Gas dissolution rate in water

[yobacan]

Hello,

I am trying to calculate the gas loss rate in a pressure vessel, which is connected to a pipeline. The upper part of this cylindrical tank will be filled with air under pressure, and the lower part will be filled with the fluid pumped, which is normally water. The air will be compressed when the pressure in the fluid goes up and viceversa, preventing the pressure in the pipeline to rise or dip too much). In other words, the air acts as a cushion. This device is known as air compressor pressure vessel.

The idea is to pump air through a valve in its upper head and then let the air reach an equilibrium with the water (normally at a point in the middle part of the tank, which is vertical). The problem is that I haven't been able to find the gas dissolution rate in the water. I have studied the Henry Law but that one is for static environments (like a closed can of Coke). In this case we have the pump constantly applying pressure to the water. Therefore, if air is not supplied constantly, it will be completely dissolved in water over time. I am looking for a formula that allows to measure this gas loss rate, for a specific gas, at a certain temperature, given a certain contact area and over a certain period of time.

Regards

Felipe

 

[zdas04]

You will almost certainly not find a pre-packaged rate of solution equation. Published work that I've been able to find (and I've spent a lot of time looking) is even a bit iffy on the direction of change in the amount of absorbed air in water with a change in temperature (they all seem to agree on the direction with a change in pressure, but the magnitude can be a bit iffy).

What you are describing has existed for generations as a pulsation bottle. The only difference is that the pulsation bottles have the air contained within a bladder that doesn't directly contact the pumped fluid. The reason that this solution has survived for many decades intact is that it works. Just pumping air into a vessel will not work because much more of the air will bubble through the pumped water than will ever dissolve in it.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Compositepro]

Well water systems use pressure tanks. These days they are almost always with rubber bladders to prevent the solution issue and associated maintenance problems. The bladderless tanks used air-volume control valves to add a few cc's of air to the tank with every pump on/off cycle, and vent any excess air. These failed frequently.

http://inspectapedia.com/water/Water_Tank_Air_Volume_Controls.php#Round.

There is no equation that will give you the answer you seek.

 

[zeusfaber]

I wouldn't attempt this without a nice thick bladder. One strong enough to block the entry at the bottom of the vessel and sustain the precharge pressure. Without it, every time your pipeline pressure drops below about half your intended pressure, you'll start to inject air into the pipeline.

A.

 

[yobacan]

Thanks for your research xdas04. I understand that there are tanks with bladders, and for this case, I intend to install a piston to separate air and water, and make the contact surface much smaller, but I assume that still there must be a rate of loss with time. It is a pity that there does not seem to existe a formula that can magnify this process, according to your opinions.

 

[zdas04]

If it is a pity, then develop one yourself.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[yobacan]

I would love to zdas04, but its not my area.

 

[georgeverghese]

Oxygen or nitrogen dissolution into water is very low, and would not be the explanation for the loss in air in this vessel. Air is being entrained down into the water by either poor air pressure controls and / or an undersized accumulator vessel.

A bladder makes up for these deficiencies. If for some reason a bladder type accumulator is not possible, then an oversized accumulator vessel and pressure controls with better response will help to minimise carryunder of air in the exit water stream.

 

[LittleInch]

This seems to give a fairly good result -

http://www.engineeringtoolbox.com/air-solubility-water-d_639.html

The key factor you seem to miss is that it should be the same water that is in the water below the air, at least for the first 100mm or so.

The key problem I have is the same as zuesfaber - if your inlet pressure into this vessel ever went below your air pressure when the level is at the minimum, or the whole volume is virtually air, then air will escape down the pipe.

Otherwise once the water in the vessel has equalized, then the dissolution rate will fall as most of the water is already saturated.

To avoid any of these issues, that's why bladdrrs are used, as most others have already pointed out.

If you're installing a piston, why would there be any contact or leakage?

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

 

[zdas04]

Littleinch,
He's pumping new water through the vessel continuously, The new water will not be saturated.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Is he? I've read this a few times now and can't see where there is any reference to this. In fact it looks more like a standard expansion / pressure tank every time I look at it.

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

 

[zdas04]

I can see why you'd think so. While a standard expansion tank meets the definition of a pressure vessel, when someone uses the term "pressure vessel" I see (admittedly in my mind) a flow path instead of a stand pipe. Probably just my bias.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[zeusfaber]

The only place I've seen what I'd usually call an accumulator referred to as a "Pressure Vessel" is in a (central heating) combination boiler, where the pressure vessel (complete with bladder) is generally clagged onto a dead leg of the system.

A.

 

[georgeverghese]

Could we see the process control loops on this surge pressure vessel ?

There should be a level controller resetting the setpoint of the air side pressure controller if level drops below the low limit of the operating level range.

Also, the feed pipe for the water should dip down below the low level limit to reduce air entrainment in the water exit nozzle.