Calculate pressure generated by fixed volume water in closed container

[asimpson]

sorry but I was never any good at thermodynamics but I need to show prove to some one that a fixed ammount of water in closed container can build up to explosive pressures if heated from 20 C to over 500 C.

Could anybody point me in the correct direction to calculate pressure generated.


Many thanks

 

[SnTMan]

Well, my first stop would be the steam tables...

 

[vpl]

Calculate? Why not search the web for a video demonstrating the effects? Or do a controlled experiment (i.e., you step in before you destroy property) with a double boiler, some water and a stove. Your local department store will sell you one if you don't already own a double boiler. Watching the top of the pot hitting the ceiling is usually enough proof that it's explosive for most people.

If you need the formula, try Wikipedia (come on, you call yourself a mechanical engineer!)

Patricia Lougheed

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[drawoh]

asimpson,

Ask this person how they think steam engines work. Read up on 19th century boiler explosions. They did not have the heat and the pressure we have nowadays, but they blew stuff up good.

JHG

 

[TheTick]

It's not explosive unless the vessel fails.

 

[MiketheEngineer]

If the vessel fails at 500 C - you will have one hell of a scalding explosion - BTW

 

[zdas04]

You didn't say if the vessel was full or not. The posts above assume an expansion space. Let's look at the no-expansion case.

Thermal expansion of liquid water increases pressure in a water-full vessel by about 100 psi for every 1°F so by the time you've increased temp from 20C to 100C you've raised pressure from atmospheric (assumption) to 17600 psi and probably broke the vessel you have the water in from thermal expansion (but no significant steam yet).

The handiest steam table that I have "only" goes to 3200 psi which corresponds to a boiling point of 705°F (374C) so let's assume that at 17600 psi the boiling point is above 500C. That says that heating water in a full, closed vessel from 20C to 500C would raise pressure to somewhere around 90000 psi. There are vessels in the world that can contain that pressure at that temperature, but you don't buy them at Dillards.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[MintJulep]

Here is about 220C.

http://dsc.discovery.com/videos/mythbusters-exploding-water-heater.html

 

[asimpson]

I am afraid I do not know how to read steam tables. Also it is not denied steam at high temperature in an enclosed space will produce pressure. It is a question of how small a quantity of water in a specified volume can produce a "safe" pressure (depending on strength of pressure vessel) when water is heated from 20 c to over 500.

Sorry I was not clearer.

Perhaps a guide to steam tables is needed.

Thanks

 

[kirrer]

Just for reference, above 374.1C, there is no 'boiling point' anymore - over the critical point, there is no difference to define 'liquid' vs 'gas'. As to the final pressure, it depends mostly on the ratio of water volume to total volume of the chamber, and somewhat on the elastic deformation of the chamber itself.

A different approach than suggested for the steam tables, compare specific volumes. At 20C, liquid water occupies 0.001002 m3/kg. At 500C, since fluid water exists only as steam (above critical point), we have to also determine the pressure. The max pressure in my steam table is 40,000 kPa, for which the specific volume is 0.005673 m3/kg. This means, ignoring any air & water vapor in the chamber at 20C (insignificant anyway), you would need a ratio of 5.673 of chamber volume to water volume, if you hope to keep pressure under 40,000 kPa at 500C. You can estimate expand beyond this using the ideal gas law, but something tells me you don't want to exceed 40 MPa chamber pressure (5800 PSI), unless you're building a steam engine out of extremely durable materials. I hope this explanation is clear.

 

[drawoh]

asimpson,

A "safe" pressure depends completely on what sort of pressure vessel you have. I understand that early boilers were made of wood. A quick Google search is not yielding anything for me, but I seem to recall that early steam engines ran on 2psig, and their boilers were dangerous.

JHG

 

[rb1957]

PV = nRT

T = 500degC
V = known
P = chosen
R = ? steam at 500degC
n = moles of water

 

[TheTick]

If it's an enclosed system at a constant temperature, isn't that a saturated system? Moles of water would be immaterial once saturation is reached. Yes?

 

[IRstuff]

Seems like a backward question; if you can control or alter the amount of water, why have any at all?

TTFN
faq731-376

 

[rb1957]

maybe the original question was meant to be "how much water can be in an enclosed (known) volume, at 500deg C, to create a pressure no greater than x psi?".

i thought moles as a stand-in for mass.

maybe, what is the pressure created by 1cc of water in an enclosed volume of 1 lt when heated to 500degC ?
1cc of water = ? moles,
V = 1 lt
T = 500degC
R = ? (= f(t) ?)
then P = nRT/V

 

[KENAT]

http://en.wikipedia.org/wiki/Steam_cannon

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[zekeman]

Good question.
Let us say that the safe pressure would be 20,000 psi.

Then, noting that the critical temperature of water is 374 C, then at 500 C= 932 F, the stuff is all steam.
From Keenan and keyes graphs, at 500 C and a specific volume of 0.1 ft^3/lb, the pressure is 5000 psi.To get to 20,000 psi, ( my assumed safe pressure, I assume the gas law which states that the pressure in a closed volume is directly as the weight or the pressure is inversely proportional to specific volume at a given temperature, so
to get to 20,000 psi, the specific volume would have to go to .1/4=.025 ft^3/lb.
But .025 is 40lb/ft^3
If water at room temperature is about 60Lb/ft^3, that would mean that the container would be about 2/3 full of water to start with.

 

[zekeman]

Just found a link to empirical results of super critical water at high pressure..
iupac.org/publications/pac/24/1/0013/pdf/

The graph shows at 2/3 full=.67 gm/cc, the corresponding pressure at 500 C is an interpolated value of 2 kilabars or about
29,000 psi vs my estimate of 20000.

 

[zekeman]

Correction
1.5 Kbar iinstead of 2 . So interpolatesd answer is
22,050 psi, a lot closer to my answer.