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.
Threadolets
A world-class technical reference for EPC piping engineers, procurement heads, TPI inspection agencies, and global project buyers specifying threadolets — integrally reinforced branch connection fittings that provide a threaded NPT or BSPT outlet on a run pipe, replacing the need for a branch tee, half-coupling weld, or set-on branch connection for small-bore instrument taps, drain points, vent connections, and chemical injection quill installations across Oil & Gas, Power Generation, Petrochemical, Offshore, and Industrial Piping systems.
Olet Family Classification
& Engineering Advantages
Threadolets are forged, integrally reinforced branch connection fittings that are saddle-welded to the outside of a run pipe to create a small-bore threaded branch outlet — providing a self-reinforcing connection that requires no additional reinforcement pad and no separate half-coupling or boss weld fitting for instrument tap and drain branch connections.
1.1 — Technical Definition and Engineering Rationale
A threadolet is a one-piece forged branch connection fitting that combines three functions in a single component: (1) a saddle-contoured base (the bore of the fitting base is contoured to match the OD of the run pipe — a full-encirclement saddle that provides intimate metal-to-metal contact over the full base circumference); (2) an integral reinforcement zone (the forged body provides the area replacement required by ASME B31.3 / ASME VIII Div.1 Paragraph UG-37 for the material removed by cutting the branch hole in the run pipe); and (3) a threaded outlet bore (NPT per ASME B1.20.1 or BSPT per ISO 7-1) that accepts a standard threaded pipe fitting, valve, instrument, or instrument root valve.
The engineering advantage of a threadolet over alternative small-bore branch connections is the elimination of the separate reinforcement pad — when a branch connection exceeds the “d/D” (branch-to-run diameter ratio) threshold at which the opening in the run pipe requires reinforcement per ASME B31.3, the required reinforcement area must be supplied either by a separate pad weld around the branch, a branch fitting (tee), or an integrally reinforced fitting. The threadolet provides the reinforcement intrinsically by its forged profile — no pad calculation, no pad weld, no additional inspection of a two-weld connection. This simplicity is why threadolets are the standard EPC industry solution for instrument taps, drain connections, vent connections, and small-bore process tie-ins on main pipe headers.
1.2 — The Olet Family Classification
Threadolet
A branch connection fitting with an NPT or BSPT internal thread outlet — provides a threaded connection for instruments (pressure gauges, transmitters, thermowells), instrument root valves, drain valves, vent valves, chemical injection quills, and small-bore process connections. Pressure classes 3000# and 6000# per ASME B16.11 / MSS SP-97. Branch thread sizes ½” to 2″ NPT/BSPT. Available for run pipe NPS ¾” through 24″ and larger. The most common olet type in EPC instrument piping — every instrument tap on a process pipe header in Oil & Gas, Power Generation, and Petrochemical facilities is typically made with a threadolet or sockolet. ASME B31.3 standard branch connection for threaded instrument branches.
Sockolet
A branch connection fitting with a socket-weld (SW) bore outlet — accepts a socket-weld pipe end inserted and fillet-welded into the socket. Provides a socket-weld small-bore branch connection directly on the run pipe. Used for: small-bore socket-weld piping branch connections (NPS ½” to NPS 2″ SW outlets); drain and vent connections where threaded connections are not specified in the project piping specification; and connections in services where threaded connections are restricted by the piping class (high-pressure steam, severe corrosive service). Available in the same pressure classes and run pipe sizes as threadolets. Per MSS SP-97 and ASME B16.11.
Weldolet
A branch connection fitting with a butt-weld (BW) outlet — the branch pipe end butt-welds to the weldolet outlet face. Provides a butt-weld branch connection on the run pipe without a tee fitting — used for larger-bore branch connections (typically NPS 2″ and above) and for all pressure classes where butt-weld connections are required by the piping class (typically ASME B31.3 Class 900# and above). Weldolets are available for branch/run combinations from NPS 1″ branch on NPS 2″ run through NPS 20″ branch on NPS 24″ run. The most structurally efficient branch connection type — the weldolet reinforcement profile is the standard against which other branch connections are measured for stress intensification factor (SIF) purposes.
Elbolet
A branch connection fitting that attaches to the intrados (inside radius) or extrados (outside radius) of an elbow rather than to a straight run pipe — provides a thermowell, drain, or instrument tap branch connection at the elbow tangent point where a standard threadolet or weldolet would conflict with the elbow geometry. Used at: thermowell locations on elbow piping where the piping layout requires the thermowell to be installed at the elbow rather than in the straight run; drain connections at the lowest point of an elbow system; and instrument tap connections where the straight pipe run is too short for a standard olet installation. Per MSS SP-97. Available in SW and threaded outlet ends.
Nipolet
An extended threadolet — a threadolet with an elongated threaded outlet pipe nipple integral to the fitting body, providing both the branch connection reinforcement and a short nipple in one forged component. The nipolet eliminates the need for a separate close nipple or short nipple threaded into the threadolet outlet — the integral extended nipple provides the required standout distance for the instrument root valve or vent cap to clear the run pipe insulation. Used in: insulated piping systems (the extended nipple provides sufficient standout for the instrument root valve to project through the insulation); instrument branch connections where the insulation thickness exceeds the standard threadolet height; and elevated temperature piping where thermal sleeve insulation requires additional branch standout.
Latrolet
A branch connection fitting for 45° lateral branch connections — the fitting base is contoured for 45° saddle-welding to the run pipe (vs the 90° saddle of standard threadolets and weldolets). Used where the process or instrument piping layout requires a 45° branch angle for: minimising the pressure drop at the branch point (45° branches have lower flow resistance than 90° branches in header manifolds); accommodating spatial constraints in congested piping areas; and sampling connections where a 45° branch orientation improves representative sampling of the flowing fluid. Available with threaded, socket-weld, or butt-weld outlet ends. The 45° saddle base contour requires a separate saddle profile cut from the run pipe — more difficult to weld than 90° standard olets.
1.3 — Threadolet vs Alternative Branch Connection Methods
| Method | Reinforcement | Welds Required | NDE Required | Code Acceptance | Preferred Use |
|---|---|---|---|---|---|
| Threadolet (MSS SP-97) | Integral (self-reinforcing) | 1 fillet weld | Visual + NDE per piping class | ASME B31.3 / B31.1 | Instrument taps; drain/vent; small-bore branches |
| Half-coupling weld-on | Separate pad required (>d/D limit) | 1–2 fillet welds | Visual + pad weld NDE | ASME B31.3 allowed | Low-pressure; non-critical instrument branches |
| Set-on boss (weld boss) | Limited reinforcement | 1 fillet weld | Visual | B31.3 (within limits) | Low-pressure; non-process drains |
| Branch tee (ASME B16.9) | Full tee reinforcement | 2 butt welds | Full BW NDE per class | ASME B31.3 standard | Large-bore; main branches; high-pressure |
| Stub-in welded branch | Separate pad required | 1 groove weld + pad | Full NDE | B31.3 with pad calc | Larger branches; no fittings available |
| Reinforced branch connection pad | Pad plate weld-on | 2–3 welds | Full NDE on all welds | ASME B31.3 App D | Intermediate bore where olet not available |
1.4 — Branch Reinforcement and Area Replacement Principles
t_h = Required run pipe wall thickness for pressure containment (mm) = P × D / (2 × S × E + P × Y)
d_1 = Effective branch bore diameter (mm) = branch bore ÷ sin β
β = Angle between the branch and the run pipe axis (90° for standard threadolets → sin β = 1.0)
Threadolet self-reinforcement principle:
MSS SP-97 threadolets are designed so that the forged body provides cross-sectional area equal to or greater than A_required for all listed run pipe and branch size combinations within the fitting’s rated pressure class. The engineer does not need to calculate A_required when using a listed MSS SP-97 threadolet — the fitting is pre-qualified by the standard for all tabulated run/branch size combinations.
For run pipe sizes and branch sizes outside the MSS SP-97 table: area replacement calculation per ASME B31.3 Paragraph 304.3.3 must be performed by the piping engineer to confirm that the selected threadolet provides adequate reinforcement for the specific run pipe schedule and material.
Run pipe: 6″ NPS, Schedule 40 (t = 7.11 mm), ASTM A106 Gr.B, design pressure 50 bar, temperature 150°C
Branch: 2″ NPT threadolet (Class 3000#), outlet for instrument root valve
MSS SP-97 lists: 2″ threadolet on 6″ pipe (within standard run pipe table) — reinforcement is pre-qualified
Action: select MSS SP-97 compliant 2″ × 6″ threadolet in ASTM A105N (matching run pipe carbon steel) → weld per WPS → no separate area calculation required.
Submit your run pipe size, branch size, pressure class, material, and quantity for a documented RFQ within 24 hours.
Size Reference
& Standards Compliance
Threadolet dimensions — saddle bore contour, body height, wall thickness, and outlet thread form — are governed by MSS SP-97, ASME B16.11, and the applicable pressure class (3000# or 6000#). All applicable standards are supported at RR Hydraulic with full certification.
Submit run pipe NPS, branch NPT size, pressure class, material, and quantity to sales@rrhydraulics.com for a certified offer.
2.1 — Threadolet Pressure Classes and Working Pressure Reference
| Pressure Class | ASTM A105N (Carbon Steel) | ASTM A182 F316L (SS 316) | ASTM A182 F51 (Duplex 2205) | ASTM A182 F11 (1¼Cr-½Mo) | Temperature Limit |
|---|---|---|---|---|---|
| 3000# (3M) | 515 bar / 7,500 psi | 260 bar / 3,780 psi | 450 bar / 6,525 psi | 515 bar / 7,500 psi | Per material ASME allowable stress table |
| 6000# (6M) | 1035 bar / 15,000 psi | 515 bar / 7,500 psi | 900 bar / 13,050 psi | 1035 bar / 15,000 psi | Per material ASME allowable stress table |
2.2 — Threadolet Size Availability (MSS SP-97)
| Branch (NPT / BSPT) | Minimum Run Pipe (NPS) | Maximum Run Pipe (NPS) | Available Pressure Classes | Material Grades Available |
|---|---|---|---|---|
| ½” | ¾” | 24″ | 3000# / 6000# | A105N, F304, F316, F316L, F51, F53, F11, F22, Inconel |
| ¾” | 1″ | 24″ | 3000# / 6000# | A105N, F304, F316, F316L, F51, F53, F11, F22, Inconel |
| 1″ | 1½” | 24″ | 3000# / 6000# | A105N, F304, F316, F316L, F51, F53, F11, F22, Inconel |
| 1½” | 2″ | 24″ | 3000# / 6000# | A105N, F304, F316, F316L, F51, F53, F11, F22 |
| 2″ | 3″ | 24″ | 3000# / 6000# | A105N, F304, F316, F316L, F51, F53, F11, F22 |
2.3 — Applicable Standards and Compliance Framework
MSS SP-97
Integrally Reinforced Socket Weld, Threaded, and Buttwelding Branch Outlet Fittings — Socket Welding, Threaded, and Buttwelding Ends. The primary standard governing threadolets, sockolets, weldolets, elbolets, nipolets, and latrolets. MSS SP-97 specifies: minimum wall thickness for the fitting body at each pressure class and size; the saddle bore contour requirements (bore must match the run pipe OD within stated tolerances to ensure full bearing contact for the fillet weld); outlet thread dimensions per ASME B1.20.1 (NPT) or ISO 7-1 (BSPT); and the marking requirements (nominal size, pressure class, material grade, heat number). ASME B16.11 cross-references MSS SP-97 for branch connection fittings in the forged fittings standard.
ASME B16.11
Forged Fittings, Socket-Welding and Threaded. Governs the pressure-temperature ratings, materials, dimensions, tolerances, marking, and testing of forged socket-weld and threaded fittings including threadolets. ASME B16.11 establishes the pressure class system (3000# and 6000# for threaded fittings) and the material groups (1.1 through 2.2) that determine the allowable working pressure at each temperature. For EPC projects governed by ASME B31.3 (Process Piping): all threadolets must comply with ASME B16.11 as the listed fitting standard — non-listed fittings require engineering qualification per B31.3 paragraph 304.7.2.
ASME B31.3
Process Piping — the governing piping design code for most EPC process plant piping systems. B31.3 Paragraph 304.3.3 covers the area replacement requirements for branch connections — the basis for specifying an integrally reinforced fitting (threadolet) vs a separate reinforcement pad. B31.3 Paragraph 304.7.2 governs unlisted fittings and requires that any fitting not manufactured per a listed standard be qualified by burst test or detailed stress analysis. All ASME B16.11 / MSS SP-97 threadolets are “listed fittings” per B31.3 — they do not require individual qualification beyond compliance with the manufacturing standard. Use of non-standard or non-listed threadolets on B31.3 piping requires the Responsible Engineer’s written concurrence per project piping specification.
ASTM A105N / A182
ASTM A105N: Forgings, Carbon Steel, for Piping Components — the material standard for carbon steel threadolets. The “N” suffix indicates normalised heat treatment, providing improved toughness and consistent mechanical properties vs non-normalised A105. ASTM A182: Forged or Rolled Alloy and Stainless Steel Pipe Flanges and Fittings — covers all alloy and SS threadolet materials: F304 (SS 304), F316 (SS 316), F316L (SS 316 low carbon), F51 (Duplex 2205), F53 (Super Duplex 2507), F11 (1¼Cr-½Mo), F22 (2¼Cr-1Mo), F5 (5Cr-½Mo), F9 (9Cr-1Mo), and F91 (9Cr-1Mo-V modified). EN 10204 3.1 on the forging heat certificate is mandatory for all EPC threadolet supply.
NACE MR0175 / ISO 15156
For threadolets in H₂S sour service piping: ASTM A105N carbon steel threadolets must be heat-treated to achieve hardness ≤ 22 HRC per NACE MR0175; the normalisation of A105N typically achieves this, but individual hardness verification per lot is mandatory for sour service supply. ASTM A182 F316L (SS 316L, ≤ 22 HRC) and F51 Duplex 2205 (≤ 28 HRC) are NACE-compliant for sour service. The fillet weld HAZ (heat-affected zone) of the threadolet-to-run-pipe weld must also comply with NACE MR0175 hardness limits — PWHT (post-weld heat treatment) may be required for A105N carbon steel threadolets on sour service piping to control HAZ hardness below 22 HRC.
ASME B1.20.1 (NPT)
Pipe Threads, General Purpose — governs the NPT (National Pipe Taper) thread form in threadolet outlets. NPT thread tolerance per ASME B1.20.1: thread pitch diameter measured by L1 taper ring gauge (for external NPT) or L1 plug gauge (for internal NPT — the thread inside the threadolet outlet). Internal NPT thread in the threadolet: 100% Go/No-Go L1 plug gauge on all threadolets for EPC supply. BSPT thread (ISO 7-1) is the alternative thread form for international (non-US-coded) EPC projects — verify the project piping specification for the required outlet thread form before ordering. NPT and BSPT are NOT interchangeable in the outlet thread — specify explicitly.
ASME B31.1 (Power Piping)
Power Piping — for threadolets in power generation piping systems (boiler feed water, steam, condensate, fuel oil). ASME B31.1 Paragraph 104.3.1 covers branch connections and references ASME B16.11 for forged branch connection fittings. B31.1 typically requires more conservative design margins and PWHT at lower thresholds than B31.3 — verify the applicable piping code on the project piping specification. For power generation EPC projects: ASME B31.1 applies to the power island piping; ASME B31.3 applies to the process and utility piping outside the power island boundary. Threadolets in B31.1 systems require PWHT per B31.1 Paragraph 132 where the run pipe requires PWHT.
EN 13480 / PED 2014/68/EU
EN 13480 (Metallic Industrial Piping) is the European equivalent of ASME B31.3 for CE-marked process piping in European projects. EN 13480 Part 4 (Fabrication and Installation) addresses branch connections and references EN 10253-4 (butt-welding pipe fittings) and manufacturer’s standards for integrally reinforced fittings equivalent to MSS SP-97. Threadolets for CE-marked piping systems under PED 2014/68/EU must comply with EN 13480 design requirements and must be supplied with an EN 10204 3.1 Declaration of Conformity. CE marking on the threadolet fitting is required for Category III and IV pressure equipment under PED — verify the PED category with the project pressure equipment engineer.
Forging Process
& Welding Requirements
Threadolet material must match or be compatible with the run pipe material for the weld qualification — mismatched materials require a dissimilar metal weld procedure qualification. RR Hydraulic supplies threadolets in all standard ASTM grades with full EN 10204 3.1 / 3.2 forging heat traceability.
3.1 — Material Grade Reference for Threadolets
| ASTM Grade | Material Type | Temp Range | NACE | Compatible Run Pipe | Application Service |
|---|---|---|---|---|---|
| A105N | Carbon steel (normalised) | −29°C to +427°C | Cond. (≤22 HRC) | ASTM A106 Gr.B; A53; A333 Gr.6 | General EPC process piping; utilities; steam |
| A182 F304 | SS 304 (A2) | −196°C to +538°C | Good | ASTM A312 TP304; A316 TP304 | General chemical; food; pharmaceutical |
| A182 F316L | SS 316L (A4) | −196°C to +454°C | Very Good | ASTM A312 TP316L; A358 TP316L | Corrosive chemical; offshore process; marine |
| A182 F51 | Duplex 2205 | −50°C to +315°C | Very Good | ASTM A790 S31803 | Offshore sour+Cl⁻; seawater; aggressive chemical |
| A182 F53 | Super Duplex 2507 | −50°C to +260°C | Excellent | ASTM A790 S32750 | Seawater injection; extreme chloride; offshore |
| A182 F11 | 1¼Cr-½Mo alloy | −29°C to +593°C | No | ASTM A335 P11; A691 Gr.1¼Cr | High-temp steam; reformer; power generation |
| A182 F22 | 2¼Cr-1Mo alloy | −29°C to +649°C | No | ASTM A335 P22; A691 Gr.2¼Cr | Very high-temp; hydrogen service; HHTHP |
| A182 F91 | 9Cr-1Mo-V (P91) | −29°C to +649°C | No | ASTM A335 P91 | Ultra-high-temp power; ultra-supercritical steam |
| A182 F347 | SS 347 (Nb-stabilised) | −196°C to +538°C | Good | ASTM A312 TP347 | High-temp SS; sensitisation prevention in service |
| Inconel 625 (N06625) | Ni-Cr-Mo superalloy | −196°C to +980°C | Excellent | ASTM B444 N06625 | Extreme corrosion + high-temp; subsea; acid |
3.2 — Manufacturing and Quality Control
3.2.1 — Forging Process
Threadolets are manufactured by hot forging — the billet of bar or bar-cut blank is heated to forging temperature (1100–1250°C for carbon steel; 1000–1150°C for SS grades) and pressed or hammered in a closed die to form the forged profile: the saddle base contour, the body wall, and the outlet boss. Forging produces a continuous grain flow through the fitting body that provides superior fatigue resistance compared to castings or machined fittings. The forged profile then undergoes heat treatment (normalise for A105N; solution anneal for SS grades; Q+T for alloy grades), followed by machining of the saddle bore contour (to match the specified run pipe OD), the outlet bore, and the NPT/BSPT thread.
3.2.2 — Saddle Bore Contour Machining
- Saddle bore radius: The inside bore radius of the threadolet base must match the outside radius of the run pipe OD — a threadolet ordered for 6″ NPS must have a saddle bore radius machined to match the 6.625″ OD of standard 6″ NPS pipe. If the wrong saddle radius is used, there will be gaps between the fitting base and the pipe surface, creating an incomplete root pass in the fillet weld and stress concentration at the root void
- Saddle bore tolerance: Per MSS SP-97, the saddle bore radius tolerance is ±0.5 mm — verified by radius gauge or CMM on sampled lot. A threadolet with a saddle bore undersized for the run pipe OD will rock on the pipe surface and cannot be properly tack-welded in position before root welding
- Contour grinding / CNC machining: The saddle bore is CNC-milled or ground from the forged profile — CNC machining provides the tighter saddle radius tolerances required for critical service (NACE, cryogenic, high-cycle fatigue) threadolet applications where voids at the root of the fillet weld are not acceptable
- Run pipe OD verification: Before ordering threadolets, verify the actual OD of the run pipe on site or from the pipe mill certificate — pipe manufactured to ASME B36.10M dimensional standard has a defined OD per NPS designation, but heavily corroded, used, or non-standard pipe may have a different OD than the nominal standard; a threadolet with the wrong saddle radius cannot be corrected in the field
3.2.3 — Welding Requirements and PWHT
Fillet Weld Design and Procedure
Threadolets are attached to the run pipe by a full-encirclement fillet weld around the base of the fitting. The fillet weld size is specified by the piping engineer — typically equal to the minimum wall thickness of the run pipe or the minimum wall thickness of the fitting base, whichever is lesser. The weld must be made per a qualified Welding Procedure Specification (WPS) that matches the base metal combination (run pipe material + threadolet material). For carbon steel threadolets on carbon steel run pipe: ASTM A105N threadolet with ASTM A106 Gr.B run pipe requires WPS qualification per ASME IX with matching P-Number grouping (both are P-No. 1 Group 1 — no dissimilar metal issue).
PWHT Requirements
Post-weld heat treatment (PWHT) is required when: (1) the run pipe requires PWHT per ASME B31.3 Table 331.1.1 based on material P-Number and wall thickness threshold; (2) the service is NACE sour per NACE MR0175 and the carbon steel weld HAZ hardness must be controlled to ≤ 22 HRC; or (3) the alloy steel run pipe (P5, P11, P22, P91) requires PWHT per the applicable code for the P-Number group. PWHT temperature range for carbon steel (P-No. 1): 595–700°C hold; alloy steels: per ASME B31.3 Table 331.1.1. PWHT must include the threadolet fitting — the fitting is in the heat-affected zone of the weld and must be heated uniformly to avoid thermal distortion and residual stress concentration.
NDE Requirements by Piping Class
Non-destructive examination requirements for threadolet fillet welds are specified by the project piping class in the piping specification: Normal Fluid Service (ASME B31.3 Category D/Normal): visual examination only on fillet welds (B31.3 Table 341.3.2). Severe Cyclic Service or Category M (toxic/lethal fluids): 100% radiographic or UT examination of all fillet welds is impractical — MT or PT on the completed fillet weld surface is the standard NDE for threaded branch connection fillet welds in Category M service. NACE sour service: Brinell hardness survey (BHN) on the weld HAZ at regular intervals around the fillet weld after welding and before PWHT, and after PWHT, to verify ≤ 22 HRC compliance.
Dissimilar Metal Weld Considerations
When the threadolet material differs from the run pipe material (dissimilar metal weld — DMW): a separate WPS qualification for the dissimilar combination is required per ASME IX. Common DMW combinations: F316L SS threadolet on A106 Gr.B carbon steel run pipe (for instrument taps on carbon steel headers with SS instrument tubing); F11 alloy threadolet on A335 P11 alloy run pipe (same material — not actually dissimilar); F51 Duplex threadolet on A312 TP316L SS run pipe (dissimilar microstructure — WPS qualification required). Dissimilar metal threadolet-to-run-pipe welds require particular attention to preheat and interpass temperature control to prevent hydrogen cracking at the carbon steel HAZ side of the weld.
P91 / F91 Special Welding Requirements
ASTM A182 F91 (9Cr-1Mo-V modified) threadolets on ASTM A335 P91 run pipe in ultra-supercritical steam piping require the most stringent welding controls of any standard EPC piping material: minimum preheat 200°C; maximum interpass temperature 300°C; immediate PWHT after welding to prevent martensite hardening and hydrogen cracking (P91 must not cool below 120°C before PWHT); PWHT at 760–790°C for 1 hour per 25 mm wall thickness; hardness survey post-PWHT (target 180–250 HBW for P91); independent third-party weld procedure qualification review by the Project Engineer before production welding commences. F91 threadolets are a critical weld item — incorrect PWHT or hardness outside the specified range is a code non-conformance requiring weld removal and replacement.
Duplex / Super Duplex Welding Controls
ASTM A182 F51 (Duplex 2205) and F53 (Super Duplex 2507) threadolets require controlled welding to maintain the correct ferrite-austenite balance in the weld and HAZ: heat input control (0.5–2.0 kJ/mm — too low creates excessive ferrite in the weld; too high causes sigma phase or austenite grain growth); preheat typically not required; maximum interpass temperature 150°C; solution annealing at 1020–1100°C (2205) or 1025–1125°C (2507) after welding in the workshop (field welded threadolets typically cannot be solution-annealed — verify with the piping engineer). Ferrite content verification per ASTM E562 (40–60% ferrite for 2205; 40–50% for 2507) on each welded threadolet lot is mandatory for offshore and sour service supply.
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from certified forging billets to final inspected and packed threadolet shipment. Dimensional inspection, thread gauging, saddle radius verification, hardness testing, material certification, and complete EPC export documentation packages are standard on all project-grade threadolet 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 piping threadolets |
| 3.1 | Forging heat-traceable mech + chem | Mandatory for all EPC threadolets | All process piping, utility, and offshore threadolets |
| 3.2 | 3.1 + TPI countersign | Offshore critical; NACE sour; nuclear; lethal service | NACE sour; offshore safety-critical; nuclear; Category M |
4.3 — Applications by Industry
Instrument Pressure Taps on Process Headers
The dominant threadolet application in EPC — ASTM A105N (Class 3000#) or A182 F316L (for corrosive services) ½” NPT threadolets welded to process pipe headers for pressure transmitter, pressure gauge, and pressure switch root valve connections. Every pressure tap on an EPC piping system (thousands on a large gas processing plant) uses a threadolet as the ASME B31.3 branch connection fitting. A105N is specified where the run pipe is ASTM A106 Gr.B carbon steel; F316L where the run pipe is stainless or the service is corrosive. The threadolet outlet threads directly into the ½” NPT root valve, which connects to the instrument impulse line. EN 10204 3.1 minimum; heat number marked on every fitting per MSS SP-97.
Offshore Platform Process Piping
F316L SS or F51 Duplex 2205 threadolets (Class 3000#, ½”–2″ NPT) for instrument taps, drain and vent connections on offshore platform process and utility piping. SS 316L (F316L) is specified where the run pipe is SS 316L for produced water, methanol injection, or chemical treatment piping. Duplex 2205 (F51) for threadolets on sour crude and wet gas piping with combined H₂S + chloride service conditions. All offshore threadolets: EN 10204 3.1 minimum; 3.2 with DNV or Lloyds countersign for safety-critical process piping; PMI on 100% of lots; ferrite count for Duplex lots; individual hardness for NACE lots. Threaded connection type per project piping specification — most offshore projects specify NPT per ASME B1.20.1.
High-Temperature Alloy Steel Power Piping
ASTM A182 F11 (1¼Cr-½Mo) or F22 (2¼Cr-1Mo) threadolets (Class 3000# or 6000#) for instrument taps on high-temperature steam and hot-oil piping in power generation plants, refineries, and petrochemical process units. F11 and F22 alloy threadolets require pre-heat before welding (typically 200°C for F11; 250°C for F22) and PWHT after welding per ASME B31.1 / B31.3 P-Number table. The PWHT requirement must be incorporated into the piping spool fabrication sequence — threadolet welding and PWHT must be done before any downstream thermowell, instrument, or valve is attached to the outlet thread. P91 (F91) threadolets for ultra-supercritical steam: full F91 welding qualification per EPRI TR-117073 guidance.
NACE Sour Service Drain and Vent Branches
ASTM A105N (normalised, ≤ 22 HRC individual hardness) threadolets for drain and vent connections on sour crude, H₂S-containing gas, and amine treating piping in NACE MR0175 sour service. Individual piece hardness verification (Brinell on every fitting) is mandatory — the normalisation of A105N typically achieves ≤ 22 HRC (≤ 237 HB), but individual pieces outside the normalisation furnace hot-spot may exceed the limit. PWHT of the threadolet fillet weld may also be required for sour service carbon steel piping — verify per the project NACE piping specification. EN 10204 3.2 with NACE compliance statement for all sour service threadolet supply; TPI co-sign for offshore sour service critical piping.
Chemical Injection Quill Branch Ports
F316L (SS 316L) or F51 Duplex 2205 threadolets (Class 3000#, 1″ or 1½” NPT) as the process pipe branch connection for chemical injection quill body installations — the chemical injection quill body threads into the threadolet NPT outlet and protrudes through the branch hole into the flowing process stream to inject corrosion inhibitor, scale inhibitor, hydrate inhibitor, or biocide into the centre of the flow. The threadolet branch bore must be clean, full-thread, and free of weld spatter inside the outlet bore (any restriction in the threadolet outlet bore causes turbulence at the quill entry point and reduces mixing effectiveness). 100% outlet bore visual inspection after welding and PWHT for chemical injection threadolet connections.
LNG and Cryogenic Piping Instrument Branches
ASTM A182 F304L or F316L (low-carbon austenitic SS) threadolets (Class 3000#) for instrument tap connections on LNG process and cryogenic utility piping at temperatures to −196°C. Austenitic SS (304L/316L) maintains adequate toughness at cryogenic temperatures without the impact testing required for carbon steel at these temperatures — ASTM A350 LF2 carbon steel with Charpy CVN testing at −46°C is the alternative for carbon steel LNG utility piping. The forging heat must be certified with impact test results at the design minimum temperature. No carbon steel threadolets on LNG cryogenic piping below −46°C unless A350 LF2 with impact certification is specified. Full EN 10204 3.1 with Charpy test results on the lot certificate.
4.4 — Export Packaging Specification
- Threadolets individually packed per combination (run pipe size × branch thread size × pressure class × material grade) in polybag or plastic box — never mix sizes or material grades; F316L and A105N threadolets of the same branch size are visually identical except for the material marking; mixing creates a critical material substitution risk on NACE and SS-specified piping systems
- Heat number marking on every threadolet per MSS SP-97 — the heat number is the traceability link to the EN 10204 3.1 MTC; any threadolet with illegible or missing heat number marking must be re-marked or removed from the supply batch
- NPT/BSPT outlet thread protected with plastic thread protector cap — protects both the thread form and the thread surface from damage during ocean freight; thread protector must be correct for the thread size and not over-tightened (over-tightened thread protectors can score the NPT/BSPT first threads and create a false gauge reading on installation)
- SS 316, Duplex, and alloy steel threadolets in dedicated grade-labelled polybags; segregated from carbon steel A105N; iron contamination from CS packaging onto SS fitting surfaces causes corrosion at the saddle bore (which becomes the weld face at installation) — a contaminated saddle bore can cause weld porosity or incomplete fusion in the first weld pass
- ISPM-15 heat-treated timber crates or export cartons for international shipment; individual lot polybags in grade-labelled compartments within the crate; desiccant sachets for ocean freight to humid tropical destinations
- Documentation in waterproof pocket: EN 10204 3.1/3.2 MTC, mechanical test certificate, Charpy impact certificate (cryogenic grades), hardness certificate (NACE lots — individual piece), thread gauge report (100% L1 plug gauge), saddle radius inspection report (100%), dimensional inspection report, PMI report (SS/Duplex/alloy), ferrite count certificate (Duplex/Super Duplex), FAI report, and where applicable PWHT records
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 threadolets | Forging heat-traceable; one MTC per forging heat |
| 02 | Chemical Composition Report | Certified lab analysis per ASTM A105N / A182 | Mandatory | All alloying elements per grade limits |
| 03 | Mechanical Properties Report | UTS, yield, elongation, reduction of area | Mandatory | Per ASTM A105N / A182 for grade; one test per forging heat |
| 04 | Hardness Test Report | ASTM E10 Brinell / E18 Rockwell C | Mandatory — NACE (per piece); all lots (sampled) | A105N NACE ≤ 22 HRC per piece; F51 Duplex ≤ 293 HB per lot |
| 05 | Charpy Impact Test Report | ASTM A370 — at design min temp | Mandatory — cryogenic grades; A350 LF2 | Test temp; average CVN J-values per heat lot |
| 06 | Thread Gauge Report (NPT / BSPT) | ASME B1.20.1 / ISO 7-1 L1 plug gauge | Mandatory — 100% all threadolets | L1 plug ± 1 turn tolerance; calibrated gauge cert |
| 07 | Saddle Bore Radius Inspection Report | Radius gauge / CMM vs specified run pipe OD | Mandatory — 100% all EPC threadolets | ±0.5 mm tolerance; 4 positions around circumference |
| 08 | Dimensional Inspection Report | Per MSS SP-97 / ASME B16.11 | Mandatory | Body height, wall thickness, outlet bore, markings |
| 09 | PMI Report (XRF) | Per lot — SS / Duplex / alloy steel | Mandatory — all non-CS lots; individual for NACE | F316L vs F304; F51 vs F53; F11 vs F22 vs F91 |
| 10 | Ferrite Content Report | ASTM E562 metallographic | Mandatory — F51 Duplex; F53 Super Duplex | 40–60% (F51); 40–50% (F53); cross-section photograph |
| 11 | NACE Compliance Statement | Hardness + heat treatment declaration | Conditional — sour service threadolets | Individual piece Brinell ≤ 237 HB (≤ 22 HRC); heat lot ref |
| 12 | First Article Inspection (FAI) Report | Project-specific format | Mandatory — new configurations; new projects | All parameters; before batch production |
| 13 | TPI Witness Certificate | SGS / BV / DNV / Lloyds | Conditional — EN 10204 3.2; offshore; nuclear; NACE | Co-witness; dimensional + PMI + hardness |
| 14 | ISO 9001:2015 Certificate | Third-party QMS certification | Mandatory — EPC projects | Scope covers forged branch connection fitting manufacture |
| 15 | Country of Origin + Packing List | Chamber of Commerce / item-level | Mandatory | HS tariff code; heat number per line item |
| 16 | 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 forging billet procurement and heat traceability, hot forging process qualification (die temperature, forging temperature, forge ratio), heat treatment process control (normalising cycle for A105N; solution annealing for SS and Duplex; Q+T for alloy grades), saddle bore contour CNC machining qualification (radius tolerance per specified run pipe OD), NPT/BSPT thread machining and gauge protocol (100% L1 plug gauge, calibrated gauge records), hardness testing procedure, PMI procedure, ferrite count test procedure, and full material traceability from billet heat to dispatched threadolet. Mandatory for all EPC, offshore, and safety-critical process piping threadolet procurement qualification.
ASME B31.3 / ASME B16.11
ASME B31.3 (Process Piping) and ASME B16.11 (Forged Fittings) together constitute the complete design and material code framework for EPC threadolet specification. B31.3 sets the design requirements (area replacement, fitting material group, PWHT requirements, NDE requirements); B16.11 sets the manufacturing and dimensional requirements (pressure class, wall thickness, marking). A threadolet that complies with B16.11 is a “listed fitting” per B31.3 Paragraph 304.7.2 — the engineer can use it in a B31.3 piping system without individual engineering qualification beyond verifying that the material grade, pressure class, and size are correct for the service conditions.
MSS SP-97
MSS SP-97 (Integrally Reinforced Socket Weld, Threaded, and Buttwelding Branch Outlet Fittings) is the manufacturing standard that specifically governs threadolet, sockolet, and weldolet fittings. The standard provides: minimum wall thickness tables by pressure class and size; saddle bore contour requirements; the run pipe and branch size combinations covered (the “listed” combinations); and the marking requirements. Compliance with MSS SP-97 is explicitly referenced in ASME B16.11 as the basis for listing branch connection fittings as acceptable forged fittings. Any threadolet supplied without MSS SP-97 compliance documentation is an unlisted fitting under ASME B31.3 and requires the Responsible Engineer’s written qualification concurrence.
PED 2014/68/EU / EN 13480
PED 2014/68/EU (Pressure Equipment Directive) and EN 13480 (Metallic Industrial Piping) are the European regulatory and design framework for threadolets in CE-marked European process piping systems. EN 13480 Part 4 Annex B covers integrally reinforced branch fittings equivalent to MSS SP-97 products. PED compliance requires: Declaration of Conformity from the manufacturer for Category I/II fittings; CE marking; and notified body assessment for Category III/IV. For European project threadolet supply: verify the PED category of the piping system with the project pressure equipment engineer before specifying the required certification documentation — Category determines whether a manufacturer Declaration of Conformity suffices or whether notified body approval is required.
Submit your run pipe NPS, branch thread size, pressure class, material grade, and quantity to RR Hydraulic for a complete, certified commercial offer.