A flange is a protruding rim or collar that connects two sections of pipe, or connects a pipe to a valve, pump, or other piece of equipment. Think of it as a lip that lets you bolt two components together with a gasket sandwiched in between, creating a sealed joint you can later take apart without cutting the pipe. Flanges are one of the most fundamental components in industrial piping, found everywhere from water treatment plants to offshore oil platforms.
How a Flange Joint Works
A flange joint has three parts: two flanges (one on each side), a gasket between them, and a set of bolts holding everything together. The gasket fills the tiny gap between the two flange faces so nothing leaks. The bolts, evenly spaced around the circumference, apply pressure to compress the gasket into a tight seal.
Getting the bolt tightening force right is critical. Too little force and the gasket won’t fully compress to fill the gap between the sealing surfaces. Too much force can crush the gasket or warp the flange itself, both of which cause leaks. This is why industrial settings often use calibrated torque wrenches and follow specific tightening sequences.
The real advantage of flanged connections over welded joints is maintenance access. When a valve needs servicing or a section of pipe needs inspection, you unbolt the flange, do the work, and bolt it back together. No cutting, no re-welding, no destroying the system.
Common Flange Types
Different piping situations call for different flange designs. The five most common types each solve a specific problem.
- Weld neck flanges have a long, tapered hub that gets butt-welded to the pipe. The taper distributes stress gradually rather than concentrating it at the base, making these the go-to choice for high-pressure and high-temperature pipelines. They’re the most robust option and the most expensive.
- Slip-on flanges are simple rings that slide over the end of a pipe and get welded in place on both the inside and outside. They cost less than weld neck flanges and work well for lower-pressure systems where extreme strength isn’t required.
- Blind flanges have no hole through the center. They bolt onto the end of a pipe, valve, or pressure vessel to seal it off completely, useful for terminating a line or closing off a branch that may be used later.
- Socket weld flanges have a recessed area where a small-diameter pipe fits inside, then gets welded around the outside. They’re a space-saving alternative to weld neck flanges on small-bore lines, though they’re generally not recommended for critical high-pressure service.
- Threaded flanges screw onto pipes with external threads, requiring no welding at all. They look similar to slip-on flanges but have internal threads bored to match the pipe diameter. These are practical for small lines where welding is difficult or where the system needs to be assembled and disassembled frequently.
A sixth type, the lap joint flange, works as a two-piece assembly. A stub end gets butt-welded to the pipe, and the flange ring slides freely behind it. Because the flange never contacts the fluid, it can be made from a cheaper material than the stub end. This design is popular in systems that handle corrosive fluids where the wetted parts need to be stainless steel or a specialty alloy but the structural ring does not.
Flange Face Types
The face of a flange is the flat or machined surface where the gasket sits. Three face types dominate industrial piping, and each pairs with specific gasket styles.
Raised face (RF) flanges have a small circular platform that sticks out slightly from the body, typically 1/16 inch for lower pressure classes (150 and 300) and 1/4 inch for higher classes (400 through 2500). The gasket seats only on this raised ring, which concentrates bolt force onto a smaller area for a tighter seal. RF flanges are the standard in refineries, power plants, and chemical processing.
Flat face (FF) flanges have a smooth surface that runs flush across the entire face. A full-face gasket covers the whole surface. You’ll find these on PVC and fiberglass piping, low-pressure water and air lines, and connections to cast iron pumps and equipment. They’re the norm for lighter-duty applications.
Ring type joint (RTJ) flanges have a precision-machined groove cut into the face that accepts a metal ring gasket, either oval or octagonal in cross-section. When the bolts tighten, the soft metal ring deforms into the groove, creating a metal-to-metal seal that holds up under extreme pressure and temperature. RTJ flanges are standard on oil and gas platforms, petrochemical plants, and high-pressure steam systems.
Gasket Selection by Face Type
The flange face determines what kind of gasket you use. Flat face flanges typically pair with non-metallic gaskets like rubber or PTFE (a chemical-resistant polymer). Raised face flanges commonly use spiral wound gaskets, which are semi-metallic, with alternating layers of metal and filler material wound into a ring. RTJ flanges require solid metal ring gaskets that can deform under high bolt loads without failing.
Using the wrong gasket type for a given face is a common cause of joint failure. A soft rubber gasket on a high-pressure RTJ flange would blow out; a rigid metal gasket on a flat-face cast iron flange could crack the flange body.
Materials and Pressure Ratings
Most industrial flanges are made from forged steel rather than cast, because forging produces a denser, stronger grain structure. The two most common material specifications are carbon steel (designated A105 under ASTM standards) for general service at ambient and elevated temperatures, and stainless steel (A182, with grades like F304 and F316L) for corrosive environments or high-temperature service.
Flanges are rated by pressure class: 150, 300, 600, 900, 1500, and 2500. These numbers don’t correspond directly to pounds per square inch. Instead, each class defines a maximum working pressure at a given temperature. A Class 150 carbon steel flange, for example, handles roughly 285 psi at ambient temperature but less at higher temperatures because the metal loses strength as it heats up. Higher classes use thicker flanges, larger bolts, and tighter tolerances.
Standards for Large Diameter Flanges
Standard flanges up to 24 inches in diameter follow the ASME B16.5 specification. Above 24 inches, a separate standard, ASME B16.47, takes over and splits into two series.
Series A flanges (originally known as MSS SP-44) are thicker, heavier, and stronger. They handle more external loading, offer ring type joint options from Class 300 through 900, and are the typical choice for new pipeline construction and equipment installations. Series B flanges (formerly API 605) use more bolts but of smaller diameter, and they have a smaller bolt circle. That smaller bolt circle actually reduces flange movement after installation, which can be an advantage. Series B flanges cost less and are popular for refurbishment or replacement jobs where the existing infrastructure dictates the dimensions.
Both series define weld neck and blind flanges in raised face configurations, so the choice often comes down to project requirements, budget, and whether you need RTJ capability, which only Series A provides within this standard.