Will PVC be suitable for this design?

[mpm]

I am designing a hail gun for my senior design project. I am needing about 70 psi of pressure for my maximum pressure. I am going to use various diameters of Schedule 40 PVC. The max pressure rating for this diameter is 220 psi at 73 degrees F. However this is for water. I've seen articles where it says not to use PVC for compressed air because it has other energies that water doesnt.

However, I am not going to be approaching anywhere near 220 psi. Will I be ok to use this PVC with my max air pressure being 70 psi? I don't see it being a problem but want to make sure.

Also I was going to use heavy duty cement and primer. Will this cement and primer withstand 70 psi? I can't find anywhere saying otherwise.

I just want to prevent serious injury if possible.

Oh, also, the reason I am choosing PVC is because I'm on a tight and small budget.

Please let me know opinions.

 

[brewnog]

We won't be able to give you a yes or no answer without seeing the design. That 220 psi pressure limit will be for a straight section of tube, and won't take into account any joints or other stress concentrations. As you have discovered, the difference between water and air is that compressed gases will expand rapidly upon initial failure of the pipe, and may well cause the crack to propagate at a vast rate of knots, possibly taking some young limbs or lives with it. (Note that this isn't to say that hydraulics are inherently safe...)

I'm always very wary about cheaply made/poorly designed/bodged pressure systems (and I've seen the astonishing amount of damage caused when a hose blew off a pump for a forging press) so my last word will be this:

If you're genuinely concerned about safety, and if you've seen advice not to use PVC for compressed air, then don't use PVC for compressed air.




Edit: By the way, I don't work in, and am not fluent with, imperial units (mainly out of principle!). If 70psi really is a pissy amount, then please forgive me for being a wimp.

 

[mpm]

This may help clear things up a little. You still may not be able to give an answer.

I've decided to change the design slightly after typing that first one.

The pressure reservoir will be 3" Schedule 40 PVC that is 24" in length.

It will have a 3" end cap that is cemented on. I will then use a bushing or some type of reducer to get it down to 1" PVC. Then there will be a 1" Tee so that I can put the air valve in it to attach an air hose. 1" PVC will be used on the other side of the Tee. THen it goes to a 1" elbow. THen to 1" straight piece. Then another elbow. After that I will attach a air regulator with gauge. THen I will have a quick relelase valve after that with 1" PVC connecting the two. THen I will need to go back up to a 3" diamater PVC for a 36" barrel.

This may not help at all or it might. This is just the rough design. I realize that 1" PVC has a higher pressure rating; however, the pressure will be greatest at the elbows and Tee's. However with the 3" diameter as my reservoir instead of 4", the max pressure rating goes up to 260 psi.

I'm thinking as long as I don't go over 70 psi, I should be fine.

I realize that 260 psi is for water, but even so, I'm no where near that number.

Could you comment on this?

 

[Danger]

Hmmm... not an area of great knowledge for me, but my gut feeling is that you have an awful lot of potential failure points. I'd be more concerned about the joints and turns than straight sections.

Brewski, I don't know how to convert units, but a standard car tire runs about 32-35psi. I believe that someone in PF once said that hi-perf bicycle tires are about 70.

 

[mpm]

70 psi is about 483 kilopascal.

Does that help any?

 

[FredGarvin]

70 psi is about 480 kPa. It's a pressure that should be respected even though it is less than most shop air systems in industrial settings (usually 110 psi).

The one thing you need to understand about the water rating with PVC is that it is a static pressure test conducted with water being the fluid. So that rating does not take into account things like hammer effects and rapid expansions. Both of those things you will be experiencing with this gun. Also, one thing to keep in mind is that those expansions (if using solely compressed air) will cause temperature drops. PVC does not like low temps.

Since this is for a senior project (HS I am assuming), what is the number of times this thing will actually be used? It had better be used in a controlled environment. I have seen plenty of people make potato guns out of PVC with no problems, but they are exttremely simply with only one joint. If you are going to build this thing, I would have two suggestions:

Are you going to use some kind of explosive propellent like lighter fluid or is it simply going to be compressed air?

 

[mpm]

I'm actually in college. I will be using this gun to test building materials for impact damage. I'm trying to see if these coatings they put on sheet metal are actually effective or not.

I will be using compressed air only. I really don't know how much use this thing will get. I'm not really sure if people will use it after I graduate or not.

I figure I need to simplify this gun as much as possible so I don't have too many points of possible failure.

 

[FredGarvin]

You can impact test items using other methods. I use a compressed air gun to do bird strike and hail impacts for our development and certification testing. They are a bit scary and are not exactly accurate, although we have gotten pretty good aim after doing it quite a bit. Every bit of ours is stainless steel and the pressure vessel (max 100 psi) was welded by a certified welder. You really need to develop a respect for pressure vessels.

What are going to be your feedback parameters for the impact tests? How were you planning on controlling the velocity of the projectile or was that not a consideration? Obviously mass and density of the projectiles are going to be controlled.

 

[mpm]

We were planning on placing the gun before a velocity meter. We plan to have an air regulator. Then just pick a pressure and fire it past the velocity meter and get the according velocity. Once we find the pressures that relate to a certain velocity for each diameter hail, we will know what to fire the gun at.

I have respect for pressure vessels which is one of the reasons I am even bothering to find out information for this. This project has a limited budget and I don't think we could afford to build a steel pressure vessel.

We are using ASTM standards for the testing methods.

 

[FredGarvin]

The velocity is going to be pretty variable even with setting the same pressure. We have found that the valve we use (a high speed butterfly) effects the flow just enough and not repeatedly that the pressure gets us in the rough area, but we usually have to do a few practice shots to nail down a good velocity.

INSERT YOUR FAVORITE LEGAL DISCLAIMER HERE.
Here's my official stance without seeing a drawing on how you are going to do this:

Precursor: Budgetary restraints are NEVER a reason to skimp on safety. If you want to think like that, go into management. There is never an acceptable reason to be unsafe in your practices, especially when other people can be injured. If I were you I would push the safety aspect heavily to get some more funds to make this thing properly. At least make the pressure vessel out of steel with a proper safety relief and fittings so you have a greater factor of safety. Steel pressure vessels usually don't fail catastrophically under these conditions. PVC can be another story.

1) The pressures should be OK, but have your entire gun assembly properly pressure checked before operating for the first time. Also double check the air temp when pressurized. Just check that against the type of PVC you are using. There are different grades with different temperature ratings.

2) Don't use this thing in a lab setting. Use it outside away from people and have a remote actuation. If you can't do that, put a dog house or some other kind of scatter shield around it.

3) Inspect the vessel often. Ideally, inspect after every time you charge discharge the pressure vessel. Watch for cracks.

4) Make a styrofoam insert pto put in the barrel that has a cut out in the dead center of it. You can put a small laser pointer in the styrofoam insert and it makes it eassier to aim.

 

[mpm]

Thanks for all your help on this Fred. The design isn't final by any means. I am just having to do a proposal for it. I will look into steel for the pressure vessel and see what I can do. Who knows, maybe I can pull it off and get one.

Thanks again.

 

[FredGarvin]

Good luck.

 

[Integral]

May I share a story about a PVC vacuum/pressure device and safety?

A few years ago I was working with precision sandblasters, part of the process was venting our 170 PSI air/sand mixture into a vacuum dust collector system. The problem was that we needed a good free flowing vent that could with stand the abrasion caused by our 25micon aluminum oxide dust (sand). My solution was to feed our vent lines into the end of long straight section of pipe which was evacuated by the vacuum system.

I envisioned something less then .5m long. The prototype ended up being something over 1m long and, even better, when the tech I was working with, slipped the end cap with the vent hose fittings onto the 2" PVC pipe, it jammed as PVC seems to do. We thought, heck its a vacuum system what is the problem. So we installed it WITHOUT GLUE!

Didn't I mention 170psi? Well eventually, unknown to us, the long tube filled with sand clogging the system. It just so happens this cannon with its end cap was mounted horizontal at just below belt height. Of course the day came when the tube was hit with a burst of 170PSI air/sand which blew the jammed on end cap off, hitting one of our more understanding operators exactly where no man wants to be hit. The ensuing safety investigation was an embarrassment and a lesson in attention to detail. We ended up with a 1' long section of steel pipe with a screwed on end cap.

 

[FredGarvin]

Oh man...I can just imagine having to talk to our safety folks about Bob getting hit in the gnads with an endcap shot across the room.

Good lessons learned there.

 

[Danger]

While I couldn't help having a wee bit of a giggle over Integral's story, it certainly does illustrate the perversity of mechanical devices. I'm sure that the poor fellow suffered for a limited time, but a head shot could have killed someone.

 

[mpm]

I talked with my group members and we had a new idea for the gun. One that might make it cheaper, safer and easier to work with.

What is yalls opinion of this design?

We will be working in a lab so we will have a compressed air hose around us at all times. What if we made the barrel out of 3" D PVC Pipe, the converted it down to 1"D PVC, then it goes into a 1" Quick Release Valve, then it goes back to 1" D PVC then up to 2" PVC (which is the expansion chamber) then back down to 1" which is then connected to an Air regulator and just on the other side is the valve stem for the air hose to connect.

This will be in one straight line so there won't be as many possible failure points. We can just regulate how much air we want into the expansion chamber and then release it with the valve.

It would work just like a pneumatic tool pretty much.

Am I missing anything here or does this look good?

 

[Danger]

I don't have time to try and visualize it properly, but it certainly sounds better than the first one.
I should contact the Royal Canadian Air Farce at CBC and ask them how their 'chicken cannon' works.

 

[FredGarvin]

Danger, I HAVE A CHICKEN CANNON! If you're nice, I'll let you come play with it. We'll lob chickens at the Target store parking lot near by.

It sounds like you may have one too many sections in the chain (unless I am just mis reading you). You have thee right idea though. Really all you need is a tank of some kind, the valve and the barrel. The valve is a 1" valve? What kind of connections does it have?

 

[Q_Goest]

Hi MPM. I can understand why Fred and others here feel a strong twinge of concern when someone in school suggests using PVC for an air tank. We've all seen explosions from pressurized tanks and pipe, and students don't typically have the experience to know how to design and test pressurized systems. Sure, we've all seen potato guns, but we don't know what it is you're really planning to build, it sounds a bit more sophisticated. And besides, potato guns have been known to explode.

The piping codes in the US apply to these systems. This would fall under ASME B31.3, Appendix A. Note appendix A refers you to the main body of the piping code for most calculations. I'll be using this as a guide to make suggestions.

Regarding materials, PVC pipe falls under ASTM D 1785 or D 2241. The fittings fall under a variety of ASTM standards including ASTM D 2464, D 2467 and D 2468. There are others. If you decide to use PVC, one thing that would help ensure safety would be to verify one of these numbers is printed directly on all the pipe and fittings you use. If they don't have these ASTM numbers printed on them, they probably don't meet the minimum standards and shouldn't be used.

Cements used for PVC pipe should meet ASTM D 2564. Verify the stuff you purchase meets this code. Follow the directions on the can.

None of the pipe should be threaded except for purchased fittings. Everything should be glued using the appropriate cement. If you cut any threads, you're doing something dangerous.

At 70 psi, you have a roughly 3 to 1 safety factor on top of the standard safety factors. The "stress allowable" for PVC at 100 F is 1.6 ksi which says the pipe is good for roughly 210 psi operating pressure. If temperature exceeds this, the pressure rating drops off very quickly, I'd not recommend it above this temperature. The code also prohibits its use below 0 F. If the plastic gets cold, which it could because of the expanding gasses doing work on whatever, wait till it is warm again before pressurizing. Cold plastic is more likely to break.

Consider what external stresses you are imposing such as bending of the pipe between supports. The pipe should be reasonably well supported over long sections.

Once you've manufactured the piping, do a pressure test on it using water. Fill with water, make sure there is no air in the system, and then pressurize to at least 200 psi, preferably 300 psi. Hold this pressure for 5 minutes, then release the pressure and verify there are no leaks or distortion of any part. If it holds this pressure, I'd feel a lot better about using it.

I'd also suggest repeating this pressure test every 100 cycles or whenever you suspect damage or leave it sitting for extended periods of time. Cyclic pressure and other uncontrolled factors could damage the piping, regular testing will help prevent catastrophic explosions.

Verify all valves you use are rated for the given pressure.

You really should have a relief device, set at about 100 psi, immediately downstream of your regulator, just in case the regulator fails.

For a pressure source, do not use any source of pressure above 150 psi. High pressure cylinders are extremely dangerous.

Off hand, I can't think of anything else to warn you about. Maybe the others here could suggest specific considerations.

 

[Danger]

Danger, I HAVE A CHICKEN CANNON! If you're nice, I'll let you come play with it. We'll lob chickens at the Target store parking lot near by.

If I ever had any doubts about your friendship, they're gone now.

It sounds like you may have one too many sections in the chain (unless I am just mis reading you).

You might be. Your parents would understand it straight off. The Royal Canadian Air Farce is a comedy quartet with their own show on CBC. One of their segments is where viewers nominate a target for the Chicken Cannon, and the best is chosen each week. For instance, the target will be a picture of George Bush. The cannon is then loaded with appropriate ammunition such as a can of beans because that's what he's full of, a litre of oil because that's why he invaded Iraq, a splash of $20 700 Huntley Street holy water to represent his hypocritical Christianity, etc.. Then they blast the **** out of the picture with it. I think that it's just a simple compressed air gun, but a lot bigger.

 

[mpm]

I realize I made the description kinda worried.

Basically I have a 3" diameter barrel, connected to a 1" Quick Release Valve, that's connected to a 2" expansion chamber which is connected to an air regulator and on the other side of the regulator is where the air hose will be connected.

The reason I thought an expansion chamber would work is because there is more volume to work with. Since there is more volume, it would take more air to fill up the chamber to get the desired pressure. Then there would be more air to propel the ice balls. I figured this would be more efficient than just using a 1" piece of PVC as the "expansion chamber".

 

[FredGarvin]

Your parents would understand it straight off. The Royal Canadian Air Farce is a comedy quartet with their own show on CBC.

We used to watch the RCAF all the time when I was a kid. I haven't seen that show in ages. The only way for me to get CBC is with rabbit ears and I just can't bring myself to put them back up except during hockey season (which I am late for).

 

[Danger]

The only way for me to get CBC is with rabbit ears

That's the only way I get anything. I've got 3 VCR's in front of the TV, which act as pre-amps, then about 250' of coax going around the corner, over the bathroom door (held in place by a critical coathanger), up the stairs, coiled up in the hall closet with carefully placed suitcases on top of it, back out to a game/TV switchbox, into a few feet of 75ohm twin-lead with tinfoil wraps, and up through the attic hatch to a set of bunny ears. And at that, the extension cord that powers one of the VCR's and DVD player has to be lying along side it, in contact, for the picture to be good (even then, ch. 13 sucks). It took me forever to fine-tune that system.

Now that I've pretty much moved in with N/W we have cable, but I still only watch the 5 stations that I could get off the air.

 

[Russ[]]

I've made patato cannons and used ether to shoot them. I think you're fine.

 

[Danger]

I've heard of that being done a lot, but I sure wouldn't mess with it. I remember from my high-school days when some guy who didn't read the instructions squirted about half a can of Quick-Start (ether) into each cylinder of his flat-head Ford V-8 and tried to turn it over. Both heads were blown out through the hood.
It just crossed my mind to ask about fibreglass as a possible substitute for the PVC. It seems kind of ridiculous on the surface, but when I was working on the paintball markers our pressure supply was a series of 5 fibreglass tanks with a manifold, running at 2,500 psi. It was regulated down to 1,200 in the (steel) lines, then 800 going into the guns. I have no idea how thick those tank walls were, though, or what reinforcement was involved.

 

[brewnog]

It just crossed my mind to ask about fibreglass as a possible substitute for the PVC.

Steady fella...!

GRP pressure vessels (and pressure-holding piping) are made by a filament winding process, - a few filaments wound helically around a mandrel, with the resin applied to the filament as it it wound. This way, you get super-uniform, predictable wall thickness, some form of regular anisotropy, and probably some benefit from having continuous filaments too, since they're pretty much aligned in the direction your hoop stress acts. I think you'd have issues using CSM layup or anything like that, because for a pressure vessel you'd need a huge FoS and therefore need to go very thick. Probably still worth a thought, but beware if comparing filment winding with chop layup.

 

[Danger]

Roger that, Brewski. I sure didn't mean to suggest that he make his own tubing. It just occurred to me that maybe he could buy the pre-made stuff cheaper than metal. Even then, I have no idea about adhesives for it. Thanks for mentioning that; I'd hate to see him blown up over a misunderstanding.

 

[lilhudi]

hello

i today make a air cannon

1-4 inch by 2 foot pvc air resivoir

1-4 inch pvc capper

various pvc male and female adapters

a single copper ball valve

and a 6 foot barrel

i went to a park by my house to test it after the pvc cement set and pumped it up couple times with increasing air pressures
10-20-40-60-80

its fun to shot potatos

any way the pvc held up great no ruptures no leaks

i will take it up higher tomarrow -i got a air compressor

and way just thought you should know that if you correctly aply the primer and cement it should be fine up to 150 psi