spud gun/T-shirt gun valve

[fovea1959]

The team took on an off-season project that includes a T-shirt gun. It's all working, but the range is a little short. We're using a sprinkler valve to shoot (Rainbird, I believe), shot tank is going to about 50-70 PSI...

One of the students in town from another team with spudgun experience thinks that part of the problem is that the stock valve does not open quickly enough, and suggested making one of the mods commonly seen on youtube; it appears that the mods involve not actuating the valve with the solenoid, but venting the back side of the diaphragm to atmosphere to snap it open. The problem there is that you need a smaller valve to open the back side; the examples on youtube are all manually actuated.

Has anyone done a sprinkler valve mod, but kept things under computer control? I think I know how to do it using a FESTO or SMC to vent the back side, but I sure don't want to reinvent any wheels. ("I'm a programmer, Jim, not a design engineer....")

[GeeTwo]

Stop right there!

Do not use a plastic sprinkler valve (designed for water) to switch air pressure. When the valve fails, it will explode and fragment, making this a serious safety hazard.

We used a valve similar to this for our air cannon (we used a 3/4" version). Note that it is rated for air as well as water.

For the same reasons, do not use PVC or similar plastic pipes to carry air pressure.

Originally Posted by fovea1959

The team took on an off-season project that includes a T-shirt gun. It's all working, but the range is a little short. We're using a sprinkler valve to shoot (Rainbird, I believe), shot tank is going to about 50-70 PSI...

Also, in general, larger valves will allow you to get more kick out of a given air pressure. A good rule of thumb to start is to have the valve (and all plumbing from the air tank to the barrel) be at least 1/3 the diameter of the barrel.

[fovea1959]

so what did you use for barrel and 1/3xbarrel diameter tubing?

[GeeTwo]

Originally Posted by fovea1959

so what did you use for barrel and 1/3xbarrel diameter tubing?

We originally had a 2" i.d. barrel and 3/4" port valves. We increased later to a 3" barrel, which helped with the footballs we were launching by reducing friction, but did not really increase the flight length of t-shirts that we had to roll "larger" to fit. We believe this is because the air flow rate was limited by the valve. Our calculations show that we could get better range if we went to 1" valves, but we haven't tested this out yet.

We also figured out how to roll t-shirts to shoot in the cannon that requires no tape, thread, zip tie, or other binding. Lots of other good tips in that thread.

We're in the process of improving for upcoming football games. A picture of the current cannon (taken Monday evening) is here. We did not change the barrels out for metal yet, but will do so before making the switch to 1" valves. We definitely do not have any PVC upstream of the valve. We've added a speaker this year, new wheels, steerable turret, LEDs, and are working on a cowling that will make it look more like a tank.

[evanperryg]

Originally Posted by fovea1959

so what did you use for barrel and 1/3xbarrel diameter tubing?

We just use a large solenoid. The back release valve is unnecessary, in our experience.

[BBray_T1296]

Originally Posted by GeeTwo

Do not use a plastic sprinkler valve (designed for water) to switch air pressure. When the valve fails, it will explode and fragment, making this a serious safety hazard.

This is no different to using the Clippard air tanks or most other FRC actuator (electric or pneumatic) Exceed specifications and you're gonna have a bad day.

Standard water mains operate around 55-75 psi, but reach as high as 100 psi, and sprinkler valves have to be designed to function in any regional pressure condition, with safety overhead for manufacturing tolerance reasons and otherwise. Since the valve will handle 100 psi and could likely handle 120, I don't think 50-70 could be an issue.

Heck, when a sprinkler valve closes, it has to absorb all of the inertia in the flowing water as well. That just means more pressure it has to be designed to handle.

I realize that the valves aren't air rated, but that really just means nobody tried. The principle behind pressure in compressible air and incompressible water is different, but the forces at the valve are the same (psi). There is no reason it would explode at those pressures, water or otherwise.

TL;DR, if you put 300 psi behind it, yeah it will probably explode, and yeah you would get hurt. But at 70 psi, I wouldn't be worried.

[fovea1959]

Originally Posted by GeeTwo

Stop right there!

Do not use a plastic sprinkler valve (designed for water) to switch air pressure. When the valve fails, it will explode and fragment, making this a serious safety hazard.

Why is running air at 70PSI more dangerous than running water at 70PSI?

[D.Allred]

Originally Posted by fovea1959

Why is running air at 70PSI more dangerous than running water at 70PSI?

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

[DonShaw]

Here is the Robo Lions Team 1261 T Shirt Bot

http://prhsrobotics.com/outreach/tshirtbot/


Uses 3/4" air solenoids with duel stage pressure reduction from a 3000 psi scuba tank. All barrels are encased in 1/2" poly carbonate and are 3" in diameter which are polycarb as well.

[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?

[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 fovea1959

Why is running air at 70PSI more dangerous than running water at 70PSI?

There are those who will tell you that 70psi air pressure is more likely to cause a rupture than 70PSI of water pressure at the time of switching. I don't disbelieve their arguments, but the much more compelling argument to me is this: Air at 70psi has a lot more energy than water at 70psi. That is, should the thing rupture, there's a whole lot more potential for damage.

For scale purposes, I shall use a single FRC Clippard tank (574 ml). I shall also simplify the problem to assume that everything is at the same temperature, 0C. It's a bit colder than the typical FRC playing field, but if you increase the temperature or recognize that air compressors heat the air up as they go, the situation only gets more dangerous.

Acknowledging the hazards of info from Wikipedia, and promising that I will verify them against the CRC Handbook on Monday when I get back to the office:

Originally Posted by wikipedia page for "Properties_of_water"

The compressibility of water is a function of pressure and temperature. At 0 °C, at the limit of zero pressure, the compressibility is 5.1×10⁻¹⁰ Pa⁻¹.[31] At the zero-pressure limit, the compressibility reaches a minimum of 4.4×10⁻¹⁰ Pa⁻¹ around 45 °C before increasing again with increasing temperature. As the pressure is increased, the compressibility decreases, being 3.9×10⁻¹⁰ Pa⁻¹ at 0 °C and 100 MPa.

...The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[32]

70 psi is equal to 483 kPa (from google units, but agrees well with my understanding that 1 atm ~ 14.2 psi and also ~ 10⁵ Pa).

Finally, recall that Energy is the ability to do work, so let's caculuate how much work each can do:

Let's assume that compressibility scales linearly with pressure over the range given for water. This means that at 70 psi, the compressibility at the end point is: (5.1 - (5.1 - 3.9) * 0.483 / 100) * 10^-10Pa^-1 ~ 5.095 x 10^-10Pa^-1. This is very little change (much less than the implied resolution), so I'll use a constant 5.1 * 10^-10Pa. This means that a clippard tank full of water at 70 psig which is allowed to expand to ambient will increase by about 574 ml * 5.1*10^-10Pa^-1 * 483 kPa = .141 ml (or about 1/7 of a cubic centimeter!). If it were pushing a piston it would do .141 ml * 483 kPa / 2 = 34 lPa = 34mJ, or .034 joules. For those who think in English units, a joule is about three-quarters of a foot pound, so this would lift one pound about 5/16 of an inch.

Now, let's do the same thing with air. To keep the math simpler, I'll assume that the temperature remains constant. Then, as we are talking about the same mass of air, we can use Boyle's Law, which is that PV = constant. For Boyle's Law, we must use total pressure, so that our end pressure is 1 atmosphere, or 101 kPa, and our starting pressure is 483 kPa + 101 kPa = 594 kPa. Our initial volume is again 574 ml. We have: 574 ml * 594 kPa = V * 101 kPa. Solving for V, we get 3370 ml. That is, the gas expanded by 2800 ml, or nearly three quarts. Because the pressure does not decrease linearly with volume, the average force is not just half the initial force, but a bit less. The work that can be done by this expansion is 332 l kPa, or 332 J. In English units, this will lift one pound about 244 feet!

In the actual case of an exploding tank or valve, the temperature will not remain constant. The rapidly expanding air will cool, decreasing the pressure and therefore the amount of mechanical energy released. Air is about 40 percent more compressible in the isothermal (constant temperature) than in the adiabatic (constant entropy, or very rapid) case. This means that the actual amount of energy released will be closer to 150 foot-pounds, but there's still five orders of magnitude more energy in the air than the water.

[Daniel_LaFleur]

Originally Posted by fovea1959

Why is running air at 70PSI more dangerous than running water at 70PSI?

This is a commonly asked question. and the reason is 2 fold:

1st is the compressibility of air vs the incompressibility of water. This means that when a vessel holding pressure water ruptures, all of the pressure is relieved with only a small fraction of expansion ( IE pushing of the shards). When a vessel holding air ruptures, the pressure is released over time carrying debris with it.

2nd is the failure mode of most plastics. When brittle objects rupture (such as PVC) the failure mode is to create shards that are sharp. Since air does not relieve all the pressure at once, it carries these shards with it as it expands to fill the 'new' area (relieves the pressure). This creates high velocity sharp projectiles .

Believe me when I say there is VERY GOOD REASON that most plastic volumes carry the warning that they are only pressure rated for liquids (take it from someone who has seen this up close ... and is lucky enough to still be here).

[EricH]

Just to further explain the danger of compressed air:

A sufficient volume of air at single-digit IWC (Inches of Water Column) can, if released suddenly, make its presence known throughout a brand-new building (by shaking if I recall correctly) with the building's biggest door wide open to vent any sudden releases of air straight outside. I wasn't in that building at the time--but I heard about it.

1 IWC = 0.036 PSI per Google's converter (I rounded).



And the item containing said air was, in fact, rated for gases.

[FrankJ]

We use an Asco 2/2 series main pulse valve. 8353C035. It is intended for back pulsing dust collectors. High flow rate & fast acting.