How Do You Select, Install, and Specify Them Correctly for Your Application?

Compression fittings are mechanical connectors that join two tubes or pipes, or connect a tube to a component such as a valve or instrument, by using compressive force to create a leak tight seal rather than relying on threads, welding, or adhesive bonding. They are among the most widely used pipe and tube connection methods in plumbing, hydraulics, instrumentation, refrigeration, and gas systems, valued for their ability to create reliable, pressure rated connections without specialized tools or heat application, and for the ease with which they can be disassembled and reinstalled when maintenance or system modification is required.

The direct answer for anyone evaluating compression fittings is this: a compression fitting works by tightening a nut over a deformable ring (called a ferrule or olive) that is compressed against the outer surface of the pipe or tube, creating a metal to metal seal that prevents fluid or gas leakage. The correct specification requires matching the fitting material and pressure rating to the fluid, pressure, and temperature of the application, and selecting between the two main design types (single ferrule and double ferrule) based on the vibration environment and sealing requirement. This article covers how compression fittings work, the types available, their applications, and how to install them correctly for reliable long term performance.

How Compression Fittings Work: The Sealing Mechanism

The sealing mechanism of a compression fitting relies on the plastic deformation of a ferrule (also called an olive or compression ring) as the fitting nut is tightened. The basic assembly consists of three components: the fitting body, the ferrule, and the compression nut. When the nut is tightened over the ferrule, it drives the ferrule into a tapered seat in the fitting body, compressing the ferrule radially inward against the outer surface of the pipe or tube. This compression creates a gas tight and liquid tight seal at two interfaces simultaneously: between the ferrule and the pipe outer surface, and between the ferrule and the fitting body seat.

The quality and reliability of a compression fitting seal depends critically on the deformation behavior of the ferrule material relative to the pipe material. Ferrules that are too hard will not deform adequately and will not seal; ferrules that are too soft will deform excessively and may extrude rather than creating a controlled seal. Quality compression fittings from reputable manufacturers specify ferrule hardness within a defined range of the pipe material hardness to ensure optimal deformation behavior during assembly.

Single Ferrule vs Double Ferrule Designs

Two principal compression fitting designs are used across different application sectors, and the choice between them has significant consequences for sealing performance, vibration resistance, and the range of tubing materials that can be used:

• Single ferrule design: Uses one deformable ring that simultaneously provides the primary seal against the tube outer diameter and grips the tube to resist pull out forces. Single ferrule fittings are simpler in construction, lower in cost, and easier to assemble in confined spaces. They are widely used in plumbing and general purpose hydraulic and pneumatic applications up to moderate pressures. The limitation of single ferrule designs is that the same ferrule must perform both the sealing and the tube gripping functions, which creates competing requirements for the ferrule material and geometry.
• Double ferrule design: Uses two separate ferrules with distinct functions. The front (leading) ferrule is optimized for sealing at the fitting body seat, while the rear (trailing) ferrule is optimized for gripping the tube and transmitting pull out load. Double ferrule compression fittings, such as those conforming to the Swagelok or Parker Hannifin design standards, are the standard specification for high pressure instrumentation, analytical systems, and any application involving vibration, temperature cycling, or frequent assembly and disassembly where single ferrule performance would be inadequate. They can be rated for pressures up to 700 bar or more in small tube sizes.

Types of Compression Fittings and Their Applications

Compression fittings are produced in a wide range of configurations, materials, and pressure ratings to suit the diverse requirements of the industries in which they are used. Understanding the main types and their appropriate applications is the starting point for correct fitting selection.

Compression Fitting Configurations

• Straight union: Connects two tubes or pipes of the same diameter in a straight line. The most basic configuration, used wherever a joint must be made between two lengths of tube or pipe without changing direction.
• Elbow: Connects two tubes at 90 degrees or 45 degrees, used where direction changes are required in a piping system. Available in equal diameter configurations and in reducing configurations that connect tubes of different diameters while changing direction.
• Tee: Creates a three way junction in a tube or pipe system, either in equal bore or in reducing configurations. Used wherever a branch connection must be made to a main run without interrupting flow.
• Reducing union: Connects two tubes or pipes of different diameters in a straight line, used for transitions between different pipe sizes in a system.
• Adapter (male or female): Connects a compression tube end to a threaded male or female connection, allowing compression fitted tube systems to interface with threaded valves, instruments, manifolds, and other threaded components.
• Bulkhead fitting: Designed to pass through a panel or wall, providing a sealed penetration point where a tube must enter or exit an enclosure, vessel, or bulkhead without leaking at the penetration point.

Materials Used in Compression Fittings

The material of the fitting body and ferrule must be compatible with the fluid being conveyed, the system pressure and temperature, and the environment in which the fitting is installed. The main material options and their typical applications are:

Correct Installation of Compression Fittings for Leak Free Performance

Compression fitting leaks in service are almost always caused by installation errors rather than by fitting defects, and most installation errors are preventable by following the correct procedure for the specific fitting design being used. The following procedure applies to standard single and double ferrule compression fittings in metal tubing applications.

1. Cut the tube squarely. Use a proper tube cutter rather than a hacksaw to ensure the cut end is perpendicular to the tube axis. A non square cut prevents the tube from seating fully against the fitting body shoulder, leaving a gap that the ferrule cannot bridge. After cutting, deburr the tube end inside and outside to remove any metal projections that could affect sealing or restrict flow.
2. Ensure the tube is round. Any out of round condition in the tube at the insertion point will prevent the ferrule from creating a uniform circumferential seal. If the tube has been bent and the end is slightly oval, cut back past the deformed section before assembly.
3. Slide the nut and ferrule(s) onto the tube in the correct order. For single ferrule designs, the nut goes on first with its threaded end facing the fitting body, followed by the ferrule with its taper facing toward the fitting body. For double ferrule designs, the nut is followed by the back ferrule and then the front ferrule, with ferrule orientations as specified by the fitting manufacturer.
4. Insert the tube fully into the fitting body. Push the tube end firmly against the shoulder inside the fitting body before beginning to tighten the nut. Incomplete tube insertion is one of the most common causes of fitting leaks because it leaves the ferrule in an incorrect position relative to the fitting body seat.
5. Tighten the nut to the correct torque or turn count. For double ferrule fittings, the standard initial make up procedure is to tighten the nut finger tight and then advance it a further 1.25 turns (one full turn plus one quarter turn) with a spanner. For single ferrule fittings, the manufacturer's torque specification should be followed precisely; overtightening excessively deforms the ferrule and can damage the tube, while undertightening fails to create an adequate seal. Do not use thread sealant, PTFE tape, or any sealant compound on compression fitting threads, as the sealing function of a compression fitting is provided by the ferrule against the tube and fitting seat, not by the threads.
6. Pressure test before returning to service. After initial make up, pressure test the connection at the system's working pressure with a leak detection fluid or with the system's normal fluid before covering or insulating the fitting. Any visible leak indicates either incomplete tube insertion, incorrect ferrule orientation, or insufficient nut tightening, all of which can be corrected before the fitting is put into service.

Reassembly After Disassembly

One of the practical advantages of compression fittings over welded or brazed connections is their ability to be disassembled for maintenance and reassembled reliably. For double ferrule fittings, the reassembly procedure after the first make up is different from the initial make up: the nut should be tightened to the snug position and then advanced only a small additional amount (typically one quarter turn or less) to achieve a seal, because the ferrule has already been permanently deformed to fit the tube and fitting body geometry from the initial make up. Overtightening a previously made up connection advances the ferrule further than necessary, reducing its remaining deformation capacity for subsequent reassemblies. Quality double ferrule compression fittings from reputable manufacturers are rated for multiple reassembly cycles without loss of sealing integrity when the correct reassembly torque is applied, making them genuinely reusable components rather than single use consumables.

Key Standards and Pressure Ratings for Compression Fittings

Compression fittings used in regulated applications including gas distribution, pressure vessels, and process industries must comply with applicable national and international standards that specify dimensional, material, and performance requirements. The main standards governing compression fittings in different market sectors are:

• BS EN 1254 (Parts 1 to 5): The European standard series for copper and copper alloy compression fittings used in plumbing and heating applications, specifying dimensional requirements, materials, pressure ratings, and test methods for fittings in water, gas, and heating service.
• ISO 8434 series: International standards for metallic tube connections for fluid power and general use, covering 24 degree cone compression fittings (Part 1), O ring face seal fittings (Part 2), and bite type tube end fittings (Part 3) used in hydraulic and pneumatic systems.
• SAE J514: The American standard for hydraulic tube fittings, specifying 37 degree flare fittings which share some functional characteristics with compression fittings in terms of mechanical sealing, used predominantly in mobile hydraulic equipment.
• ASTM B16.15 and ANSI standards: American standards for cast bronze threaded fittings and associated pipe fitting specifications that apply to compression fitting applications in North American plumbing systems.

Pressure ratings for compression fittings vary enormously with fitting size, material, and design. As a general guide, brass compression fittings in standard plumbing sizes are rated for 15 to 25 bar working pressure in water service at ambient temperature. Stainless steel double ferrule instrumentation fittings in small tube sizes (6 mm to 12 mm outer diameter) are commonly rated for 200 to 700 bar at ambient temperature, with ratings reducing at elevated temperatures according to the material's pressure temperature de rating curve. Always verify the pressure and temperature rating of any compression fitting against the maximum working pressure and maximum operating temperature of the specific system before installation, including any pressure surges or thermal excursions that the system may experience during normal operation or fault conditions.