Long Weld Neck Flange — Engineering Reference | RR Hydraulic

Engineering Reference Document

Long Weld Neck Flange

A world-class technical reference for EPC contractors, pressure vessel engineers, piping engineers, procurement heads, TPI inspection agencies, and global project buyers specifying long weld neck flanges on pressure vessels, columns, reactors, heat exchangers, and critical high-pressure piping nozzle connections in Oil & Gas, Petrochemical, Power, and LNG industries.

ASME B16.5 ASME B16.47 Series A/B ASME VIII Div. 1 / Div. 2 ASME B31.3 / B31.1 ASTM A105N / A182 / A350 DIN 2501 / EN 1092-1 NACE MR0175 / ISO 15156 EN 10204 3.1 / 3.2 ISO 9001:2015

Part 01 / Technical Definition

Industry Context,
Structural Role
& Load Characteristics

Long weld neck (LWN) flanges are flanges with an extended, tapered hub that functions simultaneously as a flange and as a pressure vessel nozzle neck. The elongated hub replaces the traditional short nozzle stub by integrating the neck directly into the forged flange body — eliminating one circumferential weld and reducing stress concentration at the shell-to-nozzle junction on pressure vessels, columns, reactors, and heat exchangers.

Long Weld Neck Flange — RR Hydraulic Engineering Reference

1.1 — Technical Definition and Functional Role

A long weld neck (LWN) flange — also referred to as a nozzle flange, extended neck flange, or barrel flange — consists of a standard flange body (raised face, RTJ, flat face, or tongue-and-groove) with a substantially elongated, tapered cylindrical neck that extends beyond the hub into the vessel or equipment wall thickness zone. The neck wall thickness is matched to the pressure vessel shell or head wall at the attachment weld location, providing a smooth, code-compliant transition with controlled stress intensification at the nozzle-to-shell junction.

In EPC and pressure vessel engineering, the LWN flange serves a dual structural function: it acts as both the pressure-retaining flange (sealing the flanged piping connection) and as the nozzle neck through the vessel shell or head. This dual function eliminates the short pipe stub conventionally inserted between the vessel shell and the flange, removing one circumferential butt weld — a significant benefit for radiographic inspection, weld repair risk reduction, and fatigue resistance in high-cycle pressure service.

RR Hydraulic manufactures long weld neck flanges under all applicable standards — ASME B16.5, ASME B16.47 Series A/B, ASME Section VIII, EN 1092-1, and project-specific engineering specifications — with full EN 10204 3.1 / 3.2 traceability and TPI capability.

1.2 — LWN Flange vs Standard Weld Neck vs Other Nozzle Types

Table 1.A — Long Weld Neck vs Weld Neck vs Alternative Nozzle / Flange Types

Parameter	Long Weld Neck (LWN)	Standard Weld Neck	Slip-On Flange	Socket Weld Flange	Nozzle Stub + Short WN
Hub length	Extended — matches vessel nozzle	Short standard hub	No hub — plain bore	Socket bore hub	Separate stub + WN assembly
Circumferential welds	1 (neck to vessel shell)	2 (stub-to-shell + stub-to-WN)	2 (face + back fillet)	1 (fillet)	2–3 welds
Stress at nozzle junction	Lowest — gradual taper	Higher — abrupt transition	Highest — no taper	High — fillet only	High — weld HAZ zones
NDE accessibility	Excellent — single weld, full RT	Good — two welds, full RT	Limited — fillet welds	Limited — fillet welds	Complex — multiple welds
ASME VIII application	Primary for vessel nozzles	Piping, some vessel nozzles	Low-pressure, non-critical	Not used on vessels	Large bore vessel nozzles
Pressure class	Class 150–2500 / PN 10–400	Class 150–2500	Class 150–600	Class 150–2500 (small bore)	All classes
Fatigue resistance	Highest — eliminates weld at SIF zone	Good	Poor	Moderate	Moderate
Cost	Higher (complex forging)	Moderate	Lowest	Low–Moderate	Moderate + fabrication
Delivery	Longer lead — custom forging	Standard stock	Standard stock	Standard stock	Multiple parts from stock
Preferred for	Vessel nozzles, reactors, critical piping	General process piping	Low-pressure utility	Small-bore high-pressure	Large-bore vessel nozzles

1.3 — Structural and Pressure-Retaining Function

The long weld neck flange eliminates the stress intensification problem inherent in conventional short nozzle assemblies. In a standard vessel nozzle constructed from a pipe stub plus a short weld neck flange, there are two circumferential welds — each introducing a heat-affected zone (HAZ), weld residual stress, and a potential flaw initiation site. The LWN replaces both welds with a single, accessible attachment weld at the base of the neck, where the neck OD and wall thickness are designed to match the vessel shell at that location.

Stress Intensification Factor (SIF) Reduction

The tapered neck geometry of the LWN flange distributes the stress transition from the vessel shell wall thickness to the flange hub gradually — reducing the stress intensification factor (SIF) at the shell-to-nozzle junction. Lower SIF directly extends fatigue life under cyclic pressure loading and reduces the risk of nozzle cracking in high-cycle process vessels.

Elimination of Circumferential Weld

Replacing a two-weld nozzle assembly (stub + short WN) with a single LWN attachment weld reduces the number of weld joints requiring radiographic examination (RT), magnetic particle inspection (MPI), and post-weld heat treatment (PWHT) — reducing fabrication time, inspection cost, and quality risk on each nozzle.

Pressure Vessel Code Compliance

ASME Section VIII Division 1 and Division 2 recognise LWN flanges as an integral nozzle configuration. The neck wall thickness and taper must comply with the applicable nozzle reinforcement rules in ASME VIII UG-36 through UG-45. The extended neck provides additional reinforcement area that may reduce or eliminate the need for a separate reinforcement pad (repad).

Nozzle Reinforcement Benefit

For vessel nozzles where the shell opening requires reinforcement per ASME VIII UG-37, the LWN neck contributes to the required reinforcement area calculation through the excess metal in the neck wall above the minimum required thickness. This often reduces or eliminates the repad, simplifying vessel fabrication and NDE.

Thermal Cycling Advantage

The smooth, continuous neck-to-shell transition of the LWN flange minimises thermal gradient stress during temperature transients. Vessels and reactors in high-temperature service with frequent startup/shutdown cycles benefit from the lower stress concentration at the nozzle junction — a primary advantage over the abrupt transition of a conventional stub nozzle.

Inspection Accessibility

The single external circumferential butt weld at the LWN neck-to-shell interface is fully accessible for radiographic (RT), ultrasonic (UT), magnetic particle (MPI), and post-weld heat treatment (PWHT) inspection operations — superior to the restricted access conditions on vessels with internals, insulation, or close nozzle spacing that limit the physical approach to internal welds.

1.4 — Load Characteristics and Nozzle Design Considerations

Long weld neck flanges on pressure vessel nozzles must withstand the combination of: internal pressure loads (ASME VIII design), external piping loads transmitted to the nozzle (moments and forces from thermal expansion, weight, and dynamic loads), flange bolt preload and gasket seating loads, and thermal cycling loads. These combined loads must be assessed in the nozzle design per the applicable code.

ASME VIII UG-45 — Minimum Nozzle Neck Thickness

t_n ≥ max(t_nozzle_pressure, t_nozzle_pipe)

t_nozzle_pressure = Minimum neck thickness for internal pressure containment
= P × R_n / (S × E − 0.6P) [ASME VIII UG-27]
t_nozzle_pipe = Minimum pipe wall thickness for the equivalent pipe schedule (per UG-45)
P = Design pressure (MPa)
R_n = Inside radius of nozzle (mm)
S = Allowable stress of nozzle material at design temperature (MPa)
E = Joint efficiency (1.0 for seamless forging)

LWN neck contribution to reinforcement area (A_neck):
A_neck = 2 × (t_neck − t_n) × L_neck × f_r1
f_r1 = S_nozzle / S_vessel (strength ratio); L_neck = reinforcement zone length per UG-40

Design Implication: The LWN neck contributes A_neck to the total available reinforcement area (A_1 + A_2 + A_neck ≥ A_required per UG-37). A well-designed LWN flange with sufficient neck wall thickness can eliminate the need for a separate reinforcement pad on moderate-bore vessel nozzles, simplifying vessel fabrication and reducing the number of weld joints.

Table 1.B — Design Safety Factors by Service Category for LWN Flanges

Service Category	Safety Factor	Governing Code	Key LWN-Specific Requirement
Pressure vessel nozzle (standard)	3.5:1 (UTS:allowable) per ASME VIII	ASME VIII Div. 1	UG-36 to UG-45 nozzle design; reinforcement check per UG-37
High-pressure vessel (Div.2)	2.4:1 (UTS:allowable)	ASME VIII Div. 2	Fatigue analysis mandatory; design by analysis
Severe cyclic service	Per fatigue curve	ASME VIII Div. 2 / B31.3	LWN preferred — lower SIF; fatigue assessment per Annex 3-F
NACE sour service	3.5:1	NACE MR0175 / ASME VIII	A105N normalised; max 22 HRC; PWHT weld per NACE SP0472
Cryogenic (LNG)	3.5:1 + Charpy verified	ASME VIII / A350	Impact test at design temp; fracture mechanics assessment
High-temperature (>450°C)	3.5:1; creep-limited	ASME VIII / ASME I	Allowable stress at temperature; creep-fatigue interaction check
Lethal / Category M fluid	3.5:1 + 100% RT	ASME VIII UW-2 / B31.3	100% RT on all welds; full penetration butt weld only; LWN mandatory
Offshore / subsea	Per DNV / DNVGL	DNV-ST-F101 / DNV-SE-0175	Enhanced NDE; corrosion allowance; HISC assessment for duplex

Specifying long weld neck flanges for vessel nozzles or critical piping?
Submit your nozzle schedule, pressure class, material spec, neck length, and bore for a documented RFQ within 24 hours.

Part 02 / Standards & Dimensional Design

Dimensional Design,
Neck Geometry
& Standards Compliance

Long weld neck flange dimensions are governed by ASME B16.5 (NPS ½–24), ASME B16.47 Series A and B (NPS 26–60), and pressure vessel codes including ASME VIII. The neck length, bore, and taper are typically custom-engineered to match the vessel shell thickness and nozzle geometry — requiring detailed dimensional data from the vessel engineering drawing. All standards are supported with full certification at RR Hydraulic.

Long Weld Neck Flange Dimensional Reference — RR Hydraulic

Formal R.F.Q. — Long Weld Neck Flanges for EPC / Vessel / Piping Projects
Submit NPS, pressure class, material, neck length, bore, and facing type to sales@rrhydraulics.com for a fully certified offer.

2.1 — Key Dimensional Parameters of a Long Weld Neck Flange

Unlike standard weld neck flanges where all dimensions are fully standardised in ASME B16.5 tables, the long weld neck flange has its flange body dimensions per ASME B16.5 / B16.47 but its neck length (L), neck bore (B), and neck OD at weld end are typically custom-specified by the vessel engineer to match the nozzle design. The following parameters must be defined on the purchase order:

Neck Length (L)

The extended hub length from the back of the flange to the weld end of the neck. Determined by the vessel wall thickness, insulation thickness, reinforcement zone requirement (UG-40), and accessibility for field welding and NDE. Typical LWN neck lengths range from 150 mm to 600 mm; custom lengths are manufactured to specification.

Neck Bore (B)

The inside diameter of the LWN neck at the weld end — must match the pipe or nozzle inside diameter at the vessel attachment point. Bore is specified to match the pipe schedule (e.g., NPS 4 Sch 80 bore = 97.18 mm ID) or the vessel shell opening ID. Bore machined to specified tolerance ±0.5 mm from nominal.

Neck OD at Weld End

The outside diameter of the LWN neck at the weld bevel location — must match the pipe OD or shell opening for a full-penetration butt weld. Transition taper from flange hub OD to weld-end OD must comply with ASME B31.3 / B31.1 pipe thickness transition rules: max 30° taper or per vessel drawing specification.

Neck Wall Thickness (t_neck)

The neck wall thickness at the weld end must equal or exceed the minimum required by ASME VIII UG-45 for pressure containment. It must also be compatible with the pipe or vessel shell wall thickness for welding — per ASME B31.3 transition requirements if the thicknesses differ significantly (ratio >1.5:1 typically requires a bevel or taper transition).

Weld Bevel Geometry

The weld end of the LWN neck is machined with a standard butt weld bevel — typically 37.5° ± 2.5° with a 1.6 mm root face, per ASME B16.25 or the applicable welding procedure specification (WPS). For material thickness transitions, compound bevels or internal taper per ASME B31.3 are machined to match the mating component.

Flange Face Type and Finish

The flange face (raised face, RTJ, flat face, T&G) is standard per ASME B16.5 / B16.47 for the specified pressure class and NPS. RTJ facing requires a precision-machined ring groove per ASME B16.20. Raised face surface finish: 125–250 µin Ra (3.2–6.3 µm) serrated concentric per B16.5. RTJ groove walls: Ra ≤ 1.6 µm (63 µin).

2.2 — ASME B16.5 LWN Flange: Flange Body Dimensional Table

The flange body dimensions (flange OD, bolt circle, bolt hole count and size, raised face diameter, and flange thickness) follow standard ASME B16.5 weld neck flange tables. Only the hub / neck dimensions differ from a standard weld neck flange. The following table provides ASME B16.5 flange body data for Class 300 and Class 600 — the most commonly specified classes for LWN flanges on pressure vessels.

Table 2.A — ASME B16.5 LWN Flange Body Dimensions: Class 300 and Class 600 (mm)

NPS	Pipe OD (mm)	Flange OD Cl.300	BCD Cl.300	Bolts×Dia Cl.300	RF Dia Cl.300	Flange OD Cl.600	BCD Cl.600	Bolts×Dia Cl.600	RF Dia Cl.600
2″	60.3	165.1	127.0	8×¾”	82.6	165.1	127.0	8×¾”	82.6
3″	88.9	209.5	168.3	8×¾”	107.9	209.5	168.3	8×¾”	107.9
4″	114.3	254.0	200.2	8×¾”	133.4	273.1	215.9	8×⅞”	133.4
6″	168.3	317.5	266.7	12×¾”	190.5	355.6	292.1	12×1″	190.5
8″	219.1	381.0	330.2	12×⅞”	247.7	419.1	349.2	12×1⅛”	247.7
10″	273.1	444.5	387.4	16×1″	304.8	508.0	431.8	16×1¼”	304.8
12″	323.9	520.7	450.8	16×1⅛”	362.0	584.2	495.3	20×1¼”	362.0
14″	355.6	584.2	514.4	20×1⅛”	396.9	641.4	546.1	20×1⅜”	396.9
16″	406.4	647.7	571.5	20×1¼”	450.9	704.9	603.2	20×1½”	450.9
18″	457.2	711.2	628.6	24×1¼”	501.7	774.7	660.4	20×1⅝”	501.7
20″	508.0	774.7	685.8	24×1¼”	552.5	857.3	736.6	24×1⅝”	552.5
24″	609.6	914.4	812.8	24×1½”	654.1	1022.4	876.3	24×2″	654.1

Table 2.B — ASME B16.5 LWN Flange Body Dimensions: Class 900 and Class 1500 (mm)

NPS	Flange OD Cl.900	BCD Cl.900	Bolts×Dia Cl.900	Flange Thick Cl.900 (mm)	Flange OD Cl.1500	BCD Cl.1500	Bolts×Dia Cl.1500	Flange Thick Cl.1500 (mm)
2″	215.9	165.1	8×⅞”	39.6	244.5	177.8	8×1″	50.8
3″	279.4	215.9	8×1⅛”	50.8	342.9	254.0	8×1¼”	66.5
4″	355.6	279.4	8×1¼”	57.2	431.8	330.2	8×1½”	79.2
6″	482.6	387.4	12×1⅜”	73.0	558.8	444.5	12×1¾”	101.6
8″	584.2	469.9	12×1⅝”	88.9	673.1	539.8	12×2″	123.9
10″	692.2	558.8	16×1¾”	101.6	787.4	635.0	12×2¼”	142.9
12″	800.1	647.7	20×1¾”	114.3	914.4	736.6	16×2¼”	165.1

2.3 — Pressure Rating Table

Table 2.C — ASME B16.5 Pressure Rating — Group 1.1 (A105N / A182 F1) LWN Flanges (bar g)

Temperature (°C)	Class 150	Class 300	Class 600	Class 900	Class 1500	Class 2500
−29 to 38	19.6	51.1	102.1	153.2	255.3	425.5
100	17.7	46.6	93.2	139.8	233.0	388.3
200	13.8	43.8	87.6	131.4	219.0	365.0
300	10.2	39.8	79.6	119.4	199.0	331.6
400	6.5	36.5	73.0	109.5	182.5	304.2
450	5.1	31.6	63.2	94.8	158.0	263.3
500	—	26.5	53.0	79.5	132.5	220.8
538	—	20.5	41.0	61.5	102.5	170.9

2.4 — Applicable Standards and Compliance Framework

ASME B16.5

Pipe Flanges and Flanged Fittings, NPS ½–24. Defines flange body geometry, pressure–temperature ratings, material groups, bolt dimensions, and facing tolerances for LWN flanges through NPS 24. The neck length and geometry for LWN flanges are specified by the purchaser beyond the B16.5 standard hub geometry.

ASME B16.47 Series A / B

Large Diameter Steel Flanges, NPS 26–60. Series A (formerly MSS SP-44) is used for pressure vessel nozzles and heavy-wall piping; Series B (formerly API 605) for lower-pressure large-bore applications. LWN flanges in large bore are almost exclusively Series A for vessel nozzle applications. Bolt data and flange geometry differ significantly between Series A and B.

ASME Section VIII Div. 1

Rules for Construction of Pressure Vessels. UG-36 through UG-45 govern nozzle design including LWN flanges used as integral vessel nozzles. UG-37 reinforcement area calculation, UG-40 reinforcement zone, UG-41 weld strength, and UG-45 minimum nozzle neck thickness all apply to LWN flange nozzle design.

ASME Section VIII Div. 2

Alternative Rules for Pressure Vessels — design by analysis. Required for higher-pressure vessels and vessels subject to cyclic loading. Fatigue assessment per Annex 3-F applies to nozzle junctions including LWN flange connections. Stress intensification factors (SIF) and stress concentration factors (SCF) are explicitly calculated, favouring the LWN geometry over conventional stub nozzles.

ASTM A105N

Carbon steel forgings for piping components — normalised condition. A105N is the standard NACE-compliant grade for carbon steel LWN flanges in sour service. Normalising heat treatment reduces hardness to ≤187 HB — the NACE MR0175 limit for carbon steel pressure-retaining components in H₂S-containing environments.

ASTM A182

Forged or Rolled Alloy and Stainless Steel Pipe Flanges and Fittings. Covers all alloy steel and SS LWN flanges: F11, F22 (Cr-Mo for elevated temperature), F304L, F316L (austenitic SS), F51 (Duplex 2205), F53 (Super Duplex S32750), F91 (9Cr-1Mo-V for very high temperature), and exotic grades.

ASTM A350 LF2 / LF3

Forgings for piping components with notch toughness requirements. LF2 (impact tested at −45°C) and LF3 (impact tested at −101°C) for LWN flanges on low-temperature and cryogenic vessel nozzles. Charpy impact testing per ASTM A350 Table S5 is mandatory — each individual test piece from the production forging must meet the energy requirement.

EN 1092-1 / DIN 2501

European flange standard — circular flanges for pipes, valves, fittings, and accessories. Type 11 (weld neck) and Type 13 (socket weld) are the relevant EN 1092-1 types. PN-rated flanges (PN 10 through PN 400) per EN 1092-1 are used on European-coded vessels. LWN flanges in PN rating are custom-necked per vessel engineer specification with EN 1092-1 body dimensions.

NACE MR0175 / ISO 15156

Materials for sour H₂S service in oil and gas production. Carbon steel LWN flanges (A105N): max hardness 22 HRC (237 HB). Alloy steel F11, F22: verify hardness at weld HAZ post-PWHT. SS grades (F304L, F316L, F51) are inherently NACE-compliant when hardness is within the austenitic/duplex limits. 100% hardness verification on all LWN flanges for sour service supply.

2.5 — Neck Length and Geometry: Selection Guide

Table 2.D — LWN Neck Length Selection Guide by Application

Application	Typical Neck Length (mm)	Neck OD Configuration	Bore Specification	Design Reference	Engineering Note
Vessel shell nozzle — standard	150–300	Tapered to pipe OD	Match pipe schedule ID	ASME VIII UG-45	Neck contributes to reinforcement area
Vessel head nozzle — dished head	200–400	Parallel or tapered	Match head opening ID	ASME VIII UG-36 / UG-37	Head thickness varies — match at weld
Column / tower nozzle	200–500	Tapered to match shell	Match column wall ID	ASME VIII / project spec	Long neck accommodates insulation thickness
Heat exchanger channel nozzle	150–250	Parallel — uniform wall	Match channel flange bore	TEMA / ASME VIII	Often stainless in tube-side process service
Reactor nozzle — thick wall	300–600	Heavy wall — match reactor	Match reactor bore	ASME VIII Div. 2	High-thickness: compound bevel required
Cryogenic vessel nozzle	200–400	Tapered; A350 LF3 material	Match cryogenic pipe ID	ASME VIII / A350	Impact test at −101°C; A182 F304L option
High-temperature nozzle (>450°C)	150–300	Tapered; F22 / F91 material	Match alloy steel pipe ID	ASME I / VIII	PWHT mandatory; creep assessment
Offshore / subsea vessel nozzle	200–450	Duplex / Super Duplex	Match duplex pipe ID	DNV / ASME VIII	HISC assessment; cathodic protection zone

2.6 — Facing Types for LWN Flanges

Raised Face (RF) — Standard

Most common facing for LWN flanges in process piping connections. 2 mm raised ring height for Class 150/300; 7 mm for Class 600–2500. Serrated concentric finish 125–250 µin Ra per ASME B16.5. Compatible with spiral wound, CAF, and soft gaskets. Standard for the majority of vessel nozzle flanged connections in O&G and petrochemical service.

Ring-Type Joint (RTJ) — High Pressure

Precision-machined octagonal groove per ASME B16.20. Required for Class 600–2500 in high-pressure, high-temperature, and hazardous fluid service. Metal ring (R, RX, BX) provides metal-to-metal seal — highest integrity facing type. RTJ groove dimensions are ring-number-specific; ring number must be specified on the purchase order for each NPS/class combination.

Flat Face (FF)

Full face — no raised ring. Used when mating to flat face flanges on equipment with brittle body materials (cast iron, non-metallic). Full-face gasket extends to bolt holes. Not typically used for LWN flanges on steel vessel nozzles — primarily encountered on utility connections to cast iron pump bodies or strainer housings.

Tongue and Groove (T&G)

Male (tongue) and female (groove) mating faces — gasket fully captured in groove. Used on heat exchanger channel-to-shell flanges and compressor cylinder head flanges where a fully confined gasket is required to prevent blowout under high-pressure cyclic loading. Must be ordered as matched pairs — tongue on one flange, groove on mating flange.

Part 03 / Materials & Manufacturing

Material Grades,
Heat Treatment
& Manufacturing Process

Long weld neck flange material is selected to match the connected vessel or pipe material — matching both mechanical properties and weldability. RR Hydraulic manufactures LWN flanges in all standard and exotic grades — from ASTM A105N carbon steel through Super Duplex and Inconel — with full EN 10204 3.1 / 3.2 traceability and heat treatment documentation.

Long Weld Neck Flange Materials — RR Hydraulic

3.1 — Material Grade Overview and Mechanical Properties

Table 3.A — Material Comparison: Grade, Mechanical Properties, Temperature Range, Use Case

Material	ASTM Grade	UTS (MPa)	Yield (MPa)	Elong. (%)	Design Temp Range (°C)	Max Hardness (HB)	Primary Vessel Application
Carbon Steel (N)	ASTM A105N	485	250	22	−29 to +538	187 (NACE)	General process vessels, separators, drums
CS Low-Temp	ASTM A350 LF2	485	250	22	−46 to +345	197	Cryogenic vessels, LNG, ethylene plants
CS Low-Temp	ASTM A350 LF3	450	240	22	−101 to +345	197	LNG storage, sub-zero process vessels
1.25Cr-0.5Mo	ASTM A182 F11 Cl.3	485	275	20	−29 to +593	217	Catalytic reformer, steam drums, heater pass
2.25Cr-1Mo	ASTM A182 F22 Cl.3	415	205	20	−29 to +649	241	Hydrocracker reactor, high-temp vessel nozzles
9Cr-1Mo-V (P91)	ASTM A182 F91	585	415	20	−29 to +649	250	Ultra-high-temp power vessel nozzles, boilers
SS 304L	ASTM A182 F304L	450	170	30	−196 to +425	192	Corrosive vessel nozzles, food, pharma, cryogenic
SS 316L	ASTM A182 F316L	450	170	30	−196 to +425	192	Marine, chloride-containing vessels, offshore
SS 321	ASTM A182 F321	485	205	30	−196 to +816	192	High-temp SS vessels; stabilised grade
Duplex 2205	ASTM A182 F51	620	450	25	−50 to +315	293	Offshore, sour service, seawater vessels
Super Duplex	ASTM A182 F53 (S32750)	750	550	15	−50 to +260	310	Seawater injection, high-chloride, subsea
Inconel 625	ASTM B564 N06625	827	414	30	−196 to +980	—	Extreme corrosion, very high-temp, sour+Cl⁻

3.2 — Yield Strength and Mechanical Properties by Heat Treatment

Table 3.B — Mechanical Properties by Heat Treatment Condition

Grade / Spec	Heat Treatment	UTS (MPa)	Yield (MPa)	Elongation (%)	Hardness (HB)	Charpy @ Temp (J)	NACE Compliant
A105N	Normalised 900°C / air cool	485	250	22	≤187	Not req.	Yes (≤187 HB)
A350 LF2	Normalised + impact tested	485	250	22	≤197	27 J @ −45°C	Conditional
A350 LF3	Normalised + impact tested	450	240	22	≤197	27 J @ −101°C	Conditional
A182 F11 Cl.3	Normalised + tempered 680°C	485	275	20	≤221	Not req.	Yes
A182 F22 Cl.3	Normalised + tempered 690°C	415	205	20	≤241	Not req.	Yes
A182 F91	Normalised 1040°C + tempered 760°C	585	415	20	≤250	Not req.	Yes
A182 F304L	Solution annealed 1040°C	450	170	30	≤192	N/A (austenitic)	Yes
A182 F316L	Solution annealed 1040°C	450	170	30	≤192	N/A	Yes
A182 F51 (Duplex)	Solution annealed 1020–1100°C	620	450	25	≤293	N/A	Yes (28 HRC max)
A182 F53 (S.Duplex)	Solution annealed 1025–1125°C	750	550	15	≤310	N/A	Conditional
A182 F91 (P91)	N+T as above; mandatory PWHT	585	415	20	197–250	Not req.	Yes

3.3 — Corrosion Resistance by Material vs Service Media

Table 3.C — Corrosion Resistance Matrix for LWN Flange Materials

Material	H₂S Sour*	CO₂ / Wet Gas	Cl⁻ / Seawater	Acids (HCl/H₂SO₄)	Caustic / Alkali	High Temp (>450°C)	Cryogenic
A105N CS	Conditional*	Fair	Poor	Poor	Good	Good to 538°C	Not suitable
A350 LF2 / LF3	Conditional*	Fair	Poor	Poor	Good	Limited	Excellent
A182 F11 / F22	Conditional*	Good	Poor	Poor	Good	Very Good	Not suitable
A182 F91 (P91)	Conditional*	Good	Poor	Poor	Good	Excellent to 649°C	Not suitable
A182 F304L	Fair	Good	Poor (SCC)	Fair	Very Good	Good to 816°C	Excellent
A182 F316L	Good	Very Good	Fair	Good	Very Good	Good to 816°C	Excellent
A182 F51 Duplex	Very Good	Excellent	Very Good	Very Good	Very Good	Limited >315°C	Good to −50°C
A182 F53 S.Duplex	Excellent	Excellent	Excellent	Excellent	Excellent	Limited >260°C	Good to −50°C
Inconel 625	Excellent	Excellent	Excellent	Excellent	Excellent	Excellent to 980°C	Excellent

* Sour service: A105N normalised (≤187 HB) mandatory per NACE MR0175 / ISO 15156. Alloy grades F11/F22/F91 must be verified for HAZ hardness post-PWHT. SS and duplex grades comply within standard hardness limits.

3.4 — Manufacturing Process

3.4.1 — Hot Forging: The Only Acceptable Manufacturing Method

Long weld neck flanges for pressure vessel and high-pressure piping applications must be manufactured exclusively by open-die or closed-die hot forging from ingot, billet, or bar stock conforming to the applicable ASTM material specification. The extended neck geometry and the requirement for continuous grain flow through the neck and into the flange hub — critical for fatigue and fracture resistance at the vessel-to-nozzle junction — cannot be achieved by machining from plate, welding of separate components, or casting.

Critical Procurement Warning — Fabricated vs Forged LWN Flanges: LWN flanges must never be fabricated by welding a pipe stub to a standard weld neck flange and presenting the assembly as a long weld neck flange. This practice — sometimes encountered in low-cost supply chains — produces a component that does not comply with ASTM A105 / A182 forging standards, does not have the grain continuity of a forged LWN, and introduces an uncontrolled weld into the pressure boundary. EN 10204 3.1 MTC traceable to a single forging heat is the verification tool. Demand heat-traceable MTCs on all LWN flange supply — reject any supply that cannot provide a forging lot heat number.

3.4.2 — CNC Machining Tolerances for LWN Flanges

Bore (inside diameter): ±0.5 mm from nominal — machined to match specified pipe schedule ID or vessel engineer drawing dimension
Neck OD at weld bevel: ±0.8 mm — matches mating pipe or vessel shell OD for butt weld fitup per ASME B31.3 / B31.1 fitup tolerance (max 1.6 mm misalignment for equivalent thickness components)
Weld bevel angle: 37.5° ± 2.5° from pipe axis; root face 1.6 mm ± 0.8 mm per ASME B16.25
Flange face flatness: max 0.8 mm TIR across raised face diameter per ASME B16.5
Bolt hole position: ±0.8 mm on BCD; angular spacing ±0.5° per ASME B16.5 — bolt holes must straddle the flange centreline
Neck length (L): ±3 mm from specified dimension — critical for correct positioning of nozzle relative to vessel shell wall
RTJ groove dimensions: ±0.05 mm on groove sealing surfaces per ASME B16.20 groove geometry table
Surface finish: neck OD Ra ≤ 6.3 µm; bore Ra ≤ 3.2 µm; weld bevel Ra ≤ 12.5 µm (adequate for weld fitup); RTJ groove walls Ra ≤ 1.6 µm

3.4.3 — Post-Weld Heat Treatment (PWHT) Requirements

PWHT is required after the LWN neck-to-vessel shell butt weld based on material and wall thickness as follows:

Carbon steel (A105N) per ASME VIII / B31.3: PWHT required when weld groove thickness exceeds 19 mm (¾”) — typical for Class 600+ thick-wall vessel nozzles. PWHT at 595–650°C, 1 hour per 25 mm thickness minimum.
A182 F11 (1.25Cr-0.5Mo): PWHT mandatory regardless of thickness. Temperature 620–650°C; 1 hour per 25 mm minimum. Verify HAZ hardness post-PWHT ≤ 241 HB for NACE compliance.
A182 F22 (2.25Cr-1Mo): PWHT mandatory. Temperature 690–760°C; 1 hour per 25 mm minimum. Hardness post-PWHT ≤ 241 HB.
A182 F91 (9Cr-1Mo-V / P91): PWHT mandatory at 730–800°C; controlled heating rate (≤55°C/hour above 425°C) and cooling rate (≤55°C/hour to 315°C) required per ASME B31.1 / EPRI guidelines. Hardness post-PWHT 197–250 HB mandatory.
Austenitic SS (F304L, F316L): PWHT not required and not recommended. Use low-carbon L grades to prevent sensitisation. Control weld heat input to minimise HAZ grain growth.
Duplex SS (F51, F53): Solution annealing after welding preferred; if not feasible (installed condition), control heat input ≤ 2.5 kJ/mm and use correct filler metal (matching or over-alloyed) to maintain 40–60% ferrite in weld HAZ.

Part 04 / QC, Applications & Export

Inspection & QC,
Industry Applications
& Documentation

RR Hydraulic maintains full traceability from raw forging heat to final packed shipment on all LWN flange orders. Dimensional inspection, EN 10204 3.1 / 3.2 MTRs, volumetric UT on forging blanks, NACE hardness compliance, Charpy impact test certificates, and complete EPC export documentation packages are standard on all project-grade supply.

Long Weld Neck Flange QC and Inspection — RR Hydraulic

4.1 — Inspection & NDT Protocol

100%

Dimensional Inspection

All flange body dimensions per ASME B16.5 / B16.47 tables. Neck length (L), bore (B), neck OD, and weld bevel geometry verified against vessel engineer drawing or project specification. RTJ groove dimensional inspection to ASME B16.20 where applicable.

Ultrasonic Testing — Forging

Mandatory volumetric UT on all LWN forging blanks per ASTM A388 / EN 10308 before machining. Detects internal voids, forging laps, inclusions, and segregation in the hub and neck zone — the primary load-bearing regions. Required for all Class 900+ and NACE service supply.

MPI

Magnetic Particle Inspection

Full surface MPI on all carbon and alloy steel LWN flanges after final machining. Flange face, hub, neck OD, bore, and weld bevel surface are inspection zones. Mandatory for Class 600+, NACE sour service, A182 F22 / F91 grades. Per ASTM E709 / EN ISO 9934.

LPI

Liquid Penetrant Inspection

SS, duplex, and super duplex LWN flanges — full surface coverage after final machining. Per ASTM E165 / EN ISO 3452. Mandatory for F304L, F316L, F51 (Duplex 2205), F53 (Super Duplex S32750) lots for offshore, chemical, and sour service supply.

PMI

Positive Material Identification

XRF verification on 100% of SS, duplex, and exotic alloy LWN flanges. Critical for preventing material substitution in multi-grade vessel nozzle supply (e.g., F316L vs F304L vs F51 — all appear identical). Results recorded and cross-referenced to MTC heat number on individual flange certificates.

HRB/HRC

Hardness Testing

100% Brinell hardness on all NACE sour service LWN flanges. A105N: ≤187 HB. A182 F22: ≤241 HB. Duplex F51: ≤293 HB (28 HRC). Results on individual flange MTC — not lot-average. Brinell per ASTM E10; Rockwell E18 with conversion table per ASME B16.34 Appendix.

CVN

Charpy Impact Testing

Mandatory for all A350 LF2 (test at −45°C) and LF3 (test at −101°C) LWN flanges. Three specimens per forging lot; average and minimum values reported per ASTM A350 Table S5. Results included in EN 10204 3.1 MTC with individual specimen energy values.

FAI

First Article Inspection

Complete dimensional, mechanical, chemical, NDE, surface finish, and visual verification on the first machined LWN flange of each unique configuration (NPS + class + material + neck length + facing type) per project order. FAI report released before batch production — mandatory for all new project items.

4.2 — EN 10204 Material Test Certificate Requirements

Table 4.A — EN 10204 Certificate Types and LWN Flange Application

Certificate	Content	Signatory	Standard EPC Requirement	When Mandatory for LWN Flanges
2.1	Conformity declaration only	Manufacturer	Not acceptable for pressure vessel nozzles	Never acceptable for ASME VIII vessels
2.2	Non-specific test results	Manufacturer	Not acceptable in O&G / pressure vessel EPC	Not acceptable for any pressure service LWN
3.1	Heat-traceable forging lot mech + chem results	Manufacturer’s authorised QC Inspector	Minimum for all pressure vessel and piping LWN flanges	All Class 150+ LWN flange supply
3.2	3.1 + countersigned by independent TPI (SGS / BV / DNV / Lloyds)	Manufacturer + TPI Inspector	Critical vessel nozzles, NACE, cryogenic, offshore, ASME VIII Div.2	All Class 600+, sour service, LNG, and ASME VIII Div.2 vessel supply

4.3 — Pressure Test and Weld Inspection Requirements

Long weld neck flanges are inspected as part of the completed vessel or piping system. Key inspection and test requirements at the nozzle weld:

Pre-weld inspection: Verify dimensional fitup of LWN neck to vessel shell — maximum root gap, misalignment, and bevel geometry per WPS and ASME B31.3 / VIII requirements before tack welding
In-process weld NDE: VT (visual) after each weld pass; MT or PT on root pass (where accessible) per ASME VIII UW-51 / B31.3 requirements
Full RT / UT on completed weld: Radiographic testing (RT) or ultrasonic testing (UT) on the completed LWN neck-to-shell butt weld per ASME VIII UW-11 and B31.3 Chapter VI — specific requirements depend on material, service, and joint efficiency factor used in design
PWHT verification: Time-temperature recording chart from PWHT operation; post-PWHT hardness testing at weld, HAZ, and parent metal locations per project NDE plan
Hydrostatic test (ASME VIII): Completed vessel tested at 1.3 × MAWP (Div. 1) or 1.25 × MAWP (Div. 2) after all PWHT and NDE — hold for minimum 30 minutes before inspection of all nozzle welds and flange joints
Post-hydrostatic NDE: MPI / LPI on flange face and nozzle weld area after pressure test to detect any flaws opened by the test pressure

4.4 — Applications by Industry

Oil & Gas — Production Vessels Oil & Gas — Separators & Drums Refinery — Reactors & Columns Petrochemical — Pressure Vessels LNG — Cryogenic Vessels Power — Steam Drums & Boilers Offshore — Topside Vessels Heat Exchangers — Channel Nozzles Hydrogen Plants — Reactors Subsea — Pressure Housing Nozzles Desalination — Vessel Nozzles Nuclear — Class 2 / 3 Vessels

Oil & Gas — Production Separators & Drums

LWN flanges in A105N for three-phase separator nozzles, flash drums, and scrubbers. NACE MR0175 compliance mandatory for all nozzles in H₂S-containing service — normalised A105N with ≤187 HB hardness verified 100%. EN 10204 3.2 with TPI witness for Class 600 and above. Reinforcement pad may be eliminated where LWN neck contributes sufficient area per UG-37.

Refinery — Hydrocracker & Catalytic Reformer Reactors

High-temperature reactor nozzles in A182 F22 (2.25Cr-1Mo) or F91 (9Cr-1Mo-V) for hydrocracker and catalytic reformer service at 350–650°C. PWHT mandatory on all welds — controlled time-temperature recording required. Nelson curve compliance (API 941) for hydrogen service at elevated temperature and pressure. ASME VIII Div. 2 analysis for cyclic pressure reactors.

LNG Terminals — Cryogenic Storage Vessels

Cryogenic LWN flanges in A350 LF3 for vessel nozzles at −165°C LNG service. Charpy impact testing at −196°C on special project specifications. A182 F304L used for inner tank nozzle connections in double-wall LNG storage tanks. Fracture mechanics assessment per BS 7910 for through-thickness defect tolerance at cryogenic temperature mandatory on critical nozzles.

Power Generation — Steam Drums & High-Pressure Vessels

Boiler drum and steam vessel nozzles in A105N (low-temperature), A182 F11 Cl.3 (intermediate temperature), and F22 Cl.3 / F91 (high-temperature). ASME Section I (Power Boilers) and ASME VIII govern design. PWHT on all Cr-Mo nozzle welds — production PWHT records and post-PWHT hardness certificates required for ASME Code stamped vessels.

Offshore Platforms — Topside Process Vessels

Topside separator, scrubber, and reboiler vessel nozzles in A182 F316L or Duplex F51 for marine atmosphere and chloride-containing process fluid resistance. HISC (Hydrogen Induced Stress Cracking) assessment mandatory for super duplex F53 nozzles in cathodic protection zones per DNV RP-F112. All offshore LWN flange supply with EN 10204 3.2 and DNVGL / Lloyds TPI.

Heat Exchangers — Channel and Shell Nozzles

TEMA heat exchanger channel-to-nozzle LWN flanges in A182 F316L (tube-side corrosive service) or A105N (shell-side carbon steel service). RTJ or T&G facing for high-pressure channel closure flanges. Tube-side LWN flanges in duplex or Inconel where chloride-containing process fluids require enhanced corrosion resistance. All supply with TEMA and ASME VIII material documentation.

4.5 — Export Packaging Specification

Individual LWN flanges wrapped in VCI (Volatile Corrosion Inhibitor) poly film — critical for carbon and alloy steel grades to prevent atmospheric corrosion on precision-machined bore and flange face surfaces during ocean freight and site storage
Bore protection: foam or plastic bore caps on both ends of the LWN neck — protects the precision bore and weld bevel from damage and contamination; prevents moisture entry into the bore during transit
Flange face protection: cardboard or foam disc bonded to the raised face or RTJ groove — prevents damage to the serrated surface finish or precision RTJ groove during transit and handling; re-machining at site is not acceptable
Individual item tagging: stainless steel or weather-proof polypropylene tag attached to each LWN flange with heat number, ASTM grade, NPS, pressure class, neck length, bore, facing type, and PO item number
Crating: LWN flanges bolted to ISPM-15 heat-treated timber base frames (not stacked) — flange face vertical, neck horizontal; heavy flanges (NPS 8+) individually crated and secured against movement with timber blocking and steel banding
Crate markings: Project PO, item tag, ASTM grade, NPS, class, neck length, gross weight, net weight, crate dimensions, country of origin, “PROTECT FLANGE FACE — DO NOT DROP” handling symbols per ISO 780 / ASTM D5276
Documentation: complete MTC package, dimensional inspection report, and TPI release note enclosed inside crate lid and in external waterproof document pocket — cross-referenced to forging heat number on each flange tag

4.6 — Complete EPC Project Documentation Package

Table 4.B — Full Documentation Package for EPC Long Weld Neck Flange Supply

#	Document	Standard / Format	Mandatory / Conditional	Notes
01	Material Test Certificate (MTC)	EN 10204 3.1 / 3.2	Mandatory — all pressure vessel / piping	Forging heat-traceable; one MTC per heat
02	Chemical Composition Report	Forging lot certified lab analysis	Mandatory (within MTC)	All elements per ASTM A105 / A182 / A350
03	Mechanical Properties Report	UTS, yield, elongation, RA, hardness	Mandatory (within MTC)	Per applicable ASTM material specification
04	Hardness Test Report	ASTM E10 Brinell / E18 Rockwell	Mandatory — NACE / sour service	Individual flange results; A105N ≤187 HB
05	Charpy Impact Test Report	ASTM A370 / EN 10045	Mandatory — A350 LF2 / LF3	Test temp + individual + average J-values
06	Dimensional Inspection Report	Per ASME B16.5 / B16.47 + project drawing	Mandatory	All flange body + neck geometry per spec
07	Bore Dimensional Report	Per project nozzle schedule drawing	Mandatory	Bore ID, neck length, weld bevel geometry
08	Flange Face Finish Report	ASME B16.5 / B16.20	Mandatory — RTJ facing	Ra measurement; RTJ groove depth/width
09	First Article Inspection Report (FAI)	Project-specific format	Mandatory — new items / first lot	Released before batch production
10	UT Report — Forging (Volumetric)	ASTM A388 / EN 10308	Mandatory — Class 900+, NACE, ASME VIII	Pre-machining blank inspection
11	MPI Report	ASTM E709 / EN ISO 9934	Mandatory — CS/alloy, Class 600+, NACE	Full surface after final machining
12	LPI Report	ASTM E165 / EN ISO 3452	Mandatory — SS / duplex grades	Full surface; F316L, F51, F53 mandatory
13	PMI Report (XRF)	Per lot — SS / duplex / exotic grades	Mandatory — non-CS grades	100% individual flange verification
14	TPI Witness Certificate	SGS / BV / DNV / Lloyds countersigned	Mandatory — EN 10204 3.2 orders	Witness at manufacturer works
15	NACE MR0175 Compliance Statement	Hardness + HT condition declaration	Mandatory — sour service LWN flanges	References forging heat + hardness results
16	Heat Treatment Certificate	Time-temperature chart + furnace records	Mandatory — A350; F11/F22/F91 grades	Production HT; PWHT records where applicable
17	RTJ Groove Inspection Certificate	Per ASME B16.20	Conditional — RTJ facing LWN flanges	Ring number, groove dims, finish
18	Forging Process Certificate	Manufacturer’s forging procedure record	Conditional — ASME VIII Div.2 / nuclear	Confirms open-die or closed-die forging; no bar/plate
19	ISO 9001:2015 Certificate	Third-party QMS certification	Mandatory — all EPC / ASME VIII vessel supply	Scope covering forged flange manufacture
20	Country of Origin Certificate	Chamber of Commerce	Mandatory — all export	For customs and duty classification
21	Packing List	Item-level per shipment	Mandatory	Cross-references MTC and TPI numbers
22	Commercial Invoice	Per INCOTERMS 2020	Mandatory	Includes HS tariff code
23	Bill of Lading / Air Waybill	Per freight mode	Mandatory	Issued by freight forwarder

4.7 — ISO and Quality System Compliance

ISO 9001:2015

Quality Management System covering raw forging material procurement, forging process control, heat treatment verification, dimensional inspection, NDT, surface finish measurement, bore gauge calibration, and full material traceability through all production stages. Mandatory for all EPC, O&G, and ASME VIII vessel project procurement qualification.

ISO 9606

Qualification testing of welders. LWN flanges are joined to vessel shells by butt welding in the field — all welders performing the LWN neck-to-shell welds must hold current ISO 9606-1 qualification (or AWS / ASME IX equivalent) in the applicable process (GTAW, SMAW, SAW) and material group. This applies to the field installation welding, not to LWN flange manufacture.

ISO 10474

Steel and steel products — inspection documents. Legacy framework from which EN 10204 certificate types derive. Some ASME project specifications or legacy vessel codes reference ISO 10474 Type 3.1.B (= EN 10204 3.1). Current operative standard for material certification of LWN flanges is EN 10204; ISO 10474 references are cross-mapped on request.

ISO 4413

Safety requirements for hydraulic fluid power systems. LWN flanges used on hydraulic power unit (HPU) pressure vessels, accumulators, and high-pressure hydraulic manifold housings must comply with ISO 4413 pressure containment and cleanliness requirements. Bore cleanliness post-machining: ISO 4406 code 16/13 or better for hydraulic system LWN connections.

Ready to source long weld neck flanges for your vessel, EPC, or process project?
Submit your nozzle schedule, material specification, pressure class, neck length, and quantity to RR Hydraulic for a complete, certified commercial offer.