CAN SOMEBODY TELL ME ABOUT THE METHOD TO MAINTAIN A DEW POINT INSIDE CARBON STEEL CYLINDER AFTER ACHIEVING THE SAME BY USING A DESSICANT DRYER. ?

does material of construction of cylinder critical to storage of ultra dry nitrogen ?

what hose material is suggested for transfer of this ultra dry nitrogen from one cylinder to another.

I was told a hose with PTFE inner lining is the best.

If your nitrogen goes into the vessel as a pressureized ultra-dry gas, how would additional water get in?  I may be missing something here, but getting the nitrogen to (-90C) dewpoint would preclude condensation from the few water molcules remaining.  It seems to me that at that point you need to select materials that are compatible with nitrogen (i.e., not subject to nitrogen embrittlement).

This looks like a really easy problem to me, what am I missing?

I used to work in the High Pressure Helium department of a global industrial gas company.  The material of your vessel is not that great a variable.  However, the intial preparation of that vessel is super critical.  As Montemayor so clearly stated, gas companies have this down pat. So if you are buying your gas, just make sure that you get the specifications down pat.  If this is supplied in DOT style cylinders, make sure that you understand the valve(API Cast Steel Valves) connections.

If you are trying to build your own vessel, you will need to bake it out to dry it.  SS drys better than CS and considering your moisture level of 0.1 ppm, I would only accept SS.  Also, there is an odd phenomenon that occurs if the pressure in the storage vessel drops really low.  At pressures less than, say 500 psig, you an see a marked increase in the dew point.  The crystal structure of the metal in the vessel can absorb (adsorb?) water.  At lower pressures, this moisture will move from the vessel walls and back into your nitrogen.  I can give you a good process to dry out a vessel if this is what you need.

This is a very high purity gas.  I would use nothing less than a flexible SS Inner and Outer flex hose.  I would also suggest that you consider not using compression fittings.  Orbitally weld everything.  Swagelok make a great high pressure and high purity fitting called a VCR.

You did not mention what you application is.  Most people seem to ignore the operational difficulties of using a gas this dry.  Even the most minute leak will shut you down.  If you can give us some more back ground & details, I am sure that myself and others will be willing to help.  

I have worked with several membrane projects in the past.  Most of these, however, did not have the purity requirements that you have here.  The high purity work was with another company.  What I have listed here is a time proven technique for drying cylinders.  It will not be cheap, but it works.

1)  You need a cylinder bake out setup.  Heat the cylinder to 250 deg F or so.  Using a good quality vacuum pump, pull a deep vacuum on the cylinders.  Every 24 hours, break the vacuum with -90 deg C Dew point Nitrogen.  Let this sit at 50-60 psig for about 3-4 hours.  Then repeat.  After 3 cycles, put 100-200 psig dry nitrogen and retest.  Once your cylinders are dry, you should not have a problem at the storage.
2) Swagelok Welded VCR fittings with Nickel Gaskets.  Do not use standard compression fittings.
3) Since you are already having problems with moisture, I do not recommend using any hoses.  If you must, use the ones from my previous post.  1/4" SS tubing, coiled about 3 times with a 9-12" diameter, can be used to create "pigtails" to connect to the cylinders.
4)  I would by a high quality moisture analyzer.  I have used MEECO to great success in the past.  This will not be cheap.
5)  On line response:  Keep your sample lines as small as possible (1/8" or 1/16" SS tubing).  Use only high purity diaphragm regulators like Tescom (also very expensive).  Be sure that you install bypass for purging.  The rest of the response equation will be determined by the moisture analyzer characteristics.

Is this a process where you are connecting and disconnecting to equipment?  The act of connecting to equipment will contaminate you gas faster than anything.  A varying scheme of deep vacuum with a dry nitrogen purge will help here.

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