Saftety of PVC Pipe

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ReeceEngineering
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Sat Dec 14, 2019 11:52 pm

Hello,

Over the years Ive built a handful of cannons. Including some over volumed ones with 10foot barrels. So I have a fair bit of experience with construction and such. As Ive gotten older and my conception of saftey has become more paramount Im now wondering if this is safe? PVC Pipe is certianly rated for the pressures at which we use it but its main purpose dosent involve rapid decompression (sometimes in sub freezing temps).

Right now my primary cannon is a simple PVC 4inch x 3ft tube that reduces to 1 foot of 3 inch PVC and then reduces again into a 2inch ball valve. The barrel is a 1 1/4in PVC pipe. I have fired it at up to 110psi. Its quite a lot of fun but when filled with an air compressor it creaks and cracks which is very unsetteling. Is this normal? All the jouints were treated with the purple primer and PVC glue, 48+ hour dry time. Ive been using the cannon for a little over a year now. When filled to 110psi with a bike pump it doesnt make any odd noises, or at least barely does. With the compresser I dont dare go above 90psi:



Has anybody had pressure rated pipe explode on them? Is it a known saftey precaution to fill them up really slowly?
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Moonbogg
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Sun Dec 15, 2019 4:50 pm

I don't know if it's safe or not. All I know is it scares the hell out of me. A year old PVC? It looks yellowed. Has it been exposed to the elements and sunlight? People have been using PVC for pneumatic cannons for a long time, but I wouldn't do it. That's just me though. PVC it too unreliable of a material for my liking. It's just totally unpredictable. It's intended to be used for pressurized water applications while usually being buried underground. How often do you hear of people's water PVC pipes breaking and having to be replaced? It happens, although seemingly unpredictably.
wdr0
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Sun Dec 15, 2019 5:53 pm

I have a couple PVC hand held cannons. Honestly they always make me a bit leery even though I know I glued them properly with new pressure rates parts. I have one made from sheduled 80 and I feel more comfortable with that considering it being rated double that of sch 40. Also considering how brittle PVC gets in the cold I tend not to fire them below 40 or so unless I've just brought them out from a heated area.

Specific to yours first thing I note is the cleanout plug. Everyone uses them but I'm fairly certain they are not pressure rated. I'd replace with a proper cap as at 100 psi you've got 1200+ lbs pushing on that threaded cap if it's 4" pipe.

And for what it's worth OSHA as well as all the PVC manufacturers explicitly stated PVC is not to be used for compressed gas of any kind at any pressure. Most of us seem to get away with using PVC for cannons but I cannot comfortably say it's safe. The hazards can be mitigated but still a risk
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Zeus
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Sun Dec 15, 2019 6:29 pm

The failure pressure of PVC is irrelevant, the failure mode is the critical part. The physical forces on a handheld pressure vessel while pressurised can cause more than enough stress in stress risors to promote failure, which is generally fracturing versus tearing. No other commonly used material fails in such a particularly nasty way.
/sarcasm, /hyperbole
ReeceEngineering
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Mon Dec 16, 2019 3:18 pm

Has anyone ever heard of a PVC cannon failure/explosion? What alternative material are you guys using?




Moonbogg wrote:I don't know if it's safe or not. All I know is it scares the hell out of me. A year old PVC? It looks yellowed. Has it been exposed to the elements and sunlight? People have been using PVC for pneumatic cannons for a long time, but I wouldn't do it. That's just me though. PVC it too unreliable of a material for my liking. It's just totally unpredictable. It's intended to be used for pressurized water applications while usually being buried underground. How often do you hear of people's water PVC pipes breaking and having to be replaced? It happens, although seemingly unpredictably.
Its not yellowed, its painted OD green. The barrel is unpainted but both are washed in sunlight. Its been kept indoors and at the time of that video I think it was only a few months old anyway.


wdr0 wrote:I have a couple PVC hand held cannons. Honestly they always make me a bit leery even though I know I glued them properly with new pressure rates parts. I have one made from sheduled 80 and I feel more comfortable with that considering it being rated double that of sch 40. Also considering how brittle PVC gets in the cold I tend not to fire them below 40 or so unless I've just brought them out from a heated area.

Specific to yours first thing I note is the cleanout plug. Everyone uses them but I'm fairly certain they are not pressure rated. I'd replace with a proper cap as at 100 psi you've got 1200+ lbs pushing on that threaded cap if it's 4" pipe.

And for what it's worth OSHA as well as all the PVC manufacturers explicitly stated PVC is not to be used for compressed gas of any kind at any pressure. Most of us seem to get away with using PVC for cannons but I cannot comfortably say it's safe. The hazards can be mitigated but still a risk
Interesting note about the cleanout plug. Ill replace it before shooting again. What pressure do you fill your cannons to? And what kind of valves?


Zeus wrote:The failure pressure of PVC is irrelevant, the failure mode is the critical part. The physical forces on a handheld pressure vessel while pressurised can cause more than enough stress in stress risors to promote failure, which is generally fracturing versus tearing. No other commonly used material fails in such a particularly nasty way.
What is a failure mode? And how can stress risors cause something to fail within its given pressure rating?
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farcticox1
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Tue Dec 17, 2019 5:16 am

Of course there is only one correct answer, NO, it's not safe.

https://www.usplastic.com/knowledgebase ... entkey=787

But safety is as state of mind :shock:
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mrfoo
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Tue Dec 17, 2019 11:54 am

ReeceEngineering wrote:Has anyone ever heard of a PVC cannon failure/explosion?
Yes.
ReeceEngineering wrote:What is a failure mode?
Well, one failure mode of a compressed gas container might be plastic deformation leading to a split. That's the sort of failure mode you might expect from a ductile material like steel. Another failure mode might be sudden brittle failure, where a part, or more often parts, of the container shatter, with an effect somewhat similar to that of a fragmentation bomb. That's the sort of failure mode one might expect from a non-ductile material like PVC pipe.
ReeceEngineering wrote:And how can stress risors cause something to fail within its given pressure rating?
The pressure rating for a piece of pipe or similar is proportional to the amount of stress the material can handle. It's calculated for that material in its normal usage. A stress riser is an area where stress is multiplied relative to the surrounding area, by tens, hundreds, or even thousands of times the "ambient" stress. So while the majority of the material might be well within tolerances, the area with a stress riser might well be encountering massively over-tolerance stress.

A good example of this is a pane of glass. Take a pane of glass, place it on a flat plane with a matchstick underneath it. Press gently on the elevated edges, and it will deform, then spring back to its original shape as you release the pressure. Now scribe a line across the glass, creating a stress riser, and repeat the exercise. You now have at least 2 pieces of glass.

PVC is remarkably similar to glass. You create stress risers by adding sharp edges and corners, flat caps, scratches, and can even weaken its structure by the act of gluing two pieces together.
ReeceEngineering
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Thu Dec 19, 2019 8:33 pm

mrfoo wrote:
ReeceEngineering wrote:Has anyone ever heard of a PVC cannon failure/explosion?
Yes.
I mean pneumatic cannons here. Ive had a PVC combustion one fail before. Luckily it wasnt during firing. It simply fell over and its own 3.5 foot height was enough to crack it down the middle. It had seen maybe over 100 shots before that.

mrfoo wrote:
ReeceEngineering wrote:And how can stress risors cause something to fail within its given pressure rating?
The pressure rating for a piece of pipe or similar is proportional to the amount of stress the material can handle. It's calculated for that material in its normal usage. A stress riser is an area where stress is multiplied relative to the surrounding area, by tens, hundreds, or even thousands of times the "ambient" stress. So while the majority of the material might be well within tolerances, the area with a stress riser might well be encountering massively over-tolerance stress.

A good example of this is a pane of glass. Take a pane of glass, place it on a flat plane with a matchstick underneath it. Press gently on the elevated edges, and it will deform, then spring back to its original shape as you release the pressure. Now scribe a line across the glass, creating a stress riser, and repeat the exercise. You now have at least 2 pieces of glass.

PVC is remarkably similar to glass. You create stress risers by adding sharp edges and corners, flat caps, scratches, and can even weaken its structure by the act of gluing two pieces together.
You seem to be describing the pressure rating as if the manufacturers didnt take anything into account except what the material would hold in its...say ideal shape. Wouldnt that mean all pressure ratings are totally worthless? I would expect the manufacturer to take the geometry of these parts into account during the design testing and pressure rating phases.
Well hybrid cannons are a different beast! Haha I dont think im near those stress levels.

The other cannon failure is concerning, although if a post from 2007 is about as much as an active forum has on this then it seems like PVC pipe is pretty dang safe for a cannon. The price of failure sure can be high but it seems very rare.

I must ask, what is everyone building with these days? Im sure Im not the only guy with PVC
Last edited by jrrdw on Fri Dec 20, 2019 6:26 am, edited 1 time in total.
Reason: Double post.
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mrfoo
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Fri Dec 20, 2019 3:24 pm

ReeceEngineering wrote:
mrfoo wrote:
ReeceEngineering wrote:Has anyone ever heard of a PVC cannon failure/explosion?
Yes.
I mean pneumatic cannons here
So do I.
ReeceEngineering wrote:You seem to be describing the pressure rating as if the manufacturers didnt take anything into account except what the material would hold in its...say ideal shape.
Yep, that's pretty much it, actually. Not sure what the safety margins are*, but when you start putting things together in ways that aren't "normal", creating stress risers, or even using things outside their intended use, pretty much all bets are off. Bear in mind, for example, that PVC pipe, regardless of its nominal pressure rating, is rated for a limited range of liquids at specific temperatures, and not for compressed gases. Failure modes are very different - with liquids you get a crack, maybe a bit actually breaks off, and subsequent leakage of the contents. With gases, brittle failure gets you a fragmentation bomb.

Other stuff that almost certainly aren't taken into account by the manufacturer's ratings - shock loadings caused by the movement of gas, physical shocks and loadings caused by recoil, friction damage caused by driving projectiles down the inside of a pipe, UV exposure, exposure to heat and chemical action for combustion cannons, etc, etc, etc

PVC comes with the risk of sudden, catastrophic, and unpredictable failure.

* This would suggest it's 0.5, meaning you shouldn't be exposing schedule plastic pipe and fittings to more than half their rating even when you're using the "correct" fluids.
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Moonbogg
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Fri Dec 20, 2019 5:29 pm

ReeceEngineering wrote: I must ask, what is everyone building with these days? Im sure Im not the only guy with PVC
My first cannon was a propane combustion made from ABS plastic. That was about 12 years ago I think. Since then, I've used aluminum. I've only made 2 cannons in over 10 years because I used aluminum. It tends to be a little more involved and expensive than using existing plastic plumbing parts. But yeah, 2 cannons in 10 years. luckily, however, neither of those happened to break yet either.

Also, sometimes people just get lucky.

[youtube][/youtube]

Fast forward to 1:45. I wonder how many times you could do this until it explodes? Of course, if it doesn't explode, that's when some people decide to throw some oxygen in there to eat up some of that apparent safety factor that's sadly been going to waste.
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