Pipe Flange Size Specifications & Classification: Selection Guide

This guide provides a comprehensive overview of pipe flange size specifications, types, and classification systems. Learn how to select the right flange based on standards (ASME, EN, GB), pressure ratings (Class/PN), and material for your project. 

△ For detailed ASME B16.5 and B16.47 dimension charts with complete size tables, please use our interactive tool: 

▶ [View Full Flange Dimensions Chart]

How to Choose the Appropriate Pipe Flange Sizes & Dimensions?

Selecting the correct steel flange size is critical for system safety and reliability. Follow these five steps to make the right choice for your project.

1. Select According to Pipe Diameter

The size of the flange must match the diameter of the pipe to ensure a tight, leak-free connection. For example, a 1-inch pipe requires a 1-inch flange. Only in special circumstances are flanges of different specifications required.

2. Select According to Fluid Characteristics
Different fluids have different pressure, viscosity, density, and corrosivity. Consider the following:

● Standard fluids (water, air, low-pressure steam): Standard size flanges are typically sufficient.
● High temperature / High pressure fluids: Standard or large-diameter flanges with appropriate material grades should be selected.
● Corrosive fluids: Material selection (e.g., stainless steel 316) becomes the primary factor.

3. Select According to Working Pressure
The flange's pressure rating (Class for ASME, PN for EN/GB) must match or exceed the maximum working pressure of the system. Selecting a flange with an insufficient pressure rating can lead to leakage or catastrophic failure.

4. Select According to Device Type and Installation Environment

Consider special requirements of the equipment (e.g., mechanical connection, hydraulic pressure) and site conditions that may limit flange size or type, such as space constraints or external loads.

Pipe Flange Size Specifications:

When selecting the size of a pipe flange, several factors need to be considered, including the size, type, and applicable standards and specifications of the flange. ‌

According to the nominal unit, the main specifications of the flange can be divided into DN10, DN15, DN20, DN25, etc., or they can also be divided into 3/8, 1/2, 3/4, 1, etc. according to the inch unit. As the specification grade increases, the corresponding hole spacing between bolts, the outer diameter of the flange ring, the diameter of the bolts, the number of bolt holes, and the thickness of the flange will also increase. For example, the selected specifications range from DN10 to DN65, the diameter of the bolt is mostly 14 or 18 mm, the number of bolt holes is 4, and the thickness is 14 mm, 16 mm, 18 mm, 20 mm, etc. The hole spacing of the bolts and the outer diameter of the flange will also increase.

Pipe Flange Classification Overview

Flanges can be classified in several ways. The most common classifications are by welding method, material, and standard. Below is a general reference:

1. By Welding Method & Type (Most Common):
● Flat Welding Flange (SO)
● Neck Flange
● Butt Welding Flange (WN)
● Socket Welding Flange (SW)
● Blind Flange (BL)
● Threaded Flange (THD)
● Lap Joint Flange (LJ)

2. By Material:
Carbon Steel Flanges: ASTM A105, economical for general service.
Stainless Steel Flanges: ASTM A182 F304/F316, for corrosive or high-temperature environments (316 grade offers superior corrosion resistance and high-temperature strength up to 1200-1300°C due to its Mo content).
Alloy Steel Flanges: For extreme pressure and temperature conditions.

3. By Standard:

According to petrochemical industry standards: threaded flange (PT), butt welding flange (WN), flat welding flange (SO), socket welding flange (SW), loose flange (LJ), flange build.
According to the mechanical industry standards: integral flange, butt welding flange, plate flat welding flange, butt welding ring plate loose flange, flat welding ring plate loose flange, flanged ring plate loose flange, flange cover.

Standards and Specifications for Pipe Flanges

When selecting a flange size, you must also consider applicable standards. The most common international standards include:

ANSI/ASME B16.5: Pipe Flanges and Flanged Fittings (NPS 1/2" to 24", Classes 150 to 2500) – widely used in US and international projects.
ASME B16.47: Large Diameter Flanges (NPS 26" to 60") – Series A & B.
EN 1092-1: Flanges and their joints – Circular flanges for pipes, valves, fittings and accessories, PN designated (European market).
GB/T 9119 & HG/T 20592: Chinese National Standards (compatible with both Class and PN systems).
JIS B 2220: Japanese Industrial Standard.

Choose the appropriate standard according to your project's geographic location and engineering specifications. Always ensure mating flanges follow the same standard.

Pipe Flange Size Classification by Dimensions

Flanges are categorized by their physical dimensions, primarily the connection port diameter (Nominal Pipe Size, NPS/DN) and the flange thickness. Generally, flanges with a larger connection port diameter also have a greater thickness.

While the exact dimensions are determined by the specific pressure rating (e.g., Class 150, 300) and standard, flanges are commonly referred to in the following size ranges:

● Small Size Flange: Typically used for smaller bore pipelines. The connection port diameter is generally less than 40mm (approx. NPS 1 1/2"), with a relatively thin flange thickness.
● Standard Size Flange: The most common range for many industrial applications. The connection port diameter is usually between 40mm and 400mm (approx. NPS 1 1/2" to 16"). Flange thickness is moderate, varying with the pressure rating.
● Large Diameter Flange: Used for high-flow or large-bore pipelines. The connection port diameter is typically between 400mm and 1000mm (approx. NPS 16" to 40"), with a correspondingly larger flange thickness.
● Extra Large Flange: Used for specialized heavy industrial pipe and equipment connections. The connection port diameter exceeds 1000mm (over NPS 40"), and the flange thickness is very substantial.

Flange Connection and Gaskets:

Flange connection is to fix two pipes, pipe fittings or equipment on a flange plate first, add flange pads between the two flange plates, and fasten them together with bolts to complete the connection. Some pipe fittings and equipment already come with flanges, which are also flange connections. Flange connection is an important connection method in pipeline construction. Flange connections are easy to use and can withstand greater pressure. In industrial piping, as well as in homes, the pipe diameter is small and the pressure is low, with no visible flange connections. If you are in a boiler room or production site, there are flange-connected pipes and equipment everywhere.

Flange gasket is a ring made of material that can produce plastic deformation and has a certain strength.

Most gaskets are manufactured from a variety of materials, broadly classified as non-metallic, metallic, and composite types. Non-metallic gaskets are often cut from sheets of materials like PTFE, synthetic rubber, or flexible graphite. Metallic gaskets are made from materials such as stainless steel, copper, or aluminum. Composite gaskets, such as spiral-wound or metal-jacketed gaskets, combine metallic and non-metallic elements to offer enhanced sealing performance under demanding conditions. The choice of gasket material and type depends on the specific application's temperature, pressure, and media.

Ordinary rubber gaskets are suitable for occasions where the temperature is lower than 120°C; asbestos rubber gaskets are suitable for situations where the water vapor temperature is lower than 450°C, the oil temperature is lower than 350°C, and the pressure is lower than 5MPa. For general corrosiveness Medium, the most commonly used is acid-resistant asbestos board.

In high-pressure equipment and pipelines, lens-shaped or other-shaped metal gaskets made of copper, aluminum, No. 10 steel, and stainless steel are used. The contact width between the high-pressure gasket and the sealing surface is very narrow (line contact), and the processing smoothness of the sealing surface and gasket is high.

Low-pressure small-diameter wired flanges, high-pressure and low-pressure large-diameter welded flanges are used. The thickness of the flange plate and the diameter and number of connecting bolts are different for different pressures. Different types of flanges serve distinct purposes, from water heating pipelines to critical oil and gas transportation systems. Careful flange selection is essential to ensure safe and reliable operation, especially for demanding applications.

For detailed guidance on selecting the correct gasket, please refer to our comprehensive gasket selection guide.

Conclusion:

The size specifications and classification of flanges are critical for ensuring safe and reliable pipeline systems. When selecting a flange size, always consider the pipe diameter, fluid characteristics, working pressure, device type, installation environment, and applicable standards to ensure the chosen flange meets specific engineering needs and safety standards.

For a complete listing of ASME B16.5 and B16.47 flange dimensions (including all sizes, thicknesses, bolt hole patterns, and weights), please use our detailed chart tool:
➡️ [View Full Flange Dimensions Chart Tool]

Read more: 

● Pipe Flange Production Process 

● Stainless steel pipe fittings vs. Carbon steel pipe fittings

● Flange Sealing Surface Types and Their Gasket Choices