ASME B16.9 Stub End for Piping System
In industrial piping design, to facilitate the installation, disassembly, and frequent maintenance of equipment, engineers often need a solution that provides both a reliable connection and flexible dismantling. The combination of the ASME B16.9 Stub End and the Lap Joint Flange presents an ideal choice for such scenarios.
This article will provide a detailed analysis of this special pipe fitting, helping you understand its structure, standards, specifications, materials, and applications.
What is a Stub End?
A Stub End is a short length of pipe manufactured with one end prepared for butt welding to pipe (Butt Weld End), and the other end pre-formed into a flared or lap joint configuration (Stub End Flange).
It does not have the full thickness of a standard flange. Its flared end functions by working in conjunction with a Lap Joint Flange. The clamping force from the bolts acts on the lap joint flange, which then presses against the Stub Ends lap, thereby compressing the gasket and creating the seal.
Production Standard: ASME B16.9
While ASME B16.9 primarily covers butt-welding fittings (such as elbows, tees, and reducers), it also strictly specifies the dimensions, tolerances, wall thicknesses, and technical requirements for Stub Ends.
Size Specifications
The ASME B16.9 standard clearly defines the dimensional specifications for Stub Ends, covering the common range from NPS 1/2 to NPS 24.
Nominal Pipe Size (NPS)
- Standard Range: NPS 1/2 (DN15) to NPS 24 (DN600). This is the most common size range in industrial applications.
- Large Sizes: For Stub Ends NPS 26 and larger, the ASME B16.9 standard typically does not apply. In such cases, ASME B16.48 (Steel Line Blanks and Stub Ends) or custom designs by manufacturers based on specific project requirements should be referenced.
Wall Thickness (Schedule)
The wall thickness of the Stub End must match the connected pipe. ASME B16.9 requires the wall thickness to conform to ASME B36.10M (for carbon steel) or ASME B36.19M (for stainless steel). Common wall thickness schedules include:
- Sch 10S / Sch 40S: Commonly used for thin-wall stainless steel lines.
- Sch 40 / Sch 80: The most frequently used wall thicknesses for carbon and alloy steel.
- Sch 160 / XXS: Used for high-pressure service conditions.
Material Selection Range
ASME B16.9 is a dimensional standard and does not restrict materials, but it mandates that materials comply with corresponding ASTM standards. Due to its unique advantages in connecting, stub ends offer great flexibility in material selection, primarily depending on the corrosiveness, temperature, and pressure of the conveyed medium.
Carbon Steel
Common Grades: ASTM A234 Grade WPB (the most common carbon steel grade for welded fittings).
Application: Suitable for general oil, gas, water, and other non-corrosive or mildly corrosive media.
ASTM A234 WPB Chemical Composition (wt%)
Stainless Steel
Common Grades: ASTM A403 Grade WP304/304L, WP316/316L.
Application: Widely used in chemical, food, pharmaceutical, and other industries requiring corrosion resistance. Using a Stub End + Carbon Steel Lap Joint Flange combination significantly saves alloy material costs.
| Grade WP | UNS | C | Mn | P | S | Si | Ni | Cr | Mo |
|---|---|---|---|---|---|---|---|---|---|
| WP304 | S30400 | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 8.0–11.0 | 18.0–20.0 | — |
| WP304L | S30403 | 0.03 | 2.00 | 0.045 | 0.030 | 1.00 | 8.0–12.0 | 18.0–20.0 | — |
| WP316 | S31600 | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 10.0–14.0 | 16.0–18.0 | 2.00–3.00 |
| WP316L | S31603 | 0.03 | 2.00 | 0.045 | 0.030 | 1.00 | 10.0–14.0 | 16.0–18.0 | 2.00–3.00 |
Alloy Steel
Common Grades: ASTM A234 Grade WP11, WP22, WP91, etc.
Application: Used in high-temperature and high-pressure environments, such as power plants and refinery high-temperature lines.
| Grade | WP11 CL1 | WP11 CL2/WP11 CL3 | WP22 CL1/WP22 CL3 | WP91 Type 1 | WP91 Type 2 |
|---|---|---|---|---|---|
| C | 0.05–0.15 | 0.05–0.20 | 0.05–0.15 | 0.08–0.12 | 0.08–0.12 |
| Mn | 0.30–0.60 | 0.30–0.80 | 0.30–0.60 | 0.30–0.60 | 0.30–0.50 |
| P | 0.030 | 0.040 | 0.040 | 0.020 | 0.02 |
| S | 0.030 | 0.040 | 0.040 | 0.010 | 0.005 |
| Cr | 1.00–1.50 | 1.00–1.50 | 1.90–2.60 | 8.0–9.5 | 8.0–9.5 |
| Mo | 0.44–0.65 | 0.44–0.65 | 0.87–1.13 | 0.85–1.05 | 0.85–1.05 |
Duplex/Super Duplex Steel
Common Grades: ASTM A815 Grade UNS S31803, S32205, S32750.
Application: Used in highly corrosive conditions involving chloride environments, such as offshore platforms.
| Grade | WPS31803 | WPS32750 | WPS32205 |
|---|---|---|---|
| UNS | S31803 | S32750 | S32205 |
| C, max | 0.03 | 0.03 | 0.03 |
| Mn | 2.00 | 1.20 | 2.00 |
| Cr | 21.0–23.0 | 24.0–26.0 | 22.0–23.0 |
| Mo | 2.5–3.5 | 3.0–5.0 | 3.0–3.5 |
Non-Ferrous Metals
Materials: Nickel alloys (e.g., Alloy 625, C-276), Titanium, Copper-Nickel alloys, etc.
Application: Extremely corrosive environments or special process requirements. Here, the cost-saving advantage of using a Stub End with a carbon steel flange is most prominent.
Type Classification: Long vs. Short
According to ASME B16.9, stub ends are primarily divided into two geometric shapes:
Long Stub End (Type A)
- Characteristic: Features a longer straight section (Lap Length).
- Advantage: Because the straight section is longer, when the lap joint flange is bolted tight, the flange does not directly contact the heat-affected zone of the pipe weld. This reduces stress concentration on the welded area, making the connection more reliable.
- Application: The standard choice for most industrial applications.
Short Stub End (Type B)
- Characteristic: Has a shorter straight section, with the lap closer to the welded end.
- Application: Typically used in space-constrained areas or with specific designs of cast flanges.
Additionally, based on the chamfer shape on the back side, they can be further classified, but the long and short types are the fundamental dimensional categories.
Dimensional Parameters and Minimum Wall Thickness
When selecting or designing, several critical dimensions require attention:
- A (Lap Length): The distance from the welding end to the base of the lap. This is the key dimension differentiating long and short types.
- B (Lap Outside Diameter): The outer diameter of the lap must match the facing of the mating lap joint flange and be slightly smaller than the bolt circle diameter to allow for rotation.
- F (Lap Thickness at Base): The thickness at the base of the lap must be sufficient to withstand the gasket compression force.
- G (Lap Sealing Face Diameter): The diameter of the annular surface that contacts the gasket.
- T (Minimum Wall Thickness):
The minimum wall thickness of a Stub End is not a fixed number but refers to the wall thickness of its straight section barrel. ASME B16.9 stipulates that the minimum wall thickness of the fitting body shall not be less than 87.5% of the nominal wall thickness.
This means if you order a Sch 40S Stub End, the finished products minimum wall thickness may have a 12.5% negative tolerance. However, to meet pressure ratings, it must match the pipe of the same schedule.
Thin-Wall Fabrication: For very thin-wall Stub Ends (e.g., Sch 10S), special attention is needed during the forming process to prevent cracking at the lap base. Manufacturers typically form these via cold or hot working methods and ensure the final minimum wall thickness meets service requirements.
ASME B16.9 Stub End
| NPS | O.D. of Barrel (Max) | O.D. of Barrel (Min) | Length, F (Long Pattern) | Length, F (Short Pattern) | Radius of Fillet, R | Diameter of Lap, G |
|---|---|---|---|---|---|---|
| ½ | 22.8 (0.896) | 20.5 (0.809) | 76 (3.00) | 51 (2.00) | 3 (0.12) | 35 (1.38) |
| ¾ | 28.1 (1.106) | 25.9 (1.019) | 76 (3.00) | 51 (2.00) | 3 (0.12) | 43 (1.69) |
| 1 | 35.0 (1.376) | 32.6 (1.284) | 102 (4.00) | 51 (2.00) | 3 (0.12) | 51 (2.00) |
| 2 | 62.4 (2.456) | 59.5 (2.344) | 152 (6.00) | 64 (2.50) | 8 (0.31) | 92 (3.62) |
| 4 | 116.7 (4.593) | 113.5 (4.469) | 152 (6.00) | 76 (3.00) | 11 (0.44) | 157 (6.19) |
| 12 | 328.0 (12.913) | 323.1 (12.719) | 254 (10.00) | 152 (6.00) | 13 (0.50) | 381 (15.00) |
| 24 | 616.0 (24.240) | 609.0 (23.969) | 305 (12.00) | 152 (6.00) | 13 (0.50) | 692 (27.25) |
Unit: in. (mm)
Why Choose a Stub End?
Compared to using a standard welding neck flange directly, the Stub End + Lap Joint Flange combination offers irreplaceable advantages:
Typical Application Scenarios
- Stainless Steel or High-Alloy Lines: Utilizing carbon steel lap joint flanges to reduce costs.
- Lines Requiring Frequent Cleaning: Such as in food and pharmaceutical industries, allowing easy dismantling for cleaning.
- Equipment Connections: Connecting to pumps, valves, vessels, etc., that require frequent maintenance or removal.
- Lined Piping: In plastic-lined or rubber-lined pipes, the Stub End often serves as a transition piece, protecting the lining layer.
Installation and Mating Considerations
- Flange Matching: Ensure the contour of the Stub Ends back (lap) matches the internal chamfer of the lap joint flange to guarantee uniform contact and load distribution.
- Gasket Selection: The sealing face is typically a Raised Face; choose an appropriate gasket for sealing.
- Butt Weld Quality: The welding end must be welded strictly according to the Welding Procedure Specification (WPS), ensuring proper alignment with the inner wall of the main pipe to avoid weld spatter or misalignment.