What Is a Flange Bolt?
A flange bolt is a headed fastener designed to secure two pipe flanges together and compress the gasket into a leak-tight seal. Flange bolts are manufactured as either heavy hex head bolts or stud bolts with two nuts; ASME B16.5 mandates the use of heavy hex series fasteners for pressure-rated flanged joints.
Heavy Hex vs Standard Hex Bolts
The key difference between heavy hex and standard hex bolts is the size of the head and nut across flats. Heavy hex dimensions per ASME B18.2.1 provide a larger bearing surface, which distributes the clamping load more evenly across the flange face and reduces the risk of flange face damage.
| Feature | Heavy Hex Bolt (ASME B18.2.1) | Standard Hex Bolt (ASME B18.2.1) |
|---|---|---|
| Head width across flats | Larger (e.g., 1-5/8” for 1” dia.) | Smaller (e.g., 1-1/2” for 1” dia.) |
| Head height | Taller | Shorter |
| Bearing area | ~15-20% greater | Baseline |
| ASME B16.5 compliance | Yes (required) | No (not permitted for flanges) |
| Thread series | UNC 8-thread (above 1” dia.) | UNC standard |
| Typical application | Flanged piping, pressure vessels | Structural, general purpose |
When to Use Each Type
Heavy hex bolts and stud bolts are the only acceptable fasteners for ASME B16.5 and B16.47 flanged joints. Standard hex bolts lack the bearing surface needed for controlled gasket compression and are not recognized by ASME pressure piping codes. The choice between a heavy hex bolt and a stud bolt depends on the application:
- Stud bolts (threaded rod + 2 nuts): Standard for most flanged joints; allow equal tightening from both sides
- Heavy hex bolts (headed + 1 nut): Used where access is limited to one side, or for non-critical, low-pressure services
ASTM Material Grades for Flange Bolts
| Bolt Grade | Specification | Matching Nut | Service |
|---|---|---|---|
| B7 | ASTM A193 | A194 2H | General high-temp (up to 450 degC) |
| B7M | ASTM A193 | A194 2HM | Sour service (NACE MR0175) |
| L7 | ASTM A320 | A194 4/7 | Low-temperature (down to -100 degC) |
| B8 Cl.1 | ASTM A193 | A194 8 | Corrosive environments (SS 304) |
| B8M Cl.1 | ASTM A193 | A194 8M | Marine/chemical (SS 316) |
Key Dimensional Standards
Flange bolt dimensions (head size, thread length, overall length) are governed by:
- ASME B18.2.1: Hex bolt head and nut dimensions
- ASME B16.5: Number of bolts, bolt diameter, and bolt circle for each flange NPS and class
- ASME B1.1: Unified thread form (UNC series)
The bolt length for a flanged joint depends on the flange type, gasket thickness, and number of washers. Refer to the stud bolt length calculation guide for detailed sizing.
Leave a Comment
Have a question or feedback? Send us a message.