Sand Blasting Container PRESSURE bogus

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In reading about the explosion of the sand container for sand blasting it occurred to me that there is no way that pressure at the top  of a sand column is in any way needed or helpful in pushing the sand out the bottom of the container.    Any pressure applied at the top is totally dissipated within the container.  YES, perhaps the last cupful of sand is helped out the bottom... but that would fall out anyway from gravity.      Air pressure available immediately after the sand or medium has fallen from the container.... to move it to the sand blast gun is great design. but that pressure at the top is both wasted and obviously presents the possibility of injury or death being there. Anyone have actual physics argument to convince me I am wrong ?Thanks, GregWeldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:If the preasure at the bottom is greater than gravity the sand won't feed. So depending on the preasure being put through the hose, you could potentially get no feed at all. or even blowback sending the sand out of the container making a useless mess instead of the useful mess you're trying to make.Also, The preasure in the top of the tank is not a set and forget. as the sand is used, more air is added to maintain the set preasure. It's not like a water rocket where the preasure forcing the fluid out of the rocket reduces as the fluid leaves the rocket.There are no problems. There are only solutions. It's your duty to determine the right one.Hobart Handler 210Airco 225 Amp MSM Stinger
Reply:The only unit I have had had a vent hole  at the top of the  exit tube of the sand after the bend....and no pressure to the top of the container..  What do you think about my contention that applying pressure to the top will not be translated to pressure on the sand to pushed out of the container at the bottom ?   It is not very far under a railroad track that no extra pressure  can be measured when a train is passing above... the pressure is dissipated before it gets very far....within a few feet.... this is part of my reason for thinking the way I do....Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:Think about paint sprayers.  Some use a pressure pot, some are gravity fed with a venturi.  Both can work, but require vastly different designs.
Reply:One is liquid .... which DOES transmit pressure.....the other is fine particle solid...... very different in their physical properties....Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:been using sandblasters for the last 40 yrs.,all production units that i have ever encountered have pressure pots to enshure consistant sand flow. tremendous difference between the cutting action of a pressure feed pot and a gravity feed pot that depends on differential pressure created by the venturi action of the nozzle to draw up sand into the blast stream.
Reply:Originally Posted by dbl612been using sandblasters for the last 40 yrs.,all production units that i have ever encountered have pressure pots to enshure consistant sand flow. tremendous difference between the cutting action of a pressure feed pot and a gravity feed pot that depends on differential pressure created by the venturi action of the nozzle to draw up sand into the blast stream.
Reply:Your thinking is wrong.The air pressure DOES force the sand out. I have cut OFF the air to the hose that pushes the sand and the Pressure IN the pot did indeed blow the media out, granted not with as much force but it did indeed push it out.Tim Beeker.
Reply:How did you determine that it was air pressure as opposed to GRAVITY which caused it to exit the container ?It was falling DOWN , right ? Unless you have an opening to the other side of pressure... like the gun nozzle OPEN... then the pressure is equalized around all the sand particles..... only when you have an opening for the air to flow towards does the opportunity to carry that sand with it present itself....Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:Sand is essentially a fluid or behaves as one. In the dry state its volume has no specific shape and can be pushed through an opening, duct, orifice or into the shape that contains it....much like water or any liquid fluid. Non of the above can be compressed like a gas; they will maintain their volume unlike air, acetylene, nitrogen, etc.Look up Pascal's principle. It states that " pressure applied anywhere to an enclosed, contained, incompressible fluid is transmitted undiminished to every part of the fluid and the container which contains it." So, there is the same pressure at the bottom of the pot as there is at the top.Gravity may keep the sand in the bottom of the container covering the exit from the pot, but pressure is what forces it out of the container at speed great enough to do any work.Fools speak when they want to say something; the wise speak when they have something to say
Reply:Originally Posted by GBMHow did you determine that it was air pressure as opposed to GRAVITY which caused it to exit the container ?It was falling DOWN , right ? Unless you have an opening to the other side of pressure... like the gun nozzle OPEN... then the pressure is equalized around all the sand particles..... only when you have an opening for the air to flow towards does the opportunity to carry that sand with it present itself....
Reply:Red Dog, no offense... but you are making stuff up as you go.... You can not claim sand is 'essentially a fluid' ( when it is not ) and then claim Pascal's law applies to it.  But IF IT WERE... how do you explain the fact that your definition would put the same pressure on all sides of the sand particles at the bottom of the container... thus not providing any DOWNWARD pressure... ?Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:Lol. This is entertaining.
Reply:I wonder what basis you have to say that Red dog is making stuff up as he goes?Fluid–adjective 2. pertaining to a substance that easily changes its shape; capable of flowing.Though a single grain of sand will not change it's shape, a volume of sand will easily change shape.Also, Iike any other container where liquid and compressed gas are containted in the same vessel the gas which will rise to the top due to gravity, will expand if preasure is released. In this case, the "fluid" would be forced out the exit, at the bottom because the compressed air at the top is expanding and pushing the incompressable "fluid" out of the exit. a chemical fire extinguisher for example. compressed air forcing a powder out of a preasure vessel.There are no problems. There are only solutions. It's your duty to determine the right one.Hobart Handler 210Airco 225 Amp MSM Stinger
Reply:Originally Posted by ttyR2Lol. This is entertaining.
Reply:Because sand has an ' angle of repose' .....and fluids do not.Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:I got the popcorn poppin'!!                                       MikeOl' Stonebreaker  "Experience is the name everyone gives to their mistakes"Hobart G-213 portableMiller 175 migMiller thunderbolt ac/dc stick Victor O/A setupMakita chop saw
Reply:I still point out the chemical fire extinguisher. Same general properties, same result. Compressed air forcing a solid (that moves as a fluid) out of a preasure vessel.The "fluid" (NOT LIQUID) theory, or approach, or what ever you want to call it applies. You have to consider the volume of sand as a unit, not the individual grains. You point out that a train will only compress the soil only a short distance below the track. This is because the force being applied can disepate out at any angle and over a VERY large surface area. In a preasure vessel the force is not directed out, just down.  because the walls of the vessel contain the force. By your definition the force at the bottom of the preasure vessel would not have any force above gravity applied to it. But a preasure vessel distributes force over the whole surface inside the vessel. If each grain of sand cannot be compressed, and the vollume as a whole cannot be compressed than in order for the preasure to be the same on all parts of the vessel, the force of the preasure must be transmitted through the sand. The sand, when given the oppertunity to move (through opening the exit) will then flow as a "fluid" out the exit because the compressed air being above the sand is expanding forcing the sand through the exit at a force greater than gravity.EDIT; all comments are based on a propperly oriented Preasure pot sand blaster where the exit is at the bottom.Last edited by ThorsHammer; 01-13-2012 at 07:25 PM.There are no problems. There are only solutions. It's your duty to determine the right one.Hobart Handler 210Airco 225 Amp MSM Stinger
Reply:Intermission time, Ive got to go pee and get some popcorn Mac
Reply:Originally Posted by ThorsHammerI......The "fluid" (NOT LIQUID) theory, or approach, or what ever you want to call it applies. You have to consider the volume of sand as a unit, not the individual grains..... In a preasure vessel the force is not directed out, just down.  because the walls of the vessel contain the force.EDIT; all comments are based on a propperly oriented Preasure pot sand blaster where the exit is at the bottom.
Reply:Originally Posted by mla2ofusI got the popcorn poppin'!!                                       Mike
Reply:Originally Posted by GBMUpon what basis are you allowed to conglomerate the sand to one unit for pressure purpose consideration and then allow it to flow as individual grains of sand when you want it to ? TH; I'm not. The volume of sand is flowing down as a liquid. Yes after it exits the preasure vessal it does become individual granuals of sand. Please tell me, what does water do when it leaves the barrel of a squirt gun? does it stay in 1 connected stream or does it divide into individual droplets?And how, pray tell, do  you keep that force from being directed out and direct it " JUST DOWN" ?   TH; because the force of the compressed gasses are above the sand. (simple sepparation of weights, like air rising to the surface of a glass of water when you blow it into the cup with a straw the air is lighter and thus rises to the top) The sand is thus being forced down as the compressed gasses expand.The walls contain the force and that is exactly where it is Dissipated TO . TH: you contradict your own arguement here. Because if forces is contained, it cannot dissipate. otherwise it is not being contained.Thanks for that edit... I was just about to turn my test sand tank sideways to do the rest of the analysis.... LOL
Reply:LOL, you are out of this discussion. You can not choose an inappropriate example and then claim victory when I ignore it.Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:Obviously fluid dynamics and physics were not your main focus. However, think about it like this, they call it a pressure pot for a reason. Like others said before, this is entertaining, especially to an engineer. Without the air pressure on top of the sand there would be a void as the sand was puller out of the bottom in the mixing chamber. Like a funnel shape and eventually no sand would come out, but the container would still be full.Last edited by kb1ftx; 01-13-2012 at 08:04 PM.Cain's Mechanical LLC Southwick, MAwww.cainsmechanical.comRig:-2006 Chevy 2500 utility body-Lincoln Ranger 10-Miller Spectrum -Tons of hand and power toolsAWS D1.2 Structural Steel Certification
Reply:Originally Posted by GBMLOL, you are out of this discussion. You can not choose an inappropriate example and then claim victory when I ignore it.Originally Posted by kb1ftx..... Without the air pressure on top of the sand there would be a void as the sand was puller out of the bottom in the mixing chamber......
Reply:ThorsHammer, if you want to participate go back to post number 20 and answer each of my questions.Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:Originally Posted by GBMUpon what basis are you allowed to conglomerate the sand to one unit for pressure purpose consideration and then allow it to flow as individual grains of sand when you want it to ? And how, pray tell, do  you keep that force from being directed out and direct it " JUST DOWN" ?   The walls contain the force and that is exactly where it is Dissipated TO . Thanks for that edit... I was just about to turn my test sand tank sideways to do the rest of the analysis.... LOL
Reply:Originally Posted by GBMHow did you determine that it was air pressure as opposed to GRAVITY which caused it to exit the container ?It was falling DOWN , right ? Unless you have an opening to the other side of pressure... like the gun nozzle OPEN... then the pressure is equalized around all the sand particles..... only when you have an opening for the air to flow towards does the opportunity to carry that sand with it present itself....
Reply:Boatbouy,   Since you are a member of that august group...You should be able to explain using physics the answers to my questions. But I suspect you can not.... or you would have done that instead of attacking me.....Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:Originally Posted by tnjindOf course the gun nozzle was open. How else is it going to leave the pot?That is the silliest statement I have read in a while.Just what are you trying to accomplish, especially with the title?I do, of course, like the theories being presented here.I take it you have not tested this and this is only an academic exercise.
Reply:Originally Posted by GBMBoatbouy,   Since you are a member of that august group...You should be able to explain using physics the answers to my questions. But I suspect you can not.... or you would have done that instead of attacking me.....
Reply:Originally Posted by GBMBoatbouy,   Since you are a member of that august group...You should be able to explain using physics the answers to my questions. But I suspect you can not.... or you would have done that instead of attacking me.....
Reply:Originally Posted by GBMThorsHammer, if you want to participate go back to post number 20 and answer each of my questions.
Reply:I asked this question because of the discussion about someone hurt from an explosion of one of these vesselsI do not know how that powder tank is constructed or where the compressed air is applied to the unloading.  Lets keep the example simple since sand tanks are simple.   There is no need to try to substitute some other vessel and argue that it is equivalent to the pressure sand vessel in question.  Just apply whatever physics arguments to the example at hand. As to people enjoying their sand blaster of whatever design.....that is nice.... but does not address the possibility that some of the design is there from tradition or to increase the price able to be asked for the machine... ( value added in the eyes of the buyer ) which may or may not function as they assume.As an engineer you should be able to refute the physics statements I have made IF they are not correct. I consider ' you appear to be bench racing ' as an attack. I expect you to refute any and all wrong statements of the physics I have made.  You will look really smart if you use appropriate laws of physics to show that I am wrong. Good Luck. Keep to the physics and do not throw in any spurious examples. I should not have to show by video or reference to some text why grains of sand drop out of an open hole in the bottom of a container.Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:Originally Posted by boomeradfWhere is your proof using physics that is does not work?
Reply:Sand would be too porous to form any seal/air block to obtain any kind of push under low volume arrangements. In a static situation in a tank of sand and air the pressure would be equal all around everywhere in the mixture. In a high volume airflow scenario the sand could present enough of a barrier to be moved by the high volume of air flow, whether down, up or horizontally through a tank or tube. You blow dirt out of tubes all the time. But I would imagine what we see in the pressurized sand blasting tank scenario is there is airflow from the top of the tank through the bottom in which case the sand is being 'drafted' out rather than pushed out in the technical sense. Sand on it's own tends to pack and clog the outlet but being drafted out with air keeps it more fluid."The things that will destroy America are prosperity at any price, peace at any price, safety first instead of duty first, the love of soft living and the get rich quick theory of life." -Theodore Roosevelt
Reply:At sea level there should be around 14.6 lbs psi on top of the sand.
Reply:Hmm, the higher air pressure(higher than the weight of air outside, around 15psi) inside the tank, lets say 40 psi will leave via the bottom port taking the sand with it.But if the tank has say only 40 psi and the hose is supplied with say 120 psi, then what would allow the lower pressure in the pot to escape into the higher pressure air stream? Venturi effect?Obviously I have no former education, just curious and speculating.I will follow this thread as it is an interesting topic. Thank youTim Beeker.
Reply:" I'm an idiot and can't figure out how a stupid sand blaster works, so you, my nave, must do a study in physics and show me with proofs why you think it can, to which I will say, 'You're wrong!'. I also don't write well, use an accusatory tone, and am much smarter than you are (I just choose not to use my brain so that you don't look bad). The reason I don't provide any proofs of my own theory is because I want you to be ale to do some work, which I will correct, so that yo will feel good about yourself" I may have a wee word or twa wrong in that quote, but it serves just the same. What a pompous a$$.
Reply:after reading the previous comments and explanations, i would offer comments for you to ponder. first, of course the outlet is at the BOTTOM of the pressure vessel, after all we want the SAND to come out. second, as any user of a pressure pot knows, there is a "choke valve" that you can close to shut off the supply of propellant air to the nozzle. this allows the pressure ratio between pot pressure and propellant pressure to change in favor of the pot, and usually will blow out a media clog. the resultant heavy sand flow has little force, and once the clog is cleared, the choke valve is reopened and forceful basting resumes with high volume propellant air flow. however, throughout the process, pressure was maintained on the blast media in the pot, and material was forced out the bottom of the pot by internal air pressure that was maintained.
Reply:Originally Posted by riceburnerAt sea level there should be around 14.6 lbs psi on top of the sand.
Reply:Originally Posted by GBMI asked this question because of the discussion about someone hurt from an explosion of one of these vesselsI do not know how that powder tank is constructed or where the compressed air is applied to the unloading.  Lets keep the example simple since sand tanks are simple.   There is no need to try to substitute some other vessel and argue that it is equivalent to the pressure sand vessel in question.  Just apply whatever physics arguments to the example at hand. As to people enjoying their sand blaster of whatever design.....that is nice.... but does not address the possibility that some of the design is there from tradition or to increase the price able to be asked for the machine... ( value added in the eyes of the buyer ) which may or may not function as they assume.As an engineer you should be able to refute the physics statements I have made IF they are not correct. I consider ' you appear to be bench racing ' as an attack. I expect you to refute any and all wrong statements of the physics I have made.  You will look really smart if you use appropriate laws of physics to show that I am wrong. Good Luck. Keep to the physics and do not throw in any spurious examples. I should not have to show by video or reference to some text why grains of sand drop out of an open hole in the bottom of a container.
Reply:Originally Posted by dbl612. . . the resultant heavy sand flow has little force, and once the clog is cleared, the choke valve is reopened and forceful basting resumes . . .
Reply:I worked for several years in the equipment rental business, renting compressors and blast equipment. This was equipment designed for industrial use on ships and bridges. ALL of our media pots were pressurized. Do you think the people that design this equipment really don't know what they're doing? It sounds as though you have little actual experience in this matter, for if you did you would not have started such a ridiculous discussion. Attached ImagesMiller Thunderbolt AC/DC (Sold)Miller Dialarc 250HFMiller MM251Miller MM200 (Sold)Miller MM130Miller Spot WelderVictor O/A rigMiller Spoolmatic 1 (Sold)
Reply:OK , Denrep, finally some pretty  good answers. Of course the question is still if pressure differential by itself... can move loose grains of sand through curved hoses with serious friction involved.... and if pressure can be transmitted though a column of sand. as to the pressure drop at the bottom of the tank .... is additional air flow needed in addition to the pressure at the top of the sand column to cause the sand to flow well for the purposes under consideration ? My sand blaster had nothing except gravity letting the sand out....but at the bottom of the tank a curved line with a vent hole on top allowed the use of air flow to move the sand in the rubber line to the nozzle.Then air pressure propelled it from the nozzle from another air source connected to the nozzle.  I often had a problem with restarting flow in the line .. particularly if the sand was not new and totally dry. No matter how much ' pressure differential ' I applied at the end of that line I could not get a restart without some shaking and gravity assist.  ' pressure differential' by itself will not move solids unless it can overcome whatever friction is inherent in the design...air flow does have that potential ..... is there some sort of way they facilitate the movement of air along the inside of the  hose to the nozzle ... particularly at the restart after using the line ?  Here would be the test procedure I think would show this one way or another. Take that column of sand a couple of feet high and 18 inches across.  Completely dry. and have a one inch pipe welded Horizontally to that tank 6 inches from the bottom and 3 feet long. Now, if air pressure is applied to the top .... will that sand evacuate the container down to the pipe opening due to the pressure differential between the top of the tank and the open end of the pipe ?Or is gravity needed for that evacuation ?   Meaning at best we can assume the air is assisting the evacuation but is not the prime mover ?Weldandpower Lincoln 225 AC,DC with Briggs 16hp gas engine.WW2 era Miller TIG.
Reply:Originally Posted by SandySand would be too porous to form any seal/air block to obtain any kind of push under low volume arrangements. In a static situation in a tank of sand and air the pressure would be equal all around everywhere in the mixture. In a high volume airflow scenario the sand could present enough of a barrier to be moved by the high volume of air flow, whether down, up or horizontally through a tank or tube. You blow dirt out of tubes all the time. But I would imagine what we see in the pressurized sand blasting tank scenario is there is airflow from the top of the tank through the bottom in which case the sand is being 'drafted' out rather than pushed out in the technical sense. Sand on it's own tends to pack and clog the outlet but being drafted out with air keeps it more fluid.
Reply:Originally Posted by bratkid63I worked for several years in the equipment rental business, renting compressors and blast equipment. This was equipment designed for industrial use on ships and bridges. ALL of our media pots were pressurized. Do you think the people that design this equipment really don't know what they're doing? It sounds as though you have little actual experience in this matter, for if you did you would not have started such a ridiculous discussion.
Reply:Originally Posted by boatbuoyi think the problem is that you haven't seen the blaster design. the sand exits the tank into a "t" that has air pressure (same pressure as tank) on one side and the siphon hose on the other. the air can then move the sand thru the siphon hose. without the pressure assist ,the sand would clog the siphon hose quite often.  http://s1264.photobucket.com/albums/jj481/boatbuoy2/
Reply:I have resisted continuing this discussion as I have already offered the physics to GBM ( who refuses to accept the principles of those much brighter than myself..i.e. Bernouilli, Pascal, Archimedes, Boyle). But, alas, I just find it too comical to let this sit!  Thanks to all who see my thinking and who share my simple thought processes, unlike those who actually KNOW ( like Bernouilli and Pascal and Archimedes and Boyle). I have not been put on this earth to question the aforementioned genius'.I do concede to GBM that there may well be a better mousetrap in all our futures....if that is  so, I await it rearing its head with bated breath.Thanks,......... most entertaining (Is there a better reason to subscribe to WW than to be entertained?)Fools speak when they want to say something; the wise speak when they have something to say