Compressed air to fill a tank
Compressed air to fill a tank
(OP)
Problem:
Using compressed air to empty a tanker truck.
The truck has a 3/4" quick disconnect opening and a safety relief valve set for 40#. Truck is 5000gal.
What is the best way to calculate the CFM of air I need to supply to empty tank.
Using compressed air to empty a tanker truck.
The truck has a 3/4" quick disconnect opening and a safety relief valve set for 40#. Truck is 5000gal.
What is the best way to calculate the CFM of air I need to supply to empty tank.





RE: Compressed air to fill a tank
This is not technically exact, but real close. OK? To find the approx volume you need at another pressure,
divide 5000 x 40 by the other pressure. Everything must be at the same temperature.
Example: If you have 1000 psi in the air tank, you need 5000 x 40 /1000 = 200 gallons of charged air tank capacity.
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RE: Compressed air to fill a tank
atmospheric pressure = 15
its appx 5000 x (40+15)/15 = 18,333 gal
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RE: Compressed air to fill a tank
I don't think I stated my issue clearly enough. I have a tanker truck that is filled and am using compressed air to push the material out (in lieu of pumping it out). The tanker is 5000gal. I will be supplying compressed air through the 3/4" opening. The tanker relief valve is set for 40psi. I need to figure out how much air (CFM) I need to use to pump out this tanker.
I figured the area of the 3/4" opening (0.442ft^2) and assumed a 20 ft/s air velocity and 40psi air supply (worse case scenario). This gave me 8.84CFM of air. Does this mean I only need a compressor sized for 8.84 CFM? Or am I missing something else?
RE: Compressed air to fill a tank
Gravity should give you around 200-300 gpm (25-40 cfm) depending on your system configuration assuming 3" piping. Your compressor will have to provide in excess of that for you to see an improvement.
You mean 0.442 square inches?
RE: Compressed air to fill a tank
The first thing you need to set is the rate at which you want to pump the tanker out at. If you want to empty the tank in say 30 minutes, then you will need to pump the contents at about 167 gpm. If this is too large of a flow for your piping system (truck to storage), then reduce the pump out rate accordingly. What makes the flow too large is the pressure drop that you would experience at this flow.
The pressure in the truck is back calculated. Start from storage, add the discharge liquid heads and frictional losses. This is the pressure needed in the truck. The rate of air is equivalent to the pump out rate of truck contents, 167 gpm or in terms of vapor flow, 167/7.481 to give you cfm of air (7.481 is the conversion from gal to cu. ft.).
To get the pressure regulator setting, you need to do another hydraulic calculation from the air supply to the truck using the flow of air. You would then set your pressure regulator for this pressure.
RE: Compressed air to fill a tank
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RE: Compressed air to fill a tank
Compressors are rated at free air volume, so 8cfm is 8 cubic feet of free air per minute, and you need 18,000 gallons of free air.
If you want the tanker emptied in 30 minutes then you will need a compressor that delivers 18000/30=600 gallons/minute. However as others have pointed out you may need more time as the outlet may be too small and the relief valve will blow off.
RE: Compressed air to fill a tank
I think I am starting to understand where you are coming from. I really don't have a specific time to empty requirement. I have an existing compressor and need to know if it has adequate size left to perform this task. I guess I am confused about your answers because I never see the 3/4" opening coming into play. Isn't there a fixed amount of air I can push through this opening, thereby limiting the volume of air I need from the existing compressor? I basically want to take a full tank, push in compressed air allowing the material to push out. I came up with 8.84CFM which would empty the tank in approx. 75min. I am concernign my calculaiton might be off because I am not taking air from atmospheric nor pumping into atmosperic. I am sure I have confused all now.
RE: Compressed air to fill a tank
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Compressed air to fill a tank
How do you offload now? How long does it take?
RE: Compressed air to fill a tank
RE: Compressed air to fill a tank
I suppose it does not say somewhere that it can deliver X Standard Cubic Feet per Minute at some known discharge pressure or anything usefull like that. Does it?
Where will this air go into the tank and where will its contents come out? Like air in at the top, contents out at bottom, or side or top or Air in at the bottom and contents out the bottom or what?
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RE: Compressed air to fill a tank
My compressor is sized for 523CFM @ 125psig. I need to figure out how much air I will need for this unloading to determine if the compressor is large enough, or if a second one needs to be purchased. The air enters at the top of the tank, material exits near the bottom.
RE: Compressed air to fill a tank
RE: Compressed air to fill a tank
RE: Compressed air to fill a tank
Vdot=sqrt{2[g(H1-H2)+((P1-P2)/rho)]}
Vdot is volumetric flow rate of fluid from tanker
g is acceleration of gravity
H1 is height of fluid in tanker above its outlet
H2 is height to which fluid must be pushed
P1 is pressure provided by compressor on top of fluid
P2 is the pressure in the receiving tank
rho is fluid density
It'll go slower than this because of system losses not taken into account.
As BigInch, et al, have already indicated, the compressor must provide air at the rate of (P+A)/A times Vdot to maintain the desired pressure, P. A is atmospheric pressure.
RE: Compressed air to fill a tank
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Compressed air to fill a tank
Adjusting the 523 CFM for a 55 psia discharge, I get around 200 CFM, with 18333 gal needed, say 2500 CF, I reckon he can get the air in the tank in 12 minutes, as long as the liquid can run out at 200 CFM, which I doubt. Seems like the liquid unloading rate is the critical path on this problem.
Let's drain through the 3/4" put 40 psig across an "orifice" and assume the piping is too short to matter and this stuff is similar to water.
V = 47.8 fps
3/4 in O = .00307 ft2 = .146 cfs = 8.8 cfm = 65 gal/min
OK 5000 gal out in 77 minutes
Critical time is 77 minutes + some for the hose resistance
Compressor is good, run it with a 39 psig discharge into the tank so you don't lift the relief valve.
That's all I'm doing for free.
Hope it works something like I've assumed!
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RE: Compressed air to fill a tank
http://www.vanton.com/chemgard_cgm.asp
We've pumped all sorts of acids with pumps like this. The corrosive liquid you refer to had to be pumped into the tanker in the first place, so pumps for this "liquid" exist. If the pump above works, you'd empty the tank in something like less than 10 minutes using the largest pump capacity of 600 gpm.
TTFN
RE: Compressed air to fill a tank
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Compressed air to fill a tank
http://virtualpipeline.spaces.msn.com
RE: Compressed air to fill a tank
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Compressed air to fill a tank
RE: Compressed air to fill a tank
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Compressed air to fill a tank
Hey, I'm basically a "pump guy" myself and I don't like to move liquids using compressors. Slug flow is usually not my ideal flow regime. I was just trying to stay within the bounds of the problem as originally given. There's gotta' be about 1000 ways to do this better, but he's got the compressor already.
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RE: Compressed air to fill a tank
I just picture the forces and stored energy in a 7/8s empty 5000 gal tank and the ticking clock of working the metal as it is charged and discharged a 1000 times. (Like a Hawaiian 737 only 3 times the pressure.) Shudder... about 200 tons of force trying to blow off a tank end. I just wouldn't do it. Then air compressors are typically only 15-20% eff in this day of expensive electricity 5000gals at 40psi probably costs about $2.50.
Just seems like using a mallet to paint with. Makes more sense for dry transfer.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Compressed air to fill a tank
and... not everybody can be Michelangelo.
http://virtualpipeline.spaces.msn.com
RE: Compressed air to fill a tank
Your electrical, so take a look at this one just to see if you agree... or if I should find another business.
http://www.eng-tips.com/threadminder.cfm?pid=407
http://virtualpipeline.spaces.msn.com
RE: Compressed air to fill a tank
http://www.flaminsystems.com/page/page/2470267.htm
http://virtualpipeline.spaces.msn.com
RE: Compressed air to fill a tank
There are only two issues: (1) what is the discharge total resistance to flow (the two big ones are usually the height of the discharge pipe and friction); and (2) what is the set point of the truck's PSV? I just put in a bad oil facility that goes into the top of a 750 bbl tank that is sitting on a 6 ft high pedastal (don't ask), the head is 11 psig and friction at 250 bbl/hour gets the required air pressure up to 15 psig. When the PSV is set above 15 psig then then the 80 bbl truck empties in 15 minutes. Some of the trucks PSV lifts at less than 15 psig and the result is that no liquid moves.
The size of the air inlet only comes into play up to the capacity of the compressor (for example, if you need 50 CFM and the inlet is small then the upstream pressure will increase until you get to an upstream pressure that will allow sonic flow of 50 CFM into the tank or you run out of available discharge pressure). If the inlet will easily flow as much air as required to push the liquid against the resistance then the pressure at the compressor will decrease.
Air compressor required volume flow rate is really a function of your required outlet flow rate. If you want 5,000 gallons to empty in 20 minutes then the volume that has to be replenished is 25,000 gpm (call it 173 scfm at 40 psig). With 450 psig supply pressure and 40 psig truck pressure this becomes a PSV calc that I don't have time to do right now.
David
RE: Compressed air to fill a tank
Hi BigInch;
That link is to a whole forum so I'm not understanding the"look at this one" reference.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Compressed air to fill a tank
http://www.eng-tips.com/viewthread.cfm?qid=159572
http://virtualpipeline.spaces.msn.com
RE: Compressed air to fill a tank
I can vouch for electripete.. He knows his stuff.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Compressed air to fill a tank
RE: Compressed air to fill a tank
http://virtualpipeline.spaces.msn.com
RE: Compressed air to fill a tank
RE: Compressed air to fill a tank
http://virtualpipeline.spaces.msn.com