Compressed Air "Cannon"

[CanuckMiner]

I have a problem which at first glance appears easy (at least I thought it was), but for some reason, I can't seem to work it out. Any ideas or solutions would be appreciated.

I am designing a system that has a compressed air pipe which more or less acts as an air receiver. Dimensions are 16" Std pipe, approximately 18 feet long. Pressure is in the order of 100psig. Downstream of this "receiver" is a series of reducers to get to 6" dia., a long radius elbow, and a 6" dia. butterfly valve. Downstream of the valve is a 6" Sch.40 "header" which branches off to six runs of 2" dia. pipe of various lengths (say 30 feet to 50 feet in length, average of about 40 feet). At the end of each of the 2" branches is a 2"x1" reducer (a "nozzle" of sorts).

Initially, the 16" dia. receiver is filled with compressed air (+/- 100psig), the valve is closed and the piping downstream of the valve is empty (i.e. open to atmosphere). What I want to determine/estimate is the time it takes for the air receiver to empty it's contents. I am trying to find a simplified approach to do this to get a rough idea of the time. I have been trying to use Bernoulli, but the compressibility messes me up.

One of the rough ways I tried was to assume the compressed air could not travel any faster than Mach 1. I came up with a figure of something like 0.6 seconds to empty based on maximum air velocity of Mach 1, but it doesn't sound reasonable. (I don't think the air will travel anywhere near Mach 1.)

I appreciate any help that anyone can offer.

Cheers,
CanuckMiner

 

[butelja]

Your systemm has transient compressible flow in a network. There is no simple way to model this. There are expen$ive $y$tem$ that can model these $cenario$. Two that come to mind are AFT Arrow from Applied Flow Technology and EASY5 (developed by Boeing) available from Mechanical Dynamics, which is part of MSC. Possibly you could obtain demo or evaluation software from one of these companies. Probably not if you tell them what you are doing.

I have somewhat mixed feelings about offering advice when it relates to something that could be used for dubious or nefarious purposes. You do realize that your air resevoir has the stored energy of ~0.5 lb TNT!?!? While you may intend to lob potatoes or something RELATIVELY harmless, what's to say you aren't an Al Queda, Islamic Jihaad, or Hamas operative who is trying to lob some improvised destructive devices at innocent people?

 

[TD2K]

I was watching a show on Discovery and they were covering one of these events (not to say that Butelja's concerns are unfounded). This is a lot more popular than I realized, some people had gone back and built trebuchets (medieval seige machines) and other ones as well (wouldn't want to personally stand beside some of these after listening to the 'developers' talk about some of the failures) They had a pumpkin shooter essentially what the poster is talking about that could throw a pumpkin a very impressive distance AND hit the target.

Some people have wayyyyy too much time on their hands :-D

 

[CanuckMiner]

For the record, I am not trying to lob explosives, ballistics, or any type of fresh produce.

The system will be used to move ore/rock in a chute in an underground mine. Occasionally rocks (ranging in sizes from a few inches to several feet across) will "plug up" the chute. Blasting the hang-up with a shot of compressed air can reportedly alleviate the problem, depending on the size of the rock(s) and the nature of the hang-up. I have my doubts as to how well the system will work as designed but I am only a part of the "team".

So to address your "concerns": nefarious...no; dubious...you bet.

Cheers,
CanuckMiner

 

[butelja]

I would think that for this application, you would want as "sharp" a pulse as possible, by having the quickest opening valve available. This would be similar to a baghouse dust collector pulse valve. These are typically diaphragm type quick exhaust valves. The same type of valves are also used on "air cannons" on dry bulk material storage bins to break up blockages.

Smaller ones are usually direct actuated via solenoid. Larger ones are usually air pilot operated. The largest I've come across is a 3" nominal size with a Cv of 140. They will go from closed to full open in a matter of milliseconds. You might need multiple pilot operated ones in series depending upon your flow requirements.

 

[TD2K]

I think the pumpkin sounds like more fun.

 

[quark]

To all!

Einstein felt the same way (ofcourse after demolition occured) for writing a letter ot FD Roosewelt about making a bomb (or he was persuaded to do so).

I am just trying to check the relevance

Regards,