Compression Tube Fitting

The Compression Tube Fitting is a precision-engineered double-ferrule fitting designed to create reliable, leak-tight connections on instrument and process tubing. Widely used across oil & gas, petrochemical, and analytical instrumentation industries, these fittings deliver consistent performance under demanding pressure and temperature conditions without the need for welding, threading, or special tooling.

Double-Ferrule Mechanism

The double-ferrule design uses two separate ferrules — a front (leading) ferrule and a back (trailing) ferrule — that work in concert to grip and seal the tubing. When the fitting nut is tightened, the back ferrule drives the front ferrule forward and inward, causing it to swage onto the tube OD and create a gas-tight metal-to-metal seal. The back ferrule simultaneously provides a spring-like grip that resists vibration and pull-out forces.

This two-stage action separates the sealing function from the gripping function, resulting in superior leak integrity compared to single-ferrule designs. The connection is also remakeable: the fitting can be disassembled and reassembled multiple times while maintaining its seal.

Materials of Construction

Fittings are available in two primary body materials to suit different service environments:

• 316 Stainless Steel (SS 316): Offers excellent corrosion resistance in aggressive media, including chloride-bearing environments. Suitable for high-pressure and high-temperature service. Ferrules are typically 316 SS as well.
• Brass: A cost-effective option for non-corrosive service, instrument air, and general-purpose applications. Brass fittings are compatible with copper and brass tubing.

All wetted surfaces are precision-machined to tight tolerances. Electropolished finishes are available on stainless steel variants to reduce surface roughness and improve cleanability.

Tube Size Range and Compatibility

Compression tube fittings are available across a broad range of tube outside diameters (OD) to accommodate both imperial and metric tubing standards:

• Imperial (inch): 1/8" OD through 2" OD
• Metric: 6 mm OD through 50 mm OD

The fittings are compatible with seamless stainless steel, copper, and other ductile metal tubing that meets the required wall thickness and hardness specifications. Tubing must be harder than the ferrule material to ensure proper swaging.

Working Pressure and Temperature

Maximum working pressure is up to 420 bar (6,092 psi) depending on tube size, wall thickness, and body material. Smaller tube sizes and heavier wall schedules support the highest pressures. The fittings are rated for service temperatures from -65°C to +232°C (-85°F to +450°F), covering cryogenic, ambient, and elevated-temperature process conditions.

Typical Applications

• Oil & gas upstream, midstream, and downstream instrumentation lines
• Analytical instrumentation: gas chromatographs, process analyzers, sample conditioning systems
• Hydraulic and pneumatic control systems
• Chemical and petrochemical process piping (small-bore instrument tubing)
• Power generation and utilities

Installation Notes

Installation requires no special tools, welding, or tube preparation beyond a clean, square cut. The standard procedure is as follows:

1. Cut the tube squarely and deburr the end.
2. Insert the tube fully into the fitting body until it bottoms out against the tube stop.
3. Hand-tighten the nut, then advance with a wrench 1¼ turns (for initial make-up) to swage the ferrules onto the tube.
4. For remakes, retighten to the original position plus a slight additional advance until snug.

Pre-swaging tools are available for high-volume installations to ensure consistent ferrule set before final assembly.

Compliance and Standards

Compression tube fittings in this range are manufactured and tested in accordance with internationally recognised standards to ensure material integrity and pressure-containing performance:

• ASTM A276 / A479 — Stainless steel bar and wire for body and ferrule material
• ASME B31.3 — Process piping design and pressure integrity
• ISO 8434-1 — Metallic tube connections for fluid power and general use
• NACE MR0175 / ISO 15156 — Sour service compatibility (H₂S environments) for applicable grades