Understanding Flange Types Under ASME B16.5 Standard — A Complete Guide

By Fulihua Pipeline Equipment | www.flhmetal.com

Introduction

When engineers and procurement professionals specify flanges for industrial piping systems, ASME B16.5 is one of the most widely referenced standards in the world. Developed by the American Society of Mechanical Engineers (ASME), this standard covers pipe flanges and flanged fittings for nominal pipe sizes from NPS ½ through NPS 24, across pressure classes ranging from Class 150 to Class 2500.

But what exactly are the different flange types defined under ASME B16.5, and how do you choose the right one for your application? This guide breaks down each flange type, explains its design characteristics, and outlines its typical use cases — so you can make informed decisions for your pipeline projects.

What Is ASME B16.5?

ASME B16.5 is the standard that defines the dimensions, tolerances, pressure-temperature ratings, materials, and testing requirements for pipe flanges and flanged fittings. It is widely adopted in the oil and gas, petrochemical, power generation, and industrial pipeline industries across North America and internationally.

Key parameters covered by ASME B16.5 include:

• Size range: NPS ½ to NPS 24 (DN15 to DN600)

• Pressure classes: Class 150, 300, 400, 600, 900, 1500, and 2500

• Materials: Carbon steel, stainless steel, alloy steel, and special alloys

• Face types: Raised Face (RF), Flat Face (FF), Ring Type Joint (RTJ)

Flange Types Under ASME B16.5

ASME B16.5 recognizes several distinct flange types, each designed for specific installation methods, pressure requirements, and applications. Here is a detailed overview of each type.

1. Weld Neck Flange (WN)

Also known as: High Hub Flange

The Weld Neck flange is considered the most robust and reliable flange type available under ASME B16.5. It features a long tapered hub that gradually transitions from the flange body to the pipe wall — a design that significantly reduces stress concentration at the base of the flange.

How it connects: The pipe is inserted into the bore of the flange and joined with a full penetration butt weld, creating a continuous, uninterrupted flow path.

Key characteristics:

• Long tapered neck provides excellent structural integrity

• Ideal for high-pressure and high-temperature applications

• Allows for accurate radiographic inspection of the weld

• Bore matches the pipe inside diameter, minimizing turbulence

Typical applications:

• Oil and gas transmission pipelines

• High-pressure steam systems

• Subsea and offshore piping

• Critical process lines in petrochemical plants

Pressure class: Suitable for all pressure classes — Class 150 through Class 2500

2. Slip-On Flange (SO)

The Slip-On flange is one of the most commonly used flange types due to its ease of installation and lower cost compared to the Weld Neck flange. As the name suggests, the pipe simply slips through the bore of the flange and is then welded in place — both on the inside and outside — to secure the connection.

How it connects: The pipe slides through the flange bore and is secured with two fillet welds — one on the inside face and one on the outside.

Key characteristics:

• Simple and quick installation

• Lower initial cost than Weld Neck flanges

• The double fillet weld provides adequate strength for moderate conditions

• Bore is slightly larger than the pipe outside diameter to allow slip-fit

Typical applications:

• Water treatment and supply systems

• Low to medium pressure pipelines

• HVAC and utility piping

• General industrial applications

Pressure class: Commonly used in Class 150 and Class 300; less suitable for very high-pressure or cyclic-load applications

3. Blind Flange (BL)

A Blind flange is a solid disc with no bore — it is used to close off the end of a pipeline, nozzle, or pressure vessel opening. Despite its simple appearance, the Blind flange is actually subjected to significant bending stress when under pressure, making it one of the most highly stressed flange types.

How it connects: Bolted directly to a mating flange or nozzle facing, using the standard bolt circle pattern.

Key characteristics:

• Solid face with no center bore

• Used to permanently or temporarily terminate a pipeline

• Can be removed for future access or expansion

• Must be designed to withstand full system pressure

Typical applications:

• Pipeline end closures

• Vessel inspection openings and manholes

• Future pipeline expansion points

• Pressure testing isolation points

Pressure class: Available in all ASME B16.5 pressure classes — Class 150 through Class 2500

4. Threaded Flange (TH)

Also referred to as a Screwed Flange, the Threaded flange features a tapered internal thread (NPT — National Pipe Taper) that allows it to be screwed directly onto a male-threaded pipe end — no welding required. This makes it particularly useful in situations where welding is hazardous or impractical.

How it connects: Screwed onto the pipe using matching NPT threads. No welding is required, though a seal weld may be added for additional security.

Key characteristics:

• No welding required for installation

• Ideal for hazardous or flammable environments where welding is prohibited

• Quick installation and removal

• Less suitable for applications with severe temperature cycling or vibration

Typical applications:

• Pipelines handling flammable or explosive media

• Remote locations where welding equipment is unavailable

• Low-pressure utility systems

• Fire protection pipelines

Pressure class: Typically used in Class 150, 300, and 600

5. Socket Weld Flange (SW)

The Socket Weld flange features a socket — a recessed bore — into which the pipe end is inserted before being secured with a single fillet weld on the outside. A small gap (approximately 1.6 mm) is left between the pipe end and the socket bottom to accommodate expansion during welding.

How it connects: The pipe is inserted into the socket bore and a single fillet weld is applied around the outside of the hub.

Key characteristics:

• Suitable for small-diameter, high-pressure piping

• Better fatigue resistance than Slip-On flanges

• The socket provides initial alignment, simplifying installation

• Not recommended for corrosive or abrasive services due to crevice at pipe-socket interface

Typical applications:

• High-pressure hydraulic lines

• Small-bore pipelines in chemical and petrochemical plants

• Steam and condensate systems

• Instrumentation and control piping

Pressure class: Commonly used in Class 150 through Class 2500 for small pipe sizes

6. Lap Joint Flange (LJ)

The Lap Joint flange is always used in conjunction with a Stub End (also called a Lap Joint Stub End). The flange itself does not contact the process fluid — instead, it acts as a backing flange that can freely rotate around the Stub End. This rotation makes bolt hole alignment extremely easy during installation.

How it connects: A Stub End is butt-welded to the pipe. The Lap Joint flange slides over the pipe and backs up against the Stub End. The flange rotates freely, simplifying bolt hole alignment.

Key characteristics:

• The flange can be rotated for easy bolt hole alignment

• Ideal for systems requiring frequent disassembly for maintenance or inspection

• The Stub End (not the flange) contacts the process fluid — so only the Stub End needs to be made from expensive corrosion-resistant material

• The backing flange can be made from carbon steel even in corrosive services

Typical applications:

• Systems requiring frequent maintenance or inspection

• Pipelines handling corrosive fluids where cost savings are important

• Low-pressure applications

• Piping systems with difficult alignment requirements

Pressure class: Typically Class 150 and Class 300

Flange Face Types Under ASME B16.5

In addition to the flange type, the sealing face type is equally important. ASME B16.5 recognizes the following face types:

Face Type	Abbreviation	Description
Raised Face	RF	Most common. A small raised ring on the face concentrates the gasket load
Flat Face	FF	The entire face is flat. Used with cast iron or non-metallic flanges
Ring Type Joint	RTJ	A metal ring gasket sits in a machined groove. For high-pressure, high-temperature applications

Pressure-Temperature Ratings

One of the most important aspects of ASME B16.5 is its pressure-temperature (P-T) rating system. Each material group has defined maximum allowable working pressures at various temperatures across seven pressure classes:

Class	Approximate Maximum Pressure (Carbon Steel A105 at 38°C)
150	19.6 bar (285 psi)
300	51.1 bar (740 psi)
400	68.1 bar (990 psi)
600	102.1 bar (1,480 psi)
900	153.2 bar (2,220 psi)
1500	255.3 bar (3,705 psi)
2500	425.5 bar (6,170 psi)

Note: Actual ratings depend on the material group and operating temperature. Always consult the full ASME B16.5 standard tables for your specific material and temperature.

How to Select the Right Flange Type

Choosing the correct flange type requires considering several factors:

1. Operating pressure and temperature Higher pressure and temperature applications generally require Weld Neck flanges due to their superior structural integrity.

2. Pipe size Socket Weld flanges are typically limited to smaller pipe sizes (NPS 3 and below), while Weld Neck and Slip-On flanges cover the full size range.

3. Service conditions For corrosive, flammable, or high-cycle fatigue services, Weld Neck flanges are preferred. Threaded flanges are ideal where welding is not permitted.

4. Maintenance requirements If the pipeline requires frequent disassembly, Lap Joint flanges offer the easiest access.

5. Budget Slip-On flanges offer a lower initial cost for moderate-service applications where Weld Neck performance is not required.

Fulihua — Your ASME B16.5 Flange Manufacturer

At Fulihua Pipeline Equipment, we manufacture all ASME B16.5 flange types across the full range of pressure classes and materials — from standard carbon steel A105 to stainless steel 316L and high-alloy grades such as F11, F22, and F91.

Our capabilities:

• Size range: DN15 – DN1500 (NPS ½ – NPS 60)

• Pressure classes: Class 150 – Class 2500

• Flange types: WN · SO · BL · TH · SW · LJ

• Face types: RF · FF · RTJ · MFM · TG

• Standards: ASME B16.5 · EN 1092-1 · ГОСТ 33259-2015 · GB/HG · JIS

• Materials: A105 · A350 LF2 · 304/316L · F5/F11/F22/F91 · Duplex 2205 and more

• Production capacity: 20,000 metric tons per year

• Lead time: Standard orders 15–30 days | Custom flanges available

• Response time: Commercial quotation within 12 hours

Whether you need standard catalogue flanges or fully custom engineered solutions, our team is ready to support your project from inquiry through delivery.

Contact Us

Email: sales@flhmetal.com

Website: www.flhmetal.com

Address: Office 101, Fulihua building, Green Intelligent Foundry & forging Industrial Park, Quanlin Town, Sishui County, Jining City, Shandong Province, China

Send us your specifications — standard, size, pressure class, material, quantity — and receive a competitive quotation within 12 hours.

Fulihua Pipeline Equipment — Built on Strength. Proven by Quality.