Flange Connection
Pipe Fittings / FlangeWhat Is a Flange Connection
A flange connection joins two pipe sections, a pipe to a valve, or a pipe to a piece of equipment by bolting two flat-faced or raised-face rings together with a gasket sandwiched between them. The bolts provide the clamping force that compresses the gasket and creates the seal. Flanged connections are the most widely used detachable joint in process piping because they allow disassembly for inspection, maintenance, and equipment replacement.
In a typical process plant, hundreds or thousands of flanged joints connect piping to valves, heat exchangers, vessels, pumps, and instruments. The integrity of every flange connection depends on correct flange type selection, proper gasket choice, appropriate bolt material, and careful assembly — a single poorly made joint can leak, causing safety hazards, environmental releases, and costly shutdowns.
Flange Types
| Type | Description | Best For |
|---|---|---|
| Weld Neck | Long tapered hub butt-welded to pipe — highest integrity joint | High-pressure, high-temperature, severe-service, and cyclic loading |
| Slip-On | Pipe slips through; fillet-welded both inside and outside | Moderate-pressure, non-critical service; easier alignment than WN |
| Socket Weld | Pipe inserts into socket; single fillet weld outside | Small-bore (NPS 2 and below), high-pressure instrument connections |
| Blind | Solid disc with bolt holes — no opening | Line termination, pressure testing, blanking off unused nozzles |
| Lap Joint | Flange slides over a stub end; bolted to a matching flange | Systems requiring frequent disassembly; alloy pipe with carbon flanges |
| Threaded | NPT or BSPT threads — no welding needed | Low-pressure, non-critical, small-bore air and water lines |
Weld-neck flanges are the standard for ASME B31.3 process piping in Class 300 and above. The long tapered hub provides a smooth transition from the pipe wall to the flange, minimizing stress concentration and making the butt weld easy to radiograph. For less severe service, slip-on flanges offer lower cost and easier fit-up but provide lower pressure integrity.
Pressure Classes and Ratings
| ASME Class | Rating at 38 °C (bar) | Typical Use |
|---|---|---|
| Class 150 | ~20 bar | Low-pressure utility, water, HVAC |
| Class 300 | ~51 bar | General process, moderate pressure |
| Class 600 | ~102 bar | High-pressure process, steam |
| Class 900 | ~153 bar | Very high pressure, some refining |
| Class 1500 | ~255 bar | High-pressure injection, HP steam |
| Class 2500 | ~425 bar | Ultra-high pressure, specialty |
These are approximate; actual pressure-temperature ratings vary with material. ASME B16.5 provides exact rating tables for each material group at each temperature. The rating drops as temperature rises — a Class 300 carbon-steel flange rated at 51 bar at 38 °C falls to about 31 bar at 300 °C.
Common Materials
| Material | ASTM Specification | Notes |
|---|---|---|
| Carbon Steel | A105, A350 LF2 | General service; LF2 for low-temperature |
| Low-Alloy Steel | A182 F5, F9, F11, F22, F91 | High-temperature steam and hydrogen service |
| Stainless Steel | A182 F304, F316, F347 | Corrosive, food-grade, cryogenic |
| Duplex | A182 F51, F53, F55 | Offshore, chloride, high-strength |
| Nickel Alloys | B16.5 Gr. B, C, Hastelloy, Inconel | Severe corrosive, high-temperature |
Gaskets and Bolting
Gaskets
- Spiral-wound: Alternating layers of V-shaped metal strip and filler (graphite or PTFE). The industry standard for process piping — handles a wide range of temperatures and pressures.
- Ring-type joint (RTJ): Solid metal ring (octagonal or oval) that seats in grooves machined into the flange faces. Used for Class 600 and above and for high-pressure, high-temperature hydrocarbon service.
- Sheet gaskets: Compressed fiber, PTFE, or graphite sheets. Used for low-pressure, non-critical services.
- Kammprofile: Metal core with graphite or PTFE facing layers. Excellent sealing, tight bolt load control.
Bolting
- ASTM A193 B7 / A194 2H: The standard carbon-alloy bolting for most process flanges up to about 400 °C.
- ASTM A193 B16: For higher temperatures (up to about 540 °C) where B7 loses strength.
- ASTM A193 B8 / B8M: Stainless-steel bolts for corrosion resistance, but lower strength than B7.
- ASTM A320 L7: Low-temperature bolting for cryogenic service.
Relevant Standards
- ASME B16.5 — Pipe flanges and flanged fittings (NPS ½ through 24, Class 150–2500)
- ASME B16.47 — Large-diameter steel flanges (NPS 26 through 60)
- ASME B16.36 — Orifice flanges
- ASME B16.48 — Steel line blanks
- ASME PCC-1 — Guidelines for pressure boundary bolted flange joint assembly
- API 6A — Wellhead and tree flanges (ring-type joint)
- EN 1092-1 — European steel flanges (PN system)
Typical Applications
- Process Piping: Weld-neck flanges at all equipment connections, valve connections, and piping junctions in refineries and chemical plants.
- Power Plants: Alloy-steel weld-neck flanges with RTJ gaskets on main steam and feedwater.
- Offshore: Duplex or 6MO flanges with alloy bolting for seawater and produced-water service.
- Pipelines: Class 600 and above flanges at block valve stations, launcher/receiver connections.
- HVAC: Class 150 slip-on or threaded flanges in chilled water and condenser water.
Advantages and Limitations
Advantages
- Can be disassembled — allows maintenance, inspection, and equipment removal
- Standardized dimensions — parts from different manufacturers are interchangeable
- Wide range of sizes, pressure classes, and materials
- Reliable sealing when correctly assembled
Limitations
- Potential leak path — every flange is a potential source of fugitive emissions
- Heavy — flanges add significant weight compared to welded joints
- Requires careful assembly — proper bolt tension, gasket selection, and alignment are critical
- More expensive than welded joints in material and labor
Installation Best Practices
- Follow ASME PCC-1: Use calibrated torque wrenches or hydraulic bolt tensioners. Torque values depend on bolt size, material, and lubricant.
- Lubricate bolt threads and nuts: Use an appropriate anti-seize compound. Unlubricated bolts give wildly inconsistent clamp load for the same torque.
- Tighten in a cross-pattern: Follow the star-pattern tightening sequence in PCC-1 to seat the gasket evenly.
- Check alignment before bolting: Flange faces must be parallel within the code tolerance. Forcing misaligned flanges together pre-loads the joint and leads to leakage.
- Inspect gasket seating surfaces: No scratches, pitting, or corrosion. RTJ grooves must be clean and within tolerance.
- Replace gaskets every time: Never reuse a gasket. Used gaskets have taken a compression set and will not reseal reliably.
Practical tip: For critical flanged joints (high-pressure hydrocarbon, toxic, or high-temperature steam), specify hydraulic bolt tensioning rather than torque. Tensioning delivers a more accurate and consistent clamp load because it is not affected by friction variations in the threads and under the nut. The difference in leakage rates between torqued and tensioned joints is significant, especially on large-diameter flanges.