I am building an air cannon with a 100 psi compressor, 2.5" PVC barrel, and a tennis ball projectile. What type of valve should I use?

 

 

I would suggest some type of ball valve rated above 100 psi for your cannon. They make 2.5 inch ball valves like this, but they are very expensive. So I would suggest you properly seal one end of your PVC and insert a smaller adapter that you could connect to a smaller ball valve, like 1/2 inch, as it would be more cost effective.

 

However, when you are making connections be very careful to make sure the connections are secure as anything under pressure can cause great harm if a connection were to fail.

Ball valves can take too long to open to a sufficient flow coefficient for the short pressure profile of an air cannon. In addition as DJBruce indicated they are expensive. If you do use a ball valve you will definitely need a very large one to overcome the low initial flow coefficients when the valve is opened just a bit. To see this observe the profile of cross sectional area as a function of % open and contrast that to a slide valve. I can help you determine the required valve size based on the desired energy/pressure profile you are looking for based on muzzle velocity. No sense guessing.

 

In addition ball valves are unbalanced and commercial ones are designed to seal tight thus for both reasons require too much opening energy. I suggest you obtain or construct an inexpensive balanced slide valve. For only 100 psi you can make or buy one made of thin pvc or similar. Consider using lubricated O rings for seals or add a small bleed hole to your pressure chamber to adjust for leakage. Spring load the thing so it opens as rapidly as possible. Good luck.