PVF Reference Library · Technical Reference

Flange Types (Weld Neck, Slip-On, Blind, Socket, Lap Joint, Threaded)

Pipe flanges come in six main styles: weld neck, slip-on, socket weld, lap joint, threaded and blind. This sheet explains how each attaches to the pipe, where it is used and what else must match.

Ref PV-0018ASME B16.5ASTM A105ASTM A182

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A flange joins pipe, valves and equipment with a gasket and bolts so the joint can be taken apart. ASME B16.5 covers steel flanges from NPS 1/2 through 24 in pressure classes 150 through 2500. Carbon steel forged flanges are usually ASTM A105. Stainless flanges are usually ASTM A182 grades such as F304L and F316L. The flange style describes how it attaches to the pipe; the face type (raised face, flat face or ring joint) describes how it seals.

How to identify

Look at the hub and bore. A weld neck has a long tapered hub with a beveled end. A slip-on has a short hub and a bore slightly larger than the pipe OD. A socket weld has a bored socket with a shoulder inside. A lap joint has a flat back and a rounded bore edge and sits loose behind a stub end. A threaded flange has internal pipe threads. A blind flange has no bore. The stamp shows size, class, material, face and bore schedule where it applies. See the flange class identification chart.

Flange styles

Style	Attaches by	Typical use	Notes
Weld neck	Butt weld at hub	High pressure, high temperature, cyclic service	Bore must match pipe schedule
Slip-on	Fillet welds inside and outside	General service, lower pressure	Easy to align; lower fatigue strength than weld neck
Socket weld	Fillet weld at socket	Small-bore high-pressure lines	Leave expansion gap at socket bottom
Lap joint	Loose behind a welded stub end	Stainless or alloy lines, frequent take-apart	Flange can be carbon steel when stub end is alloy
Threaded	Pipe threads	Where welding is not allowed, small bore	Not for cyclic or high-temperature service
Blind	Bolts over an open end	Closing lines, test points, future connections	Must be the same class as the mating flange

Key points

Mating flanges must match in size, class and face type. A raised face flange against a flat face cast iron flange can crack the cast iron.
Weld neck flanges must be ordered with the bore to match the pipe schedule.
Gaskets must match the face type, class and service. See the spiral wound gasket color code chart.
Bolt count, bolt size and bolt circle depend on size and class. See the flange bolt pattern lookup guide.
Pressure-temperature ratings for each class are in ASME B16.5. See pressure class and flange ratings explained.

Common mistakes

Ordering a weld neck without the bore schedule.
Mixing Class 150 and Class 300 flanges.
Bolting a raised face steel flange to a flat face cast iron flange.
Bottoming the pipe in a socket weld flange with no gap.

Governing standards: ASME B16.5 (pipe flanges and flanged fittings, NPS 1/2 through 24); ASTM A105 (carbon steel forgings); ASTM A182 (alloy and stainless forgings).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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