A flange is the protruding rim that enable pipes, valves and other equipment to form a connection. Flanges also increase strength at the joint. They enable you to create a piping system while also allowing for fast disassembly. By providing access points, inspections or modifications can be carried out with relative ease.
How you make a pipe-flange connection varies. It depends on the type and the requirements of your piping system. Some are weld-on pipe flange, while others can be screwed on. The type of flange you choose will also depend on issues such as pressure capacity and application. But first, let’s look at common flange types.
While specialty pipe flanges are available, there are six main types. Here they are, along with how to secure the flange on a pipe.
They’re used in low-pressure systems on smaller pipes with thick walls. They also make it easy to connect and disconnect pipe systems without disrupting the entire system for maintenance or to make adjustments. To attach the flange on a pipe, the bore’s female thread is screwed onto the pipe’s external threads. These flanges are not welded on.
Typical use:
The simple design is intended for small-size and high-pressure piping that do not transfer highly corrosive fluids. Socket weld flanges are attached by inserting the pipe into the socket and applying one fillet weld around the outside of the flange. First, the pipe is inserted in the socket of the flange. When it reaches the
bottom of the flange, the pipe is lifted out slightly by 1/16" (1.5mm) and welded. This gap allows for thermal expansion created by welding, minimizing the probability that the weld will crack. Not suitable for highly erosive or corrosive applications, as the gap is vulnerable to corrosion between the pipe end and the socket’s shoulder.
Typical use:
Slides over the pipe and used with a stub end. Also known as loose-ring flanges, and back-up flanges. These flanges are used on piping fitted with lapped pipe or with lap joint stub ends. With the stub end, the lap joint flange is typically used in systems requiring frequent dismantling for inspection and cleaning. Another advantage is its ability to swivel and align with bolt holes. As the flange never comes into contact with the fluid, the flange is highly durable and can be re-used.
Not recommended in extreme or high-pressure temperature applications.
Typical use:
These low-pressure flanges are thinner than most other flanges. With an inside diameter slightly larger than the pipe’s outside diameter, the flange slips onto the pipe. A fillet weld is applied at the top of the flange and at the bottom. The welds enhance strength and prevent leakage. Also known as hubbed flanges. Installation of slip-on pipe flanges is easy and therefore low cost.
The speed at placing the flange on the pipe saves costs, but those savings are reduced with the additional costs of two fillet welds which are needed for proper installation.
Typical use:
The lack of an inner hole enables blind flanges to seal off the end of pipe systems, preventing flow. This makes it easier and more cost efficient to carry out pressure tests. The blind flange connection is also an ideal pipeline flange. You can stop the flow of fluid and safely add new pipes or new lines to the pipeline.
Without blind flanges, shutdowns and repairs would be incredibly difficult to handle. While shutoff valves solve the problem of stopping flow, the location of the valve can be a problem. For example, if the valves are a mile or two away, then you’re looking at a significant amount of wasted fluid.
Blind flanges are installed with bolts, so they’re also easy to remove.
Typical use:
Also known as weld bend flanges. Their long necks are butt welded to a pipe. The flange’s bore matches that of the pipe, reducing turbulence and erosion. This flanged connection relocates stress to the pipes, ensuring a decrease in high-stress concentration at the bottom of the flange. When installing, weld neck pipe flanges must be positioned parallel at the time of fitting. Flanges at opposite ends of a pipe should typically have the same bolt-hole direction too.
Typical use:
Type
Pressure capacity
Pipe sizes
Use for
Threaded
Low
Small
Attaching without welding
Lap joint
Low
All
Systems that need frequent disassembly
Slip on
Low
All
Low installation cost; easy assembly
Blind
Very high
All
Flow pressure testing; closing pipes
Welded neck
High
All
High-pressure systems; extreme temperatures
The flange face is the area on the head where your gasket will go. The three most common types are:
Flanges with different faces should not be mated. For example, a raised face to flat-face flange connection will result in leakage from the joint, per ASME code B31.3.
The most common type is the raised face flange. Its name comes from the raised gasket surface, above the bolting circle face. The raised face concentrates more pressure on a smaller gasket area. In turn, this increases the joint’s pressure containment capability.
The height of the raised face is determined by the flange’s pressure rating. Likewise, the higher the pressure rating, the bigger the flange diameter, the more bolts needed and the thicker the flange.
Raised face vs. flat face flange, also called full face flange, isn’t an issue. They play different roles. Instead of a raised face, this is a flat surface. Consequently, the gasket surface is the same plane as the bolt frame, covering the flange from its inside diameter to outside diameter.
High temperature, high-pressure flanges or rather, flanges used in extreme environments, utilize a ring-type-joint face. These flanges often have a raised face with a ring groove machined into it. They can also have grooves cut into their faces with steel ring gaskets. The flanges seal when the bolts are torqued, compressing the gasket between the flanges into the grooves.
The purpose behind a raised face flange is to concentrate more pressure on a smaller gasket area, increasing the joint’s ability to contain the pressure. Ring-type-joint face flanges don’t use gaskets. The groove within the flange enables the ring to center itself when the bolts are torqued. As the process pressure increases, so, too, does the sealing pressure.
Typically used:
Raised face flange
Flat face flange
Ring-type- joint face
Process plants (chemical, etc.)
l
Oil & gas
l
l
Valves
l
Cast-iron equipment
l
Low-pressure water pipe systems
l
Severe applications: high pressure and high temperature (up to 1,382⁰F/ 750⁰C)
l
The pipe flange face is vulnerable to damage during handling and transport, so you need to think about flange covers. This flange cover shown here is made of durable and flexible polyethylene. It protects both raised and flat face flanges, along with full-face gaskets.
Flange protectors are also critical. The studded full face flange protector here is also made of polyethylene and provides full coverage by inserting the lugs firmly into the bolt holes.
Understand more about flange covers in our guide, Why Flange Covers are Essential.
The raised face flange protector shown here has an adhesive backing for fast application.
You can learn more about flange protection in Pipe & flange protection: a quick buyer’s guide.
Whether steel pipe flanges or a plastic flange, you should base the material you choose on its application. Typically, the materials should match your pipe material, but again, it depends on your application. For example, if your piping system is used for air or other non-corrosive applications, then your flanges and pipes may not need to be of the same corrosion-resistant material as acidic or caustic liquids.
Otherwise, if you’re interested in say, carbon steel slip-on flanges, you need to match the grade steel you choose to the pipe. Common flange materials include:
Carbon-steel flanges are alloyed primarily with carbon. Carbon steel has a high hardness and strength that increases with carbon content but lowers ductility and melting point. Carbon steels range from mild and low, with 0.16—0.29% carbon to ultra-high carbon steel, with around 1–2% carbon. Steels with carbon content above 2% are considered cast iron.
Just as you can enhance properties in plastics with additives, you can alloy steels with other elements to enhance the properties of your chose steel. Common alloys include molybdenum and chromium. Through different elements, you can increase a steel’s strength, ductility, corrosion resistance, and machinability.
Stainless steel is alloyed with chromium in amounts above 10%. It’s chromium that gives stainless steel a higher corrosion resistance than carbon steel, which easily oxidizes from air and moisture exposure. Stainless steel is ideal for corrosive applications that also require high strength. Different grades of stainless steels yield different outcomes. You can learn more in our guide, Understanding stainless-steel grades.
When iron is alloyed with carbon, silicon, and other alloyants, the result is cast iron. Cast irons have good fluidity, castability, machinability, and wear resistance. They tend to be brittle to a degree with low melting points.
This is a low-density metal with medium strength. Malleable and ductile, it’s more corrosion resistant than typical carbon and alloy steels. Aluminum is suitable for flanges that need both strength and low weight, such as for irrigation applications.
A PVC flange is low cost and durable. PVC pipe flanges also have the advantage of being both chemical and corrosion resistant. Additives can make PVC more flexible and softer. PVC flange connections can provide extra protection to underlying pipes. They’re also popular for water-treatment processes, favored by the agricultural industry and used in domestic plumbing. Lightweight and easy to install, PVC flanges are perfect for PVC pipe systems.
For all flange types excluding ring-type-joint faces, you’ll need gaskets. Choose them based on factors such as operating temperature, the fluid being conveyed by the pipeline, flange type, size, pressure class or rating and other specifications. First, however, you need to know material properties, which can be enhanced with additives.
Suitable for
EPDM
Natural rubber/SBR
Neoprene
Nitrile
Butyl
Silicone
Keytones
l
Hot & cold water
l
Alkalis
l
Acids & bases
l
l
Heat
l
l
Oils
l
Hot oils
l
Ozone
l
l
l
Weather resistance
l
l
l
Abrasion resistance
l
l
Low-moisture permeability
l
Low-gas permeability
l
l
Seawater applications
l
l
High temperatures
l
Low temperatures
l
Vibration suppression
l
General chemical resistance
l
Animal fats
l
Flange dimensions are determined by the nominal pipe size (NPS) and the pressure class that your application requires. The higher the pressure rating of the class, the higher the size and dimensions of the flange. Typically, a 1/2" pipe flange will belong to the 150-pound pressure class. For threaded flange, a 4" pipe flange is the largest size available.
Large diameter flanges are in higher pressure classes. Below are the different classes:
Flange types and sizes vary, but they’re manufactured according to the standards set by organizations. For example, a long weld neck flange ASME B16.5 conforms to ASME standards, and might differ slightly from another standard. ASME B16.5 swivel flange dimensions – a type of weld neck flange – have equivalents in EN and MSS standards.
(If you see, for example, ANSI 150 swivel flange dimensions, this is incorrect. ANSI at one time published standards and started working with ASME in 1988. At the time standards appeared as ANSI/ASME, but by 1996, ASME had taken over standards.)
So your first task is determining the standard you’ll work to, which will likely follow what’s commonly used in your area.
Size standards are set according for each pressure class.
The example here is a threaded flange, both raised face and flat face, according to ASME B16.5. The pressure class is 150.
Flange NPS
I.D.
O.D.
Bolt circle (BC)
Raised face (R)
Raised face (RF)
H
Raised face thickness (T)
T1
Flat face thickness (T2)
Bolt hole (B)
No. of bolt holes
½
0.93
3.50
2.38
1.38
.063
1.19
.62
.38
.56
.62
4
¾
1.14
3.88
2.75
1.69
.063
1.50
.62
.44
.56
.62
4
1
1.41
4.25
3.12
2.00
.063
1.94
.69
.50
.63
.62
4
1 ¼
1.75
4.62
3.50
2.50
.063
2.31
.81
.56
.75
.62
4
1 ½
1.99
5.00
3.88
2.88
0.63
2.56
.88
.62
.82
.62
4
2
2.50
6.00
4.75
3.62
0.63
3.06
1.00
.69
.94
.75
4
2 ½
3.00
7.00
5.50
4.12
0.63
3.56
1.12
.82
1.06
.75
4
3
3.63
7.50
6.00
5.00
0.63
4.25
1.19
.88
1.13
.75
4
3 ½
4.13
8.50
7.00
5.50
0.63
4.81
1.25
.88
1.19
.75
8
4
4.63
9.00
7.50
6.19
0.63
5.31
1.31
.88
1.25
.75
8
5
5.69
10.00
8.50
7.31
0.63
6.44
1.44
.88
1.38
.88
8
6
6.75
11.00
9.50
8.50
0.63
7.56
1.56
.94
1.50
.88
8
8
8.75
13.50
11.75
10.62
0.63
9.69
1.75
1.06
1.69
.88
8
10
10.88
16.00
14.25
12.75
0.63
12.00
1.94
1.13
1.88
1.00
12
12
12.94
19.00
17.00
15.00
0.63
14.38
2.19
1.19
2.13
1.00
12
14
14.19
21.00
18.75
16.25
0.63
15.75
2.25
1.32
2.19
1.12
12
16
16.19
23.50
21.25
18.50
0.63
18.00
2.50
1.38
2.44
1.12
16
18
18.19
25.00
22.75
21.00
0.63
19.88
2.69
1.50
2.63
1.25
16
20
20.19
27.50
25.00
23.00
0.63
22.00
2.88
1.63
2.83
1.25
20
22
22.19
29.50
27.25
25.25
0.63
24.25
3.13
1.75
3.07
1.38
20
24
24.19
32.00
29.50
27.25
0.63
26.12
3.25
1.82
3.19
1.38
20
DN
D
(diameter)
k
(PCD)
b
(thickness)
d2
(diameter of bolt hole)
Bolt size
No. of holes
Weight (kg)
10
90
60
16
14
M12
4
0.72
15
95
65
16
14
M12
4
0.81
20
105
75
18
14
M12
4
1.14
25
115
85
18
14
M12
4
1.38
32
140
100
18
18
M16
4
2.03
40
150
110
18
18
M16
4
2.35
50
165
125
18
18
M16
4
2.88
65
185
145
18
18
M16
8
3.51
80
200
160
20
18
M16
8
4.61
100
220
180
20
18
M16
8
5.65
125
250
210
22
18
M16
8
8.13
150
285
240
22
22
M20
8
10.5
200
340
295
24
22
M20
8
16.5
250
395
350
26
22
M20
12
24.1
300
445
400
26
22
M20
12
30.8
350
505
460
26
22
M20
16
39.6
400
565
515
26
26
M24
16
49.4
450
615
565
26
26
M24
20
63.0
500
670
620
28
26
M24
20
75.2
600
780
725
34
30
M27
20
124
700
895
840
38
30
M27
24
183
800
1015
950
42
33
M30
24
29.7
900
1115
1050
46
33
M30
28
374
1000
1230
1160
52
36
M33
28
492
1200
1455
1380
60
39
M36
32
842
Free CADs are available for most solutions, which you can download. You can also request free samples to make sure you’ve chosen the right product for what you need.
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When purchasing flanges, you must consider the flange material in addition to physical measurements and bolt hole alignment. The chemical composition and physical properties are important in determining whether a material will be appropriate for a given application. You can look to a combination of standards to guide your decision.
Pipes and flanges must be durable enough to withstand heat, pressure, vibrations, and corrosion, while still providing a leak proof seal. Flange class helps determine bolt hole alignment, pressure and temperature rating, and dimensional standards, however, flange material specifications must also be considered based on your application:
The manufacturing method is also important to understand. There are typically three methods used:
Choose the right flange material based on its intended use and the requirements specific to that application. Generally, your flange material will match your pipe material, but this is not always the case. Consider the strengths and weaknesses of each material when designing a piping system; the material should suit the application requirements or it may fail prematurely.
Pipe material is chosen based on variables associated with an application, including:
Certain fluids, for example, fuel or corrosive liquids, require pipes and flanges manufactured of stainless steel, alloy, or other corrosion resistant materials. Pipes and flanges used for air or other non-corrosive applications may not require the same corrosion resistant material as acidic or caustic liquids.
The standards dictate the chemical composition, dimensions, and other specifications for flanges. Generally, your flange and pipe material should meet the same standards.
American Society for Testing and Materials (ASTM) standards—displayed as the “grade” of the material used to manufacture the flange—is determined by the metal’s chemical composition. More specifically, this standard dictates how much carbon and other chemical elements are allowed to make up the raw material.
Guobiao (GB) standards are issued by the Standardization Administration of China (SAC) and include:
American Water Works Association (AWWA) standards include specifications for steel and stainless steel. AWWA flanges include:
American National Standards Institute (ANSI) and American Society of Mechanical Engineers (ASME) are the industry standards for pipeline flanges in the United States. These standards include B16.1, B16.5, and B16.47 and cover steel, forged, or cast flanges.
Deutsches Institut für Normung (DIN, German Institute of Standardization) is an international standard that applies to steel and stainless steel metric flanges and commonly in maritime applications.
It is also important to consider the requirements around where the flange material is sourced from. For many applications it may be fine to us import material in order to save on cost, but often Buy America or Domestic material may be required.
The Buy America standard was developed for government funded transportation projects to provide preference to domestically produced materials, but there are provisions that allow for materials to be sourced from a list of approved countries if it is a small enough part of the total contract, and domestic product availability is low or price is too high.
Our Domestic flanges meet the American Iron & Steel (AIS) standard, which allows imported raw components of the metal, but requires that the flange material be melted and manufactured in the United States.
Domestic flanges are able to be substituted in Buy America applications, and Buy America flanges can be substituted for applications where Import flanges are acceptable, but they can’t be substituted in the other direction.
Consider the pipe material, pressure and temperature ratings, flange dimensions, and tolerances, to determine the appropriate flange material. ANSI ratings provide information on pressure and heat tolerances.
Fitting type is an important consideration as well. Does your fitting require welding? If so, the material you use should allow welding. For non-welded styles, such as lap joint or threaded flanges, weldability is less of a concern, and other requirements will take precedence.
When choosing the ideal flange material, refer to the standards required for your intended application, consider the pipe material that will connect to the flange, and ensure the flange material you choose is rated for the temperature and pressure it must withstand. Or, contact API International, Inc. for assistance with standard fittings or custom machined parts or for further information.