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Valco Compression Fittings

The two piece compression fitting, in which a ferrule is compressed onto the tube as a nut is tightened, offers reliability in high pressure situations and in connecting metal tubing. One piece compression fittings (nut and ferrule combined) are generally not as effective, since the sealing surface rotates as the tightening force is applied.

Valco excels in all critical areas of the design and manufacture of fittings. Quality considerations, which cannot be ignored if an analytical system is to reach and maintain optimum performance levels, include interchangeability, counterbore tolerances, ID/OD concentricity, mixing potential, cleaning procedures, and the method employed to “make up” the ferrule on the tube.

No Tubing Deformation

The basic concept of compression fittings carries the inherent danger of tube deformation. While some manufacturers emphasize this positively as a method of insuring that the tubing doesn’t blow out of the ferrule, the flow anomalies introduced by the restricted ID make these fittings a poor choice for most instrument applications.

ID restriction in common compression fitting

Valco metal ferrules cut a ring near the end of the tube, which prevents tube release at high press-ures without significantly deforming and restricting the tube interior. Because our ferrules have a sharp edge at the ID near the nose, this usually takes only about 1/4 turn beyond the point where the ferrule first starts to grab the tubing. There is so little tube distortion that they are routinely used with glass-lined tubing! Only Valco’s polymer fittings rely on friction to hold a tube.

No ID restriction in Valco compression fitting

Interchangeability

Valco fitting details are designed with a consistent pilot depth, permitting reliable interchangeability as connections are revised or fittings are replaced. This interchangeability extends throughout the Valco and Cheminert fitting and valve product lines. Indeed, the Valco standard has been so widely copied that Valco and Cheminert fittings are, in general, fully interchangeable with those of our major competitors. In initial installations, Valco ferrules will often improve other manufacturers’ fitting connections.

Because of variation in tubing OD and in pilot and taper designs from manufacturer to manufacturer, the amount of tubing extending beyond the made up ferrule can vary. (The most radical variation is in the fittings manufactured by Waters. Based on the old Swagelok design, they have a pilot depth considerably longer than standard.) Figure 1 shows a properly made up fitting. If that same fitting is installed in a detail which was designed for a slightly longer tube extension (as in Figure 2), dead volume will be introduced. In the opposite case, with the pilot shorter than the pilot depth (Figure 3), the tube will bottom out before the ferrule has sealed. However, our tests prove that except in the most extreme cases, a Valco ferrule will “creep” on the tubing until it reaches the bottom of the ferrule taper, making a proper seal.

Reliably Clean

Our state of the art CNC machines use water-based lubricants. After each part comes off the machine, it is cleaned with water-soluble detergents and then rinsed in hot deionized water. Finally, every metal fitting that we make is given a thorough cleaning with steam from deionized water at 140°C. The practical result of the extra effort is this: you don’t have to be concerned about solvent residues.

More about Valco fittings:
• Precision machining, finishing, and tolerances
• Comparison of compression fitting designs
Go to Valco fittings products list

Read about Cheminert fittings:
• High pressure
• Low pressure