How Does Structural Pipe Flange Work?

How Flange Work

A flange is created when two opposing surfaces are intentionally pressed together in order to create a leak tight seal. To obtain a seal, force must be applied and maintained to each of the opposing flange faces. As many flange faces have manufacturing imperfections (scratches, dents, pits etc.), it is necessary to put a softer material between the two mating sealing surfaces to obtain the seal; this softer material is the gasket.  

Read more

Flange Assembly

Basic Flange Math

To understand how flanges work, we must first understand the concept of pressure. Pressure is defined as:

Pressure = Force / Area

P = F / A

Flanges seal because pressure is applied to the mating sealing surfaces; this pressure is known as the ‘gasket compression’ or ‘sealing pressure’. The applied pressure causes the two faces to either:

• Crush a gasket between the two mating faces.
• Press the two mating faces against each other.

In the gasket example, the gasket is deformed due to the pressure applied; this deformation causes the gasket to ‘flow’ into any surface imperfections that may be present on either sealing face. Because the surface imperfections have been filled by the gasket material, leakage is no longer possible.

The second example assumes no gasket is present and that two flange faces are pressed together. It is hard to create a leak tight seal using this method, although it is possible if the surfaces are well machined and very clean. The sealing pressure applied will often need to be significant, as the flange surface may be manufactured from metal, which does not easily deform under pressure (material and flange class dependent). Metal to metal flange face sealing is expensive and thus not common.

To create the necessary sealing pressure, the variables of force and area can be adjusted.

• Force refers to the tightening torque (bolting load) applied to the mating flange faces when the nuts on a flange assembly are tightened. Force (F) depends upon the torque (T) applied, torque friction (K) and nominal bolt diameter (D). The force described is classed as ‘bolt pretension’ or ‘bolt preload’, or ‘bolt prestress’, and is represented by the equation F = T/(KD)
• Area refers to the size of the sealing face area.

The amount of pressure on the flange sealing faces corresponds to the amount of force applied when tightening the flange assembly. Thus it is possible to regulate the pressure by adjusting the amount of effort that is exerted when tightening the bolts during flange assembly. 

The sealing area of a flange cannot be as easily adjusted as the force used during assembly. A larger sealing face requires more force to obtain a certain amount of pressure, compared to when using a smaller sealing face. The below example highlights this point, but without the use of units.

Example

A given flange assembly requires a pressure of 10 to seal. This can be achieved by applying a lot of force onto a small sealing face:

Pressure = Force / Area

10 = 40 / 4

Or, it is possible to decrease the size of the sealing face (area) and thus reduce the amount of force required to create the same amount of pressure3:

10 = 20 / 2

The relationship between pressure, force, and area, can be briefly summarised:

Decreasing the sealing face area leads to a decrease in the force required to create a given amount of pressure.
Increasing the sealing face area leads to an increase in the force required to create a given amount of pressure.

The amount of force that can be applied to a flange assembly is limited because of problems relating to physical strength (nuts are often hand-tightened), gasket blow-out4, and stripping5 of the flange bolt threads; but these problems can be overcome if the size of the sealing face is reduced. The type and size of the sealing face used will be dictated by relevant piping standards once the temperature and pressure rating of the flange is known. 

Based on what has been discussed in this section, it can be determined that flanges required to seal at higher pressures, have smaller sealing faces. It is possible for a viewer to guess the pressure at which a system operates by visually inspecting the flange sealing faces e.g. large flange sealing faces indicate low pressure systems.

Flange Defining Factors

Flanges are categorised based upon certain criteria, and these categories are usually defined by relevant piping standards and specifications (discussed later). A flange is defined by

• Type – the geometry of the flange as a whole. Welding neck, slip-on, and socket weld, are examples of different flange types.
• Face – the sealing area of the flange. Flat face, raised face, and ring type joint, are examples of different flange faces.
• Standards and Specification – flanges are manufactured to comply with given standards and specifications. Standards and specifications dictate the dimensions, geometry, schedule, and material, of a given flange (to name a few factors). 
• Dimensions – the dimensions of a flange’s hub, face, blade etc. Dimensions depend upon nominal pipe size (NPS) and the pressure class required for a given application.
• Nominal Pipe Size (NPS) – a dimensionless unit of measurement defining the size of the item (pipe, fitting etc.) that connects to the flange.
• Pressure Class – the pressure-temperature rating of the flange for a given material. Despite the name ‘pressure class’, this factor is material and temperature dependent.
• Material – the material from which the flange is manufactured e.g. cast iron, carbon steel, stainless steel etc.
• Schedule (SCH) – a pipe’s thickness/schedule. The schedule of a pipe is relevant only for welding neck and lap-joint flanges because the schedule of these flanges must match the associated pipe schedule to which they are connected. The other flange types either slide partly into, screw into, or penetrate through, their associated flange, thus the flange schedule does not need to match the pipe schedule. The schedule is relevant for swivel-ring flanges, but these have limited application and will not be discussed further.

All of the aforementioned bullet points will be discussed in a logical order in the coming sections. For now, it’s important to realise that flanges are not unique items. Flanges are manufactured for a specific purpose, with many design factors already considered. Should a flange ever fail, the exact same flange can -theoretically- be ordered to replace its predecessor6; this has significant real-world benefits, which will be discussed later in the Standardisation section. 
 

Flange Types, Faces, and Surfaces - Explained!

This video is part of our Piping Flange Fundamentals Video Course

Additional Resources

http://www.wermac.org/flanges/flanges_raised-face_flat-face_ring-type-joint.html

https://www.kamleshmetal.com/flanges-faces-types.html

https://www.theprocesspiping.com/introduction-to-flanges

What Are Pipe Flanges and How Do They Work?

Offering a reliable way to connect pipe systems with the various equipment, valves, and other components of virtually any processing system, flanges are the second most used joining method after welding.

You will get efficient and thoughtful service from Haihao Group.

Explore more:
What Makes CRFH COIL Essential for Your Projects?
Top 5 Benefits of Heavy Duty H Beams for Construction Projects
Why Choose Coil Coating - AZZ
Unlocking Success: Mastering the HR Strip Strategy
7 Key Benefits of Using Hot Rolled A572 Beams in Construction
Unlocking HR Strip: Your Guide to Modern Workforce Management
What Factors Influence Structural Beam Purchasing Decisions?

Using flanges adds flexibility when maintaining piping systems by allowing for easier disassembly and improved access to system components.

A typical flanged connection is comprised of three parts:

• Pipe Flanges
• Gasket
• Bolting

In most cases, there are specific gasket and bolting materials made from the same, or approved materials as the piping components you wish to connect. Stainless Steel flanges are some of the most common. However, flanges are available in a wide range of materials so matching them with your needs is essential.

Other common flange materials include Monel, Inconel, Chrome Moly, and many others depending on the application.

The best option for your needs will depend on both the system in which you intend to use the flange and your specific requirements.

Common Flange Types and Characteristics

Flanges are not a one-type-fits-all sort of solution. Sizing aside, matching the ideal flange design to your piping system and intended usage will help to ensure reliable operation, a long service life, and optimal pricing.

Here’s a look at the most common flange types available.

Making the Connection: Flange Facing Types

Flange design is only the start when considering the ideal flange for your piping system. Face types are another characteristic that will have a major impact on the final performance and service life of your flanges.

Facing types determine both the gaskets needed to install the flange and characteristics related to the seal created.

Common face types include:

• Flat Face (FF): As the name suggests, flat face flanges feature a flat, even surface combined with a full face gasket that contacts most of the flange surface.
• Raised Face (RF): These flanges feature a small raised section around the bore with an inside bore circle gasket.
• Ring Joint Face (RTJ): Used in high-pressure and high-temperature processes, this face type features a groove in which a metal gasket sits to maintain the seal.
• Tongue and Groove (T&G): These flanges feature matching grooves and raised sections. This aids in installation as the design helps the flanges to self-align and provides a reservoir for gasket adhesive.
• Male & Female (M&F): Similar to tongue and groove flanges, these flanges use a matching pair of grooves and raised sections to secure the gasket. However, unlike tongue and groove flanges, these retain the gasket on the female face, providing more accurate placement and increased gasket material options.

Many face types also offer one of two finishes: serrated or smooth.

Choosing between the options is important as they will determine the optimal gasket for a reliable seal.

In general, smooth faces work best with metallic gaskets while serrated faces help to create stronger seals with soft material gaskets.

The Proper Fit: A Look at Flange Dimensions

Apart from the functional design of a flange, flange dimensions are the most likely factor to impact flange choices when designing, maintaining, or updating a piping system.

However, you must consider how the flange interfaces with the pipe and the gaskets in use to ensure proper sizing.

Common considerations include:

• Outside diameter: The distance between two opposing edges of the flange face
• Thickness: A measure of the thickness of the outer attaching rim
• Bolt circle diameter: The distance between opposing bolt holes when measured from centre to centre
• Pipe size: A designation of the pipe size with which the flange corresponds
• Nominal bore size: A measurement of the flange connectors inner diameter

Flange Classification & Service Ratings

Each of the above characteristics will have an influence on how the flange performs across a range of processes and environments.

So how can you tell which flanges are up to the task and which are not?

Flanges are often classified based on their ability to withstand temperatures and pressures.

This is designated using a number and either the “#”, “lb”, or “class” suffix. These suffixes are interchangeable but will differ based on the region or vendor.

Common classifications include:

• 150#
• 300#
• 600#
• 900#
• #
• #

Exact pressure and temperature tolerances will vary by materials used, flange design, and flange size. The only constant is that in all cases, pressure ratings decrease as temperatures rise.

Flange Standards and Markings

To help make comparison easier, flanges fall under global standards established by the American Society of Mechanical Engineers (ASME) -- ASME B16.5 & B16.47.

If you’re attempting to replace or verify existing parts, all flanges must include markers -- typically on their outer perimeter -- to aid in the process.

These markers also follow a strict order:

• Manufacturer logo or code
• ASTM material code
• Material Grade
• Service rating (Pressure-temperature Class)
• Size
• Thickness (Schedule)
• Heat Number
• Special designations, if any -- for example, QT for Quenched and tempered or W for repair by welding

This guide offers a solid foundation of the basics of flange design and how to choose the ideal flange for your piping system. However, with a wide range of stainless steel flanges and other flange materials available, it is impossible to list every configuration, detail, or consideration.

Should you have questions, the Technical Sales Experts at Unified Alloys are ready to help. Serving industries and businesses across North America and Canada for more than 40 years, we understand the complexities of alloy piping and the needs of your industry. Call us today for additional information and to find the ideal flange, piping, and components for your next project.

Are you interested in learning more about Structural Pipe Flange(pt,tr,es)? Contact us today to secure an expert consultation!