Analytical Instrumentation
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On the other tube, a pressure gage has been installed. In the case of the AFP ™ new fitting with sealing lip (Figure 13c), the small holes are located between the sealing lip and the sealing area between the ferrule and the fitting body, as this is the area to be tested for this fitting.
Afterward, the evaluation was done with the test bench shown in Figure 13. Purified argon flows at atmospheric pressure through axial tube of the fitting, while the brazed tubes of the fitting are
pressurized with pure N2. The fitting is connected to a Plasma Emission Detector tuned to measure the intensity of the N2 emission line at 337.1 nm. Other types of N2 sensitive detectors could also be used. Such systems have found leaks that were undetectable with a helium based mass spectrometer leak detector. This set up is also very sensitive to measure inboard leakage.
Indeed the fitting is surrounded or "immersed" in a "sea" of air, which has about 79% N2. Compared to the 5 ppm of helium normally found in atmospheric air, which the helium mass
spectrometer based leak detector relies on to find such a leak. The N2 as a tracer is more sensitive and the system is much less complex.
Figure 12: Difference in rates of rise and fall of the pressure in the dead volume Ferrule Modifications Results
With these modifications, there is less torque required to get the same and even better sealing level of previous design. Less torque reduces further the risk to get ferrule rotation when making connection.
Figure 12 shows the difference between rates of rise and fall of the dead volume pressure. Test is done with similar set-up of Figure 7; the only difference is the use of AFPTM extended tip ferrule.
As all components, except the ferrule, are the same in both tests, leak rate at the end of tubing is assumed the same. The higher rate of rise/fall of pressure while using this new ferrule shows that the dead volume is pressurized (or depressurized) faster, indicating that the dead volume is smaller. Note: This test is done with a union body that doesn't have a sealing lip. Exact same tubing outside diameter was used in both tests.
Fittings Performance Comparison
This set up is used by AFP™ for the leak rate certification of all our valves. The gas is passed through a 0.5 micron particle filter before being introduced to the fitting under test. This is to make sure that fitting performance test will not be affected by particles being introduced into the fitting. This system has 4 to 5 times the sensitivity for leak detection than a standard helium mass spectrometer leak detector, see ref. [3].
Once the test bench properly installed, we pressurized the test area with a specific pressure of
N2; the signal increasing as the detector sees the increasing amount of N2 coming from the badly sealed area. So, we tightened the nut to eliminate the leak and get back system baseline.
Once done, we recorded both pressure and torque, and repeated these steps for many pressure and for all three fittings.
Ones can see on Figure 14 the torque requirement reduction given by the AFPTM new improvements, compare to old existing design. Furthermore, our experience has demonstrated that ¼-past tube gripping is not enough to achieve proper sealing while using those old design unions. So, torque requirement is much higher getting of the old design union the same sealing level.
Figure 13a: Test apparatus for sealing of old design fitting
Figure 14: Torque required achieving adequate sealing Conclusion Figure 13b: Test apparatus for sealing of AFP™ new fitting without sealing lips
We have demonstrated why the characteristics that make an excellent industrial fitting are giving some serious problems in high performance analytical applications. While the existing single ferrule based analytical fitting avoids problems generated by swaging action, its required rotating torque for dimension of 1/8"OD and higher generates serious problems.
Modification of ferrule, nut designs and fitting detail, as described previously, has resulted in tremendous benefits, like eliminating torque related problems (ferrule rotation and twisting), reducing dead volume, better sealing and increasing the possible number of remakes.
However, ones should not forget that an important part of the fitting is the quality of the tubing. Reader comments are welcome. The author may be contacted at:
ygamache@afproducts.ca
Reference Figure 13c: Test apparatus for sealing of AFP™ new fitting with sealing lips
To further validate the benefits brought by our improvements over existing design, we did comparison tests of sealing performance of all the three fittings related here: old design fitting, and new AFP™ fittings with and without the sealing lip. To do so, we needed specifically modified fittings.
An old design Zero Dead Volume fitting (Figure 13a) and an AFP™ new fitting without sealing lip (Figure 13b) have been changed to allow the pressurization of the internal volume between the threads and the sealing area between the ferrule and the fitting body.
Small holes in these areas have been done and external 1/16"OD tubes have been brazed on the both sides of fitting bodies. One the tube is used to send in pure nitrogen in the tested area.
[1]: Agilent, 6890 User's Manual and, Site Preparation and Installation Manual. Manual. [2]: Swagelok, Tube Fitter's Manual by F.J. Callahan. [3]: Varian Mass Spectrometer Manual, Part Number 0981-6999-09-070, September 1995, P.P. 3-10, 3-11
Trademark
AFP™ - trademark of Analytical Flow Products company VICI®
- registered trademark of Valco Instruments Company Inc.
Upchurch Scientific® - registered trademark of IDEX Health & Science LLC. Waters®
- registered trademark of the Waters Corporation See our disclaimer notice available on our website at
www.afproducts.ca August / September 2011 •
www.petro-online.com
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