Calculating Minimum Hole Size for Steam Pressure Relief in a Sealed Container

[Mosio]

Hi,
Consider I have a sealed container with some water in it, put it on a normal stove. Obviously after some time it will explode due to the steam pressure. To avoid this, I drill a hole so the steam can escape.
Is there a minimum diameter for this hole?
How should I calculate it? Assuming 1 litre of water and a methane burning temperature as source of the heat.
Many thanks in advance.

 

[Mosio]

To clarify, I need this high temperature/pressure steam for cleaning and cooking, much like a coffee maker.

 

[Tazerfish]

First of all, I don't think it is a good idea to build high-pressure containers yourself.
Especially since they are filled with superheated water which can really explode and burn you like hell.

I wanted to write some more but then I realized I know desperately little I know about high-pressure cooking pots.
Either you do some research on your own, imitating how something is industrially produced is usually a good idea for DIY projects, but seriously I beg you to just buy one of them.
It is not like they are worth a fortune.Safety first.
PS: I was always scared of these things since one of them exploded in my grandmother's house and spewed hot water everywhere.That lead to some nasty burns ...
EDIT: Sorry, that this isn't very helpful but just overthink it once more whether you still want to build one, ok ?

 

[Mosio]

...
just overthink it once more whether you still want to build one, ok ?

Thanks, you're right its dangerous and I'm sorry about your bad experience. But you know, diy things are much more customizable. That's why I'm seeking for information and on the other hand I can't trust that overseas junks. By the way thanks.

 

[billy_joule]

The required size of the hole is dependent on the rate of heat addition and If you have to ask you're out of your depth.
Pressure vessels are very dangerous, this thread should be locked.

 

[Tazerfish]

I just found out you are not the first one attempting this :
https://www.physicsforums.com/threa...from-scratch-need-help-with-equations.752003/

 

[berkeman]

If you have a hole in your pot that you are boiling, there is nothing dangerous. Thread closed.

 

[berkeman]

If you have a hole in your pot that you are boiling, there is nothing dangerous. Thread closed.

Upon further review, it probably would be good to discuss what size of hole is needed to ensure safety. Thread is re-opened.

 

[Nidum]

If a boiler is required to generate steam at any significant pressure then relying on a plain hole as a pressure limiting device is not a good thing to do .

Real boilers usually have safety valves which allow steam pressure to rise up to a preset limit and then not rise any further .

Control of steam pressure is really a matter of proper boiler management . Adjusting the water feed and firing rate so that steam is being generated at the pressure required and at the same rate as it is being drawn off for use .

 

[Nidum]

This is an excellent series of tutorials on steam technology :

http://www.spiraxsarco.com/resources/pages/steam-engineering-tutorials.aspx

 

[Mosio]

Thank you very much Nidum, Tazerfish for the info, I was wondering why this thread should be closed.
It seems it requires a huge amount of knowledge and facilities however I can't stop temptation to build something small like a coffee maker :D

 

[Nidum]

Your boiler is similar in size to those used in model steam engines .

Have a look on this site :

http://www.model-engineer.co.uk

 

[Tazerfish]

I just want to make this a theoretical problem ...
We can assume that the pressure inside the cooker is approximately the saturation vapor pressure at that temperature.So basically a complicated almost exponential function of temperature.
Then we can use Bernoulli to calculate the outflow through the opening at that pressure. Because we know the temperature and the volumetric flow rate we can then calculate the thermal energy leaving the system per unit time. In other words the power.
And in a stable state the power that is beeing supplied equals the power flowing out.
I guess we could derive a formula from these considerations which would give you a relationship between the power flowing in, the opening, the Temperature (and the pressure).
So you could plug in the maximum amount of pressure and the maximum of heat flow into the system and you would get out a minimum necessary hole size.

EDIT: 1.You would lose a lot of heat constantly using the " simple-hole" design(steam escapes)
2.the hole would have to be quite small (haven't crunched the numbers yet/just guessing)(hard to build)
3. How would you know how much pressure the container could safely withstand ?(for using the equation)
4. You would have to let it stand very long after turning off the heat, for it to cool down to 100 degrees Celcius, so it isn't superheated anymore and it cannot explode

To sum up: the "simple hole" is not really a good idea.
(though it might make an interesting physics problem)
You should go with an adjustable valve which "triggers" at a certain pressure by itself.
And either a secondary valve to release the pressure after use or just the valve from before with some modifications so it can be set to "stay open".

 

[russ_watters]

Locked again pending moderation.

 

[berkeman]

I was wondering why this thread should be closed

We do not allow dangerous discussions here at the PF, which is why this thread has been problematic. There has been some good safety advice in the thread, but it is time for it to remain closed.