Instrumentation tubing: measurement and sample lines with twin ferrule fittings

Instrumentation lines are small in diameter, but they are the eyes and ears of the plant. A micro-leak on the line to a pressure transmitter leads to a false reading, and unnecessary dead volume in a sample line leads to a misleading analysis. That is why, in instrumentation tubing, the tube, fittings and installation workmanship must all be right together, not just individually.

How do twin-ferrule fittings work?

As the nut is tightened, the front ferrule seats between the tapered bore of the body and the tube surface to create the seal. The back ferrule pushes the front ferrule along the axis and grips the tube. Splitting sealing and grip between two separate parts increases connection reliability on lines subject to vibration and allows the fitting to be disassembled and remade during maintenance.

Scope

• Union: joins two tubes in a straight line
• Elbow: 90° change of direction and tubing in tight spaces
• Tee: branch takeoff from the line, outlets for gauges and sampling points
• Connector: connects the tube to a threaded port (BSP, NPT)
• Adapter: transition between different connection types
• Small-bore stainless steel tube, needle valves and gauge connections

Tube selection

Tube outside diameter, wall thickness and material compatibility are evaluated together. Imperial and metric tubes are not interchangeable: although 6 mm tube and 1/4 inch (6.35 mm) tube look close in size, they will not seal reliably with the same fitting. Wall thickness must suit the working pressure and the range recommended by the fitting manufacturer. For the ferrule to grip the tube properly, the tube must not be harder than the fitting material; this is why seamless, annealed stainless steel tube is generally preferred. AISI 316/316L tube and fittings are used together in plants that require corrosion resistance.

Installation

Clean tube cut

The tube is cut square to its axis with a tube cutter or a suitable cutting fixture, and the inside and outside burrs are removed. An angled cut or burr prevents the ferrule from seating properly. There must be no scratches on the tube surface where the ferrule will seat.

Inserting the tube fully

The tube is pushed in until it bottoms against the shoulder in the fitting body. If the tube is not fully inserted, the ferrule grips it at the wrong point and the connection may leak under pressure.

Correct wrench turns

The nut is first tightened by hand, then turned with a wrench by the number of turns specified by the fitting manufacturer, while the body is held with a second wrench. Under-tightening causes leaks; over-tightening damages the ferrule and tube. On connections that are disassembled and remade, tightening again follows the manufacturer's instructions.

Support and routing

Lines are supported with clamps, bends are not made right at the base of a fitting, and flexibility is allowed for thermal expansion. A clean route that reduces the number of tees and elbows cuts down on both leak points and dead volume.

Quality control

• Check that the tube and fitting measurement system (imperial or metric) and material match
• Check of the cut, burrs and tube surface
• Check of nut tightening and ferrule seating
• Pressure and leak-tightness verification depending on the application
• Capping of open ends before shipping and installation

Common installation mistakes

• Tightening the nut without pushing the tube all the way to the shoulder
• Fitting imperial fittings onto metric tube, or vice versa
• Mixing bodies, nuts and ferrules from different manufacturers
• Tightening the nut more or less than the manufacturer specifies
• Cutting the tube at an angle and not removing the inside and outside burrs

Applications by sector

Process measurement and sample lines in chemical plants; pressure and temperature measurement connections in food and beverage plants; instrument lines exposed to saltwater environments on offshore platforms and ships; measurement lines on hydraulic test benches in aviation; compact test and control units in the defense industry; pressure sensing lines on pump skids in fire suppression systems.

Lead time

Fittings and tube from stock are typically ready to ship within 1-3 business days. If a custom adapter or non-standard transition is required, standard production takes an average of 5-10 business days, and custom-size parts are scheduled within an average of 10-20 business days after technical approval.