IDB-GSK-031
Static seals · gaskets · flanges · seating stress
Gasket selection guide
Choosing a gasket or static face seal — material by media and temperature, the seating stress and bolt load it needs, flange finish, and the failure modes behind most leaks.
Abstract
A gasket seals a static joint between two faces that can't be machined fine enough to seal on their own. Bolt load crushes the gasket to fill the surface texture (seating), and the remaining clamp must resist the internal pressure trying to unseat or blow it out. Choosing well means matching the material to the media and temperature, then providing enough — and uniform — bolt load.
Section 1 frames static-seal options. Section 2 is gasket types and materials. Section 3 covers seating stress and bolt load (the m and y factors). Section 4 is flange and joint design. Section 5 covers liquid / form-in-place sealants. Section 6 is failure modes and a checklist.
1.Static seal options
For a non-moving joint you have several choices; pick by pressure, temperature, media and how the faces are made:
- Gasketa compressible sheet/ring clamped between flanges; best for larger flat joints and field-serviceable connections.
- O-ring (in a groove)better for moderate pressure with a machined gland; lower bolt load, reusable (see the O-ring selection guide).
- Liquid / form-in-place sealantRTV silicone or anaerobic; fills the gap, no cut part, great for covers and irregular faces.
- Metal seal (RTJ, C-ring)extreme pressure/temperature, precision flanges.
A gasket wins when faces are large, flat and bolted; an O-ring wins when you can machine a groove and want a low-bolt-load, reusable seal.
2.Gasket types and materials
| Type | Max temp | Pressure | Media / strength | Notes |
|---|---|---|---|---|
| Elastomer sheet (NBR/EPDM/FKM/Si) | by elastomer (≤~200 °C) | low–mod | per elastomer compatibility | Soft, low bolt load, cheap; covers/low-pressure |
| Compressed fibre (CNAF) | ~400 °C | high | oils, water, steam | General industrial workhorse |
| PTFE (virgin / expanded) | ~250 °C | mod–high | near-universal chemical | Use expanded PTFE to avoid cold-flow creep |
| Flexible graphite | ~450 °C+ | high | steam, chemical | High temp; conforms well |
| Spiral-wound (metal + filler) | high | high | flanged piping | Resilient, recovers after bolt relaxation |
| Metal ring joint (RTJ) | very high | very high | oil & gas | Needs grooved flanges |
| Cork / rubber, paper fibre | low | low | oils, fuels (light) | Pans, covers, low-duty |
| Liquid (RTV / anaerobic) | ~200–300 °C | low–mod | per chemistry | Gap-filling (RTV) or rigid metal-to-metal (anaerobic) |
Match the gasket material to the fluid exactly as you would an O-ring — the elastomer/PTFE/graphite compatibility rules are the same.
3.Seating stress and bolt load
A gasket needs two things from the bolts, and the design must satisfy both:
1. Seat it — crush the gasket enough to conform: minimum seating stress y (MPa) over the gasket area. W_seat = y · A_gasket. 2. Keep it seated — resist the pressure end-load plus a margin: the m (maintenance) factor sets the residual stress needed under pressure. W_operate = (pressure end load) + m · p · (contact area).
Bolt load must cover the worse of the two. These tie straight into the Bolted joint reference — the gasket sets the required preload, and bolt count/size/torque deliver it.
| Gasket material | m factor | y seating (MPa) |
|---|---|---|
| Soft elastomer (<75 Shore A) | 0.5–1.0 | 0 – 1.4 |
| Elastomer with fabric | ~1.25 | ~2.8 |
| Compressed fibre (1.5 mm) | ~2.0 | ~11 |
| PTFE | ~2.0 | ~9 |
| Spiral-wound (SS + graphite) | ~3.0 | ~69 |
| Soft aluminium (flat metal) | ~4.0 | ~60 |
Values are indicative (ASME VIII Div 1, App. 2 style). Softer gaskets need far less bolt load — but creep more.
4.Flange and joint design
- Surface finish: soft gaskets actually seal better on a slightly rough, concentric- or spiral-serrated face (Ra ~3.2–6.3 µm) that "bites"; PTFE and metal seals want smoother, flatter faces.
- Flatness & rigidity: flanges must stay flat under bolt loadthin or widely-spaced-bolt flanges bow between bolts and leak there. Keep bolt spacing tight and flanges stiff.
- Tighten uniformly in a star/cross pattern in steps to spread the load and avoid cocking the gasket.
- Don't over-compress (crush/extrude the gasket) or under-bolt (leak). Use a gasket with the right thickness and widthwider needs more total load, thinner creeps less.
- Blow-out: confine the gasket (raised face, groove, or O-ring style) for high pressure so internal pressure can't push it out.
5.Liquid and form-in-place sealants
- RTV silicone (FIPG): flexible, gap-filling, peelable; good for sheet-metal covers and joints that flex or have imperfect faces. Cure needs air/moisture; respect skin-over time.
- Anaerobic (flange sealant): cures rigid in the absence of air between close-fitting metal faces; no gap, high strength, excellent on machined housings. Needs clean, close-contact metal.
- Both replace a cut gasket where tooling a gasket isn't worth itbut require clean, degreased surfaces and correct cure before pressurising.
6.Failure modes and checklist
| Mode | Cause | Fix |
|---|---|---|
| Leak at assembly | insufficient seating stress / uneven bolt load | more/uniform preload, correct gasket, star-pattern torque |
| Leak over time | bolt relaxation, gasket creep (esp. PTFE) | re-torque after seating, use spiral-wound/ePTFE, Belleville washers |
| Crush / extrusion | over-compression, gasket too soft/wide | harder gasket, confine it, control bolt load |
| Chemical attack / swell | wrong material | reselect per media compatibility |
| Thermal relaxation | high-temp creep loses clamp | high-temp gasket, live-loading (springs), re-torque hot |
| Blow-out | pressure exceeds confinement | confined/raised-face joint, higher bolt load |
6.1Selection checklist
- Media & temperaturechoose the material that survives both (Section 2), exactly as for an O-ring.
- Pressuresteady and peak; confine the gasket for high pressure.
- Seating & operating loadfrom y and m, size the bolt preload (link to the Bolted joint reference).
- Flangeadequate finish, flatness and tight bolt spacing; star-pattern tightening.
- Servicewill it be re-opened? Cut gasket / O-ring for serviceable; sealant for permanent.
- Relaxation planre-torque schedule or live-loading for creep-prone or hot joints.