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PSI Ratings...Scared to shoot...And many other questions!

A place to ask general spud cannon related questions.
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PSI Ratings...Scared to shoot...And many other questions!

Unread postAuthor: isu22andy » Thu Apr 17, 2008 7:06 pm

1) I recently bought some SCHAU 40 4 Inch diameter pipe that has a psi rating of 220 but my barrel (2inch) has a rating of 280psi... Is this ok? I dont want crap to blow up in my face....
2)Also my fitting dont have a PSI rating on them but are just as thick as the pipes.... Am I alright?!
3)Oh another question...Where do I mount the 2 screws for the spark gap? Right in the side of the barrel or in a fitting?!?!
4) Also whats another precaution that I can take if this thing blows up? How many shots do they last?!?!
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Unread postAuthor: Sticky_Tape » Thu Apr 17, 2008 7:10 pm

if it is a combustion it won't blow up witha rating of 220.
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Unread postAuthor: bigbob12345 » Thu Apr 17, 2008 7:12 pm

1.It will not blow up in your face unless you go above 150psi
2.If it says nsf-pw on it then you are alright but if it says nsf-dwv on it then you need to replace them as there is a good chance it will go BOOM
3.Through the double wall beetween the chamber and fitting
4.If you use pressure rated fittings and pipe then there is no need for any other precautions. though a safety pop off valve is always a good addition

Please search next time
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Last edited by bigbob12345 on Thu Apr 17, 2008 7:13 pm, edited 1 time in total.
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Unread postAuthor: STHORNE » Thu Apr 17, 2008 7:12 pm

1) yes

2) do they say nsf-dwv or nsf-pw on the outer edge? If dwv, then they are not safe to use if you are planning on using it for a pneumatic spudgun but is ok for basic combustions. But if it says pw, then your alright for either one.

3)For optimum performance, the best place is to mount the spark gap in the exact center of your chamber...so mount it in the pvc pipe, not a fitting.

4)Depending on the priming/cementing job and the quality of the pvc, a spudgun can last as long as you can.
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Unread postAuthor: DYI » Thu Apr 17, 2008 7:23 pm

Well, propane/air combustion will generate a maximum of about 129 psig, and that's in a closed chamber. If the pipe is rated for more than that, then there's a very low chance of it exploding, unless the pipe material is something like PVC that doesn't cope well with shock loads.

Fittings in many materials don't have printed ratings on them. If the fittings are plastic, they should say NSF-PW. If they're any type of metal fittings, they should list a rating on them, but you can usually tell by the thickness.

Mounting the spark gaps in the barrel wouldn't make much sense, as their's no flammable gases in it. No one has done enough accurate testing to figure out for sure where the gap should be in a combustion - it requires equipment more accurate than most of us have access to. Ideally, you want the flame front to propagate in spherical mode as far as possible.

This depends on the material. If it's steel, it will rust eventually, but this will be noticeable. If it's something that doesn't oxidise readily, but still metal, it should last longer than you unless you abuse it. If it's PVC or CPVC, it will be degraded by UV light over time, and probably has an operation lifespan of a few years, assuming you don't leave it out in the sun's rays. If it is ABS, you don't need to worry about it degrading in your lifetime.

A precaution to take if it blows up? Have a first aid kit handy, definitely including burn dressings, antiseptics, painkillers, and tweezers.
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Re: PSI Ratings...Scared to shoot...And many other questions

Unread postAuthor: Ragnarok » Thu Apr 17, 2008 7:51 pm

1) Easily more than enough. Even the most powerful combustions will only peak about around 70-90 psi.
Pressure ratings also have a safety rating of usually 3:1 to the approximate burst pressure.

2) It does depend a little, but I would suspect that if it maintains the same thickness as the pipe, you should be all right.

3) Please ignore STHORNE on this one (No offence intended to STHORNE though). Although a spark near the middle is slightly better for performance, it's not particularly safe, and creates a notable weak point in your launcher.
It should really be done through a fitting in the chamber. Holes in a pipe will reduce the load it can take, so if the screws are mounted through a double thickness of pipe, where the pipe is already twice as strong, you are not creating as significant a structural weakness.

4) Few of the techniques people come up with are any use. Chicken wire, jean fabric or duct tape wound around the chamber or barrel are next to useless at containing any shards should the pipe shatter.

As DYI says, keep a decent first aid kit around stocked with the basics, hopefully you'll never need it (if only I had such luck), but if you do, and you haven't got one, then you'll regret it.

However, a first aid kit, although it will deal with mild accidents, it will do nothing to return your eyesight or hearing if either of those get damaged, nor would anything else do much to help - I seriously recommend you buck the decision many people make and get some decent safety goggles and ear defenders.
Thick (but tight) gloves are also a good move, provided that they don't interfere with the safe operation of the cannon. Leather biking gloves are a good choice, as work/DIY gloves are often loose and clumsy.

Get in the habit of always using your safety equipment, and never allowing yourself to misuse or misbehave with your cannon.

As for how many shots a launcher lasts... it depends on how well it was made and stored.
More often, the question is how long it lasts, not shots. If stored well, in dry conditions away from sunlight, then a PVC launcher could survive maybe a decade and still be safe and usable (but don't hold me to that).
Of course, eventually repeated shooting will slowly weaken the pipe, but this is unlikely to show up for many hundreds or thousands of shots under the kind of loads you'll be using.

Really, by the time the cannon is becoming worn out and too old to use, you'll probably be long bored of it and will have built another one.

To expand on Bigbob's short comment, these questions have been asked before, and the answers aren't too deeply hidden if you know how to use the search function. However, I won't get at you for that, because few new people do know how to use it, and often aren't even aware of it's presence.
I don't often use the search function, and seldom do I use it to seek information on a subject, usually I use it for finding something I was already aware of and wanted to refer back to - so I can't really get up on a high horse to complain at you.

However, it is a useful tool - you can find it on the toolbars across the top of the forum page.

In some ways it doesn't hurt for these questions to be asked again, because it allows them to be re-evaluated and corrected over time. It's not unknown that the answers to common questions will change. The obvious example would be the "PVC vs. ABS" debate, which although still split on opinion has rather swung towards ABS rather than the earlier mantra of PVC.
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Unread postAuthor: isu22andy » Thu Apr 17, 2008 9:10 pm

The fittings are nsf-dwv and I'm making a basic combustion cannon using starting fluid or something along those longs....So I should be alright...??? So should I mount the 2 screws in the fitting or just on the pipe?!?!
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Unread postAuthor: Pilgrimman » Thu Apr 17, 2008 9:24 pm

1. If the pipe has a pressure rating over 200 psi, you will almost certainly be okay. As Ragnarok said, the pipe is rated with a safety factor. The highest pressure you will generate is probably in the neighborhood of 80 psi.

2. If the fittings have "NSF-pw" on them, they are okay to use. If you want a visual on pressure fittings as opposed to DWV fittings (the bad kind), go to the wiki.

3. If you have to drill into the chamber, always do it on a fitting so that you drill through 2 layers of PVC.

4. Anything else Ragnarok said before.

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Unread postAuthor: pizlo » Thu Apr 17, 2008 11:00 pm

Well this is a great site, get your questions answer half a dozen times in a few hours :)
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Unread postAuthor: Ragnarok » Fri Apr 18, 2008 5:40 am

isu22andy wrote:The fittings are nsf-dwv

Not completely ideal, as usually (but not exclusively) this marks weaker parts without quality testing - but if you think they're the same thickness as the pipe, they should be strong enough for this application.

However, I suggest that you try to get NSF-PW parts for your next launcher, as although the more substantial NSF-DWV parts will be adequate for a fairly simple combustion, they won't necessarily be safe for more powerful variants.

So should I mount the 2 screws in the fitting or just on the pipe?!?!

A launcher is only as strong as it's weakest part, so it should go through a fitting. A hole anywhere will weaken the launcher by a significant factor, but if that hole is in a section that is twice as strong in the first place, your "weak point" will be twice as strong, and thus your launcher will be safer.

Never drill through a single layer if there is any way (and really I mean any way) you can avoid it.
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Unread postAuthor: THUNDERLORD » Fri Apr 18, 2008 8:29 am

isu22andy wrote:The fittings are nsf-dwv and I'm making a basic combustion cannon using starting fluid or something along those longs....So I should be alright...??? So should I mount the 2 screws in the fitting or just on the pipe?!?!


The key word in this reply above "starting fluid"
Wouldn't trust that in PVC

I think it is something like 4 times more flammable than gasoline?
Also most common brands seem to have a type of Alcohol which is extremely corrosive to most plastics and adhesives.
Engines that run on high % Alcohols have special fuel lines.

:?: I have actualy been wondering the corrosive effect of alcohols on pvc considering the possibility of using 190 proof drinking type alcohol(Everclear is no longer sold but a moonshine version maybe).

I got a very good mixture (by luck) of starting fluid in a combustion I had a long time ago and it was the last time I shot it.

I've seen copper fail in HVAC work and Plumbing, it gets a slit running along it's length usualy. scary but not too dangerous.(350+psi)

If threaded steel/iron pipes fail the end cap(s) can shoot off almost like a shotgun slug.
And Pvc can shatter into little shrapnel going high velocity.

Something to think about with bbmg's also that they're filled with bb's.

Just stay within the limits(specifications) and don't do anything without (like ton's of) overview. (150 psi is way below that for the examples I gave).

BTW:
------Some good alcoholic beverages when spudders drink: Vodka( for obvious reasons), cheap beer(because some of the cans can be shot),
Or everclear 190 proof/ Bacardi 151 (can be used as fuel)-----------
Maybe I should do a poll.
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Unread postAuthor: bigbob12345 » Fri Apr 18, 2008 8:37 am

Umm your a little wrong there
Starting fluid wont generate any more pressure than propane or MAPP which we would all deem suitable for pvc.
and about the corrosion thing there was a test done with gasoline to see if it corrodes pvc and it was compared to nail varnish or something like that. They soaked peices of pvc in the chemical for a while(cant remember how long)and then took them out and ran a number of tests on them and found that gas did not affect the pvc at all. and gas is very similar to starter fluid or lighter fluid.
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Unread postAuthor: MaxuS the 2nd » Fri Apr 18, 2008 9:03 am

bigbob12345 wrote:1.It will not blow up in your face unless you go above 150psi


Hang on a sec, he's using pipe rated to 220psi..
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Unread postAuthor: Pilgrimman » Fri Apr 18, 2008 9:09 am

Yeah, but would YOU pump up a pneumatic to 220 psi? You might as well go with metal. I personally would crap my pants using large diameter PVC over 150 psi :D
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Unread postAuthor: THUNDERLORD » Fri Apr 18, 2008 9:17 am

bigbob12345 wrote:Umm your a little wrong there
Starting fluid wont generate any more pressure than propane or MAPP which we would all deem suitable for pvc.
and about the corrosion thing there was a test done with gasoline to see if it corrodes pvc and it was compared to nail varnish or something like that. They soaked peices of pvc in the chemical for a while(cant remember how long)and then took them out and ran a number of tests on them and found that gas did not affect the pvc at all. and gas is very similar to starter fluid or lighter fluid.


You are thinking of starter fluid for charcoal. And I am thinking of starting fluid for cars(the kind in the spray can used for spraying into carberators and air intakes when your car/engine won't start.

They are two totally different things!!!

Starting fluid for cars generates more pressure than gas, that's why it starts the engine when gasoline won't.

Thanks for the info on gas for pvc corrosion.
But my question(still wondering) was about different types of alcohol.

EDIT: This is a perfect example of why I said never do anything without ton's of overview.
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