Weld Neck Flange
Weld neck flanges feature a long tapered hub that is butt-welded to the pipe, providing excellent stress distribution and making them the preferred choice for critical, high-pressure, and high-temperature applications. The smooth transition from flange thickness to pipe wall thickness minimizes turbulence and erosion at the joint, while the butt weld allows for full radiographic inspection of the connection.
Our weld neck flanges are forged to ASME B16.5 dimensions with precisely machined raised face (RF) or ring-type joint (RTJ) facings. Available in pressure classes from Class 150 to Class 2500, these flanges are suitable for temperatures ranging from cryogenic (-45\u00b0C with A350 LF2) to elevated service (up to 540\u00b0C with alloy steels). Each flange is marked with material grade, heat number, pressure class, size, and manufacturer identification for full traceability.
ASME B16.5 / EN 1092-1
ASTM A105 / A350 LF2
DN15-DN1200, Class 150-2500
Applications
- High-pressure steam and boiler systems
- Critical process piping in refineries
- Offshore platform riser connections
- Cryogenic and low-temperature service
- Nuclear and power generation piping
Quality Assurance
All Weld Neck Flange products undergo NDT testing with MTC per EN 10204 3.1. Third-party inspection by SGS, BV, TUV available.
Weld Neck Flange Technical Specifications & Dimensions
The following table summarizes the key technical parameters for our weld neck flanges manufactured to ASME B16.5 and EN 1092-1 standards. Custom sizes and materials are available upon request.
| Parameter | Specification |
|---|---|
| Standard | ASME B16.5 / EN 1092-1 / JIS B2220 / GOST 33259 |
| Size Range | DN15 (1/2") to DN1200 (48") |
| Pressure Class | Class 150, 300, 600, 900, 1500, 2500 |
| Face Type | Raised Face (RF), Flat Face (FF), Ring Type Joint (RTJ) |
| Material Grades | ASTM A105, A350 LF2 (LTCS), F304/F316 (SS), F22 alloy, F91 |
| Welding Bevel | ASME B16.25 butt-weld preparation, 37.5 degree bevel |
| Hub Type | Tapered hub for butt-welding, long neck (LWN) available |
| Surface Finish | 125-250 Ra (RF), 32-63 Ra (RTJ grooves) |
Manufacturing Process & Quality Control
Our weld neck flanges are manufactured through a controlled forging and machining process that ensures structural integrity and dimensional accuracy:
- Raw Material Inspection: Spectrometer analysis (OBLF GS1000) of every heat to verify chemical composition meets ASTM A105 or specified grade.
- Forging: Upset forging of round bars to form the flange blank, ensuring grain flow follows the flange profile for maximum strength.
- Heat Treatment: Normalizing at 870-900C followed by air cooling to achieve uniform microstructure and specified mechanical properties (yield, tensile, elongation).
- Rough Machining: CNC turning of hub, bore, and outer diameter to within 0.5mm of final dimensions.
- Drilling: CNC drilling of bolt holes to ASME B16.5 hole pattern tolerances (bolt circle diameter, hole spacing).
- Finish Machining: Precision turning of raised face or RTJ groove to specified surface finish (Ra 125 for RF, Ra 32-63 for RTJ).
- NDT Inspection: 100% ultrasonic testing per ASTM E2375, magnetic particle testing on machined surfaces, dye penetrant on RTJ grooves.
- Dimensional Verification: CMM (Coordinate Measuring Machine) check of bolt circle, hole positions, flange thickness, and face flatness.
- Final Marking & Coating: Stamping with heat number, size, class, material grade per MSS SP-82; anti-rust oil coating or hot-dip galvanizing as required.
Every shipment includes a Mill Test Certificate (EN 10204 3.1) with chemical analysis, mechanical test results, and NDT reports. Third-party inspection by SGS, BV, TUV, or Lloyds Register can be arranged at the buyer's request.
Installation & Welding Guidelines
Proper installation of weld neck flanges is critical for leak-free performance in high-pressure piping systems. Follow these industry-standard practices:
Pre-Weld Preparation
- Verify flange bore matches pipe OD (check bore tolerance per ASME B16.5).
- Clean the weld bevel and adjacent pipe surface to bare metal (remove oil, rust, paint).
- Align flange face perpendicular to pipe axis (within 1 degree) to prevent bolt stress.
- Preheat carbon steel to 5C minimum; low-temp materials (A350 LF2) require 95C preheat.
Welding Procedure
- Use qualified welding procedure (WPS/PQR) per ASME Section IX.
- GTAW (TIG) root pass with argon purge for critical service; SMAW or GMAW fill passes.
- Interpass temperature control: maintain 5C preheat between passes on carbon steel.
- 100% radiographic testing (RT) of butt welds per ASME B31.3 for high-pressure service.
- Post-weld heat treatment (PWHT) at 600-650C for thickness over 19mm or when specified.
Gasket & Bolt Selection
- Raised Face: use spiral wound gaskets (CGI) with graphite filler for Class 600 and above.
- RTJ: use oval or octagonal ring gaskets matching groove R or RX number per ASME B16.20.
- Lubricate bolt threads and follow ASME PCC-1 torque sequence (star pattern, 3 passes).
- Torque to 50% of bolt yield strength for standard service, 70% for high-pressure.
Weld Neck Flange vs Other Flange Types
Understanding when to specify a weld neck flange versus alternatives helps optimize both cost and reliability:
| Flange Type | Pressure | Best Application | Cost |
|---|---|---|---|
| Weld Neck | Up to Class 2500 | High-pressure, high-temp, critical service | High |
| Slip-On | Up to Class 600 | Low to medium pressure, utility piping | Low |
| Blind | Up to Class 2500 | Pipe ends, pressure vessel closures, test points | Medium |
| Socket Weld | Up to Class 2500 | Small bore (DN15-DN80), high pressure | Medium |
Weld neck flanges are specified wherever the consequences of a joint failure are severe: high-pressure steam lines, hydrocarbon processing, nuclear systems, and subsea pipelines. For non-critical, low-pressure utility water or air lines, slip-on flanges offer adequate performance at lower cost. Our engineering team can advise on the optimal flange type for your specific operating conditions.