ASME B16.5 Flange Standard: Comprehensive Technical Guide

What Is ASME B16.5?

ASME B16.5 is the international standard that defines dimensions, tolerances, material requirements, and pressure-temperature ratings for pipe flanges and flanged fittings. Applicable to pipes ranging from NPS 1/2" to NPS 24", this standard serves as the fundamental reference for the design of petrochemical, energy, water treatment, and general industrial facilities.

Scope and Limits

Size Range

• Minimum: NPS 1/2" (DN 15)
• Maximum: NPS 24" (DN 600)
• For NPS 26" and above, ASME B16.47 applies

Pressure Classes

ASME B16.5 defines seven distinct pressure classes:

• Class 150 — Low-pressure applications
• Class 300 — Medium pressure
• Class 400 — Medium-high pressure
• Class 600 — High pressure
• Class 900 — Very high pressure
• Class 1500 — Extra high pressure
• Class 2500 — Ultra high pressure

For NPS and DN conversions, use our DN-NPS Conversion Tool.

Flange Types

Weld Neck Flange (WN)

• Butt-welded to the pipe
• Highest structural integrity
• Preferred for high pressure and temperature applications
• Allows radiographic testing

Slip-On Flange (SO)

• Slipped over the pipe and fillet welded
• Easy installation and lower cost
• Low-pressure applications between Class 150-300
• Approximately 30% more economical than WN

Blind Flange (BL)

• Used for pipe line termination or temporary closure
• Used in pressure testing
• For future expansion points

Threaded Flange (TH)

• Where welding is not possible
• Small-diameter lines (NPS 1/2"-2")
• Low pressure, low temperature

Socket Weld Flange (SW)

• Small-diameter high-pressure lines
• Ideal for NPS 1/2"-2"
• Pipe end seats into the flange socket

Lap Joint Flange (LJ)

• Used together with a stub end
• At points requiring frequent disassembly
• Economical solution in corrosive environments

Material Groups

ASME B16.5 classifies materials into groups for pressure-temperature rating:

Group 1 (Carbon Steel)

• A105: Most common forged carbon steel flange material
• A350 LF2: Low-temperature service (down to -46°C)
• Operating temperature range: -29°C to 427°C

Group 2 (Cr-Mo Alloy)

• A182 F11: 1-1/4Cr-1/2Mo, up to 538°C
• A182 F22: 2-1/4Cr-1Mo, up to 593°C
• High-temperature petrochemical service

Group 3 (Stainless Steel)

• A182 F304/304L: General purpose austenitic
• A182 F316/316L: Enhanced corrosion resistance
• Chemical, food, and pharmaceutical industries

Pressure-Temperature Rating

Critical point: The allowable working pressure of a flange varies with temperature.

Example: A105 - Class 150

Temperature	Allowable Pressure
-29°C → 38°C	19.6 bar
100°C	17.7 bar
200°C	15.3 bar
300°C	13.0 bar
400°C	10.2 bar

For detailed pressure-temperature calculations, visit our Pressure Class Calculator.

Gasket Face Types (Facing)

• RF (Raised Face): Most common, standard gasket surface
• FF (Flat Face): Mating with Class 150 cast iron
• RTJ (Ring Type Joint): High pressure, metal ring gasket

Determining Flange Dimensions

Parameters to be determined for correct flange selection:

1. Nominal pipe size (NPS)
2. Pressure class (Class)
3. Flange type (WN, SO, BL, etc.)
4. Material (A105, A182 F316, etc.)
5. Gasket face (RF, RTJ, etc.)

For flange dimension details, use our Flange Dimensions Tool.

Aktif Çelik ASME B16.5 Flange Supply

At Aktif Çelik, we supply forged steel flanges compliant with ASME B16.5 from our extensive stock range. We provide flanges in A105, A182 F304/F316, and A350 LF2 materials, in all pressure classes from Class 150 to Class 2500, in WN, SO, BL, and SW types. Contact our technical team for EN 1092-1 (DIN) flange equivalents and custom dimension requests.