Advanced Research Manufacturing - ARM Inc.

A purifier with every component and all plumbing sections welded together would provide the best assurance of purity from the supplier's factory to the end use, but it would dramatically increase the cost of maintenance such as replacing a failed component or spent purifier vessel. To accommodate servicing the various components in high purity applications face seal fittings are used with any component that may for whatever reason need to be removed for service or replacement.

A face seal connection consists of two fittings with toroidal seal rings machined into the face of the fittings, and male and female nuts that provide the sealing force between the toroidal seal rings and the flat metal gasket positioned between them. Unlike typical elastomeric O-ring seals that rely on the softness of the o-ring to fill scratches in the sealing surfaces, the face seal design works because of the precision machining used to fabricate the fittings and the surface roughness of the toroidal seal rings in the face of the fittings.

The roughness of a surface is typically defined by the term Ra. As described in ASME B46.1, Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Simply put, Ra is the average of a set of individual measurements of a surface's peaks and valleys.

A face seal fittings' toroidal seal ring is machined and electro-polished to a surface roughness Ra of 0.25 micrometers. So how deep is a .25 micrometer "scratch" in the sealing surface? An average human hair is ~90 micrometers in diameter. A 0.25 micrometer "scratch" in the surface is ~1/360th the diameter of a human hair! You would not be able to detect a "scratch" that shallow with the naked eye, let alone by running your fingernail across it. A molecule of N2 is slightly larger than O2 but both are close to 0.0003 micrometers in diameter. This means that the diameter of a N2 or O2 gas molecule is only ~1/1000 the depth of an average acceptable "scratch" in a 0.25 micrometer Ra finish.

So what does all this mean? A typical manufacturer's spec for leak rate of properly assembled metal gasket face seal fitting is 4 x 10-11 standard cc/sec of Helium, the smallest gas molecule. Leak rates for N2 and O2 would certainly be less due to being larger molecules. The typical recommended tightening for face seal fittings using stainless steel or nickel gaskets is to tighten the nut to finger tight, then with wrenches tighten an additional 45 degrees or 1/8 a full turn. The surface roughness (or smoothness might be a better way to describe it) of the sealing surfaces of face seal fittings and gaskets is paramount to making a leak tight seal. Care must be exercised to protect the sealing surfaces (both the fittings and gasket) from damage. When installing gaskets and making face seal connections, take care to avoid particulate contamination on the sealing surfaces. Particulate during gasket installation can make an indentation in the sealing surface, and can lead to failing a pressure decay test, which could lead to over tightening the nuts causing yet further damage. Particulate present may not cause a leak initially, but if the gasket is ever replaced, any indentation it leaves in the sealing surface may prevent a leak tight seal with a new gasket. If removing a component it is best practice to protect any exposed face seal fitting with a new gasket and mating blank fitting tightened per the manufacturer's recommendation. If that is not possible, using aluminum foil and/or a plastic dust cap will afford some protection.

Getter technology is well known for gas purification. Getter purifiers can be operated heated or unheated based on the gas being purified and the impurities being removed. Getter purifiers all have a finite life, essentially a limit to the amount of impurities they can remove from the gas stream.

Lifetime is typically derived from a calculation based on the input impurity level and flow rate of the gas being purified. Purifiers are typically designed and sized to provide 1 year of operation based on these parameters before needing replacement or regeneration.

But what happens if the flow rate was unknowingly more than average causing a shorter than calculated lifetime? The logical assumption would be premature impurity breakthrough, which
in the best case would be identified by decreasing process performance, and in the worst case, damage to products/equipment. There is anecdotal evidence that other things could happen. From breakdown of the gettered impurities and subsequent release into the gas stream, to cyclic burst releases of larger quantities of specific impurities.

ARM Inc. is investigating! We are scheduling end of life testing of specific purifiers and will be writing and releasing a paper on the results, these results will also be reported here.