ASME B16.5-2017 — Pipe Flanges and Flanged Fittings (NPS 1/2 Through NPS 24)

28 10 月, 2026
ASME Standards

📐 ASME B16.5-2017 is one of the most widely used standards in piping engineering. It covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings in sizes NPS 1/2 through NPS 24. Requirements are stated in both SI (Metric) and U.S. Customary units, making it the common design and procurement reference for flanged connections in the oil, chemical, power, and metallurgical industries, and a key component referenced by the ASME Boiler and Pressure Vessel Code (BPVC).

💡 In practice, B16.5 flanges are the global “interface language” of piping: any manufacturer’s flanges, bolts, and gaskets built to B16.5 are interchangeable and can form a reliable, leak-tight joint.

1. Scope and General Framework 📌

This Standard covers two product categories:

Flanges: rating classes 150, 300, 400, 600, 900, and 1500 in sizes NPS 1/2 through NPS 24, and class 2500 in sizes NPS 1/2 through NPS 12.
Flanged fittings: classes 150 and 300 in sizes NPS 1/2 through NPS 24; classes 400 through 1500 in sizes NPS 1/2 through NPS 24, and class 2500 in sizes NPS 1/2 through NPS 12 (U.S. Customary units only are provided in Nonmandatory Appendix E for the 2500 class).

The Standard limits itself to flanges and flanged fittings made from cast or forged materials; blind flanges and certain reducing flanges may be made from cast, forged, or plate materials. Requirements and recommendations for flange bolting, gaskets, and joints are also included.

⚠️ B16.5 has a limited scope: it applies only to cast or forged flanges and flanged fittings. It does not cover special welded flanges, hubbed flanges of other design, or non-standard connections — always verify the product is within the scope before specifying.

2. Material Groups and Selection 🔧

Materials are grouped in Table 1A based on identical or closely matched allowable stress and yield strength values. Where values are not identical for every material in a group, the lowest value is used, so a single pressure-temperature rating table can safely cover the whole group. Note that material groups are not numbered consecutively; numbers not appearing in this Standard may be found in the material tables of ASME B16.34.

Material Category	Typical Grades	Typical Class Range	Typical Applications
Carbon steel	A105 (forging), A216 WCB (casting)	Class 150–600	General oil, chemical, water services
Low-temperature carbon steel	A350 LF2	Class 150–600	Low-temperature and cold-climate piping
Austenitic stainless steel	F304, F316; CF8, CF8M castings	Class 150–1500	Corrosive media, food, pharmaceutical
Alloy steel	F11, F22 (Cr-Mo)	Class 150–1500	High-temperature/high-pressure steam, hydrotreating
Duplex stainless steel	F51, F53, etc.	Class 150–1500	Offshore, sour-gas and highly corrosive services

💡 Material selection key: B16.5 ratings are built on allowable stress. Always cross-check media corrosivity, design temperature, and design pressure, and consider additional material restrictions in ASME B31.3 (Process Piping).

3. Pressure-Temperature Ratings 🔥

Pressure-temperature ratings are the core of B16.5. For Class 300 and higher, the rated working pressure is established by the equation:

p_t = C₁ × S₁ × P_r / 8750

where p_t is the rated working pressure of the specified material at temperature T; P_r is the pressure rating class index (equal to the class designation for Class 300 and above); S₁ is the selected allowable stress of the material at temperature T; and C₁ is a unit conversion constant (C₁ = 10 with MPa/bar units, C₁ = 1 with psi units). Ratings for Class 150 are established separately per para. A-2.4 of the Nonmandatory Appendix.

⚠️ As temperature rises, allowable stress falls, so the permitted working pressure drops. Never size a flange on its ambient rating alone — always read the rating at the design temperature or you risk overpressure.

Rating Class	Approx. Ambient Rating	Common Flange Types	Typical Applications
Class 150	~285 psi (1.96 MPa)	Plate, slip-on, threaded, socket-weld	Low-pressure water, fire protection, general process
Class 300	~740 psi (5.1 MPa)	Weld-neck, slip-on	Medium-pressure chemical and refinery
Class 600	~1480 psi (10.2 MPa)	Weld-neck primarily	High-pressure steam and process lines
Class 900	~2220 psi (15.3 MPa)	Weld-neck	High-pressure hydrotreating, very high-pressure steam
Class 1500	~3600 psi (24.8 MPa)	Weld-neck	High-pressure oil/gas, high-pressure chemical
Class 2500	~6000 psi (41.4 MPa)	Weld-neck (heavy wall)	Ultra-high-pressure services, nuclear auxiliary systems

4. Flange Facing Types and Sealing 🔩

B16.5 defines several flange facing types; the facing choice determines gasket type and sealing reliability:

Raised Face (RF): the most common facing, used with flat or spiral-wound gaskets.
Flat Face (FF): the full flange face is the sealing surface; typical for cast-iron and low-pressure flanges.
Ring Joint (RTJ): for high-pressure service with metallic octagonal ring gaskets — reliable and high-temperature capable.
Large/Small Male-Female: aids centering; suited to medium and high pressure.
Tongue-and-Groove: gasket is confined; used for toxic media and strict sealing duties.

🚨 Facing surface finish directly determines seal quality. The Standard limits surface roughness and permissible imperfections; inspect faces for scratches and pits before installation and never grind RTJ grooves with abrasive paper.

5. Dimensions, Tolerances, and Marking 📏

The Standard gives outside diameter, thickness, bolt-hole diameter and bolt-circle diameter, hub dimensions, and tolerances for each class and size. As a special convention, bolt diameters and flange bolt-hole diameters are expressed in inch units only, even in the metric version.

💡 Units convention: B16.5 states values in both SI and U.S. Customary units, but the two systems are not exact equivalents. Specify one system in your documents and purchase orders — mixing values from the two systems constitutes nonconformance.

For marking, the Standard requires the manufacturer’s name or trademark, material identification, and pressure class on each flange, with additional marks for blind flanges, to ensure traceability.

6. Engineering Practice Essentials 🛠️

🔍 Selection workflow: set design temperature and pressure → read the rating table for the material group → determine class → pick nominal size from pipe size → choose flange type and facing → match bolts and gasket.
🔧 Bolting and gasket matching: preload must compress the gasket enough; in high-temperature service account for bolt creep relaxation — consider alloy bolting (e.g., B7, B16) where needed.
📊 Hydrostatic testing: system tests must not exceed the flange rating at the test temperature.
📋 Quality systems: Nonmandatory Appendix D describes manufacturer quality-system program requirements; request conformance statements and material certificates from suppliers.

7. Frequently Asked Questions ❓

❓ Q: What is the difference between B16.5 and B16.47?
A: B16.5 covers NPS 1/2 through NPS 24, while B16.47 covers large-diameter flanges NPS 26 through NPS 60. Choose the standard by nominal size.

❓ Q: The Class 150 ambient rating is about 285 psi — does that mean it cannot exceed this pressure?
A: Ratings vary with temperature. The flange is acceptable as long as the rating at design temperature is not less than the design pressure; additionally check combined stresses from bending moment and axial load.

❓ Q: Can B16.5 flanges be connected to flanges from other standards?
A: Same-class, same-size B16.5 flanges share standardized bolt-circle dimensions and can generally interconnect, but facing type, gasket matching, and material allowable stresses must be verified one by one to avoid leaks.

🔍 Q: Why are bolt-hole diameters given in inch units only?
A: This is an explicit convention of the Standard (para. 1.6) that keeps bolt sizes globally interchangeable, since inch-dimensioned bolts circulate most widely worldwide.

ASME B16.5-2017.pdf