low presure flow from a hig pressure tank 1

[hyposmurf]

I post this topic with all the data I have and in a true situation.
Fact : to fill with air a 1500 liters [about 400 Gallons] film bag to up a 0.050 Bar [0.7251887 PSI or 20 " water column ] pressure in about no more than 15 seconds
Source of air . 7 bar [about 100 PSI]
Mean of delivery : a coiled hose on a reel.
Mean of operating : a high pressure valve on the reel incoming before the rotary joint.
Limitations: the hose could no be more than 50 mm [2"] internal diameter. The hose will be unreel up to 6 meters and reeled up while filling the film bag as it comes out from two rolls that close this end.-
My main question : how shall be pressure rated the hose , or better: how it will be the pressure profile in the hose in all it's length.
The hose will never be closed in it's end and the bag will act as a fuse because it will not sustain more than 0.10 bar [ or 4" Water column].
A shut off safety valve could be used after the main valve before the rotating joint.
Some like the valve used at air starters on airplanes turbine motors where the air at 500 PSI is regulated at 45 PSI flow and if the pressure reach more than the rated pressure of the hose , it shut of the flow.

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[TD2K]

0.7 bar is about 10 psi, not 100 psi. Since you talk about a 'high pressure valve before the reel', I'm assuming the 0.7 bar is a typo and you do mean 7 bar or 100 psig.

To estimate the pressure profile through the hose, calculate the air flow you need to pressure the film bag in the time you need.

Calculate the pressure drop through the hose for that flow, the smaller the hose, the higher the pressure drop. My guess is you will find most of the pressure drop is being taken across the high pressure valve at the inlet to the hose.

Buy a hose rated for your air system pressure. It's not going to cost much more and it avoids any nasty surprises if someone decides to install a valve on the far end for some reason. If you buy a hose rated for your air pressure then all issues with safety valves, shut-off valves, etc go away. Keep it simple would be my advice.

 

[hyposmurf]

Of course it was a typo.
It is 7 Bar.
There is not a fact of cost , it is handling by the reel problem
I need the lightes hose that will resist the pressure.
I think that the pressure will be high at the hose start.
But do not know how much.

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[TD2K]

The amount of air needed to fill the bag to the pressure in the 15 seconds is only about 9 scfm. For 1/2" hose and allowing for 100 ft, you'd need less than 1 psi at the hose inlet downstream of your valve at the start of the filling (when the bag is at atmospheric pressure and still less than 2 psi when the bag is at 0.05 bar.

Since your hose is only 6m long, the hose pressure drop will be almost zero.

 

[hyposmurf]

Hi TDK , maybe I made some other typo, so I put it clear

Volumen to fill = 1500 liters or 53 CF
Time to fill the volumen = 15 sec.
53 CF
so in 1 sec = ------- = 3.46 cfs or 208 CFM
15 SEC

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[TD2K]

You haven't included the fact that you are only increasing the pressure in the bag by 0.05 bar. Your flow (one complete bag volume or 208 cfm) would increase by pressure in the bag by 1 atm, much more than 0.05 bar.

Look at it this way. If the pressure didn't change in the bag at all, the required air flow would be zero wouldn't it? If you were increasing the pressure by 1000 bar, you would obviously need more air yet that effect is not taken into account in your formula.

I used the ideal gas equation:

n = PV/ZRT

P is the change in pressure
V is the volume of the bag
Z taken as 1, compressibility factor
R gas coefficent
T temperature, I took it as 60F as you didn't specify it.

Working through this, to raise the pressure in the bag by 0.05 bar you need 0.005965 lbmoles or 2.26 scf (379.6 scf/lbmole). This flow is over 15 seconds or about 9 scfm.

Sorry for the mix in units.

 

[quark]

TD2K,

Congratulations for being Tip Master of the Week.

hyposmurf,

I got a little bit higher value of 10.44cfm than what TD2K got. But the calculation given by TD2K may be more precise, for he actually considered mass flow rate first.

For a simple calculation you can proceed with,

1500x(1.0132+0.05)/1.0132 = 1574liters or 74liters(2.61cu.ft) to be filled extra. Then flowrate will be 2.61*4 = 10.44cfm.

If the film bag is initially flattened and its water volume is 1500liters then you may have to go with 55.58 cf/15sec or 222cfm.

If you can get a very precise pressure regulator(which I doubt) you need not worry about pressure profile in the pipeline.

Regards,

 

[katmar]

I think the misunderstanding here is because the bag is initially flat and totally deflated, so it needs to be filled with a volume of 1500 litres (1.5 m3). Also the pressure of 0.05 bar has to be a gage pressure.

The density of air at 20 deg C and 0.05 bar gage (1.05 bar abs) is 1.25 kg/m3. The air to be transferred through the hose is therefore 1.5 x 1.25 = 1.875 kg. If this must be done in 15 seconds the flowrate is 0.125 kg/s.

Assuming that the air enters the 6 metre long hose at 7 bar gage (i.e. that the inlet valve is not a restriction) and you want a flow of 0.125 kg/s, you will get sonic choking in the hose if the inside diameter of the pipe is less than 17 mm.

You can use any hose with a larger diameter than this. I would not be concerned about the pressure profile down the hose. The entire hose should be rated for the full supply pressure, plus a bit of safety margin.

I think your biggest problem is going to be to be able to close the supply valve quickly enough when the bag reaches its target pressure. And the noise could be a problem with the sonic flow.

 

[TD2K]

Katmar may have hit it on the head and the same thing occured to me in the shower this morning (wonder if my brain had been chewing on this last night).

If the bag has to be inflated to 0.05 bar (yes, I assumed this was gauge but I assumed it was the differential pressure added to the 1300 litrs volumen to get the amount of air that had to be added) from a compressed roll of bags, then the real pressure increase is 1 atm plus 0.05 bar or about 1.064 bar or 21 times my estimate of the air flow.

 

[hyposmurf]

Katmar , as understood all the facts .
Thas the way I need to do, but I need a flexible hose to do the task.
About pressure, I will releif it at the hose end before entering the bag.
Thanks for your analisis.

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