off-season launcher

[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

[Bryan Herbst]

Originally Posted by Da Kid

Tanis, by the way you answered the questions it seems you have a different set up than we plan.

this is what we are planning to do:
Main tank(120 psi)-->pressure regulators-->A secondary tank for each of the three barrels(different psi's)-->Sprinkler valve-->actual barrels.

Ah, you are correct there. We have one barrel hooked up through a sprinkler valve to our tank. On the other end of the tank we have a compressor with regulators.

Though I am by no means an expert in pneumatics and the underlying physics, I am fairly certain that a pre-charged 5 gallon tank would prove just as problematic. Keep in mind that 120 PSI means 120 pounds of pressure per square inch. As you have 5 gallons worth of volume, you still have far more total stress on the compressor.

I could be wrong there though, so don't quote me!

[PAR_WIG1350]

Originally Posted by Tanis

Ah, you are correct there. We have one barrel hooked up through a sprinkler valve to our tank. On the other end of the tank we have a compressor with regulators.

Though I am by no means an expert in pneumatics and the underlying physics, I am fairly certain that a pre-charged 5 gallon tank would prove just as problematic. Keep in mind that 120 PSI means 120 pounds of pressure per square inch. As you have 5 gallons worth of volume, you still have far more total stress on the compressor.

I could be wrong there though, so don't quote me!

Now, I know you said not to quote you, but I just wanted to clarify something(s). I don't know if this is what you were trying to say, but if it is, then it is just an extension on that.

1) The piston in the compressor has a set (frontal) surface area [which for the sake of simplicity we will say is one square inch]. If the compressor is filling a 1-inch cube to 120 psi, it would need to push with 120 lbs of force to move the piston (1x120=120). Now, if we increase that to 5 gallons, 1x120 still comes out to be 120 lbs. The volume of the tank doesn't change the amount of force needed to push the piston in the compressor. The (frontal) surface area of the piston and the pressure are the only factors involved in this.

2) Volume and surface area are not related in any particular manner, without specifying a specific shape such as a sphere. Thus, the total force applied to a vessel at any given pressure not directly related to the vessels volume, while it is directly related to its surface area.

3) The compressor will still have to do more work to compress a larger volume of air simply due to the fact that more air must be moved, however, at a given pressure, work per compressor cycle (piston stroke) is constant.