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Compressed air to fill a tank

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.  

RE: Compressed air to fill a tank

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

RE: Compressed air to fill a tank

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

RE: Compressed air to fill a tank

(OP)
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?

RE: Compressed air to fill a tank

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?  

RE: Compressed air to fill a tank

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.

RE: Compressed air to fill a tank

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

RE: Compressed air to fill a tank

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.

RE: Compressed air to fill a tank

(OP)
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.

RE: Compressed air to fill a tank

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

RE: Compressed air to fill a tank

FlyFisher,

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

RE: Compressed air to fill a tank

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.

RE: Compressed air to fill a tank

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

RE: Compressed air to fill a tank

(OP)
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.

RE: Compressed air to fill a tank

Are the trucks carrying a liquid?

RE: Compressed air to fill a tank

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

RE: Compressed air to fill a tank

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.

RE: Compressed air to fill a tank

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. smile  

Hope it works something like I've assumed!

http://virtualpipeline.spaces.msn.com

RE: Compressed air to fill a tank

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



RE: Compressed air to fill a tank

I put in a magnetic drive pump and have run it for 21 years straight without failure, 24/7 (184,000hrs.)  You won't touch this with any air compressor.lol

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Compressed air to fill a tank

Any pump will do the job.  If you don't care about the time it takes.  So, go ahead and do it.  If it is too slow, get a larger pump.  I would be very concerned about the safety aspects.  When the volume is mostly air at 40 psi there is a tremendous amount of stored energy and you have a bomb waiting to explode.  Bleed the air off as quickly as possible, don't let people within a few multiples of the largest measurement of the tank.  Tie the tank down to prevent liftoff if there is a failure in the bottom.  Restrict it's ability to move horizontally if there is a failure in an end.  You can kill a lot of people and cause a lot of damage.  I saw a 10 inch diameter pipe only 15 feet long shoot across the shop and imbed itself in solid metal.  It was filled to about 45 psi and there was a failure in the factory made longitudinal weld.

RE: Compressed air to fill a tank

I'm with you IFRs, 5000gal liquid tank pumped to 40PSI scary.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Compressed air to fill a tank

Keith, sounds like you have as much respect for small amounts of pressure as I do for small amounts of current.  That's a good thing.  Supposedly there's a relief valve (fuse) that we all hope is in working order. smile

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.

http://virtualpipeline.spaces.msn.com

RE: Compressed air to fill a tank

BigInch; Yeah, I understand your points.thumbsup2

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

Ya, but take it easy.  I just think, relatively speaking, that a Sears spray painting air tank is probably a lot more dangerous.  I'm sure these days its welded up in somewhere I'd rather not think about, where nobody's heard of any QC and the relief valve is plugged up with paint.  My exposure-wise risk assessment is, its not so bad.  

and... not everybody can be Michelangelo.  

http://virtualpipeline.spaces.msn.com

RE: Compressed air to fill a tank

slow down guys.  Every field vac truck that I've ever seen does exactly what flyfisher is trying to do.  They have a compressor driven by the truck's PTO.  When they are filling the truck the compressor sucks on the tank to create a pretty good vacuum.  When emptying the truck it discharges into the truck.  Either direction it is an air compresor and there are millions of them in continious service in oil fields across the world.

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

Thanks zdas04. Very interesting. I stand...um ..er more informed.

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

I'm staying out of that one. lol
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

It's been a good daypipe

RE: Compressed air to fill a tank

There is a big difference between a tiny Sears pressure vessel way over designed and a huge tank not designed for pressure.  The relief valve was not designed for high flow rates, it can not and will not prevent a blow out.  Vacuum trucks are again designed for the service they see.  If you want to do this, recognize the hazards and take steps to mitigate the dangers.  Some NDE may be in order to document the integrity of the welds and connections.  I'm not saying be afraid of it, but be knowledgeable of what you are doing, think it through with others, make a plan with safety in mind, execute by the plan and wear your PPE.  Document what you do and the hazards you mitigate against.  If anything goes wrong you won't have to say "well duh - it blew up".

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