Dry Nitrogen 2

[drax]

I have a submersible tank that our customers fill with 2-3 psi of Nitrogen. One engineer told me that this could be detrimental to the product causing rust and corrosion by forcing nitrogen into a tank that already contains some oxygen. I'm not very good at this..but is this bogus? I know that the tanks are not vaccuumed or anything before the Nitrogen is put in, but what exactly is dry nitrogen?

thanks in advance

 

[MortenA]

Dry nitrogen must be nitrogen gas with a very low water dew point.

Best regards

Morten

 

[Montemayor]

drax:

By "submersible" do you mean a tank that is going to be used underwater as a Nitrogen source? Something doesn't make sense:

1) You say your customers fill it with 2-3 psig of N2 (I assume you mean gauge pressure, otherwise it's a vacuum);
2) You add that someone told you that this could be detrimental to the product causing rust and corrosion by forcing nitrogen into a tank that already contains some oxygen;
3) You further state that the tanks are not vaccuumed or anything before the Nitrogen is put in.

What is confusing is not knowing what your product in the tank is. Is it supposed to be N2 - or O2? Also confusing is the claim that N2 addition is causing rust (WRONG!) and that the tank already contains O2 (how could it, if your customers filled it with N2?).

The confusion aside, dry N2 can be nothing more than what it is labeled: Nitrogen gas that has no (or nil) water moisture or humidity - just as MortenA states. But what does the moisture (or lack of it) have to do with N2 or O2. Can you clear me up?

Art Montemayor
Spring, TX

 

[drax]

Thanks for you guys reply. We have a electrical switch in a housing that is made to be submersible. The housing has been designed to be pressurized with 2 to 3 psig of Nitrogen gas to help keep water or other fluids from ingressing in. One engineer in my company, who claims to know everything, says that introducing Nitrogen into this tank will cause rust because the O2 is already inside??? This is where I am confused...and not very tuned to gases or this subject matter...pardon my ignorance, but what happens to the oxygen that exists in the tank when N2 is introduced. The tank is not filled under any sort of vaccuum and lets say some air with moisture was in the tank when N2 was injected. What is that scenero if any?

thanks!!!

 

[Montemayor]

drax:

It turns out that the guy "who claims to know everything", if he said that N2 causes rust and corrosion, doesn't know beans about what he's talking about.

Oxygen, not Nitrogen, is the main cause of "rust" (ferric/ferrous oxide). In fact, if you want to deter, or arrest" the rusting of steel/iron, you use Nitrogen (as a blanket) to displace the 21% of Oxygen in the air atmosphere. I'm not trying to be insulting by stating that this is high school chemistry; rather, I'd like to point out how basic and straight-forward the process of "rusting" and corrosion is: it's the formation of oxides - which starts with the prescence of Oxygen, not Nitrogen (which is basically an inert, non-reactive gas).

There are other processes that contribute to "corrosion" - acids, salts, bases,etc. But the definition of rust is still: the formation of brittle coating on iron when attacked by moist air and composed essentially of hydrated ferric oxide. Oxygen can also react with copper, silver, and other metals to form the corresponding oxides and give different types of "rusts".

Your scenario is that if the N2 reduces (dilutes) the quantity of Oxygen in your submerged tank, any potential rusting will be inhibited - not increased.

I hope this explanation helps you respond to your critic. Nitrogen might react (in rare, special cases) but I doubt it if we are talking about copper/brass electrical contacts or switches.

Art Montemayor
Spring, TX

 

[drax]

Thanks for the great post! I think I have enough ammo to derail the chest pounder. I never thought about N2 used as a oxygen reducer, it has always been touted as a pressure to keep crap from getting in but now I see the other advantage.

thanks!

Mark

 

[IRstuff]

Not sure how your tank is constructed, but many similar applications have two valves, one for the supply and one for exhaust. This would allow you to fully purge both oxygen and moisture; you'd run nitrogen in with the exhaust valve open for some duration, close off the exhaust valve, then allow the box to reach the desired pressure and then close off the supply valve

TTFN

 

[25362]

I think one should better look into the rusting effect of moisture. Depending on the temperature of the tank contents and the air RH, the pressurizing (however small) may bring about condensation of the original moisture and start rusting steel. Therefore, either "pressure purging" or "sweep-through purging" should be considered to reduce the dew point. Meaning a "critical" RH value which for steel may be about 50%-60% at the prevailing temperatures. Assuming a starting RH1=100%, and a final RH2=50%, the purging can be estimated as follows.

For pressure purging the recognized formula would be:

number of cycles=ln(RH2/RH1)/ln(14.7/17.7)=ln0.5/ln0.83=3.7, namely 4 cycles.

For sweep purging (if two tank openings are available):

the amount of nitrogen: V ln(RH1/RH2)= V 0.69, namely one "tank volume" would suffice.

Of course, if the original moisture were, say RH1=70%, and the final requested RH2=60%, no purging may be required.

On the other hand if a lower dew point or moisture concentration is required the purging requirements should be recalculated.

I wonder if I am on the right track, and this is the point the "know all" colleague wanted to rise.

 

[drax]

Could you explain pressure purging and how to check the RH
inside the enclosure. Its about the size of a fridge and normally it is pressurized with 2 to 3 lbs. of nitrogen, the contents inside are copper bars, wires, and zinc plated steel components

thanks
Mark

 

[25362]

Pressure purging is simply adding nitrogen to the maximum allowable pressure, waiting a few seconds for complete diffusion to take place, and letting this gas out back to atmospheric pressure. Repeating this cycle several times, as needed, would bring down the dew point (water content) as well as the oxygen content to as low a practical level as desired.

Humidity can be measured on the gas upon letting it out (after pressurizing) by any psychrometric method.

 

[quark]

I am not very clear about the condensation due to pressure rise. RH is the ratio of partial pressure of water vapor at the given temperature to the partial pressure of water vapor at saturation temperature(this is not changing in this particular case). Also as the total pressure of a mixture is equal to the sum of partial pressures of its constituents, I doubt about any pressure rise of water vapor in the mixture due to nitrogen introduction though the total pressure increases.

Why can't you have your electrical enclosure suitable to IP56 or better?

Regards,




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