A guide to flanges
Different types of flanges
This guide answers that question and covers other topics, such as:
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.
Understanding Flanges
Understanding flanges is essential for anyone working with piping systems, as they play a crucial role in the system's overall functionality and safety. Flanges serve as connection points that allow for easy assembly, disassembly, inspection, and maintenance of various pipeline components. Knowledge of flange standards, such as ANSI, ASME, and DIN, helps ensure compatibility across different systems and applications. Each flange has specific pressure ratings, known as classes, which denote the maximum pressure it can withstand, ranging from low to high-pressure environments. Selecting the appropriate flange requires assessing factors such as material compatibility, pressure, temperature conditions, and environmental exposure. Proper understanding and selection can prevent potential issues such as leaks, corrosion, and mechanical failure, making flange knowledge integral to the successful operation of piping networks.
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.
Pipe-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.
Threaded flanges
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:
- Flammable, hazardous or explosive applications where welding is dangerous
- Ideal in restricted spaces where welding flange to pipe can’t be carried out
Socket weld flanges
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, minimising 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:
- Hydraulic pipes
Lap joint flanges
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:
- Low-pressure applications
- When flange needs frequent dismantling for maintenance
Slip-on flanges
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:
- Cooling and firefighting water lines
- Process lines for oil, gas and steam
Blind flanges
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:
- Testing pipe pressure
- Creating access points to piping systems
- Seal a piping system temporarily to make repairs, or permanently
Welded neck flanges
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:
- Piping systems with repeating bends
- In conditions with wide fluctuations in temperature and pressure
- Volatile and hazardous fluids
Characteristics of Flanges
Flanges are essential mechanical components that connect pipes, valves, pumps, and other equipment in piping systems, enabling efficient flow and maintenance access. They are typically circular in shape and are characterized by their strength, durability, and ability to withstand high pressures and temperatures. Flanges are available in various materials, including stainless steel, carbon steel, and alloys, each suited for specific applications and environments. Their surfaces are often finished with coatings or treatments to prevent corrosion and ensure longevity, especially in demanding industrial settings. Standard characteristics of flanges include dimensions such as outside diameter, bolt circle diameter, and thickness, which follow specific codes and standards. Proper selection of flange characteristics is crucial for compatibility and efficiency in piping systems. Overall, understanding these features helps in selecting the right flange for optimal performance and safety.
Flange types: at a glance
|
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 |
What is flange facing?
The flange face is the area on the head where your gasket will go. The three most common types are:
- Raised face (RF)
- Ring type joint (RTJ)
- Flat face (FF)
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.
Raised face flanges
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.
Flat face 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.
- Full face is designed to avoid the bending that flanges undergo as bolts are torqued.
- Cast iron can break during this process, which is why this material is often used to make flat face flanges. The design prevents this problem from happening.
Ring-type-joint face flange
High temperature, high-pressure flanges or rather, flanges used in extreme environments, utilise 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.
Ring joint flange vs raised face
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 centre itself when the bolts are torqued. As the process pressure increases, so, too, does the sealing pressure.
Face flanges: at a glance
|
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 |
Protecting flanges
The pipe flange face is vulnerable to damage during handling and transport, so you need to think about flange covers. The 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.
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.
Flange materials
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
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.
Alloy steel
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
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.
Cast iron
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.
Aluminium
This is a low-density metal with medium strength. Malleable and ductile, it’s more corrosion resistant than typical carbon and alloy steels. Aluminium is suitable for flanges that need both strength and low weight, such as for irrigation applications.
PVC
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, favoured by the agricultural industry and used in domestic plumbing. Lightweight and easy to install, PVC flanges are perfect for PVC pipe systems.
Gasket materials
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 sizes
ASME & EN
If using ASME standards, 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:
Pressure classes
- 150 lbs
- 300 lbs
- 400 lbs
- 600 lbs
- 900 lbs
- 1500 lbs
- 2500 lbs
Sizing
Flange types and sizes vary, but they’re manufactured according to the standards set by organisations. 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.
Sizes: inches
|
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 |
EN sizes
The UK has its own standard for pipe flange (BS4504). However, as the UK was part of the EU for so long, the European standards (EN) became the norm and are still used.
Where North America uses pressure class, Europe uses pressure ratings. EN flange-size charts are specified by PN, meaning ‘pressure nominale’. For example, a flange labelled as PN16 operates at up to 16 bar. The bar pressure system is the foundation of metric pressure pipe systems. Bar is not recognised by the International System of Units, which uses pascals instead. As a point of reference, a bar is exactly 100,000 pascals, or 14.5 pounds per square inch (psi).
Common PN ratings:
- PN6
- PN10
- PN16
- PN25
- PN40
- PN64
- PN100
PN ratings can go higher, of course. You’ll also see PCD on sizing charts. PCD stands for Pitch Circle Diameter, which is the diameter of a circle that goes through each of the bolt holes. It’s also worth noting that where ASME uses nominal pipe size (NPS), the metric equivalent is called diameter nominal or DN.
Our sizing example here is for a PN10 blind flange using standard EN1092-1.
EN sizes: millimetres
|
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 |
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