Lap Joint Flange and Stub End: Selection, Materials, and Standards for Pipe Systems

A DN200 cooling water line that needs to be disassembled twice a year is a clear candidate for a lap joint flange and stub end assembly. Instead of cutting and re-welding a weld neck flange, the maintenance crew can rotate the loose flange, slide the pipe apart, and replace only the stub end. Jiangyin Zhonghai Precision Machinery Co., Ltd. manufactures such assemblies with full in-house forging, CNC turning, drilling, and inspection, producing the flange ring and the stub end in the same controlled batch.

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How a Lap Joint Flange and Stub End Work Together

A lap joint flange only clamps; the stub end does the sealing. That division of labor is why this assembly dominates maintenance-heavy piping systems.

The stub end is a piece of pipe whose end is flared outward at a 37.5 degree angle. The lap joint flange, which has a bored hole larger than the pipe OD, slips over the pipe before the stub end is welded. When bolts are tightened, the flange presses against the back of the stub end flare and the gasket is compressed between the stub end face and the mating flange.

This design handles two jobs separately. The flange absorbs the bolt loads and allows the pipe to be rotated for alignment. The stub end carries the pressure boundary and contacts the process fluid. In practice, this separation means that a carbon steel flange can be paired with a corrosion-resistant stub end in a stainless steel or duplex line, cutting the total component cost without reducing the pressure rating of the joint.

Sealing Behavior and Gasket Selection for Stub End Faces

Sealing quality is determined by the stub end face, not by the flange ring or the bolt load alone. Roughness, flatness, and gasket type define the leak-tightness of the joint.

In a lap joint assembly, the flange rotates freely but does not make fluid contact. The stub end face, which is usually finished with a 0.8 to 3.2 micrometer Ra surface, creates the actual seal with the gasket. The flange thickness and bolt torque only provide the force needed to compress the gasket against that face.

• Ra 0.8 micrometer to 1.6 micrometer stub end faces suit spiral wound gaskets in Class 150 and Class 300 service.
• Ra 1.6 micrometer to 3.2 micrometer faces are common for non-metallic gaskets in low-pressure water or air lines.
• Ra 0.8 micrometer or better is required for metal gaskets and RTJ ring joints in high-pressure classes.

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Lap Joint vs Weld Neck: Cost, Stress, and Maintenance

Choose a weld neck flange when cyclic loading dominates and a lap joint with stub end when frequent disassembly and material economy are the priority.

The table below summarizes the trade-offs an engineer should weigh before selecting a weld neck flange or a lap joint assembly.

Material Selection and Standards for Lap Joint Assemblies

Material pairing between the stub end and the flange is the core decision because each part sees a different mix of stress and corrosion.

The stub end must match the pipe material to avoid galvanic or crevice corrosion at the weld. The flange can be a different grade, provided the pressure class and bolt loads are satisfied. In many chemical plants, the flange ring is A105 carbon steel while the stub end is F316L stainless steel, because the process fluid only touches the stub end.

For non-standard dimensions or special wall thicknesses, a forged non-standard component is often the fastest route. Many suppliers, including Jiangyin Zhonghai Precision Machinery Co., Ltd., accept drawings for custom stub ends and matching industrial application scenarios. This makes a lap joint and stub end assembly suitable for retrofits where the pipe schedule differs from standard catalog dimensions.

Custom Non-Standard Flanges for Specialized ApplicationsTailor-made flanges engineered to your drawings or key parameters. With capabilities up to 4 meters diameter and 10 tons weight, these components meet unique structural requirements and ensure precise integration.View Product →

How to Order Lap Joint Flanges and Stub Ends: Specifications That Matter

A complete order must cover standard, pressure class, material grade, size schedule, face finish, and inspection level.

One common mistake is ordering a flange ring to a pressure class while the stub end is made from a pipe schedule that is too thin. The flange can clamp correctly, but the stub end will creep at the flare under pressure. The six steps below reflect a practical ordering sequence used by piping engineers.

1. Choose the standard: ASME B16.5 or ASME B16.9 for the stub end, plus DIN/EN or JIS when the system runs on those codes.
2. Set the pressure class: Class 150, 300, 900, 1500, or 2500; each changes the flange ring thickness and bolt diameter.
3. Match material grades: A105 carbon steel, F304/F316 stainless, duplex, or nickel alloys for corrosive service.
4. Define size and pipe schedule: the stub end wall thickness follows the pipe schedule, not the flange class.
5. Specify the face finish: raised face, flat face, or ring joint face; include the required gasket contact surface roughness.
6. State the inspection level: EN 10204 3.1 material certificates, PMI testing, hydraulic testing, and NDT reports.

Frequently Asked Questions

What is the difference between a lap joint flange and a weld neck flange?

Both are used at pipe ends, but a weld neck flange is welded directly to the pipe and develops full joint strength through the neck, while a lap joint flange is not welded at all. In a lap joint assembly, the stub end is welded to the pipe and its flared face serves as the sealing surface. This means a lap joint flange can be reused after maintenance, whereas a weld neck flange is generally cut off and discarded once the pipe is altered.

Can a lap joint flange be used in high-pressure applications?

Yes. ASME B16.5 lap joint flanges are available up to Class 2500. In classes above Class 900, the face configuration is usually ring joint instead of raised face, and the stub end material must have sufficient wall thickness to handle the pressure without creeping at the flared section.

When should I replace the stub end in a lap joint flange assembly?

Replace it whenever the flared face shows grooves, corrosion pitting, or deformation from repeated bolt tightening. A gasket that leaked under pressure can cut a line into the stub end face, making a reset ineffective even if the flange itself is in perfect condition. The flange ring can usually be reused, but not the stub end.

What standard covers lap joint flanges and stub ends?

ASME B16.5 governs the flange dimensions and pressure ratings, while ASME B16.9 covers the butt-welded stub end dimensions. EN 1092 and JIS B2220 are the common alternatives in European and Japanese systems. A manufacturer holding ISO 9001 certification plus PED/TUV/TS credentials can supply these components with traceability.

Ordering from a manufacturer that forges both components in-house reduces the risk of flange and stub end mismatch, shortens lead time, and gives you a single source for material certificates.