What Is a Stud Bolt? Flange Fastener
A stud bolt is a fully threaded steel rod with two heavy hex nuts, used to clamp flanges together and compress the gasket to form a pressure-tight seal. Stud bolts are the standard fastener for flanged joints in oil and gas, petrochemical, and power generation piping systems. The threaded rod is called the “stud” and the nuts are tightened from both ends to apply uniform clamping force.
When to Use Stud Bolts
Stud bolts are required in virtually all ASME-flanged piping systems. They differ from standard hex bolts by having no head; just thread on both ends with a nut on each side. This design allows equal load distribution from both directions and is critical for high-pressure, high-temperature flange assemblies.
Specific use cases:
- All flanged joints per ASME B16.5 (NPS 1/2 to NPS 24) and ASME B16.47 (NPS 26 to NPS 60)
- Pressure vessel and heat exchanger flange connections
- Pipeline flange joints per ASME B31.3, B31.1, B31.4, and B31.8
- Any bolted joint requiring controlled bolt torque
Stud Bolt Components
| Component | Specification | Details |
|---|---|---|
| Threaded rod (stud) | ASTM A193 (high-temp), A320 (low-temp) | Fully threaded; UNC thread series (imperial) |
| Heavy hex nuts (x2) | ASTM A194 | One nut per end; heavy hex per ASME B18.2.2 |
| Thread form | ASME B1.1 (Unified) | UNC 8-thread series for diameters above 1” |
| Length measurement | FTF (first thread to first thread) | FTF = OAL minus 1/4 inch (typical) |
Common ASTM Materials
| Grade | Specification | Material | Service |
|---|---|---|---|
| B7 | ASTM A193 | Chromium-molybdenum alloy steel | High-temperature (up to 450 degC) |
| B7M | ASTM A193 | B7 with restricted hardness (22 HRC max) | Sour service per NACE MR0175 |
| L7 | ASTM A320 | Same chemistry as B7, impact tested | Low-temperature (down to -100 degC) |
| B8 Class 1 | ASTM A193 | SS 304 (carbide solution treated) | Corrosion-resistant, low strength |
| B8M Class 1 | ASTM A193 | SS 316 (carbide solution treated) | Marine, chemical service |
| B16 | ASTM A193 | Cr-Mo-V alloy steel | High-temperature (above 450 degC) |
The matching nut grades are:
| Stud Grade | Nut Grade | Nut Specification |
|---|---|---|
| A193 B7 | A194 2H | Carbon steel, hardened |
| A193 B7M | A194 2HM | Restricted hardness for NACE |
| A320 L7 | A194 4/7 | Low-temperature impact tested |
| A193 B8 | A194 8 | SS 304 |
| A193 B8M | A194 8M | SS 316 |
Stud Bolt Sizing
Stud bolt diameter and length are determined by the flange size, pressure class, and face type. The ASME B16.5 bolt chart specifies the number of bolts, bolt diameter, and bolt length for each flange combination.
| Flange NPS | Class 150 (Bolt Dia x Qty) | Class 300 (Bolt Dia x Qty) | Class 600 (Bolt Dia x Qty) |
|---|---|---|---|
| 2” | 5/8” x 4 | 5/8” x 8 | 5/8” x 8 |
| 4” | 5/8” x 8 | 3/4” x 8 | 3/4” x 8 |
| 6” | 3/4” x 8 | 3/4” x 12 | 7/8” x 12 |
| 8” | 3/4” x 8 | 7/8” x 12 | 1” x 12 |
| 10” | 7/8” x 12 | 1” x 16 | 1-1/8” x 16 |
Full bolt charts are available in the ASME B16.5 bolting reference.
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