Relief case water vaporisation in low pressure tank

[erbru]

Hi all
This is related to my thread on the vacuum pulling due to steam condensation in the atmospheric tanks.

http://www.eng-tips.com/viewthread.cfm?qid=412869

I am trying to quantify all the possible relief scenarios
One of the cases (which I think is not decisive) is a blocked-in vent during the heating up phase (as hot water is added to cold tanks during the sanitsation).
Hot water (85°C) is added to the tank (total volume empty about 43 m3) at a rate of 5.5 m3/h. The tank pressure builds up during this period due to the increase of water in the vapour space and the expansion of the air as well. The tank can probably take only 50-100 mbar of overpressure.

What is the best way of estimating this flow?
Should I take the hot water inflow, convert it to a heat input in kJ/s (total enthalpy of water at this temperature), assume all this heat is used to vapourise water then use the latent heat of water (kJ/kg) to determine the kg/s of water vapour evacuated?
Surely I also need also to account for the increase in air pressure as the tank heats up and evacuate some of the air.
Do I use ideal gas law to calculate the mass of air to evacuate...
But the PSV will be evacuating humid air and not just air or water vapour?

Do I take psychometric charts and look at the specific volume at the start and end temperatures ...but then I need to know the rate at which the temperature is increasing and does the specific volume increase go up at higher rates at higher temperatures (yes).
Do I use empirical data on the tank temperature warming up (available from site) or just the heat input from the hot water stream all converted into increased air enthalpy (conservative).
At 50°C in the tank, without venting, the pressure is already 100 mbar above atmospheric pressure.

 

[erbru]

On a little further refelction, isn't the simplest sizing basis to say the tank is empty, heats up extremely quickyl and all the hot water coming in turns to water vapour and all of this water vapour needs evacuating to keep the pressure down. That means my sizing case is 5500 kg/hot water --> 5500 kg/h steam.

 

[erbru]

Unfortunately this case is excessively high and won't be relieved through a 50 mm orifice which is what I have available. I need to get a more reasonable flow.

 

[LittleInch]

Hmmm,

I think you're going a bit far here. It sounds like you're using all the energy to boil off water - not surprising you're getting a bit of a large flow....

First start with your liquid inflow, this equates directly to air being emitted at effectively the same rate given your over pressure is so low.

I would then look at the gas being heated from ambient to 85C in some reasonable time period - say 1 minute, using ideal gas law to generate a volume flow. I think this is the casue of most of your extra vapour / gas being lost through the vent.

I'm not sure you will actually get much water vapour being emitted when all the tank is doing is filling. The vapour pressure isn't above atmospheric and the over pressure won't allow free vapourisation. once you start spraying it you will get some water loss into the vapour space.

If you start with air space at say 20C (293K) then increase it to 50C (323K), for the same volume you have a pressure increase of 110mbar. Matches pretty well to your data supplied.

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[Compositepro]

The air in the tank will heat-up and saturate with water almost immediately (~1 min., as little Inch says). After that the further addition of water will only displace some moist air.

 

[LittleInch]

I hadn't really seen the second post. Clearly unless you are supplying super heated water (>100C,>1.01 bara)this won't just instantly flash off to produce steam. That's a very very conservative assumption.

Just try this at home - make a cup of coffee and then seal it with a bit of cling film - result - clingfilm just stays there. doesn't expand until it blows off.

Try it on the outlet of a boiling kettle though and you're in a different space altogether.


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