Four Tube Fitting Styles from Low to High Pressure

Instrumentation and fluid control systems use fittings, valves, and tubing to transfer, measure, and control flow, pressure, and temperature. The fitting style affects pressure capability, assembly time, maintenance, vibration resistance, and cost.

Four common tube fitting styles are compression fittings, cone and thread fittings, butt weld fittings, and push-to-connect pneumatic fittings.

How Tube Fitting Styles Differ

Tube fittings are usually specified by tube outside diameter, such as 1/4, 3/8, 1/2, or 3/4 inch, or by metric tube sizes such as 6, 10, 12, or 20 mm. Many styles are available as straights, elbows, tees, crosses, reducers, and adapters to other thread types such as NPT or SAE.

The right style depends on pressure, vibration, available tools, installation environment, maintenance needs, and whether the tube connection must be taken apart later.

1. Compression Fittings

Compression fittings are common because they are easy to install. The fitting usually includes a body, nut, and one or two ferrules. As the nut is tightened, the ferrule grips the tube OD and creates the seal.

This style does not require coning, threading, or welding equipment. The tube is inserted into the fitting and tightened to the required torque.

Compression fittings also work well with thinner wall tubing, which can improve flow and make tube bending easier. The tradeoff is that thinner wall tubing may not be suitable for higher pressure ranges.

2. Cone and Thread Fittings

Cone and thread fittings are used in very high-pressure systems and are often associated with applications up to about 15,000 PSI, with some specialty systems going much higher.

The tube end is coned and threaded so it can seat firmly into the fitting body. This can create a strong connection, but it requires special tools, training, more installation time, and careful cleaning because cutting operations can introduce metal shavings and oil into the tube.

Cone and thread connections can be disassembled for maintenance, but field preparation is more involved than a compression fitting.

3. Butt Weld Fittings

Butt weld fittings permanently join the fitting and tube by welding. A well-formed weld gives strong vibration and fatigue resistance because the parts become a continuous metal connection.

The disadvantages are cost and maintenance difficulty. Welding equipment, trained labor, and additional time make this style expensive. Field repair can also be difficult if the joint is inaccessible or the maintenance team cannot cut and re-weld the connection.

4. Push-To-Connect Pneumatic Fittings

Push-to-connect pneumatic fittings use an internal collet and external release mechanism to connect tubing. The tube is pushed into the fitting, and disconnection is also quick.

These fittings are commonly used with polyurethane or nylon tubing in compressed-air systems. They are available in many body shapes and connection combinations, including metric tube and BSP thread options.

They are fast and convenient, but they are not a substitute for high-pressure hydraulic tube fittings. The pressure, media, tubing material, and application should determine which fitting style is used.