The Complete Guide to Pipe Flanges: Types, Standards, Materials, and Selection

Flanges — The Unsung Heroes of Industrial Piping

Every process plant, refinery, power station, and offshore platform is held together, in a very literal sense, by flanges. These circular discs with bolt holes allow piping systems to be assembled, disassembled, inspected, and modified without cutting pipe. They provide the mechanical joint between equipment nozzles, valves, instruments, and pipe segments that must be opened periodically for maintenance.

Despite their ubiquity, flanges are frequently misunderstood or under-specified. An improperly selected flange — wrong pressure class, incompatible facing, insufficient material grade for the service temperature — is a point of failure waiting to happen. Industry incident databases contain numerous records of flange joint failures attributed to misspecification, improper bolt-up, or incorrect gasket selection.

At Remax Forge & Fittings, we manufacture flanges for every major industry sector, in materials ranging from carbon steel to exotic nickel alloys. This guide consolidates the technical knowledge our engineering team draws on daily, structured to help you make better flange specification decisions.

Flange Types — Understanding the Design Options

The term "flange" encompasses a wide variety of designs, each suited to different installation requirements, pressure-temperature conditions, and maintenance philosophies. ASME B16.5 (Pipe Flanges and Flanged Fittings, NPS 1/2 through NPS 24) and ASME B16.47 (Large Diameter Steel Flanges, NPS 26 through NPS 60) are the primary dimensional and rating standards in North America and most international markets.

Weld Neck Flange (WN)

The weld neck flange is the workhorse of high-pressure, high-temperature piping. Its defining feature is a long, tapered hub that is butt-welded to the pipe. The taper distributes stress away from the weld and into the pipe body — an important characteristic in cyclic service.

Weld neck flanges are required by many engineering specifications for:

• Pressure Class 900# and above
• High-temperature steam service
• Cryogenic service
• Cyclic or fatigue-sensitive service
• Any application where full radiographic examination of the weld is required

The radiographic examination advantage is critical: because the bore of the weld neck flange matches the pipe bore exactly, the butt weld can be fully examined by radiography. Other flange types use fillet welds that cannot be radiographed.

Slip-On Flange (SO)

The slip-on flange slides over the pipe end and is secured with two fillet welds — one inside the bore at the pipe end, and one at the back of the hub. It is less expensive than a weld neck flange and easier to fit up during installation, since exact pipe cut length is not as critical.

Slip-on flanges are suitable for:

• Low to moderate pressure service (generally Class 150# and 300#)
• Non-cyclic service
• Applications where the additional fillet welds are acceptable per the governing piping code
• Plant utilities, cooling water, and general service piping

The primary limitation of slip-on flanges is that the double fillet weld is more susceptible to fatigue cracking under cyclic loading than the butt weld of a weld neck flange. ASME B31.3 allows slip-on flanges but imposes restrictions on their use in severe cyclic conditions.

Blind Flange (BL)

A blind flange is a solid disc used to close the end of a piping system, nozzle, or pressure vessel. It is the most stressed of all flange types because it must resist the full pressure acting on its face area, in bending, without the benefit of pipe wall support.

Blind flanges are used for:

• Isolating the end of a pipeline for pressure testing or decommissioning
• Closing vessel nozzles that may be needed for future connections
• Temporary isolation during construction and commissioning

Because of their high bending stress under pressure, blind flanges are conservatively rated. The minimum thickness of a blind flange is greater than a comparable weld neck or slip-on flange face.

Socket Weld Flange (SW)

Socket weld flanges are used primarily for small-diameter, high-pressure piping — typically 2 inches and below. The pipe is inserted into a socket (counter-bore) in the flange bore and a single fillet weld is applied at the hub face.

A critical assembly detail: the pipe must be backed out approximately 1/16 inch (1.6 mm) from the socket bottom before welding, to allow thermal expansion during welding and prevent socket cracking. Many field failures at socket welds are attributable to violating this requirement.

Socket weld flanges are widely used in:

• High-pressure instrument lines and sample connections
• Chemical injection and hydraulic control piping
• Small-bore process piping in Class 600# through 2500# service

Threaded Flange (TH)

Threaded flanges attach to the pipe through internal threads — no welding is required. This is both their primary advantage and primary limitation. In flammable or hazardous service, most piping codes prohibit or restrict threaded connections because thread forms are susceptible to leakage under thermal cycling and vibration.

Threaded flanges are acceptable for:

• Low-pressure, low-temperature, non-hazardous service
• Installations where welding is not possible (fire hazard environment)
• Instrument connections and gauge points in appropriate service

Lap Joint Flange (LJ)

Lap joint flanges are used with stub ends and are free to rotate around the pipe. This rotation makes bolt-hole alignment trivial during installation and is particularly valuable when the pipe or equipment has a fixed orientation.

They are used when:

• Frequent dismantling is required (heat exchanger channel covers, filter housings)
• Bolt-hole alignment is difficult
• The stub end is made of a corrosion-resistant alloy but the flange backing ring can be made of carbon steel to reduce cost

Orifice Flange

Orifice flanges are a specialized pair of weld neck or slip-on flanges drilled with tapped pressure tap ports for differential pressure flow measurement. They accommodate an orifice plate between the flange faces. Orifice flanges are always supplied in sets and must be installed in matched pairs.

Flange Facing Types — The Critical Sealing Interface

The flange face is the machined surface that contacts the gasket and creates the seal. Selecting the wrong facing for your gasket type and service conditions will result in a leaking joint.

Raised Face (RF)

The raised face is the most common facing type, used with flat ring gaskets and spiral wound gaskets. The face is raised 1/16 inch (Class 150# and 300#) or 1/4 inch (Class 400# through 2500#) above the flange bolt circle.

The raised area concentrates the bolt load onto the gasket area, increasing seating stress and improving leak tightness. The face surface finish is typically specified in terms of arithmetic average roughness (Ra) — commonly 125 to 250 microinches AARH for spiral wound gaskets, or 63 microinches for ring type joint facings.

Flat Face (FF)

Flat face flanges have no raised portion — the entire flange face, including the area inside and outside the bolt circle, is at the same level. Flat face flanges are used when connecting to equipment or piping with flat-face flanges, such as cast iron valves and pumps.

Critical: never mate a raised face flange against a flat face cast iron flange. The raised face point-loads the cast iron, which can fracture. When connecting to cast iron equipment, convert both flanges to flat face with full-face gaskets.

Ring Type Joint (RTJ)

RTJ flanges have a machined groove in the face that accepts a metallic ring gasket — either octagonal or oval cross-section. When the bolts are tightened, the ring is compressed into the groove, creating an extremely tight metal-to-metal seal.

RTJ facings are specified for:

• High-pressure service (typically Class 900# and above)
• High-temperature service
• Hydrocarbon service where zero leakage is required
• Wellhead and Christmas tree equipment per API 6A

The metal ring gaskets must be softer than the flange material to prevent damage to the machined groove. For 316 SS flanges, use 316 SS rings. For carbon steel flanges in sour service, soft iron rings may be specified.

Tongue and Groove (T&G), and Male-Female (M&F)

These specialty facings mate with corresponding counterpart faces. They provide positive gasket positioning and are used in applications requiring precise gasket alignment, such as heat exchanger covers, pump casings, and other high-integrity joints.

Flange Pressure Classes — Understanding the Rating System

ASME B16.5 defines seven pressure classes for flanges: 150#, 300#, 400#, 600#, 900#, 1500#, and 2500#. These designations do not directly indicate the maximum allowable working pressure — rather, the pressure rating of a flange is a function of both its class and its material group at a given temperature.

How to Read Pressure-Temperature Tables

The pressure-temperature (P-T) rating tables in ASME B16.5 group materials into Material Groups (1.1, 1.2, 1.3, 2.1, 2.2, etc.). The allowable pressure for each class decreases as temperature increases, reflecting the reduction in yield strength at elevated temperatures.

For example, a Class 150# flange in Group 1.1 (which includes A105 carbon steel) is rated at approximately 285 psi at ambient temperature, but only about 150 psi at 500°F (260°C) and drops further as temperature rises. A Class 600# flange in the same material at ambient temperature is rated at approximately 1,480 psi.

Choosing the Right Class

The selection process is straightforward once you know:

• The maximum operating pressure at the flange location
• The maximum operating temperature at the flange location
• The material group of the flange material
• The applicable design code safety factor (typically the pressure is multiplied by 1.5 to establish the design pressure)

Look up the material group in ASME B16.5 Appendix E, then find the class whose P-T rating at your maximum temperature exceeds your design pressure. Always round up — never select a class whose rating is marginally above design pressure without considering future operating scenarios or pressure relief set points.

Flange Materials — Matching Metal to Service

Material selection for flanges follows the same principles as for any pressure-retaining component: you need sufficient strength at the operating temperature, adequate corrosion resistance for the fluid, compatibility with the gasket and bolt materials, and compliance with the applicable code and client specification.

Carbon Steel Flanges

• ASTM A105 is the most widely used carbon steel forged flange material for ambient to elevated temperature service. It covers carbon steel forgings for piping components with operating temperatures up to approximately 800°F (427°C).
• ASTM A350 LF2 is specified for low-temperature service down to -50°F (-45°C), with mandatory Charpy impact testing to verify notch toughness at the design minimum temperature.
• ASTM A694 grades (F42 through F80) are used for high-pressure gas transmission pipelines, with yield strength grades matching common pipeline steel grades.

Carbon steel flanges are suitable for: steam systems, compressed air, hydrocarbon service without wet H₂S or CO₂, cooling water (with corrosion allowance), general process fluids with appropriate inhibition.

Stainless Steel Flanges

• ASTM A182 F304/304L — Austenitic stainless, excellent corrosion resistance in oxidizing environments, food and pharmaceutical service, cryogenic service. The "L" grade (low carbon) is preferred for welded construction to minimize sensitization risk.
• ASTM A182 F316/316L — Superior corrosion resistance to F304 due to molybdenum addition. The preferred grade for chemical process, marine, and pharmaceutical applications.
• ASTM A182 F321 — Titanium-stabilized austenitic, resistant to sensitization in high-temperature service (800°F–1,500°F range).
• ASTM A182 F347/347H — Niobium-stabilized, similar to F321, preferred for refinery furnace outlet piping and high-temperature heat exchangers.

Duplex and Super Duplex Stainless Steel

• ASTM A182 F51 (UNS S31803/S32205) — Duplex stainless, approximately twice the yield strength of austenitic grades, excellent resistance to chloride stress corrosion cracking. Used in offshore, desalination, and chemical processing.
• ASTM A182 F53 (UNS S32750), F55 (UNS S32760) — Super duplex, even higher strength and corrosion resistance. Specified for highly aggressive chloride environments such as seawater cooling, bleaching, and produced water handling.

Alloy Steel Flanges

• ASTM A182 F11, F22 — Chromium-molybdenum alloy steels for high-temperature service (up to 1,100°F). Standard in power generation, reformer piping, and refinery high-temperature circuits.
• ASTM A182 F91 — Advanced 9Cr-1Mo-V alloy, capable of operating at temperatures up to 1,200°F (650°C). Increasingly specified in ultra-supercritical power generation and refinery fired heater piping.

Nickel Alloys

• Inconel 625 (UNS N06625) — Exceptional corrosion resistance in highly aggressive environments including concentrated acids, seawater, and flue gases. Frequently specified for offshore riser flanges and chemical reactor nozzles.
• Hastelloy C-276 (UNS N10276) — Outstanding resistance to reducing acids, chlorides, and mixed acid environments. Standard in FGD (flue gas desulfurization) systems and chemical plant waste acid handling.
• Monel 400 (UNS N04400) — Copper-nickel alloy with exceptional resistance to hydrofluoric acid and seawater. Specified in HF alkylation units and marine systems.

Bolting and Gaskets — The Complete Flange Joint System

A flange is only as reliable as the complete joint system — including the bolts, nuts, and gasket. Under-specifying any of these components negates the pressure capability of the best flange.

Bolting Standards

ASTM A193 B7 stud bolts with ASTM A194 2H heavy hex nuts are the standard pairing for Class 150# through 2500# carbon and alloy steel flanges in most process service. B7 is a chromium-molybdenum alloy steel that is heat-treated to achieve high strength.

ASTM A193 B8M (316 SS) stud bolts with ASTM A194 8M nuts are used for stainless steel flanges in corrosive service.

For sour service (H₂S-containing), bolting must comply with NACE MR0175/ISO 15156, which limits the hardness and heat treatment of bolts to prevent sulfide stress cracking. B7M (softer B7 variant) is the standard sour-service bolting.

Gasket Types and Seating Requirements

Proper Bolt Torquing

Gasket seating requires a specific compressive load achieved through controlled bolt torquing. Under-torquing leaves the gasket insufficiently seated — a recipe for leakage. Over-torquing can crush spiral wound gaskets, extrude soft gaskets, or yield studs, all of which reduce the effective bolt load in service.

For critical joints, hydraulic bolt tensioning tools are used to achieve precise, uniform bolt loads. Remax Forge & Fittings' technical team can provide bolt torque recommendations for specific gasket/flange/stud combinations on request.

Inspection, Testing, and Documentation Requirements

Mill Test Certificates (MTCs)

Every flange shipment should be accompanied by MTCs that document:

• Heat number and lot number
• Chemical composition (carbon, manganese, phosphorus, sulfur, silicon, chromium, molybdenum, etc.)
• Mechanical properties (yield strength, tensile strength, elongation, reduction of area, Charpy impact values if required)
• Confirmation of compliance with the applicable ASTM standard and heat treatment condition

Remax Forge & Fittings provides EN 10204 Type 3.1 MTCs (issued by the manufacturer's quality department) as standard, and Type 3.2 (countersigned by an independent inspection body) on request for critical applications.

Dimensional Inspection

All flanges are dimensionally inspected against ASME B16.5 or B16.47 requirements, including:

• Flange face outside diameter and bore diameter
• Bolt circle diameter and bolt hole diameter
• Raised face diameter and height
• Overall flange thickness and hub dimensions
• Flatness of the sealing face

Hardness Testing

As-forged and heat-treated flanges are hardness tested to verify proper mechanical properties. For A105 carbon steel, maximum hardness is 187 HBW. Duplex and super duplex flanges are hardness tested to confirm phase balance — excessive ferrite or sigma phase will be indicated by hardness outside the acceptable range.

Positive Material Identification (PMI)

For alloy, stainless, and exotic alloy flanges, positive material identification using X-ray fluorescence (XRF) or optical emission spectroscopy (OES) confirms that the correct alloy has been used. This is mandatory on many oil & gas and petrochemical projects where alloy mix-up has caused catastrophic failures.