spud gun/T-shirt gun valve

[fovea1959]

Originally Posted by D.Allred

Air is a compressible fluid. Water is not. Rapid expansion of compressed air when released will turn debris into shrapnel. Do not use any valve unless it is rated for compressed air.

David

ah. There's the answer. It's not that the valve is more likely to fail at 70PSI air than 70 PSI water, it's that *if* it fails, the damage is more catastrophic. makes sense.

ok, so I shouldn't use Schedule 40 PVC for the tank or anything "upstream" of the shooting valve, what *do* I use?

[hank2247]

For what it's worth, my team has been using 1" sprinkler solenoids on our T-shirt cannon robot for over 3 years without any problems from the valves. (We use them up to 120 Psi)

[GeeTwo]

Originally Posted by fovea1959

ah. There's the answer. It's not that the valve is more likely to fail at 70PSI air than 70 PSI water, it's that *if* it fails, the damage is more catastrophic. makes sense.

ok, so I shouldn't use Schedule 40 PVC for the tank or anything "upstream" of the shooting valve, what *do* I use?

We're using mostly galvanized pipe; I think there are a couple of pieces of black iron in there, too. Here's a picture with the cowling off.

We're getting air from a scuba tank, using a regular scuba regulator to get down to about 170 psi. We then use a regulator similar to the FRC KoP regulators to bring it down below 60 psi (we typically shoot about 50 psi) and direct that to a cast iron accumulator (painted green, next to the scuba tank). I believe we then have 1-1/2" galvanized to the tee, and 1" galvanized to the 3/4" solenoid valves (you can only see the brass side here, right next to Leevi's hand; the solenoid is behind that). Then there's a 3/4" quick disconnect, some factory-terminated reinforced rubber hose, a black iron street elbow, and finally the first bit of PVC in the expansion to the barrel. We will likely be switching to all galvanized (apart from the tank, solenoid valve, and hose) for next year.

Another reason not to use PVC, as has been noted before, is its failure mode. It tends to break, whereas most metals and fiber/wire reinforced hoses will tear. Therefore, PVC and similar plastics are more likely to become shrapnel than one twisted and torn piece.

[GeeTwo]

Originally Posted by hank2247

For what it's worth, my team has been using 1" sprinkler solenoids on our T-shirt cannon robot for over 3 years without any problems from the valves. (We use them up to 120 Psi)

NASA ran over 100 shuttle missions before a chunk of shuttle insulation struck the leading wing edge, resulting in the demise of Columbia and her crew. Just because you get a way with something for a while doesn't make it safe.

[hank2247]

True, but everything on the cannon is rated for a much greater pressure than we ever put through it including the sprinkler valve. we also have built in points of failure to avoid any PVC explosions or fragmentation. (pressure cylinders are not made from PVC)

[weberr]

How to inspectors test compressed air tanks and steam boilers? They fill them with water and hydro test them. If the tanks fail, they just split and water squirts out and falls to the ground. When a steam boiler ruptures with steam, bad things happen. http://lubbockonline.com/stories/073...l#.VeNOZvlViko

Now when an air tank goes, it isn't pretty: https://www.youtube.com/watch?v=KVP_A7eGYxw

When using piping or valves, check for the "WOGS" rating. W= Water, O=oil, G= Gas, and S=Steam. Some valves might have 100W 60GO 40S, meaning 100psi for water, and 60psi for both oil or gas, and 40psi for steam.

So in short, 100psw water is not the same as 100psi Steam, nor 100psi Oil, and not 100psi Gas. You can have a really bad day if you mix them.

I had a neighbor use plastic water pipe to plumb his garage shop, it was left over from replacing his water feed lines in the house. The house pressure was 65psi and rated for 100psi. He ran 80psi air in the same pipe. One day there was a large bang as he was working in the garage. They took a 3" long peice of the plastic pipe out of his chest. Needless to say, when he recovered out went the plastic pipe and in went black pipe.

Be safe. Go the distance make sure you stay safe.

[Mr V]

The other thing that no one has brought up concerning PVC is that it is degraded by UV rays by making it even more brittle. Like all plastics it also off gasses, also increasing its brittleness. So the risk of failure increases with time.

[Mykey]

I will not repeat the value of having the correct materials for the job - safety first!

That being said, many of the valves we are talking about have a diaphragm in them that is held in place using a spring. If you change (lower) the pressure applied by the spring it results in a more abrupt and greater release of air flow. If you lower it too much then the valve lets go on its own when the pressure reaches the lowered break point.

So... replace the spring with a weaker one. Or the "poor teams" method is to clip small pieces off of the spring bit by bit until it has the pressure setting you want.

[Daniel_LaFleur]

Originally Posted by hank2247

True, but everything on the cannon is rated for a much greater pressure than we ever put through it including the sprinkler valve. we also have built in points of failure to avoid any PVC explosions or fragmentation. (pressure cylinders are not made from PVC)

1> everything but the PVC parts may be rated for high pressure, but PVC is NOT rated for gas.

2> universal gas laws -- pressure is exerted on all surfaces equally. Thus your PVC parts are seeing the same pressure as all other items regardless of their pressure ratings.

3> T-Shirt cannons are, by design, to be near the public (within shooting distance). Knowingly using non-gas rated parts in a gas (air) cannon may be gross negligence should something happen (I'm not a lawyer but this seems a reasonable argument).

All I'm saying is in this lawsuit culture, you may want to consider overdesigning a bit for safety.

[GeeTwo]

Originally Posted by Mykey

So... replace the spring with a weaker one. Or the "poor teams" method is to clip small pieces off of the spring bit by bit until it has the pressure setting you want.

And my advice is not to do this - get the right thing to start with, unless you know what you're doing (and by that I mean certified). There's a reason that FRC rules do not permit any modification of pneumatic devices, apart from cutting tubing and making adjustments that were specifically designed to be adjusted by the user/consumer. You got it in one: .

[Mykey]

Originally Posted by GeeTwo

And my advice is not to do this - get the right thing to start with, unless you know what you're doing (and by that I mean certified). There's a reason that FRC rules do not permit any modification of pneumatic devices, apart from cutting tubing and making adjustments that were specifically designed to be adjusted by the user/consumer. You got it in one: .

You are correct. It is by far better to have the correct component from the start. The high pressure hydraulic systems I am responsible for are bad enough. Pneumatics are a whole new level of scary. On reflection it is not a good idea for the average person to play with the inner workings of a valve system.

Unfortunately, the majority of T-shirt launchers are not based on well engineered designs. They seem to be based on you-tube videos and the desire to shoot things. Thankfully most of us who have managed to make it to adulthood have managed to suppress the urges that led us to playing with fireworks, slingshots, pea shooters and home made darts.

Although, I have to admit I still have some of that garage inventor in my blood... must resist the urge...

To those who will be making T-shirt launchers: They are inherently dangerous. Please o' please if you are doing this stay conservative (high pressure ratings for lower pressure applications)!

[FrankJ]

Originally Posted by GeeTwo

And my advice is not to do this - get the right thing to start with, unless you know what you're doing (and by that I mean certified).

I will bite. So how did you get certified?

[GeeTwo]

Originally Posted by FrankJ

I will bite. So how did you get certified?

Sorry, did I imply that I was certified for pneumatics work? One of our mentors, Isaac, is a certified pneumatics tech for Ingersoll Rand. While we've never spoken at length about it, I understood that there was a combination of course work, practical, OJT involved. I'm just a physicist who can crunch enough numbers to know that this is too ugly for a jack-of-all-trades to play around with.

I never had a biology course after high school, but I also advise that you don't perform surgery on your family members without the appropriate medical certifications.

[FrankJ]

Originally Posted by GeeTwo

Sorry, did I imply that I was certified for pneumatics work? One of our mentors, Isaac, is a certified pneumatics tech for Ingersoll Rand. While we've never spoken at length about it, I understood that there was a combination of course work, practical, OJT involved. I'm just a physicist who can crunch enough numbers to know that this is too ugly for a jack-of-all-trades to play around with.

I never had a biology course after high school, but I also advise that you don't perform surgery on your family members without the appropriate medical certifications.

Sorry I was not questioning your credentials, although it does read like that. From your posts, you seem well informed about what you write about. I meant to how does some one get certified in pneumatics in general.

Generally speaking techs work from a set of rules where engineers are able to work from first principles. In the real world this can get very blurry. I know techs & mechanics that know for more about their specific subject than the engineers that are supervising them. The world needs both kinds & hopefully that is what we are developing.

[fovea1959]

to all: thanks for all the input and discussion. We scrapped the PVC tanks and valves (kept PVC barrels), went to steel 1/2 gallon air tanks (holed sawed a hole in one end to remove the 3/8" fitting and had a 3/4" or 1" bung heliarc'd in). Valves are 12V metal valves (used for clearing the bags in dust collection systems?).

MrBasse shared a photo album with *their* shooter, and gave permission for me to link to the album. They have "cartridges" that they can preload... https://drive.google.com/folderview?...&usp=sha ring

I'll post pictures of where we ended up after scrapping the PVC tanks and valves.