Timing the Life of My Nitrogen Cylinder

If the flow were truly unrestricted the cylinder pressure would equal atmospheric instantaneously. Pressure regulators are restrictive to flow.

I meant that there was no restriction of flow beyond the regulator. What additional information would you need to answer my question?

With a truly unrestricted flow it will take zero time. You need a restriction to slow down reaching atmospheric pressure. One could also claim that it will never become completely empty, even if one uses a vacuum pump.

what additional information do you need to be able to answer this?

Some indication as to why it is of any interest would be a good start.

As you say, every regulator has a small orifice which limits the flow to a certain extent. Indeed it would be quite difficult to design a pressure regulator that did not have this feature, as the functioning of the regulator would then be totally dependent on whatever is attached. It would be potentially very dangerous to have a totally uncontrolled flow anyway. This example:

http://www.mips.ie/wp-content/uploads/2011/10/MIPS_Medical-Accessories-Offer.pdf

has a specified flow limit of 150 l/min, although it is intended to be used with a flowmeter calibrated for only 0-15 l/min. That will do as an order of magnitude. The 210 cu ft is about 5900 litres. A full nitrogen cylinder is at a pressure of about 2000 psi, so the residual volume at 10 psi can be neglected for the calculation. 5900 litres at 150 l/min empties in about 39 minutes, for a starting figure.

Thank you! a straight answer

Well if your pressure regulator is an absolute pressure regulator at 10 psi, you will be in for a surprise how long it will take for your tank to reach 1 atmosphere of pressure.

Tell the teacher your dog ate the homework. Information still missing.

What is an orifice? _{(Hint: it ain't where your daddy goes to work)}

Gas Laws

what information would you need?

Have you reviewed any of the previous hints? Investigated 'orifice'? (That didn't come out quite right) Read the instructions that came with the regulator?

I need to know that you can understand our suggestions. When you come back with some comprehension instead of just needs, you might get some help.

thanks for nothing

GIGO

As far as I know an orifice is a hole, opening or cavity, but that word isn't used once in the gas laws link you've sent me, nor are any synonyms I can think of. Please keep nudging me along.

Well I promised that if you showed at least a little comprehension that you would get help. You're correct that an orifice is a hole. For fluid flow an orifice can be used to control the flow rate of the fluid. If the link I provided doesn't suit you you could choose from one of these instead.

Now there is a problem I see with your approach. Nitrogen and oxygen are gasses that readily mix. We are breathing a mixture right now. Blowing pure nitrogen into an open oxygen bottle will just leave you with air in this bottle.

Now there's an additional safety problem. AFAIK oxygen bottles use a special thread for a variety of safety reasons. The most prominent one is that oxygen will support life. Putting any other gas but oxygen into an oxygen bottle can easily be considered a criminal act.

What is the nitrogen being used for? How many cfm are you using?

The nitrogen is being used to purge oxygen from a tank. I don't have a flow meter connected to the bottle or regulator so I don't know how many cfm I'm using.

Get a magic marker and a big baloon. Mark the valve and the Ni tank. Put the baloon over the orifice, turn the valve on, to the mark and stop the flow when the baloon reaches a certain size. You have a volume/time relationship now, go figure the rest out.

Good Luck.

P.S. You need to understand the Gas laws anyway.

At 10 PSI a 1/4 inch orifice will flow much less fluid/gas than a 6 inch orifice.

Without knowing the size of the orifice, and length of restriction, no calculation of flow can be made....Now I can tell you that at 20psi being fed into an OD 1/2" pipe with .375 ID and 1 ft long, will flow at 50.53 cfm....at 20psi a 1/4" pipe 1.2" long flows at 28.05cfm...and a 1/8" pipe 4.5cfm...but these are just ballpark

If you had a 10 psi and .09" orifice you might be around 2.4 cfm....

Are you purging these for transport? and the recipient knows that they have nitrogen not oxygen inside?

My previous statement of "purging oxygen from a tank" while accurate, was also misleading. The tanks I was referring to are for blending wine. I sparge out the oxygen to minimize oxidation. I use a disolved oxygen meter to measure the concentration before moving wine into the tank. Thanks for the input.

We may be having a problem with the English language here. Writing from the UK, I understand a tank as a container, frequently cuboidal in shape, which contains fluids at an atmospheric or low pressure. Flushing one of these with nitrogen to get the oxygen out would be an acceptable procedure if the contents are changed by the presence of oxygen. If, however, we are using the word tank to mean what we Brits call a cylinder (for pressurised gases), then I fully concur with the opinion already expressed, that putting a gas into a container intended and labelled for a different gas is inherently dangerous. I once had to deal with the consequences of a nitrous oxide cylinder which had been filled with carbon dioxide, and I have no wish to repeat the experience.

I'm having difficulty thinking of this as anything other than homework.

Nothing the OP has provided suggests anything else.

Under your conditions, the regulator will be "wide open." How fast it empties the cylinder depends on the size (capacity) of the regulator, which has not been mentioned. You could simply time it once.

I'd suggest calling the original supplier of the O2 cylinder and seeing if they'll take on the job. At least they'll know which Federal regulations apply to this particular task and have the equipment to do it safely and legally. Their name & contact information will be on the label on the shoulder of the tank.

If your question is actually "what is the volume of Nitrogen I have used flushing in to an oxygen tank at atmospheric pressure"

Then at constant temperature, the product of an ideal gas's pressure and volume is always constant.

So if your cylinder has 210 cu ft @ 2100 psi and your pressure drops to 1000 psi you will have used 1100 cu ft of nitrogen. How long it takes is irrelevant in this case so long as you try to keep the temperatures constant or wait till the cylinder temperature returns to its original temperature.. If you have other volumes and pressures you can interpolate this yourself.