Compressed air to fill a tank

[flyfisher812]

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.

 

[BigInch]

Well, I guess you need 5000 gal at 40 psig.

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.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

PS: If you are looking for how much air you need at atmospheric pressure, that means you will be venting through the open 3/4", not venting through the 40 psig setting relief valve,

atmospheric pressure = 15

its appx 5000 x (40+15)/15 = 18,333 gal

http://virtualpipeline.spaces.msn.com

 

[flyfisher812]

BigInch,
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?

 

[blackwed]

It'll take some more info to give you what you want. Are you using air to force liquid out now or are you wanting to speed up a current gravity process? What's in your truck? What size is the offload piping system onboard and offboard? Any flow controls, pumps, meters, air eliminators, relaxation chambers, etc., in the system? Where's the receiving tank (elevation) in relation to the truck? Can the receiving tank support increased flow rates and velocities? You don't want to screw up any gauging system probes or high level floats. Can the system deal with increased static charge from the higher flow velocities?

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?

 

[pleckner]

You need to do a hydraulic calculation as you would when sizing a pump; finding system liquid heads and frictional losses.

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.

 

[BigInch]

Let's let him state the problem more clearly as Blackwed suggested before we go adding more unknowns. I think there's enough already.

http://virtualpipeline.spaces.msn.com

 

[StephenA]

As Biginch says, just before the tanker is empty the pressure in the tanker will be 40psi and the volume of compressed air will be 5000 gallons.

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.

 

[flyfisher812]

StephenA/BigInch,
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.

 

[Cockroach]

I guess it depends on how much time you need to spend emptying the unit. You need a flow rate in addition to the line sizes.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[blackwed]

FlyFisher,

How do you offload now? How long does it take?

 

[dtn6770]

Aside from the fundamental hydraulics and discussions to date, verify that the safety relief valve is functional and suitable for the situation you’re working towards. It may be set at 40# but what’s its sizing basis and can it pass X-cubic feet of compress air if it needs to. Just a heads up.

 

[BigInch]

Well then I would start with trying to find out the capacity of the existing compressor you are planning to use.

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?

http://virtualpipeline.spaces.msn.com

 

[flyfisher812]

BigInch,
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.

 

[blackwed]

Are the trucks carrying a liquid?

 

[flyfisher812]

They are carrying a corrosive liquid (hence we can't use a pump).

 

[blackwed]

I'm not giving engineering advice or direction because this is outside my field of expertise, but try this for an approximation. ME's please fix it as required.

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.

 

[itsmoked]

523CFM @ 125psig = 150hp ?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BigInch]

He can only use 40 psig of that or he lifts the relief valve, so assuming he can do that, he can also put P=40.

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!

http://virtualpipeline.spaces.msn.com

 

[IRstuff]

I would revisit the assertion "They are carrying a corrosive liquid (hence we can't use a pump)." See:

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