- The flange joint — gasket, bolts, and flange faces — is a system, not individual components — a high-quality valve flanged to the pipe with incompatible gaskets or under-torqued bolts will leak regardless of valve body integrity, and this is one of the most common sources of process fluid releases in plant operations.
- Gasket selection must account for fluid chemistry, temperature, pressure, and face type simultaneously — a gasket that’s chemically compatible with the fluid but rated for insufficient temperature, or incompatible with the flange face type, will fail in service.
- Bolt torque is an engineering calculation, not a gut feel — under-torqued bolts allow gasket relaxation and leakage; over-torqued bolts crush soft gaskets beyond recovery and can yield studs — the correct torque is specific to the bolt size, material, lubrication, and gasket type.
Flange joint integrity doesn’t get the same engineering attention as valve selection, and yet flanged joints account for a disproportionate number of process plant leaks. The joint is an assembly — gasket, bolts, nuts, and flange faces — and every component must be appropriate for the service. I’ve investigated leaks that were traced directly to the wrong gasket type, to insufficiently torqued bolts, or to gaskets reused from a previous outage. All preventable. All the result of treating the flange joint as a commodity rather than an engineered assembly.
Flange Face Types and Their Gasket Requirements
Raised Face (RF)
The raised face is the standard flange face type for most process applications in ASME Class 150 through Class 2500. The raised face is a slightly elevated ring (the sealing surface) machined concentrically on the flange face. The gasket sits within the bolt circle, covering the raised face area. ASME B16.5 defines raised face dimensions and surface finish (typically 125–250 μin Ra, or 3.2–6.3 μm Ra, for spiral wound and sheet gaskets).
Raised face flanges are used with most gasket types: spiral wound gaskets (most common for process service), flat sheet gaskets, Kammprofile, and double-jacketed gaskets.
Ring-Type Joint (RTJ)
The ring-type joint uses a metal ring gasket (oval or octagonal cross-section) seated in machined grooves in the flange faces. The metal ring deforms slightly under bolt load to seal against the groove faces. RTJ flanges provide excellent sealing for high-pressure, high-temperature service (Class 600 and above where the consequences of leakage are severe) and are the standard for wellhead and subsea connections.
RTJ grooves must be precisely machined to defined tolerances — a worn or damaged groove cannot seal reliably and must be re-machined or the flange replaced. Ring gaskets are single-use: once compressed, the ring must be replaced at every disassembly (it’s plastically deformed during installation).
Flat Face (FF)
Flat face flanges have no raised area — the gasket covers the entire flange face including the bolt holes. Flat face is used for lower-pressure services and particularly with cast iron or non-metallic flanges where the raised face contact stress could crack the brittle flange face. Full-face gaskets are used with flat face flanges.
Critical: do not mate a raised face steel flange against a flat face cast iron flange — the raised face edge of the steel flange will bear directly on the cast iron face and crack it. This is a common installation error when steel valves are connected to cast iron piping or equipment.
Gasket Types for Process Service
Spiral Wound Gaskets
Spiral wound gaskets (SWGs) are the dominant gasket type for process plant flange joints. They consist of a V-profile metal strip (typically 316SS or carbon steel) wound spirally with a soft filler material (typically PTFE or graphite) between each metal layer. The alternating metal and filler construction provides resilience (the metal springs back if bolt load relaxes) and sealing (the filler fills any surface imperfections).
An SWG has three components: the winding itself, an inner ring (prevents inward buckling of the winding under bolt load and provides a centering guide), and an outer ring (centering guide that locates the gasket on the raised face and prevents over-compression). The inner ring is essential for high-pressure service (ASME B16.20 requires inner rings for Class 900 and above). Omitting the inner ring in high-pressure service risks inward gasket blowout.
SWG filler materials: PTFE (broad chemical compatibility, suitable to ~260°C); flexible graphite (high temperature to 650°C in non-oxidising service, excellent chemical resistance except strong oxidisers); mica (extreme temperature, steam service above 650°C). Match filler material to service fluid and temperature.
Sheet Gaskets (Compressed Fibre, PTFE, Graphite Sheet)
Sheet gaskets are cut from flat sheet material. They’re simple, low-cost, and appropriate for lower-pressure and lower-temperature applications. Compressed fibre/fibre-reinforced gaskets (e.g., Klinger, Flexitallic Thermiculite) are widely used in utility and moderate process service. Full-sheet PTFE gaskets for corrosive chemical service. Expanded graphite sheet (Grafoil) for high-temperature service.
Sheet gaskets have lower bolt load retention than spiral wound — they relax more over time, particularly at elevated temperatures, and may require re-torquing after thermal cycling. They are not suitable for high-pressure service or where flange face finish is poor.
Ring-Type Joint (RTJ) Metal Gaskets
Oval or octagonal cross-section solid metal rings in soft iron, low-carbon steel, 316SS, or Inconel. Selected based on compatibility with the process fluid and temperature. Octagonal rings are preferred where possible — they make better contact with the groove faces. Used for Class 600+ and all wellhead service.
Kammprofile Gaskets
Kammprofile (or grooved metal core with soft facing) gaskets have a solid metal core with concentric serrated grooves, faced with a thin layer of graphite or PTFE. The serrated grooves deform the facing material to create multiple concentric sealing lines. They provide excellent sealing with lower bolt loads than equivalent spiral wound gaskets and high resistance to blowout. Used in large-diameter flanges and heat exchanger applications where spiral wound would require very high total bolt loads.
Bolt and Stud Material Selection
Bolting materials must withstand the combination of pre-stress (bolt load during installation), service stress (pressure and thermal loads in service), and environmental attack (corrosion in the service environment). ASME B16.5 and ASME B16.34 define the required bolting materials for each pressure class and flange material combination.
Standard bolting materials:
- ASTM A193 B7 / A194 2H: chrome-moly alloy steel studs (B7) with heavy hex nuts (2H). The standard combination for all carbon and alloy steel flanged joints in process service. B7 has a minimum yield strength of 125 ksi (862 MPa) for diameter up to 2.5″. Temperature range -45°C to +425°C.
- ASTM A193 B8M / A194 8M: 316SS studs and nuts. For use with 316SS flanges in corrosive service. Lower strength than B7 (80 ksi yield) — requires larger bolts to achieve equivalent joint load. Temperature range -196°C to +425°C (appropriate for cryogenic).
- ASTM A193 B7M / A194 2HM: NACE-compliant low-alloy steel bolting for sour service. Same alloy as B7 but with controlled hardness (Brinell ≤235) to meet NACE MR0175. Lower pre-stress capability due to hardness limitation.
- ASTM A193 B16: higher-strength bolting for high-temperature service above 425°C; chrome-moly-vanadium alloy.
- Inconel 718 / 625: for very high temperature (>550°C) or highly corrosive service where B16 is insufficient.
Bolt Torque Calculation
The correct bolt torque is calculated from the required gasket stress (seating stress to create initial seal plus service stress to maintain seal under operating conditions), the bolt size and thread pitch, and a nut factor (K) that accounts for friction in the threads and under the nut face.
T = K × d × F where T = torque (N·m), K = nut factor (typically 0.12–0.20 for lubricated bolts; 0.20–0.30 for dry), d = nominal bolt diameter (m), F = required bolt force (N).
The ASME PCC-1 standard (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) provides the authoritative methodology for bolted flange joint assembly, including torque tightening, hydraulic tensioning, and cross-pattern tightening sequences. Using PCC-1 procedures reduces scatter in achieved bolt load and improves joint reliability.
Always lubricate studs and nut bearing faces before torquing — dry threads have high friction variability, meaning the same torque can produce bolt loads varying by 50% or more. Use a consistent lubricant (Never-Seez, molybdenum disulphide paste, or anti-galling thread compound) and document the nut factor used in the torque calculation.
Gasket Installation Best Practices
- Inspect flange faces before assembly — any scratches, gouges, or radial marks across the sealing face must be addressed before installing a new gasket
- Clean flange faces — remove old gasket material completely; any remaining material will cause uneven gasket compression
- Use a new gasket every time — reusing compressed gaskets from previous assemblies is not acceptable practice
- Verify gasket is centred on the raised face before installing bolts
- Hand-tighten all bolts before torquing
- Tighten in cross-pattern sequence (opposite pairs) in multiple passes — typically four passes: 30%, 70%, 100% of target torque, then a final circumferential pass to verify all bolts are at target torque
- Re-torque after first heat-up if graphite gaskets are used — thermal relaxation is significant