Danger of compressed air.

[BigMac23]

I am trying to quantify the amount of potential energy created when compressing air. Specifically related to the volume of air compressed.

Example:

A vessel is being hydrostaticly tested (water), but has air traped inside. It is pressured to 2000psi.

Scenario #1: pressurization occurs with 1 cubic inch of air.

Scenario #2: pressurization occurs with 1 cubic foot of air.

Is one scenario more dangerous than the other (more potenial energy) and if so, how much so?

Are there other factors to take into account?

I only know enough that i don't want to be around if a vessel containing a compressed gas was to let go.

Thanks in advance for you input.

 

[CESSNA1]

BigMac23: hydrostatic pressure tests are ALWAYS performed with water. The less air you have trapped, the better off you will be. There actually should be no trapped air.

The energy in the trapped air is simply V*P. Where V, in inches^3 is the volume of trapped air and P, in #/in^2 is the pressure. That will give you the potential energy of the trapped air.

With air the pressure vessel will explode like a bomb and scatter schrapnel all over the place. With water it will simply rip open and you will get wet.

In either case do not be in the area of the test item.

Regards
Dave

 

[Barry1961]

Here is a link that goes through the math.

http://www.combro.co.uk/nigelh/diver/tank.html

There are a few ways to calculate the energy you have available. To get a good feel for it you need to incorporate time into the formula. The length of time will depend on the size of the orifice or breach.

Barry1961

 

[rob768]

Hmmm, I followed that link. That's scary. Never thought of it that way.

 

[Barry1961]

Machinery's Handbook has the formulas for calculating the work required to compress air in both adiabatic and isothermal compression. The actual should be near the median between the two. They have a chart showing the work in foot-pounds from 5 to 100 psig. A few of the values off the chart showing the actual work required to compress 1 cubic foot of air from 1 atmosphere (14.7) are.

10 psig-1154.6 ft-lb
20 psig-1971.4 ft-lb
50 psig-3619.8 ft-lb
100 psig-5327.9 ft-lb

If this work was done or expended in a short period of time the results would be note worthy. Of course the actual damage would depend on several factors.

Barry1961

 

[IFRs]

We had an aluminum tube 10" diamter x 20' long rupture, fly across the shop and imbed itself in a stack of metal. No one badly hurt but we started using tie-down, restraints, pressure regulators, more QC/QA on the our supplier's side, etc. Can be done, if care and special equipment is use.

 

[Tofflemire]

Hi All,

Our rule of thumb for compressed air is anything discharge (explosion) over 3 PSI of compressed air and projectiles will break skin. We keep our pressure testing in house to 3 PSI and if we require higher pressures we use water, grease or hydraulic oil.

Tofflemire