Raised face vs flat face flanges —
what the difference means
for gasket selection
The structural difference — what each face type actually is
A raised face flange has a machined sealing face that is slightly elevated above the surrounding flange face area. Under common ASME B16.5 raised face conventions, raised face height is often 1.6 mm (1/16 inch) for lower pressure classes such as Class 150 and 300, and 6.4 mm (1/4 inch) for higher pressure classes. Always confirm the actual face height from the applicable flange standard, class, size and equipment specification. Other flange standards may specify different raised face heights.
When two raised face flanges are bolted together, the gasket sits between the two raised rings. Bolt load is concentrated across the area of the raised ring — a smaller area than the full face — which increases gasket seating stress over a smaller contact area.
Raised face flanges are common in industrial pipework systems — steel pipe, process plant, oil and gas, steam systems.
A flat face flange has a completely flat sealing surface across the full face diameter with no raised ring. The gasket typically extends to the outer edge of the flange face, covering the full area including the bolt holes.
Full face coverage distributes bolt load across the entire face area. This protects the flange from bending under bolt load — which is critical for cast iron flanges, glass-lined equipment and other brittle or non-metallic flanged components that may crack if load is concentrated at the inner bore area.
Flat face flanges are common on cast iron valves and fittings, glass-lined vessels, pump bodies, and various non-metallic equipment.
How to identify which type you have
Visual identification is usually straightforward on a clean flange face:
- Raised face: look at the flange face surface from the side. There is a visible raised ring — a small step up from the outer face area to the inner sealing ring. The inner ring often has a concentric serrated or spiral machined finish. The outer area (between the raised ring and the bolt holes) is lower.
- Flat face: the entire flange face — from bore to bolt holes — is at the same level. No step, no raised ring. The surface is flat across the full diameter.
- On drawings and specifications: raised face is typically designated RF. Flat face is designated FF. These designations appear on flange datasheets, piping and instrumentation diagrams, and material requisitions.
When the face is corroded, coated or obscured: do not guess from visual inspection alone if the face condition makes it ambiguous. Check the equipment documentation, the piping specification, or the relevant flange standard for the nominal pressure class and material. Cast iron flanges covered by standards such as BS 4504 or ASME B16.1 are commonly supplied as flat face, while carbon and alloy steel flanges to ASME B16.5 are commonly supplied as raised face unless otherwise specified. Confirm the actual face designation from the flange marking, drawing, equipment manual or applicable standard.
Why mixing face types causes problems
Do not bolt a raised face steel flange directly to a cast iron flat face flange with a ring gasket. Where such a connection cannot be avoided, the face geometry should be aligned in accordance with the governing standard or equipment manufacturer's requirements. A common engineering approach is to remove the raised face or otherwise bring both mating faces into a compatible geometry, where this is permitted by the applicable standard, equipment manufacturer and competent engineering review. Gasket selection should follow only after the face geometry has been confirmed.
Which gasket goes where
| Connection | Gasket form | Notes |
|---|---|---|
| RF to RF | Ring gasket — bore ID to beyond the raised ring OD | Standard configuration. Gasket sits between the two raised rings. Does not extend to bolt holes. |
| FF to FF | Full face gasket — bore ID to bolt holes OD, with bolt holes | Standard for cast iron, glass-lined and non-metallic equipment. Full face coverage distributes load and protects against bending. |
| RF to FF | Engineering review required — face geometry and gasket form must be confirmed | Direct assembly with a ring gasket is generally not recommended because it can concentrate load and damage the flat face flange. |
| RF to FF (alternative) | Application-specific approved arrangement only | Use only where the equipment manufacturer or governing standard explicitly permits the face/gasket combination. |
Gasket material selection — face type affects form, not material grade
The flange face type determines the form factor of the gasket — ring or full face — but it does not itself determine the material grade. Material selection is driven by the medium, operating temperature, pressure and any applicable approvals.
- Full face gaskets for flat face flanges — the same fibre grades used in ring gaskets are available in full face form. The material selection logic is the same: heating water, potable water, steam, gas, refrigerant or food contact each point toward different grades and approvals. The gasket covers more area and has bolt holes, but the material grade selection follows the same application criteria.
- Ring gaskets for raised face flanges — the inner diameter and outer diameter must match the raised face ring geometry of the specific flange standard and pressure class. Standard non-metallic gasket dimensions are governed by ASME B16.21 (flat non-metallic gaskets for ASME B16.5 flanges) and EN 1514-1 (non-metallic gaskets for EN flanges). ASME B16.20 covers metallic and semi-metallic gaskets — spiral wound, jacketed, ring joint — which is a different gasket family from the fibre and graphite flat gaskets covered here.
When ordering a replacement gasket: identify the flange standard (ASME B16.5, EN 1092-1, BS 4504 etc.), the pressure class or nominal pressure rating, the nominal pipe size, and the face type — RF or FF. These parameters usually define the starting point for the correct gasket form factor. The application conditions then define the material grade.
A gasket ordered by pipe size alone without the flange standard and face type will not necessarily be the correct form factor for the joint.
Common places where this matters in practice
- Pump connections: many pump bodies, especially cast iron pump casings, use flat face flanges. Connecting them to mating steel pipe flanges (typically raised face) without addressing the face type mismatch is a common source of both leaks and cracked pump flanges.
- Valve connections: cast iron valves — gate, globe, check — are often flat face. The mating pipework flanges may be raised face. Same issue as pump connections.
- Heat exchangers: some heat exchanger nozzle flanges are flat face, particularly on non-metallic or glass-lined units. Verify face type before ordering gaskets for heat exchanger connections.
- Older pipework systems: older cast iron pipework systems — heating mains, water distribution — often use flat face flanges throughout. Replacing sections with steel pipe using standard raised face flanges requires attention to the interface connections.
- Import/export equipment: Mixed-standard installations — where equipment manufactured to ASME standards is connected to equipment manufactured to EN or other standards — are a common place where face type mismatches occur. The actual flange face designation should always be confirmed from the standard and equipment documentation, not assumed from the regional origin of the equipment.
Three checks before you order or assemble. One: face type — RF or FF, confirmed visually or from the flange spec. Two: flange standard and pressure class — ASME B16.5, EN 1092-1, BS 4504 — these set the dimensions. Three: material grade for the medium and temperature. Get any one of those wrong and the joint may not assemble correctly or may not stay sealed.
The face type determines the gasket form. The application determines the material grade. Both must be correct for the joint to seal reliably.
Raised face flanges take ring gaskets between the raised rings. Flat face flanges take full face gaskets across the full bore-to-bolt-hole area. Mixing face types without accounting for the geometry difference causes uneven compression, leak paths and — with cast iron flat face flanges — potentially cracked flanges. Identify the face type first, then select the correct form factor and material grade for the application.