What Is a Flange?

Key Takeaways

• A flange is a bolted, gasketed connector used to join pipes, valves, pumps, and equipment in industrial and commercial piping systems, making maintenance and modifications far simpler than permanent welds alone.
• Flanges make systems easier to build, maintain, and modify because they allow controlled disassembly without cutting or rewelding pipe sections.
• PLYET Industrial Supply helps buyers choose the right flange type, material, size, and rating for real-world U.S. applications in oil and gas, water treatment, HVAC, food and beverage, and more.
• Most flanges in North American process piping follow ASME standards and use American inch-based dimensions and class ratings such as Class 150, 300, 600, and higher.
• This guide walks through flange basics, common types, facing styles, dimensions, ratings, and practical selection tips so you can order with confidence.

What Is a Flange?

A flange is a flat, usually round metal plate with a central opening and evenly spaced bolt holes around its outer perimeter. Its purpose is simple but critical: to connect pipe systems, valves, pumps, and other system components using bolts and a gasket that creates a pressure-tight seal. Think of it as the mechanical handshake between two pieces of piping or equipment.

In a typical flanged connection, you have two flanges positioned face-to-face with a gasket sandwiched between them. When the bolts are tightened in a crisscross pattern, the gasket compresses and deforms just enough to fill any microscopic surface irregularities, creating a reliable seal that can withstand pressures up to 2500 PSI or more, depending on the class rating. This controlled compression is what separates a flange joint from simpler push-fit or threaded connections.

Flanges are used anywhere future access is anticipated. You will find them at pump connections, valve stations, heat exchangers, tank nozzles, meter runs, and instrument taps throughout virtually any processing system. They are strategically placed where maintenance, inspection, or modification will eventually be needed.

Common flange materials supplied by PLYET Industrial Supply include forged carbon steel, like ASTM A105, for general service, stainless steel grades such as 304/304L and 316/316L for corrosion resistance in food, beverage, or chemical plants, and specialty alloys like Hastelloy or Inconel for high-temperature or sour service environments.

Compared to other connection methods, flanges offer a practical balance. Welding creates permanent joints with excellent strength but requires cutting and rewelding for any future work. Grooved couplings allow quick assembly but may not handle extreme pressures or corrosive media as well. Threaded fittings work for smaller lines but can loosen under vibration. Pipe flanges give you strength, flexibility, and controlled access in one package.

PLYET Industrial Supply focuses on ASME-style industrial flanges commonly used in U.S. plants, refineries, commercial buildings, and municipal systems. All dimensions follow American inch-based standards, and class ratings align with what North American engineers and maintenance teams expect.

What Does a Flange Do in a Piping or Plumbing System?

Picture this scenario: a pump in a municipal water treatment plant needs to be pulled for bearing replacement. Without flanges, your team would need to cut the suction and discharge piping, remove the pump, then field-weld new connections when reinstalling. That process could take days and require hot work permits in sensitive areas. With flange connections, the same job might take a few hours of unbolting, pump removal, gasket replacement, and reassembly.

Core flange functions in a piping system include joining pipe sections into manageable spools, connecting piping to inline equipment like pumps, compressors, strainers, and pressure vessels, terminating lines using blind flanges, and providing access for instrument connections or flow meters. They serve as strategic access points where the system was designed to come apart.

Flanges are typically located every 20 to 50 feet in process lines, at every equipment tie-in, and anywhere future disassembly is expected. Valve stations in boiler rooms, heat exchanger nozzles that require tube bundle inspection, and control valve assemblies in refineries are all common examples of flange placement.

Compared to simple couplers, flange joints handle higher pressures and temperatures. A Class 600 carbon steel flange performs at 1440 PSI at 100°F, far exceeding what most mechanical couplings can achieve. Flange connectors also allow controlled bolt-up using standard torque procedures, ensuring even gasket stress for optimal sealing performance.

PLYET Industrial Supply helps customers place and select flanges in ways that reduce downtime and make future service work safer and faster. Whether you are designing a new system or maintaining piping systems that have been running for decades, the right flange choices save real money.

Common Flange Types and Characteristics

This section gives you a buyer’s overview of the main flange types you will see on drawings and quotes, and why each one exists. Understanding the characteristics flanges offer helps you match the right style to your specific application.

Different flange types are optimized for pipe size, pressure rating, temperature range, corrosion resistance, welding preferences, and ease of installation. Some are designed for high-pressure critical service, while others prioritize quick assembly in low-pressure utility systems.

PLYET Industrial Supply can source or stock all of these standard welded flange types and other welded flange types in typical ASME pressure classes, including Class 150, 300, 600, 900, 1500, and 2500. All sizes follow American inch-based conventions from 1/2 inch up through 60 inches and beyond for specialty applications.

Threaded Flanges

A threaded flange, sometimes called a screwed flange, has tapered internal threads that accept a pipe with a matching male thread based on NPT standards common in U.S. applications. The pipe with external threads simply screws into the flange bore.
Threaded flanges are typically used on smaller line sizes where welding is difficult, not permitted, or undesirable. Near flammable atmospheres, in galvanized plumbing systems, or where equipment must be installed quickly without welders, a screwed flange offers significant advantages.

Benefits include no welding required in many cases, faster installation times (often 30 to 60 minutes versus several hours for welded joints), and easier replacement. The flange still provides a bolted gasketed seal when mated to another flange or equipment.

Limitations exist as well. Threaded flanges are generally best for moderate pressure and clean service applications, typically on line sizes up to around 4 inches, depending on design and code requirements. Thread galling can become an issue in high-vibration environments.

PLYET can help verify thread compatibility and pair your threaded flange with the appropriate gasket and facing style for your service conditions.

Socket Weld Flanges

Socket weld flanges feature a recessed socket that the pipe slips into, and then the joint is secured with a fillet weld around the outside at the hub. The pipe does not pass through; it stops at the bottom of the socket with a small gap to accommodate weld shrinkage.

These flanges are commonly used on smaller diameter lines, typically 1/2 inch to 2 inches, where a smooth internal bore and good fatigue performance are priorities. High-purity pharmaceutical systems, instrument tubing runs, and chemical process lines often use socket weld flanges ideal for these demanding applications.

Socket weld flanges provide good strength for small bore piping without requiring full penetration butt welds like weld neck flanges. There is no internal weld bead protruding into the flow path, which reduces turbulence and crevice corrosion concerns.

Code limitations do apply. ASME B31.3 has specific notes about socket weld use in certain turbulent flow conditions. PLYET encourages buyers to confirm their specific process conditions and consult our team for help matching socket weld flanges to the right applications.

Slip-On Flanges

A slip-on flange slides over the outer diameter of the pipe, then is secured with fillet welds on both the inside and outside edges. This double-weld design provides strength while still offering easier alignment than weld neck styles.

Slip-on flanges are widely used in general industrial and commercial piping where pressures and temperatures are moderate and installation flexibility matters. They are popular in sizes from small-bore up through large diameter piping, where a backing flange approach simplifies fit-up.

Advantages include easier alignment during installation, broad availability across common sizes, and usually lower initial cost compared to weld necks. For many facilities, slip-ons represent the most economical choice for routine service.
Proper welding on both sides is essential for strength and leak prevention. PLYET supplies slip-on flanges with raised face or flat face options, depending on your equipment and gasket requirements.

Common use cases include chilled water loops, low-pressure steam systems under 50 PSI, and general utility piping found in facilities across the U.S.

Lap Joint Flanges

Lap joint flanges use a two-part system: a loose flange and a separate stub end that is butt-welded to the pipe. The stub end creates the sealing face while the lap joint flange itself rotates freely around it.

This free rotation makes bolt hole alignment straightforward, even in tight spaces or skewed piping runs where other flange interfaces might fight you every step of the way. If your flange bolt holes are off by even a few degrees on a weld neck, you have a problem. With a lap joint, you simply rotate the backing flange into position.

Lap joint flanges are popular in systems that require frequent disassembly, such as food and beverage processing, pharmaceutical production, and CIP (clean-in-place) applications, where equipment is broken down regularly for cleaning.

Because the loose flange does not contact the process fluid in most designs, you can sometimes use a less expensive material for the lap joint while reserving corrosion-resistant alloys for the stub end alone. This approach can reduce costs significantly when working with expensive materials.

PLYET can match stub end and lap joint materials to both your budget and your corrosion requirements.

Weld Neck Flanges

A weld neck flange features a long tapered hub that transitions smoothly from the flange thickness down to the pipe wall. The neck flange is butt-welded directly to the pipe using full penetration welds, creating the strongest possible flanged connection.
This type is the gold standard for high-pressure, high-temperature, or cyclic service applications. Refineries, power plants, high-pressure steam lines, and hydrocracker units rely heavily on weld neck flanges because butt welding produces joints with excellent fatigue resistance and structural integrity.

Proper full-penetration welding and radiographic inspection are critical to weld neck performance. When done correctly, these joints achieve near 100% integrity and can extend service life two to three times longer than lesser joint types in demanding cyclic operations.

The matching inside diameter between the flange bore and pipe schedule reduces turbulence and erosion at the connection point. This smooth flow path improves efficiency and prevents localized wear in the most demanding process piping applications.

PLYET routinely helps mechanical engineers and maintenance teams specify weld neck flanges by schedule, material grade, and ASME class rating.

Blind Flanges

A blind flange is a solid plate with bolt holes, but no center bore. It closes or caps the end of a pipe, nozzle, or pressure vessel opening.
Blind flanges isolate sections of a system for hydrostatic testing, maintenance, or future expansion without cutting and rewelding piping. When secured with correct gaskets and proper bolt torque, blinds provide a strong, removable pressure boundary that can handle full system pressure.

Common uses include dead-ending headers, closing spare nozzles on tanks and vessels, and isolating lines during plant outages when portions of the system must remain depressurized while other sections stay in service.

PLYET provides blind flanges in a full range of ASME classes and sizes, including raised face, flat face, and ring type joint styles to match your existing flange connections.

Specialty Flanges
Beyond the common flanges discussed above, several specialty flanges serve specific engineering requirements:

Specialty Type	Primary Use
Reducing flanges	Transition between two different pipe sizes without a separate reducer
Orifice flanges	Accommodate flow measurement devices with integral pressure taps
Long weld necks	Provide extended hubs for vessel nozzles or thick-wall applications
Expanding flanges	Connect smaller pipe to larger equipment nozzles
Nipo-flanges	Offer integral reinforcement for branch connections

 

These specialty flanges often support custom or engineered applications. PLYET recommends consulting early in the design phase if your project calls for unusual flange requirements. Even if your drawing specifies a less common style, PLYET can usually source or propose an equivalent solution meeting ASME and project requirements.

Flange Facing Types: How the Seal Is Made

The flange facing is the machined surface that directly contacts the gasket and determines how the joint seals. Choosing the right facing type matters because mismatched flange interfaces can damage gaskets, reduce contact area, and cause leaks.

All major PLYET-supplied flanges are available with standard ASME-compliant facing types suitable for common gasket materials used in U.S. plants. Whether you need soft material gaskets for general service or metallic gaskets for extreme conditions, the facing must match.

Smooth and Serrated Faces

A smooth face has a relatively even machined surface, typically 125 to 250 microinches RMS roughness. Smooth faces are often paired with metallic gaskets or composite gaskets in carefully controlled applications where surface finish consistency is critical.

Serrated faces, sometimes called phonographic faces, feature fine concentric or spiral grooves that help grip softer gaskets and improve sealing under bolt load. These grooves bite into materials like compressed fiber, PTFE, or elastomer-based sheets, boosting seal pressure by 20 to 30 percent compared to smooth faces.

For most general industrial applications using soft material gaskets, serrated faces provide the most reliable seal. PLYET can help match your gasket style and facing finish to pressure, temperature, and media for optimal gasket performance.

Raised Face, Flat Face, and Ring-Type Joint

A raised face flange has a small projecting rim around the central bore, typically 1/16 inch high for Class 150 through Class 400. This raised face concentrates bolt load on a smaller annular area, improving sealing performance in most industrial services.

Flat face flanges spread the gasket load evenly across the entire flange face. They are commonly used when connecting to equipment made from brittle materials like cast flanges on older pumps or valves. The even load distribution reduces bending stresses that could crack cast iron or similar materials.

Ring joint face flanges use a machined groove that accepts a soft metal ring gasket for high pressure and high-temperature service. RTJ styles including R, RX, and BX configurations are common in refinery applications, upstream oil and gas facilities, and other demanding environments requiring two flanges with metal-to-metal contact and fire-safe sealing per API specifications.

When specifying flanges, verify the facing type shown on your drawings and confirm that both mating flanges and gaskets are compatible. PLYET is available to assist whenever facing requirements are unclear.

Flange Dimensions: Getting the Fit Right

Correct flange dimensions are critical for bolt alignment, gasket seating, and compatibility with existing piping and equipment. Even small mismatches can prevent assembly or cause joint failures.

Key dimensions buyers should know include:

• Nominal pipe size (in inches)
• Flange outside diameter (OD)
• Bolt circle diameter
• Number and size of bolt holes
• Flange thickness
• Bore diameter (matching pipe schedule)

For reference, a 4-inch Class 150 raised face flange per ASME B16.5 has an outer diameter of 9 inches, a bolt circle of 7.88 inches, eight 3/4-inch bolts, and a thickness of 15/16 inch. These standardized tables make it possible to match existing installations accurately when ordering replacements.

Gasket seating surface diameter flange classification must match your selected gasket to avoid blowouts or sealing problems. The gasket inner diameter should align with the flange bore while the gasket outer perimeter must seat properly within the raised face or full flange face area.

PLYET invites you to provide existing flange markings or measurements so our team can cross-reference and supply compatible parts. Even legacy installations with painted-over markings can usually be identified through careful dimensional checks.

Flange Classification and Service Ratings

Flange class ratings like Class 150, 300, 600, 900, 1500, and 2500 indicate the pressure-temperature capability according to ASME standards. These are not simply direct PSI values.

A Class 300 flange does not mean 300 PSI. The actual allowable pressure varies with both material grade and operating temperature. Forged flanges made from different alloys have different pressure-temperature curves even at the same class rating.
For example, an ASTM A105 carbon steel Class 150 flange is rated for 285 PSI at 100°F but drops to approximately 140 PSI at 800°F. As temperature increases, allowable pressure decreases. ASME tables provide exact values organized by material group.

In practical terms, a Class 150 carbon steel flange safely handles typical building water or HVAC service, while a Class 600 flange might be selected for higher-pressure hydrocarbon lines where more robust flange design is required. Alloy steel and chrome moly materials extend capabilities further for high-temperature steam or corrosive service.

PLYET serves as a resource to translate your process conditions—fluid type, pressure in PSI, temperature in °F—into an appropriate flange class and material selection. Whether you are replacing a single flange or specifying pipe fittings for an entire project, we can help identify the ideal flange for your needs.

Flange Standards and Markings

In the U.S., most industrial flanges follow ASME B16.5 for sizes typically 1/2 inch through 24 inches, or ASME B16.47 for larger diameter flange classification above 24 inches. These flange standards govern dimensions, tolerances, materials, and marking requirements.

Typical markings stamped on the outer rim of a flange include:

• Manufacturer identification
• Nominal flange size (e.g., NPS 6)
• Pressure class (e.g., CL300)
• Material grade (e.g., A105N)
• Applicable standard (e.g., B16.5)
• Heat number for traceability

These markings allow you to identify and reorder matching flanges even when original documentation is missing. If you can read the rim, you have most of what you need to find a replacement.

PLYET Industrial Supply ships flanges with clear markings and documentation, simplifying quality control, receiving inspection, and future maintenance. Staying within recognized ASME standards helps ensure interchangeability, safety, and code compliance for U.S. projects subject to requirements like ASME B31.1 for power piping or B31.3 for process piping.

How to Choose the Right Flange with PLYET Industrial Supply

Selecting the right flange comes down to several key decision points:

Decision Point	What to Consider
Flange type	Weld neck, slip-on, threaded, socket weld, lap joint, blind, or specialty
Material	Carbon steel, stainless steel, alloy steel, or exotic alloys
Size	Nominal pipe size and schedule
Facing	Raised face, flat face, ring type joint
Gasket	Soft material, spiral wound, or metallic
Class rating	150, 300, 600, 900, 1500, 2500

 

Before calling PLYET, gather basic system data: fluid type, operating pressure (PSI), temperature (°F), pipe size and schedule, and any applicable code requirements. This information allows us to recommend the right combination without multiple repeated bends back and forth to clarify details.

PLYET supports both maintenance and repair orders for single flanges and full project packages where standardizing flange selections across an entire facility makes sense. Common flange materials like forged carbon steel and stainless steel are stocked for fast turnaround, while specialty alloys and other common flange materials can be sourced as needed.

For edge cases involving corrosive chemicals, high-temperature steam, sour service environments, or aggressive thermal cycling, lean on PLYET’s technical experience. The flange production process and material selection for these demanding applications require careful attention that goes beyond picking a size from a catalog.
Contact PLYET Industrial Supply for application-specific guidance and reliable, U.S.-standard flange supply. Whether your project involves steel plate fabrication, manhole covers, or critical process equipment, we are ready to help.

FAQ: Flange Basics Buyers Often Ask

These concise answers address questions that buyers and maintenance teams commonly ask but may not be fully covered in the main text.

How do I tell what class my existing flange is?

The fastest method is to read the markings stamped on the outer rim. You should see the nominal size, class (such as “CL150” or “300#”), material grade, and the applicable ASME standard.

If markings are unreadable, painted over, or worn away, you can measure the bolt circle diameter, count and measure the bolt holes, and measure the outside diameter. PLYET can cross-reference those measurements to flange tables and identify the correct class.

Send clear photos and measurements to PLYET for confirmation before ordering replacement flanges. This simple step prevents costly ordering mistakes.

Can I mix different flange classes in the same system?

Mixing classes is sometimes done, but the entire system is always limited by the lowest-rated flange and gasket in the line. You cannot exceed the weakest link.
Physically bolting a higher-class flange to a lower-class flange may be possible, but you can encounter sealing problems and bolt-length issues if dimensions do not match properly. Bolt protrusion differences and gasket seating area mismatches create real problems.

Keep flange classes consistent where practical and consult PLYET before mixing ratings to avoid safety and code compliance problems.

Do both mating flanges need to have the same facing type?

In almost all cases, yes. Mating flanges should have compatible facings—raised face to raised face, flat face to flat face, or ring joint face to ring joint face—to seal properly.
Mismatched facings reduce gasket contact area, damage gaskets, and lead to leaks or bolt overstress. A raised face mated to a flat face without proper gasket accommodation is a common cause of joint failures.

Double-check your drawings and existing hardware, and reach out to PLYET if you suspect a mismatch in your system. Correcting facing incompatibility before assembly saves significant headaches.

What torque should I use when tightening flange bolts?

Proper torque depends on bolt size and grade, gasket type, flange material, and desired gasket stress. There is no single universal value that applies to all flange joints.
Follow the gasket manufacturer's torque tables and your plant’s established procedures. Always use a cross-pattern tightening sequence, typically bolted in multiple passes, to load the gasket evenly and prevent warping or uneven compression.

PLYET can point you toward appropriate torque guidelines for common gasket and flange combinations, but plant engineering always has final responsibility for specifying installation procedures.

Can I reuse flanges and gaskets after disassembly?

Flanges themselves are often reusable if they are not damaged, warped, or badly corroded. Inspect the sealing face for gouges, scratches, or pitting that could prevent proper gasket seating. Check bolt holes for elongation or thread damage.

Gaskets, however, are generally considered one-time-use components in pressure piping per guidelines like ASME PCC-1. Once a gasket has been compressed and the joint opened, that gasket should be replaced with a new one before reassembly.

Contact PLYET for replacement gaskets and to evaluate whether older flanges in your facility should be upgraded during scheduled outages rather than reused indefinitely.