14.2 Flanges, Gaskets & Piping Bolted-Joint Assembly
Key Takeaways
- ASME B16.5 flange pressure classes (150, 300, 600, 900, 1500, 2500) state a pressure-temperature rating, not a fixed pressure — the allowable pressure falls as temperature rises.
- A raised-face steel flange must never be bolted directly to a flat-face cast iron flange, because the raised face acts as a fulcrum and cracks the brittle casting.
- Spiral-wound gaskets require both an inner ring and an outer centring ring on raised-face flanges to prevent inward buckling and to centre the gasket.
- ASME PCC-1 bolt-up practice uses a numbered cross pattern in four passes — snug, 30%, 60% and 100% of target torque — followed by at least one rotational round pass.
- Flange alignment must be corrected before bolting: rotational misalignment of bolt holes and gaps between faces must never be pulled closed with the flange bolts.
Sub-tasks D-19.02/.06/.08 and A-3.05 (Uses fastening and retaining devices) meet at the flanged joint. A leaking flange is the most common piping call-out a millwright receives, and almost all of those leaks come from assembly errors rather than gasket defects.
Flange Types
| Type | Construction | Use |
|---|---|---|
| Weld neck | Long tapered hub butt-welded to the pipe | Highest integrity; high pressure, cyclic and high-temperature service |
| Slip-on | Slides over the pipe, filleted inside and out | Lower cost, easier fit-up; lower fatigue strength |
| Socket weld | Pipe bottoms in a socket, single external fillet | Small bore, NPS 2 and under |
| Lap joint | Loose backing flange over a stub end | Frequent dismantling, expensive alloys (only the stub is alloy) |
| Threaded | Screwed onto NPT pipe | Low pressure, no welding permitted areas |
| Blind | Solid disc | Positive isolation and vessel closures |
Flange Faces
| Face | Description | Gasket |
|---|---|---|
| Raised face (RF) | Gasket surface raised roughly 1.6 mm (Class 150/300) above the bolt circle | Ring gasket inside the bolt circle |
| Flat face (FF) | Full flat surface | Full-face gasket covering the bolt circle |
| Ring type joint (RTJ) | Machined groove for a metal ring | Oval or octagonal soft-iron/stainless ring; very high pressure |
The critical compatibility rule: never bolt a raised-face steel flange to a flat-face cast iron flange. The raised face bears only on the centre of the brittle casting and acts as a fulcrum, so tightening the bolts bends and cracks the cast flange. The correct practice is to machine the steel flange to flat face and use a full-face gasket, which spreads the load out to the bolt circle.
The gasket surface finish matters too. Standard raised faces are serrated (a concentric or spiral phonographic groove, typically 125–250 micro-inch Ra) to bite into a soft gasket. Facing a flange smooth for a soft gasket invites blow-out.
Pressure Classes
ASME B16.5 classes — 150, 300, 600, 900, 1500 and 2500 — are pressure-temperature ratings, not fixed pressures. A Class 150 carbon steel flange is rated near 285 psig at ambient but only about 150 psig at 260 C and less as temperature climbs further. Class number is not the working pressure. Matching flange class, bolting material and gasket to the actual service pressure and temperature is a design requirement, not a field improvisation.
Gasket Selection
| Gasket | Construction | Service |
|---|---|---|
| Compressed non-asbestos sheet | Aramid or glass fibre with elastomer binder | General water, air, low-pressure process |
| PTFE (virgin or expanded) | Chemically inert fluoropolymer | Aggressive chemicals; expanded PTFE conforms to damaged faces |
| Graphite (flexible) | Exfoliated graphite, often with a metal insert | High temperature, steam |
| Spiral wound | Alternating V-shaped metal strip and filler wound in a spiral | The industrial standard for steam, hydrocarbons and higher pressures |
| Ring joint (RTJ) | Solid metal oval or octagonal ring | Very high pressure and temperature; wellheads, high-pressure headers |
| Metal jacketed | Soft filler in a metal envelope | Heat exchangers |
A spiral-wound gasket on a raised face requires two rings:
- The outer centring ring locates the gasket concentric to the bore and acts as a compression stop so the winding is not crushed.
- The inner ring prevents the winding from buckling inward into the pipe bore under pressure, and is mandatory for PTFE-filled gaskets and for high-pressure and vacuum service.
Colour coding on the outer ring edge identifies the winding metal, and a stripe identifies the filler material — worth knowing when verifying that the right gasket is being installed.
Never reuse a gasket, and never stack two gaskets to close a gap. Never apply thread sealant or "goop" to a gasket face on a properly finished flange.
Flange Alignment Before Bolting
The joint must fit before any bolt is loaded. Common acceptance criteria used in industry (verify against the project specification):
| Condition | Typical tolerance |
|---|---|
| Parallelism (gap variation across the face) | Within about 0.8 mm (1/32 in) measured at four points 90 degrees apart |
| Centreline offset (concentricity) | Within about 1.5 mm (1/16 in) |
| Rotational bolt-hole alignment | Bolts must pass freely by hand through both flanges |
| Face-to-face gap | Gasket thickness plus a small tolerance; must not require bolt pull |
Never use the flange bolts as a come-along. Pulling misaligned flanges together with the studs preloads the joint permanently, transfers that force straight into the connected pump, compressor or vessel nozzle, cocks the gasket, and guarantees an eventual leak. Correct the pipe support, the spool length or the hanger instead.
Standard flange practice is to orient bolt holes straddling the vertical and horizontal centrelines ("two-holing"), so no bolt sits at top dead centre and gauge or drain connections stay clear.
Bolt-Up: The ASME PCC-1 Method
ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) is the accepted reference. The objective is uniform gasket stress, not maximum torque.
- Inspect and clean. Wire-brush the flange faces, check for radial scratches across the seating surface, and confirm both faces and the gasket are clean, dry and undamaged.
- Use new bolting where specified, and lubricate the stud threads and nut bearing faces with the specified compound. Torque values assume a stated nut factor; an unlubricated stud may reach only 40–60% of the intended preload for the same torque.
- Number the bolts around the flange (1, 2, 3 ... clockwise) so the pattern can be followed and recorded.
- Fit the gasket dry and centred. Do not use adhesive or grease to hold it.
- Hand-tighten and check gap at four points before applying torque.
- Apply torque in a criss-cross (star) pattern in stages:
- Pass 1 — snug, roughly 20–30% of target
- Pass 2 — about 50–70% of target
- Pass 3 — 100% of target
- Pass 4 — a rotational (clockwise) round pass at 100%, continuing until no nut turns further
- Re-check after 4 hours or after the first thermal cycle where the specification calls for it, because gaskets relax (short-term creep) and bolts lose preload.
8-BOLT FLANGE CROSS-PATTERN SEQUENCE
1
7 4
6 5 tightening order:
1 - 2 - 3 - 4 - 5 - 6 - 7 - 8
3 8 (each bolt roughly opposite
2 the previous one)
For large-diameter and high-class flanges, hydraulic torque wrenches or hydraulic bolt tensioners are used to achieve accurate, repeatable preload; tensioners stretch the stud directly and avoid friction scatter entirely.
Leak Diagnosis on a Flanged Joint
| Symptom | Likely cause |
|---|---|
| Leak at one segment of the circumference | Uneven bolt load, a flange out of parallel, or a damaged face at that point |
| Leak appearing after the first heat-up | Gasket relaxation — the joint needed a hot re-torque |
| Gasket blown out radially | Under-bolted, wrong gasket class, or spiral-wound installed without an inner ring |
| Gasket crushed and extruded | Over-torqued, wrong gasket for the class, or missing outer centring ring |
| Repeat failures on the same joint | Pipe strain forcing the joint — investigate supports, not the gasket |
A NPS 6 raised-face carbon steel flange must be connected to the flat-face cast iron suction flange of an existing pump. What is the correct method?
Why does ASME PCC-1 practice require flange studs and nut bearing faces to be lubricated with a specified compound before torquing?
A spiral-wound gasket in a Class 300 steam header blows out inward, with the winding collapsed into the pipe bore. What was most likely wrong with the installation?