Force generate by compressed nitrogen at different volume

[yudhichen]

Dear All,

I have a question regarding compressed gas. I'm doing some experiment for testing some seal (see hatch area on sketch below) at 10,000PSI by using nitrogen gas. in this experiment, i accidentally got some different result based on two different situation. you can see my experiment sketch in this picture.

case 1:
I apply nitrogen at confined space with volume 1 (V[sub]1[/sub]) to reach 10,000PSI at room temperature. the seal was able to sustain 10,000PSI without leakage.
case 2:
I apply nitrogen at confined space with volume 2 (V[sub]2[/sub]) which i'm sure larger than V[sub]1[/sub] to reach 10,000PSI at room temperature. It took longer time to build up pressure than case 1 (longer time leads to more consumption of nitrogen gas occupied at confined space) and the seal was only capable to sustain up to 8,000PSI before it leak.

from this two experiment, one thing that I'm sure, even at the same pressure, the seal shall be exposed to higher load at case 2 with bigger volume. Any body can help me to explain why and what is load comparison [in force unit, e.g. lbf] that exposed to the seal?

I try to get some sense from ideal gas law PV = mRT. in this particular case, i can cancel out pressure (P), R, and temperature (T) since this experiment was done at room temperature ambient and same pressure 10,000PSI, which leaves volume (V) and mass (m).

then i compare case 1 and case 2: V[sub]1[/sub]/m[sub]1[/sub] = V[sub]2[/sub]/m[sub]2[/sub].
if V[sub]1[/sub] = 10cu.in., V[sub]2[/sub] = 1,700cu.in. then m[sub]2[/sub] shall be 170 times of m[sub]1[/sub].

from this idea, i'm thinking that the load that exposed to the seal in case 2 shall be 170 times higher than case 1. Can somebody help me to cross check my understanding or correct me on how do i get load comparison between these 2 cases? Do i need to consider Z (compressibility factor) in this situation?

 

[marty007]

Generally, as long as the seal is the same diameter and geometry, volume should have no impact on the seal. All that matters is the local pressure acting on the seal, and any external loads (piping loads for example).

Where is the component supported in the two experiments?

Is it at the same temperature?

 

[yudhichen]

Dear marty007,

Thanks for sharing some thought.

Actually, i also think that volume has no impact to the seal.
however, these two results made me think that volume (V) might play some role on it as bigger volume shall expose more load to the seal.
you can see this sketch how i perform the experiment.

the green hatch is the confined volume where i pressurize the seal. as i explain, case 1, the seal can sustain up to 10ksi,
while case 2, seal only sustain up to 8ksi.
Since case 2 has big volume (V[sub]2[/sub] = 170 times of V[sub]1[/sub]), i notice it takes time to pump the nitrogen to that cavity (and consume more gas as well).
those experiments were run at the same temperature.

Do you think volume has no impact on this case?

 

[LittleInch]

The key difference is the force on plug 2 in case 2. This will cause a bending moment on item 1 which isn't there in case 1. I suspect this is the cause of your difference. 10000 psi is a lot of force on that plug

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

 

[marty007]

Those are two completely different loading cases, but volume is not the thing causing you trouble. LittleInch is exactly right.

In case 2, you are adding significantly more axial force on the plug, which translates to the seal. The additional force will be (pi/4)*D^2*(10,000 psi).

Even if your vessel only has an ID of 2in, you would be talking about 31,416 pounds of additional force, this is significant!

 

[yudhichen]

Dear all,

Now i think this become make sense.
I elaborate both of your comments, seems that 'plug 2' is the culprit.
I add some dimension for better comparison, it gives more than 50% additional load to the seal.

Anyway, thanks for the input. it proves that volume has no issue.