Does API 650 require WPS qualification, or can we use prequalified procedures?

API 650 does not have a prequalified WPS concept. All welding on the tank shell, floor, roof, and nozzles must be performed to a qualified WPS supported by a PQR. API 650 Appendix I sets the qualification requirements, but it also accepts ASME Section IX qualification as an alternative.

Can an AWS D1.1 WPS cover API 650 tank shell welds?

No. AWS D1.1 governs structural steel connections and is not recognized by API 650 as an acceptable qualification standard. You need separate WPS/PQR records qualified under API 650 Appendix I or ASME Section IX for any weld that API 650 governs.

What welds on a tank project fall under AWS D1.1 rather than API 650?

Structural steel welds that support the tank but are not part of the tank pressure boundary typically fall under AWS D1.1: skirt supports, anchor bolts and base plates, structural gussets, stairway and platform framing, and pipe support brackets welded to structural steel. The tank shell, floor plates, roof, and nozzle reinforcement pads are API 650 scope.

Does API 650 require the same NDE methods as AWS D1.1?

API 650 has its own NDE requirements, independent of AWS D1.1. Shell joint radiographic testing requirements depend on the course thickness and shell weld joint efficiency. API 650 also addresses MT and PT for specific components. Shell RT acceptance criteria are defined within the API 650 standard, not in AWS D1.1.

API 650 Tank Welding vs. AWS D1.1: Which Code Applies

Fab shops building storage tanks alongside structural steel often confuse which welding code governs which welds. This guide clarifies when API 650 applies, when AWS D1.1 governs, and how to manage WPS requirements when both codes are in scope.

A structural fabricator who has just won a bid to build a carbon steel storage tank for a petroleum terminal often assumes the existing AWS D1.1 WPS library can cover the work. After all, the base material is A36 or similar, the filler metal is E7018, and the shop knows how to make groove welds. The assumption is understandable—and wrong. API 650 operates on its own qualification framework, the NDE requirements differ, and mixing up which code governs which component on the same project creates audit problems and can invalidate welds.

This article clarifies the boundary between API 650 and AWS D1.1 on a tank project, what qualifications each code requires, and how a fab shop can run both programs without gaps.

What API 650 Covers

API 650, Welded Tanks for Oil Storage, governs the design, fabrication, erection, and inspection of above-ground, vertical, cylindrical, closed- and open-top welded steel tanks. The scope covers the pressure boundary: shell courses, annular floor plates, floor plates, fixed and floating roof structures, roof-to-shell connections, nozzles, manways, and reinforcement pads. Shell seam welds, floor-to-shell junction welds, and nozzle attachment welds are all under API 650 jurisdiction.

API 650 is not just a structural welding code with different paperwork. It addresses:

Weld joint efficiency factors that affect the allowable stress design of shell courses
Hydrotest requirements after erection
NDE requirements scaled to shell thickness and design category
Material requirements for low-temperature service (Appendix A) and high-strength steel (Appendix S)

What AWS D1.1 Covers on the Same Project

Structural steel associated with the tank but outside the tank pressure boundary is typically governed by the applicable structural code—most commonly AWS D1.1 where the contract or EOR specifies it. This includes:

Structural support skirts (free-standing tanks on reinforced concrete pads typically have no skirt, but elevated tanks and vessels may have structural steel support frames)
Stairways and platforms welded to the tank structure or freestanding alongside it
Pipe rack structural steel serving the tank farm
Anchor bolt assemblies and base plates welded to structural steel anchorage
Mechanical equipment support frames (pump bases, compressor skids) nearby

If the contract designates AWS D1.1 for structural steel, those welds require a D1.1-compliant WPS backed by a valid PQR—or use of prequalified procedures per Clause 5. None of those D1.1 qualifications carry over to the API 650 tank proper.

In some contracts, especially for smaller storage tanks in non-petroleum service, the owner or engineer may specify AWS D1.1 for all welding on the project, effectively choosing a structural welding qualification framework for the tank. This is rare for production API 650 tanks, but it does occur. The specification controls—know what your contract says before starting WPS development.

WPS Qualification Under API 650

API 650 Section 7 covers fabrication requirements. For welding procedure qualification, API 650 Appendix I is the vehicle. Appendix I has two paths:

Path 1 — API 650 Appendix I direct qualification: The fabricator writes a WPS, welds test assemblies, and conducts mechanical tests in accordance with Appendix I. The test requirements are similar in structure to ASME IX: tensile tests, guided bend tests, and where applicable, impact tests.

Path 2 — ASME Section IX qualification accepted: API 650 explicitly accepts WPS/PQR records qualified to ASME Section IX as an alternative to Appendix I qualification. This is a common path because many fab shops that build both pressure vessels and tanks already maintain an ASME IX program. A single ASME IX PQR can support both ASME pressure vessel work and API 650 tank work if the essential variable coverage is sufficient.

What is NOT accepted: AWS D1.1 qualification is not listed as an acceptable alternative in API 650. A WPS backed only by a D1.1 PQR is not qualified for API 650 shell, floor, or roof welds.

Key Essential Variable Differences

For fabricators transitioning from a D1.1 program to API 650, the most important practical difference is the qualification system:

No prequalified WPS in API 650. Every weld must be supported by a tested procedure. There is no equivalent to AWS D1.1 Clause 5. Even a simple E7018 fillet weld on a thin shell plate requires a tested WPS.

Filler metal classification is an essential variable. As with AWS D1.1, changing the AWS filler metal classification or group typically requires requalification.

Joint design category. API 650 distinguishes between butt joint types and specifies which are acceptable for which shell joint configurations. The allowed groove geometries for shell seam welds are more restricted than the full prequalified joint library in AWS D1.1.

Position. Like AWS D1.1, welding position affects qualification range. A tank builder typically qualifies in the 1G (flat) and 2G (horizontal) positions because tank shell seam welds are typically horizontal and floor welds are flat. Overhead and vertical are less common but require separate qualification coverage if they appear on the project.

Impact testing. For tanks designed to API 650 Appendix A for low-temperature service, the WPS must include CVN impact testing as part of the PQR. The required impact test temperature and minimum energy values are defined by Appendix A. This parallels the supplementary essential variable framework in AWS D1.1 Table 6.8 for CVN—but the temperature values and acceptance criteria differ.

NDE Under API 650 vs. AWS D1.1

AWS D1.1 NDE requirements (RT, UT, MT, PT) are defined in Clause 8 based on joint type, thickness, and whether the structure is statically or cyclically loaded. API 650 NDE requirements are entirely separate and defined in API 650 Section 8.

Shell seam radiography under API 650 depends on the weld joint efficiency used in design. A joint efficiency of 1.0 (full radiography) requires radiographic examination of all vertical seam welds and a defined percentage of horizontal seam welds in each shell course. A lower joint efficiency allows reduced examination with a corresponding design penalty on allowable stress. Most new tanks are designed with full joint efficiency to maximize the wall thickness benefit, so full RT on vertical seams is typical.

Floor plate welds receive vacuum box testing for leaks rather than RT or UT. This is a straightforward leak test method that has no equivalent in AWS D1.1.

Nozzle and fitting welds receive MT or PT depending on the design category and nozzle wall thickness.

API 650 NDE acceptance criteria are defined within the standard—they are not interchangeable with AWS D1.1 Table 8.1 or Table 8.2 acceptance criteria even if the same inspection method is used.

Welder Qualification

Tank fabricators must qualify welders under the same framework as the WPS: API 650 Appendix I or ASME Section IX performance qualification. Welder performance qualifications from an AWS D1.1 program do not satisfy API 650 requirements.

If the shop uses ASME Section IX to qualify its tank welders, those qualifications cover both pressure vessel and API 650 work—a significant administrative efficiency for a shop running both programs.

Continuity requirements: API 650 Appendix I does not contain an explicit continuity period identical to AWS D1.1 Clause 6.4.1. However, many shop quality programs impose a similar continuity requirement (typically six months) by procedure, and owner specifications for refinery and terminal tank work frequently mandate it. ASME IX QW-322 has its own six-month continuity period. Understand which document governs welder continuity for the contract you are running.

Running Both Programs in One Shop

The most common administrative error in a dual-code shop is using D1.1 WPS numbers on API 650 work orders or vice versa. Prevent this with clear WPS identification:

Prefix D1.1 WPS identifiers with a clear code indicator (e.g., D1.1-SMAW-001)
Prefix API 650 / ASME IX WPS identifiers separately (e.g., API650-SMAW-001 or IX-SMAW-001)
Ensure your welding traveler clearly references the governing code and the specific WPS applicable to each weld category on the job

A CWI overseeing both structural steel and tank work on the same project needs to verify that the correct WPS is assigned to each weld—structural steel frame gets the D1.1 WPS, shell seam gets the API 650 WPS. Mixing them does not create a compliant weld under either code.

For shops managing a growing WPS library across multiple codes and projects, see our comparison of manual spreadsheet WPS tracking vs. purpose-built software. To review how AWS D1.1 governs structural work connected to tank structures, see our guide on WPS documentation for a structural welding program audit.

Rule library based on AWS D1.1:2025; verify against your governing edition. API 650 and ASME Section IX requirements are independent standards — consult the current edition of each for your specific project. This article does not constitute engineering or legal advice.

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