Flanges

Overview

Joints That Can Be Opened Again

A flange is a bolted joint: two mating flanges, a gasket and a set of bolts. Unlike a welded joint it can be dismantled, so flanges are used wherever a line must be opened for maintenance, inspection or equipment removal – at pumps, valves, vessels and instruments.

Four things must match on both sides of the joint: size, pressure class, facing and standard. Get any one wrong and the bolt holes, the gasket or the rating will not line up. The sections below explain each choice.

Forged, not cast
Class 150 to 2500, PN 6 to PN 400
Matching pipe, fitting & flange grades
EN 10204 3.1 mill test certificates

Quick Facts

Types: Weld neck, slip-on, blind, socket weld, threaded, lap joint, reducing, orifice
Size: NPS ½" – 24" (B16.5) · 26" – 60" (B16.47)
Class: 150, 300, 400, 600, 900, 1500, 2500
PN range: PN 6 – PN 400 (EN 1092-1)
Facing: RF, FF, RTJ, tongue & groove, male & female
Certification: EN 10204 3.1 / 3.2, NDT reports, TPI

Flange Types

Types in Cross-Section

Each drawing is a half section through the flange – the dashed line is the pipe centreline, and the orange face is the gasket seating surface.

Weld Neck (WN)

Tapered hub butt welded to the pipe, transferring stress into the pipe wall. The strongest flange and the standard choice for high pressure, temperature cycling and critical service.

Butt weld · Class 150 – 2500

Slip-On (SO)

The pipe slips through the bore and is fillet welded inside and outside. Easier to align than a weld neck and lower cost – widely used for low and moderate pressure.

Double fillet weld · Class 150 – 600

Blind (BL)

A solid disc with no bore, bolted on to close off a line, vessel nozzle or valve. Used for terminations, future connections and pressure testing.

Solid · Class 150 – 2500

Socket Weld (SW)

The pipe seats in a machined counterbore against a shoulder and is fillet welded outside. Suits small-bore, high-pressure lines.

⅛" – 4" · Class 150 – 1500

Threaded (THD)

Screwed onto threaded pipe (NPT or BSP) with no welding – for utilities, low-pressure lines and areas where hot work is not permitted.

½" – 4" · NPT / BSP

Lap Joint (LJ)

A loose backing flange over a welded stub end. The flange rotates freely for bolt alignment and never touches the fluid – so an alloy stub end can pair with a carbon steel flange.

With ASTM A403 stub end

Reducing (RF)

Full-size flange face with a smaller bore, so a line can step down in size at the joint without an extra reducer and weld.

Two sizes in one flange

Orifice

Supplied in pairs with radial pressure tappings and jack screws, to hold an orifice plate for flow measurement.

Pairs with jack screws

Long Weld Neck (LWN)

An extended hub that acts as the vessel nozzle itself – common on pressure vessels, columns and heat exchangers.

Vessel & nozzle service

Spectacle Blind & Spacers

A solid disc and an open ring joined together – rotate it to block or open a line for isolation during maintenance. Also supplied as separate spacers and paddle blanks.

ASME B16.48

Plate / Slip-On Plate

Cut from plate with a flat face and no hub – an economical option for low-pressure water, air and structural connections.

Flat face · low pressure

RTJ Flanges

Weld neck or blind flanges machined with a ring groove for a metal ring gasket – the standard for high pressure and high temperature service.

Class 600 and above

Face Types

The Gasket Seating Surface

Both flanges in a joint must have the same facing – and the gasket must suit it.

Raised Face (RF)

The most common facing – a raised ring that concentrates bolt load on a smaller gasket area. Standard height 1.6 mm up to Class 300 and 6.4 mm from Class 400 upwards.

Flat Face (FF)

The whole face is flat and the gasket covers it fully. Used with cast iron equipment and GRP, where a raised face could crack the mating flange.

Ring Type Joint (RTJ)

A machined groove takes a solid metal ring gasket (oval or octagonal, type R, RX or BX). Metal-to-metal sealing for high pressure and temperature.

Tongue & Groove / Male & Female

Matching raised and recessed faces locate the gasket precisely and prevent it being blown out – used on heat exchangers and pump covers.

Standard face finish for RF and FF flanges is a serrated spiral or concentric finish of 125–250 µin Ra (3.2–6.3 µm), which grips the gasket. Smooth or other finishes can be machined on request.

Classes & Sizes

Pressure Class, PN & Size Range

The class number is a rating designation, not a pressure – the actual allowable pressure falls as temperature rises.

ASME Class	PN equivalent	Typical service	Common facing
Class 150	PN 20	Water, air, low-pressure utilities and general process	RF, FF
Class 300	PN 50	General process piping, steam, chemical service	RF
Class 400	PN 68	Intermediate ratings where 300 is not sufficient	RF
Class 600	PN 110	High-pressure process, steam and hydrocarbon lines	RF, RTJ
Class 900	PN 150	High-pressure oil & gas, boiler feed	RTJ
Class 1500	PN 260	Wellheads, high-pressure hydrocarbon service	RTJ
Class 2500	PN 420	Extreme pressure – small-bore, high-integrity joints	RTJ

Size Range

ASME B16.5: NPS ½" – 24", Class 150 – 2500
ASME B16.47: NPS 26" – 60", Series A & Series B
EN 1092-1: DN 10 – DN 1200, PN 6 – PN 400
Other: JIS 5K/10K/16K/20K, DIN, IS 6392, AWWA, customer drawing

How to Identify a Flange

1. Outside diameter: Measure across the flange edge
2. Bolt circle: Diameter through the centres of opposite holes
3. Bolt holes: Number of holes and hole diameter
4. Thickness & bore: Flange thickness and bore size / schedule

Send us these four measurements – with the facing type – and we can identify the size, class and standard for you.

Materials & Grades

Grades We Supply in Flanges

Flanges are forged products – the "F" grades correspond to the pipe and fitting grades of the same material.

Material Family	Grades	Standards	Typical Service
Austenitic Stainless Steel	F304, F304L, F316, F316L, F321, F347, F310	ASTM A182	Chemical, food & dairy, pharma, general process
Duplex & Super Duplex	F51 (S31803), F60 (S32205), F53 (S32750), F55 (S32760)	ASTM A182	Seawater, desalination, oil & gas, chemical tankers
Nickel Alloys	Inconel 600 / 625, Monel 400, Hastelloy C-276 / C-22	ASTM B564	Severe corrosion, high temperature, sour service
Carbon Steel	A105, A105N	ASTM A105	Steam, water, oil and general process piping
Alloy Steel	F5, F9, F11, F22, F91	ASTM A182	Refinery and boiler high-temperature lines
Low-Temperature Steel	LF2, LF3, LF6	ASTM A350	LPG, cryogenic and cold-climate piping
High-Yield Carbon Steel	F52, F60, F65	ASTM A694	High-pressure gas transmission lines
Titanium	Grade 2, Grade 5	ASTM B381	Seawater, chemical plant, desalination
Copper Alloys & Aluminium	Cupro-nickel 90/10 & 70/30, aluminium bronze AB2, C11000, 6061	ASTM B171, B150, B16, B209	Seawater and marine systems, heat transfer, light-duty service

Full Grade Details Equivalent Grades Ask for a Grade

Standards

Specifications We Supply To

A flange order needs both a material specification and a dimensional standard.

ASME B16.5Pipe flanges and flanged fittings, NPS ½ – 24

ASME B16.47Large-diameter flanges NPS 26 – 60 (Series A & B)

ASME B16.36Orifice flanges

ASME B16.48Line blanks – spectacle blinds, spacers and paddle blanks

ASME B16.20Metallic gaskets – ring joint, spiral wound and jacketed

ASTM A182Forged stainless and alloy flanges, fittings and valve parts

ASTM A105Carbon steel forgings for piping applications

ASTM A350Low-temperature carbon and alloy steel forgings

ASTM A694High-yield forgings for pipeline flanges and fittings

ASTM B564Nickel-alloy forgings

EN 1092-1Steel flanges, PN designated – DN 10 to DN 1200

MSS SP-25 / SP-44Marking of valves, fittings and flanges; line-pipe flanges

See the full standards directory

Manufacturing

Forged, Machined & Drilled

Flanges are forged so the grain flow follows the shape – stronger than cutting the same profile from plate.

01

Billet Cutting

Round bar or billet of the ordered grade is cut to weight, carrying its heat number through to the finished flange.
02

Heating & Forging

The billet is heated and forged in dies – or upset and ring rolled for larger sizes – producing a near-net shape with continuous grain flow.
03

Heat Treatment

Solution annealing for stainless and duplex; normalising, quenching and tempering for carbon and alloy steel – developing the properties the specification requires.
04

Machining

The flange is turned to outside diameter, thickness, hub and bore, and the face is machined with the specified serrated finish or RTJ groove.
05

Drilling

Bolt holes are drilled on the bolt circle to the number, diameter and straddle position required by the standard.
06

Inspection, Marking & Packing

Dimensional and NDT inspection, marking to MSS SP-25 with grade, size, class and heat number, then face protection and packing.

Testing & Inspection

Checked Before Dispatch

Material tests on the certificate, plus NDT and dimensional checks on request.

Chemical Analysis

Composition of the forging heat, reported on the mill test certificate.

ASTM E415
ASTM E1086

Mechanical & Hardness

Tensile, yield, elongation and hardness after heat treatment – hardness limits matter for sour service.

ASTM A370
ASTM E10 / E18

Impact Testing

Charpy V-notch at sub-zero temperature for A350 LF2 and other low-temperature grades.

ASTM E23
ASTM A370

Ultrasonic Testing

Volumetric examination of forgings for internal soundness, to the acceptance level your specification sets.

ASTM A388
ASME Sec. V

Dye Penetrant & MPI

Surface crack detection on the face, bore and machined areas.

ASTM E165
ASTM E709

Corrosion Testing

Intergranular corrosion for stainless; ferrite count and intermetallic phase checks for duplex forgings.

ASTM A262
ASTM A923

Dimensional Inspection

Outside diameter, bolt circle, hole size and number, thickness, bore, raised face height and face finish.

ASME B16.5
ASME B16.47

PMI & Certification

Positive material identification and mill test certificates as per EN 10204 3.1 (3.2 with third-party inspection).

XRF / OES
EN 10204

Packing & Delivery

Faces Protected in Transit

A damaged gasket face means a leaking joint – so faces travel protected.

Face Protectors

Plastic or wooden face covers guard the machined gasket surface and RTJ grooves against knocks.

Crates & Pallets

Small flanges in boxes; larger flanges strapped to pallets or in wooden crates for export.

Rust Prevention

Carbon steel flanges are oiled or varnished; stainless is wrapped and kept away from carbon steel.

Marking to MSS SP-25

Manufacturer, material grade, size, class and heat number stamped on the flange edge, matching the certificate.

Applications

Where These Flanges Are Used

At every joint that has to come apart again.

Oil & Gas / Refineries

Process piping, wellheads, manifolds and pipeline connections

Chemical & Petrochemical

Reactors, columns and corrosive service in alloy grades

Power Plants

Steam and feedwater lines, valves and turbine connections

Pressure Vessels

Long weld neck nozzles, manways and heat exchanger covers

Water & Desalination

Pumping stations, RO skids and seawater systems

Pharma, Food & Dairy

Hygienic process lines and equipment connections

Marine & Shipbuilding

Ballast, bilge and seawater systems in duplex and bronze

Fabrication & Job Work

Spool fabricators, skid builders and maintenance shops

How to Order

What to Specify in Your Enquiry

Send these details and we can quote accurately – usually the same working day.

1
Flange typeWeld neck, slip-on, blind, socket weld, threaded, lap joint…
2
Material gradee.g. ASTM A182 F316L or A105
3
Size & boreNPS, plus schedule for weld neck bores
4
Class or PNClass 150 – 2500, or PN 6 – PN 400
5
FacingRF, FF, RTJ, tongue & groove – and face finish if specified
6
StandardASME B16.5, B16.47 Series A/B, EN 1092-1, JIS, IS
7
Quantity & certificationPieces, EN 10204 3.1 / 3.2, NDT, TPI

Frequently Asked Questions

Weld neck or slip-on – which should I use?

A weld neck has a tapered hub butt welded to the pipe, so stress is carried smoothly into the pipe wall. It is the choice for high pressure, high temperature and cyclic or critical service. A slip-on slides over the pipe and is fillet welded inside and out – cheaper, easier to align and perfectly adequate for lower-pressure lines.

What does "Class 150" actually mean?

It is a rating designation, not a pressure. A Class 150 flange in one material holds a different pressure than the same class in another, and the allowable pressure drops as temperature rises. Always read the pressure-temperature rating table for your material group in ASME B16.5.

When do I need an RTJ flange instead of raised face?

Ring type joint flanges use a solid metal ring in a machined groove, giving metal-to-metal sealing that suits high pressure and temperature – typically Class 600 and above, and common in oil and gas. Both flanges in the joint must have matching RTJ grooves and the correct ring number.

Can I bolt an ASME flange to an EN or DIN flange?

Not normally. ASME classes and EN PN ratings have different outside diameters, bolt circles and hole patterns, so the holes will not line up. Both sides of a joint must be to the same standard – or a special adaptor flange must be made.

What face finish do you supply?

Standard raised and flat faces are machined with a serrated spiral or concentric finish of 125–250 µin Ra (3.2–6.3 µm), which helps the gasket grip. Other finishes can be machined when the specification calls for them.

I have an old flange – can you identify it?

Yes. Measure the outside diameter, the bolt circle diameter, the number and size of the bolt holes, the thickness and the bore, and tell us the facing type. Send those with a photo and we can work out the size, class and standard, then quote a replacement.