Types of Pipe Flanges Explained

  • A clear overview of the most common pipe flanges used in industrial piping systems. Includes diagrams, pressure classes, material options, and the best use cases for each flange type.

1. Weld Neck Flange

The weld neck flange is one of the strongest and most reliable flange types used in industrial piping. It features a long tapered hub that transitions smoothly into the pipe, reducing stress concentr­ations at the base of the weld. Because of its strength and flow characteristics, weld neck flanges are commonly used in high-pressure, high-temperature, and critical service environments.

[Insert Weld Neck Flange Diagram Here]

Key Features

  • Long tapered hub for stress reduction
  • Full-penetration butt weld for maximum integrity
  • Ideal for high-pressure and high-temperature systems
  • Excellent fatigue resistance

Common Materials

  • Carbon steel (A105)
  • Stainless steel (304/316)
  • Alloy steel (A182 F11, F22, etc.)

Typical Pressure Ratings

  • Class 150
  • Class 300
  • Class 600
  • Class 900 and above

Best Use Cases

  • Oil & gas transmission lines
  • Chemical and petrochemical plants
  • High-pressure steam systems
  • High-temperature pipelines
  • Cryogenic service (with specific materials)

Advantages

  • Exceptional structural integrity
  • Best option for high-pressure service
  • Smooth stress distribution reduces cracking
  • Reliable long-term sealing

Disadvantages

  • More expensive than slip-on or threaded flanges
  • Requires skilled welding
  • Not ideal for systems needing frequent disassembly

2. SLIP-ON FLANGE

The slip-on flange is widely used in low-pressure and non-critical piping systems. It slides over the pipe and is welded on both the inside and outside, providing moderate strength and easy installation. Slip-on flanges are commonly used in water lines, cooling systems, and low-pressure steam applications.

[Insert Slip-On Flange Diagram Here]

Key Features

  • Pipe slides through the bore before welding
  • Welded on both sides with fillet welds
  • Easier alignment during installation
  • Lower cost compared to weld neck flanges

Common Materials

  • Carbon steel (A105)
  • Stainless steel (304/316)
  • Alloy steel grades
  • Galvanized options for corrosion resistance

Typical Pressure Ratings

  • Class 150
  • Class 300

Best Use Cases

  • Water distribution lines
  • Cooling and chilled-water systems
  • Low-pressure industrial service
  • General-purpose piping
  • Non-critical process lines

Advantages

  • Low cost
  • Easy to align and install
  • Requires less welding skill
  • Works well for moderate temperatures

Disadvantages

  • Not suitable for high-pressure service
  • Lower fatigue resistance
  • Fillet welds provide less structural strength
  • Requires accurate measurement of pipe insertion depth

3. BLIND FLANGE

A blind flange is a solid, non-bored flange used to close the end of a piping system, pressure vessel opening, or valve connection. It creates a strong, leak-proof termination point and allows future access for inspections, modifications, or system extensions. Because blind flanges must withstand full system pressure, they often require greater thickness, especially in larger sizes.

[Insert Blind Flange Diagram Here]

Key Features

  • Completely solid—no bore
  • Provides isolation for piping, nozzles, and valves
  • Can be drilled for ports or instrumentation
  • Ideal for hydrotesting and shutdown operations

Common Materials

  • Carbon steel (A105)
  • Stainless steel (304/316)
  • Alloy steel
  • Duplex stainless steel

Typical Pressure Ratings

  • Class 150
  • Class 300
  • Class 600
  • Class 900 / 1500 / 2500

Best Use Cases

  • Permanent or temporary pipe termination
  • Hydrostatic testing
  • Isolation points for maintenance
  • Pressure vessel ends (ASME B16.5 & B16.47)
  • Systems requiring future expansion

Advantages

  • Extremely strong sealing capability
  • No internal flow turbulence
  • Simple, reliable design
  • Ideal for testing and shutdown operations

Disadvantages

  • High bending stress in large diameters
  • Heavy in higher pressure classes
  • Must be fully removed for access
  • Gasket selection is critical

4. SOCKET WELD FLANGE

A socket weld flange is designed for small-diameter, high-pressure piping. The pipe is inserted into the flange’s recessed socket and welded around the outside, creating a strong, smooth-bore connection suitable for high-pressure and leak-sensitive systems.

[Insert Socket Weld Flange Diagram Here]

Key Features

  • Pipe fits into a machined socket for alignment
  • Requires only an external fillet weld
  • Smooth internal bore for improved flow
  • Typically used on ½″ to 2″ pipe

Common Materials

  • Carbon steel (A105)
  • Stainless steel (304/316)
  • Alloy steel (F11, F22, etc.)

Typical Pressure Ratings

  • Class 150
  • Class 300
  • Class 600
  • Class 900 / 1500 (special use)

Best Use Cases

  • High-pressure steam lines
  • Chemical process piping
  • Hydraulic and high-pressure systems
  • Small-diameter pipelines
  • Leak-critical applications

Advantages

  • Stronger than slip-on flanges for small pipe
  • Smooth bore improves flow
  • Easy alignment
  • Low leakage risk

Disadvantages

  • Not ideal for corrosive fluids (crevice corrosion)
  • Must control pipe insertion depth
  • Not suitable for large pipe sizes
  • Thermal expansion may stress welds

5. LAP JOINT FLANGE

A lap joint flange is used with a stub end and allows the flange to rotate freely around the pipe. This simplifies alignment and reduces stress during bolting, especially in large-diameter or difficult-to-position piping systems. Because the flange does not contact the process fluid, it can be made from a lower-cost material while the stub end provides corrosion resistance.

[Insert Lap Joint Flange Diagram Here]

Key Features

  • Two-piece system: stub end + loose flange
  • Flange rotates for easy bolt alignment
  • Reduced risk of flange face wear
  • Flange can be made from cheaper materials

Common Materials

  • Stub ends: stainless steel, duplex, alloy steel
  • Flanges: carbon steel, stainless steel, alloy steel

Typical Pressure Ratings

  • Class 150
  • Class 300
  • Class 600

Best Use Cases

  • Corrosive fluid systems
  • Food and pharmaceutical industries
  • Piping needing frequent disassembly
  • Large-diameter systems
  • Stainless steel pipelines with carbon steel flanges

Advantages

  • Easy maintenance and alignment
  • Lower overall cost with mixed materials
  • Reduced damage to sealing surfaces

Disadvantages

  • Not suitable for high-pressure systems
  • Lower structural strength than weld neck or socket weld
  • Stub end finish quality must be high

6. THREADED FLANGE

A threaded flange, also called a screwed flange, connects to a pipe using internal threads rather than welding. This makes it ideal for systems where heat should be avoided, such as flammable environments or locations with limited welding access. Threaded flanges are most commonly used in low-pressure applications and small pipe sizes.

[Insert Threaded Flange Diagram Here]

Key Features

  • Internal threads (typically NPT) connect to pipe
  • No welding required
  • Easy installation and removal
  • Ideal for small pipe diameters

Common Materials

  • Carbon steel (A105)
  • Stainless steel (304/316)
  • Alloy steel
  • Galvanized steel

Typical Pressure Ratings

  • Class 150
  • Class 300
  • Class 600 (less common)

Best Use Cases

  • Flammable or explosive environments
  • Natural gas and fuel lines
  • Compressed air systems
  • Cooling water systems
  • Temporary or easily removable piping

Advantages

  • No welding required
  • Fast installation
  • Good for hazardous environments
  • Easy to remove and replace

Disadvantages

  • Not suitable for high pressure or temperature
  • Threads may loosen over time
  • Not recommended for larger pipe sizes
  • Requires precise thread matching

7. ORIFICE FLANGE

An orifice flange is a specialized flange pair used to hold an orifice plate, enabling flow measurement for liquids, gases, and steam. These flanges include pressure tap holes so differential pressure can be measured across the orifice plate. They are widely used in refineries, petrochemical plants, power plants, and other process industries.

[Insert Orifice Flange Diagram Here]

Key Features

  • Holds an orifice plate between the flange pair
  • Includes pressure tap holes (typically ¼")
  • Supports differential pressure measurement
  • Typically installed in matched flange pairs

Common Materials

  • Carbon steel (A105)
  • Stainless steel (304/316)
  • Alloy steel
  • Duplex stainless steel

Typical Pressure Ratings

  • Class 300
  • Class 600
  • Class 900
  • Class 1500 / 2500

Best Use Cases

  • Liquid, gas, and steam flow measurement
  • Oil and gas refineries
  • Chemical and petrochemical industries
  • Power plants
  • Process lines requiring accurate metering

Advantages

  • Highly accurate for flow measurement
  • Reliable, standardized design
  • Works with multiple orifice plate styles
  • Easy installation and maintenance

Disadvantages

  • Creates a permanent pressure drop
  • Requires accurate installation & calibration
  • Not suitable for dirty or particulate-filled fluids
  • More expensive vs. standard flanges

8. Weld Neck Flange vs Slip-On Flange

Key differences include:

  • Design & Strength:
    Weld neck flanges feature a long tapered hub that is welded directly to the pipe using a full-penetration weld. This design provides excellent stress distribution and makes them suitable for high-pressure and high-temperature applications. Slip-on flanges slide over the pipe and are welded around the outside, offering lower strength compared to weld neck flanges.
  • Installation:
    Slip-on flanges are easier and faster to install, requiring less precision and welding time. Weld neck flanges require proper alignment and skilled welding, which increases installation time.
  • Cost:
    Slip-on flanges are generally more economical due to lower material and fabrication costs. Weld neck flanges are more expensive but provide greater reliability in critical services.
  • Typical Applications:
    Weld neck flanges are commonly used in high-pressure, high-temperature, and critical piping systems such as oil and gas or chemical processing. Slip-on flanges are typically used in low-pressure, non-critical applications where cost and ease of installation are priorities.

Choosing between a weld neck flange and a slip-on flange depends on system pressure, temperature, service conditions, and budget considerations.

9. ANSI & ASME Pipe Flange Standards

Pipe flanges are manufactured according to established industry standards to ensure compatibility, safety, and performance in piping systems. The most commonly referenced standards are ANSI and ASME.

  • ANSI (American National Standards Institute):
    ANSI historically defined pressure class designations such as Class 150, 300, 600, 900, 1500, and 2500. While ANSI no longer publishes flange standards directly, the term “ANSI flange” is still widely used in the industry to refer to these pressure classes.
  • ASME (American Society of Mechanical Engineers):
    ASME publishes the current technical standards for pipe flanges. One of the most common standards is ASME B16.5, which covers pipe flanges and flanged fittings from NPS ½ through NPS 24. Larger sizes are typically covered under ASME B16.47.
  • Pressure Classes:
    Flange pressure classes indicate the maximum allowable pressure at a given temperature. Common classes include Class 150, Class 300, Class 600, and Class 900. Higher classes are designed to withstand higher pressures and temperatures.
  • Why Standards Matter:
    Using flanges manufactured to ANSI and ASME standards ensures proper fit, interchangeability, and safe operation across piping components from different manufacturers.

Understanding these standards helps engineers and buyers select the correct flange for their specific application and operating conditions.

Flange Selection Summary
When to Use Each Type

Flange Selection Summary — When to Use Each Type

Choosing the correct flange depends on pressure rating, temperature, pipe size, corrosion conditions, and how often the system requires disassembly. This chart provides a quick reference.

Flange Type Best For Not Ideal For
Weld Neck High pressure, high temperature, critical service Low-budget systems, frequent disassembly
Slip-On Low–medium pressure, general-purpose piping High pressure or fatigue-sensitive systems
Blind Line termination, hydrotesting, isolation Very large sizes (high bending stress)
Socket Weld Small-diameter high-pressure systems Corrosive fluids, large pipe sizes
Lap Joint Corrosive service, easy alignment, frequent disassembly High-pressure applications
Threaded Low-pressure, hazardous environments (no welding) High-pressure or high-temperature service
Orifice Accurate flow measurement Dirty/particulate fluids, minimizing pressure drop

Conclusion & Recommended Guides

Understanding the different types of pipe flanges—and when to use each one—is essential for designing safe, reliable, and cost-effective piping systems. Each flange has specific strengths, pressure ratings, and ideal applications. Selecting the correct flange type helps improve performance, reduce maintenance costs, and extend the lifespan of your pipeline.

For deeper learning, explore the following guides:

If you need help selecting the right flange type or you’d like to request technical assistance, feel free to contact us anytime.