Piston valve

A piston valve is a pilot operated valve. It is very similar to a diaphragm valve in theory, but replaces it's flexible diaphragm with a rigid cylinder. There are 2 varieties; barrel sealing and chamber sealing.

Barrel sealing valve

In a barrel sealing valve, the piston seals against the breech of the barrel. This is the most common piston valve design.

construction

The piston is often a well fitting cup-shaped object, such as a end cap. To provide a sealing face, a piece of rubber is attached, typically with a bolt. In the simplest case, equalization is accomplished by allowing the air to leak past the piston and into the chamber. Some people will machine their own pistons, in which case they often build O-rings into them. With the O-rings, you need to make a small equalization hole; sometimes this is fancied up to be a homemade check valve.

Because a piston is heavier and harder than a diaphragm, and PVC is somewhat brittle, it is recommended that a bumper of sorts is built into your pilot volume. Common bumpers include heavy-duty rubber hose, and such. This is labeled "F" in the "use, barrel sealing" diagram.

These valves are often used on coaxials and over/unders. In an over/under, the valve is built in a "T" fitting. The barrel is put co-axially through one end, the pilot and piston are put in the opposite one, and a chamber is connected to the perpendicular opening with a 90° elbow.

Use

top=pressurized, bottom=firing

Operation:

1. Air is added by filling behind the piston (C).
2. The piston (E) slides forwards, and seals against the barrel (A). (Alternatively, one can use a spring to move the piston forwards, which allows one to fill [slowly at first, to allow the pressure to equalize] from the chamber.)
3. More air is added, and leaks around the piston (or through a small equalization hole) and into the chamber (B), filling it to the desired pressure.
4. The pilot valve (D) behind the piston is opened, the pressure in the pilot volume (C) drops, and the higher pressure in the chamber area pushes the piston away from the barrel.
5. The air flows around from the chamber, and into the barrel, propelling the projectile.

Chamber sealing piston valve

construction

The piston of a chamber-sealing piston valve has to seal on both the chamber port and to the pilot volume. This requires that the piston be machined with O-rings. A small equalization hole is required; sometimes this is fancied up to be a homemade check valve.

The piston is almost always housed in a "t" fitting.

Because a piston is heavier and harder than a diaphragm, and PVC is somewhat brittle, it is recommended that a bumper of sorts is built into your pilot volume. Common bumpers include heavy-duty rubber hose, and such. This is labeled "F" in the "use, chamber sealing sealing" diagram.

These valves are often used on over/unders, as the flow is already turned around 90^o.

Use

top=pressurized, bottom=firing

Operation:

1. Air is added by filling behind the piston (C).
2. The piston (E) slides forwards, and seals against the barrel (A). (Alternatively, one can use a spring to move the piston forwards, which allows one to fill [slowly at first, to allow the pressure to equalize] from the chamber.)
3. More air is added, and leaks through the equalization hole in the piston and into the chamber (B), filling it to the desired pressure.
4. The pilot valve (D) behind the piston is opened, the pressure in the pilot volume (C) drops, and the higher pressure in the chamber area pushes the piston away from the chamber.
5. The air flows out of the chamber, and into the barrel, propelling the projectile.

Common traits

1. The performance of these valves can be calculated with the GGDT.
2. If your valve honks, it is probably a good idea to invest in a better pilot valve, though this is more of a problem with barrel-sealing valves.
3. In both cases, provided there is a good deal of space around the barrel, the full flow potential of the valve is realized when the piston has moved back 1/4th of the barrel's inside diameter (I.D./4). The derivation of this formula is as follows:

Given: A = Pi(R)^2

      C = 2PiR
      T = Piston travel
      R = I.D. of barrel / 2

When the piston moves back, the smallest amount of area exposed is either the area of the circular cross section of the barrel or the area exposed by the piston, which is the side of a cylinder, the bases being the barrel and piston face. The optimal ratio of area exposed between the 2 spots is 1:1, which means the exposed areas should be the same. So one sets the equations A and CT equal, that is A = CT.

Pi(R)^2 = 2PiR(T)

Solving for "T" results in:

R/2 = T which, if one substitutes I.D./2 for R, results in:

I.D./4 = T

1. Pneumatic actuation is not mandatory; mechanical means can be used to hold the valve shut, rather than using the force of the air.

The operation of barrel sealing valves falls into 3 catagories which is related to the dimensions of the barrel seal or seat to the piston outside diamter.

Large Ratio Piston Valve

When the ratio is large with either a large piston or small barrel, most of the force to open the valve is controlled by the differential of the chamber pressure and pilot area. As the valve opens, the small area that becomes exposed to the chamber as it opens contributes little to the opening speed. This large area ratio valve are noted for reliable operation as they have the ability to open pistons that don't slide well. Due to the pressure still in the pilot area as they open, as the chamber vents out the barrel, it is often faster than the pilot, so the force used to open the valve vanishes and the pilot pressure unable to keep up with the drop, forces the valve to close. If this repeats as the pilot vents, it is noticable as honking.

Small Ratio or QEV

As the ratio of the seat diameter to piston diameter is reduced, the area that the chamber is exposed to is reduced, to the point that the large area of the pilot is able to hold the piston closed until most of the pilot pressure is vented. Even when the pilot area is vented to a low pressure, the initial opening force is low because the area of the piston exposed to the chamber pressure is small as most of the face of the piston is exposed to no pressue in the barrel. This low opening force can cause many headaches with stuck pistons that sometimes fail to fire, and the equalization port may vent enough volume to prevent the pilot area from reaching a low enough pressure to open the valve. The lower the seat to diameter ratio; the lower the inital force the chamber can apply to open the valve, and the faster they snap open when they do get over the initial opening.

One attractive feature of this valve is once it unseats and starts to open, the chamber pressure now acting on the face of the piston causes a large spike in opening force as the large barrel facing area is now exposed to the chamber pressure. This type of valve snaps open and is known as a Quick Exhaust Valve or QEV.

1:1 Ratio or QDV

If the ratio of the piston diameter is reduced to the diameter of the valve seat in a spool valve, then the valve won't open even when the pilot area is not under pressure. If the chamber is pressurised and no pressure is in the pilot area, the valve has the least back pressure of any of the piston valves and therefore the fastest opening when the valve is unseated by force. This last catagory of piston valve is known as a Quick Dump Valve. To operate this valve in an air cannon, consideration must be made to mechanicaly move the piston and when the piston moves, the force will avalanche quickly as the piston face becomes exposed to the chamber pressure. Two cannons on spudfiles use a rope pull and a rod with a "sliding" piston so the valve force is not able to shove the rod in the operator's hand. Decoupling the triggering mechanism from the piston prevents the trigger from adding to the moving mass of the piston.