Steam from a low pressure vessel 2

[DDoreee]

Hello,
I have a vessel with steam and water at around 0.5 bar. When I open the valve between the top of this container and the exterior that is air at atmospheric pressure I see a lot of steam coming out. I was expecting the air to fill the vessel, so why is the steam streaming out if it’s at a lower pressure?

 

[LittleInch]

Because it's still at a higher pressure than the air outside? once you release the water probably starts boiling as the pressure is released.

Never seen a car radiator overheat before?

For a proper answer please provide temperature of the "steam and water"?

This is your first post here, so I'll let you off a bit, but this is a forum for professional engineers, not a forum like Quora...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SnTMan]

Gauge pressure, absolute pressure

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[DDoreee]

It’s a depression since 0.5 bar is less than 1 atm. the temperature is the boiling temperature for water at 0.5 bar, about 82 C.

A condescending comment after a wrong answer isn’t what I was expecting.

 

[LittleInch]

Well giving us a pressure that is open to misunderstanding and 1 1/2 lines of text, I'm not quite sure what you expected. Go back and look at the first post and then think - if someone else asked this question, what information would I need to answer it?

Normally people would say 0.5 bara. "Bar" is very commonly held to be bar (g).

So even at what is now presumably 82C water, some steam and now at 1 bara / 0barg, that's still fairly hot water.
What's the air temp outside the vessel?
Volume of the vessel?
How fast did you open the valve?

How long a period did the opening take? initially yes air would probably rush in, but then quickly get saturated with the steam.

If you want to engage and get a "proper" response", then you're going to need to provide a lot more details...
diagrams, drawings, sketches and photos always help.

What you describe is difficult to visualise and yes it seems a little odd, but we can only see what you tell us.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

From your initial post, I would assume the 0.5 bar is gauge pressure and that absolute pressure inside the vessel is that plus local atmospheric pressure. In that case, opening a valve should show a lot of steam blowing out under pressure (and not wispy puffs). If that is not the case, please advise more details as noted above.

 

[DDoreee]

The volume is 1.2 dm3. The valve is opened with an ECU very quickly. I am not allowed to show pictures or technical material. I thought I was simply unaware of a phenomenon related to steam at low pressure to cause this effect, so I would have been satisfied with a general answer in order to know where to search for a more in-depth explanations if that would have been the case.
If any more information would be helpful I would be happy to answer if I can and if asked without any personal comments that I don’t care about.

 

[LittleInch]

So 1.2 litres?

How much is water and how much is steam?

There may be some shock issue going on here, but what size and type is the valve?

The initial air would only be 50% of the vapour space so could rush in in less than a second?





Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DDoreee]

The valve is 1 cm in diameter, the steam is about 60% of the volume. I think it’s caused by a turbulent motion, but I don’t know how likely it is

 

[LittleInch]

Sounds vey likely to me if this valve opens very rapidly. The air rushes in as a cone of gas presumably down the centre? pushing the steam around the outside until about 1/2 second later the pressures equalise and the steam starts coming out.

When you do things very suddenly there's a whole heap of transient things going on which are difficult sometimes to visualise.

Have you tried a CFD analysis if you're that interested? Or just open the valve slower or use a smaller valve?

you probably won't tell us but why are you doing this? What are you trying to find out?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DDoreee]

That is something, obviously reported. The funny thing is that I am not from a third world country and I didn’t receive the right answer. Please know a person from a first world country is laughing at you. I guess you didn’t know that not everyone in the West uses the imperial system.

P.s. I guess I forgot to say thank you for laugh, clown

 

[DDoreee]

I am working on the board for control of the device, so this is more of a curiosity of mine. The day after tomorrow I should be able to check more closely, so I’ll check if your answer can be it, maybe a coworker of mine might come up with the answer

 

[LittleInch]

DDoreee,

You need to learn to not be so thin skinned on this site - it doesn't get a good response. MJC is a bit old and crotchety for sure, but just avoid any personal digs or responses. If you don't like a response ignore it or answer the technical question. Just give as much info as you can upfront - drip feeding information isn't a good way.

The only way you'll find out I think is to slow it all right down or let the air in slower via a smaller valve or hole. This violent sudden opening means it's difficult to see what's happening unless you're running a high speed camera, plus you can't see air move, but you can see steam...

dm3 is not a common unit in many places, trust me. It may be in scientific usage, but not in industry to any great extent.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DDoreee]

I know I should say this. But how am I the thin skinned when the old racist came out raging about the imaginary scenario he painted in his mind. I am sorry about drip feeding information, but I was thinking that I was just ignorant about this phenomenon and it was more obvious to explain for an expert in the field. The dm3 is just what I read from the data sheet.

 

[Compositepro]

Doreen, reread your question. What you describe is impossible. You appear to recognize this fact and therefore ask your question.
I can only guess what is really happening. My guess is that when you open the valve, cold air rushes in (which you do not mention). This air then mixes with the low pressure steam and heats up, as well as heating from the hot water below it. This air then expands and slowly comes out as hot air containing ~50% water vapor, which you describe as "steam". The rate at which this "steam" comes out will be high at first and then drop to almost zero in a few seconds. Another important observation that you do not mention.

 

[DDoreee]

Yeah, that is what happened. I posed the questioned because I was surprised of what I saw and I had two possible explanations in mind:
- there is a phenomenon I am not aware of that can explain this and an expert might quickly direct me to it;
- the steam and air are mixing fast enough to look like the steam is coming out.

At this point the second possibility is the only one I think is possible, but I’ll check again as soon as I am able. Sorry for the confusing way I opened the thread.

 

[SnTMan]

DDoreee, just to confirm, your vessel is operating BELOW atmospheric pressure, yes?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[DDoreee]

Yes

 

[georgeverghese]

You should be using a compound pressure guage to read pressures below atmospheric; is this what you've used to get this reading of 0.5bar abs?

 

[DDoreee]

Yes the instrument for the measurement is for a pressure below 1 atm and the device is supposed to have a depression at that point.