FLANGE BASICS: FUNCTIONS,
DESIGNS, AND OTHER
CONSIDERATIONS
A look at the common characteristics you should consider when choosing a flange for
your piping system
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.
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.
THREADED FLANGES
Also known as a screwed flange, this style has a thread inside the flange bore which fits with
the matching male thread on the pipe or fitting. The threaded connection means you can
avoid welding in many use cases. Simply match the threading to the pipes you wish to
connect.
SOCKET-WELD FLANGES
Ideal for smaller pipe diameters in low-temperature and low-pressure scenarios, socket-weld
flanges feature a connection in which you place the pipe into the flange and then secure the
connection with a single multi-pass fillet weld. This makes this style simpler to install than
other welded flange types while avoiding the limitations associated with threaded ends.
SLIP-ON FLANGES
Slip-on flanges are very common and are available in a large range of sizes to accommodate
systems with higher flow rates and throughput. Simply match the flange to the outer diameter
of the pipe you intend to connect. Installation is slightly more technical as you’ll need fillet
weld both sides to secure the flange to the pipe.
LAP JOINT FLANGES
Featuring a two-piece design, lap joint flanges require butt welding of the stub end to the
pipe or fitting with the use of a backing flange to create the flanged connection. This design
makes this style popular for use in systems with limited physical space or systems which
require frequent dismantling and maintenance.
WELD NECK FLANGES
Like lap joint flanges, weld neck flanges require butt welding for installation. However, their
integrity, performance in systems with multiple repeat bends, and the ability to use them in
high-pressure and high-temperature systems make them a leading choice for process piping.
BLIND FLANGES
Used for terminating or isolating piping systems, blind flanges are essentially boltable blank
discs. When installed properly and combined with the correct gaskets, they can achieve an
outstanding seal which is easy to remove when needed.
SPECIALTY FLANGES
The flange types listed above are the most common. However, there is a range of additional
specialized flange types available to suit a range of uses and environments. Other options
include nipoflanges, weldoflanges, expanding flanges, orifice, long weld neck and reducing
flanges.
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 selfalign 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#
1500#
2500#
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.
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