Never Use PVC!

[MCRKilljoy]

Hey guys, PVC Pipe is very dangerous! PVC chambers can explode. PVC is not made for compressed air. So, I used black iron pipe for my pneumatic cannon. I did have a PVC barrel. I shot AA batteries, and since there was no pressure, I thought a PVC barrel would survive. Well, it shattered. My copper barrel is fine.

[farcticox1]

hmmm, but it is pressure rated.

[Gippeto]

Hey guys, PVC Pipe is very dangerous! PVC chambers can explode. PVC is not made for compressed air. So, I used black iron pipe for my pneumatic cannon. I did have a PVC barrel. I shot AA batteries, and since there was no pressure, I thought a PVC barrel would survive. Well, it shattered. My copper barrel is fine.

What on earth gave you the impression there's no pressure in a barrel?

Pvc does have it's limitations, but are you sure you understand WHY it failed? From your chicken little post, I rather doubt it.

[jrrdw]

Live and learn! Lets keep this thread educational... :bigsmurf:

[mark.f]

To throw in my (subjective) experience, I've seen far more barrel failures (especially towards the attachment to the chamber) than chamber failures when using PVC. As Gippeto hinted, there's a reason, and it's simply because there's a lot of stress on that particular section of barrel due to its own weight, combined with the higher pressure experienced there compared to the rest of the barrel.

PVC has the ability to be used safely, although at a certain point it's not worth it when you have higher pressures available.

[D_Hall]

hmmm, but it is pressure rated.

Yes, it's pressure rated... For liquids.

Seriously, there's a huge difference between pressure rating something for liquids and pressure rating something for gases. Put it this way: Why do you think hydrostatic pressure testing involves water rather than air? Answer that question and you're well on your way to understanding why the requirements for a pressure vessel containing a liquid is very different from those for a vessel containing a gas.

[farcticox1]

well

http://www.vinidex.com.au/technote-pare ... gas-lines/

[Xamllew]

I think it's fairly safe to use, especially after seeing this



But yeah, PVC can shatter pretty spectacularly under the right circumstances, and unlike copper or steel, which might banana peel upon failure, PVC will act like a frag grenade and shatter into tiny pieces.

[mrfoo]

Seriously, there's a huge difference between pressure rating something for liquids and pressure rating something for gases.

There is.

Why do you think hydrostatic pressure testing involves water rather than air?

The important bit of hydrostatic testing is not the "hydro" bit, which is all about safety during the test, but rather the "static" bit. Static pressure testing is, as the name suggests, testing for (eventually) the ultimate burst pressure of a vessel under static or near-static conditions, i.e. how much pressure can this vessel hold before it fails. There is zero difference regarding rating for liquids or for gases in this respect, as it's an absolute measure of pressure. The reason it's done with largely incompressible liquids is to do with energy release *after* failure; with an incompressible fluid, failure is "crack, leak", with a compressible fluid, failure is "crack, explode".

Once we hit dynamic conditions, things change enormously, and incompressible fluids can be far more dangerous than compressible ones. Consider hydraulic shock (also known as "water hammer"). At that point, a whole load of things come into play - fluid density, vessel rigidity, speed of sound in the fluid, and magnitude of change of velocity in the fluid.

[MCRKilljoy]

Yes, it's pressure rated... For liquids.

Seriously, there's a huge difference between pressure rating something for liquids and pressure rating something for gases. Put it this way: Why do you think hydrostatic pressure testing involves water rather than air? Answer that question and you're well on your way to understanding why the requirements for a pressure vessel containing a liquid is very different from those for a vessel containing a gas.

PVC pipe can hold the pressures used in spudguns, but if the spudgun is bumped or falls over, it can explode with deadly shrapnel. Air can be compressed, so it can store large amounts of energy, which causes PVC to explode and also is the concept behind a compressed air spudgun. Water won't cause an explosion because water can't be compressed, so there is little stored energy. As a result, the pipe will split.

[jimmy101]

The important bit of hydrostatic testing is not the "hydro" bit, which is all about safety during the test, but rather the "static" bit. Static pressure testing is, as the name suggests, testing for (eventually) the ultimate burst pressure of a vessel under static or near-static conditions, i.e. how much pressure can this vessel hold before it fails. There is zero difference regarding rating for liquids or for gases in this respect, as it's an absolute measure of pressure.

It is more complex than your simplistic view. Yes, the failure pressure is independent of the medium but the RATING is not. The RATING is not simply the failure pressure, or the failure pressure + safety factor. The RATING takes into account both the CHANCES of failure and the CONSEQUENCES of failure. The CONSEQUENCES of the failure of a container of pressurized liquid are much less than the CONSEQUENCES of the failure of a container of pressurized gas. Therefore the RATING of a container that contains pressurized liquid is different than the RATING of a container for pressurized gas even if they have the same failure rate characteristics.

[jimmy101]

PVC pipe can hold the pressures used in spudguns, but if the spudgun is bumped or falls over, it can explode with deadly shrapnel.

Unless it is a combustion gun, in which case bumping or knocking it over is unlikely to do anything. (Accidental triggering is possible but that is true for both pneumatic and combustion guns that are loaded.)