Pipe Flanges Overview

Figure 1: Threaded pipe flange

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Pipe flanges are one of the most used connection types for pipes. These connections consist of three different parts: the pipe flange, a gasket, and bolting. They are used for various industrial applications, pressure classes, and media. They can connect piping in a system, connect components such as valves, containers, vessels, and many more applications. These connectors benefit from a straight-through connection without loss of flow or pressure. This article explores the most common pipe flange variants, their uses, and their most commonly used applications.

Table of contents

How do pipe flanges work?

Flanges are used to bolt piping components together, such as pipes, valves, fittings, pressure components, and more. Joining is done using bolts that join two parts together, with a gasket in between to ensure the connection is leak-free. Piping is connected to the flange by use of a threaded connection, a weld such as the connection in Figure 2, or through the use of a stub-end.

Figure 2: The components of a pipe flange connection: pipe (A), flange (B), gasket (C), bolting (D), and weld (E).

Standard types of pipe flanges

Threaded Flange

Threaded flanges are joined using the male thread on a pipe and the female thread inside the bore of the flange. Typically, these do not need to be welded together, although doing so will strengthen the connection. If the connection is not welded, a thread tape or liquid sealant is recommended. This flange type is generally used for relatively low pressure and low temperatures, such as water and compressed air.

In gasoline stations and other areas where the risk of explosions is high, threaded flanges are mandatory. Welding in these environments would pose a significant danger.

Figure 3: Threaded flange

Welding neck flange

A welding neck flange has a long-tapered hub that can be welded to a pipe. This type of pipe flange is used for high-pressure applications with high or low temperatures and offers an unrestricted flow of the media in the piping. Because there is no obstruction, there are no issues with turbulence, erosion, or corrosion of the joints. The pipe and flange should be connected through a full penetration V-shaped butt weld. The purpose of this type of weld is to transmit the full load-carrying capacity of the components.

Figure 4: An example of a welding neck flange connection. Pipe to flange v-shaped butt weld (A), pipe (B), flange (C), and bolting (D).

Long welding neck

Like the weld neck flanges, although these flanges have an extended neck that is not tapered. The neck, in this case, acts as a bore extension. Industrial weld neck flanges are used in high-pressure applications and high temperatures. These flanges usually have the same bore sizes as pipes in the same system, although this can be adjusted as needed.

Applications include those in the oil, gas, and petrochemical industries. They are often used as nozzles and to replace pipe sections. They can withstand higher pressures and find their use in water mains and factory piping systems. Long neck welding flanges usually have a square end instead of the beveled edge available on welding neck flanges.

A butt weld joint connects the weld neck flanges. For high-pressure applications such as drilling rig standpipes, welding the neck will make a strong connection and is a good option for pressure vessels.

Figure 5: Welding neck flanges

Slip-on flanges

These flanges are similar to threaded flanges in appearance but lack the thread inside the flange. Instead, the pipe is held in place by two fillet welds, one on the outside of the flange and one on the inside. Also known as tee joints, these welds consist of two pieces of metal perpendicular to one another. The ID of the flange is slightly larger than the OD of the pipe. This allows the pipe to slide into the flange snuggly before being welded into place.

Figure 6: Slip-on flange

Socket weld flange

These flanges are used primarily for small-size and high-pressure pipes. The pipe is slotted into the flange and held there using a solid fillet weld outside the socket flange. There should be an expansion gap between the shoulder and the end of the pipe. These flanges are not suitable for highly corrosive environments as the expansion gap between the pipe end and the socket's shoulder is susceptible to crevice corrosion.

Figure 7: Socket weld flange pipe

Lap joint flange

The use of lap joint flanges effectively minimizes the cost of connections in stainless steel or nickel alloy pipeline systems. These flanges are always used in combination with stub ends so that the flange itself can be of a lower grade, cheaper, material, such as carbon steel, while the stub end, which comes into contact with the media in the system, can be of the same material as the piping system. The pipe can be welded in place with a butt weld for extra strength.

Figure 8: Components of a lap joint flange connection (left) and a lap joint flange (right). The pipe is welded with a butt weld on the back of the stub end (B), the lap joint flange (A) slides over the pipe (C), and is held in place by the pressure from the bolts.

Blind flange

Blind flanges seal or close a pipeline to block fluid flowing. It is easy to access the pipeline when the blind flanges are unbolted so that the operator can perform activities inside the terminal end of the pipeline. These are mounted against flat face-type joints.

Figure 9: Blind flange

Other flange types

Besides the more standard flange types mentioned above, more varieties have been designed for specialistic applications. Below we cover these specific types:

Nipoflange

Nipoflanges are used for pipeline branches at 90 degrees. Nipoflanges are formed by combining a welding neck flange with a forged Nipolet in a single piece of forged steel. Installing a Nipoflange involves welding the Nipolet part on the run pipe and bolting the flanged portion to the branch pipe.

Weldoflange

A Weldoflange is similar in design to a Nipoflange; they are a combination of a weld neck flange and a branch fitting connection called a Weldolet. Weldoflanges are made out of a single piece of solid forged steel.

Swivel flange

The swivel ring flange allows quick alignment of bolt holes on mating flanges. This feature is ideal for installing large pipelines, offshore and subsea pipelines that transport oil, gas, hydrocarbons, and other demanding fluids in petrochemical applications. Swivel flanges combine a welding neck on one pipe and the swivel on the other pipe. They are then connected by the rotation of the swivel flange on the pipe so that the bolts can be connected easily.

Expander flange

Expander flanges are used to increase the bore of the pipeline from one specific point to another or connect pipes to other mechanical devices such as pumps, compressors, and valves with different inlet sizes. Expanding flanges can only increase the run pipe bore by a maximum of two sizes. A standard flange and a buttweld reducer should be used for larger pipe bore increases.

Reducer flange

The function of a reducer flange is opposite that of an expander flange. A reducer flange allows the pipeline bore to be decreased. It is only safe to reduce the bore of the run pipe by one or two sizes; otherwise, a buttweld reducer and a standard flange must be used.

Storz flange

Storz couplings are often used and designated as fire hose couplings. However, they have wide usage in many industries, including refineries, agricultural, construction, maritime safety, and military use due to their resistance against corrosion, acids, and water. The storz flange has a storz coupling on one side, and a flat face on the other.

Pipe flange face variants

The choice of face type will significantly impact flanges' performance and service life. A flange's facing determines both the type of gasket required for installation and the characteristics of the seal it creates. Below is an overview of the most common face variants.

Figure 10: Face variants: Flat face (FF), raised face (RF), lap joint (LJ), ring joint (RTJ), male and female (M&F)

• Flat Face (FF): A flat face flange has an even, flat surface combined with a full-face gasket covering a large portion of the flange surface.
• Raised Face (RF): These flanges are equipped with a small raised ring around the bore and a gasket inside the bore circle.
• Lap Joint (LJ): The lap joint connection means that the stub end facing is also the flange facing; this makes it similar to the raised face.
• Ring Joint Face (RTJ): The grooves and raised areas in these flanges match. This design provides a reservoir for gasket adhesive and allows the flanges to self-align during installation.
• Male & Female (M&F): The gasket is secured with matching grooves and raised sections, similar to tongue and groove flanges. In contrast to tongue and groove flanges, these retain the gasket on the female face, providing more accurate positioning and a more comprehensive range of options for gasket materials.
• Seal finish: Many types of faces have either a serrated or smooth finish. It is important to choose between the options because this will determine the optimal gasket for a reliable seal. Metallic gaskets are best used with smooth faces, while soft gaskets benefit from serrated faces.

Common pipe flange materials

Carbon steel, high yield carbon steel, alloy steel, stainless steel, duplex, and super duplex steel, are the most common materials used for pipe flanges. However, nickel alloy grades such as Inconel, Incoloy, Hastelloy, and Monel are also used for some applications. For more information on the chemical properties of these materials, refer to our chemical resistance page.

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Carbon steel

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Flanges made from Carbon steel have excellent chemical and mechanical properties. In addition to having excellent fatigue strength, high strength, superior chemical resistance, and high toughness, carbon steel is also highly resistant to stress corrosion cracking. This makes it an ideal material for connecting pipes.

Alloy steel

Compared to carbon steel, alloy steel contains a more significant amount of chromium and molybdenum. For applications requiring high pressures and temperatures, alloy steel is ideal. As compared to carbon steel, alloy steel is more corrosion resistant.

Duplex steel

Duplex steels combine austenite and ferrite phases. Duplex steel is corrosion-resistant and more rigid than ferritic steel. The material is highly corrosion resistant in chloride and sulfur environments. Due to its magnetic properties, duplex steel can be used in many complex industrial applications.

Stainless steel

Nickel (Ni), chrome (Cr), and molybdenum (Mo) distinguish stainless steel material from other materials. Stainless steel is popular because of its superior corrosion resistance. It can be used to fabricate pipe flanges of all sizes and types. Learn about stainless steel in our 304 vs. 316 stainless steel comparison.

Pipe flange classification and pressure rating

The pressure rating for a pipe flange, sometimes called class, #, or Lbs, is the maximum allowable pressure a pipe flange can withstand at increasing temperatures. There are seven different flange pressure ratings: 150, 300, 400, 600, 900, , and according to ANSI/ASME B16.5.

This becomes especially important when choosing the correct flange for your application. For example: if we take two flanges with the same bore, 10 inches, the same material, carbon steel, but different pressure ratings, 300#, and #, then:

• The lower-rated flange will be less robust, smaller, and lighter
• The higher-rated flange will be stronger, bigger, and heavier
• The 300# will be rated for a pressure of 50.1 bar at 50° C
• The # will be rated for a pressure of 250.6 bar at 50° C

In this introduction to flanges, we&#;ll cover the different types of flange attachment methods, which flanges to choose for a variety of applications, and an overview of pressure and temperature ratings and how they apply to different types of piping systems.

What Are Flanges and How Do They Work?

A flange is a protruding rim, lip, or rib that is used to increase strength or provide an attachment method to connect pipes, valves, and other equipment to form a piping system and to create an easy access point for cleaning, inspection, or modifications.

Flange Attachment Methods and Purposes

Flanges can be welded or screwed on, depending on the type of flange and piping system requirements. When selecting a flange type, some important aspects include pressure capacity, pipe size, and intended application.

• Ring or slip-on flanges have a center hole, and they slide onto a pipe end. The flange face extends beyond the pipe end to be welded into place.
• Lap-joint flanges are similar to slip-on flanges but are used in conjunction, which means that the flange doesn&#;t have contact with the medium in the pipe. These flanges don&#;t require welding, which allows them to rotate freely to line up with the mating flange.
• Threaded flanges are often used for smaller pipe sizes and do not require welding, which makes them a convenient option&#;however, they are not suitable for piping systems with thin walls.
• Socket weld flanges are used for small, high-pressure applications. The pipe is inserted into one end, then a fillet weld is applied around the outside of the flange. An expansion gap is required between the flange and pipe to reduce weld stress.
• Weld neck flanges feature a long, tapered hub that provides reinforcement and improved distribution of stress for use in high-pressure, sub-zero, or elevated-temperature applications.
• Blind flanges are flat, with no bore, and are used to finish off a piping system.

Common Flange Materials

Flange material can affect everything from connection methods to pressure rating, so it is an important consideration. While there are a variety of options, the best flange material varies by application requirements.

Some common flange materials include:

• Steel &#; The most common flange material, is preferred for strength
• Stainless Steel &#; Preferred for its higher resistance to corrosion and rust
• Aluminum &#; Lightweight material ideal for corrosion resistance
• Ductile Iron &#; A lower-cost option for backing flanges that can be used because the flange doesn&#;t come in contact with the medium

Flanges come in a variety of surface finishes, which determine the roughness or smoothness of the finished flange face. Flange faces may offer a smooth finish or can have serrations&#;small grooves on the flange face. The appropriate finishing technique depends on the intended service use.

The AWWA standard finish is between 24-55 serrations per inch and is suitable for a range of applications. Serrations may be continuous spirals or evenly spaced concentric grooves. Other finishing options include a mill (or rolled) finish and surface flanges that offer no finishing except where machining is required.

Coatings protect the flanges from corrosion or damage through shipping, storage, and use. These coatings include:

• Light oil or other rust-prohibiting material
• Zinc
• Paint
• Epoxy
• Galvanized

Outside of flange material and coatings, it is also important to differentiate between the different flange material sources.

• Import &#; These are popular due to their lower cost, but in some cases, like for government projects, they may not be appropriate.
• Buy America &#; This standard provides a preference for 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, domestic product availability is low, or the price is too high.
• Domestic &#; These flanges meet AIS (American Iron & Steel) standards, which allows imported raw components of the metal, but requires that the flange material be melted and manufactured in the United States.

How Are the Most Common Flanges Used?

To determine the correct flange for your application, consider flange size and attachment method, materials used to manufacture the flange, the pressure class rating, bolt hole size and placement, and which standards your flanges must meet. Refer to flange dimensions when selecting your pipe fittings&#;most often listed as outside diameter (OD), inside diameter (ID), bolt hole diameter and spacing, and thickness (T). Refer to flange dimensional tolerances if necessary specifications aren&#;t available, or consider a custom-machined fitting.

Slip-On Flanges

Slip-on or ring flanges are often used for high-temperature, low-pressure piping systems and can be used in applications with higher flow rates. Also called a &#;ring flange,&#; this style flange slides onto a pipe end through a center hole and the flange face extends beyond the pipe. The flange is then welded into place for a secure fit. Ring flanges may be manufactured to ANSI and AWWA standards, are available in Class 150 through , and are available in flat face or raised face styles.

Ring-type joint flanges are similar to ring or slip-on flanges, but are preferred for high-pressure applications as this type is paired with a gasket for improved sealing.

Lap-Joint Flanges

Lap-Joint flanges, also called back-up or backing flanges, are paired with stub ends or angle face rings. The backing flange slides over the pipe end, and welding is not required. This solution is ideal for ensuring flange bolt holes line up correctly as the backing rings are able to rotate to match bolt holes&#;connect the stub end and backing flange, line up the connection points, then align the bolt holes to complete the installation. Lap-Joint flanges are suitable for piping systems that require regular maintenance or that are dismantled often.

Threaded Flanges

Threaded flanges attach to pipes with threaded ends, no welding is required&#;making threaded flanges ideal in small spaces or areas where welding is not possible. This convenient option is often used for smaller diameter pipe sizes and high-pressure applications, but should not be used in a piping system with thin walls. Threaded flanges are available in a variety of materials and dimensions to meet a wide range of specifications.

Blind Flanges

Blind flanges are available with the same thicknesses and bolt hole patterns as other flange types, but are manufactured with no bore. They are used to finish off a piping system and can create an efficient, leak-proof seal when paired with a gasket and properly seated.

Weld-Neck Flanges

Weld-neck flanges feature a long, tapered hub that is welded to the pipe. Weld-neck flanges are used for high-pressure and sub-zero to high-temperature applications to distribute piping system stress and provide reinforcement.

Socket Weld Flanges

Socket weld flanges offer a higher fatigue strength over a slip-on flange when used for small pipe sizes and high-pressure applications. Choose socket weld flanges based on class and pressure rating, as well as pipe schedule. To assemble, the pipe is inserted into one end, then retracted slightly to form an expansion gap to reduce weld stress. Then, a fillet weld is applied around the outside of the flange. This assembly allows smooth fluid flow with no leakage.

Reducing Flanges

Reducing flanges are used in pipe-to-pipe connections when there is a change in pipe size. The bore matches the smaller pipe size so the larger and smaller pipes fit together properly. Reducing flanges are available in weld-neck, slip-on, and threaded styles.

Flanged Pump Connectors

Flanged pump connectors include flanges on either end of a hose&#;generally made of flexible braided steel or stainless steel&#;and are used to absorb pump vibration and reduce stress on the piping system. They&#;re also suitable when adjusting for minor misalignment. Pump connectors are also available with male X male threaded ends.

Overview of Flange Types and Benefits

Type of FlangePressure CapacityBenefits and UsesSlip-On FlangeLowLower-cost option; fits over the pipe for a snug fitLap-Joint FlangeLowEasier bolt hole alignment; good for applications that require frequent dismantlingThreaded FlangeLowNo-weld attachment for small-diameter applicationsBlind FlangeHighestTo seal off piping systems and provide easy accessWeld-Neck FlangeHighReduces pipe stress in high-pressure applicationsSocket Weld FlangeHighSmooth fluid flow, no leakage

Common Piping and Flange Standards

Flange standards cover everything from dimensions, materials, maximum pressure capabilities, temperature ratings, manufacturing technique, and other technical details. These classifications help determine which flange is appropriate for which applications.

There are a variety of piping standards, including these most common standards:

• ANSI/ASME (American National Standards Institute and American Society of Mechanical Engineers) is the industry standard for pipeline flanges in the United States.
• MSS (Manufacturers Standardization Society) is a standard used in steel pipelines.
• API (American Petroleum Institute) covers large diameter carbon steel flanges used in high-pressure oilfield applications.
• AWWA (American Water Works Association) is a set of waterworks industry standards that cover water treatment and supply.
• DIN (&#;Deutsches Institut für Normung&#; German Institute of Standardization) is a European metric standard that is used internationally, often in maritime applications.

ANSI Flange Temperature and Pressure Ratings

Flanges are rated based on the maximum pressure they can withstand at a particular temperature. There are seven main ANSI pressure ratings, from #150 to #. Flange performance is determined by the rating&#;generally, the lower the class number, the lower the pressure capability. This rating may be listed in a number of ways, including pressure rating, #, Lb(s), or class. This means that 300 Lb, 300#, and Class 300 all refer to the same pressure class or rating.

AWWA Flange Standards, Ratings, and Size Range

This table lists flanges by AWWA standard, class and drill pattern, PSI rating, and size to assist in determining the appropriate flange based on these specific requirements.

StandardClass/Drilling PatternPSI RatingRing/Slip-On Size RangeBlind Size RangeHub Size RangeAWWA C207 SteelClass B, # psi4&#; &#; 144&#;4&#; &#; 72&#;N/AAWWA C207 SteelClass D, # psi (4&#; &#; 12&#;), 150 psi (all other sizes)4&#; &#; 144&#;4&#; &#; 72&#;4&#; &#; 96&#;AWWA C207 SteelClass E, # psi4&#; &#; 144&#;4&#; &#; 72&#;4&#; &#; 96&#;AWWA C207 SteelClass F, # psi4&#; &#; 48&#;4&#; &#; 48&#;N/AAWWA C228 Stainless SteelClass SA, # psi (SPFH Series)2&#; &#; 12&#;2&#; &#; 8&#;N/AAWWA C228 Stainless SteelClass SB, # psi2&#; &#; 72&#;2&#; &#; 72&#;N/AAWWA C228 Stainless SteelClass SD, # psi (4&#;&#; 12&#;) 150 psi (all other sizes)2&#; &#; 72&#;2&#; &#; 72&#;N/AAWWA C228 Stainless SteelClass SE, # psi2&#; &#; 72&#;2&#; &#; 72&#;N/AAWWA C228 Stainless SteelClass SF, # psi4&#; &#; 48&#;4&#; &#; 48&#;N/A

The best flange style, material, and rating will ultimately depend on the intended application. We manufacture standard and custom flanges to meet the highest quality standard. Our full line of flanges can be viewed here.

If our standard flanges don&#;t meet your requirements our Custom Machining Shop allows the manufacture of custom parts as well as modifications to stock items, so you can get exactly what you need.

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