Double Ferrule Fittings

DOUBLE FERRULE FITTINGS

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

Double ferrule fittings also are known as tube fittings are special fitting types used for tightening tubes to prevent leakage of the pipeline. The double ferrule fittings are made from brass and stainless steel (SS316) materials. These fittings are also known as instrumentation fitting and are produced in sizes ranging from 1/8” to 2” and 6mm to 42mm in outer diameter (OD).

The double ferrule fittings have a maximum working pressure of 670 Bar, and measurement is based on the tube material and thickness of the tube wall. Double ferrule fittings are used in a variety of industries like oil and gas, chemical, fuel, etc. majorly, these fittings are used for metal to metal sealing of tubes. The body design comes straight with stepped shoulders and nuts.

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1. What is a Double Ferrule Fitting?

SS316 Union Tee

Double ferrule fittings are fittings used to seal metal to metal connection. These fittings are also known as instrumental fittings or tube fittings. They are produced according to the specifications of the industry intended for. The complete double ferrule fitting has four components which are the body, nuts, front ferrule, and back ferrule.

After installation, a tube is added and this makes up whole 5 components for the double ferrule fittings. History has it that the design of the double ferrule fittings has been in the industry since 1945 and the popular market brands available are made in the USA by Parker A-lok and Swagelok.

The double ferrule fittings are commonly available in Stainless steel and Brass materials. A few times, the steel material could also be used for production. To increase the corrosion resistance of the fittings and durability level, some processes are carried out. They are Passivation with stainless steel, acid cleaning with brass, and white zinc electroplating with chrome VI free or steel.

2. How does a Double Ferrule Fitting work?

The double ferrule fitting works based on some factors. The factors that can affect the working pressure of the fittings are:

Fitting Material

Here’s a table showing the elevated temperature de-rating factors:

Temperature ⁰F (⁰C)	Brass	Stainless Steel SS316
100 (38)	1.00	1.00
200 (93)	0.80	1.00
300 (149)	0.78	1.00
400 (204)	0.50	0.97
500 (260)	0.13	0.90
600 (316)	-	0.85
700 (371)	-	0.82
800 (427)	-	0.80
900 (482)	-	0.78
1000 (538)	-	0.77
1200 (649)	-	0.37

SS316 Reducing Union

An illustration for this table can be used for a tube under room temperature with an outer diameter size of 10mm x 1.5mm fitting and thickness can accommodate a working pressure of a maximum of 380 Bar. But, when the temperature changes to 371C or 700F, as a result of the de-rating factor of 0.82, the working pressure also reduces to 311.6 Bar. This drop-in working pressure will have an impact (negatively) on the effectiveness of the system.

Tubing wall thickness

Double ferrule tube fittings are measured to the highest working pressure based on the thickness of the wall tubes because it controls the working pressure of the tubes.

The table below shows the working pressure for SS316 (Fractional Seamless Stainless Steel) (ASTM A269/A213)

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

To calculate allowable pressure, the parameters to use are the maximum outer diameter and minimum wall thickness. For instance, a stainless steel tube with an outer diameter (OD) size of ½” x 0.035 thickness can accommodate a maximum working pressure of 2600 PSI. Thus, the double ferrule fittings that can be used for the tube compression is a maximum of 2600 PSI.

The table below shows the Working pressure for Metric Seamless Stainless Steel SS315 tubing (ASTM A269/A213).

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	-	-	-	-	-	-	-	-
15	180	220	280	340	390	430	-	-	-	-	-	-	-
16	170	200	260	320	360	400	-	-	-	-	-	-	-
18	-	190	240	300	330	370	-	-	-	-	-	-	-
20	-	170	210	260	290	320	370			-	-	-	-
22	-	150	190	230	260	290	330	380			-	-	-
25	-	130	170	210	230	260	300	340	320		-	-	-
28	-	-	-	180	200	230	260	300	280	330		-	-
30	-	-	-	-	180	200	230	260	260	310	Value	-	-
32	-	-	-	-	170	190	210	240	240	290	330	-	-
38	-	-	-	-	160	170	200	230	230	240	380	310	-
42	-	-	-	-	-	160	200	200	180	210	250	280
50	-	-	-	-	-	-	-		150	180	200	230	260

Here’s another illustration, a stainless steel SS316 with an outer diameter size of 10mm x 1.5mm thickness will accommodate a maximum working pressure of 380 Bar.

Fitting Seal

The temperature level of an O-ring sealed with elastomer can reduce the fitting’s temperature level. The table below shows the O-ring temperature ratings:

SS316 Red Union Coupling

Elastomer Seal Material	Working Temperature
PTFE (Teflon)	-30C – 315C (-22⁰F – 600⁰F)
NBR (Perbunan)	-25C – 110C (-13⁰F – 230⁰F)
FKM (Viton)	-28C – 204C (-20⁰F – 400⁰F)

A tube with an outer diameter size of 10mm x 1.5mm thickness, sealed with NBR, can perfectly work with a temperature between -25⁰C to 110⁰C.

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3. What is the purpose of a Ferrule Fitting?

The double ferrule fitting is suitable for use in different applications. More so, the fitting is easy to install. The connections can be done without welding or threaded ends. This also helps reduce cost and the fittings are reusable. The advantages of double ferrule fittings are:

High resistance to corrosion and chemical exposure
Ability to withstand high pressure and temperatures
High ductility and strength
Ability to withstand exposure to harsh weather and environment.

The double ferrule fittings are applied in the following industries:

Pharmaceutical industry
Refineries
Shipbuilding
Oil and Gas industry
Instrumentation
Vacuum application, etc.

Front Ferrule

Back Ferrule

4. How do you fit a Double Ferrule Fittings?

Installation double fittings are critical to creating a reliable connection. Please refer to the ferrule fitting assembly guideline below:

STEP 1
Remove all substances like dirt and other contaminants from the tube surface. Then, cut the tube to the desired length.

STEP 2
Take out the nut and ferrules from the fittings end connection and discard the ferrules.

STEP 3
Place the tube into the double ferrule fitting. Also, ensure the tube is well inserted into the fitting.

STEP 4
While holding the fitting body steady, hand tightening the nut 1/4 turns. For small-size fitting bodies like 1/16”, 1/8”, 3/16”, 2mm, 3mm, 4mm, tighten the nut 1/8 turn.

STEP 5
Insert the tube fitting until it rests firmly on the shoulder of the tube fitting body. Using your hand, tighten the nut to the fitting assembly. Ensure you do not over tighten the nut to avoid damage to the fittings. For tube size bigger than 1” or 25mm, remove and discard the nuts and ferrules from the fitting set, followed by inserting the preswaged tube adapter.

STEP 6
Mark the nut at 6 o’clock position and while holding fitting body, tighten the nut 1 1/4 turns to the 9 o’clock position.