Twin / Double Ferrule Fittings

Tube Fittings are designed to meet the specifications required by industrial applications. Twin ferrule fittings are often referred to as double ferrule fittings or instrumentation fittings. There are two popular brands for twin or double ferrule fittings which are both from the United States, Swagelok and Parker A-lok.

A twin or double ferrule fitting is made up of four components, which are fitting body, back ferrule, front ferrule, and nut. The two ferrule design provides outstanding leak-tight connections as it has better tube-gripping action and improved resistance to vibration. In a twin or double ferrule fitting design, the back ferrule moves the front ferrule forward to spring-load the fitting assembly and seal with the fitting body, and create the tubing seal. The back ferrule also swages the tube to provide the grip needed to keep the fitting and tubing in place.

All assembly and reassembly action is transferred axially through the tube, resulting in leak-proof sealing. As a result, the tube does not move radially during installation, and it is not strained. The mechanical integrity of the system is preserved. The twin or double ferrule fitting has close tolerance control in machining, surface smoothness, and hardness of each component.

Twin / Double Ferrule Tube fittings

Twin or double ferrule tube fittings are available in Stainless Steel, Steel and Brass Material. Additional processing had been carried out, to protect fittings from corrosion. The additional process is passivization (stainless steel), white zinc electroplating with chrome VI free (steel), and acid cleaning (brass).

Twin or double ferrule fittings are measured in metric (mm) and inches sizes ("OD). The metric sizes range from 2mm to 50mm, while inch sizes range from 1 1/16"OD to 2"OD. The pressure rating is determined by the tubes’ wall thickness, with varying tube materials. For more information regarding the working pressure, please refer to HSME M Tube Fittings .

Twin or double ferrule fittings are simple to install. Welding or threaded ends are not required for the connections. It shifts to cost-cutting, resulting in less reliance on manpower. They may be re-used several times while maintaining a cheap material cost. The following are some of the advantages of utilizing a twin or double ferrule fitting:

• Exceptional ductility and tensile strength
• Excellent corrosion and chemical resistance
• High-pressure and high-temperature tolerance
• Ability to resist severe weather and the environment

Twin or double ferrule fittings are generally supplied in a wide range of sectors as following:

• Shipbuilding
• Chemical processing
• Oil and gas applications
• Power generation
• Pharmaceutical
• Semiconductors
• Vacuum Application

Before inserting the tube into the fittings, make sure that the tube ends are well prepared, and then lightly deburr the inner and outer diameters. After that, insert the tube, until it reaches the ends of the fitting shoulder, and hand-tighten with the nut. Scribe the nut vertically at 6 o’clock position, and the wrench tighten with 1 1/4 turns horizontally, while holding the body firmly with a second wrench to the 9 o’clock position.

Ferrule Fittings (Body)

Working Pressure of the fittings is rated to the working pressure of tubing as listed in the tubing tables. Pressure ratings are based on fully annealed seamless stainless steel tubing ASTM A269/A213 and TP 316/316L, 304/304L or equivalent. Tubes are to be free from scratches and suitable for bending and flaring with hardness 90 HRB or less.

Allowable working pressures are calculated using S value of 20 000psi (1378 bar) for ASTM A269 tubing at -20 to 100°F (-28 to 37°C) as listed in ASME B31.3 Process Piping Code and ASME B31.1 Power Piping Code.

Table 1: Fractional Seamless Stainless Steel Tubing

Working pressure are shown in psi.

Table 2: Metric Seamless Stainless Steel Tubing

Working pressure are shown in bar.

Tube Fittings temperature ratings vary in accordance to the body material as per below:

Body Material Temperature Rating

There are reductions in the nominal pressure (PN) caused by the temperature of the fluid medium or surroundings. Please refer to the table below:

Elastomer's Temperature Rating

When combing seal material, the temperature rating of seal may become the limiting factor on the fitting's temperature rating. Kindly refer to the temperature rating below:

Twin or Double Ferrule Fittings, which are also called flareless fittings can be found in the IMPA Marine Stores Guide, which is the world’s leading reference guide for maritime purchasing and supply. For details on the reference codes of flareless fittings, please kindly see below:

For Catalogue Pages of Stainless Steel Flareless Male Elbows, please click here .

i. Stainless Steel Flareless Male Connectors

ii. Stainless Steel Flareless Male Elbows

iii. Stainless Steel Flareless Female Connectors

iv. Stainless Steel Flareless Unions

v. Stainless Steel Flareless Reducing Unions

vi. Stainless Steel Flareless Bulkhead Unions

vii. Stainless Steel Flareless Tee Unions

viii. Stainless Steel Flareless Nut

viiii. Stainless Steel Front Ferrule

x. Stainless Steel Rear Ferrule