Flange Types and Sealing Methods

In modern industrial piping systems, pipe flanges play a crucial role as indispensable connecting components. Whether in petrochemical, power, water supply and drainage, or food and pharmaceutical industries, flanges are ubiquitous wherever pipeline transportation is involved. This article will detail the structure, types, sealing methods, and marking methods of flanges, providing authoritative reference for engineering design and procurement.

 

Flange Connection Structure

A flange is typically disc-shaped with bolt holes on its outer circumference and a central opening for the pipe or equipment to pass through.

 

During installation, two flanges are placed opposite each other, a gasket is placed between them, and bolts are passed through the bolt holes and tightened, thus tightly connecting the pipe or equipment and ensuring that the medium inside the pipeline does not leak. It also provides a certain degree of support and positioning for the piping system.

 

The choice of different materials depends on factors such as the characteristics of the medium inside the pipeline, the working pressure, and the temperature.

 

For example, stainless steel flanges are often used in chemical pipelines transporting highly corrosive media; while carbon steel flanges are sufficient for ordinary water pipeline systems.

 

Types of Flanges

Common pipe flanges include slip-on flanges, weld neck flanges, socket flanges, loose flanges, and threaded flanges.

 

1. Slip-on Flanges

Slip-on flanges are divided into plate slip-on flanges and neck slip-on flanges. They have a simple structure and low cost, and are suitable for pipe connections with relatively low pressure and small diameter.

 

They are widely used in general civil building water supply and drainage and low-pressure industrial pipelines.

 

2. Socket Flanges

Socket flanges are suitable for small-diameter pipes. The pipe is inserted into the socket of the flange and welded, resulting in a strong connection and good sealing performance.

 

3. Weld Neck Flanges

Weld neck flanges are connected to the pipe by butt welding. The weld quality is high, and they can withstand higher pressure and temperature.

 

They are commonly used in high-temperature and high-pressure pipeline systems in industries such as petrochemicals and power.

 

4. Loose Flanges

Loose flanges are characterized by not being directly welded to the pipe, but rather connected via flanges, steel rings, or other connecting parts.

 

They are suitable for non-ferrous metal pipelines or applications requiring frequent disassembly, reducing damage to the pipe base material.

 

5. Threaded Flanges

Threaded flanges connect to pipes using threads, making installation and disassembly convenient.

 

They are generally used in piping systems with low pressure, small pipe diameter, and frequent maintenance requirements.

 

Flange Sealing Surface Types

Flange sealing surfaces are classified as flat face (FF), raised face (RF), male and female face (MFM), tongue and groove (TG), and ring joint (RJ).

 

1. Flat Face (FF)

The sealing surface is a smooth plane, suitable for low pressure and non-toxic media, generally used for water pipelines and other connections with low requirements.

 

2. Raised Face (RF)

Also known as a flat face flange, the sealing surface is flat, resulting in a large contact area with the gasket. After pre-tightening, the gasket can easily extend or move to the sides, resulting in poor sealing performance. It is only suitable for low-pressure applications.

 

3. Male and Female Face (MFM)

Consists of a concave face and a convex face. They must be used in conjunction with a convex face during installation.

 

This type of flange offers better sealing performance than a flat face flange and is commonly used for medium-pressure pipeline connections carrying toxic or hazardous media.

 

4. Tongue and Groove Seal (TG)

The sealing surface consists of a tongue and a groove, providing excellent sealing performance.

 

It is generally used for connections of flammable, explosive, toxic media, and high-pressure pipelines.

 

5. Ring Joint Seal (RJ)

A ring joint seal flange has an annular trapezoidal groove cut into the flange's raised face as the sealing surface.

 

Like the tongue and groove seal, the flange must be axially separated during installation and disassembly. The possibility of axial separation of the flange must be considered in pipeline design.

 

This sealing surface is specifically designed to mate with solid metal gaskets machined into octagonal or elliptical shapes to achieve a sealed connection. It offers good sealing performance, requires strict installation, and is suitable for high-temperature and high-pressure conditions, but demands high precision in sealing machining.

 

Flange Marking

The outer ring or near the sealing surface of the flange usually displays the following key information:

1. Nominal Diameter (DN)

This indicates the diameter of the pipe to which the flange will connect, usually in millimeters (mm). For example, "DN100" indicates that the flange is suitable for pipes with a nominal diameter of 100 mm.

 

2. Nominal Pressure (PN)

This indicates the design pressure the flange can withstand, commonly measured in megapascals (MPa) or pounds per square inch (psi). For example, "PN16" indicates a nominal pressure of 16 bar.

American Standard flanges use the CLASS designation, such as "CLASS 150," indicating a pressure rating of 150 pounds.

 

3. Material Designation

The flange material determines its strength, corrosion resistance, and applicable environment. Common material designations include:

Carbon steel: such as "A105," indicating forged carbon steel.

Stainless steel: such as "304" or "316," representing different types of stainless steel.

Alloy steel: such as "F22," indicating low-alloy steel.

 

4. Standard Designation

Flanges must be designed and manufactured to conform to specific standards, such as:

ANSI/ASME B16.5: American Standard flange.

EN 1092-1: European Standard flange.

GB/T 9119: Chinese National Standard flange.

 

5. Heat Treatment Condition

Some flanges will indicate the heat treatment process, such as "N" (normalizing) or "QT" (quenching and tempering).

 

Summary

Selecting the appropriate flange type and sealing method is a crucial step in ensuring safety, durability, and economy in piping system design.

Low-pressure, non-hazardous media → Slip-on or threaded flanges, flat or raised face seals;

High-pressure, high-temperature, or hazardous media → Butt-welded, tongue and groove, or ring face flanges, stainless steel or alloy steel.

 

Scientific and reasonable flange selection can significantly reduce pipeline leakage risks and maintenance costs, and improve project reliability.