Comprehensive specifications for ANSI B16.1 cast iron flanges and ANSI-adopted steel flange standards
The American National Standards Institute (ANSI) is a private, non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. ANSI does not develop standards itself; rather, it accredits and coordinates standards development by industry groups, professional societies, and trade associations.
For flange standards, ANSI has adopted specifications developed by the American Society of Mechanical Engineers (ASME) and other technical organizations. This means ANSI B16.5 and ASME B16.5 refer to the same technical standard, and ANSI B16.1 is the standard for cast iron pipe flanges.
ASME develops the technical specifications for flange standards based on industry experience and engineering requirements. ANSI then reviews and accredits these standards as American National Standards. The result is that ANSI-designated standards (like ANSI B16.1 and ANSI B16.5) carry both designations interchangeably in modern usage.
ANSI B16.1 specifies cast iron pipe flanges and flanged fittings for use with ferrous piping. This is one of the oldest flange standards still in use, with origins dating back over a century. It defines dimensions, materials, and pressure ratings for cast iron flanges in three primary classes: 25, 125, and 250 PSI.
Cast iron flanges are used primarily in low-to-medium pressure applications and offer excellent value for gravity-fed systems, water distribution, and legacy installations. While steel has largely replaced cast iron in new construction, ANSI B16.1 flanges remain essential for system maintenance and compatibility with existing infrastructure.
Pressure Rating
25 PSI
Material
Cast Iron
Temperature Limit
Up to 300°F
Typical Applications
Water systems, low-pressure gas, gravity feed systems
Pressure Rating
125 PSI
Material
Cast Iron
Temperature Limit
Up to 366°F saturated steam
Typical Applications
Water supply, steam (saturated), compressed air, general industrial
Pressure Rating
250 PSI
Material
Cast Iron
Temperature Limit
Up to 406°F saturated steam
Typical Applications
High-pressure water systems, steam systems, industrial compressed gas
ANSI B16.5 is identical to ASME B16.5 and specifies steel pipe flanges, flanged fittings, and valves. It supersedes ANSI B16.1 for all modern industrial applications due to superior strength, reliability, and pressure handling capabilities.
For a complete deep dive into ANSI B16.5 / ASME B16.5 specifications, see our ASME Flanges Deep Dive.
Proper dimensioning is critical for flange selection and installation. Understanding key flange dimensions ensures correct bolt selection, bolt spacing, and sealing integrity. All ANSI flanges must comply with standardized dimension tables.
Complete dimension tables for all ANSI flange sizes and classes are available in our Resources section. These tables provide exact measurements for procurement, engineering, and installation verification.
View Dimension TablesBolt pattern information is essential for procurement of bolts, studs, and nuts. The table below shows common bolt patterns for popular ANSI B16.5 flange sizes. Always verify exact dimensions from the flange standard or manufacturers' data sheets.
| Size | Class | Bolt Circle (inches) | No. of Bolts | Bolt Hole Dia. | Flange Thickness |
|---|---|---|---|---|---|
| 4" | 150 | 5-1/8" | 4 | 7/8" | 1-3/16" |
| 4" | 300 | 5-3/4" | 8 | 7/8" | 1-1/2" |
| 6" | 150 | 7-1/2" | 4 | 1" | 1-7/16" |
| 6" | 300 | 8-1/2" | 8 | 1" | 1-3/4" |
| 8" | 150 | 9-5/8" | 4 | 1-1/8" | 1-5/8" |
| 8" | 300 | 11" | 8 | 1-1/8" | 1-15/16" |
| 10" | 150 | 11-3/4" | 4 | 1-1/4" | 1-13/16" |
| 10" | 300 | 13-1/2" | 8 | 1-1/4" | 2-3/16" |
| 12" | 150 | 14" | 4 | 1-3/8" | 1-15/16" |
| 12" | 300 | 16" | 8 | 1-3/8" | 2-5/16" |
Understanding the differences between ANSI, ASME, and API standards is crucial for selecting the correct flange for your application. Each standard was developed with specific industries and use cases in mind.
| Aspect | ANSI | ASME | API |
|---|---|---|---|
| Scope | North American industrial standards | North American industrial & power generation | Oil and gas industry focus |
| Coverage | All industries (cast iron, steel) | Primarily steel and alloy | Large diameter, high-pressure |
| Pressure Ratings | Classes 25, 125, 250 PSI | Classes 150, 300, 600, 900, 1500, 2500 PSI | Classes 150, 300, 600, 900, 1500 PSI |
| Size Range | NPS 1/2" to 24" | NPS 1/2" to 24" (B16.5) / 26-60" (B16.47) | Up to 120" OD |
| Primary Material | Cast Iron preferred | Steel, Stainless, Alloy | Carbon Steel |
| When to Use | Legacy systems, low-pressure, water | Modern industrial, chemical, power | Oil & gas, extreme pressure/temp |
ANSI designations cover general industrial applications across all industries. ANSI B16.1 is preferred for legacy systems and cast iron applications where pressure requirements are low.
Best for: General purpose, legacy systems, water applications, low pressure
ASME standards represent the current state-of-the-art for industrial flanges. ASME B16.5 and B16.47 are the preferred specifications for modern engineering projects across all industries.
Best for: Modern industrial, power generation, chemical, thermal systems
API standards are specifically developed for the oil and gas industry. API 605 addresses large-diameter flanges used in pipeline and offshore applications with extreme conditions.
Best for: Oil & gas, crude oil pipelines, extreme pressure/temperature
Technically, ANSI B16.5 refers to the American National Standard version of ASME B16.5. ASME B16.5 is the technical standard published by the American Society of Mechanical Engineers, and it has been adopted as an ANSI standard. For practical purposes, they are identical specifications and can be used interchangeably. Most specifications now reference ASME B16.5 as the primary standard.
ANSI B16.1 cast iron flanges are available from NPS 1/2" through 24". However, most applications use sizes from 2" to 12". Larger sizes (18", 24") exist but are less common due to the weight and brittleness of cast iron at those sizes. For sizes above 24", ASME B16.5 steel flanges are the standard choice.
ANSI flanges follow North American (imperial) dimensions and are primarily used in North America, parts of South America, and some Asian countries. European systems use DIN EN 1092-1 (metric), which is not directly interchangeable. However, ANSI-to-metric flange adapters are available for projects requiring integration between systems.
In modern engineering specifications, reference ASME B16.5 or ASME B16.47 rather than ANSI B16.1 for most applications. ASME standards are more current and widely recognized. For cast iron applications, specify ASME B16.1 when cast iron is required. Always include: nominal size, pressure class, flange type (slip-on, welding neck, socket weld, threaded, or blind), material grade, and any special requirements (coating, drilling, facing).
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