Everything You Need to Know About Flanges for Pipes

FLANGE FOR PIPES/PIPING

WHAT IS A FLANGE?

A flange is a mechanical device to connect pipes, valves, pumps, and other equipment to form a piping system. It consists of a plate or ring that provides a mounting surface for attaching two sections of pipe or connecting a pipe to a valve, pump, or other device. Flanges are designed to be bolted together with a gasket between them to provide a tight, leak-proof seal.

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Flanges are widely used in various industries, including oil and gas, chemical processing, power generation, and water treatment, due to their ease of assembly and disassembly, which facilitates inspection, cleaning, and modifications to the piping system.

Flanges can be produced by casting or forging steel. In the oil and gas industry, forged steel flanges are preferred due to their intrinsic strength and durability.

FLANGE KEY PARAMETERS

When selecting (and ordering to suppliers) flanges for piping systems, several key measures and specifications must be considered to ensure compatibility with the system, proper fit, and reliable operation under the intended service conditions. These measures include diameter, pressure class, flange facing type, and material grade.

Here&#;s a breakdown of these critical flange specifications:

1. Flange Type and Specification

The first parameter to consider when ordering a flange is the shape/design: welding neck, slip-on, threaded, lap joint flange. The 13 types of standard/non-standard flanges are thoroughly described in this article.

Together with the type, the buyer shall indicate also the applicable specification, i.e. ASME B16.5, ASME B16.47, type A, EN -1, JIS, UNI, DIN, etc.

2. Nominal Pipe Size (NPS) and Diameter

• Nominal Pipe Size (NPS): Refers to the standardized size designation for pipes and the corresponding flanges. It&#;s not the actual dimension of the pipe or flange but a nominal specification that indicates compatibility.
• Outside Diameter (OD): The external diameter of the flange, which is larger than the OD of the pipe to accommodate bolting around the perimeter.
• Bore Diameter: The inner diameter of the flange that matches the inner diameter of the pipe, ensuring a smooth flow of the medium.

Note: for welding neck and socket-weld flanges, the pipe schedule shall also be communicated to the manufacturer/supplier. This is a piece of information that is typically missing in requests for proposals, tenders, or purchase orders but it&#;s necessary.

Refer to this article for more information about NPS.

3. Pressure Class (i.e. Flange Rating)

• Flanges are classified according to their maximum allowable pressure at a specific temperature. The pressure-temperature ratings are defined by ASME B16.5 for sizes up to 24 inches. Common classes include:
  • Class 150
  • Class 300
  • Class 600
  • Class 900
  • Class
  • Class
• The class designation determines the thickness of the flange and the size and number of bolting holes required to withstand the pressure.

4. Flange Facing Type & Flange Surface Finish

The flange face is the surface area that contacts the gasket. The type of flange facing affects the seal quality and is selected based on the fluid, pressure, and operational conditions. Common types include:

• Flat Face (FF): A flat surface that mates with another flat-faced flange, typically used for low-pressure applications.
• Raised Face (RF): Features a raised surface around the bore, which concentrates the gasket compression, making it suitable for a wide range of pressures.
• Ring Type Joint (RTJ): Designed for high-pressure/temperature applications, it uses a metal ring as a gasket seated in a grooved face.

According to the ASME B16.5 specification, the raised face is the standard facing for flanges (different facings, like RTJ or flat face FF, have to be ordered specifically).

The flange surface finish can be smooth, stock, concentric serrated, etc.

5. Material Grade

The material of the flange must be compatible with the piping system and the medium it will carry. Material choice impacts the flange&#;s strength, corrosion resistance, and suitability for specific temperatures and pressures. Common materials include:

• Carbon Steel: Widely used for its strength and versatility. (ASTM A105 is a common specification)
• Stainless Steel: Offers corrosion resistance for aggressive media. (e.g., ASTM A182 F304/304L, F316/316L)
• Alloy Steel: Used for special applications requiring high strength or resistance to heat and corrosion. (e.g., ASTM A182 F11, F22)

6. Cathodic Protection

Cathodic protection is a method used to prevent corrosion in metal structures by making them the cathode of an electrochemical cell. In the context of piping systems, flanges are critical points where corrosion can lead to leaks or failure, potentially causing safety hazards and operational issues. To mitigate this risk, especially in environments prone to causing corrosion, flange isolation kits are employed as part of a cathodic protection strategy. Learn more about the use of flange isolation kits to prevent corrosion and provide cathodic protection.

7. Other Technical Considerations

• Hub Design: Some flanges, like weld neck and slip-on, have different designs regarding their connection to the pipe (e.g., a long tapered hub for weld neck flanges).
• Gasket Surface: The surface finish of the flange face affects the seal quality and compatibility with different gasket materials.

Selecting the right flange involves a detailed understanding of these key measures and how they interact with the system&#;s design requirements, ensuring a secure, leak-proof connection that can withstand operational demands.

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WHAT IS A FLANGED JOINT?

A flanged joint is a type of connection used in piping systems, equipment, and structural components that involves the use of flanges to mechanically join two sections. This joint is formed by bolting together a pair of flanges with a gasket between them to ensure a tight, leak-proof seal. Flanged joints are widely utilized across various industries&#;such as oil and gas, chemical processing, water treatment, and power generation&#;due to their reliability, ease of assembly and disassembly, and the ability to connect different types of components, such as pipes to valves, pumps, and tanks.

Flanged joints are used to connect pipes with other piping equipment (for example, another length of pipe, a valve, or a pressure vessel). Hundreds, even thousands, of flanged joints are necessary for a piping system to function.

A flanged joint comprises:

• Flanges: Flat, circular disks with bolt holes around the perimeter. The mating flanges (the &#;main&#; and the &#;companion&#; flange) are welded, screwed, or otherwise attached to the ends of pipes, valves, pumps, or other components. Flanges come in various types (weld neck, slip-on, blind, threaded, lap joint, and socket weld) to suit different applications and service conditions.

• Gasket: A sealing element placed between the flanges to prevent leaks. Gaskets are made from various materials, including rubber, graphite, PTFE, and metal, chosen based on the fluid properties, pressure, and temperature of the system.

• Bolts and Nuts: Used to bolt the flanges together, compressing the gasket to create a tight seal. The number, size, and material of the bolts and nuts depend on the flange specifications and the operating conditions.

Flanged joints are effective and economical ways to connect piping components, and feature the following characteristics:

• Leak-Proof Seal: When properly assembled, flanged joints provide a reliable seal that prevents the escape of liquids or gases, ensuring the safety and efficiency of the piping system.

• Ease of Assembly and Disassembly: Flanged joints can be easily bolted together or taken apart, facilitating quick installation, maintenance, repair, or modification of the piping system without the need for cutting or welding.

• Flexibility: Flanged joints accommodate slight misalignments and are suitable for both permanent and temporary connections, offering flexibility in system design and layout.

• Pressure and Temperature Tolerance: Designed to withstand the specific pressure and temperature conditions of the application, flanged joints are suitable for a wide range of operating environments.

Welded or Threaded Connections

A steel pipe can be welded on the flange (typical for weld neck, socket weld, slip-on, and lap joint flanges) or screwed into it (this is the case of threaded flange). Welded connections between pipes and flanges are used for high pressure and temperature applications, and diameters above 2 inches. Threaded connections are used for small-diameter piping systems that are not subject to mechanical forces such as expansion, vibration, contraction, or oscillation.

Insulation Kits

Flange insulation kits can also be used for flanged connections/joints, to protect the corrosion on flanges, bolts, and gaskets generated by the static currents running through the pipeline (this is called &#;cathodic protection&#;).

To prevent dangerous leakages in the pipeline, flanged joints shall be executed by trained personnel only (the standard TSE &#; TS EN Part 1-4, &#;Flanges and their joints&#; is the reference norm).

The image shows the elements of a flanged joint:

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