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Explosion!!!

Discussion in 'Special Effects' started by danl, Dec 24, 2006.

  1. danl

    danl Member

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    i am thinking of rigging an "explosion" for our show... (we're doing "les mis")

    i'm trying to brainstorm a way that i can rig an explosion on the back side of the barracade... i was thinking of using foam bricks/stones/wood that would be launched over the barricade (toward the audience) with a bunch of smoke... all i could think of was a catapult-type rigging, but thought i'd drop in here for some possible other ideas...

    and... go!
     
  2. Van

    Van CBMod CB Mods Premium Member

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    Just an Idea, Try rigging a 4- 6" pvc pipe with an endcap. Tap it with a good sized hole 1" or so, either hook directly to a compressor ar use a protable air tank and rig either an electric solenoid or manual valve in-line. Stuff the pipe with some foam bricks, foam faux wood, some Rye flour. You should probably put a piece of cheese cloth in the bottom to act as wadding so the air doesn't just blow by the bricks and stuff.
    Do not use Talc as a "dust" use only rye flour it is the most hypo-allergenic and least irratating dust alternative I have found. If my description is too confusing go take a look at your local rental house and ask to see a conffetti cannon. Basically just upgrade the size. Go to Grainger.com to find the air valves / solenoids, You want the highest flow valve you can get. it's not about total psi it's about volume, and getting as much of that volume transferrred as quickly as possible.
     
  3. TupeloTechie

    TupeloTechie Active Member

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    we did les mis last year, we used a small flashpot, and a fog machine, then we had the crew throw faux bricks and wood over the baricade, it was pretty simple, but it worked

    I have also made one of thoes "confetti canons" If you need help on making one, look up "phnematic spud guns" because there made the same way, I used a sprinkler valve from the homedepot, it was to actually launce confetti, but we had alot of it in there, so faux bricks and wood would prabally work good.
     
  4. jwl868

    jwl868 Active Member

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    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
     
  5. TupeloTechie

    TupeloTechie Active Member

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    They do make pvc rated at higher psig levels, the stuff we used was 300psig rated, but we never gave it over a hundred psig, we used just those tanks that you buy to refill your car, and usually filled it to 75psig, I guess we were not being as safe as possible(espicially since we were using pyrotechnics without any trianed personal)

    I would listen to joe, and either build it out of steel pipe, or buy one from a respected company, there pretty useful for a lot uses, so it might be a good addition to your fx collection.
     
  6. Van

    Van CBMod CB Mods Premium Member

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    Joel most of your statements on this subject appear to be assumptive. I have never see a pvc manufacturer make the statement listed above. that would be very similar to saying, " don't use our pipe cause it'll explode the first time you get a water hammer in your house !" Do you know what a water hammer is ? I don't have time to delve into every one of your points right now. I will get back to this. Now if you have a reference to which you can refer me I'd be happy to take a look at it . Beleive me I'm very much in favor of Safety and taking precautions where precautions are necessary. I feel you are painting with much to broad of a brush on this one.
     
  7. jwl868

    jwl868 Active Member

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    van

    An excellent reference is Chemtrol (now owned by NIBCO), specifically their Thermoplastic Piping Catalogue which can be found online at http://www.nibco.com/cms.do?id=2&pId=1 and then clicking the Chemtrol Thermoplastic Piping Catalogue button in the lower right. At the bottom of each page is a warning against using PVC in compressed air systems.

    Regarding water uses, most manufacturers (such as Chemtrol) have design guidelines in their catalogues. And designers and plumbers will follow these guidelines to prevent a water hammer problem. A properly designed water system will not be damaged by water hammer.

    The issue with air is the likelihood of oil in compressed air that will weaken the PVC.

    And I may have used a broad brush on the pressure ratings in that smaller diameter piping has a much higher pressure rating, up uo 300 psig. However, many PVC valves and fittings are only rated to 150 psig, regardless of diameter.


    Joe
     
  8. Van

    Van CBMod CB Mods Premium Member

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    Ok See, that's much more in line with what I was thinking of. If this explosion piece were made from a 4" piece of PVC, had debris in it, and was piped to using a rated air hose, it shouldn't require much more than 10-15 psi to get quite a large amount of thrust behind the "stuff" which is well within safety tolerances of sch 40 pipe. As to oil being present in the air supply, a standard evaproative water trap < air dryer> installed in line should do nicely. In a properly maintained air system there should be nothing but air and possibly some water comming through the line,and the water is usually only what is left after the inline filter. As this would be for one production, I don't think the danger represented by oil degradation of the PVC is going to amount to very much at all, probably a much greater risk is represented by improper storage of the PVC in the first place. The Number one culprit in failure of PVC piping is UV from the sun. If you ever find a plumbing supply or home supply store storing thier PVC outdoors, do not buy from them. It takes an extremely short amount of time in the direct sunlight for a PVC pipe to break down because of UV radiation. In the case of Sun damaged PVC an explosion resulting in small sharp shards of really hard PVC is a real possibility. In a standard failure of PVC the result is usually a split running the length of the pipe, in the area of the failure. It looks kind of melted in a way, kind of like a pair of lips on the side of the pipe. It is however important to remember top do the math when deling with things like PSI and PSIG, for instance, in a test of fitting pratices for 4" pvc pipe 48" long you are supposed to pressurize the pipe to 3.5psig that brings the total amount of force on the pipe to somewhere around 8,600. Quite an explosion indeed if something were to fail.
    I would have to ammend my earlier post to represent what Joel posted however.
    Instead of tapping the end of a piece of PVC I would probably be much more inclined to buy the proper fittings with threads and install a pnuematic hose adapter that way. Actually tapping sch#40 PVC is not advised in any higher pressure application you can tap sch#80 for use with other fittings but why bother? when all the fitting are available right there at your local hardware store. Thanks for catching that Joel I was just kinda shooting from the hip < so to speak >
     
  9. jwl868

    jwl868 Active Member

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    No Problem, van. I usually cite my statements better.

    But I am pretty sure that the failure mode of PVC is shards and pieces. Cetainly, this is the case when using compressed air.

    It also occurs to me that I know this because of my line of work. One could also get this information from a PVC pipe salesman. On the other hand, I don't think thare are any such warnings (product guides) at a Lowes or Home Depot and maybe not even from a regular hardware store.

    [By the way, I don't follow the 3.5 psig to 8,600.]

    Joe
     
  10. jason0

    jason0 Member

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    so basically, a pneumatic spud gun. I've built several. My personal rule of thumb is to never exceed 1/3 the weakest link.

    do not try to tap pvc pipe. such a bad idea.

    and you dont need a compressor. If it's a one-off, a bike pump will work fine. It will take 3-5 minutes though. get a pump with a pressure gauge.

    get a 10 foot (standard length) stick of 4" pvc. make sure its pressure rated. it will be printed right on the pipe itself. cut it to whatever length, probably 4 foot chamber, 6 fooot barrrel. To do the fitting, take an old bike tire. cut the valve out, leaving a circle around it. drill a hole in the endcap (not the pipe) and epoxy it in. This is good to around 80 psi, and if it fails will fail safely. It will also fail before the pipe. primed and cemented fittings are rated for more than teh pipe's pressure but read the directions, because its easy to not do thsi step correctly. In the middle, a solenoid valve is nice, but these are expensive, and not amazingly reliable. A simple ball valve will do, and will allow higher flow. You will want a pvc reducer to 1.5 or 2 inches, then get a threaded fitting to put on the end of a short piece of the smaller diameter pipe. put the valve on that, and repeat on the barrel. It works nicely.
     
  11. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    To take this thread in a different direction. I would be more inclined to go with flash pots and a little fog machine action. You need to find an expert to help you do it safely (if you can't find a stage tech who knows what they are doing look for a magician or magic store). Depending what type of flash powder you use you can either get more smoke or more light flash.
     

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