Twin Ferrule Straight Union

A twin-ferrule union is a fitting that joins two instrumentation tubes of the same outside diameter in a straight line. Each end has a nut, a front ferrule and a back ferrule. When the nut is tightened, the front ferrule seats against the body and the tube to provide the seal, while the back ferrule grips the tube to strengthen retention against vibration. The connection requires no welding and produces a clean, repeatable makeup on the tube.

It is used in the impulse lines of pressure and level transmitters, in sampling and analyzer systems and in hydraulic control panels; in chemical and food plants, on offshore platforms, in instrumentation lines in ship engine rooms and in aviation test equipment.

• Tube OD (inch or metric), wall thickness and hardness must be compatible with the fitting; some inch and metric sizes are very close to each other and must not be mixed up.
• Stainless steel tube must be annealed and have a scratch-free surface; longitudinal scratches compromise sealing.
• Nuts, ferrules and bodies from different manufacturers must not be intermixed.

Installation note: Cut the tube square and deburr it, then push it in until it bottoms against the shoulder in the body. After finger-tightening the nut, turn it with a wrench by the number of turns given in the manufacturer's instructions; the makeup can be verified with a gap inspection gauge.