Ferrule Fittings

Double ferrule fittings are visually identifiable by the stepped shoulders on straight body and nut. It is commonly made from stainless steel SS316 and brass material. Straight body and nut are made through close grain forgings.

It allows the connection of two straight tubes, either in inches or metric size. The size (OD – outer diameter) is available is 6mm to 42mm and 1/8” to 2”. Double Ferrule tube fitting, also know as instrumentation fittings, makes use of a two ferrule mechanical grip design to secure tubing connections.

Size	1/8"OD to 2"OD
Material	Stainless Steel SS316 Carbon Steel Brass
Working Pressure	Up to 670 bars
Connection Type	Compression end

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Double Ferrule Fittings

What are Double Ferrule Fittings

1. What are double ferrule tube fittings?

Double ferrule fittings are composed of four precision parts: body, nut, front and back ferrule. The addition of tubing makes it a five component upon installation. The two ferrules separate sealing and tube gripping functions. The front ferrule creates a seal against the fitting body and on the tubing outside diameter. As the nut is turned, the back ferrule compresses the front ferrule and forms an effective tube grip. The market well-known brands are Swagelok and Parker A-lok from United States.

Union Coupling

Double ferrule fittings are supplied for installation in extensive applications:

Onshore oil and gas
Shipbuilding
Refinery
Instrumentation
Power generator
Pharmaceutical
Chemical and Petrochemical
Semiconductors
Vacuum Application
Gas Application

2. How are double ferrule tube fitting used?

When the nut is screwed onto the body and is tightened against the tapered area of the body, the edge is compressed tightly against the tube by curling inward. The back ferrule that is located between the nut and body rolls up as the front ferrule rolls and bites into the tube and achieves the connection between fitting and tube as well as a non-leakage sealing effect.

The fitting performs leak proof sealing with all assembly and reassembly motion being transmitted axially through the tube. This results in no radial movement of the tubing upon assembly and the tubing is not stressed. The mechanical integrity is maintained. The double ferrule fitting has a close tolerance control in machining, surface smoothness and hardness of each part.

Union Tee

3. How does a double ferrule fitting work?

There are various factors that affect the double ferrule fitting working pressure:

3.1 Tubing wall thickness

Tube fittings are rated to the maximum working pressure according to its tubing wall thickness installed. The tube wall thickness governs the tubing working pressure.

Tube OD (inch)	WT (inch) 0.028"	WT (inch) 0.035"	WT (inch) 0.049"	WT (inch) 0.065"	WT (inch) 0.083"	WT (inch) 0.095"	WT (inch) 0.109"	WT (inch) 0.120"	WT (inch) 0.134"	WT (inch) 0.156"	WT (inch) 0.188"
1/8"	8500	10900
3/16"	5400	7000	10200
1/4"	4000	5100	7500	10200
5/16"		4000	5800	8000
3/8"		3300	4800	6500	8600
1/2"		2600	3700	5100	6700
5/8"			2900	4000	5200	6000
3/4"			2400	3300	4200	4900	5800	6400
7/8"			2000	2800	3600	4200	4800	5400	6100
1"				2400	3100	3600	4200	4700	5300	6200
1 1/4"					2400	2800	3300	3600	4100	4900
1 1/2"						2300	2700	3000	3400	4000	4900
2"							2000	2200	2500	2900	3600

Table 1: Fractional Seamless Stainless Steel SS316 Tubing (ASTM A269/A213)
*Working pressure are shown in psi

Weldable Bulkhead

Allowable pressure calculations are calculated with maximum outer diameter and minimum wall thickness. For an example, stainless steel SS316 tubing size outer diameter 1/2"OD x 0.035 thickness can withstand working pressure up to 2600 PSI. Therefore, double ferrule fittings used to compress the tube are able to withstand up to 2600 PSI.

Tube OD (mm)	WT (mm) 1.0	WT (mm) 1.2	WT (mm) 1.5	WT (mm) 1.8	WT (mm) 2.0	WT (mm) 2.2	WT (mm) 2.5	WT (mm) 2.8	WT (mm) 3.0	WT (mm) 3.5	WT (mm) 4.0	WT (mm) 4.5	WT (mm) 5.0
6	420	520	670
8	310	380	490
10	240	300	380
12	200	240	310	380	430
14	180	220	280	340	390	430
15	170	200	260	320	360	400
16		190	240	300	330	370
18		170	210	260	290	320	370	Value
20		150	190	230	260	290	330	380	Value
22		130	170	210	230	260	300	340	Value	Value
25				180	200	230	260	300	320	Value
28					180	200	230	260	280	330	Value
30					170	190	210	240	260	310	Value
32					160	170	200	230	240	290	330
38						140	170	200	230	240	280	310
42								170	180	210	250	280	Value
50								Value	150	180	200	230	260

Table 2: Metric Seamless Stainless Steel SS316 Tubing (ASTM A269/A213)
*Working pressure are shown in bar

Another example as follows, stainless steel SS316 tubing size outer diameter 10mm x 1.5mm thickness can withstand working pressure up to 380 Bar. Therefore, double ferrule fittings used to compress the tube are able to withstand up to 380Bar.

3.2 Fitting material 

Temperature °F (°C)	Stainless Steel SS316	Brass
100 (38)	1.00	1.00
200 (93)	1.00	0.80
300 (149)	1.00	0.78
400 (204)	0.97	0.50
500 (260)	0.90	0.13
600 (316)	0.85	-
700 (371)	0.82	-
800 (427)	0.80	-
900 (482)	0.78	-
1000 (538)	0.77	-
1200 (649)	0.37	-

Table 3: Elevated Temperature de-rating factors

For an example, the same tube with outer diameter of size 10mm x 1.5mm thickness and fitting can withstand working pressure up to 380 Bar under room temperature. However, under the temperature 700°F or 371°C, due to de-rating factor of 0.82, the fitting working pressure will drop to 311.6 Bar and it would affect the would system running pressure.

3.3 Fitting seal (optional)

When rubber elastomer seal is used in fitting, the temperature rating of the O-ring may be the limiting factor on fitting’s temperature rating.

Elastomer Seal Material	Working Temperature
NBR (Perbunan)	-25°C to 110°C (-13°F to 230°F)
FKM (Viton)	-28°C to 204°C (-20°F to 400°F)
PTFE (Teflon)	-30°C to 315°C (-22°F to 600°F)

Table 4: O-ring temperature ratings

For an example, the same tube with outer diameter of size 10mm x 1.5mm thickness, with NBR seal, the fitting can work under temperature from -25°C to 110°C.

4. What are the purposes of a double ferrule fitting?

Reducer

Double ferrule fittings can be easily installed. The connections do not require welding or threaded ends. It turns to cost reducing which leads to lesser fabrication on manpower. They can be re-used and keep a low material cost. The benefits of using double ferrule fitting are:

Offer maximum strength and ductility
Chemical resistant and high corrosion resistance
Hold extreme temperatures
High pressure resistant in harsh or high stress nuclear environment

5. How do you use a double ferrule compression fitting?

Double ferrule fittings do their job effectively when assembled correctly. Often, leakages may still occur due to a loose connection or an incorrect tube fitting. Fittings assembly is easy to learn and initiate.

First, cut your tube to the required length neatly, make sure that you have squared ends without any burr.
Then, one must push the tube inside the twin ferrule fitting firmly, till it reaches a stop inside.
Next, mark the starting point at 6 o’clock position before tightening nut.
Follow by simply turning the nut 1-1/4 times and make sure the tube is strongly secured. The starting point is placed at a 9 o’clock position.
The best part about twin ferrule fittings is that they are durable, and so they can be used many times. The parts may be disassembled and reassembled for a different tube. A twin ferrule fitting ensures that the pieces never lose shape or strength due to any excessive fluid pressures.

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