Pipe Flanges Information

Pipe flanges are protruding rims, edges, ribs, or collars used to make a connection between two pipes or between a pipe and any type of Fitting or equipment component. Pipe flanges are used for dismantling piping systems, temporary or mobile installations, transitions between dissimilar materials, and connections in environments not conducive to solvent cementing.

Flanges are simple mechanical connectors that have been used successfully for high-pressure piping applications. They are well understood, reliable, cost-effective, and readily available from a wide range of suppliers. In addition, the moment-carrying capacity of flanges is significant compared to other mechanical connectors. This is an important feature for systems that experience pipe-walking or lateral buckling from temperature and pressure variations (e.g., deep water lines). Flanges can be designed to meet a wide range of application requirements such as high-temperature and corrosion resistance. 


Pipe flanges have flush or flat surfaces that are perpendicular to the pipe to which they attach. Two of these surfaces are mechanically joined via bolts, collars, adhesives, or welds.

This video depicts the layout of a flange connection using bolts.

Typically, flanges are attached to pipes via welding, brazing, or threading.

Welding joins materials by melting the workpieces and adding a filler material. For strong, high-pressure connections of similar materials, welding tends to be the most effective method of flange connection. Most pipe flanges are designed to be welded to pipes.

Brazing is used to join materials by melting a filler metal which solidifies to act as the connector. This method does not melt the workpieces or induce thermal distortion, allowing for tighter tolerances and clean joints. It also can be used to connect very dissimilar materials such as metals and metalized ceramics.

Threading is applied to flanges and pipes to allow the connections to be screwed together in a manner like nuts or bolts.

While the method of attachment can be a distinguishing feature, there are other considerations more important to pipe flange selection. Factors an industrial buyer should consider first are the flange’s physical specifications, type, material, and performance features most suitable for the application. 

Physical Specifications

Primarily, a flange must fit the pipe or equipment for which it is designed. Physical specifications for pipe flanges include dimensions and design shapes. 

Flange Dimensions

Physical dimensions should be specified to size flanges correctly.

  • Outside diameter (OD) is the distance between two opposing edges of a flange’s face. This can a
  • Thickness refers to the thickness of the attaching outer rim and does not include the part of the flange that holds the pipe.
  • Bolt circle diameter is the length from the center of a bolt hole to the center of the opposing hole.
  • Pipe size is a pipe flange’s corresponding pipe size, made according to accepted standards. It is usually specified by two non-dimensional numbers, nominal pipe size (NPS) and schedule (SCH).
  • Nominal bore size is the inner diameter of the flange connector. When manufacturing and ordering any type of pipe connector, it is important to match the bore size of the piece with the bore size of the mating pipe.

Flange Faces

Flange faces can be manufactured to many custom shapes-based design requirements. Some examples include:

  • Flat
  • Raised face (RF)
  • Ring type joint (RTJ)
  • O-ring groove

Types of Pipe Flanges

Pipe flanges can be divided into eight types based on design. These types are blind, lap joint, orifice, reducing, slip-on, socket-weld, threaded, and weld neck.

Blind flanges are round plates with no center hold used to close the ends of pipes, valves, or equipment. They assist in allowing easy access to a line once it has been sealed. They can also be used for flow pressure testing. Blind flanges are made to fit standard pipes in all sizes at higher pressure ratings than other flange types.

Blind flange. 

Lap joint flanges are used on piping fitted with lapped pipe or with lap joint stub ends. They can rotate around the pipe to allow for an easy alignment and assembly of bolt holes even after the welds have been completed. Because of this advantage, lap joint flanges are used in systems requiring frequent disassembly of the flanges and pipe. They are like slip-on flanges but have a curved radius at the bore and face to accommodate a lap joint stub end. The pressure ratings for lap joint flanges are low but are higher than for slip-on flanges.

Typical lap joint flange.

Slip-on flanges are designed to slide over the end of piping and then be welded in place. They provide easy and low-cost installation and are ideal for lower pressure applications.

Typical slip-on flange.

Socket weld flanges are ideal for small-sized, high-pressure piping. Their fabrication is like that of slip-on flanges, but the internal pocket design allows for a smooth bore and better fluid flow. When internally welded, these flanges also have fatigue strength 50% greater than double welded slip-on flanges.

Diagram for a typical socket weld flange 

Threaded flanges are special types of pipe flange that can be attached to the pipe without welding. They are threaded in the bore to match external threading on a pipe and are tapered to create a seal between the flange and the pipe. Seal welds can also be used along with threaded connections for added reinforcement and sealing. They are best used for small pipes and low pressures and should be avoided in applications with large loads and high torques.

Typical threaded flange

Welding neck flanges have a long-tapered hub and are used for high pressure applications. The tapered hub transfers stress from the flange to the pipe itself and provides strength reinforcement that counteracts dishing.

Typical welding neck flange

TypePressure CapacityPipe SizesApplications / Advantages
BlindVery highAllClosing pipes, flow pressure testing
Lap jointLowAllSystems requiring frequent disassembly
Slip-onLowAllLow installation cost, simple assembly
Socket weldHighSmallSmooth bore for better fluid flow
ThreadedLowSmallAttachment without welding
Welding neckHighAllHigh pressures and extreme temperatures

Overview and comparison of pipe flange types by pressure capacity, suitable pipe sizes, and applications or advantages.

Special Flange Designs

Some flange types can be designed to incorporate special functions, such as size reduction or orifice mounting.

Orifice flanges are used in place of standard flanges to allow an orifice meter to be installed on the flange. Orifice plate carriers are designed into the flanges for fitting meter connections. These meters are used to measure the flow rate through the system at that point.

Typical orifice flange | 

Reducing flanges are used in place of standard flanges to allow for a change in pipe size. The flange consists of one specified diameter with a smaller diameter bore size. Except for the bore and hub dimensions, a reducing flange has dimensions of the larger pipe size. Welding neck, slip-on, and threaded flanges can be reducers, and are considered an economical means to make a pipe size transition.

Various reducing flanges and a typical cross-sectional diagram.

Materials of Construction

Pipe flanges can be made from several varied materials depending on the piping material and the requirements of the application. Selection depends on factors such as environmental corrosion, operating temperature, flow pressure, and economy. Some of the most common materials include carbon steel, alloy steel, stainless steel, cast iron, copper, and PVC.

Carbon steel is steel alloyed primarily with carbon. It has a high hardness and strength which increases with carbon content but lowers ductility and melting point. For more information on carbon and alloy steels, please visit the Carbon Steels and Alloy Steels area on Engineering360.

Alloy steel is steel alloyed with one or more elements which enhance or change the steel’s properties. Common alloys include manganese, vanadium, nickel, molybdenum, and chromium. Alloy steels are differentiated based on standard grades. For specific information on individual types of alloying elements, please visit the Metals and Alloys section on Engineering360.

Stainless steel is steel alloyed with chromium in amounts above 10%. Chromium enables stainless steel to have a much higher corrosion resistance than carbon steel, which rusts readily from air and moisture exposure. This makes stainless steel better suited for corrosive applications that also require high strength. For more information on stainless steel alloys, please visit the Stainless-Steel Alloys area on Engineering360.

Cast iron is iron alloyed with carbon, silicon, and several other alloyants. Silicon forces carbon out of the iron, forming a black graphite layer on the exterior of the metal. Cast irons have good fluidity, castability, machinability, and wear resistance but tend to be brittle with low melting points. For more information on cast irons, please visit the Cast Irons area on Engineering360.

Aluminum is a malleable, ductile, low-density metal with medium strength. It has better corrosion resistance than typical carbon and alloy steels. It is most useful in constructing flanges requiring both strength and low weight. For more information on aluminum, please visit the Aluminum and Aluminum Alloys area on Engineering360.

Brass is an alloy of copper and zinc, often with additional elements such as lead or tin. It is characterized by good strength, excellent elevated temperature ductility, reasonable cold ductility, good conductivity, excellent corrosion resistance, and good bearing properties. For more information on brass and other copper alloys, please visit the Copper, Brass, and bronze Alloys area on Engineering360.

PVC or polyvinyl chloride is a thermoplastic polymer that is inexpensive, durable, and easy to assemble. It is resistant to both chemical and biological corrosion. By adding plasticizers it can be made softer and more flexible.

Performance Features

Performance features are properties of a flange that may be dependent on several other factors but are nonetheless important to consider. These properties include weight, ease of assembly, and durability.

Weight is the mass or heaviness of a flange. It is dependent on size and material density. Industrial buyers should consider the strength of the pipe or pipe supports when dealing with large or high density flanges to ensure the weight can be properly supported.

Ease of assembly is a qualitative measure of the efficiency of the assembly and disassembly process. For applications where a flange is used as a temporary attachment or fix, the ease of set up and take down time can be very important.

Durability is the strength or toughness of a pipe flange under stress or pressure. Durability is dependent on the compatibility of the flange design with the pipe and the material strength. Flanges operating at high pressures require durable seals to operate effectively. Pipe flange products generally have a pressure rating that defines the maximum pressure the flange is designed to hold.


Flanges - Diagram

150lb Flange

Length through Hub Y

Nominal BorePipe OD AFlange dia. BRaised Face dia. DFlange Thickness TWelding NeckSlip-on Screwed Socket-weldBolt PCDNo. of BoltsBolt Hole dia.
2534.1107.9 50.814.3  55.6 17.5 79.4 15.9
3242.9117.563.5 15.9 57.1 20.6 88.9 15.9 
4048.4127.073.0 17.5 61.9 22.2 98.4 15.9 
5060.3152.492.1 19.0 63.5 25.4 120.6 19.0 
6573.0177.8104.8 22.2 69.8 28.6 139.7 19.0 
8088.9190.5127.0 23.8 69.8 30.2 152.4  419.0 
90101.6215.9139.7 23.8 71.4 31.7 177.8 19.0 
100114.3228.6157.2 23.8 76.2 33.3 190.5 19.0 
125141.3254.0185.7 23.8 88.9 36.5 215.9 22.2 
 168.3279.4215.9 25.4 88.9 39.7 241.3 22.2 
200 219.1 342.9 269.9 28.6 101.6 44.4 298.4 22.2 
350 355.6 533.4 412.7 34.9 127.0 57.1 476.2 12 28.6 

Approximate Mass (kg)

Nominal Bore mmWeld NeckSocket Weld Slip-on Screwed Blind
450 68.0452.62102.51
500 81.6565.32123.38
600 118.8491.63203.21


300lb Flange

Length through Hub Y

Nominal BorePipe OD AFlange dia. BRaised Face dia. DFlange Thickness TWelding NeckSlip-on Screwed Socket-weldBolt PCDNo. of BoltsBolt Hole dia.
2534.1123.8 50.817.561.927.088.9 419.0
3242.9133.363.5 19.065.1 27.098.4419.0
4048.4155.673.0 20.6 68.330.2114.3419.0
5060.3165.192.1 22.269.833.3127.0819.0
6573.0190.5104.8 25.476.238.1149.2822.2
8088.9209.5127.0 28.679.442.9168.3822.2
100114.3254.0157.2 31.785.747.6200.022.2
125141.3279.4185.7 34.998.450.8234.9822.2
 168.3317.5215.9 36.598.452.4 269.91222.2
200 219.1 381.0269.9 41.3111.161.9330.21225.4
350 355.6 584.2412.7 54.0142.976.2514.32031.7

Approximate Mass (kg)

Nominal Bore mmWeld NeckSocket Weld Slip-on Screwed Blind
15 0.910.73  0.76
201.41 1.25 1.13 
251.81 1.36 1.77 
322.27 2.04 2.68 
403.06 2.81 2.83 
503.74 3.13 3.52 
655.56 4.54 5.44 
807.37 6.12 7.26 
909.53 7.71 9.98 
10011.79 9.53 11.79 
12515.42 12.70 15.88 
15019.96 16.33 20.87 
20032.21 25.40 38.10 
25044.00 35.38 55.34 
30064.41 50.80 86.18 
35084.37 74.39 107.05 
400111.58 101.60 145.15 
450 138.35 126.10 181.89 
500 174.63 149.69 231.33 
600 247.21 222.26 342.92 

600lb Flange

Length through Hub Y

Nominal BorePipe OD AFlange dia. BRaised Face dia. DFlange Thickness TWelding NeckSlip-on Screwed Socket-weldBolt PCDNo. of BoltsBolt Hole dia.
2534.1123.8 50.817.561.9 27.0 88.9 419.0
3242.9133.363.5 20.666.728.698.4419.0
4048.4155.673.0 22.269.831.7114.3 419.0
5060.3165.192.1 25.473.036.5127.0819.0
6573.0190.5104.8 28.679.441.3149.2822.2
8088.9209.5127.0 31.782.546.0 168.3822.2
90101.6228.6139.7 34.985.749.2 184.1825.4
100114.3273.0157.2 38.1101.654.0 215.9825.4
125141.3330.2185.7 44.4114.360.3266.7828.6
 168.3355.6215.9 47.6117.566.7 292.11228.6
200219.1 419.1269.9 55.6133.376.2349.21231.7
350 355.6 603.2412.7 69.8165.193.7527.02038.1

Approximate Mass (kg)

Nominal Bore mmWeld NeckSocket Weld Slip-on Screwed Blind
450 217.27215.46301.64
500 312.98277.60387.82
600 443.16370.13521.63


Tolerances of ASA 150, 300 amd 600 Flanges to ANSI B 16.5 : 1968

Inside Diameter250 mm and under+ 0.8
 – 0
300 mm and under+ 1.6
 – 0
Outside Diameter600 mm and under± 1.6
Thickness450 mm and under+ 3.2
 – 0
500 mm and over+ 4.8
 – 0
Spot FacingSpot facing on the back of the flange shall be parallel to the flange face within± 1°
Outside Diameter of Hub 300 mm and smaller+ 2.4
 – 1.6
300 mm and over± 3.2
Total Depth450 mm and smaller+ 3.2
 – 0.8
450 mm amd over+ 4.8
 – 1.6
DrillingBolt Circle± 1.6
Bolt Spacing± 0.8
Chamfer AngelAll Sizes± 2.5°

Carbon Steel Flanges for Oil, Gas and Petro-chemical industries to ANSI B 16.5 and BS 1560 A105 Normalised.

Flange Specifications

15mm – 32mm

Flanges - 15 mm Nominal Bore Pipe Table

40mm – 80mm

Flanges - 40mm - 80mm Nominal Bore Pipe Table

100mm – 200mm 

Flanges - 100mm-200mm Nominal Bore Pipe Table

250mm – 400mm

Flanges - 250mm Nominal Bore Pipe Table