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Threaded Flanges
A comprehensive engineering reference for EPC contractors, piping engineers, procurement heads and TPI agencies — covering threaded flange types, thread forms, pressure-temperature ratings, dimensional standards, facing types, material grades, surface treatments and full project documentation requirements.
Threaded Flange Types, Thread Forms
& Engineering Selection Principles
NPT · BSPT · BSPP · RF · FF · RTJ · Class 150–2500
Definition and Engineering Function
A threaded flange (also called a screwed flange) is a pipe flange with a tapered or parallel internal thread bore that allows it to be screwed directly onto the end of a threaded pipe or fitting — without requiring welding to the pipe. The flange face then bolts to a mating flange in the standard manner using stud bolts, hex bolts, a gasket and nuts. This two-stage connection (screwed onto pipe + bolted to mating flange) is the defining engineering characteristic of the threaded flange and distinguishes it from weld neck, slip-on and socket weld flanges which all require welding to the pipe.
The threaded flange is specified in process piping wherever welding to the pipe is impractical, prohibited, or uneconomical: in non-weld zones (atmospheric areas where hot work permits are difficult to obtain), on galvanised or coated pipe (where welding would destroy the corrosion protection), on field-assembled small-bore instrument connections (where threaded assembly is faster than field welding), and in Class 150 water and utility services (where the low operating pressure makes the mechanical thread engagement sufficient for the service conditions). The threaded flange is limited to relatively small bore sizes (typically NPS ½" to 4" per ASME B16.5) because the taper thread engagement in larger diameters becomes increasingly difficult to make leak-free, and the structural asymmetry of a screwed connection introduces bending loads that are not acceptable in high-pressure piping systems.
The ASME B16.5 pressure-temperature rating of a threaded flange is the same as the equivalent weld-neck flange at the same material and pressure class — the flange body itself has the same rating. However, ASME B31.3 Process Piping Code restricts threaded flanges to non-severe service (no cyclic loading, no fatigue-sensitive service, no vibration). This is because the thread engagement creates a stress concentration at the thread root and a potential crevice corrosion site at the thread-pipe interface that reduces the long-term fatigue life of the connection below that of a welded joint. Additionally, ASME B16.5 restricts the thread form to NPT (tapered) for pressure-retaining service — parallel (BSPP) threads are not leak-tight without a sealant and are not permitted in pressure service without a qualified sealant system.
Thread Forms — NPT, BSPT, and BSPP
Threaded Flange vs Weld Neck Flange — Engineering Comparison
Flange Facing Types
| Thread Form | Standard | Taper | Thread Angle | Pressure Service | Primary Application |
|---|---|---|---|---|---|
| NPT | ASME B1.20.1 | 1:16 (1/16"/ft) | 60° | Yes — ASME B31.3 | US / international process piping, pressure service |
| BSPT (Rc/R) | BS EN ISO 7-1 | 1:16 | 55° | Yes — with EN codes | European / Asian process piping, EN 1092-1 flanges |
| BSPP (Rp/G) | BS EN ISO 228-1 | None (parallel) | 55° | With sealant only | Hydraulic fittings, utility services, instrument connections |
| PT (Rc) — JIS | JIS B 0203 | 1:16 | 55° | Yes — JIS codes | Japanese plant, refinery, offshore — JIS B2220 flanges |
Dimensional Data, Pressure Classes
& Pressure–Temperature Ratings
Class 150–2500 · PN6–PN250 · Bolt Circle · PCD · Face Dia.
ASME B16.5 Threaded Flange — Class 150 Dimensions
| NPS | OD (mm) | PCD (mm) | No. Bolts | Bolt Size | Flange Thick. (mm) | RF Dia. (mm) | Bore (NPT) |
|---|---|---|---|---|---|---|---|
| 1/2" | 88.9 | 60.3 | 4 | 1/2" ×1-3/4" | 11.2 | 34.9 | 1/2" NPT |
| 3/4" | 98.4 | 69.9 | 4 | 1/2" ×1-3/4" | 12.7 | 42.9 | 3/4" NPT |
| 1" | 108.0 | 79.4 | 4 | 1/2" ×1-3/4" | 14.2 | 50.8 | 1" NPT |
| 1-1/4" | 117.5 | 88.9 | 4 | 1/2" ×1-3/4" | 15.7 | 63.5 | 1-1/4" NPT |
| 1-1/2" | 127.0 | 98.4 | 4 | 1/2" ×2" | 17.5 | 73.0 | 1-1/2" NPT |
| 2" | 152.4 | 120.7 | 4 | 5/8" ×2-1/2" | 19.1 | 92.1 | 2" NPT |
| 2-1/2" | 177.8 | 139.7 | 4 | 5/8" ×2-1/2" | 22.4 | 104.8 | 2-1/2" NPT |
| 3" | 190.5 | 152.4 | 4 | 5/8" ×2-1/2" | 23.9 | 127.0 | 3" NPT |
| 4" | 228.6 | 190.5 | 8 | 5/8" ×2-1/2" | 23.9 | 157.2 | 4" NPT |
ASME B16.5-2017. All dimensions in mm except bolt sizes (inch). OD = flange outside diameter; PCD = bolt circle diameter; RF Dia. = raised face diameter; Bore (NPT) = thread designation per ASME B1.20.1. Flange thickness measured at the thinnest point of the flange hub. For Class 300 and higher, OD and PCD increase — see ASME B16.5 Tables 9–14.
Pressure–Temperature Ratings — Key Material Groups
| Pressure Class | Rating at 20°C (bar) | Rating at 100°C (bar) | Rating at 200°C (bar) | Rating at 300°C (bar) | Rating at 400°C (bar) | Max Temp (°C) |
|---|---|---|---|---|---|---|
| Class 150 | 19.6 | 17.7 | 15.1 | 13.4 | 11.5 | 538 |
| Class 300 | 51.1 | 46.6 | 39.8 | 35.1 | 30.0 | 538 |
| Class 600 | 102.1 | 93.2 | 79.6 | 70.2 | 60.1 | 538 |
| Class 900 | 153.2 | 139.8 | 119.4 | 105.3 | 90.1 | 538 |
| Class 1500 | 255.3 | 232.9 | 199.0 | 175.5 | 150.1 | 538 |
| Class 2500 | 425.4 | 388.1 | 331.7 | 292.5 | 250.2 | 538 |
ASME B16.5-2017 Table 2-1.1, Material Group 1.1 (A105, A182 F316, A182 F304 and equivalents). Ratings de-rate with increasing temperature — verify the actual operating temperature against the P-T table for the specific material group before specifying flange class. Threaded flanges in ASME B31.3 normal service are limited in practice to Class 150 for most water/utility services and Class 300–900 for small-bore process connections.
| DN | NPS Equiv. | OD (mm) | PCD (mm) | No. Bolts | Bolt Size | Flange Thick. (mm) | RF Dia. (mm) |
|---|---|---|---|---|---|---|---|
| DN 15 | 1/2" | 95 | 65 | 4 | M12 | 14 | 45 |
| DN 20 | 3/4" | 105 | 75 | 4 | M12 | 16 | 58 |
| DN 25 | 1" | 115 | 85 | 4 | M12 | 16 | 68 |
| DN 32 | 1-1/4" | 140 | 100 | 4 | M16 | 18 | 78 |
| DN 40 | 1-1/2" | 150 | 110 | 4 | M16 | 18 | 88 |
| DN 50 | 2" | 165 | 125 | 4 | M16 | 20 | 102 |
| DN 65 | 2-1/2" | 185 | 145 | 8 | M16 | 22 | 122 |
| DN 80 | 3" | 200 | 160 | 8 | M16 | 24 | 138 |
| DN 100 | 4" | 220 | 180 | 8 | M16 | 24 | 158 |
EN 1092-1:2018 Type 13 (Screwed Flange), PN 16. Thread bore per EN 10226-1 (BSPT) for pressure service or EN ISO 228-1 (BSPP) for hydraulic/utility service. Bolt sizes are metric per EN 1092-1 — compare with ASME B16.5 inch bolts. EN 1092-1 uses DN / PN designation system vs ASME NPS / Class system.
Governing Standards
ASME B16.5 is the primary standard for pipe flanges and flanged fittings from NPS 1/2" through NPS 24", covering threaded flanges in Classes 150 through 2500. It specifies dimensions, tolerances, facing types, pressure-temperature ratings, bolting dimensions and materials. ASME B16.47 covers large-diameter flanges NPS 26" through NPS 60" (threaded flanges are not typically used at these sizes). EN 1092-1 is the European equivalent, covering flanges in DN 10 through DN 4000 at pressure ratings PN 2.5 through PN 400. JIS B2220 is the Japanese standard, widely used in Asian refinery and petrochemical projects. ASME B31.3 governs the design and construction of process piping systems and imposes additional requirements on threaded flange use (non-severe service only). ASME B1.20.1 defines NPT thread dimensions; BS EN ISO 7-1 defines BSPT; BS EN ISO 228-1 defines BSPP.
// P_T_rating from ASME B16.5 Table 2 (material group) at operating temperature T_op
// P_op = design pressure including pressure surges and safety factor per design code
// WORKED EXAMPLE: A105 threaded flange, P_op = 25 bar, T_op = 150°C
// Class 150 rating at 150°C ≈ 16.4 bar → INSUFFICIENT (25 > 16.4)
// Class 300 rating at 150°C ≈ 43.2 bar → SUFFICIENT (25 < 43.2)
Class_selected = Class 300 // Use Class 300 A105 threaded flange for this service
Material Grades, Mechanical Properties
& Surface Treatments
Galvanised · Painted · Passivated · NACE · Cryogenic
| Grade | ASTM Standard | ASME B16.5 Group | Yield (MPa) | Temp Range (°C) | Corrosion | Key Service |
|---|---|---|---|---|---|---|
| A105 | ASTM A105 | 1.1 | 250 min | −29 to +538 | Low | Standard carbon steel — most common threaded flange material |
| A350 LF2 | ASTM A350 | 1.1 | 250 min | −46 to +343 | Low | Low-temperature carbon steel — Charpy tested to −46°C |
| A182 F11 | ASTM A182 | 4 | 205 min | −29 to +593 | Low | 1-1/4Cr-1/2Mo — elevated temperature service |
| A182 F22 | ASTM A182 | 5 | 205 min | −29 to +649 | Low | 2-1/4Cr-1Mo — high-temp, hydrogen service |
| A182 F304 | ASTM A182 | 2.1 | 205 min | −196 to +816 | High | Austenitic SS — general corrosion service |
| A182 F316L | ASTM A182 | 2.2 | 170 min | −196 to +816 | Very High | Low-carbon SS — offshore, chloride, chemical |
| A182 F51 (Duplex) | ASTM A182 | 3.1 | 450 min | −50 to +300 | Very High | Duplex 2205 — offshore, sour service, seawater |
| A182 F53 (Super Duplex) | ASTM A182 | 3.2 | 550 min | −50 to +280 | Extreme | Super duplex — severe sour service, subsea |
| B564 N06625 (Inconel 625) | ASTM B564 | — | 276 min | −196 to +980 | Extreme | High-temp, acid, chemical, cryogenic service |
| B564 N10276 (Hastelloy C276) | ASTM B564 | — | 283 min | −196 to +1038 | Extreme | Most corrosive environments — HCl, H₂SO₄, FGD |
Material Selection Guidance
ASTM A105 is the standard forgeable carbon steel material for threaded flanges in normal temperature carbon steel service (−29°C to +538°C). It covers all ASME B16.5 pressure classes (150 through 2500) and is compatible with hot-dip galvanising, epoxy coating and primer-paint surface treatments. A105 is the most widely stocked grade worldwide and is the default specification unless service conditions require a different material. For low-temperature carbon steel service below −29°C, ASTM A350 LF2 is required — it is Charpy impact tested to −46°C and is the standard LTCS (low-temperature carbon steel) flange material for LNG, cryogenic utility and cold-climate piping.
ASTM A182 F11 (1-1/4Cr-1/2Mo) and F22 (2-1/4Cr-1Mo) threaded flanges are specified for elevated temperature service in refinery, power generation and hydrogen service where A105 has insufficient creep resistance above 400°C. F22 is rated to 649°C and is used in high-temperature hydrogen process piping (hydrocracker, reformer) where the Nelson Curves (API RP 941) must be satisfied. F11 and F22 are also required in ASME B31.3 Category M fluid service at elevated temperature. Both grades require PWHT (post-weld heat treatment) if any welding is performed on the assembled piping system.
Threaded flanges in H≶S-containing service must comply with NACE MR0175 / ISO 15156. Carbon steel A105 is typically compliant (hardness ≤22 HRC / 237 HB) but must be verified on each heat/lot — some A105 forgings can exceed NACE limits if improperly heat-treated. Alloy steel A182 F11 and F22 are NACE-compliant at restricted hardness levels (≤22 HRC / 22 HRC respectively, per NACE Part 2). Stainless steel A182 F316L is NACE-compliant per Part 2 without restrictions for austenitic SS. Duplex 2205 (F51) and super duplex (F53) are NACE-compliant within their hardness limits per NACE Part 3. For offshore sour service threaded flanges, EN 10204 3.2 MTC with full cross-section hardness survey, PMI and TPI countersignature are mandatory.
| Finish | Standard | Thickness (µm) | Temp Limit (°C) | Thread Zone | Notes & Application |
|---|---|---|---|---|---|
| Plain / Bare | — | — | — | Uncoated | Stainless and alloy steel flanges; carbon steel for immediate installation |
| Primer paint (rust preventive) | Project spec | 25–75 | — | Protect or clear | Carbon steel storage protection; applied before dispatch; remove from thread bore and face before installation |
| Hot-dip galvanise | ASTM A153 Class D | 45–86 | 200 | HDG may block thread | Carbon steel threaded flanges for water / utility outdoor service; re-tap NPT bore after galvanising |
| Epoxy spray coat | BS EN 13438 | 75–150 | 120 | Protect thread bore | Exterior of carbon steel flanges; internal thread bore must be masked during application |
| Fusion-bonded epoxy (FBE) | AWWA C550 | 300–500 | 80 | Bore masked | Water utility threaded flanges; potable water compatible; bore and face masked |
| Yellow zinc chromate | Project spec | 8–20 | 150 | Thin — minimal impact | Temporary protection of machined faces and threads during storage / transit |
| SS passivation | ASTM A380 | N/A | 870 | Passivated | All stainless and duplex threaded flanges; restores passive oxide film; no dimensional change |
| Electropolish + passivation | ASTM A967 + A380 | −5 to −25 µm | 870 | Bore electropolished | Food-grade, pharmaceutical, cleanroom SS threaded flanges; Ra ≤ 0.8 µm on bore |
Hot-dip galvanising on threaded flanges deposits zinc inside the NPT bore, reducing the effective bore size and potentially blocking thread engagement. NPT threads must be re-tapped after galvanising. Specify "HDG + re-tapped NPT bore" on purchase orders for galvanised threaded flanges. Primer paint applied to flange faces and thread bores must be removed prior to installation — paint on the gasket seating surface or thread bore will cause both gasket seal failures and thread engagement problems.
Inspection, QC Protocols, Applications
& Export Documentation
Process Piping · Utility · Instruments · Offshore · Hydraulic
Inspection and Quality Control
Threaded flange dimensions are verified per ASME B16.5 Table 9–14 (ASME) or EN 1092-1 (EN) for each heat/lot. Critical dimensions include: outside diameter (OD), bolt circle diameter (PCD), flange thickness, raised face diameter, raised face height, and bore diameter. The flange face finish must be measured by profilometer to confirm surface roughness within ASME B16.5 Table 2 limits: serrated concentric or phonographic finish for RF (125–250 Ra), smooth for RTJ groove walls (≤3.2 Ra), smooth for FF with full-face gaskets. Thread gauge inspection per the relevant thread standard (ASME B1.20.1 for NPT, ISO 7-1 for BSPT, ISO 228-1 for BSPP) must be performed from the thread entry end on each flange — Go/No-Go ring and plug gauges used.
NPT thread gauging for threaded flanges uses a precision taper ring gauge (Go and Not-Go) per ASME B1.20.1. The acceptance criterion for the thread bore is: the large end of the plug gauge must be flush with (or within ±1 turn of) the thread entry face. A threaded flange bore that is cut too shallow produces an NPT thread that projects excessively beyond the pipe, reducing the effective thread engagement length. A bore cut too deep produces insufficient projection, meaning the pipe and flange thread do not engage over the required minimum length. Both conditions cause premature joint leakage under pressure cycling or temperature change. For Class 600 and above threaded flanges, NPT thread gauging must be performed 100% per unit (not just sampled) for safety-critical piping service.
Hardness testing (Brinell HB per ASTM E10 or Vickers HV per ISO 6507) is performed on each heat/lot for all alloy and stainless grades. For NACE sour service flanges, hardness must be verified ≤22 HRC (237 HB) on the flange body and on the threaded bore section specifically — thread cutting can locally work-harden the bore surface above NACE limits. PMI (Positive Material Identification) by XRF or OES spectrometry is performed 100% on all stainless, duplex, super duplex, Inconel and Hastelloy threaded flanges to confirm alloy identity.
Shell pressure testing (hydrostatic) of threaded flanges is performed per ASME B16.5 Paragraph 8.2 when specified by the purchaser. The test pressure is 1.5× the Class 150 rating at ambient temperature (or for higher classes, 1.5× the class rating at ambient temperature). The test is applied with the NPT bore plugged or with a test pipe stub threaded in. Hydrostatic testing is not routinely performed on stock flanges but is mandatory for project supply when called for in the purchase order or Piping Material Specification (PMS/MDS).
Applications by Industry
Threaded flanges are the standard terminal fitting for small-bore (NPS 1/2" to 2") instrument take-off connections, pressure gauge tappings, thermometer wells, chemical injection points, drain and vent connections on process vessels and piping. The threaded flange screws directly onto the instrument nipple or threaded pipe stub without welding, allowing instrument connections to be made, changed, or re-routed without hot work. In refinery and petrochemical plants, Class 300 or Class 600 A105 threaded flanges with NPT bore are the standard for all normal-service instrument connections; Class 900 for high-pressure services.
Class 150 A105 HDG threaded flanges are the standard for above-ground water mains connections, fire water system instrument take-offs, cooling water system vents and drains, and general utility piping connections where welding is not economical or not permitted (galvanised piping systems, systems containing combustibles or oxygen). The low operating pressure of water utility service (typically 10–15 bar maximum) is well within the Class 150 rating (19.6 bar at 20°C for A105). NPT thread connection with PTFE tape or pipe thread compound provides a reliable leak-free joint at these pressure and temperature conditions.
On offshore platforms, stainless steel 316L (A182 F316L) and duplex 2205 (A182 F51) threaded flanges with NPT bore and raised face are used for chemical injection quill connections, methanol injection points, corrosion inhibitor injection flanges and small-bore process sampling connections. The no-weld installation eliminates hot-work permit requirements in Zone 1/Zone 2 hazardous areas, making threaded flanges the preferred terminal fitting for chemical injection and sampling systems on operating platforms. EN 10204 3.2 MTC, PMI, NACE hardness compliance and TPI countersignature are standard offshore project requirements.
Threaded flanges with BSPP bore (G-thread per ISO 228-1) are used extensively in hydraulic power unit manifold connections, pneumatic control panel connections and instrumentation tubing terminations. These are not pressure-rated service applications in the ASME B31.3 sense — they are below the code pressure threshold — and the parallel BSPP thread with an O-ring or DOWTY seal face provides a reliable, easily assembled connection for hydraulic and pneumatic service at moderate pressures. Hydraulic system flanges per SAE J518 and ISO 6162 use a different standard (O-ring face seal) and are distinct from process pipe threaded flanges.
Electropolished stainless steel 316L threaded flanges (A182 F316L) with passivation per ASTM A380 and BSPP bore are used in food processing and pharmaceutical plant for connecting sample valves, CIP (Clean-In-Place) connection points, sight glasses and small-bore flow measurement instruments. The electropolished bore surface (Ra ≤ 0.8 µm) and threaded connection eliminate crevices and allow full CIP cleaning of the bore. 3-A Sanitary Standards and EHEDG guidelines specify surface finish and material requirements for food-contact flanges.
Export Packaging and Preservation
- Threaded flanges packed individually in heat-sealed polypropylene bags or bubble wrap sleeves; each bag labelled with PO number, standard (ASME B16.5 / EN 1092-1), NPS/DN, pressure class, material grade, thread form (NPT/BSPT/BSPP), facing type (RF/FF/RTJ), heat/lot number and quantity
- Flanges faces (gasket contact surfaces) protected with cardboard or foam inserts; protectors must prevent damage to the precision-machined serrated finish (AARH 125–250) which is critical for gasket sealing
- Thread bore sealed with PVC threaded plugs before packaging to prevent contamination of NPT threads during transit and storage
- RTJ groove flanges must have individual ring groove protective inserts to prevent damage to the precision-machined oval/octagonal groove profile during transit
- Bags packed in double-wall corrugated cartons with foam void fill; cartons on ISPM-15 heat-treated timber pallets with stretch wrap and steel strapping for export
- MTC, dimensional inspection report (OD, PCD, flange thickness, RF height), thread gauge certificate (NPT/BSPT/BSPP), face finish report, hardness survey, PMI (for alloy/SS/duplex), hydrostatic test report (if specified), all project documents in waterproof sealed envelope attached to crate exterior
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.2 for offshore / NACE / high-temperature / safety-critical service |
| 02 | Dimensional Inspection Report | ASME B16.5 / EN 1092-1 | AQL 1.0 per ISO 2859; OD, PCD, thickness, RF height, bore as mandatory chars |
| 03 | Thread Gauge Certificate | ASME B1.20.1 / ISO 7-1 / ISO 228-1 | Go/No-Go ring gauge per heat lot; 100% for Class 600+ safety-critical service |
| 04 | Face Finish Report | ASME B16.5 Table 2 / project spec | Profilometer Ra — 125–250 AARH for RF; ≤3.2 Ra for RTJ groove; stated per lot |
| 05 | Hardness Test Report | ASTM E10 / ISO 6507 | Per heat/lot; cross-section mapping for NACE sour service flanges |
| 06 | PMI Report (XRF / OES) | Project specification | 100% of SS, duplex, super duplex, Inconel, Hastelloy threaded flanges |
| 07 | Chemical Analysis Report | ASTM A105 / A182 / A350 | Included in MTC — verify against grade limits incl. CE for weldable grades |
| 08 | Charpy Impact Test Report | ASTM A350 / project spec | Mandatory for LTCS A350 LF2; specified for low-temp service below −29°C |
| 09 | Hydrostatic Test Report | ASME B16.5 Para.8.2 | When specified in PO or PMS/MDS; 1.5× class pressure at ambient |
| 10 | Heat Treatment Certificate | ASTM A105 / A182 / A350 | Required for F11, F22 and Q&T grades; normalise and temper for A105 if specified |
| 11 | Surface Coating Certificate | ASTM A153 / A380 / A967 | Required for all coated finishes; HDG re-tapped bore verification stated |
| 12 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include flanges and forged fittings manufacture |
| 13 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports threaded flanges to ASME B16.5, EN 1092-1 and JIS B2220 in all pressure classes (Class 150 through Class 2500, PN 6 through PN 400) across the full material range: A105, A350 LF2, A182 F11/F22, A182 F304/F304L/F316/F316L, A182 F51/F53/F60, B564 Inconel 625, Hastelloy C276 and titanium. Thread forms: NPT, BSPT, BSPP, PT (JIS). Facing types: RF, FF, RTJ. Full EN 10204 3.1/3.2 MTC, thread gauge certificates, face finish reports, PMI, Charpy impact, hydrostatic test, NACE hardness mapping and TPI witness by BV/DNV/Lloyds/SGS/TÜV. 48-hour express dispatch on standard in-stock sizes.