Flange Types Guide

Flange class comes from pressure and temperature. Flange type does not. Type comes from what the joint is there to do: let a line be broken for maintenance, positively isolate it, hold an instrument, take an abrupt size change, or allow the pipe to rotate during assembly. Decide the job first and the type usually follows without argument, which is why this flange types guide is organised by function rather than by picture. Ranges are underpipe fittings and flanges.

The Standard Types by Job

Type	The job it does	What to watch
Weld neck	Full strength permanent joint on pressure and cyclic service	The bore is ordered to a schedule. A mismatch leaves a permanent step in the line
Slip-on	Lower cost joint where alignment tolerance helps	Two fillet welds, and the setback that makes the inner one possible
Blind	Closes a line, and provides positive isolation	This is what a permit to work names, because it is not a valve
Threaded	Joint without welding, small bore, often where welding is not permitted	The thread carries both seal and load, which sets a floor on pipe wall
Socket weld	Small bore joint that can be broken	Exists so the line can be dismantled, not for pressure capability
Lap joint with stub end	Rotating flange for alignment, and the flange never touches the process	Carbon steel backing flange on an alloy line is the cost case

The Specialty Types

Type	The job it does
Orifice	Carries the pressure tappings for flow measurement. The bore is an input to the flow calculation, so it is measured rather than assumed
Spectacle blind	Permanent, visible isolation that can be swung between open and blanked. Thickness follows class and size, so large high class blinds become lifting items
Large diameter, ASME B16.47	NPS 26 and above. Series A and Series B are not interchangeable
Reducing	Removes a reducer and a weld, and replaces them with an abrupt entry. Not suitable upstream of a meter
Anchor	Transfers axial thrust into a structure. Bought like a flange, never bolted, and made to drawing
RTJ, ring type joint	High pressure and high temperature sealing. The ring must be softer than the groove, or the flange takes the damage

Series A and Series B Is the One to Check

On large diameter flanges, ASME B16.47 defines two series and they do not mate with each other:

Series A is heavier, with fewer, larger bolts
Series B saves weight using more, smaller bolts on a thinner flange, which leaves less tolerance during bolt-up

An enquiry for a B16.47 flange that does not state the series is incomplete, and the mistake is not discovered until two flanges meet.

Facings

Facing	Where it is used
Raised face, RF	The general default across most classes
Flat face, FF	Where the mating component is cast iron or another brittle material that a raised face would crack
Ring type joint, RTJ	High pressure and temperature, with a metal ring in a machined groove

The surface finish on a raised face is machined to a specified roughness range, and it matters in both directions. Too rough and the gasket cannot seal; too smooth and it can leak because the gasket has nothing to grip. The finish is specified, not left to the shop.

The Bore Question

This is the recurring error on weld neck flanges. The flange bore is ordered to match a pipe schedule, and if it does not match the pipe actually supplied, the result is a step at the weld. That step:

Disturbs flow, which matters most upstream of instrumentation
Creates a stress concentration in cyclic service
Cannot be corrected after welding without cutting the joint out

State the schedule for the flange bore explicitly, alongside the pipe schedule. The two are separate line items on an order even though they describe one joint. The dimensional background is in thepipe schedule guide.

What a Complete Flange Callout Contains

Type and facing
Pressure class
Size, and for NPS 26 and above the B16.47 series
Bore schedule, for weld neck and other bored types
Dimensional standard: ASME B16.5, B16.47, B16.36 for orifice, B16.48 for line blanks
Material specification and grade, with the class where the grade has one, such asA105 orA350 LF2
Face finish requirement
Certification level, from theMTC and EN 10204 guide

Flange Types: Buyer Questions

Does pressure decide flange type?

No, pressure and temperature decide the class. Type follows what the joint has to do: be broken for maintenance, positively isolate, carry an instrument, or allow rotation during assembly.

Why does the weld neck bore matter?

It is ordered to a schedule, and if it does not match the pipe supplied the joint has a permanent internal step that disturbs flow and concentrates stress.

Are ASME B16.47 Series A and Series B interchangeable?

No. Series A is heavier with fewer larger bolts and Series B uses more smaller bolts on a thinner flange, and they do not mate, so the series must be stated.

When is a flat face used instead of a raised face?

Where the mating component is cast iron or another brittle material that a raised face would crack by loading it unevenly.

Can a flange face be too smooth?

Yes. The machined finish is specified as a range, because too rough prevents sealing and too smooth leaves the gasket nothing to grip.

What makes a blind flange different from a valve?

It provides positive isolation rather than closure, which is why a permit to work names the blind rather than the valve upstream of it.

Applying This to an Enquiry

State the type, class, facing, face finish and, on weld neck flanges, the bore schedule. The bore is the item most often left off, and it is the only one on that list that cannot be corrected after the joint is welded.

Quotations are available in USD, OMR, AED, SAR, QAR, KWD and BHD, on EXW, FOB, CFR or CIF terms, with EN 10204 3.1 certificates as standard and 3.2 on request. Lead time is confirmed at quotation.

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Flange Types: Chosen by What the Joint Has to Do