Aug. 04, 2025
Piping flanges are key elements of a piping system and are used in various industries, such as oil and gas, petrochemical, manufacturing, etc. These flanges connect pipes to valves or equipment or with another pipe spool and form a tight seal that prevents leaks and ensures efficient fluid flow within the system.
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Several types of piping flanges are available, each with unique features and benefits. You must understand the differences between these flanges to choose the right one for a particular application. In this article, I have listed the different types of piping flanges, their function, and applications in various piping systems. The following types of flanges are covered.
The pipe flanges are the second most used joining method after welding. They are used when joints need dismantling. It provides flexibility for maintenance. Flange Connects the pipe with various equipment and valves. Breakup flanges are added to the pipeline system if regular maintenance is required during plant operation.
A flanged joint comprises three components; flanges, gaskets, and bolting, and is assembled by a pipefitter. Special controls are required to select and apply all these elements to attain a joint with acceptable leak tightness.
However, using a flange connection in underground piping is not advisable when it is supposed to be buried. The flange is also the most common source of leaks and fires in a process plant. There are a variety of flanges available to suit the requirements. A flange can be classified in several alternate ways based on the following:
A threaded Flange is also known as a screwed flange, and it has a thread inside the flange bore, which fits on the pipe with a matching male thread on the pipe. This type of joint connection is speedy and simple but unsuitable for high pressure and temperature applications. Threaded Flanges are mostly used in utility services such as air and water.
Socket-Weld Flanges have a female socket in which the pipe is fitted. Fillet welding is done from outside on the pipe. Generally, it is used in small bore piping and is only suitable for low pressure and temperature application.
Slip-On flange has a hole with a matching outside diameter of pipe from which pipe can pass. The flange is placed on pipe and fillet welded from both inside and outside. Slip-On Flange is suitable for low pressure and temperature application.
The lap flange has two components: a stub end and a loose backing flange. The stub end is butt-welded to the pipe, and the backing flange freely moves over the pipe. The backing flange can be of a different material than stub material and normally of carbon steel to save the cost. A lap flange is used where frequent dismantling is required, and space is constrained.
Weld neck flange is the most widely used type in process piping. It gives the highest level of joint integrity as it is butt-welded with a pipe. These types of flanges are used in high pressure and temperature application. Weld neck flanges are Bulky & costly with respect to other types of flanges.
The blind flange is a blank disc with a bolt hole. These flanges are used with another type of flange to isolate the piping system or terminate the piping as an end. Blind flanges are also used as a manhole cover in the vessel.
A reducing flange is used in place of a standard flange to allow for a change in pipe size. This flange eliminates the need for a standard reducer in piping. 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 standard pipe flange size and is considered economical to make a pipe size transition.
These flanges are available in weld neck, slip-on, and threaded end types. Reducing flanges are an economical way to make transitions between pipes of different sizes, however, due to high-pressure loss, they are rarely used in piping. ASME B16.5 covers the dimensions of reducing flanges.
Image source: www.msmmfg.com
It is similar to a weld neck flange but increases the size of the pipe to the first or second larger size. It is an alternative to using a reducer and weld neck flange. Useful for connecting to valves, compressors, and pumps.
These flanges are specially designed. Dimensions of the weld neck are in line with the connecting pipe, other dimensions are in accord with standard ASME B16.5 flanges.
Image source: www.msmmfg.com
Flangeolet is a combination of two olets (mainly weldolet or nipple) and a flange as you can see in the image that the hub length is longer, like a long neck weld flange. It is a 90 Degree branch connection used for high-pressure piping.
It reduces the two weld joints more than the traditional Olet + Pipe + Flange connection and one weld joint in the case of the Olet + Flange connection. You can see the image of the flangeolet branch.
1. Weld Neck Flange
A weld neck flange features a long tapered neck that gradually narrows to match the pipe thickness. It connects to pipes or equipment through butt welding. This design ensures even stress distribution at the weld, reducing pressure concentration at the connection point, which enhances the reliability and durability of the weld. Weld neck flanges are commonly used in high-temperature, high-pressure industries such as oil, gas, and chemicals.
2. Slip-On Flange
A slip-on flange looks like a disc with an annular flange. It is installed by sliding it onto the pipe end and then connecting it to the pipe or fitting it with two fillet welds, one on the inside and one on the outside. The inner diameter of the slip-on flange is slightly larger than the outer diameter of the pipe, so precise alignment is not required. However, it is typically used in low—to medium-pressure systems such as water treatment, HVAC, and chemical piping systems.
3. Socket Weld Flange
A socket weld flange has a recessed area or socket in the center of its connecting end. The socket’s inner diameter typically matches the pipe’s outer diameter to be connected. The pipe is inserted into this recess and then welded along the flange’s inner edge to join the pipe and flange. This flange is usually used in small-diameter piping systems. However, because this welding method effectively isolates the weld from the fluid inside the pipe, socket weld flanges are commonly used in high-pressure and high-temperature environments, such as in chemical, oil and gas, and pharmaceutical industries.
4. Lap Joint Flange
A lap joint flange consists of a loose ring and a short pipe end (a stub end). The loose ring resembles a disc with a large hole in the middle, with an inner diameter more significant than the outer diameter of the pipe. The short pipe end is welded to the pipe and has a shape similar to the face of a regular flange, allowing it to rotate freely. This design means the lap joint flange does not require precise alignment during installation, making it easy to connect with other flanges. It is primarily used in low-pressure and non-critical piping systems that require frequent disassembly and maintenance.
5. Threaded Flange
A threaded flange is typically round, and its most notable feature is the threaded inner hole, which matches the external threads of the pipe, allowing it to be screwed onto the pipe. It is used when welding is impossible and is commonly found in low-pressure, low-temperature piping systems such as water supply and drainage.
6. Blind Flange
A blind flange, with no holes in the center, is typically used to seal the end of a pipe or an opening in a pipeline. It is commonly used for maintenance, repair, testing, and similar purposes.
Flanges are typically classified by their sealing surfaces, determining the connection method. The main types include:
1.Flat Face（FF）Flange
The flat face flange features a flat sealing surface, providing a large contact area with the pipe or fitting. Gaskets are usually made of rubber or non-metallic materials. It is typically used for connecting low-pressure or non-critical sealing pipe systems.
2. Raised Face (RF) Flange
The raised face flange features a slightly raised sealing surface, which helps increase the contact pressure between the flange and the pipe or fitting. Common gasket materials include non-metallic materials (such as rubber and asbestos) and metallic materials (such as stainless steel and copper). Raised face flanges are typically used in medium to high-pressure systems to ensure a reliable seal.
3. Male-female (MFM) Flange
The characteristic feature of a Male-Female (MFM) flange is its sealing surface, which is designed with complementary raised and recessed structures: one flange has a protruding sealing surface (male face). In contrast, the other has a corresponding concave sealing surface (female face). This design ensures precise alignment during flange assembly and provides enhanced sealing performance. Common gasket materials include metallic materials like stainless steel and non-metallic materials like rubber or flexible asbestos. It’s important to note that the gasket must match the raised and recessed structure of the flange.
Male-female flanges are commonly used in the chemical and petroleum industries for pipeline systems requiring high-pressure and high-temperature environments.
4. Tongue and Groove （TG) Flange
The tongue and groove face flange features a sealing surface designed with interlocking tongue and groove structures: one has a circular protrusion (tongue face), while the other has a corresponding circular groove (groove face). This sealing surface enhances performance, typically using metallic materials such as stainless steel and soft iron for gaskets. It is suitable for pipeline systems in the chemical, petroleum, and natural gas industries that require higher sealing requirements.
5. Ring Type Joint (RTJ) Flange
The ring-type joint (RTJ) flange has a sealing surface with a circular groove that holds a metal ring gasket. The gasket is usually oval or octagonal in cross-section and made from hard metals like stainless steel or nickel alloy to ensure strength and sealing performance under high pressure. This type of flange is suitable for pipelines requiring very high sealing performance.
1. ASME Flange
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ASME/ANSI B16.5: Applicable to pipe flanges and flange fittings with nominal diameters ranging from NPS 1/2 to NPS 24.
ASME B16.47: Applicable to large diameter flanges, divided into Series A and Series B, with diameters ranging from NPS 26 to NPS 60.
2. DIN Flange
Din : Applicable to steel pipe flanges with a nominal pressure of PN 16.
Din : Applicable to steel pipe flanges with a nominal pressure of PN 25.
3. EN Flange
EN -1: Applicable to steel pipe flanges with nominal pressures ranging from PN 2.5 to PN 400 and sizes from DN 10 to DN .
4.JIS Flange
5.GB/T Flange
GB/T : Applicable to steel pipe flanges with nominal pressures ranging from PN 2.5 to PN 40 and sizes from DN 10 to DN .
6. API Flange
API 6A: Applicable to wellhead and Christmas tree equipment in the oil and gas industry, including flanges, fittings, valves, and related equipment, with pressure ratings ranging from to psi.
7.AWWA Flange
AWWA C207: Applies to steel and plate flanges, including weld neck flanges, slip-on flanges, and lap joint flanges, with pressure ratings of Class B, D, E, and F. It is primarily used for water systems.
When measuring flanges, it’s essential to use appropriate tools to ensure accuracy and precision. The tools you will use most often include:
Caliper: Used to measure diameters, thickness, and heights.
Tape Measure: Suitable for measuring larger outer diameters and bolt circle diameters.
Inside Micrometer: Used to measure bore diameters.
Thickness Gauge: Used to measure flange thickness and raised face height.
Before using measuring tools, ensure they are calibrated. Repeat measurements multiple times to obtain accurate data and adhere to relevant standards and regulations to ensure measurements fall within specified tolerances.
Follow these testing steps according to the corresponding specifications and drawing requirements:
Step 1: Use a caliper or tape measure to measure the maximum outer diameter of the flange.
Step 2: Use a caliper or tape measure to Measure the bolt circle diameter.
Step 3: Measure the diameter of individual bolt holes using a caliper.
Step 4: Measure the thickness of the flange using a caliper.
Step 5: Measure the height difference between the flange sealing face and the main flange surface.
Step 6: Measure the diameter of the flange’s intermediate holes using a caliper or inside micrometer.
Step 7: Record the number of bolt holes and measure the diameter and length of bolts to determine the appropriate bolt specifications.
Suppose you need to check the elemental composition and hardness of materials on-site. In that case, you can use a spectrometer and portable hardness tester. For more precise identification, sampling on-site and sending samples to a laboratory for testing is necessary to obtain the corresponding test reports.
The material for flanges needs to be selected based on characteristics such as operating environment, pressure, temperature, and the nature of the medium. Apart from aluminum, copper alloys, plastics, and composite materials, commonly used metallic materials include carbon steel, stainless steel, alloy steel, and nickel-based alloys. You can learn about the common alloy grades and their applications. For more details, please refer to:
Carbon Steel Flange
Carbon steel flanges commonly use materials such as ASTM A105 and, for low-temperature applications, ASTM A350 LF2. They are frequently employed in the petroleum, natural gas, and chemical industries. Due to their high material strength, they are suitable for high-pressure applications. However, their corrosion resistance is average, often requiring coatings or galvanization treatments.
Stainless Steel Flange
The commonly used grades for stainless steel flanges are ASTM A182, F304, and F316. Due to its high chromium content (at least 11% chromium), stainless steel forms a stable chromium oxide layer that prevents further oxidation and corrosion. Therefore, stainless steel exhibits excellent corrosion resistance. Flanges made from this material are commonly used in the chemical, pharmaceutical, and water treatment industries, where corrosion resistance is required for pipeline systems.
Alloy Steel Flange
Joint chromium-molybdenum alloy steel flanges, such as ASTM A182 F11 and F22, contain chromium and molybdenum elements that enhance the material’s resistance to high temperatures and oxidation. Therefore, chromium-molybdenum alloy flanges are commonly used in pipelines and equipment under high temperature and high-pressure conditions, such as in chemical and power industries.
Nickel Alloy Steel Flange
Nickel-based alloy flanges include Inconel (corrosion and high-temperature resistant Inconel 600 / corrosion and fatigue resistant Inconel 625), Incoloy (high-temperature and oxidation resistant Incoloy 800 / corrosion resistant Incoloy 825), Hastelloy (corrosion-resistant Hastelloy C276), Monel (Monel 400), and acid-resistant Alloy 20. Due to their unique resistance to high temperatures, high pressures, and corrosive environments, nickel alloy steel flanges are widely used in various industrial applications.
Flange sealing relies on the flange gasket, nuts, and bolts. These components are crucial to the flange connection system. Selecting the correct ones when using flanges ensures optimal sealing performance.
The flange gasket is a sealing material placed between flange faces. As mentioned in the section on flange sealing face classifications, its primary function is to fill the gaps between the flange faces and prevent leaks.
Standard flange gaskets are categorized by material into non-metallic, semi-metallic, and metallic types. They are used in different applications: low-temperature and low-pressure systems, medium to high-pressure and high-temperature environments, and applications requiring high temperature, high pressure, mechanical strength, and corrosion resistance.
Non-metallic materials include rubber, flexible graphite, PTFE (polytetrafluoroethylene), and fibers. Semi-metallic materials include spiral wound gaskets and metal composite gaskets. Metallic materials include aluminum, stainless steel, and copper.
Flange nuts are used to secure bolts in flange connections. When selecting nuts, you should choose based on the application. Incorrect selection can lead to seal failure or reduced durability. Common types of nuts include:
Hex nuts are the most common type and are suitable for various flanges. Heavy hex nuts are used for high-load connections and high-strength bolts. Lock nuts, designed with anti-loosening features, are ideal for flange connections in vibrating environments.
Joint flange forming processes mainly include forging, casting, and rolling, each used in different scenarios:
Forged flanges are commonly used in high-pressure and high-temperature environments such as oil and gas, chemical processing, and power generation.
Cast flanges are typically used in applications without high strength, such as water treatment and HVAC systems.
Rolled flanges are usually used to manufacture large-diameter flanges. They are often found in low to medium-pressure pipeline projects.
No matter which forming process is used, the following production steps must be followed:
Step 1: Material Selection
Select materials according to requirements. After the raw materials pass inspection, cut them into sizes and shapes suitable for processing.
Step 2: Forming
Forging: Heat the raw material and use forging equipment and molds to press it into shape.
Casting: The raw material of metal is melted, poured into molds, and cooled to form. This method can produce flanges with complex shapes.
Rolling: Roll the metal into a specific shape, then cut it to form.
Step 3: Heat Treatment
To improve the material’s hardness, strength, and wear resistance, the formed flange blanks must undergo heat treatment according to specifications or design requirements, such as annealing, normalizing, quenching, and tempering.
Step 4: Machining
Finishing flanges requires specialized equipment, such as CNC lathes, milling machines, and grinders, to ensure dimensional accuracy and surface finish:
Turning and Milling: Used for machining the flange’s outer diameter, sealing surface, and bolt holes.
Drilling and Tapping: Used for machining bolt holes to ensure connection with pipes or equipment.
Grinding and Polishing: Used to optimize the surface finish and flatness of the flange.
Step 5: Inspection and Testing
After production and processing, flanges need to undergo rigorous quality inspection to be deemed qualified, including:
Dimensional Inspection: This has been mentioned earlier and includes detailed flange measurements.
Surface Quality Inspection: This mainly checks the surface finish, especially the sealing surface, and the overall appearance for any scratches or flatness issues that might affect quality.
Material Performance Testing: This includes hardness and tensile tests to verify that material performance meets the standards and design requirements. For example, low-temperature performance testing is also required for 350LF2 material.
Step 6: Marking and Packaging
After passing inspection, the flange needs to have necessary identification information printed on its surface, serving as its certification. Then, it should be securely packaged for transportation and storage. Particular attention should be given to protecting the sealing surface to prevent wear and impact during transportation, which could cause unnecessary damage.
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