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I need a fitting, but which one?

12 Apr 2012

We (tech service) get calls all the time about fittings, especially the ones we sell which are manufactured by Swagelok, Parker, and Valco. It seems like customers are either very familiar with fittings, or they know virtually nothing about them. I will try and provide some information that will hopefully take some of the confusion out of these different fittings.

First, the Swagelok and Parker fittings that Restek sells are designed for similar applications. Although each of these companies offer a huge variety of products, we only sell the items usually requested by our customers in the field of chromatography. These fittings are commonly used to plumb gases to a gas chromatograph, install packed columns, install gas filters (traps), etc…


A few things to keep in mind:

1. We sell both brass and stainless steel fittings. Brass is for use with copper tubing, and stainless steel is for use with stainless steel tubing. If you try and use a brass fitting with stainless steel tubing, brass isn’t hard enough for the ferrule to “bite” into the stainless steel tubing; therefore, you will not be able to obtain a proper seal. In addition, connecting two dissimilar metals may lead to galvanic corrosion

2. The fittings we sell may have compression or National Pipe Thread tapered ends (NPT). Compression ends are made for a tube which has a specific outside diameter (OD). For example, a ¼” compression fitting is designed to connect a section of tubing which has an OD of ¼”. When the nut (which contains the ferrules) is tightened, it compresses the ferrules onto the tubing to make the seal. Keep in mind that most metal tubing is designated by its OD while most plastic tubing is designated by its ID (and usually needs a plastic fitting to obtain a proper connection).

3. NPT (ends) look very different than compression (ends). NPT has no nuts or ferrules, but instead relies entirely on a thread connection. Like plastic tubing, the product designation for NPT (and pipe in general) is the internal diameter (ID), not tubing OD like compression. To help clarify the difference between compression and NPT, the photo below shows a fitting with male NPT end on the left, and compression end on the right.


4. The metal ferrules that are included in the compression end(s) can be replaced with ferrules of different materials. For example, if trying to connect a fitting to a ¼” OD glass tube, you would want to replace the metal ferrules with a softer material like graphite. Similarly, you would want to replace the metal ferrules with PTFE ferrules if using the fitting with PTFE tubing.

5. Finally, unlike compression, NPT fittings can be either male or female. Just remember that the threads on a male fitting are on the outside, and threads on a female fitting are on the inside.


We also sell Valco fittings. These are considered low (or zero) dead volume fittings, meaning they are designed to eliminate any dead space (volume) within the fitting. Common uses for these fittings are to connect capillary columns and other small ID tubing where dead space may cause chromatography (or other) problems such as tailing peaks. These fittings usually include stainless steel ferrules. Just like with other compression fittings, the metal ferrules can be replaced with Valcon Polyimide ferrules when connecting tubing other than stainless steel. Note that these fittings, while containing nuts and ferrules like other compression fittings, have a very different style (see photo below), and are not compatible/interchangeable with the Swagelok / Parker fittings described above.


A few final thoughts on the topic of fittings:

For gas flow rates less than 5mL/min, consider Valco connectors to minimize dead volume. For the most inert fittings, consider Siltek/Sulfinert Treated products. When in doubt as to which fitting to use, always ask someone first, as using the incorrect fitting can be dangerous if a proper seal is not made. Remember, we are just a phone call or e-mail away.