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Precision pressure control of Oxygen and Nitrogen gas
2

Precision pressure control of Oxygen and Nitrogen gas

Precision pressure control of Oxygen and Nitrogen gas

(OP)
Hello

I am trying to bring a pressure of Nitrogen or Oxygen gas from around the 300Bar relative (4351.1Psi) to just over the atmospheric pressures. To do this the first thing that came to mind was to use a needle valve. However I am not sure if this is the best way to go?
I am trying to keep the weight and size to minimum. Therefore I am trying to avoid using any intermediary pressures reducers if possible. I don’t know of any valves which will operate over such a large range, if you know of any please let me know.
Also if anyone knows any other ways of doing this please let me know.
    
Thank you

RE: Precision pressure control of Oxygen and Nitrogen gas

Look at divers scuba gear they have a pressure regulator that reduces the pressure from 300 bar to almost atmospheric pressure.

RE: Precision pressure control of Oxygen and Nitrogen gas

Some of the analyzer pressure regulators such as GO can do this for small flow rates.  However, there is no mention of the volumetric flow in the post.  One solution would satisfy 1/4" OD tubing, a different solution would apply to 8 NPS pipe.  BTW, the solution for oxygen could be very different than for nitrogen.

RE: Precision pressure control of Oxygen and Nitrogen gas

Trying to do it with a needle valve is pretty tough.  As pressure in the tank drops you'll have a smaller dP across the valve and therefore a smaller flow rate.  The needle valve would require constant adjustment, like you get from any number of regulators.  For single-human size flow rates, the diver's regulator above is a good solution.  For larger flow rates I'd look at Fisher or Norseal regulators.

The only thing good to say about trying to do it with needle valves is your capital is lower, but you need constant vigilance to maintain a constant downstream pressure.

David

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

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RE: Precision pressure control of Oxygen and Nitrogen gas

>> As pressure in the tank drops you'll have a smaller dP across the valve and therefore a smaller flow rate. <<
Splitting hairs:  The drop will be critical thus independent of P2.  You'll still get a decaying flowrate with a fixed position of a needle valve when the tank pressure decays,  just because the P1 decreases.  

The regulators that come on compressed gas bottles do just the task that is being requested.  AND they are certified oil-free if rated for use on oxygen.  

RE: Precision pressure control of Oxygen and Nitrogen gas

why not use regulators designed for n2 & o2 bottles.  You must also becareful with o2 regulators they must not contain oil based lubricants.

RE: Precision pressure control of Oxygen and Nitrogen gas

(OP)
Thank you all for your reply you have been most helpful. I took israelkk’s advice and went to the local dive shop and asked about their regulators. They were quite bulky compared to what I needed. But I think I have to live with this for my functional tests. I thank zdas04 for reminding me about the variable pressure if I use a needle valve. However to shade some more light on the situation the flow rates I am looking for is quite small in the vicinity of 4 L/min ( approx. 0.14 cubic feet/min).
From the research I have conducted (google and local dive shop) it looks like I have to design some thing my self if I need to bring the size and weight down to what I need. If anyone know any good references in the area let me know.
Thank you

RE: Precision pressure control of Oxygen and Nitrogen gas

You still haven't defined what you mean by "precision" wrt to the pressure you wish to maintain.  I mention it because precision regulators are usually physically larger than 'regular' regulators.

If you seriously intend to design your own regulator, you will need to buy a book or two.

But even before doing that, search for free literature on safe handling of oxygen.  It may contain some surprises.

Mike Halloran
Pembroke Pines, FL, USA

RE: Precision pressure control of Oxygen and Nitrogen gas

Any medical equipment shop will have oil/grease free regulators for the O2 supply.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Precision pressure control of Oxygen and Nitrogen gas

JimCasey,
Flow across a nearly closed needle-valve seat is considerably more complex than simply opening a hole between a high-pressure source and a low pressure sink.  With the length of the seat you get a definate pressure gradient over the distance.  The pressure gradient will often (some would say usually) cause the flow to never see enough dP at any given point to put you in critical flow at that point.  I've seen very small upstream pressure changes have a significant impact on the volume flow rate out a needle valve when it is throttled very close to shut.

David

RE: Precision pressure control of Oxygen and Nitrogen gas

Per chance are you trying to mix one of the diving gases, Nitrox, Heliox, Trimix, etc.

I must caution as I know of two fatalities associated with supposedly off standard mixtures of these gases. Even though the mixing of these gases is straightforward there are several input variables that can complicate the calculations. A gas mixture computer is almost a necessity.

RE: Precision pressure control of Oxygen and Nitrogen gas

(OP)
MikeHalloran; by percussion I mean input pressure drops from 300 Bar to around 2 Bar (over long time maybe days) and output pressure of below 0.5 Bar is maintained. Pressure drop of 1.5 Bar across the regulator is an assumption as I don’t have the specification of such device yet.

Unclesyd; I am not trying to mix diving gases but my application is similar to diving applications, hence my trip down to the local dive shops. If there are any guidelines regarding oxygen or nitrogen handling please let me know, web links will be fantastic.

Thanks you all as your input has been invaluable, especially with the links and suggestion of suppliers.
Happy New Year!

RE: Precision pressure control of Oxygen and Nitrogen gas

If you will need any quantity over a period of time you should take a look at a membrane separation system (21%-40% O2), like the Nuvair.  This is one of sveral on the market.

http://www.northshorecompressor.com/nitrox.html

RE: Precision pressure control of Oxygen and Nitrogen gas

(OP)
Inclesyd; I had a look at the northshore compressor website. What they had looked bulky and I don't think I am able to compact the device in to miniature size. Unless you know of a way.
Thank you.

RE: Precision pressure control of Oxygen and Nitrogen gas

If you look at the home base you will see that the membrane portion of the system is quite small. One of the determinants of the membrane size is the required flow so there is a possibility of reduction in size here. No matter what your approach you will still have to have the HP bottles with or without a compressor.

Can you comeback with a little more information about what you are trying to accomplish.

http://www.nuvair.com/shopsite_sc/store/html/nitrox_traveler.shtml

RE: Precision pressure control of Oxygen and Nitrogen gas

You also might want to take a look at the "Nitrox Stik".

For information you need to contact the following company.

http://www.envirodive.com/nitroxinfo.php

RE: Precision pressure control of Oxygen and Nitrogen gas

You could try a pressure regulator for a paintball marker, they are small and light weight. Also look at http://www.beswick.com/

RE: Precision pressure control of Oxygen and Nitrogen gas

(OP)
EdDanzer; I have already sent an email to these people and am waiting for their feedback. Their regulators do look promising however the input pressure is lower than what I require (approx 300bar). I am considering placing a pressure reducing valve or something like that to bring down the pressure to a lower range so that I can use their regulator. Thanks you for the link.
I am not quite sure if such a thing as pressure reducing valve exist? A restriction which has a large pressure drop across it (200-250 bar)? If anyone know of such a device please let me know. Otherwise does anyone know how to build one? I can think of a few ways to achieve this but I will be experimenting.
Thank you and regards

RE: Precision pressure control of Oxygen and Nitrogen gas

In pneumatics a pressure reducing valve is called a regulator. All these devices have a seat that closes as the pressure balance is met. The reason I suggested a paintball marker (gun) is these are good to 5,000 psi / 340 bar. You may need two regulators, on for primary pressure reduction (paintball marker) and one for fine control if the flow rates vary much, and you may need a relief valve or relieving regulator if there is much temperature variation when holding pressure.

RE: Precision pressure control of Oxygen and Nitrogen gas

(OP)
EdDanzer
I am not specifically looking for a primary stage pressure regulator as such and am trying to avoid primary stage regulator as size and weight are important, however what I was thinking about was orifice which will have a large pressure drop across it. However I am not sure if this is possible as I have not seen an orifice used in such an application before. I hope this clarifies my intent.

RE: Precision pressure control of Oxygen and Nitrogen gas

An orifice will only work if the high pressure never changes pressure, or temperature. If either changes the flow rate will change which will affect the down stream pressure.
More detail about your application will make it easier to provide better suggestions.

RE: Precision pressure control of Oxygen and Nitrogen gas

(OP)
EdDanzer
As highlighted in the posting above I am trying to regulate the pressure from a gas cylinder to a tube for human consumption. I need a regulating system to bring the pressure down from the cylinder to a level where it is appropriate for human consumption. However size and weight is a factor therefore trying to avoid a secondary regulator if possible.

I think the orifice is appropriate because the primary pressure drop does not need to be accurate as long as it is with in the range of the input of the secondary regulator. Therefore the variance can be large coming out of the orifice.
The secondary stage regulator will regulate the pressure to the set value.

Let me know if my thinking is correct?

RE: Precision pressure control of Oxygen and Nitrogen gas

You can buy air in high pressure bottles.  But you seem to be proposing separate oxygen and nitrogen bottles.  Is it your intention to feed the human some odd or variable N/O mixture?  If so, you need at least two regulators and a mixer, and probably a computer.

Is it your intention to just let the gas flow continuously, or do you need a demand flow regulator, like on a scuba pack?  An orifice would not serve as a pre- regulator, unless you are prepared to dump the gas that's not being inhaled, continuously.  This is because if you stop the flow, the pressure drop across the orifice goes to zero.
The demand flow regulator is less wasteful of expensive compressed gas.

Scuba systems used to have single- stage regulators, with a large diaphragm and mechanism attached to the air bottle, and large low pressure hoses to route the air to the diver.  

For decades they have mostly used two-stage regulators, with a small pre-regulator on the bottle, and a demand flow regulator of modest proportions at the diver's mouth.  Total volume of this mechanism is a bit smaller than the older system, and it doesn't suffer the problem of the large hoses getting full of nasty tasting gunk.

Mike Halloran
Pembroke Pines, FL, USA

RE: Precision pressure control of Oxygen and Nitrogen gas

(OP)
Mike
You asked:
You can buy air in high pressure bottles.  But you seem to be proposing separate oxygen and nitrogen bottles.  Is it your intention to feed the human some odd or variable N/O mixture?  If so, you need at least two regulators and a mixer, and probably a computer.

No this not my intention at all. My intention is to feed constant stream of the gas at constant flow and pressure (to the user).

Is it your intention to just let the gas flow continuously, or do you need a demand flow regulator, like on a scuba pack?
No I my intention is for a continuous flow of gas

An orifice would not serve as a pre- regulator, unless you are prepared to dump the gas that's not being inhaled, continuously.  This is because if you stop the flow, the pressure drop across the orifice goes to zero.
I agree. Intend for the gas to flow through the orifice first then the regulator

The demand flow regulator is less wasteful of expensive compressed gas.
agree but in my situation; this is acceptable

Scuba systems used to have single- stage regulators, with a large diaphragm and mechanism attached to the air bottle, and large low pressure hoses to route the air to the diver.
I will be most interested to learn about the mechanism which was used and how it worked. Do you have any references or can you explain how it worked?

For decades they have mostly used two-stage regulators, with a small pre-regulator on the bottle, and a demand flow regulator of modest proportions at the diver's mouth.  Total volume of this mechanism is a bit smaller than the older system, and it doesn't suffer the problem of the large hoses getting full of nasty tasting gunk.
I will be interested to learn how design changes influenced resolution of these situation!

I am new in this field and the only experience I have is in design of valves and regulators is in the natural gas and water and at lower pressures. Therefore any info in this area is most welcome and I thank you in advance.

RE: Precision pressure control of Oxygen and Nitrogen gas

(OP)
MikeHalloran

Thanks for the links.

If you have more information it will be fantastic. Especially if you know of any hand books which detail the deferent regulator designs and norms with in the industry. ponder

RE: Precision pressure control of Oxygen and Nitrogen gas

I think we've pretty much reached the limit of what I know or can find out quickly about gas regulators.

Mike Halloran
Pembroke Pines, FL, USA

RE: Precision pressure control of Oxygen and Nitrogen gas

Since flow, weight and size requirements have not been specified, I can only offer rule of thumb guidance...

1.  For constant output pressure, you do need a pressure regulator.  Any other type of valve would require external feedback or adjustment to control pressure.
2.  For this high of a pressure drop, you need to heed manufacturers pressure ratings.
3.  Since you are working from cylinder gas (a variable source as the pressure decays as contents are used) you must use multiple stages of regulation or balanced valve design regulators.  Not too many options with a balanced valve at those inlet pressures though...
4.  Two stage regulators are available where the inlet pressure effect will be almost insignificant.
5.  Make sure your regulators are compatible with oxygen.  Typical oxygen regulators are free of contaminants, unapproved lubricants and are also typically brass construction.

Hope these guidelines help.

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