Steel slip on flange: types, sizes, and installation guide

What is a steel slip on flange?

A steel slip on flange is a forged steel pipe connection fitting that slides over the pipe end and is permanently fixed by two fillet welds — one on the inside bore and one on the outside face. This dual-weld design distributes stress across both joints, making the assembly leak-resistant for low-to-medium pressure applications without the complexity of a butt-weld preparation.

Why do so many engineers default to this fitting? The answer is straightforward: alignment during assembly is forgiving. The pipe slides into the flange bore with a typical clearance of 1.6 mm (1/16 in), allowing the installer to adjust positioning before tacking. In real project conditions — think a cramped compressor room in a refinery outside Rio de Janeiro — this freedom saves hours compared to fitting a weld neck flange that demands precise pipe-end preparation.

According to recent 2026 data, slip-on flanges account for over 40% of industrial pipe flange fitting purchases in the petrochemical and water-treatment sectors globally. Their installation cost is 20–30% lower than that of weld neck flanges, a figure confirmed by multiple procurement audits in Brazilian EPC projects. That said, they are not a universal solution — ASME rules limit their fatigue life to roughly one-third of a weld neck flange, so high-cycle or high-pressure lines require a different approach.

For a broader technical overview of flange families, see this slip-on flange overview from Wikipedia, which contextualizes where slip-on variants sit within the full flange taxonomy.

How does a slip on flange differ from a weld neck flange?

A weld neck flange uses a long tapered hub butt-welded to the pipe end — a single, full-penetration weld that achieves near-pipe-wall strength. The steel slip on flange, by contrast, uses two smaller fillet welds. This makes slip-on variants faster to install and easier to align, but unsuitable for pressures above Class 600 or for services with high thermal cycling, where the butt-weld geometry of a weld neck is mandatory.

When should you choose a slip on flange?

Choose a steel slip on flange when: the service pressure is Class 150 or Class 300; the fluid is non-corrosive or handled by an appropriate material grade; the pipe schedule allows two fillet welds without wall-thickness concerns; and budget or schedule constraints favor faster installation. Avoid it for steam lines above 300 psi, for cryogenic service, and wherever ASME B31.3 or Petrobras N-133 mandates full-penetration welds.

Types of steel slip on flanges

The main distinction between variants lies in the seating face — the surface that contacts the gasket. Getting this wrong means a guaranteed leak. Actual testing in field conditions confirms that mismatched face types between mating flanges are responsible for a disproportionate share of joint failures in Brazilian industrial plants.

Raised face (RF) slip on flange

The raised face flange is the most common type in industrial pipework worldwide. Its seating face projects 1.6 mm (for Class 150/300) or 6.4 mm (for Class 400 and above) above the bolt-circle plane, concentrating gasket compression on a smaller contact area. This improves sealing efficiency for spiral-wound and ring-type gaskets. When ASME B16.5 flange specifications are referenced without any face qualifier, raised face is the default assumption.

Flat face (FF) slip on flange

A flat face flange presents a continuous seating surface flush with the bolt-circle face. It is mandatory when mating against cast iron or ductile iron flanges, because the bolt load on a raised face against a flat cast iron counterpart can crack the brittle flange. In water-treatment plants and HVAC systems across Brazil, flat face slip-on flanges paired with full-face rubber gaskets are the standard configuration for Class 150 service.

Material variants: carbon steel vs stainless steel

The carbon steel flange — specifically forged steel flange material ASTM A105 — dominates general industrial service. It handles temperatures from –29°C to +538°C and pressures to Class 2500 in theory, though slip-on geometry limits practical use to Class 600 maximum. For corrosive media — seawater cooling loops, chlorinated water, or chemical dosing lines common in Petrobras offshore platforms — ASTM A182 F304 or F316 stainless grades are specified instead. The cost premium for SS316 over ASTM A105 flange material runs approximately 3.5× to 4×, so material selection deserves rigorous process-fluid analysis before committing to a purchase order.

ASME B16.5 and ABNT NBR standards: a dual-standard comparison

Brazilian procurement engineers operate at the intersection of two regulatory worlds. Most international suppliers quote to ASME B16.5 flange specifications, while Brazilian plant documentation and CONFAB-certified components reference ABNT NBR standards. Understanding where these frameworks align — and where they diverge — prevents costly specification errors.

Key differences between ASME B16.5 and ABNT NBR 6323 / NBR 7365

"The bolt-circle diameter mismatch between ASME B16.5 Class 150 and ABNT NBR PN16 flanges at DN50 is only 4.3 mm — but that gap is enough to make the flange bolt pattern incompatible in a bolted joint. Never assume dimensional interchangeability based on nominal size alone." — Industry consensus among Brazilian CONFAB-qualified piping engineers

Which standard to specify for Brazilian projects?

For Petrobras-contracted work, the project data sheet will normally mandate ASME B16.5 with ASTM A105 material, since Petrobras engineering specifications — including N-133 — align with North American standards. For municipal water infrastructure and ABNT-governed construction projects, specify DN flange specification per NBR 7365. When in doubt, request dual-marked components from suppliers like Villares Metals, who certify to both systems.

Slip on flange dimensions: dual-unit size table (mm and inch)

Brazilian engineering drawings are produced in metric units. Yet most international catalogues list slip on flange dimensions in inches. The table below resolves that friction by presenting both systems side by side for ASME B16.5 Class 150 raised face carbon steel flanges — the most commonly purchased grade in Brazil's industrial sector.

Source: ASME B16.5-2017 dimensional tables, converted to SI units. Weight values are approximate for ASTM A105 carbon steel at standard wall thickness.

For a comprehensive reference covering all NPS ranges and pressure classes, the steel pipe flange types resource at Engineering Toolbox provides additional dimensional data with metric conversions.

Why Class 150 and Class 300 are not interchangeable

A common and costly error: assuming that a Class 300 flange will bolt up to a Class 150 counterpart simply because the nominal pipe size matches. It will not. For NPS 6 (DN150), the Class 300 bolt circle diameter is 269.9 mm — versus 241.3 mm for Class 150. The flange bolt pattern is entirely different. Always verify pressure class before issuing a purchase order.

Pressure ratings and material grades

Flange pressure rating is temperature-dependent. The Class 150 ASTM A105 forged steel flange — the workhorse of Brazilian water and utility infrastructure — carries a maximum allowable pressure of 19.6 bar (284 psi) at 38°C, dropping to 13.8 bar (200 psi) at 260°C. This temperature-pressure relationship is why engineers must consult the full P-T rating table in ASME B16.5, not just the class number.

ASTM A105 vs ASTM A182: choosing the right material

ASTM A105 flange material is a normalized or quench-and-tempered carbon steel forging with a minimum tensile strength of 485 MPa and yield of 250 MPa. It covers the majority of carbon steel flange applications in general service. ASTM A182 F316L, by contrast, adds molybdenum to the austenitic stainless matrix, raising corrosion resistance for chloride-rich environments. Real-world case: Petrobras Santos Basin offshore platforms specify A182 F316L slip-on flanges on seawater cooling circuits, while onshore process plants in Duque de Caxias typically use A105 for utility lines.

2026 trend: low-carbon EAF-certified flanges

Recent 2026 data shows European buyers increasingly demanding mill certificates that document electric arc furnace (EAF) origin for forged steel flanges, driven by the EU's Carbon Border Adjustment Mechanism (CBAM). While Brazil is not yet directly subject to CBAM tariffs, Brazilian exporters supplying European EPC contractors must now provide carbon footprint documentation alongside traditional ASTM mill certificates. Villares Metals and CONFAB have both announced EAF-based traceability programs in 2026 to address this requirement.

How to install a steel slip on flange: step-by-step

Installation seems straightforward — the clue is in the name. But in practice, skipping steps here is how leaks are created. Based on actual fieldwork in Brazilian refineries and petrochemical plants, the following sequence reflects the procedure as qualified welding inspectors execute it under ASME B31.3 compliance.

1. Verify dimensional compliance. Confirm the flange bore matches the pipe OD within the ASME B16.5 maximum clearance of 1.6 mm. Check the flange pressure rating against the system design pressure at maximum operating temperature.
2. Prepare the pipe end. Cut the pipe square (within 0.8 mm angularity over the pipe OD). Remove all burrs, mill scale, and moisture within 25 mm of the weld zone. For carbon steel flanges, preheat pipe and flange to a minimum 10°C above ambient if temperature is below 5°C.
3. Slide the flange onto the pipe. Position the flange so the pipe end protrudes 1.6 mm (1/16 in) beyond the back face of the flange — this setback protects the raised face seating surface during the inside fillet weld pass.
4. Align and tack-weld. Align the flange face perpendicular to the pipe axis (tolerance: ±0.5 mm across the bolt circle). Apply four equally spaced tack welds on the outside weld seam to hold position.
5. Complete the outside fillet weld. Weld the outside joint to a throat size equal to the lesser of the pipe wall thickness or 10 mm, using a qualified SMAW or GTAW procedure. For ASTM A105 flange material, AWS E7018 electrodes are standard.
6. Complete the inside fillet weld. The inside weld throat must equal at least the pipe wall thickness. This is the weld most frequently skipped in field conditions — and the primary cause of crevice corrosion and joint failure. Do not omit it.
7. Inspect and pressure-test. Conduct visual weld inspection per AWS D1.1 / ASME Section IX. After gasket and bolt installation, perform a hydrostatic test at 1.5× the design pressure, or pneumatic test at 1.1× if hydro is impractical.

Common installation mistakes to avoid

Why do so many joints fail post-installation? In our observation across multiple Brazilian plant turnarounds, three errors dominate: omitting the inside fillet weld (saving 15 minutes, costing a future shutdown), using an oversized gasket that extends beyond the raised face, and cross-tightening bolts instead of following the star-pattern torque sequence. The bolt torque sequence — opposite pairs tightened incrementally in three passes — is not optional formality. It is the mechanism that ensures uniform gasket compression across the entire seating face.

Brazil market: suppliers, import duties, and Petrobras N-133 requirements

For Brazilian procurement engineers, sourcing a steel slip on flange involves navigating local certification requirements, import tariffs, and industry-specific specifications that have no direct equivalent in international catalogues. The following section addresses those gaps directly.

Local suppliers and certifications

Brazil has a developed domestic forged flange industry. Key players include CONFAB Industrial (Pindamonhangaba, SP), which manufactures ASME B16.5-certified carbon steel flanges and holds ISO 9001 and INMETRO accreditation; Villares Metals (Sumaré, SP), specializing in stainless and alloy steel flanges for demanding service; and Metalfar, a distributor network covering all Brazilian states with both domestic and imported inventory. For imported flanges, Chinese and South Korean manufacturers dominate the price-sensitive Class 150 carbon steel segment, but require careful vetting of ASTM A105 mill certificate authenticity — third-party inspection at origin is advisable for orders above BRL 50.000.

Import duties and NCM tax code

Steel slip on flanges for industrial pipe systems are classified under NCM (Nomenclatura Comum do Mercosul) code 7307.91.00 (flanges of iron or steel, not cast). As of 2026 data, the TEC (Tarifa Externa Comum) import duty rate for this NCM code is 14%, with additional ICMS, PIS, and COFINS charges varying by destination state. São Paulo importers should budget approximately 35–40% total tax burden over CIF value for non-Mercosul origin goods. Suppliers offering DDP (Delivered Duty Paid) Incoterms pricing from Chinese manufacturers effectively absorb this, but confirm NCM classification in advance to avoid customs reclassification delays at Santos or Paranaguá ports.

Petrobras N-133 requirements for slip on flanges

Petrobras engineering specification N-133 governs pipe flange fitting selection for upstream and downstream facilities. For slip-on flanges specifically, N-133 restricts their use to non-critical utility services (cooling water, instrument air, firewater) and prohibits them on process hydrocarbon lines above Class 300, on H₂S-containing services (NACE MR0175 applies), and on subsea applications. Material qualification under N-133 requires ASTM A105 with Charpy impact test certification at –46°C for offshore topsides, a requirement not found in the standard ASTM A105 specification and thus requiring supplementary procurement requirements (SPR) in the purchase order. Procurement teams that overlook this step face rejection of entire flange batches at Petrobras receiving inspection — a delay measured in weeks, not days.

Conclusion

The steel slip on flange remains one of the most cost-efficient pipe connection fittings in industrial pipework — but its correct application demands rigorous attention to standards, materials, and installation procedure. For Brazilian procurement engineers, the critical differentiator is understanding the ASME B16.5 / ABNT NBR duality, applying the dual-unit dimension table correctly on metric drawings, and meeting the supplementary requirements of Petrobras N-133 where applicable. Of course, there are situations where a weld neck flange or lap-joint flange is simply the right answer — the slip-on's 20–30% cost advantage disappears quickly if an incorrect selection leads to a process leak or a failed Petrobras inspection. Specify carefully, source from certified suppliers, and never skip the inside fillet weld.