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Unread 28-06-2010, 08:52
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Re: Most Powerful FIRST Team created T-Shirt Cannon

Just a note about safety. In industry, there are several factors that are considered when characterizing the "safety" of a device or design. When considering a dangerous event that might occur, the two most important are:
  1. Likelihood of the event occurring
  2. Cost of the event occurring.

If either one of these is high, that device or design is considered unsafe, even if the other is low. The reason PVC is not rated for pressurized gas is the cost of failure, not the likelihood of failure. When gas is pressurized in a reservoir, energy is stored (not so much the case with non-compressible fluids like water or oil). If the reservoir fails, that energy is released. If the material is brittle plastic, that energy release is almost instantaneous, and is accompanied by flying shrapnel.

Metal reservoirs can also fail catastrophically. If you look at pressure vessels like SCUBA tanks, you will see dates stamped on the side. Each SCUBA tank must have a visual internal inspection every year (to look for corrosion or visible flaws), and a hydrostatic test every three years (the tank is filled with water and pressurized to measure the growth of the tank. Water is used instead of air so that if the tank fails, little or no energy will be released). With higher pressures and higher volumes come more energy and more danger. Here is the result of a SCUBA tank failure.



So, PVC pipe is not more likely to fail when pressurized with gas instead of water, but the cost of that failure, in terms of safety, is much higher. Please consider the cost of failure of any design you wish to build, as well as the likelihood of that failure.
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