1. Overview & Standards
Flanged connections are critical pressure boundaries in pipeline and process systems. Proper bolt sizing, material selection, and tightening procedures ensure gasket compression and joint integrity.
Image: Bolted Flange Joint Cross-Section
Raised-face flange with spiral-wound gasket, showing stud bolt, nut, and bolt circle diameter.
Raised-face flange with spiral-wound gasket, showing stud bolt, nut, and bolt circle diameter.
Key Standards
| Standard | Scope |
|---|---|
| ASME B16.5 | Flange dimensions, pressure ratings, bolt sizes for NPS 1/2-24 |
| ASME B16.47 | Large diameter flanges NPS 26-60 (Series A and B) |
| ASME PCC-1 | Bolted flange joint assembly guidelines, torque procedures |
| ASTM A193/A194 | High-temperature bolting materials (B7, B16, B8) |
| ASTM A320 | Low-temperature bolting materials (L7, L43) |
Stud Bolt vs. Machine Bolt
Stud bolts (threaded rod with two nuts) are standard for pipeline and pressure vessel flanges because they:
- Allow equal clamping force on both flange faces
- Easier to remove (nuts run off, stud pulls out)
- Accommodate flange-to-flange variations
- Required by ASME B31.3 for Class 300+ service
Critical: Under-tightened bolts allow gasket blow-out; over-tightened bolts yield and lose preload. Target 40-60% of bolt yield strength provides adequate gasket seating with safety margin for thermal cycling.
2. Bolt Sizing per ASME B16.5
ASME B16.5 specifies bolt diameter, quantity, length, and bolt circle diameter for each flange size and pressure class combination.
Pressure Classes
ASME B16.5 defines six pressure classes. The class number roughly indicates pressure rating at 100°F:
| Class | Rating @ 100°F (psig) | Typical Application |
|---|---|---|
| 150 | 285 | Low-pressure gas, water service |
| 300 | 740 | Natural gas gathering/transmission |
| 600 | 1,480 | High-pressure gas, compressor stations |
| 900 | 2,220 | Very high pressure, CO₂ pipelines |
| 1500 | 3,705 | Offshore, high-pressure process |
| 2500 | 6,170 | Ultra-high pressure service |
Bolt Requirements by Flange Size
Image: Bolt Circle Diameter (BCD)
Plan view of flange showing bolt holes arranged on bolt circle, with BCD dimension.
Plan view of flange showing bolt holes arranged on bolt circle, with BCD dimension.
| NPS | Class 150 | Class 300 | Class 600 | Class 900 |
|---|---|---|---|---|
| 2" | 4 × 5/8" | 8 × 5/8" | 8 × 3/4" | 8 × 7/8" |
| 4" | 8 × 5/8" | 8 × 3/4" | 8 × 7/8" | 8 × 1-1/8" |
| 6" | 8 × 3/4" | 12 × 3/4" | 12 × 7/8" | 12 × 1-1/8" |
| 8" | 8 × 3/4" | 12 × 7/8" | 12 × 1" | 12 × 1-3/8" |
| 10" | 12 × 7/8" | 16 × 1" | 16 × 1-1/8" | 16 × 1-3/8" |
| 12" | 12 × 7/8" | 16 × 1-1/8" | 16 × 1-1/4" | 16 × 1-5/8" |
| 16" | 16 × 1" | 20 × 1-1/4" | 20 × 1-3/8" | 20 × 1-7/8" |
| 24" | 20 × 1-1/4" | 24 × 1-1/2" | 24 × 1-3/4" | 24 × 2-1/4" |
Format: (Number of bolts) × (Bolt diameter). Full data available in ASME B16.5 Tables.
Bolt Length Determination
Stud bolt length per ASME B16.5 Table E-1 accounts for:
- Two flange thicknesses
- Gasket thickness (spiral-wound typical: 0.175")
- Two heavy hex nuts (A194)
- Thread engagement: minimum one diameter
- Protrusion: 1-3 threads beyond nut
Approximate Stud Length:
L = 2(flange thickness) + gasket + 2(nut height) + allowance
For raised-face (RF) flanges with standard gaskets, use ASME B16.5 Table E-1.
For ring-type joint (RTJ), add 0.25" to 0.50" to RF length.
3. Torque Calculations
Bolt torque creates the clamping force (preload) that compresses the gasket. The relationship between applied torque and resulting bolt tension depends on thread friction.
The Torque-Tension Equation
Basic Formula (ASME PCC-1):
T = K × d × F
Where:
T = Applied torque (lb-in)
K = Nut factor (dimensionless friction coefficient)
d = Nominal bolt diameter (inches)
F = Desired bolt tension (lb)
Convert to ft-lb:
T (ft-lb) = T (lb-in) / 12
Nut Factor (K) Values
The K-factor accounts for friction in threads and under the nut. It varies significantly with lubrication:
| Condition | K-Factor | Notes |
|---|---|---|
| Dry (as-received) | 0.20 | High variability, not recommended |
| Lightly oiled | 0.18 | Machine oil on threads |
| Heavy oil/grease | 0.16 | Petroleum-based grease |
| Anti-seize compound | 0.15 | Copper, nickel, or zinc based |
| Moly paste (MoS₂) | 0.13 | Molybdenum disulfide |
| PTFE paste | 0.12 | Teflon-based lubricant |
| Graphite | 0.10 | Lowest friction |
Lubrication is Critical: Using the wrong K-factor can result in 30-50% error in bolt tension. Always specify lubricant type in bolting procedures and use consistent lubrication.
Torque Calculation Example
Given:
• Bolt: 1" diameter, ASTM A193 B7 (S_y = 105,000 psi)
• Target stress: 50% of yield = 52,500 psi
• Lubricant: Anti-seize (K = 0.15)
Step 1: Bolt stress area (1"-8 UNC)
A_s = 0.606 in²
Step 2: Desired tension
F = σ × A_s = 52,500 × 0.606 = 31,815 lb
Step 3: Calculate torque
T = K × d × F = 0.15 × 1.0 × 31,815 = 4,772 lb-in
T = 4,772 / 12 = 398 ft-lb
Reference Torque Values
Torque for A193 B7 bolts at 50% yield stress with anti-seize (K = 0.15):
| Bolt Size | Stress Area (in²) | Tension (lb) | Torque (ft-lb) |
|---|---|---|---|
| 5/8" | 0.226 | 11,900 | 93 |
| 3/4" | 0.334 | 17,500 | 164 |
| 7/8" | 0.462 | 24,300 | 266 |
| 1" | 0.606 | 31,800 | 398 |
| 1-1/8" | 0.763 | 40,100 | 564 |
| 1-1/4" | 0.969 | 50,900 | 795 |
| 1-1/2" | 1.405 | 73,800 | 1,384 |
| 2" | 2.651 | 139,200 | 3,480 |
Image: Torque Distribution in Bolt
Pie chart showing ~50% thread friction, ~35% nut face friction, ~15% bolt stretch (useful work).
Pie chart showing ~50% thread friction, ~35% nut face friction, ~15% bolt stretch (useful work).
Limitations of Torque Method
- Friction variability: ±25% variation in bolt stress for same torque
- Only 10-15% efficiency: Most torque energy lost to friction
- Thread damage: Galled or damaged threads unpredictably increase friction
- Relaxation: Gasket and embedment relaxation reduce tension over time
For critical joints, consider hydraulic tensioning (±5% accuracy) or ultrasonic bolt measurement (±3% accuracy).
4. Bolt Materials
Bolt material selection depends on operating temperature, corrosion environment, and sour service requirements.
Standard Service: ASTM A193/A194
| Grade | Stud Spec | Nut Grade | Temp Range | Application |
|---|---|---|---|---|
| B7/2H | A193 B7 | A194 2H | -40°F to 750°F | Standard service, most common |
| B7M/2HM | A193 B7M | A194 2HM | -40°F to 750°F | H₂S service per NACE MR0175 |
| B16 | A193 B16 | A194 4 or 7 | -20°F to 1000°F | High temperature (Cr-Mo-V) |
| B8/8 | A193 B8 | A194 8 | -425°F to 1500°F | 304 SS, corrosive service |
| B8M/8M | A193 B8M | A194 8M | -425°F to 1500°F | 316 SS, better corrosion |
Low Temperature: ASTM A320
| Grade | Temp Range | Application |
|---|---|---|
| L7 | -150°F to 400°F | Cryogenic, LNG service (Charpy tested) |
| L7M | -150°F to 400°F | Low temp + H₂S service |
| L43 | -150°F to 400°F | Cryogenic, better toughness (Ni-Cr-Mo) |
Sour Service (H₂S): Per NACE MR0175/ISO 15156, use B7M (HRC ≤ 22) or L7M for sour gas service. Standard B7 (HRC ≤ 35) is susceptible to sulfide stress cracking.
Material Mechanical Properties
| Grade | Min Yield (psi) | Min Tensile (psi) | Hardness |
|---|---|---|---|
| B7 | 105,000 | 125,000 | HRC 22-35 |
| B7M | 80,000 | 100,000 | HRC ≤ 22 |
| B16 | 105,000 | 125,000 | HRC 22-35 |
| L7 | 105,000 | 125,000 | HRC 22-35 |
| B8 Class 1 | 30,000 | 75,000 | HRB 95 max |
| B8 Class 2 | 80,000 | 100,000 | HRC 35 max |
Note: B8/B8M Class 1 (solution annealed) has low strength but excellent corrosion resistance. Class 2 (strain hardened) offers higher strength but limited temperature range.
5. Assembly Procedures
ASME PCC-1 provides guidelines for bolted flange joint assembly. Proper procedure reduces leak rate by up to 80% compared to random tightening.
Pre-Assembly Checklist
- Inspect flange faces for damage, warping, corrosion (clean with wire brush)
- Verify gasket type, size, and material match specification
- Check stud bolt material, length, and thread condition
- Apply lubricant to threads and nut bearing surfaces (not gasket or flange face)
- Verify torque wrench calibration (within 6-12 months)
Tightening Sequence
Image: Cross/Star Tightening Pattern
8-bolt and 12-bolt flange diagrams showing numbered sequence (1→5→3→7→2→6→4→8) for star pattern.
8-bolt and 12-bolt flange diagrams showing numbered sequence (1→5→3→7→2→6→4→8) for star pattern.
Use cross/star pattern to distribute load evenly:
8-bolt pattern: 1 → 5 → 3 → 7 → 2 → 6 → 4 → 8
12-bolt pattern: 1 → 7 → 4 → 10 → 2 → 8 → 5 → 11 → 3 → 9 → 6 → 12
Rule: Each bolt tightened is ~180° (or maximum distance) from previous bolt.
Multi-Pass Torquing
| Pass | Torque Level | Purpose |
|---|---|---|
| 1 | 30% of target | Align flanges, initial gasket compression |
| 2 | 60% of target | Continue compression, redistribute load |
| 3 | 100% of target | Final bolt stress and gasket seating |
| 4 | 100% (verify) | Re-check all bolts, re-torque any loose |
Post-Assembly Verification
- Visual: Uniform gap around flange perimeter, 1-3 threads protruding
- Torque check: Verify 10-25% of bolts; if any >10% low, re-torque all
- Pressure test: Per code (typically 1.5× design pressure)
- Hot re-torque: For service >400°F, re-torque after 24-48 hours at temperature
Common Errors
| Error | Consequence | Prevention |
|---|---|---|
| Random tightening | Uneven gasket load, leaks | Use star pattern |
| Single-pass torque | Local over/under-compression | Minimum 3 passes |
| Dry threads | 50% higher torque needed, galling | Always lubricate |
| Over-torquing | Bolt yield, gasket crush | Calibrated wrench, proper torque |
| Misaligned flanges | Edge loading, leaks | Use alignment tools, check gap |
Success Factors: Clean flanges, correct gasket, proper lubrication, star pattern tightening, multi-pass torquing, verification pass. Following ASME PCC-1 procedure dramatically reduces leak probability.
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