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: two flanges rated to the same pressure class, a gasket that seals the facing, and stud bolts with nuts that hold the assembly together. If any of the three is under-specified, the joint fails — and a leaking flange joint in hydrocarbon service is a serious incident waiting to happen. Treat the connection as one system.
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 classes, facings and materials so that flanges from different factories can still be bolted together. 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; bore matches the pipe | Main process and hazardous service — the default in oil and gas |
| Slip-on | Placed over the pipe and fillet-welded | General purpose, lower class service |
| 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 | Small, low-pressure, non-critical duty |
Pressure classes under ASME B16.5 are 150, 300, 600, 900, 1500 and 2500. Higher classes mean thicker, stronger flanges and a higher pressure rating — and a Class 600 flange can never be mixed with a Class 300 one. Raised face (RF) is the standard facing for most classes; ring-type joint (RTJ) facings are used for higher classes and larger sizes where gasket retention is critical.
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. API 601 covers spiral wound gaskets for refinery service. 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. The established pairing for high-pressure, high-temperature hydrocarbon service is:
- Stud bolt — ASTM A193 Gr. B7 — A chromium-molybdenum carbon steel, quenched and tempered, with the strength to hold a Class 150–2500 joint at high flanging pressure.
- Nut — ASTM A194 Gr. 2H — A heavy hex nut of low-alloy steel, hardened, designed so a failure happens in the nut threads rather than the bolt.
- Threads and dimensions — Threads per ANSI B1.1 (class 2 is common); dimensions per ANSI B18.2. Bolts are supplied with one or two heavy hex nuts as specified.
- Corrosion-resistant service — For aggressive service, A193 Gr. B8M (316 stainless) or low-hardness variants are used instead of plain B7.
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 · gaskets · stud bolts & nuts
