Need Help figuring out this gas cylinder volume issue

[bspayne1983]

First post but this issue has me mind boggled. I work for a compressed gas cylinder test lab and we are doing some gas cycle testing. I am testing 300 liter composite gas cylinder and filling it with nitrogen to 5000 psi, it takes 4 hrs to fill with 2 Bauer high pressure compressors running simultaneously. I want to reduce the filling time by reducing the internal volume of the cylinder by filling it 90% full with HDPE plastic rods. So, that’s what I have done but it still takes the exact same of nitrogen to fill it with reduced volume as it did with original volume(300L). I don’t understand how this is possible? If anyone has any insight it would be greatly appreciated

 

[Baluncore]

Welcome to PF.

So, that’s what I have done but it still takes the exact same of nitrogen to fill it with reduced volume as it did with original volume(300L).

How do you measure the volume of nitrogen needed to fill the cylinder to 5000 psi ?
Do you measure the weight when the cylinder is empty and when full ?
Do you measure the time needed to reach 5000 psi ?

Is nitrogen moving into the volume of the HDPE polymer ?
Most dry polymers will absorb 2% to 5% water by volume over time.
Test that, by venting the cylinder quickly to atmospheric pressure, does N2 continue to diffuse from the HDPE for some time afterwards ? Does the pressure rise again ?

It worries me that you are using gas pressure and not a liquid to test the cylinder.
Why do you not pressure test with a safe liquid, like water?
You are building a "fragmentation grenade" by including HDPE rods inside the 5000 psi cylinder.

 

[berkeman]

It worries me that you are using gas pressure and not a liquid to test the cylinder.
Why do you not pressure test with a safe liquid, like water?

Very good point. Hydrostatic testing is a standard for pressurized gas cylinders (like O2 cylinders used in EMS and hospitals)...

https://en.wikipedia.org/wiki/Hydrostatic_test

 

[Dullard]

When things make no sense, it usually means that one (or more) of your "facts" is wrong. Things always make sense (except quantum mechanics).

 

[russ_watters]

How compressible is HDPE?

 

[256bits]

I am testing 300 liter composite gas cylinder and filling it with nitrogen to 5000 psi, it takes 4 hrs to fill with 2 Bauer high pressure compressors running simultaneously

Why two compressors?
What type?
What is the SCFM of the compressors?

4 hours fill time seems to be a problem - do you have a leak in your plumbing?
What is the source of N2? - Another bottle of N2, or generation of N2 from the air while pumping?

 

[Baluncore]

The bulk modulus of HDPE starts out at about 5 GPa for low pressures.
In this case, 5000 psi is = 34.5 MPa.
34.5 MPa / 5 GPa = 0.0069 = 0.7% volume reduction.

https://www.researchgate.net/public...-and-Youngs-modulus-for-a-HDPE-and-b-LDPE.png

 

[Mr B Jones]

First post but this issue has me mind boggled. I work for a compressed gas cylinder test lab and we are doing some gas cycle testing. I am testing 300 liter composite gas cylinder and filling it with nitrogen to 5000 psi, it takes 4 hrs to fill with 2 Bauer high pressure compressors running simultaneously. I want to reduce the filling time by reducing the internal volume of the cylinder by filling it 90% full with HDPE plastic rods. So, that’s what I have done but it still takes the exact same of nitrogen to fill it with reduced volume as it did with original volume(300L). I don’t understand how this is possible? If anyone has any insight it would be greatly appreciated

Maybe the gauge saying 300L is actually pressure gauge. Gases have constant pressure/volume values so displacing volume "tricks" gauge as it is measuring the gas pressure. I worked as a plumber and we had to apply static pressure tests on natural gas lines where the gauge was required to be at highest point. My plumber and I theorized about the placement all afternoon. The water supply lines underwent similar static pressure test requiring the gauge be at apex as well but hydraulic pressures and gas pressures are affected differently by their respective environments. (I'm not a dr, but I play one t the job site.)

 

[Lnewqban]

Welcome, @bspayne1983 !

The problem seems to be in reaching that high pressure while the valves of the compressors fight the gas from flowing back into the cylinder.

Are your compressors working in series or as two stages?
Are them rated to reach that high pressure?

Note that volumetric capacity of most compressors is given for less than 200 bars.

The higher the output pressure to reach, the higher the temperature and the less the mass that goes into the tank for each stroke.

Fill the tank with a suitable liquid and use a pump if at all possible, or minimum amount of gas, if not.

Copied from
https://www.bauer-kompressoren.de/f...ist/serviceunterlagen/betreiberhandbuch_e.pdf

 

[Nik_2213]

A couple of thoughts:
Is there an inter-cooler, else the hot air being pumped into tank is cooling there, reducing effective pressure.

Is pressure 'stroke' of one compressor thwarting other ? By analogy with exhaust manifold of IC engines, where unfortunate configuration gives 'badly tuned' standing waves etc for too much back-pressure on one or more cylinders, reducing gas clearance and efficiency...

FWIW, please, please take heed of those very under-stated cautions about hydraulic rather than pneumatic pressure testing: Safety videos of their comparative failure modes show difference between 'Oops' and 'Boiler Explosion'. In case of latter, 'Be NOT There'...

 

[Tom.G]

difference between 'Oops' and 'Boiler Explosion'. In case of latter, 'Be NOT There'...

Not to worry, if there for the latter, he will definitely not be Anywhere afterwards.

 

[Baluncore]

FWIW, please, please take heed of those very under-stated cautions about hydraulic rather than pneumatic pressure testing:

I work for a compressed gas cylinder test lab and we are doing some gas cycle testing.

We know that the OP is fully aware of the difference between hydraulic and gas pressure testing. In a way, part filling the cylinder with HDPE, is reducing the stored energy, while making the cyclic testing quicker, and more economic.

The repeated warning is for newcomers to the field, who need to be reminded of the stored energy present in compressed gas, and the destruction caused, when a pressure vessel fails. Being aware saves a lot of paper work, and the coroners time.