Views: 17 Author: Rachel Publish Time: 2025-08-20 Origin: Site
Stub end, also known as a lap joint stub end, is an important part of piping. It works with lap joint flanges to make connections in pipe systems that are easy to access and reuse. The stub end is welded to the pipe, while the matching lap joint flange slides freely over it.
Its main benefits are:
• Free movement of the stub end for easy bolt hole alignment.
• Lower cost than welded flanges, with no need for pipe cutting or welding.
They only need to match the stub end and pipe materials, and the flanges can use cheaper materials, thus reducing your costs.
Stub end is a pipe fitting component used to create a removable, bolted flange connection in piping systems. Constructed from materials matching the pipe's corrosion resistance requirements—such as stainless steel, carbon steel, or exotic alloys. The stub end features a flared, machined face for sealing and a straight hub welded to the pipe end.
Its design complies with standards including ASME B16.9, with the following common variants:
Type A: Square-cut back.
Type B: Tapered back for vibration resistance.
Type C: Narrow hub for low-pressure threaded/socket weld flanges.
Its critical features include:
Easy Assembly: A freely rotating flange enables effortless bolt-hole alignment.
Cost Efficiency: Only the stub end contacts the fluid, allowing the flange to be made from cheaper materials like carbon steel.
Before welding, slide the lap joint flange onto the pipe. Then, butt-weld the stub end to it, ensuring full penetration and alignment. The flange remains loose, sliding forward to mate with the stub end's flared face.
A gasket is placed between the flanges, then the bolts are uniformly tightened in a star pattern to compress it.
This system offers:
Quick disassembly for maintenance/inspection.
No pipe cutting required.
Ideal for corrosive services, large diameters, and frequent access.
However, stub ends are unsuitable for high-pressure/temperature applications, where weld neck flanges are preferred.
Stub End Type A (Square-Cut Back)
Stub End Type A has a square-cut end and meets the dimensional standards of ASME B16.9 and MSS SP-43. Its hub length is typically twice the pipe wall thickness, ensuring structural integrity during welding. This type interfaces precisely with standard lap joint flanges, providing a flat sealing surface for gasket compression.
Type A dominates general industrial applications due to its straightforward design and universal compatibility. It is used in chemical processing, water treatment, and medium-to-low pressure systems. In these applications, corrosion-resistant SS316 stub ends are paired with cheaper carbon steel flanges to save costs.
The stub end is butt-welded to the pipe, while the lap joint flange remains free to rotate, allowing for easy bolt alignment.
Stub End Type B (Tapered Back)
Stub End Type B incorporates a tapered transition between the flared face and hub, adhering to ASME B16.9 specifications. This design reduces stress at the weld, improving resistance to vibration, thermal cycles, and fatigue in dynamic systems.
Its application targets high-vibration environments such as compressor stations, pump discharge lines, and reciprocating machinery connections. The tapered design reduces crack propagation under cyclic loading, although it requires careful welding to preserve its stress-distribution.
Type B is compatible with standard lap joint flanges. However, it is less common than Type A and is typically made for industries like oil and gas or power generation.
Stub End Type C (Narrow Hub)
Stub End Type C has a short hub and fits threaded or socket weld flanges per ASME B16.11. Fitted for tight spaces or infrequent maintenance, this compact design suits low-pressure, small-bore pipes (typically ≤ NPS 4").
Common applications include instrument air lines, hydraulic systems, and auxiliary utility services in refineries or pharmaceutical plants. The stub end is attached to the pipe by socket welding or threading, and then the lap joint flange is slid over it. However, Type C precludes use in high-stress or corrosive services.
Long Pattern Lap Joint Stub Ends
Long Pattern Lap Joint Stub Ends offer a key advantage:
Feature: An extended barrel (≥3x pipe thickness per MSS SP-43).
Benefit: This allows for greater misalignment between the pipe and flange.
This long pattern suits severe thermal expansion and contraction in high-temperature pipelines, like refinery furnaces or steam distribution.
The elongated barrel also enhances vortex shedding resistance in external flow applications. To avoid gasket misalignment, they are made of creep-resistant ASTM A312 TP316L or A333 Gr.6 alloy and used with long weld neck flanges.
Installation requires meticulous thermal gap calculation to prevent flange binding under operating conditions.
Short Pattern Lap Joint Stub Ends (SP Series)
Short (SP) pipe fittings have a short pipe diameter and are mainly used in narrow spaces.
Governed by MSS SP-43, SP-series components have at least 40% shorter barrels than equivalent Type A or B stub ends.
This short pattern minimizes weight while retaining a raised lap face for compatibility with standard lap joint flanges. Short pattern stub ends are used in low-pressure utility systems like instrument air, cooling water, and fuel gas lines.
However, their reduced hub height limits angular deflection and restricts application to stationary pipe runs without significant bending moments.
Materials lean towards cost-efficient alloys like ASTM A403 304L or galvanized ASTM A53B. Crucially, installation demands precision alignment tools to avert bolt-hole offset due to reduced tolerance margins for radial adjustment.
Step 1: Preparation and Flange Positioning
Begin by ensuring the pipe end is clean, beveled, and free of contaminants. Slide the lap joint flange onto the pipe before welding, orienting its bolt holes to align with the mating flange. Verify the flange faces are parallel and the flange rotates freely along the pipe. This step is critical because once the stub end is welded, the flange cannot be added or removed.
Step 2: Welding the Stub End
Butt-weld the stub end's hub to the pipe end using a full-penetration weld. Align the stub end concentrically with the pipe to avoid misalignment-induced stress. For Type B stub ends, ensure the beveled transition remains intact post-weld. After welding, inspect for defects via NDT methods.
Step 3: Flange and Gasket Assembly
Slide the lap joint flange forward until it contacts the stub end’s raised face. Place a gasket centered on the stub end’s sealing surface. Never position the gasket against the lap joint flange—it must seat flush against the stub end’s machined face. Bring the mating flange into alignment, ensuring bolt holes are concentric.
Step 4: Bolting and Tightening
Insert bolts through both flanges, hand-tightening nuts initially. Follow a star pattern to tighten bolts gradually in multiple passes. Use a calibrated torque wrench to achieve even compression, adhering to ASME PCC-1 torque values.
Verify uniform gasket compression and flange face parallelism. Post-installation, pressure-test the joint per system requirements.
Corrosive Fluid Handling Systems
Stub ends are essential in pipelines conveying aggressive media where corrosion resistance is critical. By welding a corrosion-resistant stub end to the pipe, the lap joint flange can be standard carbon steel.
This costs by 40–60% in chemical processing, desalination, and pulp/paper plants.
High-Maintenance Process Equipment
Ideal for frequent disassembly, stub ends provide rapid access to pump connections, heat exchanger headers, and filter housings. The lap joint flange's free rotation simplifies bolt alignment during reassembly, eliminating pipe cutting or re-welding. This reduces downtime in pharmaceutical cleanrooms, food/beverage facilities, and refinery turnaround operations.
Large-Diameter Piping Networks
For pipes ≥ NPS 12", stub ends enable modular installation. To install, first butt-weld the stub end to the pipeline, then slide the lighter lap flange into position.
This method is indispensable in water distribution mains, offshore platform risers, and power plant cooling circuits, where alignment flexibility accelerates construction.
Thermal/Vibration-Prone Services
In systems experiencing thermal cycling or mechanical vibration, Type B (tapered back) stub ends mitigate fatigue failure. The tapered hub redistributes stress away from the weld, preventing crack initiation.
This design is mandated for gas turbine exhausts, LNG transfer lines, and reciprocating pump piping in energy facilities.
Space-Optimized Utility Services
Short-pattern stub ends (Type C) resolve clearance challenges in instrumentation panels, hydraulic skids, and modular process units. Their compact hub suits threaded/socket weld connections for low-pressure air, water, or drain lines ≤ NPS 4". The design avoids flange interference in congested areas like control rooms or packaged equipment.
Stainless Steel Stub Ends
Stainless steel like ASTM A403 Grades 304/316/316L is the dominant material for stub ends in corrosive or hygienic services. Its chromium-nickel content provides inherent resistance to oxidation, acids, and chlorides, making it ideal for chemical processing, food/beverage, and marine applications.
Austenitic grades 304/316 balance cost and performance, while 316L's low carbon content prevents weld decay. Duplex/super duplex steels offer enhanced strength and pitting resistance for offshore oil/gas systems. Compliance with ASTM A182/A473 ensures mechanical integrity under ASME Class 150–600 pressures.
Carbon Steel Stub Ends
Carbon steel and low-alloy steels are cost-effective choices for non-corrosive, general-industrial applications. Their high tensile strength suits water, steam, oil, and gas utilities in power plants, HVAC, and fire protection systems.
ASTM A105 or A860 materials provide weldability and impact resistance. Carbon steel stub ends pair with identical flanges, while galvanized variants prevent rust in atmospheric exposures. Avoid in corrosive media unless clad or coated.
Nickel Alloy Stub Ends
For high temperatures (>800°F/427°C) and concentrated acids or sulfidic environments, nickel alloys are essential. Alloy 625 (UNS N06625), Hastelloy C-276 (N10276), and Monel 400 (N04400) deliver unmatched corrosion/erosion resistance. These alloys comply with ASTM B564/B366 and serve critical roles in:
Sulfuric/nitric acid reactors.
Offshore sour gas pipelines.
Nuclear waste handling.
To learn more about Stub End products, contact us.
