Is Rust & Oxygen Rich Air a Hazard 3

[apatjr]

We're working on a process where nitrogen is extracted from compressed instrument quality air by removing O2 from the air. This extracted oxygen along with regular air is vented to atmosphere through a carbon steel pipe at near atmospheric pressure. This process increases the precentage of oxygen enriched air in the vent from the normal 21% oxygen to 25-30%. The amount of enrich air is about 200 SCFM through a 3" pipe.

Our customer states that an oxygen rich environment along with rust is a known hazard.

Is this a true statement at atmospheric pressure?

 

[nbucska]

False

<nbucska@pcperipherals DOT com> subj: eng-tips

 

[quark]

I couldn't get your actual concerns about hazards.

Some nitrogen generators come with dual beds for moisture and oxygen adsorbtion. Both moisture and oxygen will speed up rusting process in the exhaust piping. If you have separate towers for drying and oxygen removal then the problems will be less.

It is always better to exhaust the regeneration fluid to a safe height(away from hydrogen and other autoignition sources).

Regards,

 

[rnd2]

nbucska is correct. Iron oxide is already oxidised. There has to be reactive, combustible material present together with increased oxygen concentration for a hazardous reaction to occur.
Oxygen enriched air is more likely to cause spontaneous combustion to a combustible material than regular air at the same temp.

 

[Montemayor]

apatjr:

You will get faster (than usual internal rusting of the carbon steel vent pipe. I know first-hand because this is what I observed when we vented 99.99% pure Oxygen to the atmosphere while preferentially producing Hydrogen through the electrolysis of water. I have two comments to make besides the above answer:

1) The rusting of a vent pipe is not considered a "hazard" by me as much as it is considered a maintenance problem. quark is correct in challenging your concern about hazards. The only hazard I can identify in this application is that the rusted-out pipe may fall on somebody’s head in future years. Additionally, if rust were a hazard, there would be safety questions so numerous, most plants in the USA would not be allowed to operate.

2) But why use a carbon steel vent pipe? I stopped doing this with the electrolytic cells and used PVC pipe instead to vent the pure Oxygen. The PVC is maintenance free, won't rust, is cheaper, and I didn't have to paint it. It worked for years, until we decided to route the Oxygen to a reciprocating compressor and fill cylinders with the gas. I used a PVC pipe to take the gas to the compressor.

I hope this experience helps.


Art Montemayor
Spring, TX

 

[steveen]

Art is on the right path.

At high pressures and flowrates there are design guidelines for transporting oxygen. Has to do with removal of the oxide film or rust on the inside of the pipe. If the oxide film is continually removed, the pipe will fail quickly.

Should the oxygen or enriched air find a heat source and combustible, the resulting fire will be something to tell your grandchildren about..... See reports on the Apollo fire for the hazards of working in a pure O2 environment.

If the vent is near a combustible, consider using clay based material for vent piping as PVC will melt and burn, also a PVC pipe will ultimately fail from exposure to sunlight, so in about 10 years, you'll have to replace the tailpipe.

 

[BenThayer]

agree with the other basic facts. given 30% O2 at essentially 0 psig, there would be little danger.

with regards to the Apollo incident. as i recall they were also doing a pressure test of the capsule. since they could not do 15 psi differential by pulling vacuum, they had 15 psig in the capsule and using pure Oxygen.

the partial pressure of O2 went from normal of ~3 psia to ~30 psia.

 

[TangoCleveland]

Take a look at NFPA 53, "Recommended Practice on Materials, Equipment, and Systems Used in Oxygen-Enriched Atmospheres".

Larry