Overview
A ring gasket is a flat, ring-shaped seal that sits inside the bolt circle of a flange, unlike a full face gasket that extends over the bolt holes. It is defined by inside diameter, outside diameter and thickness, all derived from nominal pipe size and flange pressure class.
What a Ring Gasket Is in Flange Language
Flange sealing splits into two geometries, and the choice decides where the bolt load lands.
The first is the inside bolt circle, or IBC, style. Its outside diameter stops short of the bolt holes, so the gasket ring sits captured inside the circle of fasteners. That profile matches the raised face of a raised-face flange, which is why a ring gasket is the default there.
The second is the full face type, which reaches the flange edge and carries punched holes.
When Full Face Wins
Flat face flanges take full face gaskets. So do cast iron and plastic flanges, because those materials are weak in bending and a ring-type gasket levers the flange outboard until it cracks. Raised face flanges take ring-style parts. Get this backwards on a cast iron pump housing and the result is a broken flange, not a weep.
The Three Dimensions That Define a Ring Gasket
Inside diameter should follow the pipe bore. Undersize it and the part intrudes into the flow path, where it erodes and tears. Oversize it, and the bare flange face meets the fluid.
Outside diameter is governed by the bolt circle. The gasket ring must clear every bolt hole with margin for the fitter to centre it. Thickness is the third variable.
Nominal bore alone does not define a ring gasket. An NB 100 part for class 150 and one for class 300 are different items, because class 300 flanges are larger, with a wider bolt circle. That is the most common ordering error, so quote the class and flange standard with the size.
How to Read a Ring Gasket Size Chart
A size chart is a lookup table, not a design document. These are its columns.
| Column | What it tells you |
|---|---|
| Nominal bore (NB / DN / NPS) | Pipe size label, not a dimension |
| Pressure class | 150 to 2500, and it shifts the OD |
| Inside diameter | Bore of the finished part |
| Outside diameter | Outer edge, clearing the bolt holes |
| Thickness | Drives seating stress and creep |
| Bolt circle diameter | Hole centres, setting the OD limit |
| Bolt holes | Count and diameter, for full face parts |
| Standard reference | ASME B16.21, B16.5 or B16.47 |
A worked confirmation runs as follows. Maintenance asks for a rubber gasket ring on an NB 150 line. Establish the flange standard, class and face type first. Say the answer is ASME B16.5, class 150, raised face. Read the raised face diameter and bolt circle from B16.5, then check the ring gasket dimensions against ASME B16.21, which covers non-metallic flat gaskets. Above that range, ASME B16.47 covers series A and series B flanges, which are not interchangeable. Verify against the standard, not a supplier table alone.
Thickness: Thinner Than Instinct Suggests
Thicker feels safer, and for most joints it is not. A thinner section has less material to creep and extrude.
Thickness earns its place when the faces cannot be trusted. Pitted faces, a flange that distorts through thermal cycling, or faces that are not parallel need material to conform to. Use the thinnest ring gasket the flange condition allows, then reface at the next shutdown. Elastomers generally need in the region of 15 to 30 per cent compression to seal.
Material Forms, From Rubber to Metal
Geometry is one decision, material the other. A soft rubber ring gasket and a graphite one of the same size need very different bolt loads.
- Rubber. Nitrile for oils at roughly -30 degC to +120 degC, EPDM for steam at approximately -50 degC to +150 degC, silicone to about +230 degC.
- PTFE. Near inert across roughly -200 degC to +260 degC, though virgin grades cold flow.
- Compressed fibre. The general choice for water, oil and mild chemicals.
- Graphite. High temperature duty and thermal cycling, usually with metal reinforcement.
- Metal. Solid or jacketed rings where a soft ring gasket would extrude.
Hardness counts as much as polymer choice, and gasket rubber commonly falls between 40 and 80 Shore A. A softer rubber gasket ring seats at lower bolt load but extrudes sooner.
Measuring When There Is No Drawing
A part that has spent years under bolt load has flowed outward, thinned and possibly swollen. What you measure is the deformed shape, not the specification.
Measure the flange instead. Take the raised face outside diameter, the pipe bore and the bolt circle across opposing hole centres. Those numbers identify the flange in the standard, and the standard gives you the ring gasket.
Where Ring Joint and O-Ring Geometries Sit
A ring joint gasket is a solid metal ring seating into a machined groove on an RTJ flange. It is called out by ring number, in styles R, RX and BX.
An O-ring gasket seals by squeeze inside a groove rather than by clamped face load, and is sized by inside diameter and cord diameter.
Why Buyers Across India Choose DeePee Rubber
Correct geometry is half the job. The part still has to be cut accurately, from a compound matched to the media, in a thickness the flange can seat. DeePee Rubber has manufactured industrial rubber products and precision sealing components since 1965, supplying industries where reliability decides uptime. Send your flange standard, pressure class, face type and operating conditions, or share a drawing, and the DeePee Rubber team will confirm the ring gasket dimensions against the relevant standard before anything is cut. Request a quotation with those details and avoid the error that stalls most shutdowns.
Conclusion
Sizing a ring gasket is a short chain of checks, and most leaking joints failed because one link was skipped. Bore, class, standard, face type. Get those four right and the diameters fall out of the published tables without argument. Choose thickness against the real condition of the faces, and material against media and temperature rather than what is on the shelf. An O-ring gasket answers a different problem, so match the geometry to the flange in front of you. Once the numbers are settled, hand them to a manufacturer who will check them again.
Key Takeaways
- Order a ring gasket by nominal bore, pressure class and flange standard, never pipe size alone.
- Raised face flanges suit ring-style parts; flat face and cast iron need full face.
- Use the thinnest section the flange face allows, then reface rather than thicken.
- A rubber ring gasket cut in-house still needs its ID and OD from the flange standard.
- Measure the flange, not the failed part, when no drawing exists.
Frequently Asked Question
A ring gasket stops inside the bolt circle and has no bolt holes, concentrating load on a narrow band. A full face part reaches the flange edge with matching holes. Raised face flanges normally take the first, cast iron flanges the second.
No. The bore may match, but flange diameter, bolt circle and bolt pattern all change with class. A ring gasket size chart therefore splits every nominal bore across class 150, 300, 600 and above.
Not in practice. A ring joint gasket is metal, seats in a machined groove and seals by plastic deformation at high pressure, and it is ordered by ring number. A flat ring type gasket is a soft sheet part clamped between two faces.
