What is ASME B16.5? – Standard for Pipe Flanges and Flanged Fittings

ASME B16.5 is one of the most widely used piping standards in the oil & gas, chemical, power, and process industries. It standardizes the design of pipe flanges and flanged fittings for sizes NPS ½ through NPS 24, covering pressure classes 150, 300, 400, 600, 900, 1500, and 2500. By defining pressure-temperature ratings, dimensions, materials, tolerances, and testing requirements, it ensures global compatibility between piping systems, gaskets, and bolting assemblies.

For engineers, the code provides a ready reference for selecting the correct flange type, size, and pressure class based on operating conditions. Using ASME B16.5 correctly minimizes the risk of leaks, ensures mechanical integrity, and facilitates interchangeability between manufacturers worldwide.

Introduction

ASME B16.5 provides engineers with a standardized framework for designing and procuring pipe flanges and flanged fittings. It ensures mechanical integrity, global compatibility, and safe operation across various industrial applications.

Scope of ASME B16.5

The scope defines what the standard covers and its limits:

Pipe Size Range: NPS ½″ to 24″ (larger flanges covered under ASME B16.47).
Flange Types: Weld Neck, Slip-On, Blind, Socket Weld, Threaded, Lap Joint, Ring-Type Joint.
Pressure Classes: 150 to 2500.
Materials: Carbon steel, stainless steel, alloy steel, nickel alloys, etc., as referenced in ASTM standards.
Applications: Process piping, refinery units, LNG facilities, offshore structures, boilers, and pressure vessels.

Engineering Tip: When specifying flanges, always check if the design pressure and temperature fall within the ranges given in ASME B16.5 Tables 1A/1B. If your size is larger than NPS 24, refer to ASME B16.47 instead.

Pressure-Temperature Ratings

ASME B16.5 provides tables listing the maximum allowable pressure at specific temperatures for each material group. Each flange is assigned a pressure class (150, 300, 600, etc.), which does not directly equal pressure (psi) but represents a range depending on material and temperature. As temperature increases, the allowable pressure rating decreases.

Engineer’s Use Case: Selecting a Class 300 flange in ASTM A105 carbon steel at 200°C? Check Table 2-1.1: the allowable pressure is 740 psi. Even if your line pressure is only 400 psi, ensure temperature derating is acceptable.

Materials and Specifications

ASME B16.5 lists permitted materials by referencing ASTM standards. Common examples:

Carbon Steel: ASTM A105, A350 LF2 (low-temp service)
Stainless Steel: ASTM A182 F304/F316
Alloy Steel: ASTM A182 F11, F22 (high temperature)
Nickel Alloys: ASTM B564 grades for corrosive/cryogenic service

Engineering Tip: Ensure selected material matches the service fluid, design temperature, and corrosion allowance. For sour service (H₂S), compliance with NACE MR0175 is usually required.

Flange Types and Applications

Flange Type	Description	Typical Use
Weld Neck (WN)	Long tapered hub, butt-welded	High pressure, high temperature critical service
Slip-On (SO)	Pipe slides into bore, fillet welded	Low-pressure water lines, utilities
Blind (BL)	Solid plate, no bore	Closing ends, hydrostatic testing
Socket Weld (SW)	Pipe inserted into socket, fillet welded	Small-bore, high-pressure lines
Threaded (THD)	Internal threads, no welding	Low-pressure, maintenance-friendly
Lap Joint (LJ)	Two-piece with stub end	Lines requiring frequent disassembly
Ring Type Joint (RTJ)	Machined groove for metal gasket	High-pressure, leak-critical services

Engineering Tip: Always match flange face (RF, FF, RTJ) with the correct gasket type. Using a flat gasket on an RTJ flange will cause leakage.

Dimensions and Tolerances

The code provides detailed dimensional tables (Tables 2–11) covering:

Outside diameter of flange
Bolt circle diameter and number of bolt holes
Flange thickness (T)
Hub dimensions (for weld neck)
Raised face height and diameter
Tolerances on machining

Engineer’s Use Case: For a 6-inch Class 150 Weld Neck flange: OD = 11.00″, Bolt Circle = 9.50″, Bolt Holes = 8 × 0.75″, Thickness = 1.12″. This ensures proper bolting and gasket seating across manufacturers.

Marking, Testing, and Quality Assurance

Markings: Flanges must be stamped with size, pressure class, material grade, heat number, and manufacturer’s name or trademark.
Hydrostatic Testing: Fittings are tested at 1.5 times design pressure.
Quality Control: Manufacturers must follow ASME and ASTM requirements for mechanical tests, chemical composition, and heat treatment.

Engineering Tip: During inspection, always verify flange markings match the material test certificate (MTC) before installation.

Integration with Other Codes

ASME B16.5 flanges are not standalone — they integrate with:

ASME B31.3 (Process Piping): For piping design, stresses, and flexibility.
ASME Section VIII Div. 1 (Pressure Vessels): For vessel nozzle connections.
API 6A / API 607: For oilfield and fire-safe applications.

Engineering Tip: If your project involves pressure vessels, always check compatibility between B16.5 flange rating and vessel design pressure as per ASME VIII.

Conclusion

ASME B16.5 provides engineers with a standardized, reliable framework for designing and procuring flanges and flanged fittings. By carefully selecting the correct pressure class, material, and flange type, engineers can ensure safe and leak-free operation across a wide range of industries. When integrated with ASME piping and pressure vessel codes, B16.5 becomes a critical reference point for mechanical integrity in oil & gas, power, and chemical plants.

For precise design and manufacturing, always consult the official ASME B16.5 standard. Using these reference tables and diagrams improves design speed and reduces errors while staying compliant with industry standards.