Weld neck flanges and threaded flanges both join a flange to pipe, but in opposite ways. The first one is welded on, and the other screws onto the pipe. That single difference affects pressure limits, installation, and long-term reliability. And that is why the choice between them requires a closer look. This guide explains how each flange works, compares them side by side, and covers what to consider before choosing one for your specific need.
What Is a Weld Neck Flange?
A weld neck flange is recognized by its extended, tapered neck and beveled end, both machined to accept a full-penetration butt weld to the mating pipe. The taper gradually reduces the flange’s thickness down to the pipe wall thickness. That spreads stress across a longer section of material instead of concentrating it at a single point. Our guide to what a weld neck flange is covers weld neck flanges in detail.
Weld neck flanges are forged, most often from ASTM A105 carbon steel or ASTM A182 stainless and alloy steel. They are made to ANSI/ASME B16.5 for NPS ½″ through 24″ in Classes 150 through 2500, and to ANSI/ASME B16.47 for NPS 26″ through 60″ in Classes 75 through 900. Common configurations include standard, long, reducing, raised-face, and flat-face weld necks, as covered in our guide to weld neck flange types and applications. In larger sizes and higher pressure classes, weld neck flanges are usually the only type specified. They are the industry standard for high-pressure, high-temperature, cyclic, and heavy-use applications across process plants, power facilities, and pipelines.
What Is a Threaded Flange?
A threaded flange is a mechanical piping component that connects to a pipe using screw threads instead of welding. The flange is designed with an internally threaded (commonly tapered) center that screws onto matching external pipe threads (commonly tapered). Threaded flanges are also often called companion flanges. The thread form is NPT (National Pipe Taper), defined by ASME B1.20.1 standard. That tapered bore is what sets a threaded flange apart from every other flange type.
Threaded flanges are mainly standardized under ANSI/ASME B16.5 for NPS ½″ through 24″. API International threaded flanges are machined from A105 forged carbon steel and 304/316 forged stainless steel. They are used mostly in low-to-moderate pressure and non-critical piping. Threaded flanges are typically limited to smaller diameters and to applications where temperature fluctuations are minimal. And they are also frequently used to create reducing connections.
Pipe Connection Methods
A weld neck flange is joined to the pipe with a single full-penetration butt weld, made through a beveled V-groove in the flange hub and the pipe end. A threaded flange is assembled by screwing it onto the pipe. No heat, filler metal, or welding procedure is involved in a threaded connection.
Once either flange is connected to the pipe, it joins to its mating flange the same way any ANSI/ASME B16.5 joint does. The bolts supply the clamp load, and the gasket between the two faces makes the seal. On a threaded flange, thread sealant seals the thread joint on the pipe itself. A welded joint has no such leak path to manage.
Welding Restrictions and Site Conditions
Threaded flanges exist for situations where welding is impractical, restricted, or unsafe. The typical cases are explosive or flammable atmospheres where hot work permits are difficult to obtain, remote locations where welding equipment is not available, and systems that need regular disassembly. Low-pressure utility connections are another common fit. Threaded flanges are also one of the most practical small-bore connection options.
Pressure, Size, and Service Limits
Weld neck flanges are available across the full range of pressure classes, 150 through 2500 under B16.5, and they are the standard choice for high-pressure and high-temperature service.
Threaded flanges are used mostly in low-to-moderate pressure, non-critical piping and in smaller diameters. They are most commonly manufactured and used in Class 150, 300, and 600. In Class 900 and above, ANSI/ASME B16.5 gives threaded flanges lower pressure-temperature ratings than weld-end flanges of the same class. Threads weaken the pipe wall and are prone to leakage under vibration and thermal cycling.
! In most cases, weld neck or socket weld flanges are the better option at those classes. If a threaded flange is required, engineering review is needed before installation.
Temperature swings and vibration
Temperature swings and vibration are the other limit. Repeated expansion and contraction fatigues the thread sealant layer and can reduce thread engagement over time. Threaded flanges are not recommended where thermal cycling or vibration is present. A welded joint does not loosen this way.
Threaded Flange Suitability by Pressure Class
The table below summarizes how common threaded flanges are in each class. Our guide to using threaded flanges covers the pressure-temperature ratings behind it.
Pressure Class | Use for Threaded Flanges | Notes |
Class 150 | Very common | Low-pressure steam, water, utilities, instrument air |
Class 300 | Very common | General process piping, moderate steam |
Class 600 | Common | Moderate-pressure service |
Class 900 | Limited | Reduced pressure-temperature rating; weld neck or socket weld usually preferred |
Class 1500 | Rarely used | Weld neck or socket weld usually preferred |
Class 2500 | Not recommended | Weld neck or socket weld usually preferred |
Unlike weld neck flanges, threaded flanges are not covered by ANSI/ASME B16.47. Above NPS 24, they are made to industry standards based on the B16.47 bolt patterns, and dimensions vary by manufacturer. The series (A or B) should be stated, and details confirmed with the end user before fabrication.
Installation Requirements
Installing a threaded flange requires matching pipe threads cut to the correct tolerance. ASME B1.20.1 defines only the thread form. It does not prescribe a sealant. The applicable piping code, such as ASME B31.3 for process piping, governs whether PTFE tape, anaerobic thread sealant, or pipe dope is required. No welding inspection, procedure qualification, or post-weld heat treatment applies.
Installing a weld neck flange requires beveling, alignment, a qualified welding procedure under ASME Section IX, and typically weld inspection before the joint enters service. Our weld neck flange welding procedure guide covers the process in detail. Threaded installation is faster and needs less specialized labor. Weld neck installation takes longer but produces a joint suited to far more demanding conditions.
Comparison Overview
Property | Threaded Flange | Weld Neck Flange |
Connection method | Screws onto a threaded pipe end; no welding required | Full-penetration butt weld |
Bore | Tapered internal NPT thread | Smooth bore with a beveled weld end and tapered hub |
Pressure classes | Most common in Class 150, 300, and 600; reduced ratings in Class 900 and above | Class 150 through 2500 (B16.5); Class 75 through 900 (B16.47) |
Typical sizes | Smaller diameters; B16.5 covers NPS ½″ through 24″ | NPS ½″ through 24″ (B16.5) and NPS 26″ through 60″ (B16.47) |
Temperature | Best where temperature fluctuations are minimal | Full range up to the material’s rated maximum |
Thermal cycling / vibration | Not recommended; threads can loosen | Well suited; full-penetration weld resists fatigue |
Installation | No welding equipment or procedure required | Requires qualified welding procedure and welder |
Disassembly | Can be unthreaded for removal | Requires cutting the weld |
Governing standards | ANSI/ASME B16.5; thread form per ASME B1.20.1 | ANSI/ASME B16.5 and B16.47 |
Typical use | Low-pressure utility lines, small-bore piping, hot-work-restricted areas | High-pressure, high-temperature, critical service |
Selection Guidelines
- Threaded flanges are the better fit when: the system is low-to-moderate pressure, non-critical, and small-bore. Welding is restricted or impractical on site, or the system needs regular disassembly. The connection will not face thermal cycling or vibration.
- Weld neck flanges are the better fit when: the system operates at moderate to high pressure or temperature, is subject to cyclic loading, or falls under a piping code that requires a welded, code-rated connection.
Conclusion
Threaded and weld neck flanges are made for different service conditions. A threaded flange trades pressure capability and long-term reliability for a connection that requires no welding at all. And a weld neck flange trades that installation simplicity for a joint that holds up under the pressure, temperature, and cycling that most industrial piping systems face.
API International threaded flanges are machined from A105 forged carbon steel and 304/316 forged stainless steel and conform to ANSI/ASME B16.5 standards. Weld neck flanges are machined to ANSI/ASME B16.5 and B16.47. Our in-house machine shop can also add threads to any stock flange. Explore the comprehensive range of flanges in our online product catalog, or contact us for custom machining on nonstandard sizes and configurations. Get connected with a dedicated sales representative today, or call us at 503.692.3800.
Frequently Asked Questions
What is a weld neck flange?
A weld neck flange has a long, tapered hub that is butt welded to the pipe, spreading stress evenly and creating a strong, leak-resistant connection. It is the standard choice for high-pressure pipelines and critical process piping. Weld neck flanges are forged and made to ANSI/ASME B16.5 for NPS ½″ through 24″ and ANSI/ASME B16.47 for NPS 26″ through 60″.
How is a weld neck flange connected to pipe?
A weld neck flange is joined to the pipe with a full-penetration butt weld. The pipe end and hub are beveled and aligned with a controlled root gap, then tack-welded and welded to a qualified procedure under ASME Section IX. Weld inspection is typically required before the joint enters service.
When should a weld neck flange be used instead of another type?
Weld neck flanges are the right choice for high-pressure and high-temperature piping, steam systems, and any service with vibration, thermal expansion, or cyclic loading. They suit applications where a secure, lasting connection matters more than cost or ease of installation. They are also specified where the piping code requires a welded, code-rated connection.
What are threaded flanges?
Threaded flanges, also called companion flanges, have internal threads that allow them to be screwed directly onto the external threads of a pipe, with no welding required. The thread form is NPT (National Pipe Taper), defined by ASME B1.20.1. They are used mostly in low-to-moderate pressure, non-critical piping and in smaller diameters.
When are threaded flanges the right choice?
Threaded flanges are the right choice when welding is impractical, restricted, or unsafe on site, such as in explosive or flammable atmospheres, remote locations, or systems that need regular disassembly. The service should also be low-to-moderate pressure, non-critical, and in smaller diameters. Threaded flanges are not recommended where thermal cycling or vibration is present.
What pressure classes are threaded flanges suitable for?
Threaded flanges are most commonly used in Class 150, 300, and 600. In Class 900 and above, ANSI/ASME B16.5 gives threaded flanges lower pressure-temperature ratings than weld-end flanges of the same class. Class 900 is limited, Class 1500 is rarely used, and Class 2500 is not recommended. A weld neck or socket weld flange is usually the better choice at those classes.
How to install a threaded flange?
A threaded flange is screwed onto matching pipe threads until snug, with thread sealant applied as needed. The applicable piping code, such as ASME B31.3 for process piping, governs whether PTFE tape, anaerobic thread sealant, or pipe dope is required. No welding equipment, welding procedure, or weld inspection is involved. Our threaded flange guide covers assembly and sealant requirements in more detail.
What is the difference between a threaded flange and a weld neck flange?
A threaded flange screws onto a threaded pipe end and requires no welding. A weld neck flange has a tapered hub and is joined to the pipe with a full-penetration butt weld. Weld neck flanges are the standard choice for high-pressure, high-temperature, and cyclic service. Threaded flanges suit smaller diameters and low-to-moderate pressure, non-critical service, and they can loosen under vibration or thermal cycling.
How to order threaded and weld neck flanges from API International?
To order threaded or weld neck flanges from API International, determine your required pipe size, pressure class, material, connection type, and any special standards (such as ANSI/ASME B16.5 or B16.47). Contact API International’s experienced team, who will help you select the right flange for your application. Our in-house machine shop can also add threads to any stock flange.
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