Steam & Excursion > danger of steam (non-railroad)

This may be off-topic, but I consider this a warning about a rare household danger which could do some serious harm.

Apparently when boiling a pot of water, it can get super-heated and flash to steam. I had that happen to me last night. Just a six-cup saucepan, all of a sudden sent a geyser of hot water up about two feet and showered the stove top, floor and counter. I was fortunate not to be too close or I would have been severely burned. I was shaking for hours after it happened.

My advice now is to never look down into a pot of water which you are trying to boil.

Also when heating water in a microwave, place a non-metal item in the water to help break the surface tension of the water.
I use a bamboo chopstick.

The surface tension can be so strong that if you heat the water and go to grab it, it can suddenly break the surface tension and spew super heat water all over the place, including your hand and arm causing significant burns.

So just be careful heating water on top of the stove and in the microwave oven.

Pat
South Austin, TX

I always leave the lid cocked a little to allow the steam to escape. Kind of like using a safety valve on a loco boiler to prevent the same only with that it tends to send stay bolts flying until the crown sheet fails.

Bob Krieger
Cheyenne, WY

I've heard about it happening in the microwave but never on the stovetop. Interesting that it happened to you that way.

To bring it back to the topic of railroading, the water can get superheated in a steam boiler as well, causing much the same reaction as in the microwave.

The temperature of steam and boiling water under pressure increases as the pressure goes up. At 150 PSIG for example, the temperature of saturated steam is 366 degrees.

if there is a rapid pressure drop on the steam side, the water becomes superheated in relation to the dropping boiler pressure. The water begins boiling violently to let off the energy and can cause carryover of water into the throttle much more easily in that state.

Where I've seen this happen is on scale live steam, where the throttle is pulled wide with the fire being out or low. The pressure and thus temperature rapidly drop, and the water carries over into the throttle, giving you a warm shower. This can also happen with very aggressive safety valves that drop the pressure rapidly.

Why don't you get scalded from that water coming out of the boiler? When the water hits ambient pressure, it expands and cools instantly, so it is not scalding. This is why you can hold your hand up to the stream of water and steam coming out of a blowdown valve, and it just feels luke-warm.

When water is superheated at ambient temperature such as in a microwave, the pressure differential is negligible so the water is much hotter when it lands on you.

Fun with thermodynamics.

-M

Mike Massee
Tehachapi, CA
Photography, Railroading and more..

Your very serious warning reminds me of an old not-serious question:

Since the molecules in a glass of warm water are in constant motion, what are the odds of all of the molecules suddenly going in the same direction, resulting in the water rising up and out of the glass?

Thanks for the bamboo stick hint & microwave ovens ....

I have seen the water in a microwave appear to be STILL, but boil suddenly upon moving the 4 qt. glass measuring cup....
Took me by surprise...!!!

I don’t think I’d try putting my hand near an open boiler blowdown valve. 🤕

W.



Edited 3 time(s). Last edit at 10/12/17 15:59 by wcamp1472.

Bob3985 Wrote:
-------------------------------------------------------
> I always leave the lid cocked a little to allow
> the steam to escape. Kind of like using a safety
> valve on a loco boiler to prevent the same only
> with that it tends to send stay bolts flying until
> the crown sheet fails.

Actually I was not using a lid. It just flashed out of the blue. I wonder how high the lid would have been thrown if I was using one?

I did find a few reports of similar cases:

https://www.thenakedscientists.com/forum/index.php?topic=41236.0

I think the contributing factor in my case was that the pot was smaller than the electric burner, so the side of the pot was being heated too.

sptno Wrote:
-------------------------------------------------------
> Also when heating water in a microwave, place a
> non-metal item in the water to help break the
> surface tension of the water.
> I use a bamboo chopstick.

Pat, thanks for calling attention to boiling water in microwaves. I will change my practices with them, too.

Well, just so long as my coffeemaker doesn't get too excited.

I believe almost any surface roughness is enough to form a nucleation point, where the bubbles can start. This issue is very common in glass vessels, such as laboratory work, so they often drop a glass marble in the beaker. Was you pot glass?

Kimball Wrote:
-------------------------------------------------------
> I believe almost any surface roughness is enough
> to form a nucleation point, where the bubbles can
> start. This issue is very common in glass
> vessels, such as laboratory work, so they often
> drop a glass marble in the beaker. Was you pot
> glass?

no - old Revere Ware copper clad bottom and stainless steel.

That's good to keep in mind about the glass

From those of us who sometimes cook for a living, please don't be impatient and heat your water as fast as you can. Microwave is a disaster waiting to happen unless you warm water in bursts of 30 -45 seconds at a time. That way you gradually warm your liquid.

Stove top boiling should be done over medium heat with the cover on. That will warm water more quickly than if you boil over high heat with no cover at all. Remember, if you get your water too scalding or steamy, just take it off the cook top and let it sit for a few minutes. Then you can reheat it to the maximum in a very short time via stove top or microwave.

-John

Along with all the sage advice and observations presented, the issue with microwaves is also
amplified at elevation ... since water boils at a lower temperature the higher up ya go ...
be aware that if you try to microwave using your "normal times" if you are at a ski area or vacation spot
that is over 7,000 ft or so in elevation, super-heating is very common and will cause "boil over" if you
add instant coffee or whatever without using the stirring trick first ... especially if you are at
9K elevation or better ...

Odyssey,
Evergreen, CO

jcaestecker Wrote:
-------------------------------------------------------
> From those of us who sometimes cook for a living,
> please don't be impatient and heat your water as
> fast as you can. Microwave is a disaster waiting
> to happen unless you warm water in bursts of 30
> -45 seconds at a time. That way you gradually
> warm your liquid.
>
> Stove top boiling should be done over medium heat
> with the cover on. That will warm water more
> quickly than if you boil over high heat with no
> cover at all. Remember, if you get your water too
> scalding or steamy, just take it off the cook top
> and let it sit for a few minutes. Then you can
> reheat it to the maximum in a very short time via
> stove top or microwave.
>
> -John

Thanks John for your good advice.

ts1457 Wrote:
-------------------------------------------------------
> Bob3985 Wrote:
> --------------------------------------------------
> -----
> > I always leave the lid cocked a little to allow
> > the steam to escape. Kind of like using a
> safety
> > valve on a loco boiler to prevent the same only
> > with that it tends to send stay bolts flying
> until
> > the crown sheet fails.
>
> Actually I was not using a lid. It just flashed
> out of the blue. I wonder how high the lid would
> have been thrown if I was using one?
>
> I did find a few reports of similar cases:
>
> https://www.thenakedscientists.com/forum/index.php
> ?topic=41236.0
>
> I think the contributing factor in my case was
> that the pot was smaller than the electric burner,
> so the side of the pot was being heated too.

the sides being heated is an interesting factor. I will make sure my pots are larger than the electric burner they are on.....

66

Westbound Wrote:
-------------------------------------------------------

> Since the molecules in a glass of warm water are
> in constant motion, what are the odds of all of
> the molecules suddenly going in the same
> direction, resulting in the water rising up and
> out of the glass?

They are in constant motion, but not random motion; they are bouncing off one another. So the collision which pushes one molecule up will push another down.