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Guide to Flanges

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 2500, 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 Uses
Slip-On FlangeLowLower-cost option; fits over the pipe for a snug fit
Lap-Joint FlangeLowEasier bolt hole alignment; good for applications that require frequent dismantling
Threaded FlangeLowNo-weld attachment for small-diameter applications
Blind FlangeHighestTo seal off piping systems and provide easy access
Weld-Neck FlangeHighReduces pipe stress in high-pressure applications
Socket 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 #2500. 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 Range
AWWA C207 SteelClass B, #15086 psi4” – 144”4” – 72”N/A
AWWA C207 SteelClass D, #150175 psi (4” – 12”), 150 psi (all other sizes)4” – 144”4” – 72”4” – 96”
AWWA C207 SteelClass E, #150275 psi4” – 144”4” – 72”4” – 96”
AWWA C207 SteelClass F, #300300 psi4” – 48”4” – 48”N/A
AWWA C228 Stainless SteelClass SA, #15050 psi (SPFH Series)2” – 12”2” – 8”N/A
AWWA C228 Stainless SteelClass SB, #15086 psi2” – 72”2” – 72”N/A
AWWA C228 Stainless SteelClass SD, #150175 psi (4”– 12”) 150 psi (all other sizes)2” – 72”2” – 72”N/A
AWWA C228 Stainless SteelClass SE, #150275 psi2” – 72”2” – 72”N/A
AWWA C228 Stainless SteelClass SF, #300300 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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