RFQ Today
Certifications: EN 10204 3.1 / 3.2 MTRs, NACE MR0175 compliance, PWHT records, Third-Party Inspection (SGS / BV / DNV / Lloyds), and complete EPC export documentation packages.
Stud
Ends
A world-class technical reference for EPC piping engineers, procurement heads, TPI inspection agencies, and global project buyers specifying stud ends — lap-joint stub ends, loose-flange assemblies, and weld-neck stud end pipe terminations used throughout process piping to achieve the material economy of a carbon steel loose backing flange with a corrosion-resistant stainless or alloy stud end lap, to allow free rotation of the flange bolt holes for alignment, and to enable fast disassembly of corrosion-resistant piping systems for maintenance without disturbing the pipe spool welds.
Type Classification
& Material Economy
Stud ends — also called stub ends or lap-joint ends — are pipe fitting end preparations that create a lap-joint flange connection: the stud end provides the pipe connection weld and the sealing face, while a separate loose (backing) flange rotates freely around the pipe to align bolt holes without rotating the pipe spool itself.
1.1 — Technical Definition and Engineering Context
A stud end (stub end) is a short pipe fitting with a butt-weld inlet end (matching the pipe schedule and bore) and a lapped (enlarged diameter) outlet face — the lap provides the sealing face for a flat-face or raised-face gasket and serves as the retaining shoulder for the loose backing flange. The assembly — stud end welded to the pipe end, loose flange sliding freely on the pipe — is the lap-joint flange connection defined in ASME B16.5 and constitutes one of the five standard flange face types alongside weld-neck, slip-on, socket-weld, and blind flanges.
The engineering and economic rationale for specifying stud ends is threefold: (1) Material economy: The only pressure-retaining component at the flange joint is the stud end lap and the pipe — the loose backing flange carries no internal pressure load. This allows the loose flange to be manufactured from a lower cost material (carbon steel ASTM A105N or A181) even when the pipe and stud end are manufactured from expensive alloy materials (SS 316L, Duplex, Inconel, titanium) — the carbon steel flange performs the structural clamping function while the alloy stud end provides the corrosion-resistant sealing face. On large SS piping systems, this material split can reduce flange procurement cost by 60–80% compared to all-SS weld-neck flanges. (2) Bolt hole rotation: The loose backing flange rotates freely around the pipe — allowing perfect bolt hole alignment at the flanged joint without rotating the pipe spool. This is especially valuable in large-bore piping (NPS 12″ and above) where rotating a heavy pipe spool to align bolt holes is impractical. (3) Easy disassembly: Lap-joint connections can be opened for maintenance, inspection, or seal replacement without cutting or disturbing the pipe spool — the backing flanges slide back along the pipe when the bolts are removed, giving full access to the flange joint face.
1.2 — Stud End Type Classification
Type A Stud End (Long Pattern)
A stud end with a long overall length — the butt-weld end and the lap face are separated by a sufficient pipe-equivalent length that the fitting conforms to ASME B16.9 face-to-end dimensions for butt-weld fittings. The long pattern Type A stud end can be used as a pipe spool component with the lap facing any pipe length — the fitting length equals the ASME B16.9 face-to-end dimension for the corresponding 90° elbow. Type A is the standard for EPC process piping project supply where the piping specification references ASME B16.9 for wrought butt-weld fittings. Available in NPS ½”–24″ and larger; schedule-matched to the pipe; all ASTM A403 grades.
Type B Stud End (Short Pattern)
A stud end with a shorter overall length — the butt-weld end is closer to the lap face. The short pattern Type B stud end is used where the piping layout is congested and the minimum pipe spool length from the flange face to the nearest weld must be minimised. Type B dimensions per MSS SP-43 (Wrought and Fabricated Butt-Welding Fittings for Low Pressure Applications) — typically used for lower-pressure applications (ASME Class 150# and 300#) where the reduced body length is structurally adequate. Not interchangeable with Type A in high-pressure piping systems where ASME B16.9 dimensions are mandatory — verify the applicable fitting standard on the project piping specification before ordering.
Lap-Joint Backing Flange (Loose Flange)
The separate sliding flange that sits behind the stud end lap face — manufactured from carbon steel (ASTM A105N or A181 Gr.II) regardless of the pipe and stud end material in most applications. The backing flange has a bore slightly larger than the pipe OD to allow free rotation. It is not welded to the pipe — it slides freely. The backing flange bolt hole circle, bolt hole diameter, and raised face (or flat face) dimensions match the applicable ASME B16.5 or B16.47 pressure class for the nominal pipe size. For EPC supply: the backing flange is specified separately from the stud end — provide the NPS, pressure class (150#, 300#, 600#, etc.), and backing flange material when ordering.
Long Radius Elbow with Lap-Joint End
A standard long-radius (1.5D) or short-radius (1.0D) butt-weld elbow with one or both ends prepared as a stud end (lapped) rather than a standard butt-weld bevel. Used where a direction change and a flanged connection coincide at the same fitting location — eliminates a separate stud end fitting and one butt weld in the piping layout. The lapped elbow end serves as both the directional change and the flange connection point. Less common than straight stub ends but valuable in congested piping arrangements. Available in all ASTM A403 grades and schedules. Per ASME B16.9 with the lapped end profile per ASME B16.5 Appendix E.
Reducing Stud End
A stud end with a different bore diameter at the butt-weld inlet end compared to the lap face outside diameter — used to transition between two pipe schedules at a flanged connection without a separate reducer fitting. The reducing stud end achieves both the schedule/diameter transition and the lap-joint flange connection in a single fitting. Custom-manufactured to project drawing dimensions — not in standard ASME B16.9 tables. Specified when: the pipe schedule changes at a flanged break point (the upstream pipe is heavy-wall high-pressure; the downstream pipe is standard wall); or when a pipe-size reduction occurs at a flanged equipment nozzle where the equipment nozzle NPS differs from the connecting pipe NPS.
Stub End with Raised Face (RF) or Flat Face (FF)
Stud ends are available with raised face (RF) or flat face (FF) lap profiles. Raised face: the lap sealing surface is raised 1.6 mm (Class 150#/300#) or 6.4 mm (Class 600# and above) above the backing flange contact face — the standard for spiral-wound gasket or ring-type joint flange connections. Flat face: the entire lap face is flat — used for connections to cast iron or ductile iron equipment flanges where a flat-face gasket is required to prevent the brittle equipment flange from cracking under the uneven bearing force of a raised-face connection. Verify the flange facing type specified in the project piping class before ordering stud ends — RF and FF are not interchangeable in service.
1.3 — Lap-Joint vs Weld-Neck Flange: Engineering Comparison
| Parameter | Lap-Joint (Stud End + Backing Flange) | Weld-Neck Flange | Engineering Significance |
|---|---|---|---|
| Flange material | Carbon steel (backing) + alloy (stud end) | Must match pipe material throughout | 60–80% flange cost saving on SS/alloy piping |
| Bolt hole alignment | Free rotation — always alignable | Fixed to pipe — must rotate spool | Lap-joint invaluable on large-bore and tight spaces |
| Disassembly | Flanges slide back — full access | Flanges stay at pipe end | Lap-joint preferred for frequent maintenance |
| Pressure integrity | Stud end + pipe carries pressure | Flange + pipe carries pressure | Both fully code-compliant; weld-neck slightly stiffer |
| Fatigue / vibration | Lower (no hub stress concentration) | Higher (tapered hub SIF) | Lap-joint better for vibrating equipment connections |
| High-pressure service | Up to ASME Class 2500# | Up to ASME Class 2500# | Equivalent pressure ratings with correct backing flange |
| Bore match to pipe | Exact bore match (same schedule) | May have hub bore taper | Lap-joint provides smooth bore continuity for clean-in-place |
| NDE of weld | BW to pipe — standard RT/UT | BW to pipe — standard RT/UT | Identical NDE accessibility for both types |
1.4 — Material Economy Calculation
C_LJ = Cost of stud end (alloy) + backing flange (carbon steel A105N)
Example — 6″ NPS ASME Class 300# flange assembly, SS 316L vs lap-joint:
Option A: SS 316L weld-neck flange (ASTM A182 F316L, ASME B16.5 Class 300#): C_WN = 100 (index)
Option B: SS 316L stud end (ASTM A403 WP316L) + A105N carbon steel backing flange (ASME B16.5 Class 300#):
SS 316L stud end cost ≈ 20% of the full SS weld-neck flange (short length fitting vs heavy forging)
A105N backing flange cost ≈ 12% of the SS weld-neck flange (CS material vs SS)
C_LJ total ≈ 32 vs C_WN = 100 → Saving ≈ 68% per flanged connection
On a large SS 316L piping system with 500 flanged connections: 500 × (100 − 32) = 34,000 index units saved — a major procurement cost reduction on alloy piping systems.
(1) Services above ASME Class 600# where the additional stiffness of the weld-neck hub provides better joint integrity under high bolt loading
(2) Piping classes that explicitly exclude lap-joint connections (some company piping standards prohibit lap-joint on high-pressure or toxic services)
(3) Where the process fluid is so corrosive that even the external surface of the backing flange (carbon steel) would be attacked — use all-alloy weld-neck flanges
(4) Where the crevice between the stud end lap face and the backing flange bore is unacceptable for the service fluid (crevice corrosion risk in concentrated chloride service)
Submit your NPS, pressure class, pipe schedule, material, and quantity for a documented RFQ within 24 hours.
Pressure Classes
& Standards Compliance
Stud end lap dimensions — lap OD, lap height, bore ID, and overall length — are governed by ASME B16.9 (Type A) and MSS SP-43 (Type B). Lap face dimensions must match the ASME B16.5 backing flange bore for the applicable pressure class. All applicable standards are supported at RR Hydraulic.
Submit NPS, pressure class, pipe schedule, facing type, material, and quantity to sales@rrhydraulics.com for a certified offer.
2.1 — ASME B16.9 Type A Stud End Key Dimensions
| NPS | Pipe OD (mm) | Lap OD (mm) | Lap Height (mm) | Overall Length (mm) | Bore (Sch 40S) | Max Pressure Class |
|---|---|---|---|---|---|---|
| ½” | 21.3 | 34.9 | 6.4 | 38 | 15.8 | ASME Cl. 2500# |
| ¾” | 26.7 | 42.9 | 6.4 | 38 | 20.9 | ASME Cl. 2500# |
| 1″ | 33.4 | 50.8 | 6.4 | 51 | 26.6 | ASME Cl. 2500# |
| 1½” | 48.3 | 73.0 | 6.4 | 57 | 40.9 | ASME Cl. 2500# |
| 2″ | 60.3 | 92.1 | 6.4 | 64 | 52.5 | ASME Cl. 2500# |
| 3″ | 88.9 | 127.0 | 6.4 | 76 | 77.9 | ASME Cl. 1500# |
| 4″ | 114.3 | 157.2 | 9.5 | 86 | 102.3 | ASME Cl. 1500# |
| 6″ | 168.3 | 215.9 | 9.5 | 105 | 154.1 | ASME Cl. 900# |
| 8″ | 219.1 | 269.9 | 9.5 | 124 | 202.7 | ASME Cl. 900# |
| 10″ | 273.1 | 323.9 | 9.5 | 143 | 254.5 | ASME Cl. 600# |
| 12″ | 323.9 | 381.0 | 12.7 | 165 | 304.8 | ASME Cl. 600# |
| 16″ | 406.4 | 469.9 | 12.7 | 191 | 381.0 | ASME Cl. 300# |
| 24″ | 609.6 | 685.8 | 12.7 | 248 | 574.7 | ASME Cl. 150# |
2.2 — Applicable Standards and Compliance Framework
ASME B16.9
Factory-Made Wrought Buttwelding Fittings — the primary standard governing Type A stud ends (long pattern). B16.9 specifies the face-to-end (overall length), lap OD, and lap height dimensions for stub ends from NPS ½” through NPS 24″ (and by agreement to 48″). B16.9 also defines the butt-weld inlet bevel per ASME B16.25. The B16.9 face-to-end dimension for the stud end is identical to the corresponding elbow — this ensures that stud ends are dimensionally interchangeable with other butt-weld fittings in the piping layout without affecting spool length calculations. All ASME B31.3 process piping EPC projects specify stud ends to ASME B16.9 Type A for high-pressure (Class 600# and above) applications.
MSS SP-43
Wrought and Fabricated Butt-Welding Fittings for Low Pressure Applications — covers Type B stud ends (short pattern) for ASME Class 150# and 300# applications. MSS SP-43 provides reduced overall lengths compared to ASME B16.9 — acceptable for low-pressure service where the shorter fitting length does not compromise pressure integrity. Type B dimensions are widely used in SS and alloy piping for chemical and process applications at lower pressures. For EPC projects: always confirm with the project piping specification whether Type A (B16.9) or Type B (MSS SP-43) is required — the two types are not interchangeable and mixing them in a piping spool affects the spool face-to-face dimension.
ASME B16.5
Pipe Flanges and Flanged Fittings — governs the lap-joint backing flange dimensions for NPS ½”–24″. B16.5 Appendix E defines the lap joint interface dimensions: the backing flange bore (slightly larger than pipe OD to allow free rotation), the lap face OD (must match the stud end lap OD within tolerance), and the lap face height clearance. The lap OD of the stud end must conform to B16.5 Appendix E tolerance (±0.8 mm) to ensure that the backing flange freely rotates over the lap without binding and that the lap face projects into the gasket seating zone with the correct bearing load distribution. Stud ends with undersized lap OD will allow the gasket to be compressed unevenly by the backing flange; oversized lap OD prevents the backing flange from passing over the lap face.
ASTM A403
Wrought Austenitic Stainless Steel Piping Fittings — the primary material standard for SS stud ends. Covers WP304, WP304L, WP316, WP316L, WP321, WP347, and other austenitic grades in the W (wrought) and WX (wrought extra-strength) conditions. For EPC: WP316L is the standard for corrosive service stud ends (low carbon SS 316 — prevents sensitisation at the butt weld HAZ). The “WP” prefix indicates the fitting is made from wrought (not cast) material — wrought material has superior toughness, fatigue resistance, and microstructural consistency compared to cast equivalents. EN 10204 3.1 mandatory on all ASTM A403 stud end lots for EPC supply.
ASTM A234 / A420
ASTM A234: Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High Temperature Service — covers carbon steel (WPB Grade) and alloy steel stud ends (WP1, WP11, WP22, WP91). ASTM A420: Piping Fittings of Wrought Carbon Steel and Alloy Steel for Low-Temperature Service — covers low-temperature grades (WPL6 for −46°C). A234 WPB is the material standard for carbon steel stud ends in standard temperature service, and for the carbon steel backing flanges (per ASTM A105N for forged flanges or A234 WPB for wrought fittings). A234 WP11/WP22/WP91 stud ends for alloy steel piping systems at high temperature — same PWHT requirements as A182 F11/F22/F91 weldolet and fittings.
ASME B16.47
Large Diameter Steel Flanges — governs lap-joint backing flanges and stud end lap dimensions for NPS 26″–60″. Series A (formerly MSS SP-44) and Series B (formerly API 605) — both series are used in EPC large-bore piping depending on the applicable company or project piping standard. For stud ends on large-bore piping (NPS 26″ and above): the lap OD and lap height must be matched to the B16.47 Series A or Series B backing flange bore — not to the B16.5 dimensions (which cover only NPS ½”–24″). Specify the B16.47 series explicitly when ordering large-bore stud ends — “B16.47 Series A” and “B16.47 Series B” backing flanges have different bolt hole patterns for the same NPS and pressure class.
ASME B31.3
Process Piping — ASME B31.3 Paragraph 304.2.1 covers lap-joint flanges and stud ends as listed fittings under ASME B16.9. The lap-joint connection (stud end + backing flange) is fully code-compliant for all Normal Fluid service pressure classes. B31.3 Paragraph 304.5.1 notes that the backing flange does not require the same pressure rating as the stud end (the stud end carries the internal pressure), but the backing flange must be rated for the bolt preload and the external structural loads from the piping system. For high-cycle fatigue piping (severe cyclic service per B31.3): the lap-joint connection SIF may be lower than a weld-neck flange — the lap face reduces the bending moment transmitted to the pipe compared to the hub of a weld-neck flange.
EN 10253-4 / EN 13480
EN 10253-4: Butt-Welding Pipe Fittings — Part 4: Wrought Austenitic and Austenitic-Ferritic (Duplex) Stainless Steels with Specific Inspection Requirements. Governs SS and Duplex stud ends for European projects. EN 13480: Metallic Industrial Piping — Part 3 covers the design of lap-joint connections. For CE-marked piping systems under PED 2014/68/EU: stud ends (stub ends) must comply with EN 10253-4 for SS and Duplex grades, or EN 10253-2 for carbon and alloy steel, with the appropriate CE marking and EN 10204 3.1 / 3.2 material certification. ASTM A403 stud ends are accepted on EN 13480-governed projects with an ASTM-to-EN material equivalence statement reviewed by the project piping engineer.
Lap Face Machining
& Weld Requirements
Stud end material must match the connecting pipe material for the butt weld qualification — the same WPS, preheat, and PWHT requirements apply as for any butt-weld fitting of the same material. RR Hydraulic supplies stud ends in all standard ASTM grades with full EN 10204 3.1 / 3.2 forging heat traceability.
3.1 — Material Grade Reference for Stud Ends
| ASTM Grade | Material | Temp Range | NACE | Compatible Pipe | Service Application |
|---|---|---|---|---|---|
| A234 WPB | Carbon steel | −29 to +427°C | Cond. | A106 Gr.B; A53 Gr.B | General EPC; utilities; water; steam |
| A403 WP304L | SS 304L | −196 to +538°C | Good | A312 TP304L | Chemical; food; water treatment |
| A403 WP316L | SS 316L | −196 to +454°C | Very Good | A312 TP316L | Offshore; chloride; corrosive chemical; marine |
| A403 WP317L | SS 317L (higher Mo) | −196 to +454°C | Very Good | A312 TP317L | High-chloride; bleach; aggressive chemical |
| A403 WP321 | SS 321 (Ti-stabilised) | −196 to +870°C | Good | A312 TP321 | High-temp SS; sensitisation prevention in service |
| A403 WP347 | SS 347 (Nb-stabilised) | −196 to +870°C | Good | A312 TP347 | High-temp SS; stabilised weld HAZ |
| A815 WPS31803 | Duplex 2205 | −50 to +315°C | Very Good | A790 S31803 | Offshore sour+Cl⁻; seawater; aggressive chemical |
| A815 WPS32750 | Super Duplex 2507 | −50 to +260°C | Excellent | A790 S32750 | Extreme chloride; seawater injection; subsea |
| A234 WP11 | 1¼Cr-½Mo alloy | −29 to +593°C | No | A335 P11 | High-temp steam; reformer; FCC piping |
| A234 WP22 | 2¼Cr-1Mo alloy | −29 to +649°C | No | A335 P22 | Very high-temp; H₂ service; HHTHP |
| A234 WP91 | 9Cr-1Mo-V (P91) | −29 to +649°C | No | A335 P91 | USC steam; advanced power plant piping |
| A420 WPL6 | CS low-temp (CV tested) | −46 to +260°C | Cond. | A333 Gr.6 | Cryogenic CS; LPG; −46°C service |
3.2 — Lap Face Machining and Surface Finish
The lap face (the raised circular face of the stud end that contacts the gasket and the backing flange bore) is the most critical machined surface on the fitting — it is the sealing surface for the flanged joint. The lap face must be machined to: (1) the correct lap OD per ASME B16.5 / B16.47 ±0.8 mm tolerance, ensuring the backing flange can slide over the lap freely while the lap provides the correct load-bearing area for the gasket; (2) the correct lap height per ASME B16.9 tables — insufficient lap height allows the backing flange bore to bear on the pipe OD rather than on the lap face; (3) the correct surface finish for the specified gasket type — spiral-wound gaskets require a smooth phonographic finish (Ra 3.2– 6.3 µm, 125–250 AARH) on the raised face; metallic ring-type joint (RTJ) requires a smooth Ra ≤ 1.6 µm finish; PTFE full-face gaskets require a smooth Ra ≤ 3.2 µm finish to prevent gasket extrusion under bolt load.
- Lap OD verification: The lap OD is the dimension that determines whether the backing flange will slide over the lap face to its correct seated position — verify the lap OD against ASME B16.5 Appendix E before installing any stud end in the piping system. A lap OD that is too large will prevent the backing flange from passing over the lap — an error that requires the stud end to be removed and re-machined or replaced
- Lap face perpendicularity: The lap face must be machined perpendicular to the fitting bore axis to within 0.5° — a tilted lap face creates an uneven gasket load when the backing flange bolt is tightened, causing the gasket to fail on the low-load side of the joint. Perpendicularity verified by CMM on sampled lot
- Lap face surface finish: The raised face of the lap must be machined to the gasket manufacturer’s specified surface finish (phonographic serrated finish for spiral wound gaskets; smooth finish for soft gaskets and RTJ grooves). Ra measurement by contact profilometer on 100% of SS and alloy stud end lots where the lap face finish is specified in the project piping specification
- Bore match to pipe: The stud end bore ID must match the branch pipe bore ID within ASME B16.9 mismatch tolerance (±1.6 mm) — a bore mismatch greater than 1.6 mm requires a 1:4 transition taper per ASME B16.25. Verify the pipe schedule bore ID matches the stud end bore before welding — a mismatch that is only discovered after welding requires cutting out and replacing the stud end
3.3 — Butt Weld to Pipe and PWHT Requirements
Butt Weld Preparation and Root Pass
The stud end inlet end is bevelled per ASME B16.25 (standard 37.5° bevel, t ≤ 22 mm; compound bevel for heavier wall) matching the mating pipe end preparation. Root gap per WPS; full-penetration root pass using GTAW for SS, Duplex, and alloy steel stud ends. Back-purge with argon for all SS and Duplex root passes — the inside of the pipe and stud end bore is back-purged to prevent root oxidation that would create a corrosion-initiating oxide notch at the weld root inner surface. For large-bore stud ends (NPS 6″ and above): back-purge using an inflatable purge dam system positioned inside the pipe at the weld location; confirm purge gas purity (O₂ ≤ 50 ppm) before commencing root welding to prevent root oxidation.
Bore Alignment at Lap Face
The stud end must be aligned concentrically with the mating pipe end before tack welding — the bore centreline of the stud end must align with the bore centreline of the pipe within the weld fit-up root gap tolerance. Misalignment of the stud end relative to the pipe creates a stepped bore at the weld root that acts as a turbulence-generating step in the flow path (particularly significant in clean-in-place (CIP) piping systems for food and pharmaceutical applications) and a stress concentration in fatigue-sensitive systems. For large-bore stud ends (NPS 10″ and above): internal alignment clamps (spider dogs) are used to hold the stud end concentrically with the pipe bore during tack welding — the clamp is removed after tack welding before the root pass begins.
Lap Face Protection During Welding
The lap face (gasket contact surface) must be protected from weld spatter and arc strikes during the pipe butt weld — weld spatter on the lap face creates hard raised deposits that prevent proper gasket seating and cause gasket leaks at the flanged joint. Protection methods: wrap the lap face with aluminium foil or heat-resistant tape before welding; position the weld joint so that the welder is working away from the lap face; use spatter-release compound on the lap face before welding commences. After welding and PWHT: inspect the lap face for spatter, arc strikes, and discolouration — clean by light grinding with fine abrasive paper followed by passivation if the fitting is SS.
PWHT for Alloy Stud Ends (WP11 / WP22 / WP91)
ASTM A234 WP11 (1¼Cr-½Mo), WP22 (2¼Cr-1Mo), and WP91 (9Cr-1Mo-V) stud ends require the same PWHT as the equivalent A182 F-grade fittings and the pipe material after butt welding: WP11 (P-No. 5A): 595–725°C × time per B31.3 Table 331.1.1; WP22 (P-No. 5B, Gr.1): 675–750°C; WP91 (P-No. 5C, Gr.1): 760–790°C. The PWHT must cover the butt weld, the weld HAZ, and a sufficient length of the stud end body to relieve all welding-induced residual stresses. Lap face protection during PWHT: the lap face must not be mechanically distorted by the PWHT fixture or thermal gradient — support the stud end horizontally with the lap face clear of any fixture contact during the PWHT soak cycle.
Duplex 2205 / Super Duplex Stud End Welding
ASTM A815 WPS31803 (Duplex 2205) and WPS32750 (Super Duplex 2507) stud end butt welds require the same controlled welding conditions as any Duplex piping butt weld: heat input 0.5–2.0 kJ/mm; max interpass temperature 150°C; over-alloyed filler (ER2209 for 2205; ER2594 for 2507); 100% Ar or 98% Ar/2% N₂ back-purge for root passes. The stud end lap face should be masked from argon purge gas contact during root welding — purge gas condensation on the cold lap face can cause moisture contamination of the weld area. Post-weld ferrite count on the butt weld cross-section from the weld procedure qualification record to verify correct 40–60% ferrite balance.
Crevice at Lap-Backing Flange Interface
The annular crevice between the stud end lap face OD and the backing flange bore — and the crevice on the back of the lap face between the lap OD and the pipe OD — must be assessed for crevice corrosion risk in aggressive service environments. For concentrated chloride service (seawater, offshore process, chloride chemical): the crevice at the back of the stud end lap (between the lap underside and the pipe OD) can concentrate chloride and create crevice corrosion even on Duplex SS. This is the primary reason why some offshore company piping standards prohibit lap-joint connections in high-chloride or NACE sour service piping — the piping engineer must evaluate the crevice corrosion risk explicitly for these services before specifying stud ends.
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from certified pipe or plate stock to final inspected and packed stud end shipment. Lap OD verification, lap height, bore measurement, lap face finish, material certification, and complete EPC export documentation packages are standard on all project-grade stud end supply.
4.1 — Inspection & QC Protocol
4.2 — EN 10204 Material Test Certificate Requirements
| Certificate | Content | EPC Requirement | When Mandatory |
|---|---|---|---|
| 2.1 / 2.2 | Declaration / non-specific | Not acceptable for EPC process piping | Never for ASME B31.3 / B31.1 stud ends |
| 3.1 | Heat-traceable mech + chem | Mandatory — all EPC stud ends | All process, power, chemical, and offshore stud ends |
| 3.2 | 3.1 + TPI countersign | Offshore critical; NACE; nuclear; Category M | NACE sour; offshore safety-critical; nuclear; lethal service |
4.3 — Applications by Industry
SS 316L Piping with Carbon Steel Backing Flanges
The dominant stud end application in EPC process plants — ASTM A403 WP316L stud ends welded to ASTM A312 TP316L pipe, with ASTM A105N carbon steel ASME B16.5 backing flanges on all flanged connections. This combination delivers the full corrosion resistance of SS 316L at every pressure-boundary surface (pipe bore, weld, and stud end lap face) while using cost-effective carbon steel for the non-pressure-bearing backing flange structural function. On a large petrochemical facility with 2,000 SS piping flanged connections, the stud end / backing flange combination vs all-SS weld-neck flanges saves millions in flange material procurement cost. EN 10204 3.1 on all WP316L stud end lots; passivation per ASTM A967; PMI on 100% of SS lots.
Duplex 2205 / Super Duplex Offshore Process Piping
ASTM A815 WPS31803 (Duplex 2205) stud ends on offshore platform produced water, sour crude, and chemical injection piping with carbon steel backing flanges — achieving the full corrosion resistance of Duplex 2205 at every pressure boundary surface (most critical: the lap face in contact with the process gasket and the pipe bore in contact with the process fluid) while using cost-effective carbon steel for the backing flange. Note: for offshore piping classes that prohibit lap-joint connections in sour or seawater service (due to crevice corrosion concern at the lap back face), weld-neck Duplex 2205 flanges are specified instead. Verify the project offshore piping specification before ordering Duplex stud ends for applications where crevice corrosion is a concern.
Food, Pharmaceutical, and CIP Hygienic Piping
ASTM A403 WP316L stud ends with electropolished lap faces (Ra ≤ 0.4 µm) for hygienic flanged piping connections in food processing, beverage production, pharmaceutical manufacturing, and ultra-pure water systems. The smooth bore and smooth lap face of the stud end provide a crevice-free, cleanable internal surface — essential for CIP (clean-in-place) systems where the piping interior must be cleaned by circulating CIP solution without disassembly. The free-rotation backing flange allows easy alignment of flange joints without rotating piping spools during maintenance. EN 10204 3.1; FDA 21 CFR 177.2800 material compliance for direct food contact SS 316L fittings; electropolish certificate with Ra measurement.
Large-Bore Alloy Piping — Bolt Hole Alignment
WP316L, WP22, or WP91 stud ends on large-bore alloy piping (NPS 12″–24″) where rotating heavy pipe spools for bolt hole alignment is impractical during installation. Large pipe spools (NPS 16″ SS 316L, schedule 10S, 6 metres long) weigh 300–800 kg — aligning bolt holes by rotating the spool requires a crane and is potentially unsafe. The freely rotating backing flange of the lap-joint connection allows the pipe fitter to align all bolt holes from the backing flange alone without touching the pipe spool. This installation advantage is particularly significant on alloy piping systems (where pipe spool weights are higher than carbon steel equivalents of the same OD and length) and in confined offshore platform piping areas.
P91 / WP91 Steam Piping Stud Ends
ASTM A234 WP91 (9Cr-1Mo-V) stud ends for flanged connections on ultra-supercritical (USC) main steam and hot reheat piping in advanced power plants. WP91 stud ends provide matching creep-strength properties to the A335 P91 pipe at the flanged joint — using a lower-alloy stud end (WP22 or WP11) on a P91 pipe creates a creep-strength mismatch at the flanged connection that accelerates thermal fatigue damage at the dissimilar material interface. WP91 stud ends: full ASME IX WPS qualification matching P91 grade; PWHT 760–790°C × time; post-PWHT hardness survey on all weld HAZs; EN 10204 3.1 mandatory; 100% RT of butt welds. Lap face protected during PWHT to prevent distortion.
Alloy Piping Systems — Economy at Scale
The most compelling economic argument for stud ends arises on EPC projects with large quantities of alloy piping flanges — Titanium Grade 2, Hastelloy C276, Inconel 625, or Duplex 2507 piping systems where weld-neck flanges in the same material would cost 5–10× the cost of the equivalent carbon steel carbon flanges. In these ultra-high-cost alloy systems: titanium, Hastelloy, or Inconel stud ends with carbon steel backing flanges reduce the overall flanged joint cost by 70–85% compared to all-alloy weld-neck flanges. This material split is structurally and code-compliant — the only pressure-retaining alloy required is the stud end lap and the pipe bore. This makes stud ends the only economically viable flanged connection solution for exotic alloy piping at industrial project scale.
4.4 — Export Packaging Specification
- Stud ends individually wrapped — the lap face (gasket seating surface) is the most critical surface on the fitting and must arrive at the fabrication shop free of rust, scratches, and mechanical damage that would compromise gasket seating. Individually wrap each stud end in VCI poly for carbon steel; clean polyethylene sleeve for SS and Duplex; foam disc over the lap face for protection during transit
- Lap face protected from contact with other fittings — never bulk-stack stud ends with the lap face of one fitting resting against the pipe OD of another; the resulting contact mark on the lap face creates a leak path at the flanged joint. Individual cell packaging in cardboard trays for small-bore stud ends; individual foam-wrapped stacking for large-bore
- Heat number marked on every stud end per ASME B16.9 — the traceability link to the EN 10204 3.1 MTC. For alloy stud ends (WP316L, WP22, WP91): the heat number is the only field-visible identification that distinguishes the material from other alloys of identical appearance
- SS and Duplex stud ends in dedicated SS-labelled polybags; segregated from carbon steel hardware; iron contamination on the passivated SS lap face causes corrosion pitting that compromises the gasket seating surface and the corrosion resistance at the critical sealing interface
- Backing flanges (if supplied as part of the stud end assembly package) packed separately and clearly labelled — Carbon Steel / A105N / ASME Class 150#-300#-600# etc.; backing flanges are bolt-hole-circle matched to the stud end pressure class; mixing classes creates a non-compliant assembly
- ISPM-15 timber crates or export cartons; desiccant sachets for ocean freight; documentation in waterproof pocket: EN 10204 3.1/3.2 MTC, mechanical test certificate (including Charpy for cryogenic grades), lap OD inspection report (100%), lap face Ra certificate, perpendicularity report (sampled), PMI report (SS/Duplex/alloy), ferrite count (Duplex/Super Duplex), passivation certificate (SS), FAI report, and backing flange certificates (if included)
4.5 — Complete EPC Project Documentation Package
| # | Document | Standard / Format | Mandatory / Conditional | Notes |
|---|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | Mandatory — all EPC stud ends | Heat-traceable; one MTC per material heat lot |
| 02 | Chemical Composition Report | Certified lab per ASTM A403 / A234 / A815 | Mandatory | All alloying elements; Cr/Ni/Mo/C limits per grade |
| 03 | Mechanical Properties Report | UTS, yield, elongation, reduction of area | Mandatory | Per ASTM spec; one test per material heat |
| 04 | Charpy Impact Test Report | ASTM A370 at design min temp | Mandatory — WPL6; cryogenic SS; WP91 PWHT | Test temp; CVN J-values per material heat |
| 05 | Lap OD Inspection Report | ASME B16.5 Appendix E ±0.8 mm tolerance | Mandatory — 100% all EPC stud ends | Critical for backing flange assembly and gasket seating |
| 06 | Dimensional Inspection Report | Per ASME B16.9 / MSS SP-43 | Mandatory | Lap height, overall length, bore, OD at BW end |
| 07 | Lap Face Surface Finish Certificate | ISO 4287 Ra profilometer | Mandatory — SS / Duplex / alloy EPC stud ends | Ra per gasket type; phonographic pattern for SW gaskets |
| 08 | Lap Face Perpendicularity Report | CMM measurement ≤ 0.5° to bore axis | Mandatory — large bore (NPS ≥ 6″); SS; alloy | Sampled lot; large bore 100% |
| 09 | PMI Report (XRF) | Per lot — SS / Duplex / alloy steel | Mandatory — all non-CS lots; individual for NACE | WP316L vs WP304L; WPS31803 vs WPS32750 |
| 10 | Ferrite Content Report | ASTM E562 metallographic | Mandatory — WPS31803 Duplex; WPS32750 Super Duplex | 40–60% (2205); 40–50% (2507); photo of cross-section |
| 11 | Passivation Certificate | ASTM A967 | Mandatory — all SS and Duplex stud ends | Acceptance test confirms lap face passivated |
| 12 | NACE Compliance Statement | Hardness + heat treatment declaration | Conditional — sour service | Individual Brinell confirmation per piece where required |
| 13 | First Article Inspection (FAI) Report | Project-specific format | Mandatory — new configurations | All parameters including lap OD, Ra, perpendicularity |
| 14 | TPI Witness Certificate | SGS / BV / DNV / Lloyds | Conditional — EN 10204 3.2; offshore; nuclear | Co-witness dimensional + PMI + lap face Ra |
| 15 | ISO 9001:2015 Certificate | Third-party QMS certification | Mandatory — EPC projects | Scope covers stud end manufacture |
| 16 | Country of Origin + Packing List | Chamber of Commerce / item-level | Mandatory | HS tariff code; heat number per line item |
| 17 | Commercial Invoice + Bill of Lading | Per INCOTERMS 2020 | Mandatory | Freight forwarder issued |
4.6 — ISO and Quality System Compliance
ISO 9001:2015
Quality Management System covering pipe / forging material procurement and heat traceability, forming and CNC machining process qualification (lap OD machining per B16.5 tolerance; lap height; bore ID; overall length; bevel per B16.25), lap face surface finish process qualification (phonographic turning parameters; profilometer acceptance criteria), lap face perpendicularity measurement procedure (CMM setup, acceptance criteria, reporting), PMI procedure, ferrite count test procedure, passivation process control, and full material traceability from raw material heat to dispatched stud end. Mandatory for all EPC, offshore, chemical, and food/pharma stud end procurement qualification.
ASME B16.9 / B16.5 / B16.25
The three-standard framework for stud end manufacture and installation. ASME B16.9 governs the stud end body dimensions (Type A long pattern: face-to-end, lap OD, lap height, butt-weld inlet bevel); ASME B16.5 Appendix E governs the lap interface dimensions (the lap OD tolerance that ensures correct fit with the backing flange bore); ASME B16.25 governs the butt-weld inlet bevel preparation. All three must be simultaneously satisfied — a stud end with correct B16.9 lap OD but incorrect B16.5 lap OD tolerance will either not assemble with the backing flange or will not provide the correct gasket seating area. The procurement document must reference all three standards to create an unambiguous specification.
ASME B31.3 / B31.1
The governing piping design codes for stud end installation. ASME B31.3 Paragraph 304.2.1 accepts lap-joint connections as listed fittings when the stud end per ASME B16.9 and backing flange per ASME B16.5 are used. B31.3 NDE requirements for the butt weld between the stud end and the pipe are the same as for any other butt-weld fitting (RT/UT per Table 341.3.2 for the applicable service category). ASME B31.1 accepts lap-joint flanges for power piping subject to the same pressure-temperature limitations as other flange types. Stud ends on B31.1 piping: verify that the backing flange pressure-temperature rating per ASME B16.5 meets or exceeds the system design conditions at the flanged connection location.
EN 10253-4 / PED 2014/68/EU
EN 10253-4: Butt-Welding Pipe Fittings — Part 4 (Wrought Austenitic and Austenitic-Ferritic SS with Specific Inspection Requirements) governs SS and Duplex stub ends / stud ends on European CE-marked piping systems. The standard covers the material grades, dimensional tolerances, and mechanical testing requirements equivalent to ASTM A403 and A815 for European project supply. PED 2014/68/EU category applies to the piping system — for stud ends on Category III / IV pressure equipment: notified body approval is required. For NORSOK M-650 (Norwegian offshore qualification of manufacturers for special materials): ASTM A403 WP316L and A815 WPS31803 stud end manufacturers must demonstrate compliance with the relevant NORSOK MDS (Material Data Sheet) requirements for offshore piping fittings.
Submit your NPS, pipe schedule, material grade, lap facing type, and quantity to RR Hydraulic for a complete, certified commercial offer.