incomplete fusion are flagged and logged the moment they form on the bevel, well ahead of the AUT scan. On a spread where every repair means excavation, rewelding and a second inspection, finding the suspect weld while the crew is still on it is where monitoring pays back — the thermal-imaging weld comparison below shows how heat distribution alone separates a sound weld from a suspect one.

Monitoring during deposition and NDT after it are separate layers. Where a spread needs a volumetric look at tie-in welds without a radiation exclusion zone — a real constraint on live rights-of-way — an excitation-based thermal inspection route is worth evaluating as a complement to RT and AUT.

For the high-speed visual channel described above, see the PoolDrop 480 fps weld pool camera. To check whether a camera setup fits your spread and bevel access, request a welding camera system demo for girth welds.