Overview
Slip on flanges are designed with a bore large enough for the pipe to pass through the flange before welding. The flange is positioned on the pipe and is normally attached by fillet welds in accordance with the approved fabrication procedure and applicable piping requirements.
The simple geometry makes slip-on flanges common in many industrial and utility systems where the piping specification permits them. They should not, however, be ordered by nominal size alone. A proper specification includes the flange standard, NPS or DN, pressure class or PN, facing, material, bore, pipe OD/wall, required finish, inspection, and documentation.
Trade terms such as “slip flange,” “slip weld flange,” and “slip on pipe flange” usually refer to the same general flange family, but supplier terminology can vary. The drawing and referenced standard should control.
Why It Matters for Pipe Components
A slip-on flange depends on the relationship between the flange bore and pipe OD. Too little clearance can prevent assembly; too much can make alignment difficult or create a joint that does not match the intended welding detail.
Axial position matters too. The pipe end is set relative to the flange face before welding, and that position affects access to the internal fillet weld and the finished face. The required arrangement should come from the approved fabrication standard or project detail rather than a generic shop habit.
Slip-on flanges also have different load-transfer and fatigue characteristics from weld-neck flanges. This does not mean they are categorically unsafe or unsuitable. It means the piping designer should choose the flange type based on code, service, pressure-temperature conditions, cyclic loading, owner requirements, and fabrication constraints.
Methods, Parameters, or Process
| Parameter | What to confirm |
|---|---|
| Flange standard | ASME B16.5, EN 1092-1, or other project requirement |
| Nominal size | NPS or DN |
| Pressure designation | Class or PN |
| Facing | RF, FF, or other specified facing |
| Bore | Compatible with pipe OD and required clearance |
| Material | Exact material specification and grade |
| Pipe end position | Per approved joint detail |
| Welds | Size, location, process, and acceptance per WPS/project |
| Bolt drilling | Compatible with mating flange or equipment |
| Marking/traceability | As required by standard and purchase specification |
ASME B16.5 includes slip-on flanges within its dimensional scope. EN systems also include flange types that may be described commercially as slip-on or plate/hubbed flanges, but terminology and dimensions differ. Do not convert a flange from one system to another by nominal size alone.
The welding procedure should define joint preparation, weld size, sequence, consumables, preheat/PWHT where applicable, and inspection. These values depend on material, thickness, piping code, and project specification and should not be taken from a generic article.
Supplier and Inspection Checklist
- State “slip-on flange” and the governing standard.
- Include nominal size, class/PN, facing, and material grade.
- Confirm the flange bore against the actual pipe OD.
- Provide pipe wall thickness when it affects fabrication or line-class review.
- Verify flange OD, thickness, bolt circle, hole count, and hole diameter.
- Check the pipe insertion/position detail before welding.
- Use an approved WPS for the required fillet welds.
- Inspect welds using the project-specified method and acceptance criteria.
- Protect the gasket face from weld spatter and handling damage.
- Check flange orientation and bolt-hole clocking on fabricated spools.
- Maintain material traceability and MTC linkage when required.
- Confirm any project restriction on slip-on flanges before purchase.
Application to Pipe Fittings, Valves, and Flanges
Slip-on flanges are often installed on straight pipe at equipment, valve, tank, or skid interfaces. When placed close to elbows or tees, fabricators need enough straight length and access to position and weld the flange correctly.
At valves and pumps, the slip-on flange must match the mating flange standard, pressure designation, facing, and drilling. A correct pipe fit does not guarantee equipment compatibility.
For corrosion-resistant piping, slip-on flanges may be made from the same alloy as the pipe or use a project-approved material strategy. If the process fluid can contact the flange bore or weld region, corrosion compatibility should be reviewed rather than inferred from the pipe material.
FAQ
What is a slip-on flange?
It is a flange with a bore that allows it to slide over the pipe before being positioned and welded.
Are slip-on flanges welded inside and outside?
Common configurations use fillet welds at both locations, but the exact joint detail must follow the governing standard, piping specification, and approved WPS. Do not use a generic weld detail as a contractual requirement.
Is a slip-on flange the same as a lap-joint flange?
No. A lap-joint flange is a loose backing flange used with a stub end or lapped end and can rotate before bolting. A slip-on flange is welded directly to the pipe.
Can a slip-on flange be used for high-pressure service?
Suitability depends on the flange standard, class, material, design code, service, owner specification, and joint design. Some projects restrict flange types for severe or cyclic services.
Next Steps
Include the pipe OD/wall and the flange joint detail in the RFQ, especially for fabricated spools. Ask the supplier to confirm bore, drilling, facing, and welding procedure before production. Exact dimensions and weld requirements should be verified against the project-specified standard and approved fabrication documents.
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