Pressure while heated - half full or full container

[liquidFuzz]

Hi

I was in a discussion with some friends a couple of days ago. The discussion revolved the pressure raise in a liquid filled container. The question was, is the pressure behavior dependent on whether the container is filled with only water or filled 50/50 water/air. (The discussion started when an unopened beer ended up in the fire...)

My intuitive answer is that the pressure behavior while heated is dependent on the containment, but I don't know how... Nor do I know how to calculate the pressure for the liquid/gas pressure of the water. The air pressure seems pretty straight forward if the ideal gas law is used.

 

[sophiecentaur]

A beer can on the fire ic a dangerous thing because the water inside could reach a very high pressure as the vapour pressure increases greatly above 100C. The ends would blow out and the internal pressure would not be affected by the presence of some air.
If the container is totally rigid ('ideal' situation) and the 100% water is contained then the pressure will vary much more than if there is 50% air. Water (as all liquids and solids under 'normal' conditions) has a much lower expansion rate with temperature than air but its bulk modulus is much higher (it can be treated as incompressible under many circumstances).
However, a practical container could well expand more than water, when heated so the pressure inside a water filled cylinder could actually drop when it's heated. Also, if the pressure in the cylinder were reduced significantly, some of the water could vaporise. So the precise conditions would need to be known for a definitive answer.

 

[BvU]

You want to google and study the phase diagram of water.

 

[CWatters]

Liquid water is almost incompressible so a full container of water can reach very high pressures without the contents expanding much. Whereas air is much more easily compressed. So when heated the pressure in a half full bottle will increase more slowly than a full one.

However If you are testing something like a steam boiler you use a liquid like water because if you use air there will be much more energy in the tank when it reaches max pressure. If it fails then the water doesn't expand much before the pressures drops. With compressed air it expands enormously and much more dangerously.

The container matters. No container and the pressure can't increase above that of its surroundings, typically one atmosphere.

 

[liquidFuzz]

Sophi - Thanks for the concern. No one was injured by the heated beer can, some of the participants reported minor headaches the following day... In my mind I did the approximation that the container's volume was constant. And that the mixture was exactly 50/50.

CWatts - The energy is pressure times volume. However, water has more inertia so the BANG will be of a bit lower volume, sound volume that is. :-)
Or is it? If heatet water (gas) is cooled to liquid phase in a burst, or let's call it explosion.

 

[BvU]

However If you are testing something like a steam boiler you use a liquid like water because if you use air there will be much more energy in the tank when it reaches max pressure. If it fails then the water doesn't expand much before the pressures drops. With compressed air it expands enormously and much more dangerously.

There's other effects that make using water a much more dangerous proposition

 

[sophiecentaur]

You want to google and study the phase diagram of water.

The OP may need a bit of help with this or with interpreting the phase diagram. The diagram shows how the three states are present at different temperatures and pressures. In the bottom right of the diagram, there are combinations of pressure an temperature at which the vapour state exists. You can ignore the solid state for this.
http://www.science.uwaterloo.ca/~cchieh/cact/c123/phasesdgm.html shows you more than you need to know.
There is a table of temperatures and vapour pressures of water, lower down. (The temperatures are in K, not C. 273K corresponds to 0°C)) The pressure at 373k (boiling point) is 760mm of Mercury (atmospheric pressure) . Go up to 500K (227°C) and the pressure is around 20 metres of mercury. (Ignore the 400K value- it's a typo!) Thats around 26 times atmospheric pressure so your can would have burst long ago.
If the can were full of just air, the pressure would have only gone up to about 2 Atmospheres.

 

[CWatters]

There's other effects that make using water a much more dangerous proposition

Yes steam explosions are very dangerous. If the blast doesn't kill you the steam condenses on you dumping the latent heat.

However what I was referring to is hydrostatic pressure testing which is a lot safer using water than a gas...

https://en.m.wikipedia.org/wiki/Hydrostatic_test

 

[CWatters]

CWatts - The energy is pressure times volume. However, water has more inertia so the BANG will be of a bit lower volume, sound volume that is. :-)
Or is it? If heatet water (gas) is cooled to liquid phase in a burst, or let's call it explosion.

Energy is pressure * change in volume.

Compressed air expands much more than water when the pressure is accidentally released.

So let's say you had two sealed glass bottles, one full of water and one air. They should both burst at same pressure as they are identical bottles. If they both explode the one containing water should explode with less violence than the one containing air because it contains less stored energy. However there can be secondary issues, the one containing air might not get to a high enough pressure or the water may cause thermal stresses causing the bottle to shatter.

 

[anorlunda]

I wish this thread had been reported immediately instead of posting replies. It is a dangerous scenario.

Among other things, as soon as the can starts to leak, the flashing steam will blow hot coals from the fire around. There are many other ways it could be dangerous. That is what @BvU said.

Thanks for the concern. No one was injured by the heated beer can, some of the participants reported minor headaches the following da

You misunderstand the nature of this forum. It is public. Anyone, including children can read it or search for it on Google. So it is not just your safety in question, it is the safety of all people who might read it in the future. Our rules do not allow public discussion of dangerous topics.

Thread closed.