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.
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.
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.
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:
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 a continuous spiral, or evenly spaced concentric grooves. Other finishing options include a mill (or rolled) finish and cast 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:
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 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, 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.
Because 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 are flat and are available with the same thicknesses and bolt hole patterns as flanges, 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.
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. The most common types of reducing flange attachment are weld-neck, slip-on, and threaded.
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 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.
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.
|Type of Flange||Pressure Capacity||Benefits and Uses|
|Slip-On Flange||Low||Lower-cost option; fits over the pipe for a snug fit|
|Lap-Joint Flange||Low||Easier bolt hole alignment; good for applications that require frequent dismantling|
|Threaded Flange||Low||No-weld attachment for small-diameter applications|
|Blind Flange||Highest||To seal off piping systems and provide easy access|
|Weld-Neck Flange||High||Reduces pipe stress in high-pressure applications|
|Socket Weld Flange||High||Smooth fluid flow, no leakage|
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:
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.
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.
|Standard||Class/Drilling Pattern||PSI Rating||Ring/Slip-On Size Range||Blind Size Range||Hub Size Range|
|AWWA C207 Steel||Class B, #150||86 psi||4” – 144”||4” – 72”||N/A|
|AWWA C207 Steel||Class D, #150||175 psi (4” – 12”), 150 psi (all other sizes)||4” – 144”||4” – 72”||4” – 96”|
|AWWA C207 Steel||Class E, #150||275 psi||4” – 144”||4” – 72”||4” – 96”|
|AWWA C207 Steel||Class F, #300||300 psi||4” – 48”||4” – 48”||N/A|
|AWWA C228 Stainless Steel||Class SA, #150||50 psi (SPFH Series)||2” – 12”||2” – 8”||N/A|
|AWWA C228 Stainless Steel||Class SB, #150||86 psi||2” – 72”||2” – 72”||N/A|
|AWWA C228 Stainless Steel||Class SD, #150||175 psi (4”– 12”) 150 psi (all other sizes)||2” – 72”||2” – 72”||N/A|
|AWWA C228 Stainless Steel||Class SE, #150||275 psi||2” – 72”||2” – 72”||N/A|
|AWWA C228 Stainless Steel||Class SF, #300||300 psi||4” – 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.