ASME B16.5 Flange Bolt Selection: Stud Count, Grade and Length by Pressure Class

ASME B16.5: The Global Standard for Pipe Flanges

ASME B16.5 "Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard" is the most widely adopted flange standard in the world, governing the oil & gas, petrochemical, power generation, and process industries. The standard defines flange dimensions, pressure-temperature ratings, facing types, and — critically for this guide — bolting requirements including stud count, bolt diameter, and minimum bolt length for each combination of pipe size (NPS) and pressure class.

Pressure Classes Defined

ASME B16.5 flanges are available in seven pressure classes: 150, 300, 600, 900, 1500, and 2500. The class number is NOT the working pressure; it's a pressure-temperature rating system where the maximum allowable working pressure (MAWP) decreases as temperature increases. For example, a Class 300 flange in carbon steel is rated for 750 psig (51.7 bar) at 38°C but only 405 psig (27.9 bar) at 400°C.

Stud Bolt vs Through Bolt

ASME B16.5 flanges typically use stud bolts — threaded rods with a nut on each end — rather than through bolts (hex head bolt + nut). Stud bolts provide several advantages:

• No head clearance required (critical in compact piping layouts)
• Equal accessibility from either side of the flange
• Nuts can be replaced without removing the bolt body
• Reduced risk of flange face damage during assembly/disassembly

Stud Count and Diameter by NPS and Class

NPS	Class 150	Class 300	Class 600	Class 900	Class 1500
2"	8 × 5/8"	8 × 5/8"	8 × 3/4"	8 × 7/8"	8 × 1"
3"	8 × 5/8"	8 × 3/4"	8 × 7/8"	8 × 1-1/8"	12 × 1-1/4"
4"	8 × 5/8"	8 × 3/4"	8 × 1"	8 × 1-1/4"	12 × 1-1/2"
6"	12 × 3/4"	12 × 7/8"	12 × 1-1/8"	12 × 1-1/2"	16 × 1-3/4"
8"	12 × 3/4"	12 × 1"	12 × 1-1/4"	16 × 1-1/2"	20 × 2"

Note: Stud count and diameter increase with both pipe size and pressure class. This table shows a representative sample — consult ASME B16.5 Table 6 for the complete specification.

Bolt Length Determination

ASME B16.5 Appendix M provides the formula for minimum stud length: L = 2T + 2N + 2G + A, where:

• T = flange thickness
• N = nut height
• G = gasket thickness
• A = thread allowance (typically 6mm / 0.25" for thermal expansion and tolerance stack)

In practice, engineers typically specify standard lengths from ASME B18.2.6 (inch series) or ISO 272 (metric series) that meet or exceed the calculated minimum. Common practice is 2-3 threads projecting beyond the nut face when the joint is fully assembled at ambient temperature.

Thread Engagement Requirements

Minimum thread engagement — the axial length of thread overlap between bolt and nut — must be at least 1.0D (one times the nominal diameter) for steel-on-steel connections per ASME PCC-1. For nuts on softer materials (brass, aluminum, bronze bushings), minimum engagement increases to 1.5D. Insufficient thread engagement causes thread stripping under preload before the bolt reaches its design tension.

Stud Projection Beyond Nut

ASME PCC-1 requires a minimum of 1–3 threads (typically 2 full threads) projecting beyond the nut face after final torquing at ambient temperature. This provides visual confirmation of proper assembly and accommodates thermal expansion during startup. Excessively long studs (>6 threads projection) create bending moment risk if equipment vibrates.

Raised Face vs Ring-Type Joint (RTJ) Flanges

The gasket type influences bolt length requirements:

• Raised Face (RF): Most common facing for Class 150-900. Uses spiral-wound or soft gaskets (1.5–4.5mm thick).
• Ring-Type Joint (RTJ): Required for Class 1500+ and high-pressure sour service. Uses metal ring gaskets (R or RX profiles, 6–12mm tall) sitting in machined grooves. RTJ flanges are thicker and require longer studs than equivalent RF flanges.

LOKRON's ASME B16.5 Stud Bolt Supply

LOKRON maintains a large inventory of cut-to-length ASME B16.5 stud bolts in ASTM A193 B7, B7M, B16, B8, and B8M, with matching A194 nuts. We supply complete stud bolt sets (studs + nuts + washers where required) for NPS 1/2" through NPS 24" in all pressure classes, with EN 10204 3.1 certification and PED compliance. Custom lengths and special alloys (Duplex, Inconel, Incoloy) available on request.