Just to re-iterate some cautions about building do-it-yourself “confetti cannons” and the like:
Just about every
PVC pipe manufacturer's catalogue that I have seen emphatically states "DO NOT USE
PVC PIPE WITH COMPRESSED AIR OR COMPRESSED GAS".
The pressure
rating of
PVC pipe at 100 F is 150 psig ("psig" is pounds per square inch,
gauge and is relative to atmospheric pressure. An air
gauge will read "0" if it is opened to the atmosphere. People tend to
drop the "g" in common usage, but it's a sloppy habit.) The actual pressure
rating depends on the
thickness, the diameter, and temperature. But 150 psig should be considered as the starting
point. (although you might be stuck at no more that 150 psig because I believe most valves and threaded fittings are rated at 150 psig maximum.)
Compressed air, particularly from a
compressor, will have residual oil that weakens
PVC.
When
PVC pipe fails, it bursts suddenly and explosively, and splinters into shrapnel-like shards. If anyone in nearby, they will be hurt badly.
I doubt that
PVC manufacturers recommend threading/tapping brass or metal fittings into their pipe. In any case, that would become the weak link and a possible
point of failure. More importantly, you don't know the pressure
rating of such a joint. (Your 150 psig
system may now be 50 psig with a metal
fitting projectile and miscellaneous slivers of
PVC.)
PVC pipe manufacturers make bushings and reducers and adapters so that you can safely go from 6-inch
PVC down to a metal
fitting without compromising the pressure
rating of the
system (although, as noted, you may be limited to 150 psig).
Because the
barrel is open-ended, over-pressuring the
barrel is highly unlikely, but bear in mind, some yahoo might put a cap on it as a "joke". Or press the end down onto a mat "just to see what will happen". Maybe a few large diameter holes near the business end of the
barrel might provide some reasonable
safety relief to minimize the effects of accidental closure of the
system.
For air piping, use compressed air-rated hose, steel pipe, and compressed air-rated fittings. For air-line valves, use metal valves constructed for compressed air use. That is, everything between the big tube and the compressed air source has to be rated for compressed air use.
This is a
safety issue – whatever you design, you must make sure to consider what happens if you accidentally over-pressure the
system. Unless you have a
safety valve (which you could design into the
system), you must assume that you could accidentally apply the full pressure of your compressed gas source to the
system. For example, if your
compressor can only put out 100 psig, then your
system needs only be designed for 100 psig. If your
compressor can put out 175 psig, or if you are using a compressed air tank, you need to take a very careful look at what you are doing and what will happen when someone screws up or if an air pressure regulator fails.
"DO NOT USE
PVC PIPE WITH COMPRESSED AIR OR COMPRESSED GAS".
Joe