Quick Dump Valve

The Quick Dump Valve

The quick dump valve is a variation on a traditional barrel sealer piston valve. In the section on piston valves, various configurations are covered and the advantages and disadvantages of each. A quick dump valve may be used where the traditional valves shortcomings cause problems for the builder. It was noted in the piston valve section in the wiki that as a barrel sealer valve nears a 1:1 ratio, the pilot pressure needed to open the valve drops lower, and when they pop open, they do so with more force and speed.

Spool valves are balanced design valves with several o rings that move past ports and are operated by the rings crossing ports to join some in the same segment or separate them. They are noted for the ability to switch high pressures with little force to operate the valve. Typical uses are the hydraulic controls on heavy machinery such as a backhoe. Normal spool valves have both ends of the valve rod at atmospheric pressure.

A QDV when closed has the appearance of a spool valve. The air source under pressure is sealed off with an o ring on both sides of the port. What makes this different than a typical spool valve is when it is opened, the supply pressure is then applied to the end of the spool. This pressure provides a positive feedback to the spool so as it opens, the force opening it rapidly increases as supply pressure in the outlet port pushes on the piston.

Unlike a piston valve that uses pressure in a "Pilot" area to hold the valve closed, the QDV has no pressure on the pilot area of the piston to hold it closed. In this manor it is much like a spool control valve.

The first mention of the valve on Spudfiles is in this thread. One of the members noticed it before I joined Spudfiles and posted it. [1] The official contest where the design was posted is here; [2]

This is a video of the T Shirt launcher in action. They won the competition with the design. [3]

There are several QDV cannons on Spudfiles to show some options in building them. The oldest known Quick Dump Valve spudgun is shown below. It is made from non pressure rated cellular core ABS DWV pipe to test the properties of the pipe in air cannon applications. This cannon is still functional in 2009.

The oldest is constructed with ABS. The valve is located in the breech of the chamber. The barrel is a long pipe that extended all the way though the chamber with ports cut into the pipe between the chamber and barrel.

This design works well. The drawbacks are the piston needs a rod shoved into the breech rope pull hole to reset the valve between shots and the design did not use floating o rings, so it pulls a little hard.

The piston is made from a piece of PVC pipe and a couple pipe caps and o rings. Any sturdy cylindrical object can be used that will hold the pressure and forces.

For safety on this design, it is very important to remember that shoving anything into the barrel does the same thing as pulling the rope. It opens the valve. Never shove anything into this design of valve while it is under pressure. Load it and then add pressure.

The spudfiles page on the cannon is here; [4] It was posted as a materials test discussion on using unrated plastic pipe in air cannons.

Later improvements to the design included using a rod instead of a rope to open and close the valve. The piston if permitted to slide freely on the rod can be made to operate quickly without having to accelerate the control trigger rod. In operation, when the rod is pushed in, a pin on the rod pushes the piston closed. To fire the rod is pulled back through the piston which retracts the pin behind the piston out of the piston travel area. Then a nut on the end of the rod pushes the front of the piston pulling it open triggering it. Once the valve cracks open, the air pressure between the piston and projectile pushes the valve open with the same pressure it applies to the projectile.

The valve can be relocated to the barrel end of the chamber. This permits easy detachment of the barrel and the ability to attach various size barrels with minimum dead space.

A model of the T Shirt launcher shows the construction detail with the improvements. In Spudfiles, it is called the Marshmallow Cannon. It was built for a high speed marshmallow launching competition. With a removable and interchangeable barrel selection, it is easy to adapt to shooting gumballs, jawbreakers, batteries, tennis balls, and t shirts. With a 36 inch long t shirt barrel the 1 inch valve is able to toss shirts 200 feet.

The Spudfiles entry is here; [5] The spudfiles entry has links to a theory of operation video as well as more photos.

The tank as before is coaxial design with a valve body in the center. Unlike the earlier design, the barrel is removable and the valve is located at the barrel end of the chamber instead of the breech. The new design uses a rod to trigger and reset the valve.