Nipples & Couplings

Part 01

Pipe Nipple Types, Length Classifications
& Engineering Selection

Part 01 — Nipple Types, Length Classifications & Selection

Close Nipple · Short Nipple · Barrel Nipple · Hex Nipple
Swage Nipple · ASME B1.20.1 · NPT · BSPT

Pipe Nipple · Close Nipple · Short Nipple · Long Nipple · Barrel Nipple · Hex Nipple · Swage Nipple · Concentric Swage · Eccentric Swage · Bull Plug · Round Head Plug · NPT · BSPT · 

Definition and Engineering Function

A pipe nipple is a short length of pipe with threads on both ends, used to connect two female-threaded fittings, valves, instruments or equipment connections. Nipples are the fundamental threaded pipe extension fitting — they are used wherever a short threaded pipe connection is needed between two components, where the pipe cannot be extended directly to reach the mating connection, or where a precise length of threaded pipe is required in a confined space.

Nipples are manufactured from pipe per ASME B36.10M (carbon and alloy steel) or B36.19M (stainless and duplex), cut to the required length, and threaded on both ends per ASME B1.20.1 (NPT, US standard) or ISO 7-1 (Rp/Rc/BSPT, UK/European standard). The thread taper on both NPT and BSPT threads creates a self-sealing mechanical interference when tightened — the tapered threads deform slightly and wedge together, creating a metal-to-metal seal that is supplemented by PTFE tape or pipe thread sealant compound for pressure service.

Engineering Note — NPT vs BSPT Thread Compatibility

NPT (National Pipe Taper, ASME B1.20.1) and BSPT (British Standard Pipe Taper, ISO 7-1) are not interchangeable. Both are tapered thread forms with a 1:16 taper rate, but they differ in thread pitch (TPI) for the same nominal pipe size, thread form angle (NPT = 60°, BSPT = 55°), and thread crest/root geometry. Attempting to mate an NPT male with a BSPT female (or vice versa) produces a cross-threaded joint that cannot seal and will damage both fittings. Always confirm the thread standard of all mating components before ordering nipples — instrument connections in particular frequently mix NPT (North American process instruments) with BSPT (European-origin valves and fittings) in the same system, creating a specification hazard that must be resolved by specifying a conversion nipple (NPT male × BSPT male or equivalent).

Request a Formal Quotation — Nipples & Couplings, All Types & Materials

NPT / BSPT · 1/8"–12" · CS / SS 304/316 / Duplex · Close / Short / Long / Barrel · EN 10204 MTC

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Nipple Types — Engineering Descriptions

Close Nipple (All-Thread)

ASME B1.20.1 · No bare pipe

The shortest possible nipple — threads run from end to end with no unthreaded (bare) pipe section in between. Both threads meet at the centre of the nipple body. Minimum overall length is determined by the thread engagement length for the specified NPS. Used where the two female fittings must be as close together as possible. Cannot be gripped with a pipe wrench on the bare pipe — must be installed using a nipple chuck or strap wrench. Difficult to disassemble without damaging the threads.

Short Nipple

ASME B1.20.1 · Minimal bare section

A nipple with a short unthreaded (bare) pipe section between the two threaded ends. The bare section is just long enough to grip with a pipe wrench for installation and removal. Overall length typically 38–76 mm for 1/2"–2" NPS. The standard short nipple length is defined by the pipe thread engagement length plus the minimum bare pipe section needed for wrench grip. The most common nipple type for instrument connections, valve bodies, and general process connections.

Long Nipple / Barrel Nipple

Custom length · Sch 40 to XXS

A nipple with a long unthreaded centre section. Long nipples are specified by giving the total overall length (e.g. "3/4" NPT × 150 mm long"). Barrel nipples have an even longer unthreaded section — effectively a short pipe spool with threaded ends. Used where two female connections are further apart than a short nipple can bridge, or where the instrument or valve connection must be positioned at a specific distance from the process pipe. Commonly used for instrument root valves, chemical dosing connections, and sample point connections.

Hex Nipple (Reducing Nipple)

ASME B16.11 · Two different NPS

A nipple with different thread sizes on each end (e.g. 1" NPT one end, 3/4" NPT other end) and a hexagonal body for wrench engagement. Used as a reducer in threaded piping systems, eliminating the need for a separate coupling and reducer fitting. The hex body provides a wrenching flat that does not damage the pipe OD. Standard for connecting different-bore threaded connections in instrument manifolds, valve bodies and small-bore piping. Specified as Larger NPS × Smaller NPS.

Swage Nipple (Concentric/Eccentric)

ASME B16.11 / MSS SP-95 · Size reducer

A forged fitting that transitions from one pipe size to a smaller pipe size, with male NPT threads on the larger end and male NPT threads on the smaller end. Concentric swage nipples have the two bore centrelines aligned (same centreline). Eccentric swage nipples have the bottom of both bores aligned (flat bottom) — used in liquid drain lines to prevent dead legs. Available in plain end (PE) or threaded end (TE) configurations. Used for inline size reduction without requiring a separate coupling and reducer fitting.

Bull Plug / Hex Head Plug

ASME B16.11 · Thread closure

A solid forged plug with male external threads used to close off a female-threaded opening (coupling, tee branch, valve body port). Bull plugs have a round or hex external head and are the standard method for blanking off instrument, drain and vent connections in process piping. Hex head plugs provide a wrenching flat. Not to be confused with a pipe cap (which uses a female thread over the outside of a male pipe end).

Part 02

Coupling Types, Thread Standards
& Dimensional Reference

Part 02 — Coupling Types, Thread Standards & Dimensions

Full Coupling · Half Coupling · Reducing Coupling · Union
NPT ASME B1.20.1 · BSPT ISO 7-1 · Thread TPI

Full Coupling · Half Coupling · Reducing Coupling · Union · NPT · BSPT · BSPP · Socket Weld Coupling · ASME B16.11 · MSS SP-83 · Thread Engagement · TPI · 

Sourcing Nipples & Couplings for a Process or Instrument System?

All thread standards · CS / SS / Duplex · Close / Short / Long · Full / Half / Reducing Coupling · EN 10204 MTC

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Coupling Types — Engineering Descriptions

Full Coupling (Equal Coupling)

ASME B16.11 · Both ends same NPS

A short cylindrical fitting with female internal threads on both ends of the same NPS. Used to join two male-threaded pipe ends or nipples together inline. The most common coupling type in small-bore threaded piping. Per ASME B16.11, full couplings are available in Class 2000, 3000 and 6000 pressure ratings (corresponding to Sch 80, Sch 160 and Sch XXS wall pipe equivalent). Also available as socket weld couplings (SW) where the pipe ends are socket-welded rather than threaded.

Half Coupling

ASME B16.11 · One threaded end

A fitting with female internal threads on one end only — the other end has a plain weld end (butt weld or socket weld) that is welded to the main pipe run or vessel nozzle. Half couplings are the standard method for adding a small-bore threaded branch connection to a large-bore run pipe or vessel — a hole is drilled in the run pipe, the half coupling is fillet-welded over the hole (for socket weld) or butt-welded (for butt weld), and the branch instrument or valve is then screwed into the coupling. Available in Class 2000, 3000 and 6000 per ASME B16.11.

Reducing Coupling

ASME B16.11 · Two different NPS

A coupling with female internal threads of different NPS on each end — used to connect pipes or nipples of two different sizes inline. The larger-bore thread is on one end and the smaller-bore thread is on the other. Specified as Larger NPS × Smaller NPS (e.g. 2" × 1"). Eliminates the need for a separate full coupling and hex nipple reducer pair. Standard for inline size changes in threaded piping systems, chemical dosing lines, and instrument impulse lines.

Union (Three-Piece)

ASME B16.39 · Disconnectable

A three-piece fitting: two male-threaded ends (the union end pieces) that butt together face-to-face, held together by a large hex ring nut. The joint between the two end faces is sealed by a ground flat face or a seat and cone. Unions allow a threaded piping section to be disconnected and reconnected at any time without rotating the pipe — the ring nut is simply unscrewed to separate. Essential for any instrument, equipment or pipe section that must be periodically removed for maintenance or calibration. Available in iron, carbon steel, stainless and alloy materials.

Socket Weld Coupling (SW)

ASME B16.11 · Class 3000/6000

A coupling with socket (counterbore) ends into which the pipe is inserted and fillet-welded. The pipe sits in the counterbore with a small gap (typically 1.6 mm) between the pipe end and the socket bottom — this gap prevents stress concentration from differential thermal expansion of the socket and pipe wall during cyclic service. Preferred over threaded couplings for high-pressure, high-temperature or cyclic service where thread engagement may loosen. Not permitted where crevice corrosion or product purity concerns apply (internal dead leg at socket gap).

Threaded Coupling — SW × Threaded

ASME B16.11 · Mixed end type

A coupling with one socket weld end and one female threaded end. Used at the transition between socket weld piping and threaded instrument or equipment connections — for example, connecting a threaded instrument to a socket weld piping system without requiring an additional nipple and coupling pair. Allows a direct socket weld to the pipe on one side and a threaded connection on the other. Available in Class 3000 and 6000.

Table 1 — NPT Thread Data: ASME B1.20.1 (Key Sizes)

NPS	TPI	Major Dia. (mm)	Pitch (mm)	Thread Length (mm)	Engagement Length (mm)	BSPT TPI (ISO 7-1)
1/8"	27	10.29	0.941	6.8	4.1	28
1/4"	18	13.72	1.411	10.4	5.8	19
3/8"	18	17.15	1.411	10.9	6.7	19
1/2"	14	21.34	1.814	13.6	8.1	14
3/4"	14	26.67	1.814	13.9	9.1	14
1"	11½	33.23	2.209	17.3	10.4	11
1½"	11½	48.26	2.209	18.4	11.1	11
2"	11½	60.33	2.209	19.1	11.7	11
3"	8	88.89	3.175	23.4	14.7	11
4"	8	114.30	3.175	25.7	17.1	11

ASME B1.20.1-2013. All NPT threads have a 1:16 (1/16 inch per inch) taper per side. BSPT (ISO 7-1) also has a 1:16 taper but different TPI and thread form (55° Whitworth vs 60° V-form for NPT). NPT and BSPT are NOT interchangeable — always confirm thread standard of all mating components. Thread Engagement Length = effective thread length that carries the seal load — minimum insertion depth for leak-free make-up. Apply PTFE tape (2–3 wraps) or pipe thread compound (Rectorseal, Jet-Blue or equivalent) on all pressure-rated NPT and BSPT joints.

Part 03

Material Grades, Pressure Class Ratings
& NACE Compliance

Part 03 — Materials, Pressure Ratings & NACE Compliance

A105 · A106 Gr.B · A182 F304L/F316L · A815 F51
Class 2000/3000/6000 · NACE MR0175 · ASME B16.11

A105 · A106 Gr.B · A182 F304L · F316L · F321 · F51 Duplex · F53 Super Duplex · Class 2000 · 3000 · 6000 · NACE MR0175 · Galvanic · CS · SS · Inconel · 

Table 2 — Material Grades for Nipples & Couplings

Material	ASTM Grade	Yield (MPa)	Temp Range (°C)	Corrosion	Key Application
CS Forged	A105 N	≥250	−29 to +538	Low	Standard threaded fittings, couplings, process piping
CS Pipe	A106 Gr.B / A53	≥241	−29 to +538	Low	Nipples machined from pipe; utilities, oil & gas
CS − LTCS	A333 Gr.6	≥241	−46 to +343	Low	Cryogenic, LNG, low-temp process nipples
Alloy F11	A182 F11 Cl.2	≥310	−29 to +593	Moderate	High-temp refinery nipples, H₂ service
Alloy F22	A182 F22 Cl.3	≥310	−29 to +621	Moderate	High-pressure, high-temp couplings
SS 304/304L	A182 F304/F304L	≥170	−196 to +816	High	Chemical, pharma, food, cryogenic connections
SS 316/316L	A182 F316/F316L	≥170	−196 to +816	Very High	Offshore, chloride, chemical, pharma connections
SS 321	A182 F321	≥170	−196 to +816	High	High-temp SS, sensitisation resistance
Duplex 2205	A182 F51	≥450	−50 to +315	Very High	Offshore sour, seawater, chloride connections
Super Duplex	A182 F53/F55	≥550	−50 to +300	Extreme	Subsea, HPHT, severe sour instrument connections
Inconel 625	A182 F625	≥414	−196 to +980	Extreme	High-temp corrosive, acid, offshore
Monel 400	A182 F60	≥207	−196 to +480	Extreme	Hydrofluoric acid (HF), seawater, brine

Table 3 — ASME B16.11 Pressure Class Ratings for Threaded Fittings

Pressure Class	Equivalent Pipe Schedule	A105 Carbon Steel (bar) at 38°C	SS 316L (bar) at 38°C	Typical Application
Class 2000	Sch 80 equivalent	137	103	General process piping; low-medium pressure instruments
Class 3000	Sch 160 equivalent	207	155	High-pressure process piping; process instruments Class 600
Class 6000	Sch XXS equivalent	414	310	Very high-pressure; Class 1500/2500 instrumentation

ASME B16.11-2016. Pressure ratings are at ambient temperature (38°C) for the specified material. Ratings derate significantly with temperature — verify with the applicable ASME B16.11 P-T table for the specific material and operating temperature. For stainless and alloy grades, use the B16.11 ratings for the specific material group. Thread make-up quality (engagement depth, sealing compound or tape, tightening torque) significantly affects the actual leak-free pressure achieved — ASME B16.11 ratings assume properly made-up joints.

NACE Compliance for Sour Service

Nipples, couplings and plugs in sour H≶S service must comply with NACE MR0175 / ISO 15156. For carbon steel A105 forged fittings, the NACE maximum hardness is 22 HRC (237 HB) throughout the cross-section. Standard A105 normalised typically complies — but every heat must be hardness-verified and reported in the MTC. For nipples machined from A106 Gr.B pipe, the same hardness limit applies and the pipe heat must be NACE-qualified. Threaded connections in sour service require careful attention to thread compound compatibility — some thread sealants contain metallic pigments that are not compatible with H≶S service or with stainless/duplex thread surfaces.

Part 04

Applications, Installation Notes
& Quality Control and Documentation

Part 04 — Applications, Installation & QC Documentation

Instrumentation · Drain / Vent · Chemical Injection · Sample Points
Petrochemical · Offshore · Power · Chemical · Food Grade

Instrument Root Connection · Drain Nipple · Vent Nipple · Sample Point · Chemical Injection · Gauge Connection · Relief Valve Connection · Thermowell · Sealant · PTFE Tape · 

Applications by Use Case

Instrumentation Root Connections

Nipples and couplings are the primary connection fittings for instrument root connections throughout process plants — the short threaded pipe sections that connect pressure gauges, pressure transmitters, thermowells, flow elements, level gauges and sample valves to the process pipe or vessel nozzle. A standard instrument root connection assembly consists of: half coupling welded to the process pipe → close or short nipple → root valve (needle or block valve) → instrument body. The close nipple between the half coupling and root valve determines how close the valve can be positioned to the pipe, which in turn determines the degree of thermal isolation or vibration exposure on the instrument. Long nipples are specified where the instrument must be positioned away from hot piping to avoid exceeding the instrument's maximum ambient temperature rating.

Drain and Vent Connections

Nipples, bull plugs and half couplings are standard for all drain and vent connections in process piping — the small-bore (typically 1/2" or 3/4" NPT) connections at low points (drains) and high points (vents) of process piping spools used for draining and venting during commissioning, shutdown and maintenance. Close nipples or short nipples with bull plugs are used where no flow is expected during normal operation — the bull plug is removed during draining or venting operations. Drain and vent nipples in corrosive service must be in the same material as the process pipe — mixing materials (e.g. carbon steel nipple on stainless pipe) creates a galvanic corrosion risk at the weld junction.

Barrel nipples (long nipples) and couplings are used for chemical injection quills in pipelines — the injection quill is a tube that extends into the pipe bore through a threaded half coupling to inject chemical inhibitor or treatment chemical into the flow stream at the pipe centreline (rather than at the pipe wall). The injection quill assembly consists of: half coupling welded to the pipe → gate/ball valve → barrel nipple → quill tube insertion fitting. The barrel nipple length is selected so that the valve is outside the pipe insulation and accessible for operation.

Sample points in process piping — where process fluid is withdrawn for laboratory or on-line analysis — use short or long nipples, ball valves and sample probe assemblies threaded into half couplings welded to the process pipe. The length of the nipple and the depth of any sample probe into the pipe bore are specified by the process chemist or analyst to ensure that the sample is representative of the pipe stream (not a stagnant dead leg). Stainless SS 316L is the minimum material for sample connections in corrosive hydrocarbon service; duplex 2205 for sour service.

Offshore instrument hook-up piping uses nipples, couplings, unions and plugs in duplex 2205 (A182 F51) or SS 316L (A182 F316L) throughout — all offshore instrument connections in hydrocarbon service are in these materials as a minimum. Carbon steel threaded fittings are not used offshore due to corrosion. All offshore nipples and couplings require EN 10204 3.1 MTC as a minimum; 3.2 with TPI for sour service; PMI on duplex grades; hardness survey for NACE sour service.

Installation Requirements — Thread Make-Up

• Thread sealant selection — PTFE tape (2–3 wraps on clean, dry threads, wound in the direction of thread rotation) is standard for most service. Two-part thread compound (Rectorseal, Jet-Blue, etc.) is used for high-pressure or high-temperature service, or where vibration could loosen PTFE tape. Never use PTFE tape on instrument gas connections where tape fragments could reach the instrument orifice or sensor — use anaerobic thread sealant (Loctite 577 or equivalent) instead
• Thread engagement depth — minimum engagement is the thread engagement length per ASME B1.20.1 (Table 1 above). Under-engagement leaves insufficient thread contact for the seal; over-engagement can crack cast or machined female fittings. Count the wrench-tight turns from hand-tight — typically 2–3 wrench turns for NPT joints
• Galvanic compatibility — never thread a carbon steel nipple into a stainless steel coupling in a wet or corrosive environment; the galvanic couple corrodes the carbon steel nipple and seizes the joint. Use SS nipples throughout SS piping systems; use CS nipples throughout CS piping systems
• Vibration service — in high-vibration locations (compressor skids, pump discharge, engine rooms), use locking thread compound (Loctite 243 medium strength or equivalent) or check that socket weld fittings can be used instead of threaded, as threaded joints can loosen under sustained vibration

Quality Control

QC for nipples and couplings covers: (1) Thread gauging — both male and female threads checked with NPT or BSPT Go/No-Go gauge rings and plugs per ASME B1.20.1 or ISO 7-1; (2) Material verification — PMI on all SS, duplex and exotic grades; (3) Hardness — per heat for NACE sour service; (4) Length and OD — dimensional check of overall length, bare pipe section length and OD for nipples; (5) Thread compound compatibility — confirmed with process chemist for sour or corrosive service.

Export Packaging

• Nipples and couplings packed in polypropylene bags or zip-seal bags by NPS, schedule, type (close/short/long) and material; labelled with thread standard (NPT/BSPT), NPS, schedule, material grade and lot number
• Thread protectors on both ends of nipples; inside thread protectors in couplings — plastic protectors only, no metal that could damage thread flanks
• Small sizes (1/8"–1") in compartmented cartons by size and grade; larger sizes in individual poly bags in master cartons
• SS and duplex items segregated from CS items — ferrous contamination on SS thread flanks causes staining and corrosion in service
• MTC (EN 10204 3.1/3.2), thread gauge certificate, PMI report (alloy grades), hardness report (NACE sour service), dimensional inspection report in waterproof sealed envelope with each consignment