Weld Neck Flange

ASME B16.5 Class 150 flange dimensions

Flange outside diameter, minimum flange thickness, bolt circle, bolting and raised face, in millimetres, for NPS 1/2 to 12 in. These dimensions are set by the size and the pressure class and are common to the flange types on this line; the bore, hub, socket or thread detail changes with the type and is confirmed on the drawing or the quotation. Thickness includes the 1.6 mm raised face.

Data to ASME B16.5; confirm against the project specification before fabrication. Sizes above 12 in and Class 300 to 2500 follow the same layout and are quoted against the specification. Send the size, class, facing and destination port and we will confirm availability and price within one business day.

What a Weld Neck Flange Is

A weld neck flange has a tapered hub that is butt welded to the pipe end. The hub is the reason the type exists: it gradually increases the wall thickness from the pipe up to the flange, so the stress at the junction is spread over a longer distance instead of being concentrated in one corner. That is why the weld neck is the default choice for high pressure, high temperature and severe cyclic service, and for every joint that has to be examined – a butt weld can be radiographed or ultrasonically tested, a fillet weld cannot. The bore is machined to match the pipe inside diameter, so a weld neck flange is always ordered against a pipe schedule.

How to Specify a Weld Neck Flange

Six things fix the item: nominal pipe size, pressure class, material grade, bore, facing and hub length. The class is a rating class and not a pressure in psi – the permissible working pressure depends on the class together with the material group and the design temperature, so it has to be read from the pressure-temperature tables of ASME B16.5 for the actual material rather than assumed. The bore follows the pipe wall: an STD bore and a Schedule 160 bore in the same size and class are different flanges, so the wall thickness or schedule must be stated. Facings are raised face as standard, flat face where the mating component is cast iron or bronze, and ring joint where a metal gasket is required. Long hub patterns are available where a longer straight section is needed ahead of the weld.

Where It Is Used

Refinery and petrochemical process piping, compressor and pump suction and discharge nozzles, steam and condensate headers, high pressure gas lines and storage and transfer station piping, and any flanged joint at Class 900 and above. In sour service the flange is supplied to the hardness limits required by NACE MR0175, with the material grade and heat treatment agreed before the order is placed.

Facing Selection: Raised Face, Flat Face and Ring Joint

A raised face takes a sheet or spiral wound gasket and is the standard facing; the raised face height is 1.6 mm for Class 150 and 300 and 6.4 mm for Class 400 and above, so a Class 300 and a Class 600 flange of the same size do not have the same face height. A flat face is used where the mating component is cast iron or bronze and must not be overstressed by bolting – a flat face flange is always bolted to another flat face flange, never to a raised face. A ring joint facing takes a metal ring gasket seated in a machined groove; it is normal practice for high pressure and high temperature service and is less sensitive to thermal cycling than a sheet gasket.

Flange type selection: which flange for which service

The six B16.5 flange types are not variants of one item; each one solves a different problem. This table is the short version of the decision — the sections above explain the weld neck itself, and each other type has its own page.

Source: joint characteristics and service limits as described for each type in ASME B16.5 and in the piping code applied to the installation; type by type limits are described on the individual flange pages on this site.

Five checks before a flange is welded into the line

A flange that is right in the store room can still be wrong at the joint. These five checks take minutes and catch almost every flange rejection.

Does the wall thickness of the pipe change the flange?

It changes the bore, not the outside dimensions. The class fixes the flange OD, bolt circle and thickness; the pipe schedule fixes the machined bore of a weld neck flange. That is why a weld neck flange is always ordered against a pipe schedule, and why two flanges of the same size and class can be different items.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the forged flange material grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example ASTM A105 flange. The stainless grade has the higher tensile requirement but the lower minimum yield strength, so the two are not interchangeable on the strength table alone. The pressure class sets the flange dimensions, so the grade does not set the wall.

Source: specified minimum yield and tensile strength for each grade, per ASTM A105 / ASME SA105 and ASTM A182 / ASME SA182. The standard named against each grade is the governing document.

• A105
• A350 LF2
• A694 F52
• A694 F60
• A694 F70
• F11
• F22
• F91
• F304
• F304L
• F316
• F316L
• F321
• F51

Download Weld Neck Flange data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.

What decides the price

There is no list price for flanges: the same drawing can be quoted at different numbers depending on seven things. Send them and the quotation comes back within one business day.

Reply within one business day. Send the seven items above with the quantity and the destination port. Send the specification or WhatsApp the specification.