Pipe Flanges: Types, Faces, Pressure Classes & Selection Guide
Every bolted connection in a piping system depends on a flange. Wherever two pipes meet, wherever a valve connects to a pipeline, wherever a pressure vessel nozzle joins the process piping — there is a flange. A refinery can have 50,000 flanged connections. A single offshore platform can have 10,000. Every one of them must be the right type, the right material, the right pressure class, and the right face finish — or it leaks.
Flanges are always purchased alongside pipe, fittings, and fasteners. Getting the flange specification wrong holds up the entire piping package. This guide covers every flange type, face type, pressure class, material, and standard you need to specify flanges correctly for your project.
Flange Types
Weld Neck Flange (WN)
The weld neck flange is the premium choice for critical service. It features a long tapered hub that is butt-welded to the pipe. The hub's taper provides a smooth stress transition from the thick flange body to the thinner pipe wall, which reduces stress concentrations at the base of the flange.
The bore of a weld neck flange matches the inside diameter of the pipe it connects to, ensuring unrestricted flow and no turbulence at the joint. The butt weld connection allows full radiographic (RT) inspection of the joint — a requirement in most process piping codes.
Best for: High-pressure, high-temperature, cyclic service, and any application where ASME B31.3 or B31.1 codes apply. The default flange type for refinery, petrochemical, and power plant process piping. If the specification does not state a flange type, assume weld neck.
Limitations: Higher cost than slip-on. Requires precise alignment and qualified welders for butt welding.
Slip-On Flange (SO)
The slip-on flange slides over the outside of the pipe and is attached by two fillet welds — one on the inside and one on the outside of the flange. The bore is slightly larger than the pipe OD to allow insertion.
Slip-on flanges are easier to align and weld than weld neck flanges, and they cost approximately 30% less. However, the fillet weld connection is weaker than a butt weld and cannot be fully inspected by radiography.
Best for: Low-to-moderate pressure utility service (water, air, low-pressure steam), structural connections, and applications where cost reduction is a priority and the piping code allows fillet-welded connections.
Limitations: Not recommended for high-pressure, high-temperature, or cyclic service. ASME B31.3 assigns a lower strength rating to fillet-welded joints compared to butt-welded joints. Not suitable for pipe sizes above NPS 2-1/2" in high-pressure classes.
Blind Flange (BL)
A blind flange is a solid disc with no center bore. It bolts to a flanged connection to seal the end of a pipe, valve, or pressure vessel nozzle. Blind flanges must withstand the full system pressure acting across their entire face area, plus the bolt pre-load — making them the most mechanically loaded flange type.
Best for: Closing off pipe ends, isolating sections of piping for testing or maintenance, pressure vessel test connections, and future connection points. Every piping system needs blind flanges for hydrostatic testing, where line ends must be sealed while the system is pressurized.
Key point for procurement: Blind flanges are often overlooked in material take-offs. Ensure your MTO includes blinds for all test connections, future connections, and spare nozzles.
Socket Weld Flange (SW)
The socket weld flange has a recessed bore (socket) that the pipe inserts into. A single fillet weld on the outside of the flange secures the connection. The socket provides alignment and the fillet weld provides the seal and structural connection.
Best for: Small-bore piping (NPS 2" and below), high-pressure small-diameter applications, instrument connections, and process sample points. Common in chemical plants and refineries for drain and vent connections.
Limitations: Limited to NPS 1/2" through NPS 4" (most commonly NPS 2" and below). Not suitable for large-diameter or low-temperature service. A small gap must be maintained between the pipe end and the socket bottom to allow for thermal expansion — improper installation without this gap can cause cracking.
Threaded Flange (TH)
The threaded flange has NPT female threads (per ASME B1.20.1) that screw directly onto the pipe without welding. A small seal weld may be added on the outside for additional security.
Best for: Applications where welding is not possible or not permitted — such as explosive atmospheres, galvanized pipe connections, and low-pressure utility services. Also used for temporary connections and where disassembly without cutting is required.
Limitations: Limited to NPS 4" and below. Not suitable for high-temperature service (thermal cycling can loosen threaded connections). Not suitable for high-pressure or cyclic loading. Thread corrosion can make disassembly difficult over time.
Lap Joint Flange (LJ)
The lap joint flange is a two-piece assembly: a loose backing flange plus a stub end that is butt-welded to the pipe. The flange slips over the stub end and rotates freely, allowing easy bolt-hole alignment. Only the stub end contacts the process fluid.
Cost-saving advantage: When the process fluid requires corrosion-resistant material (stainless steel, nickel alloy, titanium), the expensive alloy is used only for the stub end — the backing flange can be carbon steel. This can reduce flange costs by 40–60% in exotic material applications.
Best for: Systems requiring frequent disassembly, corrosive service where material cost is a concern, and applications where bolt-hole alignment is difficult (such as offshore installations with tight tolerances).
Flange Face Types
The flange face is the sealing surface where the gasket sits. The face type must match on both mating flanges and must be compatible with the gasket type.
Face Type | Abbreviation | Description | Gasket Type | Typical Application |
|---|---|---|---|---|
Raised Face | RF | A raised ring machined on the flange face, 1.6mm (1/16") high for Class 150/300, 6.4mm (1/4") high for Class 400+ | Spiral wound, CNAF, graphite | Standard for most process piping — the default face type |
Ring Type Joint | RTJ | A machined groove in the flange face that accepts a metallic ring gasket (R, RX, or BX type) | Metallic ring gaskets (soft iron, SS, Inconel) | High-pressure (Class 600+) and high-temperature service; API 6A wellhead connections |
Flat Face | FF | The entire flange face is flat, no raised ring | Full-face gaskets (rubber, CNAF) | Cast iron flanges, AWWA water flanges, low-pressure HVAC |
Critical rule: Never bolt a raised face (RF) flange to a flat face (FF) flange. The raised face creates a bending moment on the flat face flange that can crack it — particularly with cast iron flanges. If you are connecting carbon steel pipe to cast iron equipment, machine the raised face off the steel flange to create a flat face, or use a flat face flange.
Pressure Classes
ASME B16.5 defines seven pressure classes for flanges NPS 1/2" through NPS 24":
Class | Approx. Max Working Pressure at 38°C (100°F) | Typical Application |
|---|---|---|
150 | 19.6 bar (285 psi) | Low-pressure utility, water, HVAC, general service |
300 | 51.1 bar (740 psi) | Medium-pressure process piping, steam |
400 | 68.1 bar (985 psi) | Medium-high pressure (less common) |
600 | 102.1 bar (1,480 psi) | High-pressure process piping, oil & gas |
900 | 153.0 bar (2,220 psi) | High-pressure service |
1500 | 255.1 bar (3,705 psi) | Very high-pressure service, wellheads |
2500 | 425.0 bar (6,170 psi) | Ultra-high-pressure, subsea, wellheads |
Pressure-temperature relationship: The pressure ratings above are for carbon steel (ASTM A105) at ambient temperature. As temperature increases, the allowable working pressure decreases. At 400°C, a Class 300 flange can only handle approximately 30 bar — significantly less than its ambient rating. Always check the pressure-temperature tables in ASME B16.5 for the specific material and operating temperature.
For flanges above NPS 24": ASME B16.47 covers large-diameter flanges from NPS 26" through NPS 60" in two series:
Series A (MSS SP-44) — heavier, more bolt holes. Standard for most process and pipeline applications.
Series B (API 605) — lighter, fewer bolt holes. Used in specific applications where the lighter design is acceptable.
Flange Materials
Material | ASTM Specification | Application |
|---|---|---|
Carbon steel | A105 (forged) | Standard for most process piping. Suitable to ~425°C |
Low-temperature CS | A350 LF2 (forged) | Service temperatures down to -46°C. Impact tested. |
Stainless steel 304 | A182 F304 | Corrosion resistance, moderate temperature |
Stainless steel 316 | A182 F316 | Superior corrosion resistance, chemical service |
Alloy steel Cr-Mo | A182 F11 (1.25Cr-0.5Mo) | High-temperature service (refinery, power plant) |
Alloy steel Cr-Mo | A182 F22 (2.25Cr-1Mo) | Higher-temperature service |
Alloy steel Cr-Mo | A182 F91 (9Cr-1Mo-V) | Very high-temperature (up to ~600°C) |
Duplex SS | A182 F51 (2205) | High-strength corrosion resistance, offshore |
Super Duplex SS | A182 F53 (2507) | Extreme corrosion, seawater, subsea |
Inconel 625 | B564 N06625 | Extreme corrosion and temperature |
For most projects in Africa and the Middle East: ASTM A105 carbon steel flanges cover 80%+ of requirements. A350 LF2 for any service below -29°C. A182 F316 for stainless steel requirements. Always verify the material matches the pipe specification and piping class.
How to Specify Flanges for Your Project
A correct flange specification requires these parameters — every single one:
1. Standard: ASME B16.5 (NPS 1/2"–24") or ASME B16.47 (NPS 26"–60")
2. Type: Weld Neck, Slip-On, Blind, Socket Weld, Threaded, or Lap Joint
3. Size: NPS and pipe schedule (the schedule determines the bore diameter for WN flanges)
4. Pressure Class: 150, 300, 400, 600, 900, 1500, or 2500
5. Face: RF (raised face), RTJ (ring type joint), or FF (flat face)
6. Material: ASTM specification and grade (e.g., ASTM A105, A182 F316)
7. Bore: For weld neck flanges, the bore must match the pipe ID. Specify the pipe schedule to ensure the correct bore. Example: "WN flange NPS 6", Class 300, RF, ASTM A105, bore to SCH 80"
Example purchase order line item:
50 pcs — Weld Neck Flange, NPS 6", Class 300, RF, ASTM A105, bore to SCH 40, per ASME B16.5
Common mistakes that cause rejection:
Ordering WN flanges without specifying the bore (pipe schedule)
Mixing RF and FF flanges in the same system
Specifying Class 150 for service pressures above 20 bar
Using A105 flanges for service temperatures below -29°C (use A350 LF2)
Forgetting blind flanges for test connections and future connections
Flanges + Fasteners + Gaskets: The Complete Joint
A flanged joint is a system — the flange, gasket, and fasteners must all be specified together. A common procurement mistake is ordering flanges without matching fasteners and gaskets.
For ASME B16.5 RF flanges, the typical fastener/gasket combination is:
Pressure Class | Stud Bolt | Nut | Gasket |
|---|---|---|---|
150 | ASTM A193 B7 | ASTM A194 2H | Spiral wound (ASME B16.20) with CS outer ring |
300 | ASTM A193 B7 | ASTM A194 2H | Spiral wound with CS outer ring |
600 | ASTM A193 B7 | ASTM A194 2H | Spiral wound with CS outer ring |
900+ | ASTM A193 B7 | ASTM A194 2H | Spiral wound or RTJ metallic ring |
For low-temperature service: A193 B7M or A320 L7 stud bolts with A194 2HM or 7 nuts.
For high-temperature service: A193 B16 stud bolts with A194 4 or 7 nuts.
Order everything together. When you source flanges, fasteners, and gaskets from the same supplier, you eliminate the risk of mismatched specifications and you get a single shipment to your project site.
Which Flange Type for Which Application?
Application | Recommended Flange | Why |
|---|---|---|
Refinery process piping | Weld Neck, RF, Class 150–600 | Strongest joint, full RT inspection, code-compliant |
Pipeline valve connections | Weld Neck, RF or RTJ | Must match valve flange rating and face |
Pressure vessel nozzles | Weld Neck (long welding neck) | Stress distribution, insulation clearance |
Line ends and test points | Blind | Seal system for hydrostatic test |
Small-bore drains and vents | Socket Weld or Threaded | Compact, suitable for NPS 2" and below |
Corrosive chemical service | Lap Joint + alloy stub end | Cost savings — only stub end needs alloy material |
Water distribution (AWWA) | Slip-On, FF | Lower cost, matches cast iron equipment |
Galvanized pipe connections | Threaded | No welding on galvanized surface |
Offshore platform piping | Weld Neck, RTJ, Class 600+ | High integrity, metal-to-metal seal |
Temporary / maintenance access | Blind or Lap Joint | Easy to remove and reinstall |
Supply from Kasko Makine
Kasko Makine supplies forged steel flanges in every type, size, pressure class, and material required for your project:
Types: Weld Neck, Slip-On, Blind, Socket Weld, Threaded, Lap Joint, Long Weld Neck, Spectacle Blind, Spacer/Spade
Sizes: NPS 1/2" to NPS 60" (ASME B16.5 and B16.47)
Pressure Classes: 150 through 2500
Materials: ASTM A105, A350 LF2, A182 F304/F316/F316L, A182 F11/F22/F91/F51/F53
Faces: RF, RTJ, FF
Complete piping packages: Flanges + pipe + fittings + stud bolts + nuts + gaskets — all from one source, shipped together to your project site. One purchase order, one shipment, one point of contact.
All flanges supplied with EN 10204 Type 3.1 material test reports. Third-party inspection available on request.
Request flange pricing — send your flange summary sheet, piping class details, or material take-off (MTO) to info@kaskomakine.com or contact us via WhatsApp at +90 (537) 521 1399 . We respond within 24 hours and deliver across Africa, the Middle East, Central Asia, and beyond.
FAQ SCHEMA (for structured data)
Q: What are the main types of pipe flanges?
A: The six standard types per ASME B16.5 are Weld Neck (strongest, for high-pressure service), Slip-On (lower cost, moderate pressure), Blind (seals pipe ends), Socket Weld (small-bore high-pressure), Threaded (no welding required), and Lap Joint (two-piece assembly for corrosive service). Weld Neck is the default for industrial process piping.
Q: What is the difference between Class 150 and Class 300 flanges?
A: Class 300 flanges are thicker, have a larger bolt circle, use larger bolts, and handle approximately 2.5 times more pressure than Class 150 at ambient temperature (740 psi vs 285 psi). The pressure class must be selected based on the design pressure and temperature per ASME B16.5 pressure-temperature tables.
Q: What does RF, RTJ, and FF mean on a flange?
A: RF (Raised Face) has a raised sealing ring and uses spiral wound or sheet gaskets — standard for most process piping. RTJ (Ring Type Joint) has a machined groove for a metallic ring gasket — used in high-pressure service (Class 600+). FF (Flat Face) has no raised ring — used with cast iron flanges and water service.
Q: What material is standard for carbon steel flanges?
A: ASTM A105 is the standard forged carbon steel flange material for service temperatures from -29°C to approximately 425°C. For temperatures below -29°C, ASTM A350 LF2 (impact tested to -46°C) is required. For stainless steel applications, ASTM A182 F304 or F316 are standard.
Q: What fasteners go with ASME B16.5 flanges?
A: The standard combination for carbon steel flanged joints is ASTM A193 Grade B7 stud bolts with ASTM A194 Grade 2H heavy hex nuts. For low-temperature service, A320 L7 studs with A194 7 nuts. For high-temperature service, A193 B16 studs with A194 4 or 7 nuts. Always match the fastener specification to the service conditions.
Q: What is the maximum size for ASME B16.5 flanges?
A: ASME B16.5 covers flanges from NPS 1/2" through NPS 24". For larger sizes (NPS 26" through NPS 60"), ASME B16.47 applies, which defines two series: Series A (heavier, more bolts) and Series B (lighter). Most pipeline and process applications above NPS 24" use ASME B16.47 Series A flanges.