ULTRA DRY (DEW POINT MINUS 90 DEG C) NITROGEN STORAGE 1

[BADVE]

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. any comments

 

[zdas04]

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?

David

 

[Montemayor]

BADVE:

Dave hits your problem right on the head. You couldn't raise the dewpoint of the contained, pressurized N2 unless you come up with a higher-pressured source of water and force the H20 into the compressed N2.

High purity, ultra-dry N2 is routinely supplied in compressed gas cylinders for Lab and other uses by industrial gas suppliers. I did this for many years and I venture to say that perhaps as much as thousands of these cylinders are filled and delivered each day throughout the globe. No special requirements are needed to maintain the ultra-low dew point of compressed N2 (or any other gas).

The use of a hose with PTFE inner lining is fine - as long as you don't exceed the hose's pressure rating; i.e., I'm assuming you regulate the gas outlet pressure to a value lower than the customary 2,200 -2,500 psig cylinder pressure. Why would PTFE lining be the best? I consider SS tubing (rated for 3,000+ psig) to be the best.

I'm with Dave in thinking that perhaps we missed something here in translating what you mean to say in English. Can you confirm our assumptions of your application?

 

[jcfoley]

BADVE,

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 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.

Chris Foley
Midland, TX

 

[BADVE]

Dear All

Let me elaborate the situation.

My organisation is building a Nitrogen Geneartion and distribution trolley for aerospace application.

The concept is like this.

Nitrogen is generated from atmospheric air using the membrane technology. This nitrogen is boosted from 10 bar to 350 bar using a pneumatic booster. Then this high pressure gas is passed on to a molecular sieve dryer where it is dried to -90 deg C Atmospheric dew point.

Now this ultra dry nitrogen is passed onto a storage bank of cylinders (40 litre capacity cylinders). Now if we measure the dew point of same nitrogen from bottles it is only -70 deg C atm dew point.

This means the Nitrogen is getting "WET" during its passage from the dryer to storage cylinders.

For information, the N2 gas cylinders are of 350 bar rating and are of alloy carbon steel. The tubing is 316 series stainless steel and the cylinder shut valves are of brass and fittings used are compression fittings

Can somebody tell me

1) The drying / baking method for cylinders.
2) type of fittings and leak tightness procedure for tubibg / piping
3) hose material, its scavenging / purging.
4) The dew point measurement method for this application
5) Response of on line dew point measurement system.

 

[jcfoley]

BADVE,

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.

Hope this helps.
Chris Foley
Midland, TX

 

[BADVE]

Thanks for the responses.

The ultra dry nitrogen is used in air craft for various purposes. (Tyre filling, Canopy opening, Missile charging Unit)

Hence connecting and disconnecting will be frequent.

Can someone suggest about hoses couplings with built in valves so that the gas inside the hose remains inside and is not contaminated and after reconnecting the storage trolley to air craft the fall in dew point is minimum.

Currently I am using BEKO germany make high pressure dryer, Swagelok make double compression fittings and SS-316 6 mm ID tubing.

The dew point sensing is through Panametrics make MTS-5 (moisture Target Series 5) instrument.

Badve - India

 

[jcfoley]

Badve,

1)If your end application requires a gas this dry on a mission failure process, I would NOT USE compression fittings. In fact, at this pressure I would also stay away from pipe fittings. They work fine for one or two connections, but then they will start to leak.

2)You need to do an analysis based on your connections to include 100% moisture saturated air. If the amount of dry N2 is large enough, you can prove that a small amount of air at the connection points is not significant in the overall process.

3)As for the couplings with valves, I have never used anything like this. However, this type of coupling is probalby available in the pneumatic parts market. I would not use anything but high quality, almost semiconductor quality parts if this is mission critical. Very few pneumatic components need to be this dry. I would use a good quality integral needle valve as the connection point. This would minimize the amount of moist air the N2 is exponsed to.

Chris