potential energy of a compressed gas

[jfriddell]

All:

I have been asked to assist with reworking the projectile calculations guideline for my company. We perform a significant number of high pressure hydrostatic and gas tests of pressure containing equipment.

I would like to request assistance from compression experts to assist me in determining the stored energy in a pressure containing equipment piece when I know the following variables:
1) test medium = N2 nitrogen gas
2) test pressures = 5-15 ksig
3) test volume = can be accurately determined for each test
4) test temperature = 50-70 deg F

All gas tests are performed in a water filled pit for leak observation and safety purposes.

If members can help me in determining the stored potential energy, I feel I'll be on my way. I can handle the kinematic and water drag equations once the projectile is launched, although I hope this never happens.

Thank you.

 

[iainuts]

hi jfriddell. I have to believe there are standards for this kind of thing. It sounds like DOT testing. If not, I think the best way of modeling this and showing how much energy is stored is to consider your situation to be equivalent to a cylinder with a volume equal to the volume of your test cylinder, and with a piston on the cylinder that is allowed to move as the gas expands. That means the gas inside the cylinder is undergoing an isentropic expansion from the test pressure to atmospheric. In this case, the first law reduces to dU = W or the amount of energy released during an explosion.

So determine your internal energy at pressure (specific internal energy knowing P and T and then multiplied by mass) and then again assuming it isentropically expands to atmospheric pressure. The difference is the amount of energy released.

 

[israelkk]

If the tank is filled with a gas the
Stored energy = Gas pressure X Tank volume.

This is the energy released when the tank bursts. Sometimes it is converted to the weight of TNT bomb explosion.

 

[zdas04]

You have to be pretty careful with that conversion for piping. While the energy stored in a multi-mile test can be in the kiloton range, not all of it can participate because of friction. I've been looking at this problem for a very long time and have found even the most violent gas-pressure explosions to be far smaller than their pressure*volume product would indicate (but still pretty exciting). I haven't found a "standard" that gives guidance on this arithmetic.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[jfriddell]

Members:

It seems then there is no tried and true method, theoretical or empirical, for estimating stored energy due to gas compression in a pressure containing piece of equipment.

Is this what I am hearing from the group?

I'm dealing with volumes in the 20,000 in3 to 2 in3 range depending on the size of and component(s) being tested.

 

[sailoday28]

Neglecting, electrial and magnetic effects, the stored energy should include:
1-chemical energy,
2-potential energy (due to gravity ie, elevation),
3-internal energy ( U=mu, where m= mass and specific internal energy ie,similar to Cv*T for perfect gas)
and
4-kinetic, if vessel is moving,

Note for internal energy of perfect gas, kinetic energy of molecules defines temperature.


Regards

Regards

 

[israelkk]

See MIL-STD-1522A available at

http://assist.daps.dla.mil/quicksearch/

On page 10 there is a table of stored energy up to 30000psi per volume of 1 cubic feet.

 

[pmover]

Sandia Labs in Albuquerque, NM has several gas guns as does NASA near Las Cruces, NM. These gas guns are used for hyper-velocity tests of projectiles, etc. Perhaps contacting or searching (doubt if that info is available online) their websites or perhaps researching via google may yeild the results you desire. Otherwise, what sailoday states is reasonably accurate.

good luck!
-pmover

 

[25362]

This item has been discussed in other forums. Following iainuts' message, the energy, expressed in liter-atm, that could be released in an adiabatic expansion (i.e., explosion) of an ideal gas equals

[p₁V/([γ]-1)] [1-(p₂/p₁)^{([γ]-1)/[γ]}]​

where

p₁ is the test pressure, atm
p₂=1, atmospheric pressure
V =volume of vessel, L
[γ] = 1.4 for nitrogen