Super Duper Watergun...im getting crazy..who knows one?

[cantab]

Ben, you need to listen to that video again. The pipe that shattered was one that had been in the freezer. So presumably at minus 20 or so celsius. It may not shatter the same way at room temperature.

[Ben]

That video is merely an example of the power of PVC shrapnel. Yes, PVC has much lower impact strength when cold, but the failure characteristics of a pressure vessel are essentially the same. Take it from the US government:

"When PVC piping explodes, plastic shrapnel pieces are thrown in all directions."

Edit: Failure in water guns is not likely to occur with PVC and even if it does, it will be tamer than the example video. Firstly, the amount of shrapnel definitely is related to the temperature, and water fights occur at warmer temperatures. Gas is the primary problem as gas compresses and consequently can store a great deal of energy. Water volume limits the amount of gas and when the ratio of gas to water is high, the pressure is lower. This is favorable.

[cantab]

So does the failure behaviour differ depending on whether the fluid inside is a liquid or a gas? And pertinently for water blasters, what about when you have both?

[Teh Moron]

I don't think it's the pressure as much as it is the repeated impacts from bricks. The repeated impacts probably created quite a few stress fractures in the material, and the PVC most likely failed at one point, and due to the preexisting weak points, spread to the other cracks, and set off a chain of cracking PVC. Now, I'm quite sure in more realistic situations in which the fault was solely the cause of overpressurization and not preexisting material fatigue combined with whatever variables the testers introduced, the PVC would rupture or burst, but this video does bring up a few interesting points.

Of course, that would require much testing that I am currently unable to do (both safety and financial reasons)

[Ben]

Gas stores a great deal of energy because of the volume change. When given the opportunity to, it will expand. A fracture, especially when the material fails in a way as to generate shrapnel, will send all of these pieces flying.

In comparison, incompressible fluids like water don't store anywhere near as much energy and a failure usually doesn't send pieces flying. A pipe might crack and some of the fluid will leak.

With both in the system you should focus on the gas and the possibility of the system turning into a rocket.

An interesting application of this idea if hydrotesting. Hydrotesting is commonly used to test the safety of pressurized components at a certain pressure. The procedure realizes this fact and pressurizes an incompressible fluid like water so any failure is reasonably safe.

The repeated impacts likely did fatigue the material a bit, but that test is not the only one that verifies what I say. Either way, I don't think a material taking some impacts is unrealistic for water guns.

It's a well known fact that PVC pipe and fittings will create shrapnel during failure if pressurized with gas. Read some of the link I posted from the US govt. These failure characteristics are due to the properties of PVC. PVC is a somewhat brittle plastic that can not absorb much energy prior to failure. Other plastics such as ABS, PC, and PE are very ductile and consequently would fail differently.

[cantab]

Yet the APH is your design right? Are you saying you no longer believe it to be safe?

[Ben]

I am not saying the APH is unsafe. Our use is well below the pressure rating. The amount of air is rather low and there are no parts I think are likely to break off and turn the water gun into a rocket.

I'm only saying what PVC's failure characteristics are as Teh Moron said that PVC doesn't make shrapnel. This knowledge helps make safer water guns.

The non-rated fittings I use in the guide are a little worrisome, but not much as they're typically only use water pressure. I intend to correct this problem in the future.

[Silence]

Originally Posted by Eruco

That's a dangerous assumption, In the video they're using dry ice instead of straight air and water but it's the same rupture pressure.

I can't watch the video, but I can tell you the problem was the dry ice. PVC is too brittle to store any pressure at those temperatures.

Ben, the video you linked to was pretty reassuring because I had doubts of my own about the strength of PVC (even at room temperature) when dropped or struck by something.

However, I don't buy the argument that PVC won't shatter when there's water inside. That may hold true if you fill a sealed length of pipe with water and then perform impact tests. But if there's pressurized air, then it's the same as if there were only pressurized air; the fluid will continue to expand and propel the shards after an explosion.

If you find a test of pipe containing water at high pressure (preferably air pressure), then I'll believe it. I just know that nothing I've seen has specified that.

[Teh Moron]

The points you brouught up are true. The majority of what I've learned about PVC comes from water transmission use only, where there is little energy stored in the pipes. In this case, the one burst is enough to release what little pressure there is inside the pipe, where with air, there is more energy, and one crack is likely to lead into a cascading failure.

I think what would be an interesting gun to make is a water gun with seperate chambers for air and water. The air would be behind the water and propel it, but of course this would only work in bursts due to the air filling the pipe after a few seconds.