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Technical Guide

Stud Bolt Torque Chart: ASME B16.5 Flange Bolting Class 150 to 2500

2026-05-22Β·10 min read

Why Torque Matters: The Joint Integrity Equation

Flanged joint leakage is the number one cause of unplanned process shutdowns in refineries and petrochemical plants, and the majority of those leaks are not caused by bad gaskets or substandard bolts β€” they are caused by incorrect bolt preload. Too little torque leaves the gasket under-compressed and prone to leaking under thermal cycling. Too much torque yields the stud bolt, permanently reducing clamping load. Getting the torque right requires a proper understanding of the relationship between applied torque, bolt tension, and gasket seating stress.

The Torque-Tension Relationship

Applied torque (T) converts to bolt tension (F) through the relationship:

T = K Γ— F Γ— d

Where:

  • T = applied torque (NΒ·m or ftΒ·lbf)
  • K = nut factor (torque coefficient β€” accounts for thread friction and under-nut friction)
  • F = bolt tensile load (N or lbf)
  • d = nominal bolt diameter (m or inches)

The nut factor K is the critical variable. It ranges from approximately 0.08 (well-lubricated PTFE-coated bolts) to 0.25 (dry, unlubricated threads). Using the wrong K factor β€” or forgetting to apply lubricant when your torque values assume lubricated conditions β€” can result in actual bolt tension being 50–100% different from the intended value at the same applied torque.

Typical Nut Factor Values

ConditionK FactorTorque vs Lubricated Baseline
PTFE/Xylan coated (e.g., Xylan 1424)0.08–0.10–25 to –35% torque needed
Molybdenum disulfide (MoSβ‚‚) paste0.12–0.14–10 to –20% torque needed
Oil or light machine grease0.15–0.18Baseline reference
Zinc-rich thread lubricant (Jet-Lube, Copperslip)0.17–0.20+5 to +15% torque needed
Hot-dip galvanized (HDG) with no additional lube0.20–0.22+15 to +30% torque needed
Dry, unlubricated (plain carbon steel)0.20–0.25+25 to +60% torque needed

ASME B16.5 Stud Bolt Torque Reference Table (ASTM A193 B7 / A194 2H)

The following torque values are calculated for ASTM A193 B7 stud bolts paired with A194 2H heavy hex nuts, targeting 50% of specified minimum yield strength (SMYS = 105 ksi = 724 MPa) β€” a conservative field starting point per ASME PCC-1 Appendix J. Lubrication assumption: light oil or anti-seize compound, K = 0.17.

Flange SizeClass 150Class 300Class 600Class 900Class 1500Class 2500
Format: Bolt size Γ— bolt count β†’ Target torque (NΒ·m) at 50% SMYS, K=0.17
1" NPSΒ½" Γ— 4 β†’ ~45Β½" Γ— 4 β†’ ~45⅝" Γ— 4 β†’ ~85ΒΎ" Γ— 4 β†’ ~150β…ž" Γ— 4 β†’ ~2301" Γ— 4 β†’ ~350
2" NPS⅝" Γ— 4 β†’ ~85⅝" Γ— 8 β†’ ~85ΒΎ" Γ— 8 β†’ ~150β…ž" Γ— 8 β†’ ~2301" Γ— 8 β†’ ~3501ΒΌ" Γ— 8 β†’ ~680
4" NPS⅝" Γ— 8 β†’ ~85ΒΎ" Γ— 8 β†’ ~150β…ž" Γ— 8 β†’ ~2301β…›" Γ— 8 β†’ ~4901ΒΌ" Γ— 8 β†’ ~6801Β½" Γ— 8 β†’ ~1,180
6" NPSΒΎ" Γ— 8 β†’ ~150ΒΎ" Γ— 12 β†’ ~1501" Γ— 12 β†’ ~3501ΒΌ" Γ— 12 β†’ ~6801β…œ" Γ— 12 β†’ ~9201ΒΎ" Γ— 12 β†’ ~1,800
8" NPSΒΎ" Γ— 8 β†’ ~150ΒΎ" Γ— 12 β†’ ~1501β…›" Γ— 12 β†’ ~4901ΒΌ" Γ— 12 β†’ ~6801Β½" Γ— 12 β†’ ~1,1802" Γ— 12 β†’ ~2,700
10" NPSβ…ž" Γ— 12 β†’ ~2301" Γ— 16 β†’ ~3501ΒΌ" Γ— 16 β†’ ~6801β…œ" Γ— 16 β†’ ~9201ΒΎ" Γ— 16 β†’ ~1,8002Β½" Γ— 12 β†’ ~5,100
12" NPSβ…ž" Γ— 12 β†’ ~2301" Γ— 16 β†’ ~3501ΒΌ" Γ— 20 β†’ ~6801Β½" Γ— 20 β†’ ~1,1802" Γ— 20 β†’ ~2,7003" Γ— 12 β†’ ~8,800

Note: These torque values are indicative starting points only. Final torque targets must be established through a formal flange management procedure per ASME PCC-1, taking into account gasket type, flange surface finish, operating temperature, and actual bolt lot tensile properties.

ASME PCC-1 Cross-Bolt Tightening Sequence

Applying torque in a simple clockwise sequence results in uneven gasket loading. The ASME PCC-1 cross-bolt (star pattern) sequence achieves uniform gasket seating:

  1. Pass 1 β€” Snug tight (~20% of target torque): Number bolts 1 through N from 12 o'clock position. Tighten in cross sequence to bring flange faces together.
  2. Pass 2 β€” 50% of target torque: Repeat cross sequence at 50% of final target torque.
  3. Pass 3 β€” 100% of target torque: Repeat cross sequence at 100% target torque.
  4. Pass 4 β€” Verification (clockwise): Final clockwise verification pass at 100% torque. If any bolts move before reaching target, continue clockwise passes until all bolts hold. Typically 2–4 passes for spiral-wound gaskets; up to 6 for RTJs.

Hot Torquing and Thermal Retorquing

Gasket materials β€” particularly spiral-wound types β€” experience 10–30% stress relaxation in the first thermal cycle as the gasket seats under load and temperature. ASME PCC-1 recommends a hot retorque after the joint first reaches operating temperature. This is mandatory for Class 600 and above flanges and all spiral-wound gasket applications at most major operating companies.

Hydraulic Bolt Tensioning vs Torque Wrenching

For Class 900 and above flanges, and for all large-diameter or safety-critical connections, hydraulic bolt tensioning is preferred over torquing. Tensioning applies direct axial load β€” eliminating K-factor uncertainty β€” and achieves bolt load uniformity of Β±5% versus Β±25–35% with torque wrenching. LOKRON recommends tensioning for all pressure class β‰₯ 900 flanges and Category M fluids per ASME B31.3.

Ordering ASME B16.5 Bolt Kits from LOKRON

LOKRON supplies complete ASME B16.5 flange bolt kits: ASTM A193 B7 stud bolts cut to the correct length for each flange size and pressure class, paired with A194 2H heavy hex nuts, with a single EN 10204 3.1 MTR covering both components. Specify flange size, pressure class, gasket type (spiral-wound, RTJ, or flat face), and quantity β€” LOKRON will confirm correct bolt dimensions, quantity per flange, and total kit price within 24 hours. Surface treatment options include plain, hot-dip galvanized, PTFE/Xylan coated, and Dacromet.

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Need this grade or specification?

LOKRON supplies PED-certified stud bolts and heavy hex nuts with full documentation.

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