ASME B16.5 weld neck flange is essential components in high-pressure and high-temperature pipelines. Known for their strength, leak-proof performance, and durability, these flanges are widely used in oil & gas, petrochemical, power plants, and offshore systems. Understanding their dimensions, pressure ratings and materials is key to safe and efficient piping operations.
Understanding Weld Neck Flange and Their Function
A weld neck flange is a type of flange with a long tapered hub that is butt-welded to the pipe, ensuring smooth flow and strong structure. The characteristic neck helps distribute stress, reducing the risk of failure at the weld area.
In piping systems where high pressure, high temperature or critical services are involved, weld neck flanges are preferred because their robust design handles extreme conditions better than many other flange types.
ASME B16.5 Standard
What ASME B16.5 Covers (NPS, DN, Pressure Class)
ASME B16.5 defines the dimensions, tolerances, materials, pressure–temperature ratings, and testing for pipe flanges from NPS ½″ to 24″.Pressure classes defined by ASME B16.5 include 150, 300,400, 600, 900, 1500, and 2500.
ASME vs ANSI Flange Standards
ASME sets the technical standards for flanges, like size, pressure, and material, while ANSI approves these standards for national use. Simply put, ASME writes the rules, ANSI gives them official recognition.
Weld Neck Flange Dimension
Class 300 WN Flange Dimension
| Nominal Pipe Size | A | B | C | D | E | F | G | H | I | J | |
| inch | mm | mm | mm | mm | mm | mm | mm | mm | Holes | mm | mm |
| 1/2 | 21.30 | 95.20 | 15.70 | 14.20 | 52.30 | 21.30 | 38.10 | 35.00 | 4 | 15.70 | 66.55 |
| 3/4 | 26.70 | 117.3 | 20.80 | 15.70 | 57.15 | 26.70 | 47.70 | 42.90 | 4 | 19.00 | 82.50 |
| 1 | 33.40 | 123.9 | 26.70 | 17.50 | 62.00 | 33.50 | 53.80 | 50.80 | 4 | 19.00 | 88.90 |
| 1 1/4 | 42.20 | 133.3 | 35.10 | 19.00 | 65.00 | 42.20 | 63.50 | 63.50 | 4 | 19.00 | 98.50 |
| 1 1/2 | 48.30 | 155.4 | 40.90 | 20.60 | 68.30 | 48.30 | 69.85 | 73. 15 | 4 | 22.30 | 114.3 |
| 2 | 60.30 | 165. 1 | 52.60 | 22.30 | 69.85 | 60.45 | 84.00 | 91.90 | 8 | 19.00 | 127.0 |
| 2 1/2 | 73.00 | 190.5 | 62.70 | 25.40 | 76.20 | 73.15 | 100.0 | 104.6 | 8 | 22.30 | 149.3 |
| 3 | 88.90 | 209.5 | 78.00 | 28.40 | 79.25 | 88.90 | 117.3 | 127.0 | 8 | 22.30 | 168.1 |
| 3 1/2 | 101.6 | 228.6 | 90.20 | 30.20 | 81.00 | 101.6 | 133.3 | 139.7 | 8 | 22.30 | 184.1 |
| 4 | 114.3 | 254.0 | 102.4 | 31.70 | 85.80 | 114.3 | 146.0 | 157.2 | 8 | 22.30 | 200.1 |
| 5 | 141.3 | 279.4 | 128.3 | 35.00 | 98.50 | 141.2 | 177.8 | 185.7 | 8 | 22.30 | 234.9 |
| 6 | 168.3 | 317.5 | 154.2 | 36.50 | 98.50 | 168.4 | 206.2 | 215.9 | 12 | 22.30 | 269.7 |
| 8 | 219.1 | 381.0 | 202.7 | 41.10 | 111.2 | 219.2 | 260.3 | 269.7 | 12 | 25.40 | 330.2 |
| 10 | 273.0 | 444.5 | 254.5 | 47.70 | 117.3 | 273.0 | 320.5 | 323.8 | 16 | 28.40 | 387.3 |
| 12 | 323.8 | 520.7 | 304.8 | 50.80 | 130.0 | 323.8 | 374.6 | 381.0 | 16 | 31.70 | 450.8 |
| 14 | 355.6 | 584.2 | To be specified by the Purchaser | 53.80 | 142.7 | 355.6 | 425.4 | 412.7 | 20 | 31.70 | 514.3 |
| 16 | 406.4 | 647.7 | 57.15 | 146.0 | 406.4 | 482.6 | 469.9 | 20 | 35.00 | 571.5 | |
| 18 | 457.2 | 711.2 | 60.45 | 158.7 | 457.2 | 533.4 | 533.4 | 24 | 35.00 | 628.6 | |
| 20 | 508.0 | 774.7 | 63.50 | 162.0 | 508.0 | 587.2 | 584.2 | 24 | 35.00 | 685.8 | |
| 24 | 609.6 | 914.4 | 69.85 | 168.1 | 609.6 | 701.5 | 692.1 | 24 | 41.10 | 812.8 | |
Class 150 WN Flange Dimension PDF
Class 400 WN Flange Dimension PDF
Class 600 WN Flange Dimension PDF
Class 900 WN Flange Dimension PDF
Class 1500 WN Flange Dimension PDF
Class 2500 WN Flange Dimension PDF
How to Measure a Weld Neck Flange Correctly
Measurements should include OD, bolt circle diameter, hub height, and flange thickness. Accurate measurement ensures proper assembly and gasket compatibility.
Pressure Rating Chart: How to Choose the Right Flange Class
Temperature–Pressure Relationship per ASME B16.5
Pressure rating depends on flange class and operating temperature. For ASTM A105 carbon steel:
Class 150: 19.6 bar (285 psi) at 38 °C
Class 300: 51 bar (740 psi) at 38 °C
As temperature increases, allowable pressure decreases.
| Temp., °F | Class150 | Class300 | Class400 | Class600 | Class900 | Class1500 | Class2500 |
| −20 to 100 | 285 | 740 | 985 | 1,480 | 2,220 | 3,705 | 6,170 |
| 200 | 260 | 680 | 905 | 1,360 | 2,035 | 3,395 | 5,655 |
| 300 | 230 | 655 | 870 | 1,310 | 1,965 | 3,270 | 5,450 |
| 400 | 200 | 635 | 845 | 1,265 | 1,900 | 3,170 | 5,280 |
| 500 | 170 | 605 | 805 | 1,205 | 1,810 | 3,015 | 5,025 |
| 600 | 140 | 570 | 755 | 1,135 | 1,705 | 2,840 | 4,730 |
| 650 | 125 | 550 | 730 | 1,100 | 1,650 | 2,745 | 4,575 |
| 700 | 110 | 530 | 710 | 1,060 | 1,590 | 2,655 | 4,425 |
| 750 | 95 | 505 | 675 | 1,015 | 1,520 | 2,535 | 4,230 |
| 800 | 80 | 410 | 550 | 825 | 1,235 | 2,055 | 3,430 |
| 850 | 65 | 320 | 425 | 640 | 955 | 1,595 | 2,655 |
| 900 | 50 | 230 | 305 | 460 | 690 | 1,150 | 1,915 |
| 950 | 35 | 135 | 185 | 275 | 410 | 685 | 1,145 |
| 1,000 | 20 | 85 | 115 | 170 | 255 | 430 | 715 |
Flange Class Selection Guide (150 vs 300 vs 600 etc.)
Selecting the proper class ensures safety and longevity. Over-specifying increases cost, while under-specifying risks leakage or failure.
Common Mistakes When Choosing Pressure Class
Ignoring temperature effects, material compatibility, or system cyclic loads are common errors leading to premature failure.
Flange Face Types: RF, FF, and RTJ Differences
What Is Raised Face (RF) Flange?
RF flanges have a slightly raised sealing surface. This design concentrates the gasket load in a smaller area, improving the sealing performance for general industrial applications such as oil, gas, and water pipelines.
What Is Flat Face (FF) Flange?
FF flanges have a flat, even surface. They are typically used with cast iron valves, pumps, or fittings where the mating surface is also flat. Because the entire face is in contact, they reduce bending stress on brittle materials but are not suitable for high-pressure applications.
What Is Ring Type Joint (RTJ) Flange？
RTJ flanges are designed for very high-pressure and high-temperature systems. A metal ring gasket fits into a precision-machined groove on the flange face, creating a strong metal-to-metal seal that can withstand severe service conditions.
RF, FF, and RTJ Differences
| Parameter | RF (Raised Face) | FF (Flat Face) | RTJ (Ring Type Joint) |
| Sealing Surface Form | Slightly raised area | Fully flat | Machined groove |
| Gasket Type | Soft / semi-metallic | Full-face soft gasket | Metal ring gasket |
| Pressure Range | Medium–High (Class 150–2500) | Low (Class 150) | Very High (Class 600–2500) |
| Temperature Resistance | Up to ~500 °C | Below ~200 °C | Up to ~800 °C |
| Typical Materials | Carbon steel, stainless steel | Cast iron, ductile iron | Alloy or stainless steel |
| Common Applications | General process piping | Low-pressure water or cast systems | High-pressure, high-temperature service |
| Sealing Reliability | High | Moderate | Very high |
Weld Neck Flange vs. Slip On Flange
| Feature | Weld Neck Flange | Slip On Flange |
| Strength & Fatigue Resistance | Excellent | Good |
| Leakage Resistance | Excellent | Good |
| Initial Cost | Higher | Lower |
| Installation Difficulty | Higher | Lower |
| Best Application | high pressure/high temperature | Low pressure/ambient temperature |
Flanges Materials
•Carbon Steel (ASTM A105): Standard for general service pipelines.
•Stainless Steel (A182 F304/F316): Suitable for corrosive environments; F316 preferred for chloride-rich fluids.
•Alloy Steel: For high-temperature or high-strength applications.
•Duplex & Nickel Alloys: For highly corrosive or offshore conditions.
Applications
•Oil & Gas Pipelines
•Petrochemical & Power Plants
•Offshore Installations
FAQs
What Is the Difference Between ASME and ANSI Flanges?
ASME B16.5 provides detailed pressure-temperature ratings, material requirements, and dimensional tolerances, while ANSI refers more broadly to American National Standards.
Can Weld Neck Flanges Be Used for All Pressure Classes?
WN flanges are suitable for all classes defined in ASME B16.5, but the material and hub design must match pressure and temperature requirements.
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