API 1104 is the welding standard for pipelines and related facilities in the petroleum and natural gas industry. Published by the American Petroleum Institute, it covers butt welds, fillet welds, and socket welds on carbon and low-alloy steel pipelines used for transporting oil, gas, and other products. If you’re welding cross-country pipeline, gathering lines, or pipeline tie-ins, this is the code you work under.
The standard governs procedure qualification, welder qualification, inspection, testing, and acceptance criteria. It’s a self-contained code, unlike ASME Section IX, which requires a referencing code for acceptance criteria. API 1104 gives you the complete package for pipeline welding.
Scope and Application
API 1104 applies to:
- Transmission pipelines (cross-country lines carrying oil, gas, or other products)
- Gathering and distribution lines
- Compressor stations, pump stations, and metering facilities
- Pipeline tie-ins and repairs
- Both new construction and in-service welding
It does NOT apply to:
- Process piping within refineries or chemical plants (ASME B31.3)
- Power piping (ASME B31.1)
- Structural steel (AWS D1.1)
- Non-ferrous metals
The pipe sizes covered range from NPS 2 (2-3/8 inch OD) through large-diameter transmission pipe (48 inch and larger). Wall thicknesses from standard weight through extra-heavy are covered, with specific requirements changing based on wall thickness.
Procedure Qualification (Section 5)
Before any production welding begins, the welding procedure must be qualified by testing. Unlike AWS D1.1, there are no prequalified procedures in API 1104. Every procedure gets tested.
Essential Variables
A change in any essential variable requires a new procedure qualification. API 1104’s essential variables include:
| Variable | What Constitutes a Change |
|---|---|
| Welding process | Any change (SMAW to GMAW, adding GTAW root, etc.) |
| Base material | Change in specified minimum yield strength group |
| Joint design | Major changes in groove angle, root opening, or root face |
| Wall thickness | Outside the qualified range |
| Pipe diameter | Outside the qualified diameter group |
| Filler metal | Change in classification or going from one AWS classification group to another |
| Electrical characteristics | Change from AC to DC, or polarity change |
| Position | Roll welding to fixed position (or vice versa) |
| Shielding gas | Change in gas type or mixture beyond specified tolerances |
| Direction of welding | Uphill to downhill (or vice versa) for vertical welding |
| Preheat | Decrease in minimum preheat temperature |
| PWHT | Addition, deletion, or change in temperature/time |
Note that direction of welding (uphill vs. downhill) is an essential variable in API 1104. This is specific to pipeline work, where downhill progression with cellulose electrodes (E6010/E8010) is common for root and hot passes, while uphill with low-hydrogen electrodes (E7018/E8018) is used for fill and cap on some procedures.
Destructive Testing for Procedure Qualification
API 1104 requires these destructive tests on procedure qualification coupons:
Tensile Tests: The weld joint must meet or exceed the minimum specified tensile strength of the pipe material. Specimens are pulled to failure, and the break location and tensile value are recorded.
Nick Break Tests: This is unique to API 1104. A notched specimen is broken by impact, and the exposed fracture surface is examined for defects. The fracture surface must be free of incomplete fusion, slag inclusions wider than 1/32 inch, and gas pockets larger than 1/16 inch. Nick break testing is a fast, practical test that reveals internal defects.
Bend Tests: Root bend and face bend specimens are bent 180 degrees around a mandrel. The convex surface of the bent specimen is examined for cracks and openings. No crack or opening exceeding 1/8 inch in any direction is acceptable, with exceptions for corner cracks.
| Pipe OD | Tensile Specimens | Nick Break Specimens | Root Bend | Face Bend |
|---|---|---|---|---|
| 2-3/8 to 4-1/2 in | 2 | 2 | 2 | 2 |
| 4-1/2 to 12-3/4 in | 2 | 2 | 2 | 2 |
| Over 12-3/4 in | 4 | 4 | 4 | 4 |
For wall thicknesses over 1/2 inch, side bend specimens replace root and face bends.
Welder Qualification (Section 6)
Each welder must demonstrate ability to make sound welds using a qualified procedure. The qualification test mirrors production conditions as closely as practical.
Test Requirements
The welder makes a complete circumferential butt weld on a pipe test coupon in the position(s) they’ll use in production. The coupon is then tested by one of these methods:
- Destructive testing: Nick break tests and bend tests (same criteria as procedure qualification)
- Radiography: The entire circumference is radiographed and must meet Section 9 acceptance criteria
- A combination: Some companies use RT for the initial qualification and destructive testing for specific positions
Position Groups
Pipeline welding positions differ from structural positions. API 1104 uses:
- Roll welding: Pipe rotated, welding always in flat or near-flat position
- Fixed position: Pipe doesn’t rotate, welder moves around the pipe
- Fixed position at 45 degrees: The 6G equivalent, qualifies for all positions
A welder qualified on a fixed-position test is qualified for both fixed and roll welding. A welder qualified only on roll welding cannot weld fixed position.
Single Qualification vs. Multiple Qualification
A welder can be qualified for one procedure or multiple procedures. Each procedure that differs in essential variables requires a separate qualification test. However, once qualified on a specific procedure, the welder doesn’t need to retest unless:
- They fail to weld with the process for 6 months (continuity requirement)
- The company has reason to question their ability
- They move to a new employer (employer acceptance required)
Non-Destructive Testing (Section 8 and 9)
API 1104 provides detailed requirements and acceptance criteria for NDT methods:
Radiographic Testing (RT)
Radiography is the primary NDT method for pipeline welds. API 1104 Section 9 provides acceptance criteria with specific limits for:
| Defect Type | Acceptance Criteria Summary |
|---|---|
| Inadequate penetration without high-low (IP) | Max 1 inch length, aggregate max 1 inch in 12 inches of weld |
| Inadequate penetration with high-low (IPD) | Max 2 inches, aggregate max 3 inches in 12 inches |
| Incomplete fusion (IF) | Max 1 inch length, aggregate max 1 inch in 12 inches |
| Incomplete fusion due to cold lap (IFD) | Max 2 inches, aggregate max 2 inches in 12 inches |
| Internal concavity (IC) | Total length not more than 1/2 the circumference |
| Burn-through (BT) | Max 1/4 inch diameter, max one per weld |
| Slag inclusions | Elongated: max 2 inches, isolated: max 1/2 inch width |
| Porosity | Individual pore max 1/8 inch, distribution density limits per figure |
| Cracks | Not acceptable, any size |
API 1104 also provides an alternative acceptance standard (Appendix A) based on fracture mechanics. This approach uses fitness-for-service criteria and may accept defects that would be rejected under standard criteria, if engineering analysis demonstrates the defects won’t grow to critical size during the design life.
Ultrasonic Testing (UT)
Automated ultrasonic testing (AUT) has become common on large-diameter pipeline projects. API 1104 includes provisions for AUT, which can replace or supplement radiography. AUT is faster than RT on large pipe and doesn’t create radiation safety zones.
Other NDT Methods
Magnetic particle testing (MT) and liquid penetrant testing (PT) are used for surface-breaking defects, particularly on the root and cap passes. They don’t replace RT or UT for volumetric examination but supplement them.
How API 1104 Differs from D1.1 and ASME IX
API 1104 vs. AWS D1.1
| Feature | API 1104 | AWS D1.1 |
|---|---|---|
| Application | Pipelines | Structural steel |
| Prequalified procedures | None | Yes (Clause 3) |
| Nick break test | Required | Not used |
| Acceptance criteria source | Self-contained | Self-contained |
| Welder qualification | Pipe-based | Plate or pipe |
| Direction of welding | Essential variable | Not an essential variable |
| Fitness-for-service option | Yes (Appendix A) | No |
| In-service welding | Covered (Appendix B) | Not applicable |
API 1104 vs. ASME Section IX
| Feature | API 1104 | ASME Section IX |
|---|---|---|
| Application | Pipelines | Pressure vessels, piping (via referencing code) |
| Acceptance criteria | Self-contained | From referencing code (VIII, B31.3) |
| Material grouping | Yield strength groups | P-numbers and group numbers |
| Nick break test | Required | Not required |
| P-number system | No | Yes |
| Welder qual transferability | Employer-specific | Employer-specific |
| Fitness-for-service | Appendix A | Not addressed (separate FFS standards exist) |
In-Service Welding (Appendix B)
One feature unique to API 1104 is its coverage of in-service welding on live pipelines. Appendix B addresses welding on pipelines that are operating under pressure, which introduces two major concerns:
Burn-through risk: Welding on thin-wall pipe under pressure can melt through the wall. Appendix B requires minimum wall thickness calculations and limits heat input to prevent this. Generally, pipe with wall thickness under 1/4 inch requires special engineering analysis before in-service welding.
Hydrogen cracking: The flowing product acts as a heat sink, causing rapid cooling in the weld zone. This creates hard microstructures susceptible to hydrogen cracking. In-service procedures typically require low-hydrogen electrodes, controlled heat input, and sometimes specific bead placement sequences (temper bead technique).
Qualifying an in-service procedure requires welding test coupons under simulated flow conditions that match the production environment, including flow rate and product temperature.
Common Pipeline Welding Procedures Under API 1104
Downhill SMAW (Traditional Method)
The classic pipeline procedure uses downhill progression:
- Root pass: E6010 or E8010 with cellulose coating, run downhill from 12 o’clock to 6 o’clock on each side
- Hot pass: Same electrode, run immediately after the root to refine and temper the root deposit
- Fill passes: E7018 or E8018 low-hydrogen electrode, run uphill or downhill depending on the procedure
- Cap pass: Same as fill, with a wider weave for full coverage
Combination Procedures (GTAW + SMAW)
Many modern pipeline specifications call for a GTAW root with SMAW fill and cap:
- Root pass: GTAW (TIG) for a high-quality root with full penetration
- Hot pass: SMAW with E6010 or SMAW with E8010
- Fill and cap: SMAW with E7018/E8018
This combination produces a better root than straight SMAW and is common on high-pressure transmission lines and sour service pipelines.
Mechanized GMAW
Large-diameter pipeline projects increasingly use mechanized (automatic) GMAW with bug-and-band systems. These produce consistent welds at high travel speeds, but they require their own procedure qualification and trained operators. The welding “operator” rather than “welder” distinction applies to mechanized welding.
Getting API 1104 Qualified
If you want to work pipeline, here’s the practical path:
- Learn to weld pipe in all positions, particularly downhill SMAW with 6010 for root and hot pass, 7018 for fill and cap
- Get hired by a pipeline contractor or attend a pipeline welding school that does API 1104 qualification testing
- Pass the qualification test on the contractor’s specific procedure. You’ll weld a complete joint on 6-inch or larger pipe in the fixed position
- Maintain your qualification by welding pipeline continuously. The 6-month continuity rule applies
Pipeline welding pays well because the qualification is demanding and the work is physically hard. Expect the test to include a full circumferential butt weld, typically on 6- or 8-inch schedule 80 or schedule 120 pipe, in the 6G (45-degree fixed) position. Every inch of that weld gets inspected, usually by X-ray. There’s no hiding defects.
API 1104 is revised periodically, with the most recent editions incorporating provisions for newer technologies like mechanized welding and AUT. Always confirm you’re working to the edition specified in the project contract, because acceptance criteria and essential variables can change between editions.