What is the ASME standard for flanges?

In the world of piping and pressure vessel systems, ASME flange standards play a critical role in ensuring safety, reliability, and global compatibility. Whether used in oil & gas, chemical processing, power generation, or water treatment, flanges serve as essential components for connecting pipes, valves, pumps, and other equipment.

So, what is the ASME standard for flanges? This article will explore the key ASME standards related to flanges, including their classifications, dimensions, pressure ratings, and applications—helping you make informed decisions when selecting the right flange for your system.

Overview of ASME Flange Standards

The American Society of Mechanical Engineers (ASME) has developed several key flange standards under the B16 series. These include general-purpose flange standards, large-diameter flange specifications, and specialized standards for flow measurement and material types.

Here are the most widely used ASME flange standards:

Standard	Application	Size Range (NPS)	Pressure Class	Typical Materials
ASME B16.5	General pipe flanges	1/2″ to 24″	150 to 2500	Carbon steel, stainless steel, alloy steel
ASME B16.47	Large diameter flanges	26″ to 60″	75 to 900	Carbon steel, alloy steel
ASME B16.36	Orifice flanges (flow measurement)	1/2″ to 24″	300 to 2500	Forged steel
ASME B16.1	Cast iron flanges	1/2″ to 48″	25, 125, 250	Grey cast iron
ASME B16.42	Ductile iron flanges	1/2″ to 48″	150, 300	Ductile iron

ASME B16.5: The Core Standard for Pipe Flanges

ASME B16.5 is the most commonly referenced flange standard worldwide. It defines:

Flange types: Weld Neck (WN), Slip-On (SO), Socket Weld (SW), Threaded (TH), Blind (BL), and Lap Joint (LJ)

Flange sizes: From NPS 1/2 to NPS 24 (DN 15–600)

Pressure ratings: Class 150, 300, 400, 600, 900, 1500, and 2500

Sealing face types: Raised Face (RF), Ring Type Joint (RTJ), Flat Face (FF), Tongue & Groove (T/G), Male & Female (M/F)

Materials: ASTM A105 (carbon steel), A182 (stainless and alloy steel), A216 (cast steel), and others

This standard covers over 70% of global industrial flange applications, making it the default specification for most pipeline connections.

ASME B16.47: For Large-Diameter Applications

When your pipe size exceeds 24 inches, ASME B16.47 comes into play. It defines:

Flanges from NPS 26 to NPS 60 (DN 650–1500)

Pressure classes: 75, 150, 300, 400, 600, 900

Two series:

Series A (MSS SP-44): Thicker, more robust, higher bolt count—used for critical, high-load applications

Series B (API 605): Lighter, fewer bolts, more economical—used in moderate-pressure systems

It is commonly used in long-distance pipelines, refineries, and power plants.

ASME B16.36: For Orifice Flange Installations

ASME B16.36 is a specialized standard for orifice flanges, which integrate pressure tap holes to measure flow using orifice plates and differential pressure sensors.

Sizes: NPS 1/2 to 24

Pressure classes: Class 300 to 2500

Always used in pairs, with a central orifice plate and gaskets

Sealing face: Raised Face only (no RTJ or FF)

Compatible with ASME B16.5 for bolt dimensions and outer diameter

Used extensively in oil, gas, and petrochemical industries for flow monitoring and process control.

ASME B16.1 & B16.42: Cast Iron and Ductile Iron Flanges

For non-critical, low-to-medium pressure systems:

ASME B16.1 covers gray cast iron flanges, available in Class 25, 125, and 250. These are typically used in waterworks and HVAC but are not suitable for welding or high-impact environments.

ASME B16.42 covers ductile iron flanges, offering better strength, corrosion resistance, and working pressure than gray iron. Classes include 150 and 300.

Related ASME Flange Standards

ASME B16.20: Gaskets for metallic flanges, including RTJ rings and spiral wound gaskets

ASME B16.21: Non-metallic gasket specifications (e.g., PTFE, rubber, compressed fiber)

ASME PCC-1: Guidelines for proper flange assembly, bolt tightening, and leak prevention

How to Choose the Right ASME Flange Standard?

When selecting the correct ASME flange standard, consider the following:

Pipe Size: Use B16.5 (≤ 24″) or B16.47 (> 24″)

Pressure Class: Based on working pressure and temperature

Application:

• Flow measurement: B16.36
• General pipeline: B16.5 or B16.47
• Low-pressure water: B16.1
• Mid-pressure gas or water: B16.42

Material Compatibility: Choose from carbon steel, stainless steel, ductile iron, or alloys as per fluid medium

Gasket Type and Sealing Face: RF, RTJ, FF, etc., based on sealing requirements and gasket compatibility

The ASME standards for flanges form a comprehensive and globally recognized system for specifying safe, efficient, and compatible flange connections across industries. From ASME B16.5 for general applications to B16.47 for large-diameter flanges and B16.36 for flow measurement, each standard serves a specific purpose to meet diverse engineering needs.

Understanding these standards is essential for anyone involved in piping design, procurement, or maintenance. For mission-critical applications, always refer to the latest ASME codes and consult certified suppliers or engineers.

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