Air powered sprayer

[Highbeam]

"even if it could pump to 75 psi in that size container... isn't the psi calc you made on the end caps only valid if the tank was completely full and the pressure was applied with a fluid in a sealed system ... as in a hydraulic circuit?"

Doesn't matter if it is full or partially full of liquid or air. The pressure will be marginally higher at the bottom if filled with liquid due to the weight of the liquid. It also doesn't matter whether it is sealed or not but if it is not sealed then the pressure will bleed off. I wouldn't buy a compressor that couldn't do 75 psi since the tire ratings are based on max pressure which on a 10-ply tire is way over 50, sorry I don't recall the actual. It might take all day using one of those cheesy compressors but being tractor type folks I am sure that someone along the line will hook it up to their shop compressor.

Good welds by a good welder and proper materials can withstand the forces. A guy like me with a stick welder who knows how to make the welded joints look good but maybe not make good enough welds is the scary part. If you are careful about pressures then you can spray with much lower pressures but you've got to figure that if there's a schrader valve that someone will fill er up.

 

[HomeBrew2]

........

Let's see if I can keep you on point ... can you explain where your figure of 23,561 psi came from in your original post, in the context of this thread where compressed air is charging a tank with some water in it?
Your calcualtion is correct for a specific hydraulic circuit but not for an air charged vessel.

 

[Tom_Veatch]

... can you explain where your figure of 23,561 psi came from in your original post, in the context of this thread where compressed air is charging a tank with some water in it?
...

Total force trying to separate the endcap from the body tube is equal to the pressure on the endcap X the area of the endcap.

F = PA
P = 75 psi
A = Pi X Radius Squared = 3.14159 X (10)^2 = 314.159

F = 75 X 314.159 = 23561.925 Pounds

Could also be stated as 375 Pounds per inch of circumference (weld length)

In a static situation it doesn't matter whether the pressurized fluid is a gas such as air or a liquid such as water or hydraulic fluid, the physics of the situation is the same.

If the fluid is in motion relative to the container wall, compressibility and viscosity of the fluid could have a big influence on the flow dynamics. But in a static situation such as this, it makes no difference at all.

 

[bucmeister]

I disagree with the 23,561 psi figure also. The figure is 75 psi as stated in the original point, that is the definition of psi, pounds per square inch afterall you can't extrapolate and focus that to predict failure. If that were the case then many hydraulic hoses and cylinders would explode everyday. Now as was later stated there is a concentration of force at the weld union where you have welded the joint with 7018 rod which has a strength rating of 70,000 psi so I think it will likely be able to handle the 375 psi mentioned later in the post.

 

[Tom_Veatch]

I don't think anyone is claiming a pressure of 23561 PSI. The pressure is only 75 pounds per square inch.

The units on the 23561 figure is POUNDS, a unit of force, not PSI, a unit of pressure. The force on each square inch is 75 pounds. The total force exerted over all 314.159 square inches of the endcap is 23561 pounds or 375 pounds per inch of weld.

The force we are talking about here is the total force necessary to hold the endcap in place against 75 pounds acting on each square inch of the endcap's area. That equates to a FORCE of 375 pounds distributed along each linear inch of weld. That says nothing about the stress in the weld or the likelihood of failure. It simply states how much force each linear inch of the weld around the circumference is required to withstand.

Or, think of it another way. What if you had a hydraulic cylinder that was 20 inches in diameter and a system pressure of 75 PSI. How much "push" would you expect to get from that cylinder when it extends? That's right, 23,561 pounds.

 

[HomeBrew2]

I don't think anyone is claiming a pressure of 23561 PSI. The pressure is only 75 pounds per square inch. ....

Thanks, Tom, for straightening this out. I have to admit I got caught up in Highbeam's overblown "grenade" analogy. Nobody originally talked about that kind of pressure nor using a shop compressor or anything else that would pose a serious threat.

 

[Highbeam]

Thanks Tom for clarifying, that was my math too. Lbs of force is not a pressure.

Pressure (liquid or air) applied over a certain area gives a force. I don't really know where the grenade will pop, I just don't want to be too close.

 

[Tom_Veatch]

...
I don't really know where the grenade will pop, I just don't want to be too close.

It's been a long time since I ran a stress analysis on a pressure vessel and don't have nearly enough info to attempt to do so on this one, even if I were so inclined. But if he just welded flat plates across the open ends, thats where I'd expect it to fail in a proof test.

375 #/inch is really not that high a load if there's enough meat there to hold it - and if the endplates are thick enough to support the membrane stresses - and the pipe wall is thick enough to support the induced bending from that flat end - and ... I think I'll follow your lead and stand back a ways. I would suggest that he proof it full of water with very little void/air space to reduce the grenade's kill radius.

 

[Highbeam]

When we put watermains together we do a pressure test on them. A 12" ductile iron watermain, 250 psi, and a non-welded cap, and push together gasketed joints. You can bet that there is a huge thrust block holding the whole mess from blowing apart.

 

[HomeBrew2]

When we put watermains together we do a pressure test on them. A 12" ductile iron watermain, 250 psi, and a non-welded cap, and push together gasketed joints. You can bet that there is a huge thrust block holding the whole mess from blowing apart.

Now we're up to 250 psi ... man, you gotta get outta the city more often ...