Twin Ferrule Instrumentation Fittings

Twin ferrule fittings (instrumentation fittings, also called double ferrule compression fittings) grip the tube in a controlled way on small-diameter measurement, control and sample lines, giving neat and repeatable assembly. When the nut is tightened, the front ferrule seats against the body cone and the tube to create the seal; the back ferrule grips the tube and holds it against vibration. The connection requires no welding and can be disconnected and remade. Straight unions, union elbows, union tees, male connectors and adapters are chosen to suit different line layouts.

Types of twin ferrule fittings

• Straight union: joins two tubes on the same axis.
• Union elbow: 90° change of direction; routing without bending the tube inside panels and in tight spaces.
• Union tee: branch takeoff; transmitter, pressure gauge and sample branches.
• Male connector: connects the tube to a threaded port, with NPT, BSPT or BSPP threaded ends.
• Adapter: converts between different connection types and sizes.

Selection criteria: tube size, wall thickness, port thread

• Tube OD: Inch and metric sizes are very close to each other: 1/4 inch tube is 6.35 mm, while 6 mm tube is 6.00 mm. 3/8 inch (9.53 mm) and 10 mm, and 1/2 inch (12.7 mm) and 12 mm are also frequently confused. Close sizes will not hold securely even when tightened; tube and fittings must be selected from the same measurement system.
• Wall thickness and hardness: The tube must be of a hardness the ferrule can grip; wall thickness is selected according to working pressure. Annealed seamless stainless steel tube is the common choice. For tube that will be bent, the bends are planned so that enough straight length remains before the tube enters the fitting.
• Material compatibility: When AISI 316/316L fittings are used with tube from the same family, no galvanic difference arises; they are the preferred choice in plants where corrosion resistance is required.
• Port thread: The thread of the male connector or adapter end is verified against the valve, manifold or instrument it will connect to.

Twin ferrule fitting installation

The tube must be cut square and clean, and deburred. The tube is pushed all the way to the tube stop inside the body; after the nut is tightened by hand, it is turned with a wrench by the number of turns specified by the manufacturer. After assembly, adequate tightening can be checked with a gap inspection gauge. Bodies, nuts and ferrules from different manufacturers must not be mixed. When a disconnected joint is remade, the nut is brought back to its previous position and tightened slightly. Lines must be supported with clamps; the weight of the tube and vibration must not bear on the connection.

Common mistakes

• Tightening without pushing the tube all the way to the bottom of the body
• Keeping inch and metric parts in the same box
• Wrapping sealing tape around the thread or the ferrule
• Tightening the nut beyond the number of turns specified by the manufacturer
• Connecting the tube to the fitting under tension or with misalignment

Which industries use them?

Transmitter and analyzer lines in chemical plants, instrumentation panels on offshore platforms, sample and control lines in food plants, hydraulic test benches in aviation, and compact control circuits in the defense industry. For line layout and tubing workmanship, see also our instrumentation tubing service page.