The mating surface of a flange, nozzle, or valve is called the face. The face is usually machined to create a smooth surface. This smooth surface will help assure a leak-proof seal when two flanges are bolted together with a gasket sandwiched between. Although numerous types of flange faces are produced, we will focus only on the following three:

• flat face
• raised face
• ring-type joint

Flat face

As the name implies, flanges with flat faces are those that have a flat, level connecting surface (see Figure). Forged steel flanges with a flat face flange are commonly found in 150# and 300# ratings. Their principal use is to make connections with 125# and 250# cast iron flanges, respectively. Attaching steel pipe to the cast iron flanges found on some valves and mechanical equipment always presents a problem because of the brittle nature of cast iron. Using a flat face flange will assure full surface contact, thereby reducing the possibility of cracking the softer cast iron. Figure 4-3 shows a sectional view of flange with a flat face.

Raised Face

The most common face type in use, the raised face is available in all seven of the aforementioned pound ratings. Appropriately named, this flange face has a prominent raised surface. With shallow grooves etched into the raised surface, this flange face assures a positive grip with the gasket. Flanges rated 150# and 300# have a raised face, while flanges 400# and above have a ¼ raised face (see Figure). It is important to note most dimensioning charts, including the ones provided in this text include the raised face thickness in the length dimensions for 150# and 300# flanges. However, the ¼ raised face thickness is not always included in the length dimensions for 400# and higher pound ratings. To assure accurate dimensioning, always determine if the dimensioning chart being used includes the ¼ raised face thickness for the larger pound rating flanges. The ¼ raised face thickness must be added to the dimensioning chart measurement to obtain the overall flange length if the dimensioning chart indicates it has not been added. Figure 4-5 includes a sectional view of a weld neck flange having a raised face.

Ring-Type Joint

Also known simply as ring joint, the ring-type joint does not use a gasket to form a seal between connecting flanges. Instead a round metallic ring is used that rests in a deep groove cut into the flange face (see Figure). The donut-shaped ring can be oval or octagonal in design. As the bolts are tightened, the metal ring is compressed, creating a tight seal.
Although it is the most expensive, the ring-type joint is considered to be the most efficient flange used in process piping systems. The ring and g oove design actually uses internal pressures to enhance the sealing capacity of the connecting flanges. The superiority of this seal can have its disadvantages, however. When dismantling ring joint connections, the flanges must be forcibly separated to release the ring from the groove. In crowded installations, this could cause major problems. Because of this, the ring joint flange is relegated to applications where space for maintenance and replacement are adequate.
Although available for all pound ratings, flanges with ring-type joint faces are normally used in piping systems rated 400# and higher. See Figure 4-7 for the sectional view of a flange with a ring-type joint face.