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Compressed air to fill a tank

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flyfisher812

flyfisher812

Mechanical
Jul 12, 2006
5
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.
 
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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.-
 
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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.
 
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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. :)

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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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.-
 
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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.

 
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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
 
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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[red]"look at this one"[/red] reference.

Keith Cress
Flamin Systems, Inc.-
 
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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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