Pipe Flanges Glossary

This comprehensive pipe flanges glossary serves as an essential reference for engineers, technicians, procurement specialists, and quality inspectors working with flanged connections. Covering hundreds of industry terms—from flange types and materials to testing procedures and industry standards—this resource provides the foundational knowledge needed to properly select, install, and maintain pipe flanges across all major industries. Understanding these components is crucial for ensuring system integrity, operational safety, and preventing costly leaks or failures in industrial applications.

Types of Flanges

Blind Flange: A solid disk-shaped flange without a bore, used to close off or seal the end of a piping system.

Slip-On Flange: A flange that slips over the pipe and is then welded both inside and outside to the pipe.

Weld Neck Flange: A flange with a long tapered hub that is butt-welded to the pipe, providing reinforcement at the base of the flange.

Socket Weld Flange: A flange with a socket that accepts the pipe; the pipe is inserted into the socket and welded.

Threaded Flange: A flange that has internal threads that mate with external threads on a pipe.

Lap Joint Flange: A two-piece flange assembly consisting of a stub end and a backing flange ring that can rotate freely around the stub end.

Orifice Flange: A specialized flange designed to hold an orifice plate for flow measurement.

Ring Type Joint (RTJ) Flange: A flange with a groove to accommodate a metal ring gasket.

Spectacle Blind Flange: A specialized blind flange consisting of a disk (blind) and a ring connected by a section of metal, resembling eyeglasses.

Expander Flange: A flange that incorporates a tapered section to make a transition between different pipe diameters.

Reducing Flange: A flange with a smaller bore diameter than its mating pipe diameter.

Long Weld Neck Flange: Similar to weld neck flange but with extended neck length for special applications.

Swivel Flange: A flange that can rotate to accommodate misalignment between connecting pipes.

High Hub Flange: A flange with an extended hub height for additional reinforcement.

Welding Neck Reducing Flange: A weld neck flange with a bore diameter smaller than the connecting pipe.

Nipoflange: A short pipe section with a flange on one end, used to create a flanged connection on existing pipework.

Face Types

Flat Face (FF): A flange with a completely flat sealing surface, typically used with full-face gaskets.

Raised Face (RF): A flange with a raised area around the bore that forms the gasket seating area, the most common type.

Ring Type Joint (RTJ) Face: A flange face with a specially machined groove to accommodate a metal ring gasket.

Tongue and Groove: A pair of mating flanges where one has a raised ring (tongue) and the other has a matching depression (groove).

Male and Female: A pair of mating flanges where one has a raised portion (male) that fits into a recess on the other (female).

Stock Finish: Standard machine finish on raised face (125-250 AARH).

Smooth Finish: Fine machine finish on raised face (125-200 AARH).

Serrated Finish: Concentric or spiral serrations on a raised face for better gasket grip.

Phonographic Finish: Spiral grooves on the flange face to improve gasket sealing.

Lapped Finish: A highly polished finish achieved by lapping (30-50 AARH).

Flange Components

Bolt Circle: The diameter of the circle on which the bolt holes are centered.

Bolt Holes: Holes drilled in the flange to accommodate bolts or studs.

Bore: The inner diameter of the flange that matches the pipe’s inner diameter.

Face: The sealing surface of the flange.

Hub: The thickened portion of the flange that transitions to the pipe.

Neck: The cylindrical portion of a weld neck flange between the flange face and the weld end.

Gasket Surface: The area of the flange face where the gasket seats.

Backside: The side of the flange opposite to the face.

Root Face: The flat surface at the end of the hub that is prepared for welding.

Bevel Angle: The angle of the prepared edge for welding, typically 30°, 37.5°, or 45°.

Land: The flat portion at the root of the weld preparation.

Thickness (T): The overall thickness of the flange excluding any raised face.

Raised Face Height: The height of the raised portion on a raised face flange.

Outside Diameter (OD): The overall outer diameter of the flange.

Flange Rating: The pressure-temperature rating of the flange.

Gasket Types

Full Face Gasket: A gasket that covers the entire face of the flange, including the bolt holes.

Ring Gasket: A gasket that fits within the bolt circle and only covers the raised face area.

Spiral Wound Gasket: A gasket made from a metal strip and filler material wound together in a spiral.

Ring Type Joint (RTJ) Gasket: A solid metal ring gasket used with RTJ flanges.

Metal Jacketed Gasket: A gasket with a metal outer shell and a softer inner core.

PTFE Gasket: A gasket made from polytetrafluoroethylene (Teflon) for chemical resistance.

Compressed Non-Asbestos Fiber (CNAF) Gasket: A gasket made from compressed non-asbestos fibers and rubber binders.

Corrugated Metal Gasket: A metal gasket with corrugations to provide spring loading.

Double-Jacketed Gasket: A gasket with metal covering both the inner and outer surfaces.

Kammprofile Gasket: A solid metal core with concentric grooves and a soft facing material.

Lens Ring Gasket: A specially shaped metal gasket with a lens-shaped cross-section.

Delta Ring Gasket: A triangular cross-section metal gasket used in high-pressure applications.

Graphite Gasket: A gasket made from flexible graphite material for high-temperature applications.

Metal-Wound Graphite Gasket: A combination of metal and graphite for enhanced performance.

Self-Energizing Gasket: A gasket design that uses system pressure to enhance sealing.

Flange Standards

ASME B16.5: American standard for pipe flanges and flanged fittings (NPS 1/2 through NPS 24).

ASME B16.47: American standard for large diameter steel flanges (NPS 26 through NPS 60).

API 6A: American Petroleum Institute standard for wellhead and Christmas tree equipment.

DIN Standards: German industrial standards for flanges (e.g., DIN 2527, DIN 2566).

EN 1092: European standard for flanges and their joints.

JIS B2220: Japanese Industrial Standard for steel pipe flanges.

BS 10: British Standard for flanges and bolting for pipes, valves, and fittings.

ISO 7005: International Standard for metallic flanges.

MSS SP-44: Manufacturers Standardization Society Standard for steel pipeline flanges.

AWWA C207: American Water Works Association standard for steel pipe flanges.

ANSI B16.1: Standard for cast iron pipe flanges and flanged fittings.

ANSI B16.24: Standard for copper alloy pipe flanges and flanged fittings.

ANSI B16.42: Standard for ductile iron pipe flanges and flanged fittings.

AS 2129: Australian Standard for flanges for pipes, valves, and fittings.

GOST 12820/12821: Russian standard for flanges.

KS B 1503: Korean standard for steel pipe flanges.

Pressure Ratings

Class 150: Standard pressure rating for flanges (up to 275 PSI depending on temperature and material).

Class 300: Medium pressure rating for flanges (up to 720 PSI depending on temperature and material).

Class 600: High pressure rating for flanges (up to 1440 PSI depending on temperature and material).

Class 900: Higher pressure rating for flanges (up to 2160 PSI depending on temperature and material).

Class 1500: Very high pressure rating for flanges (up to 3600 PSI depending on temperature and material).

Class 2500: Highest standard pressure rating for flanges (up to 6000 PSI depending on temperature and material).

PN (Pressure Nominal): European designation for pressure rating (e.g., PN 10, PN 16, PN 25, PN 40).

JIS 5K: Japanese pressure rating (approximately equivalent to Class 150).

JIS 10K: Japanese pressure rating (approximately equivalent to Class 300).

JIS 20K: Japanese pressure rating (approximately equivalent to Class 600).

ASA 300: Older designation for Class 300 flanges.

BS 10 Table D: British Standard pressure rating.

BS 10 Table E: British Standard pressure rating.

BS 10 Table F: British Standard pressure rating.

BS 10 Table H: British Standard pressure rating.

Material Specifications

ASTM A105: Carbon steel forgings for piping applications.

ASTM A182: Forged or rolled alloy and stainless steel pipe flanges.

ASTM A350: Carbon and low-alloy steel forgings for low-temperature service.

ASTM A515: Carbon steel plates for pressure vessels for intermediate and higher temperature service.

ASTM A516: Carbon steel plates for pressure vessels for moderate and lower temperature service.

ASTM A53: Standard specification for pipe, steel, black and hot-dipped, zinc-coated, welded and seamless.

ASTM A106: Seamless carbon steel pipe for high-temperature service.

ASTM A193: Alloy-steel and stainless steel bolting materials.

ASTM A194: Carbon and alloy steel nuts for bolts.

ASTM A234: Piping fittings of wrought carbon steel and alloy steel.

ASTM A694: Carbon and alloy steel forgings for pipe flanges for high-pressure transmission service.

ASTM A216: Steel castings for pressure-containing parts.

ASTM A217: Steel castings for pressure-containing parts suitable for high-temperature service.

ASTM A240: Chromium and chromium-nickel stainless steel plate, sheet, and strip.

ASTM A479: Stainless steel bars and shapes for use in boilers and other pressure vessels.

ASTM A536: Ductile iron castings.

ASTM B148: Aluminum-bronze sand castings.

ASTM B564: Nickel alloy forgings.

ASTM B462: Copper-nickel-zinc alloy rod, bar, and shapes.

Material Grade Designations

F304/304L: Austenitic stainless steel (18% Cr, 8% Ni).

F316/316L: Austenitic stainless steel with molybdenum for better corrosion resistance.

F321: Titanium-stabilized austenitic stainless steel.

F51/F60: Duplex stainless steel grades.

F53: Super duplex stainless steel.

F11/F22: Chrome-molybdenum alloy steel for high-temperature service.

F5/F9: Chrome alloy steel for high-temperature service.

F91: Advanced chrome-molybdenum alloy steel for high-temperature service.

WCB: Carbon steel casting grade.

WCC: Carbon steel casting grade with tighter requirements.

LCB/LCC: Low-temperature carbon steel casting grades.

Monel 400: Nickel-copper alloy for corrosive environments.

Inconel 625: Nickel-chromium-molybdenum alloy for highly corrosive environments.

Incoloy 825: Nickel-iron-chromium alloy with copper and molybdenum additions.

Hastelloy C276: Nickel-molybdenum-chromium alloy for severe corrosive services.

Titanium Grade 2: Commercially pure titanium.

Alloy 20: Nickel-iron-chromium alloy with copper and molybdenum for sulfuric acid resistance.

Flange Testing and Inspection

Hydrostatic Testing: Testing flanged joints with pressurized water to check for leaks.

Pneumatic Testing: Testing flanged joints with pressurized air or gas to check for leaks.

Ultrasonic Testing (UT): Non-destructive testing method using sound waves to detect flaws.

Radiographic Testing (RT): Non-destructive testing method using X-rays or gamma rays to detect internal flaws.

Magnetic Particle Testing (MT): Non-destructive testing method for detecting surface and near-surface flaws in ferromagnetic materials.

Dye Penetrant Testing (PT): Non-destructive testing method for detecting surface-breaking flaws.

Hardness Testing: Testing to determine the resistance of a material to permanent deformation.

Positive Material Identification (PMI): Testing to verify the chemical composition of a material.

Visual Inspection: Examination by eye to detect visible defects.

Dimensional Inspection: Checking flange dimensions against specified tolerances.

Surface Finish Measurement: Quantifying the roughness of the flange face.

Impact Testing (Charpy V-Notch): Testing the impact resistance of materials at low temperatures.

Ferrite Content Testing: Measuring ferrite content in austenitic stainless steel welds.

Heat Treatment Verification: Confirming proper heat treatment through hardness or microstructure testing.

Intergranular Corrosion Testing: Testing for susceptibility to intergranular corrosion.

Flange Installation Terms

Bolt Torque: The amount of rotational force applied to a bolt or stud during installation.

Bolt Tensioning: The process of applying a specific tension to bolts to achieve proper gasket compression.

Bolt Preload: The initial tension applied to a bolt during installation.

Flange Alignment: The process of aligning two flanges before bolting them together.

Flange Facing: The process of machining the face of a flange to achieve the required surface finish.

Bolt-Up Sequence: The specific order in which bolts should be tightened during flange assembly.

Star Pattern: A crisscross pattern used for tightening bolts to ensure even gasket compression.

Gasket Compression: The amount by which a gasket is compressed during flange assembly.

Cold Pull: Intentional misalignment of piping to accommodate thermal expansion.

Hot Torquing: Tightening of bolts while the system is at operating temperature.

Bolt Lubrication: Application of lubricant to bolts to achieve proper torque-tension relationship.

Bolt Load: The tensile force in a bolt after tightening.

Gasket Seating Stress: The compressive stress required to seat a gasket.

Gasket Operating Stress: The compressive stress required to maintain a seal during operation.

Flange Rotation: Angular displacement of a flange face under load.

Flange Separation: Axial displacement between flange faces under pressure.

Gap Measurement: Measuring the gap between flange faces before assembly.

Anti-Seize Compound: A lubricant applied to bolts to prevent galling and seizure.

Bolt Elongation: The increase in bolt length due to tensioning.

Ultrasonic Bolt Tensioning: Using ultrasonic measurement to determine bolt tension.

Flange-Related Problems

Flange Leakage: Unwanted escape of fluid through a flanged joint.

Flange Distortion: Deformation of a flange due to excessive stress or temperature.

Bolt Relaxation: The loss of bolt tension over time due to various factors.

Gasket Blowout: Catastrophic failure where a gasket is expelled from between flanges due to pressure.

Gasket Creep: Continued compression of a gasket over time under load.

Flange Corrosion: Deterioration of flange material due to chemical or environmental factors.

Stress Corrosion Cracking (SCC): Cracking induced from the combined influence of tensile stress and a corrosive environment.

Galvanic Corrosion: Corrosion caused by the contact of dissimilar metals in an electrolyte.

Bolt Fatigue: Failure of bolts due to cyclic loading.

Galling: A form of wear caused by adhesion between sliding surfaces, especially in threaded connections.

Hydrogen Embrittlement: Loss of ductility in metal caused by hydrogen absorption.

Thermal Cycling Damage: Damage caused by repeated heating and cooling cycles.

Erosion-Corrosion: Combined effect of erosion and corrosion in flowing systems.

Flange Face Damage: Scratches, nicks, or gouges on the flange face affecting sealing.

Gasket Crush: Excessive compression of a gasket beyond its design limits.

Differential Settlement: Uneven settlement of pipe supports causing stress on flange joints.

Flange Warping: Distortion of a flange face due to welding heat or improper handling.

Vibration-Induced Loosening: Loosening of bolts due to mechanical vibration.

Misalignment-Induced Stress: Stress caused by improper alignment of mating flanges.

Thermal Expansion Stress: Stress caused by differential thermal expansion in a piping system.

Bolting and Fasteners

Stud Bolt: A threaded rod with nuts on both ends, commonly used for flange connections.

Hex Bolt: A bolt with a hexagonal head.

Heavy Hex Bolt: A bolt with a larger hexagonal head than standard.

Hex Nut: A nut with a hexagonal shape.

Heavy Hex Nut: A nut with a larger hexagonal shape than standard.

Washer: A disk-shaped plate used to distribute the load from a bolt or nut.

Lock Washer: A washer designed to prevent a fastener from loosening.

Thread Pitch: The distance between adjacent threads.

Thread Series: Classifications of thread forms (e.g., UNC, UNF, UNEF).

Unified National Coarse (UNC): A common thread series with coarse pitch.

Unified National Fine (UNF): A common thread series with fine pitch.

Metric Thread: Thread conforming to the ISO metric screw thread standard.

Coarse Thread: Threads with larger pitch, providing quicker assembly.

Fine Thread: Threads with smaller pitch, providing stronger connections.

Bolt Grade: Classification of bolt strength (e.g., SAE Grade 5, Grade 8).

A193 B7: A common high-strength alloy steel bolt material specification.

A193 B8: A common stainless steel bolt material specification.

A193 B8M: A molybdenum-bearing stainless steel bolt material specification.

A320 L7: A low-temperature service alloy steel bolt material specification.

Special Flange Applications

Fire Safe Flange: A flange design that maintains a seal even during a fire.

Cryogenic Flange: A flange designed for use in extremely low-temperature applications.

High-Temperature Flange: A flange designed for use in elevated temperature applications.

High-Pressure Flange: A flange designed to withstand extremely high pressures.

Vacuum Flange: A flange designed for vacuum service (e.g., ISO-K, CF, KF).

Quick-Release Flange: A flange designed for rapid assembly and disassembly.

Dielectric Flange: A flange with electrical insulation to prevent galvanic corrosion.

Split Flange: A two-piece flange that can be installed around existing pipe.

Jacketed Flange: A flange with an outer jacket for heating or cooling service.

Rotatable Flange: A flange that can be rotated to align bolt holes.

Subsea Flange: A flange designed for underwater applications.

Compact Flange: A non-standard flange design with reduced face-to-face dimensions.

NACE-Compliant Flange: A flange that complies with NACE (National Association of Corrosion Engineers) standards for sour service.

Wafer Flange: A flange designed to be sandwiched between two conventional flanges.

Steam Service Flange: A flange specifically designed for steam applications.

Flange Design and Engineering

Flange Rating Interpolation: Determining pressure ratings between specified temperature points.

Flange Design Pressure: The maximum allowable working pressure at the design temperature.

Flange Design Temperature: The temperature used for determining the pressure rating.

MAWP (Maximum Allowable Working Pressure): The maximum pressure permitted during normal operation.

Bolted Joint Analysis: Engineering analysis of a bolted flange joint.

Flange Compatibility: Matching of different flange standards for connection.

Effective Gasket Width: The width of the gasket that actually contributes to sealing.

Gasket Factor (m): A factor representing the gasket material’s ability to create a seal.

Gasket Constant (y): The minimum seating stress required for a gasket.

Tightness Class: A classification of the required leak tightness of a joint.

Flange Moment Capacity: The ability of a flange to resist bending moments.

Stress Intensification Factor: A factor used in stress analysis to account for stress concentrations.

Flange Deflection: The bending or deformation of a flange under load.

Flange Rotation Allowance: The maximum acceptable angular rotation of a flange.

Hub Stress: Stress developed in the hub section of a flange.

Ring Stress: Stress developed in the ring section of a flange.

Flange Manufacturing

Forging: The process of shaping metal by applying localized compressive forces.

Casting: The process of pouring molten metal into a mold to create a shape.

Plate Flange: A flange cut from plate material.

Machining: The process of removing material to achieve the final dimensions and finish.

Hot Rolling: A metalworking process where metal is passed through rollers at high temperature.

Cold Rolling: A metalworking process where metal is passed through rollers at room temperature.

Heat Treatment: Thermal processes to alter the physical and mechanical properties of a material.

Normalizing: A heat treatment process to achieve a uniform grain structure.

Annealing: A heat treatment process to increase ductility and reduce hardness.

Quenching: Rapid cooling of a material to obtain desired properties.

Tempering: A heat treatment process to increase toughness after quenching.

Solution Annealing: A heat treatment process for stainless steels and nickel alloys.

Stress Relief: A heat treatment process to reduce internal stresses.

Surface Finish: The texture of a surface after manufacturing processes.

Surface Roughness: A measure of the texture of a surface, often expressed in microinches or Ra values.

Flange Dimensional Terminology

NPS (Nominal Pipe Size): A North American standard for pipe size designation.

DN (Diamètre Nominal): European standard for pipe size designation.

RF Height: The height of the raised face above the flange base.

Bolt Length: The overall length of a bolt or stud.

Grip Length: The portion of a bolt that is gripped between the nut and bolt head.

Thread Engagement: The length of thread in contact between a bolt and nut.

Hub Length: The axial length of the hub portion of a flange.

Hub Diameter: The outside diameter of the hub portion of a flange.

Back Hub Diameter: The diameter at the back of the hub where it meets the flange ring.

Flange Thickness Variation: The allowable variation in thickness across a flange.

Flatness Tolerance: The allowable deviation from a perfectly flat surface.

Parallelism: The condition of two surfaces being parallel within a specified tolerance.

Concentricity: The condition of having the same center.

Eccentricity: The distance between the centers of two circular features.

Out-of-Round: The deviation from a perfect circle.

Industry-Specific Flange Applications

Chemical Processing Flanges: Flanges used in chemical processing with special material considerations.

Petrochemical Flanges: Flanges used in petrochemical industries, often to ASME or API standards.

Oil and Gas Flanges: Flanges specific to upstream, midstream, and downstream oil and gas applications.

Power Generation Flanges: Flanges used in power plants, often for high-temperature and high-pressure steam.

Nuclear Flanges: Flanges meeting strict nuclear industry standards.

Water Treatment Flanges: Flanges used in water and wastewater treatment plants.

Food Processing Flanges: Sanitary flanges used in food and beverage industries.

Pharmaceutical Flanges: High-purity flanges used in pharmaceutical manufacturing.

Semiconductor Flanges: Ultra-high purity flanges used in semiconductor manufacturing.

Cryogenic Processing Flanges: Flanges designed for extremely low-temperature service.

Marine Flanges: Flanges used in shipbuilding and offshore applications.

Pulp and Paper Flanges: Flanges used in pulp and paper manufacturing.

Mining Industry Flanges: Flanges used in mining applications, often with abrasion resistance.

HVAC Flanges: Flanges used in heating, ventilation, and air conditioning systems.