Views: 9 Author: Rachel Publish Time: 2025-08-20 Origin: Site
Stub end, also called lap joint stub end, is a critical piping component used in conjunction with lap joint flanges to create accessible, reusable connections in pipe systems. The stub end is welded to the pipe, while the matching lap joint flange slides freely over it.
Its main features and advantages are: the stub end flange can move freely, simplifying the alignment of bolt holes during installation; at the same time, it is more cost-effective because compared with welded flanges, lap joint flanges and stub ends do not require cutting pipes or interfering with welds. 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 adheres to standards like ASME B16.9, with common variants including Type A (square-cut back), Type B (tapered back for vibration resistance), and Type C (narrow hub for low-pressure threaded/socket weld flanges).
Its critical features include compatibility with freely rotating lap joint flanges, enabling effortless bolt-hole alignment during assembly, and material cost efficiency since only the stub end (not the flange) must resist the fluid, allowing cheaper flange materials like carbon steel.
To use a stub end, first slide the lap joint flange onto the pipe; then, butt-weld the stub end’s hub to the pipe, 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 stub end and the opposing flange, and bolts are tightened uniformly in a star pattern to compress the gasket.
This system permits rapid disassembly for maintenance or inspection without pipe cutting, making it ideal for corrosive services, large-diameter lines, or systems requiring 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 features a square-cut back profile perpendicular to the pipe axis and conforms to ASME B16.9 / MSS SP-43 dimensional standards. 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 deployed in chemical processing, water treatment, and low-to-medium pressure systems where corrosion resistance dictates stub end material (e.g., SS316), while permitting cost-effective carbon steel flanges. Installation requires full-penetration butt welding to the pipe, with the lap joint flange remaining freely rotatable to facilitate 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 geometry mitigates stress concentrations at the weld junction, enhancing resilience against vibration, thermal cycling, 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 risks under cyclic loading but demands meticulous welding to maintain the bevel’s stress-distribution function. While compatible with standard lap joint flanges, Type B is less common than Type A and often requires custom fabrication for specialized projects in oil/gas or power generation industries.
Stub End Type C (Narrow Hub)
Stub End Type C, defined by a shortened hub length, is engineered for compatibility with threaded or socket weld lap joint flanges per ASME B16.11. Its compact design suits low-pressure, small-bore piping systems (typically ≤ NPS 4") where space constraints or infrequent disassembly is anticipated. Common applications include instrument air lines, hydraulic systems, and auxiliary utility services in refineries or pharmaceutical plants. Installation involves socket welding or threading the stub end to the pipe, followed by sliding the lap joint flange over the assembly. However, Type C precludes use in high-stress or corrosive services.
Long Pattern Lap Joint Stub Ends
Long Pattern Lap Joint Stub Ends feature extended barrel lengths, typically ≥3 times the nominal pipe thickness per MSS SP-43 standards, enhancing greater offset capabilities between pipe and flange centerlines. This long pattern suits severe thermal expansion/contraction in high-temperature pipelines, such as refinery furnace transfer lines or steam distribution. The elongated barrel also enhances vortex shedding resistance in external flow applications. Fabricated from ASTM A312 TP316L or A333 Gr.6 alloys for creep resistance, they pair with long-weld-neck flanges to enable structural flexibility without gasket misalignment. Installation requires meticulous thermal gap calculation to prevent flange binding under operating conditions.
Short Pattern Lap Joint Stub Ends (SP Series)
Short Pattern (SP) Stub Ends prioritize compactness, with truncated barrel lengths optimized for confined spaces—such as skid-mounted plant modules, valve clusters, or instrumentation manifolds. Governed by MSS SP-43 specifications, SP-series components exhibit ≥40% shorter barrel length versus Type A/B stub ends for equivalent pipe sizes. This short pattern minimizes weight while retaining a raised lap face for compatibility with standard lap joint flanges. Short pattern stub ends are common on low-pressure utility systems, including 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 isolates the expensive alloy material only to the wetted surface, slashing material costs by 40–60% in chemical processing, desalination plants, and pulp/paper industries.
High-Maintenance Process Equipment
Applications requiring frequent disassembly—such as pump connections, heat exchanger headers, or filter housings—leverage stub ends for rapid access. 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. Welding a heavy weld-neck flange onsite is impractical; instead, the stub end is butt-welded to the pipe, and the lighter lap joint flange is slid 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.
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