off-season launcher

[NickE]

Originally Posted by Team2374

While it is certainly better to use components that are rated for air pressure, I found in my own (not too extensive) research that large enough solenoid valves (1" diameter) would be prohibitively expensive for some teams. However, if you know of some source that has such valves for cheap, I would definitely look into it.

And before someone says that an catastrophic failure would also be "prohibitively expensive," I guess that's just something teams will have to consider. I suppose the other option is to just wrap the sprinkler valve in layers (and layers (and layers)) of duct tape.

A catastrophic failure would be more than expensive. It has the potential to seriously injure team members or spectators regardless of how much duct tape you wrap around it. Is that really a risk you, your team and your school are willing to take to save a few bucks?

[IanW]

Hence why I put "prohibitively expensive" in quotes the second time.
I would still like to know what valve 254 uses - might make my entire point moot.

[NickE]

Originally Posted by Team2374

Hence why I put "prohibitively expensive" in quotes the second time.
I would still like to know what valve 254 uses - might make my entire point moot.

From our team website:
1" NPT MAC Solenoid Valve at ~17.0 cV Flow Capacity with a 23 ms Response Time.

For the increased reliability, safety, performance and peace of mind, buying a quality valve was well worth the investment.

[OSU_Springer]

Originally Posted by NickE

From our team website:
1" NPT MAC Solenoid Valve at ~17.0 cV Flow Capacity with a 23 ms Response Time.

For the increased reliability, safety, performance and peace of mind, buying a quality valve was well worth the investment.

An ASCO 8210G004 12 VDC solenoid valve is $218 LIST. You can probably get this cheaper if you go to a distributor and explain that you are a school related team. This is a 1" NPT valve, with a full bore orifice. If you step up to the 1 1/2" NPT 8210G022 12VDC solenoid valve, you get a much higher Cv (22.5 vs 13) but your price goes up to $405.

Both of these valves are only rated for 125 PSIG, so be careful not to try climbing above that. The biggest thing with building the air cannon is making sure that you can get an appropriate amount of air into the barrel in as short amount of time as possible. That's why it seems that higher pressures are better. You'll only need a limited amount of pressure if you can get the flow high enough. This of course also depends on your secondary accumulator (tank) volume. Make it too small and you won't have enough air to accelerate the projectile to a proper velocity, make it too large and you are just wasting energy (assuming equal barrel lengths).

A note on compressor usage: Do not expect kit compressors to last very long if you are using them constantly. They are surely not rated for anything higher than a 10-15% duty cycle. This means that they are only rated to run at the pressure rating for 10-15% of time. In other words, 6-9 minutes out of every hour. Running them more than that risks overheating the compressor and burning something out.

[Bryan Herbst]

Originally Posted by NickE

A catastrophic failure would be more than expensive. It has the potential to seriously injure team members or spectators regardless of how much duct tape you wrap around it. Is that really a risk you, your team and your school are willing to take to save a few bucks?

Out of curiosity, have you ever heard of an incident involving a sprinkler valve failure?

There are plenty of stories about and warnings against using PVC to store air, but I have never before heard someone claim that using a sprinkler valve would be dangerous.

Sprinkler valves are typically very solid. I wish I had a data sheet on hand for one of them. If someone does, could you read off the ratings?

[Travis Covington]

Originally Posted by Tanis

Sprinkler valves are typically very solid. I wish I had a data sheet on hand for one of them. If someone does, could you read off the ratings?

Typically solid when used for gardening? I don't understand the argument here. Sprinkler valves are rated for water pressure. A plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would. Use a valve rated for air pressure if you are even remotely concerned with safety. Water is incompressible and behaves completely different than air when pressurized. Please refer to the countless other threads here where this has been discussed before. Bottom-line; please don't cut corners when people’s safety is at risk.

[Bryan Herbst]

Originally Posted by Travis Covington

Typically solid when used for gardening? I don't understand the argument here. Sprinkler valves are rated for water pressure. A plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would. Use a valve rated for air pressure if you are even remotely concerned with safety. Water is incompressible and behaves completely different than air when pressurized. Please refer to the countless other threads here where this has been discussed before. Bottom-line; please don't cut corners when people’s safety is at risk.

Yes, they are rated for water pressure. That doesn't mean that they are at a high risk for failure when using air. It simply means that they need to be tested before being used with compressed air.

As for "a plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would:" Not true. PVC used as an air storage tank almost always fails at the end caps or connection points, typically due to the a failure in the cement used to join two parts. If it isn't an endpoint, then it's because there was a crack somewhere along the body. If you are operating at reasonable pressures, a PVC tank will not spontaneously explode.

I have very carefully watched every thread on CD relating to air cannons and PVC air storage. I would never use PVC to store air. However, you may notice that every single team in those threads intends to use (and many have been using) sprinkler valves. None of them have reported any problems, and no one before has ever called them out on the sprinkler valve.

[Travis Covington]

Originally Posted by Tanis

Yes, they are rated for water pressure. That doesn't mean that they are at a high risk for failure when using air. It simply means that they need to be tested before being used with compressed air.

As for "a plastic sprinkler valve would likely fail in the exact same way a PVC storage tank would:" Not true. PVC used as an air storage tank almost always fails at the end caps or connection points, typically due to the a failure in the cement used to join two parts. If it isn't an endpoint, then it's because there was a crack somewhere along the body. If you are operating at reasonable pressures, a PVC tank will not spontaneously explode.

I have very carefully watched every thread on CD relating to air cannons and PVC air storage. I would never use PVC to store air. However, you may notice that every single team in those threads intends to use (and many have been using) sprinkler valves. None of them have reported any problems, and no one before has ever called them out on the sprinkler valve.

I still encourage you to do further research before using a sprinkler valve. While there may not be any documented failures that you have found thus far, the associated risks should not be taken lightly. Explaining in near absolutes how something will not fail is not helping your case here, either. I do not believe you know how all PVC storage tanks fail, and also do not believe you know all of the failure modes of a sprinkler valve. Objectivity is key, and building something safe which your school, students, and team can be certain won't harm anyone is my only concern. I posted initially because it seems like many people on these forums are not taking the safety of these cannons seriously. Please do not take my posts as personal attacks, because they are far from that. Using internet forums, youtube videos, and tech blogs to determine the safety of processes, mechanisms, materials, and hardware scares me and I hate seeing corners being cut with regard to safety in an effort to save a few bucks. Please just be as cautious as you can when proceeding.

[Akash Rastogi]

Originally Posted by Tanis

Yes, they are rated for water pressure. That doesn't mean that they are at a high risk for failure when using air. It simply means that they need to be tested before being used with

Why go through haphazard testing on a component's limits while another product is already rated by the company's engineers through rigorous testing and with a high safety factor to perform the exact same task in a safe manner? It would be like testing the limits of 150 pound strength steel cable and because under my own tests I see it is fit to lift 170 pounds of material instead of purchasing something rated at 175 pounds or 200.

The same conclusions drawn about the unintelligent PVC storage tanks should be applied to other points of potential failure. Although I'm not an engineer like Travis, common sense would tell me to just purchase the valve that can handle the pressure well beyond what you will be using. Its not something sensible to cut corners with.

+0.02