Introduction
An integral type flange is defined by how the flange, the pipe, the nozzle, or the vessel neck behave together as one structural unit under load. Round shape and bolt pattern alone do not make a flange integral.
Many engineers and buyers treat "integral" as shorthand for "welded" and move on. That shortcut can cause real problems. Getting the classification, pressure rating, material, facing, or bore wrong affects structural integrity, sealing performance, code compliance, and project timelines.
This article breaks down what an integral type flange actually means under ASME rules. You will see how it differs from loose and optional designs, how it relates to weld neck flanges, and what to check before you specify or order one.
Key Takeaways
- Flange and connected component share loads as one unit, not as separate parts
- "Integral type" is a structural design classification, not a synonym for every weld neck flange
- Check material, temperature, facing, bore, gasket, bolting, and governing code together before selecting
- Verify both ASME dimensional standards and project-specific design codes before purchase
What Is an ASME Integral Type Flange?
Under ASME pressure-vessel design rules, a flange connection is evaluated as one of three structural categories: integral, loose, or optional. This classification comes from ASME BPVC Section VIII, Division 1, Mandatory Appendix 2, which governs how bolted flange connections are calculated for pressure vessels.
Here's the practical distinction:
- Integral type: The flange and the adjoining neck, shell, or pipe wall are treated as one structural unit. The attachment must transfer load effectively between the flange and the pressure boundary.
- Loose type: The flange carries the joint load without relying on the attached component as part of the flange section. Slip-on and lap joint arrangements typically fall here.
- Optional type: The attachment sits between the two extremes and may be evaluated using either calculation route when the applicable code requirements are met.
Integral is an Appendix 2 design classification, separate from the product name on a drawing. A drawing might specify "weld neck flange" as the physical part, while a separate calculation determines whether that connection qualifies as integral. The two labels answer different questions.
Why Structural Continuity Matters
Pressure equipment sees more than static internal pressure. It sees bending moments, bolt preload, thermal expansion, and external piping loads. An integral connection gives that load path continuity, which matters most in high-pressure or high-temperature service where a weak transition point becomes the failure point.
ASME research confirms that Section VIII, Division 1 uses distinct geometric factors for integral versus loose flanges, while Division 2 applies a different equation set entirely (ASME PVP research on flange design factors). The math isn't interchangeable, and neither is the classification.
Terminology note: Before you order or design against a spec that says "integral flange," confirm whether it means the ASME pressure-vessel classification, an equipment manufacturer's integral nozzle, or just informal shorthand for "welded on." These get mixed up more often than you'd expect.
How Integral Type Flanges Compare With Weld Neck and Other Flange Arrangements
Weld neck, slip-on, lap joint, socket weld, and blind flanges are geometric product types covered by ASME B16 dimensional standards. Integral, loose, and optional are structural analysis categories under pressure-vessel flange rules. Related, but not the same system.
| Flange Arrangement | Attachment Method | Typical Structural Role | Common Use |
|---|---|---|---|
| Weld neck | Butt-welded to pipe | Often qualifies as integral | High-pressure piping, vessel nozzles |
| Slip-on | Fillet-welded over pipe | Typically loose type | Lower-pressure piping, easier fit-up |
| Lap joint | Rotates over a stub end | Loose type | Frequent disassembly, alignment needs |
| Blind | No pipe connection; caps the line | Evaluated independently | Line isolation, testing, dead-ends |
| Socket weld | Pipe inserted, fillet-welded | Varies by design | Small-bore, high-pressure lines |
The ASME Companion Guide to the Boiler and Pressure Vessel Code associates weld-neck flanges with integral-type behavior and slip-on or lap-joint flanges with loose-type behavior (ASME Companion Guide, Bolted-Flange Joints and Connections).
That is a reasonable starting assumption, but it is not automatic. The weld detail and the actual design calculation control the final classification, not the catalog name.
Where Lap Joint Flanges Fit
A lap joint flange works with a separate stub end rather than a direct weld to the pipe. Buyers pick this arrangement for easier disassembly, rotation for bolt-hole alignment, or to use a lower-cost carbon steel flange over a corrosion-resistant alloy stub end. For those conditions, it is a practical specification choice.
Avoiding Common Specification Errors
Three mistakes show up repeatedly on the buying side:
- Treating "integral" as a universal product category rather than a code-based calculation outcome
- Mixing ASME B16.5/B16.47 dimensions with non-ASME dimensional systems on the same job
- Specifying flanges by nominal size alone without checking class, facing, and material
Before selecting anything, determine whether the flange belongs to pressure-vessel construction, process piping, an equipment connection, or a temporary test setup. That answer points you to the governing code.
ASME Standards, Dimensions, and Specification Requirements
Different standards govern different parts of a flange specification, and confusing them causes procurement delays. Here's how the roles split:
| Requirement | Governing Standard | What It Covers |
|---|---|---|
| Pipe flange dimensions (NPS ½–24) | ASME B16.5 | Ratings, dimensions, materials, tolerances, marking |
| Large-diameter flanges (NPS 26–60) | ASME B16.47 | Classes 75–900, dimensions, tolerances |
| Pressure vessel design | ASME BPVC Section VIII | Integral/loose/optional classification, structural calculations |
| Process piping | ASME B31.3 | Materials, fabrication, examination, testing |
| Metallic gaskets | ASME B16.20 | Ring-joint, spiral-wound, metal-jacketed gasket dimensions |
| Nonmetallic gaskets | ASME B16.21 | Flat gasket types, sizes, materials |
Twinco Industrial's flange lineup covers ASME B16.5, plus ASME B16.47 Series A (MSS SP-44) and Series B (API 605), in sizes from ½ inch through 72 inches and pressure classes 150 through 2500.
For the same 26-inch line size, the geometry diverges fast: Series A lists a 34.25-inch outside diameter with 24 bolt holes; Series B lists a 30.94-inch outside diameter with 36 bolt holes. Series selection has to happen before the order, not after.
What to Verify Before You Order
A complete specification check should confirm:
- Standard designation - B16.5, B16.47 Series A, or Series B
- Size and pressure class - matched to the governing design code, not assumed from nominal size alone
- Material grade - carbon steel (A105N, A350 LF2), stainless (304/304L, 316/316L), alloy (F11, F22, F91), duplex/super duplex, or nickel alloys (Inconel, Monel, Hastelloy)
- Facing type - flat face, raised face, ring-type joint, or tongue-and-groove
- Bore and schedule compatibility with the mating pipe
- Gasket and bolting requirements per ASME B16.20 or B16.21
- Inspection documentation - material certs, traceability, and any required NDE
Facing selection ties directly to gasket compatibility, bolt loading, and leakage control. A raised-face flange with the wrong gasket type will leak even when every other flange detail is correct.
Always match the current code edition to the project's adopted design year. Editions change, and dimensional tables are not retroactive.
Where ASME Integral Type Flanges Are Used
Integral design earns its place where the flange-to-boundary connection has to act as one combined structural section under real stress: pressure-vessel nozzles, equipment connections, and demanding high-pressure process systems.
ASME's own engineering literature covers high-pressure integral and welding-neck flanges paired with pressure-energized ring-joint gaskets, including use in API 10,000-lb and 15,000-lb flange classes (ASME design paper on high-pressure integral flanges).
Industries where this decision comes up most:
- Petrochemical and refining - nozzle connections and high-pressure process lines
- Pipeline construction and integrity - continuous load paths under cyclic operating pressure
- Power generation - boiler and turbine piping upkeep
- Fabrication and modular construction - vessel and skid nozzles welded and rated before shipment
- Maintenance and turnaround projects - hydrostatic testing of the finished connection
When a Loose or Optional Arrangement Makes More Sense
Integral isn't always the right call. Loose or optional designs fit better when:
- Frequent disassembly or bolt-hole realignment is expected
- Corrosion-resistant alloy costs need controlling (lap joint with alloy stub end, carbon steel flange)
- The application is temporary, such as isolation during testing
- The governing code specifically permits and validates a loose or optional route for that service
A Practical Selection Workflow
- Define the service conditions - pressure, temperature, fluid, and cyclic loading
- Identify the governing design code for the equipment or piping system
- Confirm dimensions, standard series, and material against that code
- Review the mating components and sealing system together, not separately
- Collect required documentation - certs, traceability, inspection records
- Get final sign-off from the responsible design engineer
Twinco Industrial supports this process by sourcing flanges and related piping components, coordinating gasket and bolting specs with the flange order, and moving quickly on urgent procurement through its inside sales and specialty-sourcing team.
For pipeline, refinery, and turnaround work across many pressure classes and materials, a supplier who can source and verify specs in one call saves time a fragmented supply chain usually burns.
Frequently Asked Questions
What does integral flange mean?
An integral flange is attached or formed so the flange and the connected pipe, vessel, nozzle, or neck act as one structural assembly. Under ASME rules, that is a calculation-based classification, not a physical description.
What are the different types of flanges and their dimensions?
ASME design categories include integral, loose, and optional. Common physical types include weld neck, slip-on, lap joint, socket weld, threaded, and blind. Dimensions depend on the governing ASME standard, nominal size, pressure class, facing, and material.
How is an integral type flange different from a weld neck flange?
Integral type describes the structural design classification, while weld neck describes a physical flange configuration. A weld neck arrangement often behaves as integral, but the governing code calculation determines the formal classification, not the part name alone.
Which ASME standards apply to integral type flanges?
ASME B16.5 or B16.47 cover dimensions; BPVC Section VIII governs structural classification; B31.3 covers piping design. Gasket standards include B16.20 and B16.21—confirm the current edition for your application.
What information is needed to order an ASME integral flange?
Provide size, pressure class, material grade, facing, bore, and mating pipe or equipment details. Also include temperature, fluid service, governing design code, gasket and bolting needs, and required inspection documents.
Are integral type flanges suitable for high-pressure applications?
Yes, when correctly designed and specified. Suitability depends on the code calculation, pressure-temperature conditions, material selection, weld details, sealing system, and final engineering approval, not on the classification name alone.
