Author：Zhengrong Time：2026-10-12 11:55:00 Number of views：84Second-rate
Two flanges can share a size, a pressure class, and a bolt pattern and still refuse to seal, because the parts that matter are pressed face to face. The facing, the surface finish, and the gasket have to work as a set. Mismatch any one of them and the joint leaks no matter how hard the bolts are pulled.
This guide covers the three facings found in most plants, how each seals, and the rules that keep a flange supplier and an installer on the same page.
A bolted flange joint seals by squeezing a gasket between two metal faces until the gasket fills every microscopic gap. The shape of those faces controls how much pressure the gasket sees and where that pressure lands.
Get the geometry right and the joint holds for years. Get it wrong at the drawing stage and the leak appears the first time the line is pressurized, or the first time it is heated and cooled.
The raised face is the default in ASME B16.5 and appears on the great majority of process flanges. A shallow ring, usually about 2 mm high, projects above the bolting plane. The gasket sits on that ring, so the bolt load concentrates on a smaller area and produces a higher seating stress.
It suits a wide range of pressures and temperatures and pairs with spiral wound, sheet, and many other gaskets. The raised ring is easy to machine and easy to inspect for damage.
A flat face flange has no raised ring. The full face, bolt holes included, contacts the mating flange. It is common on cast iron equipment and on valves and pumps with cast bodies that are too brittle to take the concentrated load of a raised face.
Flat face joints need a full face gasket and lower bolt stress. They are usually limited to lower pressure classes and to equipment where the casting cannot be trusted with a point load. A full face gasket is essential, because a small ring gasket on a flat face has nothing to seat against.
The ring type joint, or RTJ, uses a metal ring that sits in a machined groove on each face. Bolting drives the ring into the groove, and the metal deforms slightly to form a hard, tight seal. This facing is built for high pressure and high temperature service, and it appears on wellhead, refinery, and high pressure gas duty.
RTJ facings come in several groove profiles, and the ring must match the groove. The two are not interchangeable between profiles, and a mismatched pair will never seal.
Facing geometry is only half the story. The surface finish on the face drives how the gasket bites. A common specification calls for a concentric or spiral serrated finish in a defined roughness range, measured in microinches or micrometres.
Too smooth and the gasket cannot grip. Too rough and the gasket cannot fill the valleys, so leakage paths remain. A machining mark that spirals outward, rather than a cross hatch, can literally guide the fluid toward the edge. The finish belongs on the drawing, not in a verbal instruction.
A handful of rules prevent most field failures:
Never bolt a raised face against a flat face. The raised ring floats and cannot seal.
Match the gasket to the facing, full face for flat face, ring type for RTJ grooves.
Keep the bolt class and the flange class consistent, so the joint is governed by the weaker part.
Use the specified surface finish and check it after any repair or re machining.
Tighten bolts in a cross pattern in stages, and re check after the first heat up.
Each of these is cheap at the design stage and expensive once the line is in service.
The facing only works if the bolts deliver the right load. Too little and the gasket never seats. Too much and the gasket crushes or the flange yields. A torque value from a table is a starting point, not a guarantee, because friction on the threads and the nut face changes how much of the turning effort actually becomes clamping force.
On high temperature joints the story continues after start up. The bolts expand, the gasket relaxes, and the load drops. Re tightening after the first heat cycle, done safely, restores the seal. Joints that skip that step are the ones that weep a week later, usually at a flange nobody thought to check twice.
Reach for a raised face for general process duty. Reach for a flat face when the mating equipment is a cast body that needs a gentle load. Reach for an RTJ when pressure and temperature rule out a soft gasket. Set the finish, name the gasket, and confirm the two faces are a matched pair before the order goes out.
ASME B16.5, Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, American Society of Mechanical Engineers.
ASME B16.47, Large Diameter Steel Flanges: NPS 26 through NPS 60, American Society of Mechanical Engineers.
API 6A, Specification for Wellhead and Christmas Tree Equipment, American Petroleum Institute.
ASME B31.3, Process Piping, American Society of Mechanical Engineers.
Nayyar, M. L. (Editor), Piping Handbook, 7th Edition, McGraw Hill.
Bickford, J. H., An Introduction to the Design and Behaviour of Bolted Joints, Marcel Dekker.
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