C-P Systems
What is a Raised Face Flange in Piping Engineering?
What is a Raised Face Flange in Piping Engineering?
A raised face flange is a pipe flange whose seating surface sits elevated above the bolt circle face. This elevated area is where the gasket rests and where the bolted connection creates its seal. The raised face concentrates the bolt clamping force onto a smaller area than a full-face design. This concentration increases gasket seating stress and improves the pressure-tightness of the joint. Consequently, the raised face flange is the most common flange facing type used in process plant piping worldwide.
Raised Face Dimensions Under ASME B16.5
ASME B16.5 governs the dimensions, pressure ratings, and facing requirements for raised face flanges from NPS half-inch through NPS 24 in pressure classes 150 through 2500. The standard defines two raised face heights based on pressure class. Class 150 and Class 300 flanges carry a raised face height of 1/16 inch, approximately 1.6 millimetres. Class 400 and above carry a raised face height of 1/4 inch, approximately 6.4 millimetres. For Class 150 and 300, the raised face height is included in the published overall flange dimensions. For Class 400 and above, the raised face height is additional to the published dimensions and must be added when calculating face-to-face lengths in the piping layout.
Surface Finish Requirements
The gasket contact surface of a raised face flange requires a specific machined finish to ensure the gasket seals correctly. ASME B16.5 specifies a concentric or spiral serrated phonographic finish with 30 to 55 grooves per inch and a surface roughness between 125 and 500 microinches Ra. This controlled roughness allows the gasket material to flow into the surface microtexture under bolt load and form a fluid-tight seal. A surface that is too smooth does not provide sufficient mechanical grip for the gasket. A surface that is too rough creates leak paths between the gasket and the flange face.
Applications in Piping Engineering
Standard Process and Utility Piping
Raised face flanges appear on virtually every flanged connection in process and utility piping systems operating in Class 150 through Class 2500 service. Piping class documents specify the facing type for every service in the plant. The vast majority of process piping piping classes specify raised face throughout, from low-pressure utility services to high-pressure hydrocarbon lines. Consequently, weld neck flanges with raised faces are the most frequently specified and installed flange type in any process plant.
Gasket Selection for Raised Face Connections
Raised face flanges accept a wide range of gasket types. Spiral wound gaskets are the most common choice for process services at elevated temperatures and pressures. They consist of alternating layers of metallic wire and soft filler material wound in a spiral configuration, providing resilience under cyclic loading. Flat ring sheet gaskets suit lower-pressure, ambient-temperature services. Additionally, metal-jacketed and solid metal gaskets suit very high-pressure and high-temperature services where soft filler materials would extrude or degrade. The gasket selection for a raised face connection must match the service conditions, the flange pressure class, and the facing finish.
Bolting and Assembly
Stud bolts with heavy hex nuts are the standard bolting used with raised face flanges. The bolts pass through the bolt holes in the flange body and are tightened in a sequential cross-bolt pattern to load the gasket evenly across the full seating area. Uneven bolt tightening creates high spots and low spots on the gasket seating surface. This uneven loading causes localised leakage paths. Furthermore, over-tightening compresses the gasket beyond its design limit and damages the seating surface. Controlled torque tightening to the specified bolt load is therefore essential for a reliable raised face flange seal.
Comparison with Ring Type Joint Flanges
For very high-pressure and high-temperature services, particularly in Class 900 and above, engineers specify ring type joint flanges rather than raised face flanges. RTJ flanges use a machined groove in the face that receives a solid metal ring gasket. The seal relies on metal-to-metal contact rather than a compressible gasket material. RTJ flanges provide superior sealing integrity at pressures and temperatures where spiral wound gaskets begin to relax. However, RTJ connections are more expensive to fabricate and require greater care during assembly and disassembly. Raised face flanges with spiral wound gaskets remain adequate for the large majority of process services.
Benefits of Raised Face Flanges
Concentrated Gasket Seating Stress
The raised face geometry concentrates the bolt clamping force onto the smaller seating area between the two raised faces. This produces higher gasket seating stress per unit area than a flat face design using the same bolt load. Higher seating stress improves the initial seal tightness and the ability of the joint to maintain its seal under thermal cycling, vibration, and fluctuating pressure. Consequently, raised face flanges achieve reliable sealing across a wide range of operating conditions with standard bolting and gasket materials.
Gasket Type Versatility
Raised face flanges accept flat ring gaskets, spiral wound gaskets, metal-jacketed gaskets, and solid metal gaskets. This versatility means engineers can select the gasket type that best suits the specific service temperature, pressure, fluid, and maintenance frequency without changing the flange design. Furthermore, replacement gaskets are widely available in standard sizes for all ASME B16.5 pressure classes and nominal pipe sizes, simplifying plant maintenance and spare parts management.
Universal Interchangeability
All raised face flanges manufactured to ASME B16.5 are dimensionally interchangeable regardless of manufacturer. This interchangeability allows plant operators to procure replacement flanges, fittings, and gaskets from multiple suppliers without risk of dimensional mismatch. It also allows plant modifications and expansions to connect new piping to existing raised face connections without special fabrication
Limitations to Consider
Incompatibility with Flat Face Flanges
ASME B31.3 prohibits bolting a raised face flange directly to a flat face flange unless the raised face is machined off or a full face gasket is used. When a raised face steel flange mates with a flat face flange on a cast iron pump or valve body, the raised face contacts only the inner portion of the mating flange face. The bolt clamping force then bends the outer rim of the cast iron flange. Cast iron has low tensile strength and cracks under this bending load. Consequently, engineers must verify the facing type at every connection before specifying bolted joints between dissimilar materials.
Surface Damage and Corrosion
The raised face seating surface is a precision-machined area. Physical damage from improper gasket removal, tool marks, impact, or corrosion creates defects that prevent the gasket from seating correctly. Even minor radial scratches on the seating surface provide a leak path under the gasket. Inspectors must examine the raised face surface condition before every bolted assembly. Damaged surfaces require re-machining or flange replacement before the joint can reliably seal.
Gasket Creep and Relaxation
Soft and semi-metallic gaskets on raised face flanges are subject to creep under sustained bolt load at elevated temperature. The gasket material slowly deforms, reducing the contact stress at the seating surface. This reduction in seating stress allows the joint to leak over time. High-temperature services therefore require periodic re-tightening after initial heat-up, or the selection of gasket materials with lower creep rates such as flexible graphite or solid metal designs. Furthermore, thermal cycling between ambient and operating temperature causes differential expansion between the flange body and the bolts, which further reduces bolt load over time.
Misalignment at Assembly
Raised face flanges require accurate pipe alignment before bolting. If the two mating flange faces are not parallel and concentric within the tolerances defined in ASME B16.5, the gasket seates unevenly. Uneven seating produces localised stress concentrations on one side of the gasket and insufficient stress on the other. This creates a joint that is more susceptible to leakage under operating conditions than a correctly aligned and assembled joint. Pipe stress engineers must therefore verify that nozzle loads and pipe routing do not impose angular misalignment at flanged connections during thermal expansion.
Raised Face Flange FAQ
What is a raised face flange in piping engineering? A raised face flange is a pipe flange whose gasket seating surface sits elevated above the bolt circle face. The elevated area concentrates bolt clamping force onto a smaller gasket seating area than a flat face design. This increases the gasket seating stress and improves pressure-tightness. ASME B16.5 governs raised face flange dimensions from NPS half-inch through NPS 24 in pressure classes 150 through 2500. The raised face height is 1/16 inch for Class 150 and 300, and 1/4 inch for Class 400 and above. Raised face flanges are the most widely specified flange facing type in process plant piping worldwide.
What gaskets are used with raised face flanges? Raised face flanges accept several gasket types depending on service conditions. Spiral wound gaskets are the most common choice for process services at elevated temperatures and pressures. They combine metallic winding with a soft filler material and provide resilience under thermal cycling. Flat ring sheet gaskets suit lower-pressure ambient-temperature services. Metal-jacketed gaskets suit very high-pressure services. Solid metal ring gaskets suit extreme conditions approaching the limits of the raised face design. Gasket selection must account for service temperature, pressure, fluid compatibility, and the facing finish specified on the flange.
Why should a raised face flange not be bolted to a flat face flange? Bolting a raised face flange directly to a flat face flange causes the raised face to contact only the inner portion of the mating flange face. The bolt clamping force then bends the outer rim of the flat face flange. For steel flat face flanges, this bending is generally acceptable. However, for cast iron flat face flanges on pumps and valves, the bending stress exceeds the tensile strength of cast iron, causing the flange to crack. ASME B31.3 therefore requires the raised face to be machined off, or a full face gasket used, whenever a raised face steel flange connects to a cast iron flat face flange.
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