Typical operating pressure of a gas clinder? 2

[ipocoyo]

Does anyone know how one can determine the pressure of gas inside a pressurised gas cylinder? We have a pressure regulator attached to a gas cylinder containing 9.5m3 of Nitrogen gas, with the inlet pressure showing around 140bar and the outlet at 5.5 bar. Is this possible?

Still an intern

 

[LittleInch]

Commercial gas cylinders often come at an initial pressure of 220bar, but all the vendor/ look at the data sheet.

So yes. Very possible

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Also: If you get a response it's polite to respond to it.

 

[zeusfaber]

And 300 bar is not uncommon in some parts of the world these days.

By way of education, there are lots of ways you could use the information available to you to decide whether the pressures are credible
[ul][li]Read the label on the cylinder if there is one - this is likely to tell you what pressure the filler intended to charge the cylinder to.[/li]

[li]Look at the WC stamping on the shoulder of the cylinder and divide that into the 9.5 m3 (should be about the same)[/li]

[li]Look at the WP stamping on the shoulder of the cylinder - should be about the same, and certainly not lower than the other two[/li]

[li]Look at the inlet pressure rating stamped on the body of the regulator - again, this should be similar and not lower than the first two.[/li]

[li]Look at the range (the number at the top end of the scale) of the regulator inlet pressure gauge. Very roughly, expect the other figures to be about 2/3 to 3/4 of this (This is an admittedly quick and dirty check, but more accurate than you might expect)[/li]
[/ul]
Those all tell you what might once have been in the bottle - not what's there now. If you want confidence in that, you could try something like:
[ul]
[li]Check the inlet gauge reading is no greater than the filling pressure[/li]

[li]Close the cylinder valve[/li]

[li]While watching the needles on both inlet and outlet gauges, slowly vent the outlet[/li]

[li]Check that the needle on the inlet gauge falls steadily (without getting stuck or falling off), that the outlet pressure collapses pretty much as the inlet pressure drops past the outlet pressure setting and that the inlet gauge eventually reaches zero (you're keen to make sure that the inlet gauge doesn't reach the zero stop while the outlet pressure is still being maintained and that the outlet pressure collapse doesn't happen while the inlet gauge is still indicating much larger pressures)[/li]
[/ul]
This doesn't completely calibrate the inlet gauge, but it does test for the most common defects.

As to the 5.5 bar outlet pressure: That's going to be determined by the requirements of your process and may well be adjustable on the regulator. Look at the labelling on the regulator (or apply the 2/3 rule to the outlet gauge).

As a rule of thumb, single stage regulators are usually capable of a turndown ratio of up to about 100 - that is to say that they can accurately reduce pressure down to 1/100th of the maximum inlet pressure. In your case, that would imply that anything above 2.2 to 3 bar is achievable with a single stage - so 5.5 bar doesn't sound at all alarming.

A.

 

[ipocoyo]

Hello,

Thanks for all the responses you have provided me!

Upon checking the gas cylinders, here is what is stamped:

C.P 20MPa
T.P 30 MPa
W.C 50.8L
60.2kg

I have never seen the abbreviations C.P before, would anybody know what it stands for? Also, is it safe to assume that the weight of gas inside the cylinder is 60.2kg if nothing else is stated?

Thanks again!

Still an intern

 

[LittleInch]

Mt best guess is that this is

Cylinder Pressure (max working pressure)
Test Pressure (1.5 × MAWP)
Working capacity (internal volume)
Weight of the cylinder itself.

The cylinder doesn't know what gas it will contain and therefore can't estimate the mass of gas.

But if you know volume, pressure,temperature and has them you can calculate mass.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zeusfaber]

Practically, agree with LittleInch, although a couple of the abbreviations spell out differently
CP is normally Charge Pressure (means the same thing though)
WC is normally Water Capacity (quoted in terms of something incompressible, so there's no confusing it with the amount of free gas it could hold at working pressure) Again, means the same as LI said..

 

[georgeverghese]

Typical fill pressure for N2 cylinders is 3000psig.

 

[ipocoyo]

Hello All,

If we know the volume and type of gas inside, are we not able to find the mass of gas by just using the density formula?

From there, if we know the mass, temperature and volume can we not estimate the theoretical pressure using ideal gas law?

Thanks!


Still an intern

 

[georgeverghese]

You are confusing liquid behaviour for compressible gas. Liquid density is loosely invariant with pressure, while gas density is directly related to gas pressure.

rho-gas = Molwt x Press / (zRT) - you may find this even in a high school text on chemistry or physics.

 

[ipocoyo]

Thanks for the clarification, had a little brain fart moment there. Thanks all those who helped!

Still an intern

 

[ipocoyo]

We have a compressed gas cylinder containing Argon on site at the following specifications:
Pressure = 20MPa
Volume= 10.2 m3
Temperature= 288K

I am trying to get the theoretical amount of Ar inside. I used the ideal gas law where PV=nRT but seem to be getting a result which does not look right.

PV=nRT
(2x10^7)(10.2)=n(8.314)(288)
n=85198
N=mass/MR
mass=85198(40)
mass=3407906g or 3407.9 kg
What am I doing wrong?


Still an intern

 

[LittleInch]

10 cubic metres??

That's a decent sized pressure vessel not a gas cylinder. 100 litres maybe?

You don't list units here so it's difficult to check the calculation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ipocoyo]

Hi sorry!
Here are my calculations with units:

(2x10^7Pa)(10.2m3)=n(8.314J⋅K−1⋅mol−1)(288K)
n=85198
N=mass(g)/MR
mass=85198(40)
mass=3407906g or 3407.9 kg

According to what is printed on the cylinder, it is 10.2m3.

Still an intern

 

[LittleInch]

!) When you see something strange, engage brain. A cylinder like this is clearly not 10 CUBIC METRES

That's a box 1m x 1m x 10m. Does that cylinder look that big??

The wording on the bottle, which is quite crucial, is 10.2 Sm3. That S means Standard or "atmospheric"

Hence the volume of the gas at 200 bar in that cylinder at "standard" conditions would be 10.2 m3

On a very basic p1V1=P2V2, at 200 bar that means that the volume of the gas inside the cylinder is about 0.05m3, so 50 litres.

Now density of argon gas at 300K and 1 bar a is about 1.6 kg/m3. Your bottle tells you, you have 10.2 Sm3 of the stuff so the mass of gas in the bottle is 10.2 x 1.6 = 16.32 kg

If you put 50litres into your calc you get more or less the same.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

WC = 50.8 litres per your earlier post, not 10.2m3.
We know that 1kgmole of an ideal gas occupies 23.645 Sm3 (ie at 1atm, 15.6degC or 60degF). So an easier calc is (10.2 / 23. 645) x 40 = 17.2kg of Ar. This approach will account for Ar compressibility also at 20MPa.

 

[Caloooomi]

I did chuckle at a gas cylinder weighing 3,400 kg

For the sm3, the s is important. It stands for standard, as in standard temperature and pressure. Default is 100 kPa and 0 degC.