Views: 124 Author: Monica Publish Time: 2026-03-18 Origin: Site
WN (weld neck) flange is the strongest, most inspectable flange type for high-pressure, high-temperature, and cyclic-loaded piping. Its long, tapered hub is butt-welded directly to the pipe, creating a full-penetration, leak-tight joint that transfers stress into the pipe. This guide covers ASME B16.5 dimensions (Class 150/300/600), face and neck types, weight charts, and a selection table to help you specify the correct flange.
WN flange is a pipe flange with a long, tapered hub that is butt-welded directly to the pipe, giving a full-penetration weld and reinforcing the joint.
WN flanges, also called welding neck flanges, use a long tapered hub that is welded to the pipe's end. Their primary purpose is to provide a strong, leak-proof, and reusable connection in a piping system, especially in services involving high pressure, extreme temperatures, or cyclic loading (vibration and line surge).
1. Superior Stress Distribution
The hallmark of the WN flange is its tapered hub. As the hub tapers toward the pipe's diameter, it provides reinforcement to the flange itself and transfers the stress to the pipe. This is particularly advantageous in environments prone to "line surge" or mechanical vibrations.
2. High Resistance to Fatigue
Because the connection is a butt weld, the internal bore of the flange can be machined to match the pipe's inside diameter exactly. This eliminates turbulence and erosion at the joint, significantly increasing the fatigue life of the piping system compared to slip-on or socket-weld flanges.
3. Durability Under Extreme Conditions
In our facility, we often manufacture WN flanges from advanced nickel alloys and stainless steels. These materials, combined with the WN design, allow the flange to operate reliably in cryogenic temperatures or high-heat environments where thermal expansion could otherwise cause joint failure.
4. Ease of Inspection
The butt-weld joint between a WN flange and a pipe is easily inspected via Radiographic Testing (RT) or Ultrasonic Testing (UT). This "full penetration" weld capability ensures that the integrity of the joint can be verified to the highest safety standards—a requirement in the oil and gas and nuclear industries.
Dimensional specifications for Weld Neck flanges from NPS 1/2 to NPS 24 are governed by ASME B16.5; larger sizes (NPS 26–60) follow ASME B16.47. The tables below give key dimensions for Class 150, 300, and 600.
Note: The flange bore (B) is specified by the purchaser to match the pipe inside diameter (pipe schedule). Raised face height is typically 2 mm for Class 150/300 and 6.4 mm for Class 600 and above, included in flange thickness (tf).
Table 1: ASME B16.5 Class 150 WN Flange Dimensions
NPS | DN | Flange OD (O) mm | Thickness (tf) mm | Raised Face Dia. (R) mm | Hub Dia. (X) mm | Length thru Hub (Y) mm | Bolt Circle (W) mm | No. of Bolts | Bolt Hole Dia. (I) mm |
1/2 | 15 | 90 | 11.2 | 34.9 | 30 | 48 | 60.3 | 4 | 15.9 |
3/4 | 20 | 100 | 12.7 | 42.9 | 38 | 51 | 69.9 | 4 | 15.9 |
1 | 25 | 110 | 14.3 | 50.8 | 49 | 54 | 79.4 | 4 | 15.9 |
1 1/4 | 32 | 115 | 15.9 | 63.5 | 59 | 57 | 88.9 | 4 | 15.9 |
1 1/2 | 40 | 125 | 17.5 | 73.0 | 65 | 60 | 98.4 | 4 | 15.9 |
2 | 50 | 150 | 19.1 | 92.1 | 78 | 64 | 120.7 | 4 | 19.1 |
2 1/2 | 65 | 180 | 22.4 | 104.8 | 90 | 70 | 139.7 | 4 | 19.1 |
3 | 80 | 190 | 24.3 | 127.0 | 108 | 70 | 152.4 | 4 | 19.1 |
4 | 100 | 230 | 24.3 | 157.2 | 135 | 76 | 190.5 | 8 | 19.1 |
6 | 150 | 280 | 25.4 | 215.9 | 188 | 89 | 241.3 | 8 | 22.4 |
8 | 200 | 345 | 28.4 | 269.7 | 240 | 102 | 298.5 | 8 | 22.4 |
10 | 250 | 405 | 30.2 | 323.8 | 292 | 108 | 362.0 | 12 | 25.4 |
12 | 300 | 485 | 31.8 | 381.0 | 342 | 117 | 431.8 | 12 | 25.4 |
Table 2: ASME B16.5 Class 300 WN Flange Dimensions
NPS | DN | Flange OD (O) mm | Thickness (tf) mm | Raised Face Dia. (R) mm | Hub Dia. (X) mm | Length thru Hub (Y) mm | Bolt Circle (W) mm | No. of Bolts | Bolt Hole Dia. (I) mm |
1/2 | 15 | 95 | 12.7 | 34.9 | 38 | 51 | 66.7 | 4 | 15.9 |
3/4 | 20 | 115 | 14.3 | 42.9 | 48 | 56 | 82.6 | 4 | 19.1 |
1 | 25 | 125 | 15.9 | 50.8 | 54 | 60 | 88.9 | 4 | 19.1 |
1 1/4 | 32 | 135 | 17.5 | 63.5 | 64 | 64 | 98.4 | 4 | 19.1 |
1 1/2 | 40 | 155 | 19.1 | 73.0 | 70 | 67 | 114.3 | 4 | 22.2 |
2 | 50 | 165 | 20.7 | 92.1 | 84 | 68 | 127.0 | 8 | 19.1 |
2 1/2 | 65 | 190 | 23.9 | 104.8 | 100 | 75 | 149.2 | 8 | 22.2 |
3 | 80 | 210 | 27.0 | 127.0 | 117 | 78 | 168.3 | 8 | 22.2 |
4 | 100 | 255 | 30.2 | 157.2 | 146 | 84 | 200.0 | 8 | 22.2 |
6 | 150 | 320 | 35.0 | 215.9 | 206 | 97 | 269.9 | 12 | 22.2 |
8 | 200 | 380 | 39.7 | 269.9 | 260 | 110 | 330.2 | 12 | 25.4 |
10 | 250 | 445 | 46.1 | 323.8 | 321 | 116 | 387.4 | 16 | 28.6 |
12 | 300 | 520 | 49.3 | 381.0 | 375 | 129 | 450.8 | 16 | 31.8 |
Table 3: ASME B16.5 Class 600 WN Flange Dimensions
NPS | DN | Flange OD (O) mm | Thickness (tf) mm | Raised Face Dia. (R) mm | Hub Dia. (X) mm | Length thru Hub (Y) mm | Bolt Circle (W) mm | No. of Bolts | Bolt Hole Dia. (I) mm |
1/2 | 15 | 95 | 14.2 | 34.9 | 38 | 51 | 66.5 | 4 | 15.7 |
3/4 | 20 | 118 | 15.7 | 42.9 | 48 | 57 | 82.6 | 4 | 19.1 |
1 | 25 | 124 | 17.5 | 50.8 | 54 | 62 | 88.9 | 4 | 19.1 |
1 1/2 | 40 | 155 | 22.4 | 73.2 | 70 | 73 | 114.3 | 4 | 22.4 |
2 | 50 | 165 | 25.4 | 92.1 | 84 | 84 | 127.0 | 8 | 19.1 |
3 | 80 | 210 | 31.8 | 127.0 | 117 | 100 | 168.1 | 8 | 22.4 |
4 | 100 | 273 | 38.1 | 157.2 | 146 | 114 | 215.9 | 8 | 25.4 |
6 | 150 | 356 | 47.8 | 215.9 | 206 | 140 | 292.1 | 12 | 28.4 |
8 | 200 | 419 | 55.6 | 269.7 | 260 | 162 | 349.3 | 12 | 31.8 |
10 | 250 | 508 | 63.5 | 323.8 | 321 | 184 | 431.8 | 16 | 35.1 |
12 | 300 | 559 | 66.5 | 381.0 | 375 | 200 | 489.0 | 20 | 35.1 |
Note: The flange bore (B) is specified by the purchaser to match the pipe's inside diameter, which is dependent on the pipe schedule. The raised face height is typically 2 mm (1/16 in) for Class 150 and 300, and 6.4 mm (1/4 in) for Class 600 and above, and is included in the flange thickness (tf).
WN flanges are categorized by face type (RF, FF, RTJ) and neck configuration (standard, Long Weld Neck, Reducing). Your face choice depends on pressure class and the equipment you bolt to; your neck choice depends on whether the flange also acts as a nozzle or changes pipe size.
1. Raised Face (RF) Weld Neck Flanges
The Raised Face (RF) flange is the most common type used in process plant applications, featuring a small additional thickness on the flange face to concentrate pressure on a smaller gasket area.
By raising the gasket surface above the bolting circle face, this type ensures that more clamping force is applied to the gasket. In our stainless steel production line, RF flanges are typically finished with a serrated surface to "bite" into the gasket for a superior seal.
2. Flat Face (FF) Weld Neck Flanges
Flat Face (FF) flanges have a machining surface that is in the same plane as the bolting circle face and are primarily used with cast iron or brittle equipment.
The objective of an FF flange is to avoid the "bending moment" that occurs when bolting RF flanges together. If you are connecting a stainless steel WN flange to a cast iron pump or valve, an FF flange is mandatory to prevent cracking the cast iron component.
3. Ring Type Joint (RTJ) Weld Neck Flanges
RTJ flanges feature a deep groove machined into their face, designed to house a metal ring gasket that creates a "metal-to-metal" seal under high pressure.
These are the heavyweights of the industry. Typically found in Class 600 and higher, RTJ WN flanges are used in oilfield drilling and high-pressure gas systems. The integrity of the seal increases as the internal pressure increases, making them incredibly reliable in volatile conditions.
4. Long Weld Neck (LWN) Flanges
Long Weld Neck flanges feature a straight, heavy-walled barrel instead of a tapered hub, functioning essentially as a nozzle for pressure vessels or tanks.
Because the "neck" is much longer and thicker than a standard WN flange, it provides extra reinforcement and eliminates the need for a separate piece of pipe when connecting a flange directly to a vessel wall.
5. Reducing Weld Neck Flanges
Reducing WN flanges are used to change the pipe size at a flange connection point without the need for a separate concentric reducer.
This type is highly efficient for saving space in tight manifold configurations. The hub of the flange matches the larger pipe diameter, while the bore and the face transition to a smaller size.
Weight scales with size, pressure class, and material density; the table below gives approximate weights for stainless steel 304/316 (density ~7.93 g/cm3). Multiply by the relative density of your alloy (e.g., duplex ~7.8, nickel alloys ~8.4–8.9) for other grades.
NPS (inch) | Class 150 Weight (kg) | Class 150 Weight (lbs) | Class 300 Weight (kg) | Class 300 Weight (lbs) |
1/2 | 0.5 | 1.1 | 0.7 | 1.5 |
1 | 0.8 | 1.8 | 1.1 | 2.4 |
2 | 1.6 | 3.5 | 2.3 | 5.1 |
4 | 3.9 | 8.6 | 5.8 | 12.8 |
6 | 6.8 | 15.0 | 10.5 | 23.1 |
8 | 10.5 | 23.1 | 16.8 | 37.0 |
10 | 15.2 | 33.5 | 24.5 | 54.0 |
12 | 21.8 | 48.1 | 35.0 | 77.2 |
14 | 27.5 | 60.6 | 45.2 | 99.6 |
16 | 35.0 | 77.2 | 58.5 | 129.0 |
18 | 43.5 | 95.9 | 72.0 | 158.7 |
20 | 54.0 | 119.0 | 88.5 | 195.1 |
24 | 78.5 | 173.1 | 125.0 | 275.6 |
Note: Weights are theoretical and based on stainless steel density. Actual weights may vary slightly due to manufacturing tolerances and material mill variations.
Oil and Gas Industry
In upstream exploration and downstream refining, fluids are often under extreme pressure. WN flanges, particularly in duplex or super-duplex stainless steel, are used in wellhead equipment and high-pressure manifolds where they must resist both internal pressure and external corrosive environments.
Petrochemical and Chemical Processing
In systems handling corrosive acids or volatile organic compounds, the leak-proof nature of the WN butt-weld connection is essential. Using Hastelloy C276 for the WN flange ensures that the connection point is as corrosion-resistant as the rest of the piping system.
Power Generation
In both nuclear and fossil fuel power plants, steam lines operate at temperatures exceeding 500℃. The WN flange’s ability to handle thermal expansion and contraction without developing leaks makes it the standard for high-pressure steam distribution.
Desalination and Marine Applications
For offshore platforms and desalination plants, the mechanical strength of the WN flange is coupled with high-alloy materials (like 6-Moly stainless steel) to resist chloride-induced stress corrosion cracking and pitting.
We offer full traceability, material test certificates, and custom machining to meet your specifications.
Choose a WN flange when the joint is critical, high-pressure, high-temperature, or must be radiographically inspected; choose slip-on, socket-weld, or threaded flanges only for lower-risk, cost-sensitive services.
| Flange Type | Connection | Pressure / Temperature Suitability | Relative Cost | Full-Penetration Weld / RT-UT Inspectable | Best Application |
|---|---|---|---|---|---|
| Weld Neck (WN) | Butt weld (full penetration) | High pressure & temperature, cyclic load | High | Yes / Excellent | Critical, high-pressure, inspectable piping |
| Slip-On (SO) | Two fillet welds | Low–moderate pressure | Low | No / Poor | Low-cost, non-critical lines |
| Socket Weld (SW) | Single fillet weld | Small bore, moderate pressure | Low–Med | No / Poor | Small-bore, high-pressure utility lines |
| Threaded | Screwed | Low pressure, no welding | Low | N/A | Non-welded, non-flammable areas |
| Blind | No pipe (closes end) | Matches line rating | Med | N/A | Vessel/line blanks, hydrotesting |
| Lap Joint (LJ) | Slips over stub end, bolted | Low–moderate, frequent dismantling | Med | N/A | Frequent maintenance, low-corrosion service |
Face Type Selection Table
| Face / Neck Type | Seal Mechanism | Typical Class | Use When |
|---|---|---|---|
| Raised Face (RF) | Soft gasket on raised face | 150–2500 | General process service (most common) |
| Flat Face (FF) | Full-face gasket, no bending moment | 150–300 | Bolting to cast iron / brittle equipment |
| RTJ | Metal ring, metal-to-metal | 600+ | High-pressure gas, oilfield, volatile service |
| Long Weld Neck (LWN) | Butt weld, long barrel | Any | Vessel / tank nozzles |
| Reducing WN | Butt weld, size change | Any | Change pipe size at the flange |
What is a WN flange used for?
A WN (weld neck) flange connects pipe to pipe, valves, or equipment with a full-penetration butt weld. It is used where high pressure, high temperature, vibration, or the need for radiographic inspection makes weaker flange types unsafe.
WN flange vs slip-on flange: which should I choose?
Choose a WN flange for critical, high-pressure, or inspectable service because it provides a full-penetration weld and superior fatigue life. Choose a slip-on flange only for low-cost, low-pressure, non-critical lines where a fillet weld is acceptable.
RF vs FF vs RTJ: which face type do I need?
Use RF (raised face) for general process service. Use FF (flat face) when bolting to cast iron or brittle equipment to avoid cracking. Use RTJ (ring type joint) for Class 600+ high-pressure gas and oilfield service requiring a metal-to-metal seal.
What standard covers WN flange dimensions?
WN flange dimensions for NPS 1/2–24 are covered by ASME B16.5; larger sizes (NPS 26–60) follow ASME B16.47. Material and marking requirements are given in ASTM A182 (forged flanges) and related product standards.
How do I calculate WN flange weight?
Weight depends on size, pressure class, and material density. Use the weight chart above (based on stainless steel 304/316, ~7.93 g/cm3) as a baseline, then scale by your alloy's density (duplex ~7.8, nickel alloys ~8.4–8.9 g/cm3). For exact shipping and lifting loads, request the mill weight from the supplier.
Can WN flanges be used for cryogenic or high-temperature service?
Yes. WN flanges are the preferred choice for both extremes: in cryogenic LNG and low-temperature service they avoid brittle failure when made from suitable grades (e.g., 304L/316L), and in high-temperature steam or furnace service they accommodate thermal expansion without leaking.
WN flanges deliver the strongest, most inspectable pipe joint for demanding service. Specify by ASME B16.5 dimensions, choose the face type by pressure and mating equipment, and use the selection tables above to confirm WN is the right flange type for your application.
Need a quote or custom-machined WN flanges? JN Alloy stocks WN flanges in stainless steel (304/316/316L/904L/254SMO), duplex (2205/S31803/S32205), super duplex (2507/S32760), and nickel alloys (Inconel, Incoloy, Hastelloy, Monel) with full material test certificates and custom machining. Contact our engineers: Info@jnalloy.com | +86-193-3990-0211 | www.jnalloy.com.
