Double Ferrule Tube Fittings

Products  ·  Applications  ·  Quality  ·  Resources  ·  About  ·  Contact

Home › Products › Tube Fittings › Double Ferrule Fittings

How the two-ferrule joint works

Each tube end carries a front ferrule and a back ferrule inside one nut. Hand-tightening pushes the tube into the body until it bottoms. Wrench turns drive the back ferrule forward, swaging the front ferrule onto the tube outside diameter while biting into the tube.

The front ferrule seals against the body cone; the back ferrule holds against vibration and pull-out, giving a metal-to-metal seal with no elastomer. Tighten 1-1/4 turns past finger-tight to 1 in (25 mm), 3/4 turn below, pre-swage above. Never loosen a fitting under pressure.

Materials and pressure ratings

Bodies, nuts and ferrules are 316 or 316L stainless steel. Thread pressure depends on metal at the threaded end: a 1/16 in NPT male thread is rated 11,000 psig (757 bar) in 316 stainless, or 5,500 psig (378 bar) in brass.

Thread working pressures are referenced to ANSI/ASME B31.3 at ambient temperature. Catalogue pages 1-01 to 1-70 publish no pressure or temperature column, so R-LOK issues ratings for the exact size, tube wall and material on the quotation.

Series codes and sizes

Every series code starts with S for the body: SU straight union, SRU reducing union, SBHU and SBHRU bulkheads, SMC and SGMC male connectors, SFC female connector, SSMC and SOSC O-ring seal connectors, SPWC and SSWC weld connectors, SP plug and SC connector.

Elbows cover SE and SLE, SHE, SME, SHME, SMPWE and SSWE, plus SPHE, SPHME and SPME adjustable. Branch shapes are ST, SUT, SET, SMET, SMT, SRT and SRET. Metric codes carry an M suffix; SRU-6M-4M joins 6 mm to 4 mm tube.

Standards referenced: NPT thread form per ASME B1.20.1 and SAE AS71051; ISO 7-1, EN 10226-1, BS 21, DIN 2999 and JIS B0203 taper threads; ISO 228-1, DIN 228-1, JIS B0202 and BS 2779 parallel threads; Thread working pressures referenced to ANSI/ASME B31.3 at ambient temperature; 37 degree AN flare ends to SAE practice.

Typical duties: Instrument, sample and analyser lines on gas skids; Petrochemical and chemical analyser systems; High-purity gas and chemical delivery in semiconductor fabs; Pharmaceutical and bioprocess lines needing weld-free joints; Hydrogen, battery and new energy process skids; Hydraulic and pneumatic control panels.

Double ferrule tube fitting model selection guide – SMC male connector (NPT)

Series / model	Size	Connection	Working pressure	Body material	Notes
SMC-1-1N	1/16 in. tube OD	1/16 male NPT	11000 psig (757 bar)	316 stainless steel	d min. 1.27 mm; hex h 5/16 in.; A 8.63, B 10.92, I 20.00, L 23.83 mm
SMC-1-2N	1/16 in. tube OD	1/8 male NPT	10000 psig (690 bar)	316 stainless steel	d min. 1.27 mm; hex h 7/16 in.; A 8.63, B 10.92, I 22.35, L 26.23 mm
SMC-2-2N	1/8 in. tube OD	1/8 male NPT	10000 psig (690 bar)	316 stainless steel	d min. 2.28 mm; hex h 7/16 in.; A 12.70, B 15.24, I 23.87, L 30.48 mm
SMC-2-4N	1/8 in. tube OD	1/4 male NPT	8000 psig (551 bar)	316 stainless steel	d min. 2.28 mm; hex h 9/16 in.; A 12.70, B 15.24, I 28.95, L 35.56 mm
SMC-2-6N	1/8 in. tube OD	3/8 male NPT	7800 psig (537 bar)	316 stainless steel	d min. 2.28 mm; hex h 11/16 in.; A 12.70, B 15.24, I 29.21, L 35.81 mm
SMC-3-2N	3/16 in. tube OD	1/8 male NPT	10000 psig (690 bar)	316 stainless steel	d min. 3.04 mm; hex h 7/16 in.; A 13.71, B 16.00, I 24.63, L 31.24 mm
SMC-4-2N	1/4 in. tube OD	1/8 male NPT	10000 psig (690 bar)	316 stainless steel	d min. 4.82 mm; hex h 1/2 in.; A 15.24, B 17.78, I 25.40, L 32.76 mm
SMC-4-4N	1/4 in. tube OD	1/4 male NPT	8000 psig (551 bar)	316 stainless steel	d min. 4.82 mm; hex h 9/16 in.; A 15.24, B 17.78, I 30.48, L 37.84 mm
SMC-4-6N	1/4 in. tube OD	3/8 male NPT	7800 psig (537 bar)	316 stainless steel	d min. 4.82 mm; hex h 11/16 in.; A 15.24, B 17.78, I 30.98, L 38.35 mm
SMC-4-8N	1/4 in. tube OD	1/2 male NPT	7700 psig (530 bar)	316 stainless steel	d min. 4.82 mm; hex h 7/8 in.; A 15.24, B 17.78, I 37.33, L 44.70 mm
SMC-6-4N	3/8 in. tube OD	1/4 male NPT	8000 psig (551 bar)	316 stainless steel	d min. 7.11 mm; hex h 5/8 in.; A 16.76, B 19.30, I 32.51, L 39.87 mm
SMC-6-6N	3/8 in. tube OD	3/8 male NPT	7800 psig (537 bar)	316 stainless steel	d min. 7.11 mm; hex h 11/16 in.; A 16.76, B 19.30, I 32.51, L 39.87 mm
SMC-8-8N	1/2 in. tube OD	1/2 male NPT	7700 psig (530 bar)	316 stainless steel	d min. 10.41 mm; hex h 7/8 in.; A 22.86, B 21.84, I 38.86, L 49.02 mm
SMC-8-12N	1/2 in. tube OD	3/4 male NPT	7300 psig (502 bar)	316 stainless steel	d min. 10.41 mm; hex h 1-1/16 in.; A 22.86, B 21.84, I 40.38, L 50.54 mm

Selection notes: The 1-10 ordering table for SMC (male connector) prints only model code, tube OD, NPT thread size, hex width and mm dimensions; seat/packing and Cv are not published in this section, so those columns are omitted. Working pressures are the NPT thread-end ratings printed on page 1-01 under ‘316 stainless steel – male thread’, quoted in psig and bar exactly as printed; 1-01 also publishes a brass column for the same thread sizes.

Catalogue reference 1-10, 1-01. Confirm the final rating against the data sheet issued with your order.

See also: High Purity Weld Fittings · Threaded Fittings · Tube Fittings

Frequently asked questions

What is the pressure rating of these fittings?

Catalogue 1-01 to 1-70 carries no pressure column for the joints. Page 1-01 gives 11,000 psig (757 bar) for a 1/16 in 316 stainless NPT thread down to 3,900 psig (268 bar) at 2 in, referenced to ANSI/ASME B31.3. R-LOK confirms the rating on quotation.

Does a complete fitting include both ferrules and the nut?

Assemblies such as SU, SMC and SBHU come complete with body, nut and both ferrules. Ferrules and nuts are listed separately (SN nut, SFF and SBF ferrules), so a joint is re-made without replacing the body; SFS kits hold ten front and ten back ferrules.

Can you supply NPT and BSP threads in the same order?

Yes. The tables list NPT ends per ASME B1.20.1, ISO 7/1 taper and ISO 228 parallel ends, plus SAE/MS straight-thread and 37 degree AN flare. NPT is supplied where ISO is listed and ISO where NPT is listed, so state the thread standard when asking.