2" PISTON VALVE
My goal with this valve was to come up with a design that could be built using stock PVC parts with minimal modifications.  Every peice is of-the-shelf.  Minor modification is neeeded for the piston parts, but I easily accomplished these with a few hand tools.  With a lathe and a power saw of some kind, I figure total construction time would be less than half an hour.  The key to the valve is the fact that 1.5" pipe fits perfectly inside SCHD 80 2".  I have not yet looked for other larger sizes that fit together as nicely.  For this reason, I'm not sure if the valve could easily be scaled up to larger pipe sizes.

The valve is barrel-sealing.  The chamber attaches to the top of the Tee, and the barrel to the Schd 80 elbow.

The valve is housed in a 2" T fitting and a 2" endcap connected by a peice of SCHD 80 2" pipe. 

The piston is made from a peice of 1.5" pipe and a flat 1.5" endcap.  It fits inside the 2" pipe of the housing.  I shortened the endcap a bit to give the piston more travel.  In hindsight I should have shortened it more, but I may just make another piston anyway out of SCHD 40 to lower the weight. The seal is created by a peice of 1/8" neoprene bolted to the piston face.

  A peice of 1.5" SCHD 80 pipe extends out of the Elbow. There is a ring of SCHD 80 2" permantly attached aroud it which fits in the left side of the Tee.  The assembly slides in and is secured with screws (not pictured), so that it can be removed to get the piston out if necissary.

The valve is actuated by a pilot valve screwed into the 2" Endcap of the housing (also not pictured).  When the pilot valve is opened, the piston slides open (left) and things start hauling ass out of the barrel.  A coil spring returns the piston to the closed position after firing

UPDATE - 6/8/06

It finally works!  It only took me two years of procrastinating to get this one from the drawing board to reality, but it actually fires.  Good Lord does it fire! 
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