On Mon, 01 Dec 2003 19:16:41 -0600, Richard J Kinch
wrote:

Bob Engelhardt writes:

I've been meaning to do the chemistry to show that there simply is not
enough energy from the propane in an empty tank to do any harm. But I
haven't gotten around to it (what else is new?).

Yeah, do that chemistry. You essentially have 5 gallons of steam at, what,
3000 deg F? Fair bit of pressure, that, and too quick for the overpressure
device to respond.

The theoretical maximum pressure would be less than 130psi.

Someone, please check my math.

For experimental purposes, the tank size doesn't matter.
But in order to do the math easier, we need some real numbers.
Math is easier if things are metric, so lets keep it simple.

My 100lb propane tank looks like it would hold 10 gallons of water.
So lets round up to 50 liters.

The reaction for combustion of propane is:

C3H8 + 5O2 - 3CO2 + 4H2O

One mole of propane requires 5 moles of oxygen.

The ideal gas law states that one mole of an "ideal gas"
takes up 22.4 liters volume.

So 22.4L of propane reacts with 112L O2.

But, oxygen only makes up 21% of air.

So, 22.4L of propane needs 533.3L air.

Or a ratio of 1L propane to 23.8L air.

Or, for our tank, roughly 2L propane and 47.6L air.

So lets change our tank to fit the gas.

2L propane and 47.6L air in a 49.6L tank.

After burning, the propane produced 6L CO2 and
8L H2O vapor.

So now the tank contains:
8L H2O vapor
37.6L Nitrogen
6L CO2

Or, it would if the tank would expand to keep the pressure at zero.

Since we are looking for peak pressure, we don't care what happens
when everything cools and the steam condenses.

So we will assume everything in the tank is now at 3500F, which is the
maximum temperature a propane in air flame will get.

Lets go to the ideal gas law PV=nRT

P = Pressure, and is what we are looking for.
V = Volume. In our case, the volume inside the tank is 49.6 L
n = the number of moles of gas. We have 51.6L which is 2.3 moles.
R = a constant = 0.0821 L*Atm/Mole*K
T = Temperature in Kelvin = 2200K

So, P x 49.6L = 2.3M x (0.0821 L*Atm/Mole*K) x 2200K

Which means P = 8.38 atmospheres.

1 atmosphere = 14.7 psi.

Therefore, the theoretical maximum pressure inside the tank would only
be 123 psi.

Dave