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What Are The Most Common Flange Types?

ansi/asme flange types, awwa flanges
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This blog post will cover the most common flange types used in today’s industrial applications. Flanges are essential components in industrial applications. Even though there are many different types of flanges, they all share a common goal- to connect piping systems together. Let’s explore the specifics of the seven most popular flange types, their unique characteristics, and their practical uses that make them essential in various industries.

Weld Neck Flange:

slip on flange diagram 5

Weld neck flanges perform exceptionally well in high-pressure systems. They are one of the most common flange types. Their long neck allows a seamless butt weld directly onto the pipe, leaving a smooth connection. Because of this, they are perfect for use in sectors like the energy and power generation industry, where dependability under high pressure is crucial. This style of flange connection is almost exclusively used in larger diameters and higher-pressure classifications.

Slip-on Flange:

Slip on flange diagram
awwa class 150 stainless steel slip on plate flange
The simplicity of installing slip-on flanges makes them popular among manufacturers and fabricators. Because the slip-on flange lacks a weld bevel and permits pipe length adjustment in relation to flange location, it is an easy and excellent substitute for a weld neck. The matching pipe will have plenty of room in the slip-on’s bore. This extra room provides sufficient workspace for the fabricator and welder to connect. They are frequently used in sectors like shipbuilding and construction that demand quick and effective setup. This kind of flange is widely used in applications with lower pressure. A hub found on most slip-on style flanges frequently resembles a raised face in terms of dimensions. In cases where space is restricted, and the application permits a “ring style” slip-on, it can be outfitted without a hub. A ring-style slip-on without a hub still qualifies as a slip-on flange and can be recognized as you see fit, even if the hub style is more frequently used when discussing slip-ons. Higher-pressure class slip-ons are commonly designed with a lap joint height for improved attachment.

Socket-weld Flange:

slip on flange diagram 6
Socket Weld Flange
Socket-weld flanges are resilient to high pressure and temperatures, benefiting the power generation and chemical processing industries. With a counterbore in the middle to enable a clean pipe weld into the socket, socket weld flanges are less popular but no less effective connection for flanges that require less total available space than a typical weld neck flange. These are typically supplied in smaller nominal pipe sizes, and when needed, they’re an excellent substitute for slip-on and weld necks. If the fluid is not corrosive and the pressure class is relatively low, a socket weld may be chosen for an assembly with limited space or extremely high-temperature duty.

Threaded Flange:

slip on flange diagram 3
Threaded ASME Flange
Threaded flanges are easy to install and perform well in low-pressure applications. They are a vital component in piping systems and other environments where it’s essential to maintain a tight seal without doing a lot of welding. Threaded flanges have a female NPT thread in the middle for connecting to male NPT threaded piping. They are also sometimes called companion flanges, screw flanges, or NPT flanges. Threaded flanges are frequently used to create reduced connections. In addition to the basic NPT form, as defined by the ASME B16.5 specification, several thread styles can be machined.

Blind Flange: 

slip on flange diagram 4
awwa steel blind plate flange 1986

Blind flanges are essential for closing off portions of a pipe system. They are dependable barriers in waterworks and other hazardous material-related industries, guaranteeing worker safety and stopping leaks during emergencies or maintenance. Blind flanges with raised faces frequently have a slight indentation in the middle, as seen in B16.1 and other industry standard cast steel requirements from decades ago. This recess allowed the pipe end to mate with the connecting flange and for a seal to form between the blind and the connecting flange. To produce a dished head formation at the top of the flange, the more oversized OD blind flanges have a concave recess through the blind’s body. For the most part, blind flanges won’t be produced in these forms because the earlier cast iron specification is somewhat outdated. Instead, go for a modern blind flange with or without a hub.

Lap Joint Flange:

slip on flange diagram 2
Stub End
A Lap joint flange makes the list among the other common flange types. They are also known as Backup flanges, Loose Ring flanges, Loose Hubbed flanges, and Van Stone flanges. Easily disassembled, lap joint flanges offer flexibility and are perfect for industries like the food and beverage industry that need regular maintenance or adjustments. The hubs can be adjusted to your specifications and are usually taller than a slip-on design. Higher-pressure class slip-ons are frequently designed with a lap joint height for improved attachment. Customers will occasionally choose a slip-on intended to resemble a lap joint with a machined hub if the hub height is not an issue and a lap joint is not easily accessible. Industrial flanges are used in various industries, such as structural applications and plumbing. Rolled angle rings and lap joint flanges are two parts that are essential to many processes. The main distinction between rolling angle rings and lap joint flanges is that the latter is exclusively offered in a butt-weld arrangement. Conversely, lap joint flanges are available in slip-on and butt-weld varieties. The backing flange on a lap joint flange can rotate, unlike the backing flange on a rolled angle ring.

Custom Flange:

Contrary to the name, custom flanges are common flange types with many applications and uses across various industries. Our machine shop makes it possible to use AIS and import materials to build products that satisfy various specifications. We also facilitate quick changes to our stock of goods to serve our customers better. Whether you need to reverse engineer a sample or a CAD drawing, we can work with you on prototypes, production quantity manufacturing, and even product warehousing.
custom flanges

Flange Face Types

Independent of the flange type, every flange has a face that determines how it seals against its mating component. The three standard flange face types are:
  • Raised Face (RF) — used across all ANSI/ASME pressure classes from 150 through 2500.
  • Flat Face (FF) — used primarily in water works systems and when connecting to cast iron or non-metallic equipment to prevent flange distortion, in applications up to 300 psi.
  • Ring-Type Joint (RTJ) — a metal ring gasket seated in a machined groove, used in high-pressure and high-temperature service.
Face type and surface finish must be compatible with the mating flange and the gasket. See flange facing options and how to choose for full detail.

Common Flange Types: Summary Table

Flange Type How It Connects Best For Key Point
Weld Neck Butt welded to the pipe via a long tapered hub High-pressure, high-temperature systems Spreads stress evenly for the strongest, most leak-resistant joint
Slip-On Slides over the pipe end, then fillet welded Lower-pressure lines and larger diameters Simpler and more economical; easy to align
Socket Weld Pipe seats in a socket, then fillet welded Smaller-bore, higher-pressure lines Smooth bore flow with less welding than butt-weld types
Threaded Screws onto threaded pipe, with no welding Low-pressure, non-critical systems Ideal where welding is unsafe or disassembly is frequent
Blind Solid plate bolted over the opening Sealing or isolating the end of a line Leak-proof shutoffs and future pipeline extensions
Lap Joint Two pieces with a stub end; the flange rotates Systems needing frequent disassembly Easy bolt alignment; not for high-pressure or high-vibration
Custom Machined to your specification Nonstandard sizes, bores, facings, or materials Made when off-the-shelf options do not fit

API International Machining Capabilities:

  • Machining inner diameter, outer diameter, gasket face, chamfers, and tapers up to 120″ outer diameter
  • Single point threading of inner and outer diameter with standard or metric pitch
  • Straight turning, tapers, and angles of inner and outer diameter face
  • Profile milling, facing, contouring, pocketing, boring and threading
  • Milling straight and tapered pipe threads
  • Single-part or large production orders
  • Bolt-hole drilling up to 3″ diameter
  • Turning shafts up to 60″ in length
  • Steel, Aluminum, Duplex, Super Duplex, Alloy 20, and other exotic metals
Each flange type has a unique role to play. At API International, Inc., we pride ourselves on offering a diverse range of fittings, including these essential flange types, to meet the specific needs of various industries. Contact our sales team if you have questions or need a quote. Our dedicated sales team staff are ready to help you find the industrial fittings you need for your project.

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Frequently Asked Questions

What is the difference between a plate slip-on flange and a weld neck flange?

Plate slip-on flanges are simpler and cheaper (plate material without a hub or raised face), which makes them more suitable for lower pressure lines. Weld neck flanges are stronger (forged with weld neck) and preferred for high-pressure, high-temperature applications.

What are the advantages of socket weld flanges?

Socket weld flanges offer high resistance to pressure and temperature, require less welding than butt weld types, and are easy to align during installation. They’re also available in a wide range of sizes and pressure classes.

When are threaded flanges the right choice?

Threaded flanges are ideal for low-pressure, non-critical piping systems, especially where welding is unsafe or impractical. They’re also convenient for temporary setups or systems needing frequent disassembly.

What are the disadvantages of lap joint flanges?

Lap joint flanges are not suitable for high-pressure or high-vibration systems. Their loose design can compromise sealing integrity in dynamic conditions, and they generally provide less structural strength than welded flange types.

Do blind flanges need gaskets?

Yes, gaskets are typically placed between the flange and pipe surface to ensure a secure, leak-free seal. The bolts are then tightened evenly around the flange to maintain consistent pressure and prevent leaks.

What is the difference between RTJ and raised face flanges?
  • RTJ (Ring Type Joint) flanges use a machined groove with a metal ring gasket to achieve a metal-to-metal seal, making them suitable for very high-pressure and high-temperature service.
  • RF (Raised Face) flanges use a gasket compressed on the raised surface area, making them more common in general industrial applications with moderate to high pressures.

What gasket should be used on a flat face flange?

Only a full-face gasket should be used. It covers the entire flange surface from the inside bore to the outer edge, ensuring even compression. These are typically non-metallic (rubber, fiber, PTFE).

How do I order industrial flanges from API International?

To order industrial flanges from API International, share your pipe size, the flange standard and class you need (for example, ANSI/ASME B16.5 or AWWA C207), your facing requirement, and the material or coating requirements. If your project has special requirements like corrosion resistance, high temperature service, or specific bolt hole drilling, include that as well. Contact API International’s experienced team and we will help you select the right flange for your application from our massive stock inventory or custom machine it.

Video Guide to Flanges