A question on air pressure & altitude

[Dave-h]

Good afternoon Ladies & Gents,

A question about the effect of air pressure on compressed air supply hoses.

On the web site: http://www.engineeringtoolbox.com/air-altitude-pressure-25_462.html the absolute air pressure at 0 feet and 8000 feet, are given as 14.696 and 10.91 psi respectively - a reduction of around 25% when moving from sea level to 8000 feet.

If a compressed air system that operates at 100psi at sea level, is taken to 8000 feet, would the connecting air hoses be experiencing the stress equivalent to 125psi at sea level?

Thanks

Dave

 

[quark]

First of all you have to specify whether it is gauge pressure or absolute pressure. If it is gauge pressure then net effect(the difference of internal and external pressures) on hose wall remains same.

If it is absolute pressure, then there is a difference of (14.696-10.91) = 3.786psig (higher at higher altitude)

Regards,

 

[FredGarvin]

In terms of simply the stresses induced due to the pressure in the tube/pipe, no. Since you would undoubtedly be talking about 100 psig. The age old $$\sigma = (P*R)/t$$ does not involve a difference in pressure, simply the internal. The stress induced due to pressure is a function of the material. Think of a beam being bent in a vacuum. The stresses on that would be no different that if it were done in some other atmosphere.

 

[Dave-h]

Thank you both for your replies.

Perhaps a little more explanation on my part would have helped.

The question really relates back to the hose characteristics, and whether a hose that can cope with a little over 100psi at sea level, would be able to cope at 8000 feet.

I was trying to understand what would happen if one was to take a system, charged with 100psi at sea level, up to 8000 feet. Would the pressure in the system be increased by the same proportion as the external air pressure was reduced (ie 25%) - so would the 100psi become in effect 125psi?

Thanks,

Dave

 

[Q_Goest]

If the system is charged to 100 psi gauge (psig) at sea level and sealed off, then at an altitude of 8000 feet, the pressure would be 103.786 psig, not some percentage more.

Think of it this way, the absolute pressure inside the system at sea level is 114.7 psi absolute (psia), and that stays the same regardless of altitude. Only temperature can change that number, so if your system gets cold it will decrease in pressure, and when heated it will increase in pressure.

So at sea level you have 114.7 psia on the inside and 14.7 psia on the outside, so the net difference is 100 psig which we read as "gauge pressure".

At 8000 feet, you have 114.7 psia on the inside and 10.9 psia on the outside, so the net difference is 103.8 psig. You should be able to measure that with a regular pressure gage.