Why flanged connections dominate oil and gas piping
Flanged connections give plant operators a maintenance point that welded connections cannot. Pumps, valves, vessels and instruments must be removable, and pipelines must be temporarily separated for cleaning, testing or tie-in. Every one of those joints uses the same three elements: compatible flanges, a gasket that seals the facing, and stud bolts with nuts that hold the assembly together. Incomplete specification can compromise the joint. Treat the connection as one system and follow the approved design and assembly requirements.
Flanges: ASME B16.5 types and pressure classes
The defining standard for flanges from NPS 1/2 to NPS 24 is ASME B16.5. It fixes the dimensions, pressure-temperature ratings and other requirements within its scope. Confirm the exact product and interface. Common flange types in oil and gas are:
| Flange type | How it is joined to the pipe | Typical use |
|---|---|---|
| Weld neck | Butt-welded to the pipe | Confirm bore, end preparation and project requirements |
| Slip-on | Placed over the pipe and fillet-welded | Use where permitted by the approved piping specification |
| Blind | No bore — closes the end of a line or vessel | Flange ends, valves, test boundaries |
| Socket weld | Pipe fits inside the socket, fillet-welded | Small-bore high-pressure lines |
| Lap joint | A stub end backs the loose flange | Lines that are taken apart frequently |
| Threaded | Screwed onto the pipe | Confirm thread and permitted service in the project specification |
Pressure classes under ASME B16.5 are 150, 300, 400, 600, 900, 1500 and 2500. The rating must be reviewed for the applicable material and temperature; the class number is not a pressure value in psi. Size limits depend on class, and the facing must be specified separately. Read the flange pressure class buyer guide before comparing rating descriptions or proposed substitutes.
Gaskets: sealing the face
For the joint to seal, the gasket must match the flange size, class and service. The industrial workhorse is the spiral wound gasket per ASME B16.20: a stainless steel strip winding (commonly AISI 304 or higher alloy) and a filler such as graphite, spiralled into a flat gasket that seals under bolt load and spring-back. Specify the applicable ASME B16.20 edition and project requirements. Select the winding and filler to the process fluid and temperature, not just the flange size.
Stud bolts: ASTM A193 B7 and A194 2H
The bolts that hold a flanged joint in tension are threaded rods with a heavy hex nut at each end, made to ASTM material grades. The established pairing for high-pressure, high-temperature hydrocarbon service is:
- Stud bolt — ASTM A193 Gr. B7 — Quenched-and-tempered chromium-molybdenum alloy steel. Confirm the material and mechanical requirements against the approved joint specification.
- Nut — ASTM A194 Gr. 2H — Quenched-and-tempered medium-carbon steel with specified mechanical requirements; not an intentional weak link in the joint.
- Threads and dimensions — Specify diameter, thread, length and the applicable dimensional requirements. Define how many nuts and other accessories form each ordered set.
- Other service requirements — Alternative fastener materials and coatings require technical selection for the actual environment; do not substitute grades from a generic corrosion-resistant description.
Manufacturer reference: Portland Bolt's ASTM A194 technical table identifies Grade 2H material and mechanical requirements. Use the applicable specification and project requirements for acceptance.
Specify the whole set
When adding flanged connections to your equipment list, include every element — not just the flange:
- Flange size, type and class — e.g. weld neck, size, Class 600, RF
- Material grade — e.g. ASTM A105 carbon steel
- Gasket — size, class and type — e.g. spiral wound with graphite filler
- Bolting — stud bolts A193 Gr. B7 with A194 Gr. 2H nuts, dimensions and quantity
- Material certificates — EN 10204 Type 3.1 for all pressure-containing parts
- Coatings for bolts and flanges where corrosive atmosphere
Related: flanges, fittings & bolting · gaskets & sealing products · EN 10204 certificates
Sourcing a flanged connection package? Send us your flange, gasket and bolt list and we will quote it as one matched set.
Frequently asked questions
Can a Class 300 flange be bolted to a Class 600 flange?
Do not assume flanges of different classes are interchangeable. The responsible engineer must verify the exact dimensions, facing, bolting and rating of the proposed joint. A matching bolt pattern alone does not approve a mixed-class connection.
What is the difference between RF and RTJ facing?
Raised face (RF) joints use a gasket on the raised sealing surface. Ring-type joint (RTJ) joints use a compatible metal ring gasket in the specified grooves. Facing, ring geometry and materials must match the approved connection; pressure class alone does not select the facing.
Why is ASTM A193 Gr. B7 paired with A194 Gr. 2H nuts?
B7 studs and 2H nuts are a common specified combination, but selection depends on the approved joint requirements. Grade 2H is quenched-and-tempered medium-carbon steel with specified mechanical requirements. The nut should not be described as an intentional weak point or a safeguard against overtightening.
