Installing double ferrule compression fittings correctly

Instrument tubing carries process fluid, impulse lines and air to gauges, transmitters and valves. A leak at a compression fitting is a safety issue and a nuisance, and nearly all of them come from installation errors rather than faulty fittings. A few simple steps prevent them.

How a double ferrule fitting works

The fitting has four parts: body, nut, front ferrule and back ferrule. As the nut is tightened, the back ferrule pushes the front ferrule forward and into the body taper. The front ferrule seals against the body and the tube, and the back ferrule grips the tube to hold it against pressure and vibration. The tube itself takes a permanent set, which is why the connection stays tight in service.

Choose the right tube first

Compression fittings are designed for tube of a stated outside diameter and a limited range of wall thickness. Use seamless, annealed stainless steel tube, such as 316 or 316L, with a smooth, round, scratch-free surface. The tube should be softer than the fitting, because the ferrules must bite into it. Check the outside diameter and wall against the fitting manufacturer table. Too thin a wall collapses under the ferrule grip, and too thick a wall prevents a proper bite.

Step-by-step installation

• Cut the tube square with a proper tube cutter or a saw with a guide, not an abrasive wheel
• Deburr the inside and outside of the tube end and clean away swarf
• Slide the nut and then the ferrules onto the tube in the correct order and direction, with the narrow end of the front ferrule facing the body
• Insert the tube into the fitting until it bottoms firmly against the shoulder inside the body
• Tighten the nut finger tight, and hold the body with a spanner
• Mark the nut at the six o'clock position, then turn it the specified number of turns

How many turns

For most double ferrule designs the instruction for tube sizes 1/4 inch (6 mm) and larger is one and a quarter turns from finger tight. For very small tubes, such as 1/16, 1/8 and 3/16 inch, or 2 to 4 mm, it is usually three quarters of a turn. These figures are typical, so always follow the instructions of your fitting manufacturer.

Reassembly

A fitting can usually be dismantled and reassembled many times. Insert the tube with its set ferrules into the body until the front ferrule seats, tighten the nut finger tight, and then snug it with a spanner by a small amount, often about a quarter turn, until you feel resistance. Do not apply the full initial turns again. Follow the manufacturer instructions for the number of remakes permitted.

Common mistakes

Checking the joint

Many manufacturers provide a gap inspection gauge. After the first tightening, it will not fit between the nut and body if the fitting is pulled up enough. After hydrostatic or pneumatic test, spray a leak-detection fluid on the joint and check for bubbles. Test at the working pressure and at the test pressure specified for the system.

Threads on the body end

The body usually has a male or female pipe thread on its other end, such as NPT, BSPT or BSPP. Make sure the thread standard matches the instrument or valve port, and use a suitable sealant or tape with tapered threads. BSPP threads seal with a washer or O-ring, not on the thread.

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