Sealing points for compression fittings

When I worked as a machinist, the only way we did compression fittings was to make the fitting about .001" to .002" larger than the hole. We would heat up the object about 2000 degrees to expand the object and cool the fitting to shrink it. The fitting was then slid into the hole and the temperature was allowed to normalize.

For plumbing

You are right the general description only mentions the one seal. But of course you have to consider both sides of the seal.

I believe this is the information you requested.

http://www.swagelok.com/downloads/webcatalogs/en/MS-06-16.pdf?301Red=Y

Compression fitting seal points vary slightly by manufacturer. Probably the best examples are found at Parker .com/inphorm/fittings. Swagelok talks a good talk about being the best, but you could get an equal or better for much less money from Parker or Hy-Lok. Compression fittings normally rely on interference fit between the shoulder of the major part of the fitting with the ferrule. Having said that, you really have to look at the system pressure issues. A compression fitting is not the preferred approach for high pressure >2000 PSI. With cost issues being ever present, compression fittings are used, to the detriment of the end user. That is why I make a good living in the after-market: fixing obvious errors in product selection.

After working on many varied applications I have used Swagelok compression fittings for a number of years now. There are many ways that manufacturers can create a seal, some maintain a seal between the tube and the shoulder of the fitting while others do it on the ferrule itself.

Having attended a Swagelok training course I learnt that a seal is created between the front ferrule and the throat of the fitting body while the rear ferrule does the holding or gripping of the tube. The length of tube insertion into the fitting body varies greatly from manufacturer to manufacturer. While the tube provides a support for the tubing face a seal is not always created in this area as this depends on the wall thickness, material and hardness of the tube being used.

Whatever fitting you use the largest cause of failure and leakage is due to poor installation and not following the relevant manufacturers instructions. I can only speak from experience but if the instructions are followed correctly Swagelok's products will withstand pressures high enough to burst the tubing with no leakage. The twin ferrule mechanical type of fiiting has been used on many of our applications successfully above 2000psi and some over 5000psi. They have just launched a new advanced geometry ferrule on www.swagelok.com . Our local distributor in Manchester have a great training course that our engineers and fitters have attended.

In my experience the service and product that Swagelok offer is second to non and very often the small price difference is immaterial to the total cost of our projects. Hope this helps?

You are 100% correct in you evaluation... based on my own personal experience. I have been using Swagelok fittings since the late seventies on a variety of equipment. A discussion on the sealing principles of these fittings arose and, lo and behold, I listened to "an engineer" enlighten me to the fact that Swagelok fittings seal by intimate contact with the end of the tube (at the bottom of the taper and counterbore in the fitting). Can you imagine my astonishment in that revelation!!! I think maybe I need some college level refresher courses. This is why I feel this country will find most manufacturing will be done offshore in years to come just based on competency alone. Clear minded thinkers are few and far between. Thanks for your input.

Check this link out. It explains how the Swagelok Tube Fitting works.

http://www.swagelok.com/downloads/webcatalogs/en/MS-06-16.pdf?301Red=Y