Unions

Unions join two pieces of tubing of the same OD. Select the union with the bore that matches the ID of the tubing. If the IDs are different, choose the union with a bore which matches the smaller tube bore.

• Internal unions have female threads and a fitting detail for zero volume fittings. The nuts have male (external) threads.
• External unions have male threads, requiring a nut with internal threads.
• External/internal unions have male threads on one end and female threads on the other, for connecting a standard zero dead volume fitting to an existing tube which already has an external nut made up on it.

Internal fittings are almost always the best with tubing of 1/8" OD or smaller. They make a stronger connection and offer the lower volume necessary for high performance instrumentation. Also, because 1/16" external fittings have very thin, easily distorted walls, they are not as durable as 1/16" internal fittings. In sizes larger than 1/8", external fittings are generally easier to make up because of less thread friction.

Bulkhead versions can be mounted through an instrument panel or on a bracket. The fitting body is undercut so that it bites into the panel when the mounting nut is tightened, eliminating the need for a lock washer. An O-ring can be installed between the body and the panel to allow operation in purged environments. Typically the mounting nut goes inside the instrument, so that the long threaded portion will be out of sight. In the external/internal bulkhead unions, the mounting nut is on the side with the Valco internal fitting.

Standard material is 300 series stainless.

Zero Volume vs. Zero Dead Volume

A true zero volume fitting is one in which no part of the fitting actually becomes a part of the flow path. The only Valco fittings which fit this description are our through-bore unions, which allow tubing to butt end-to-end. (So these are only zero volume if the tube ends are perfectly square.)

All other fittings are designed with zero dead volume: that is, there is no volume introduced by the fitting which is not cleanly swept.

Drawing: Zero volume vs. zero dead volume

Tech tips

Further reference

Tech tip: Adding filtering capability to a union

Filtering capability can be added to a union by inserting a screen or frit into it before making up the fittings. However, when a fitting detail has a screen or frit in it the pilot depth is reduced, so that the ferrule makes up closer to the tube end than it otherwise would.

If that tube is used in any other Valco fitting, it will introduce unswept volume. The design of our filters takes this into account, allowing our fittings to remain truly interchangeable.

Tech tip: Installing a through-bore union

Because the tubing will pass all the way through a through-bore union, we suggest making up the first tube in a standard Valco fitting to establish the proper length of tubing extending beyond the ferrule. Install this made-up connection in the through-bore union, then the second tube can be butted against it for a true zero volume connection.