Why the Face Matters More Than the Flange
A flanged joint seals by crushing a gasket between two faces with enough bolt load, evenly, and keeping it crushed through pressure and temperature cycles. The facing controls how that load concentrates: a raised face concentrates it on a narrow ring for a tight seal at moderate load; a ring-type joint concentrates it on a metal ring for the highest integrity; a flat face spreads it to protect a brittle flange. Everything else about a flange — its bolt circle, its class — exists to serve that sealing surface. Mating faces must be of compatible type: the pair is the product.
Raised Face (RF) — the Process Default
The raised face is a ring standing proud of the flange's bolt-hole circle — 1.6 mm high in Class 150 and 300, 6.4 mm in Class 400 and above — finished with concentric or spiral serrations that bite into the gasket. The raised ring concentrates bolt load on the gasket area, which is what lets RF joints seal reliably with spiral-wound, compressed fibre or elastomer gaskets across the full class range. RF is the default facing of process piping: if a line list says nothing, RF is what experienced buyers assume — but confirm, because assuming is also how errors ship.
Flat Face (FF) — Protecting the Weak Partner
The flat face is flush across the whole flange, used with a full-face gasket that covers the bolt holes. Its purpose is protective: against cast iron, cast steel of low strength, plastic or other brittle flanges, a raised face would concentrate load and can crack the mating flange as the bolts are tightened. FF spreads the load over the full face and the full-face gasket supports the brittle material right up to the bore. The rule is absolute in one direction: a steel RF flange must not be bolted directly against a cast iron FF flange without machining the raised face off (and checking the resulting thickness) — the raised ring will act as a wedge.
Ring-Type Joint (RTJ) — Metal on Metal
The RTJ face replaces the gasket surface with a machined groove in each flange, into which a solid metal ring (oval or octagonal section, in soft iron, stainless or alloy to suit the service) is crushed by the bolting. The ring seals metal-to-metal and actually seals harder as pressure rises, because internal pressure energises the ring into the groove. RTJ is the facing of high pressure, high temperature and hazardous service — Class 600 and above commonly, oil and gas production especially. Its costs are real: the flanges and rings cost more, the grooves must be protected from damage, and the joint needs high, evenly applied bolt load. In return it gives the most reliable seal in the B16.5 system.
Tongue & Groove, Male & Female
Two narrower facings complete the family. Tongue and groove pairs a raised tongue on one flange with a matching groove on the other, trapping a narrow gasket in the groove: excellent gasket retention and protection, used in high-pressure and heat-exchanger service, but the pair is captive — the joint cannot be opened sideways, and the two flanges are a matched set. Male and female is the same idea in a shallower form, the gasket sitting in the female recess. Both facings must be ordered as pairs, never as singles against an assumed mate.
Surface Finish: the Number Behind the Letters
Facings are finished to a specified roughness, conventionally in the range of 125 to 250 micro-inches AARH for RF faces with spiral or concentric serrations. The serrations are functional — they grip the gasket and stop it creeping or blowing out — and a face polished smooth by mishandling, or scored across the gasket path by careless storage, will not seal to its rating. This is why flange faces are protected with covers in transit and why a facing should be inspected, not assumed, at assembly. A radial score across the sealing ring is a reject, however good the rest of the flange is.
Matching Faces at the Joint
| Face | Mates With | Gasket | Typical Service |
|---|---|---|---|
| RF | RF | Spiral wound / sheet / elastomer | General process, all classes |
| FF | FF | Full-face sheet | Cast iron, low pressure, brittle mates |
| RTJ | RTJ (same groove) | Metal ring, oval or octagonal | High pressure / temperature, oil & gas |
| Tongue & groove | Its own pair only | Narrow ring gasket | High pressure, exchangers |
| Male & female | Its own pair only | Recessed flat gasket | Special / exchanger service |
Face Protection, Storage and Assembly
More facing failures are created in the warehouse than in the design office. Faces should travel with their protective covers on and stay covered until assembly; a flange stored face-down on concrete or stacked face-to-face without separators collects exactly the radial scores and dents that defeat gaskets. At assembly, the checks are cheap and decisive: run a fingertip across the serrations for scores, confirm the gasket is the one the facing expects (a spiral-wound gasket on an FF joint, or a full-face gasket pinched under an RF ring, are both leaks in waiting), and tighten the bolting in a star pattern in stages so the face loads evenly — a cocked flange defeats a perfect facing with a wedge-shaped gasket gap. RTJ grooves deserve one extra minute: verify the ring number matches the groove, and never reuse a crushed ring.
One facing deserves a last word because it is not machined on the flange at all: on a lap joint, the sealing face is the flare of the stub end, finished flat or serrated to suit the gasket, while the loose flange behind it has no sealing duty. Inspectors occasionally reject a lap joint flange for having "no face" — the face is on the other part, and it is the stub end that must be checked for damage and finish.
Ordering the Face Correctly
State the facing on every flange line — type, and for RTJ the ring number if known — and check it against the mating equipment, not just against the other pipe flanges: pumps, vessels and valves arrive with their own faces, and the pipe flange must match them. Our flange pages state the facings of each type's catalogue executions, from weld neck to blind; if a drawing's facing callout is unclear, send the drawing — reading it is cheaper than re-facing a delivered flange.