Flange Parts Explained: A Practical Guide to the Body, Face, Bore and Bolt Circle

A pipe flange is often described as a simple connector, but reliable service depends on several precisely made flange parts working together. The flange body carries load, the bore connects to the pipe, the face seals against a gasket, and the bolt circle creates the clamping force. A small error in one feature can cause misalignment, leakage, vibration, or a difficult installation.

This guide explains the main flange parts and the function of every flanges body part. It is written for engineers, procurement teams, installers, and maintenance personnel selecting flanges for water treatment, chemical equipment, pressure vessels, oil and gas lines, power plants, and general industrial piping.

What Are the Main Flange Parts?

The exact geometry changes with flange type, but most forged and plate flanges include a body or rim, a flange face, a bore, bolt holes arranged on a bolt circle, and a gasket seating area. Weld neck flanges also have a hub or neck. Some designs add a ring groove, a lap joint stub-end bearing surface, or a threaded bore.

Flange part	Primary job	Why it matters
Flange body or rim	Provides the structural disc around the bore	Resists bolt load, pressure load, and handling stress
Flange face	Contacts the gasket	Controls sealing performance
Bore	Forms the central flow opening	Affects pipe fit-up, flow continuity, and weld preparation
Hub or neck	Transfers load into the pipe	Improves fatigue resistance in demanding service
Bolt holes and bolt circle	Accept fasteners and apply clamping force	Must match the mating flange exactly
Gasket seating area	Supports controlled gasket compression	Helps prevent leaks at pressure and temperature

1. Flange Body: The Load-Carrying Part

The flange body, sometimes called the rim or disc, is the main mass of material between the bore and the outside diameter. When people search for a flanges body part, they usually mean this structural section. It holds the bolt holes, supports the sealing face, and distributes bolt tension around the joint.

Body thickness is not an arbitrary dimension. It is determined by the standard, nominal size, pressure class, material strength, and flange construction. A Class 150 carbon steel flange body, for example, is not designed for the same load as a Class 600 alloy steel flange. Custom flanges may require additional thickness when they are large diameter, used on equipment nozzles, or exposed to unusual external loads.

2. Flange Face: The Sealing Surface

The flange face is the machined surface that compresses the gasket. It is one of the most important flange parts because the joint cannot seal reliably without the correct face type, surface finish, gasket, and bolt load.

Common face designs include flat face (FF), raised face (RF), ring type joint (RTJ), tongue and groove, and male and female faces. An RF face concentrates gasket load on a smaller area and is common in industrial steel piping. An FF face spreads load over a wider area and is often used with cast iron equipment or low pressure service. RTJ grooves are used for metal ring gaskets in severe pressure and temperature applications.

Inspect the flange face before installation. Deep radial scratches, pitting, corrosion, distortion, or weld spatter can create a leak path. The required serration and roughness should match the gasket manufacturer recommendation and the applicable flange standard.

3. Bore: The Flow Opening and Pipe Interface

The bore is the central opening of a flange. Its diameter must be suitable for the pipe schedule or wall thickness, especially for weld neck flanges where the bore transitions into the pipe. Correct bore sizing helps maintain flow continuity and avoids an internal step that can cause turbulence, erosion, product build-up, or pressure loss.

A flange bore can be standard, reduced, threaded, or custom machined. For corrosive service, lined systems, and special pipe schedules, confirm the bore dimensions from the project drawing rather than ordering only by nominal pipe size. A nominal NPS or DN size alone does not define every bore detail.

4. Hub or Neck: Stress Transfer for Welded Flanges

The hub is the reinforced section that joins the flange body to the pipe connection. It is most visible on weld neck flanges, where the tapered neck transfers stress gradually from the flange to the pipe wall. This geometry is valuable in high pressure, high temperature, cyclic, and vibration-prone piping.

Slip on flanges have a shorter hub and are welded at the bore and outer edge. Socket weld flanges have a recessed socket for small-bore pipe. Threaded flanges use an internal thread instead of a weld connection. Each construction changes how loads move through the flange parts, which is why flange type should be selected based on service conditions rather than price alone.

5. Bolt Holes and Bolt Circle

Bolt holes are the openings around the outer area of the flange body. Their center points form the bolt circle, also called bolt circle diameter or BCD. The bolt circle must match the mating flange in diameter, hole count, hole size, and orientation. Even when two flanges have a similar outside diameter, incompatible bolt patterns will prevent assembly.

Fasteners do more than hold two metal parts together. They create the gasket seating stress. For this reason, correct bolt material, lubrication, tightening sequence, and torque control matter as much as the holes themselves. Uneven tightening can tilt the flange faces and overload one side of the gasket.

6. Gasket Seating Area

The gasket seating area is the portion of the flange face designed to contact and compress the gasket. On a raised face flange, it is the raised ring around the bore. On a flat face flange, it can extend across the full face. On an RTJ flange, the seating area is the precisely machined groove for the metal ring gasket.

Select the gasket after confirming the service medium, temperature, pressure, face type, and bolt load. Rubber, PTFE, compressed fiber, spiral wound, corrugated metal, and ring joint gaskets all need different conditions. A gasket that is correct for clean water may fail quickly in hot steam, hydrocarbons, solvents, or vacuum service.

How Flange Parts Work Together

During assembly, the pipe or equipment connects through the bore and hub. The flange body supports the bolt pattern. Bolts pull the two bodies together, and the clamping force compresses the gasket on the flange face. In service, internal pressure tries to separate the joint while temperature cycles and piping loads challenge alignment. The flange components must act as one system to retain a stable seal.

This is why a flange should never be evaluated only by its outside diameter. A technically correct order must cover standard, size, pressure rating, material, face type, bore, flange type, bolt pattern, gasket requirement, and any special machining.

Flange Parts by Flange Type

Weld Neck Flange

Key features are a long tapered hub, matched bore, flange face, and bolt circle. It is a strong choice for high pressure, high temperature, and cyclic applications.

Slip On Flange

This design has a short hub and a bore larger than the pipe outside diameter so the pipe can slip inside before welding. It is common in moderate service when installation economy is important.

Blind Flange

A blind flange has no through bore. Its solid body closes a line, nozzle, or vessel opening. The face, bolt circle, and body thickness must withstand the design pressure.

Threaded and Socket Weld Flange

These flange parts are adapted for small-bore connections. A threaded flange has internal threads, while a socket weld flange has a socket recess for the pipe end.

Lap Joint Flange

A lap joint flange works with a stub end. The loose backing flange provides the bolt circle, while the stub end provides the wetted face. This can reduce the need for expensive alloy material in the outer flange body.

Inspection Checklist Before Ordering or Installing

• Confirm the flange standard: ASME B16.5, ASME B16.47, EN 1092-1, DIN, JIS, or the project specification.
• Check NPS or DN size, pressure class or PN rating, and pipe schedule.
• Verify flange material and any required material test documents.
• Confirm face type and surface finish for the selected gasket.
• Measure bore, outside diameter, thickness, bolt hole quantity, bolt hole diameter, and bolt circle.
• Inspect the flange face for damage before bolting.
• Use a controlled cross-pattern tightening procedure.

Common Problems Caused by Incorrect Flange Components

Common issues include a bore that does not match the pipe wall, bolt holes that do not align, a wrong face type, a damaged gasket seating area, or insufficient hub strength for the piping load. These problems can lead to forced assembly, uneven gasket compression, flange rotation, leakage, and unplanned shutdowns.

For critical systems, send the drawing and mating-flange details to the manufacturer before production. This is especially important for custom flange parts, large diameter flanges, pressure vessel flanges, and replacement flanges for older equipment.

Songhai Flange Manufacturing Support

Songhai Flange supplies carbon steel, stainless steel, alloy steel, forged, plate, weld neck, slip on, blind, threaded, socket weld, lap joint, and custom flanges for global industrial projects. We can review flange body dimensions, bore requirements, face type, bolt circle, and machining details against the project drawing before manufacture.

For a quotation, provide the flange type, standard, size, pressure rating, material, face, bore or pipe schedule, quantity, drawing, and destination market. Clear technical data helps ensure every flange part is made for the intended connection.

FAQ: Flange Parts and Flange Body Components

What are the main flange parts?

The main flange parts are the body, face, bore, hub or neck where applicable, bolt holes, bolt circle, and gasket seating area.

What is the body part of a flange?

The flange body is the main structural disc or rim around the bore. It supports the flange face and bolt holes while resisting pressure and bolting loads.

Why is the flange face important?

The flange face is the gasket contact surface. Its type and condition directly affect sealing reliability.

What is a bolt circle on a flange?

The bolt circle is the diameter of the circle passing through the centers of the bolt holes. It must match on both mating flanges.

Can a damaged flange face be repaired?

Minor damage may sometimes be repaired by qualified machining, but the finished dimensions and surface condition must still meet the applicable standard and gasket requirements.

Conclusion

Understanding flange parts makes it easier to specify the right product and avoid installation problems. The flange body carries load, the face seals, the bore connects the flow path, the hub transfers stress, and the bolt circle provides controlled clamping. When these flange components are matched to the correct standard, gasket, material, and operating conditions, they form a durable and maintainable piping connection.