Throw bag launchers and DIY…

[LBF]

It's not only the PVC pipe walls giving, which most people think about, but also threaded fittings, turbulence from dimensional changes in the gas path and on and on. If you have a smaller diameter pipe with a larger gas reservoir and say a valve and threaded connections of some sort in your design, consider the force on the plastic surface area and threaded connections and even things like temperature changes due to gas expansion (Joule-Thompson effect) and the like. The stress on plastic parts can add up quickly and especially over time cause fatigue cracking, on the way to failure. Even 80 to 100 psi air is nothing to fool with especially, if it hits exposed skin. Think of injection injuries with sprayers. If you're shoulder launching your bags, well not anywhere near my face and neck thanks. Just my two cents.

(BTW for scary real life demonstrations of the power of a compressed gas, look up some of the steam explosion incidents in process industry - I've seen 2 km of 24" piping whipped off pipe supports and moved around fifteen feet - awesome power as the steam starts moving a liquid (condensate) slug around at near supersonic speeds - why we have steam traps for liquid removal!). In my books, any compressed gas demands real respect and a pause for second thought.

In my case I did do a bit of considering about this all. Thus I took certain steps, like trying to experiment towards lower pressure and making the air flow pathway as smooth and straight as possible. Not only does that reduce forces on the materials, but it also can benefit performance. The smoother the air flow and the faster I can dump the pressure down the barrel, the faster I can push the bag down the barrel with less pressure. So it’s an all-around benefit. I have no need to leave these charged up for longer than it takes me to load and get in position so temperature changes are essentially nil and fatigue is minimized. I store them out of sun and heat and with the valves open. So I think I have done a lot towards minimizing the major concerns.

BTW, steam is an entirely different animal than compressed air. It has it’s own set of rules and is arguably much more dangerous than compressed air. That’s not at all to say compressed air shouldn’t be a concern and something to exercise a certain level of caution working with, which is why I proof tested my PVC cannons to 125/130 psi (all my current compressor is capable of), even though I have no intentions of operating it at that pressure. To try and compare what I’m doing to steam would be rather like me telling you to go watch an arc flash video for a 440 volt breaker and tell you that the breakers in your house panel are dangerous. Yes, there is danger around the breakers in your house (compressed air), but you don’t have to wear a bomb suit to reset them (steam).

That all said, it is indeed good to advise caution.

 

[Reach]

To try and compare what I’m doing to steam would be rather like me telling you to go watch an arc flash video for a 440 volt breaker and tell you that the breakers in your house panel are dangerous. Yes, there is danger around the breakers in your house (compressed air), but you don’t have to wear a bomb suit to reset them (steam).

That all said, it is indeed good to advise caution.

Ok, so I must chime in on this one. After spending far too much time doing arc flash calculations in college, I discovered that the average house panel actually has a greater chance of an arc flash than many (most?) industrial panels. The AIC rating of a residential breaker is lower, cable runs are shorter, and wire sizes are actually larger than necessary for their normal loads; those three all add up to a much greater potential for an arc flash than most think.

 

[LBF]

Ok, so I must chime in on this one. After spending far too much time doing arc flash calculations in college, I discovered that the average house panel actually has a greater chance of an arc flash than many (most?) industrial panels. The AIC rating of a residential breaker is lower, cable runs are shorter, and wire sizes are actually larger than necessary for their normal loads; those three all add up to a much greater potential for an arc flash than most think.

Interesting. I would guess that the arc flash would be smaller/less violent for house panels since the volts/amps are lower than industrial panels? I’ve done electrical for quite a few years, mostly residential, but worked maintenance for a printing company for awhile and that was where someone finally said anything about arc flash. That includes some trade-school level training in residential. Kinda curious now as to why it only seems to get brought up for the industrial high volt, high amp stuff. The only somewhat logical conclusion would be that residential setting is far less violent/deadly?

 

[Reach]

Interesting. I would guess that the arc flash would be smaller/less violent for house panels since the volts/amps are lower than industrial panels? I’ve done electrical for quite a few years, mostly residential, but worked maintenance for a printing company for awhile and that was where someone finally said anything about arc flash. That includes some trade-school level training in residential. Kinda curious now as to why it only seems to get brought up for the industrial high volt, high amp stuff. The only somewhat logical conclusion would be that residential setting is far less violent/deadly?

I would guess the flash would not be smaller, or at least not sufficiently small to remove the serious hazard of an arc flash. An arc flash occurs when there is sufficient current moving through a fault, usually a short circuit, to vaporize some of the copper in the system, typically the bus bars of the panel. That vaporized copper is what creates the explosion, a piece of copper the size of a penny, when vaporized in an arc flash, causes an explosive vapor cloud the size of a Volkswagen.

This vapor cloud not only burns everything it touches with heat near that of the sun, it also copper plates it, so your exposed skin (and your lungs) will be badly burnt and copper plated. If you survive the initial blast, likely you’ll die of pneumonia from the copper in coating your burned lungs.

And to answer the other part of your question, the current available to the point of the fault is what determines the likelihood and the potential power of the arc flash. Most residential systems contain circuit breakers with a fairly low AIC (Amps Interrupting Current) rating. The AIC rating is how many amps of current flow the breaker can stop before being overcome. A circuit breaker, unlike a fuse, does not trip the instant the current exceeds the rated current, it takes time. Less time with a larger draw, but still time nonetheless.

This trip current curve chart :https://www.manualshelf.com/manual/square-d/hom120cp/specification-english/page-34.html shows just how long it takes for a very commonly installed residential breaker to trip. A 15 amp breaker, with a current draw greater than about 450 amps will take up to 0.02 seconds to trip. That is plenty of time to vaporize copper and cause an arc flash, and given enough potential available current, can exceed the 10,000 amp AIC rating, which will allow the arc flash to continue until sufficient copper has been vaporized to break the arc.

My father shared with me a photo recently of one of his job sites where an errant loader operator hit the service entrance cable just before the meter. Before the breaker on the pole tripped, the arc vaporized several inches of the 4/0 service cable and melted some of the siding, and burned the paint off the top of the meter base. I would say that proves there is sufficient power to cause an arc flash, fortunately the loader operator survived though his bucket now has a notch burned in the cutting edge.

 

[ghostice]

Great discussion guys