 Flanges.webp)
The dimensions of all weld neck flanges depend on their size and class, except for the bore. Each buyer has to specify the bore according to a pipe schedule. In case a Schedule 40 bore is ordered for Schedule 80 pipes, the resulting assembly will develop a permanent bump that disturbs fluid movement, gathers dirt and concentrates stress exactly where it was meant to. Weld neck flanges are the best choice for applications using high pressure, high temperature and cycling piping in Oman and the GCC areas. The tapered hub is what buys that. The wider range sits on the pipe fittings and flanges hub.
Why Weld Neck Is Specified
The long tapered hub is the point of the design. It carries the flange thickness down to the pipe wall gradually, so bending and thermal loads transfer into the pipe without a sudden change of section, and the butt weld sits away from the flange body where it can be fully radiographed. Three consequences follow:
- Better fatigue and cyclic performance than slip-on or socket weld joints
- Suitable for high pressure and high temperature service, including Class 900 and above
- Full volumetric inspection of the attachment weld, which slip-on fillet welds do not allow
The cost is a more expensive flange and a butt weld that needs fit-up and a qualified procedure.
Bore and Schedule Matching
The flange is ordered as a size, a class and a bore. The bore follows the pipe schedule.
| NPS | DN | Pipe OD (mm) | Sch 40 wall (mm) | Sch 80 wall (mm) | Sch 160 wall (mm) |
|---|---|---|---|---|---|
2 | 50 | 60.3 | 3.91 | 5.54 | 8.74 |
3 | 80 | 88.9 | 5.49 | 7.62 | 11.13 |
4 | 100 | 114.3 | 6.02 | 8.56 | 13.49 |
6 | 150 | 168.3 | 7.11 | 10.97 | 18.26 |
8 | 200 | 219.1 | 8.18 | 12.70 | 23.01 |
10 | 250 | 273.1 | 9.27 | 15.09 | 28.58 |
12 | 300 | 323.9 | 10.31 | 17.48 | 33.32 |
Carbon and alloy walls are to ASME B36.10M; stainless flanges are bored to the S schedules of ASME B36.19M. Where the pipe was bought to a stated wall rather than a schedule, give that wall on the flange enquiry.
Sizes are supplied to specification. Send the NPS, class, facing, bore schedule and grade.
Pressure-Temperature Ratings
Class is not a pressure in bar. It is an entry into a rating table that falls with temperature and varies with material group. For ASME B16.5 Material Group 1.1, covering A105 carbon steel forgings, the ratings at 38 degrees C are 19.6 bar for Class 150, 51.1 bar for Class 300 and 102.1 bar for Class 600. Low carbon austenitic stainless flanges fall into Group 2.3, which gives 15.9 bar, 41.4 bar and 82.7 bar for the same three classes at the same temperature. Read the class against the design temperature and the actual material group, not against the ambient carbon steel figure.
Sour Service
Under NACE MR0175 / ISO 15156, flanges in sour service carry the same requirements as the pipe. Carbon and low-alloy forgings must meet a 22 HRC hardness limit with controlled chemistry and an accepted heat treatment condition, and hardness testing on the finished forging is commonly ordered. On pipeline work, A694 high yield flanges intended for sour service need the sour requirement stated on the order, since the standard grade is not automatically supplied to it. State your sour service requirements in the RFQ.
weld neck flanges Price Factors
Each enquiry is priced on its own, because grade, class and testing scope move the figure more than quantity does. What counts:
- Grade and the alloying elements it carries
- Size, pressure class, facing and bore
- Schedule or wall the item is made to
- Production route, forged or formed from pipe or plate
- Supplementary testing and inspection scope
- Order quantity and availability
- Certificate level and witness requirements
- Delivery terms and port of discharge
Quotations are available in USD, OMR, AED, SAR, QAR, KWD and BHD, on EXW, FOB, CFR or CIF terms.
What to send for a quotation:
item type, material grade and standard, size, pressure class, facing, schedule or wall, quantity, testing, certification, inspection, destination port and required date.
Certification, Traceability and PMI
Material comes with EN 10204 3.1 certificates as standard, and EN 10204 3.2 certificates on request. Inspection by the buyer's nominated third-party agency can be arranged; state your requirement in the RFQ. Check that the heat number on the certificate matches the marking on the item itself, and that the heat treatment condition is stated, and the bore the flange was machined to. Positive material identification confirms the grade before anything is welded in. See the MTC and EN 10204 certification guide.
Delivery and Handling
Lead time is confirmed at quotation. Fittings ship bundled or crated with bevel protection; flanges ship with face and ring groove protection, since a scored face leaks whatever the gasket. In coastal Oman, salt deposits build up on anything left in the open, so a rinse and a dry before fitting up is worth the time.
Related Flanges and Fittings
- Slip-on flanges
- Blind flanges
- RTJ flanges
- Large diameter flanges
- ASTM A105 carbon steel flanges
- Buttweld fittings
Weld Neck Flanges: Buyer Questions
What bore should I order on a weld neck flange?
The bore is machined to the pipe schedule, so order it to the same schedule or wall as the pipe. A mismatch leaves a permanent step inside the joint.
When is a weld neck flange preferred over a slip-on?
On high pressure, high temperature and cyclic service, and wherever the attachment weld needs full radiography, because the tapered hub transfers load gradually and the butt weld can be volumetrically inspected.
Are ASME B16.47 Series A and Series B flanges interchangeable?
No. Bolt circles, bolt sizes and thicknesses differ between the two series, so the series must match the equipment already installed.
What pressure does a Class 300 flange hold?
It depends on material and temperature. ASME B16.5 Group 1.1 carbon steel gives 51.1 bar at 38 degrees C for Class 300, while low carbon austenitic stainless in Group 2.3 gives 41.4 bar, and both fall as temperature rises.
Which facing finish should I specify?
The gasket decides it. Spiral wound gaskets need a serrated finish within the range the gasket manufacturer specifies, so a smooth machined face can leak.
What should I send for a quotation?
Send the NPS, pressure class, facing, bore schedule, material grade, quantity, testing, certification, inspection and destination port.
Request a Quote for Weld Neck (WN) Flanges
Send the size, class, facing, bore schedule and grade with quantities. Where the bore schedule has not been stated or does not match the pipe on the same line, it is queried before the offer rather than after machining.