Ferrule Fittings: Types and Applications
A ferrule fitting is a compression-type tube connection that creates a leak-tight seal by compressing one or two tapered metal ferrules (also called olives) onto the outside surface of a tube. Ferrule fittings require no welding, brazing, or special tools and are the standard connection method for instrumentation tubing in oil and gas facilities, chemical plants, and power stations.
These fittings are available in various types and configurations for tube sizes from 1/16 in. to 2 in. OD, covering the full range of small-bore instrumentation, hydraulic, and analytical piping applications.
Types of Ferrule Fittings
Ferrule fittings are classified by the number of ferrules and the connection configuration:
| Type | Description | Common Brands |
|---|---|---|
| Single ferrule | One ferrule grips and seals the tube | Parker A-LOK, Ham-Let |
| Double ferrule | Front ferrule seals, back ferrule grips | Swagelok, Parker CPI |
| Bite-type | Single ferrule bites into tube (DIN 2353) | Parker EO, Ermeto |
| Flare-type | Tube end flared, ferrule compresses flare | AN/JIC fittings |
Double Ferrule vs Single Ferrule
| Feature | Double Ferrule | Single Ferrule |
|---|---|---|
| Ferrules | Front (sealing) + back (gripping) | Single ferrule (seal + grip) |
| Sealing mechanism | Front ferrule swages onto tube | Ferrule bites and deforms onto tube |
| Tube grip | Back ferrule provides independent grip | Same ferrule seals and grips |
| Remake capability | Multiple remakes possible | Limited remakes |
| Vibration resistance | Superior (two-point contact) | Good |
| Assembly sensitivity | More forgiving | Requires precise torque |
| Pressure rating (316SS) | Up to 15,000 psi (1/4 in. tube) | Up to 10,000 psi (1/4 in. tube) |
Ferrule Fitting Configurations
Ferrule fittings are available in various end connections and configurations:
| Configuration | Use |
|---|---|
| Union (straight connector) | Tube-to-tube connection |
| Union elbow (90 deg.) | Direction change |
| Union tee | Three-way branch |
| Male connector | Tube to male NPT/BSP thread |
| Female connector | Tube to female NPT/BSP thread |
| Bulkhead union | Through-panel connection |
| Reducer | Different tube sizes |
| Cap/plug | Line termination |
Materials and Pressure Ratings
| Material | Typical Pressure Rating (1/4 in.) | Application |
|---|---|---|
| 316 stainless steel | 10,000-15,000 psi | Process instrumentation, offshore |
| 304 stainless steel | 10,000-15,000 psi | General instrumentation |
| Alloy 625 | 15,000+ psi | Subsea, sour service |
| Alloy 825 | 10,000+ psi | Corrosive chemical service |
| Monel 400 | 10,000+ psi | Seawater, HF acid |
| Carbon steel | 5,000-10,000 psi | Hydraulic systems |
| Brass | 3,000-5,000 psi | Low-pressure utility, water |
Ferrule fittings are specified by tube OD, not pipe NPS. Common instrumentation tube sizes are 1/4 in., 3/8 in., and 1/2 in. OD. Metric sizes (6 mm, 10 mm, 12 mm) are used in European projects. Mixing imperial and metric ferrule fittings on the same project must be avoided as they are not interchangeable.
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