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Considerations of Flow, Design, and Application
Piston valves are among the best solutions for spud gunners who wish to improve the performance of their pneumatic launchers. These homemade valves can be made in a variety of sizes at relatively low cost. There are two major designs that are used, the coaxial design, in which a piston seals against the barrel, keeping the outlet of the valve closed until the piston is actuated, and a perpendicular design (exemplified by the Supah valve on <a href="http://www.spudtech.com/">SGTC</A>,) in which a piston seals against both the barrel and the pilot of the valve, keeping the inlet of the valve closed until the piston is actuated. The name coaxial comes from the idea of the barrel and the chamber sharing a mutual axis. The other type of valve is commonly called a Supah-type piston valve on the Sputech forum, but I think that a perpendicular valve would be a better name, to avoid comparisons between specific valves and to attempt to generalize amongst the group. The valve would be perpendicular because the outlet to the barrel is perpendicular to the inlet from the chamber (though the barrel and chamber would typically be parallel to one another, with the addition of an elbow to one end of the valve or the other.) Below are basic diagrams of these valves.
(Note: The pilot is the volume of air behind the piston into which the fill valve adds air to the launcher and from which the exhaust valve vents air from the launcher. It is the dark blue area to the right of the piston in both drawings below.)
A coaxial piston valve
A perpendicular piston valve
The designs of the valves really aren't that different, with the sealing face moving from the breech of the barrel (the outlet of the valve) to the outlet of the chamber (the inlet of the valve.) Coaxial valves can also be built inside of a tee, so that they are as modular and small sized as a perpendicular piston valve. Here is a design for a coaxial piston valve in a tee. Click on the picture to be taken to a larger drawing if you wish to see more details.
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Both pistons actuate by exhausting the pressurized air on one face of the piston. The resulting pressure differential throws the piston to one side, opening the valve. The main difference in performance is based on four things: how fast the piston moves, how far the piston must open to allow full airflow, how much dead air space is behind the projectile in the barrel, and the largest size of a barrel that can be fully fed by the valve.
Speed of the Piston's Motion:
The speed of the piston is based on a few things, including the size of the pressure differential across the piston, the quality of the seal around the piston, and the size of the pilot volume.
In a perpendicular piston, the pressure differential does not need to be large to begin the actuation of the piston, so it begins opening as soon as the exhaust valve is activated and the pressure differential is enough to overcome the force of friction (less than 1 psi difference would be sufficient.) As the air vents further, the differential grows very quickly, creating a force equal to the full cross-sectional area of the piston times the pressure differential. With only a small differential, the piston is subjected to sizeable forces. (For example, in a 4" valve, with a pressure differential of 10 psi, the piston experiences a force of 125.5 pounds.) The quality of the seal around the piston must be perfect for the valve to function properly, so any air exhausted from the pilot contributes to moving the piston. The volume of the pilot in a perpendicular piston must be large, because the piston must open rather far to actuate completely. This means that there is a relatively large amount of air that must be removed for a large pressure differential to be created.
In a coaxial piston, the pressure differential needs to be large for the piston to begin opening. This is because the pressure acts over a relatively small area on one face of the piston and over a large face on the other. For instance, with a 4" piston sealing against a 2" pipe, the face of the piston toward the chamber has an area of 8.1 square inches exposed to the air pressure in the chamber, while the face of the piston toward the pilot has an area of 12.55 square inches exposed to the air pressure in the pilot. This means that as long as the air pressure on the pilot-side face of the piston is at least 2/3 (roughly 8.1/12.55) of the pressure on the chamber-side face of the piston, there is a force toward the barrel that keeps the valve closed. Once the force on the chamber-side face of the piston is larger, the piston begins to open slowly. As soon as the piston opens a tiny amount, the air pressure is able to act over a much larger surface on that face, and the piston immediately experiences a very large force opening it. The quality of the seal around a coaxial piston can be arbitrarily good (even perfect if a 1 way valve is used to fill the chamber through the piston) but is typically leaky, so that air can flow out of the pilot and around the piston to fill the chamber.
When the exhaust valve is opened, this leakiness slows the rate at which the pressure in the pilot drops. If the leak is severe enough, the piston will not be able to move. However, if the area of the exhaust valve is larger than the equivalent area of the leaks, the piston will eventually actuate. Typically the leak is very small in comparison to the size of the exhaust valve. Because the distance that the piston must open is quite small for a coaxial valve, only 1/4 of the barrel's diameter (this is not true if the barrel is very large in comparison to the tee that the valve is built in,) the pilot's volume can be quite small. This means that when the exhaust valve is opened, the pressure differential is created very quickly.
Taken together, it would make sense to say that a coaxial piston would take longer to begin moving, but once the valve begins to open, the coaxial piston would open at a much faster rate than a perpendicular piston. Below are conceptual graphs showing the pressure differential across the piston, the net force on the piston (negative values are expressed as zero, positive values are in the direction of the piston opening, to the right in the drawings above.)
PLEASE NOTE: These drawings are my idea of what happens inside the valve as it opens. They are not to be taken as factual, simply as hypothetical concepts. Real experimental data would be required to do this properly. The scales do not necessarily match, though they are more or less what I would expect to see in a real graph of these data.
Distance the Piston must Move to Fully Open:
Another factor that must be taken into consideration is the distance that the piston must open before the barrel of the launcher is fully fed with pressurized air. The further the valve must open, the longer it will take before the projectile is exposed to the maximum force of the air. Additionally, a slow opening valve (like a ball valve) causes the projectile to move partially (or completely) through the barrel, causing it to be exposed to the force created by air pressure for a shorter time once the valve is fully opened. In any valve, the smallest cross-sectional area of the valve is the limit to that valve's flow rate, so as long as the smallest cross-sectional area of the valve is at least as large as the barrel's cross-sectional area, the valve is not restricting the flow of the air.
In a coaxial valve, the piston does not have to travel far for the valve to be fed. As long as the space around the barrel is 1/4 of its diameter, the piston only needs to open 1/4 of the barrels diameter to fully feed the valve. This is because the cross-sectional area through the valve will be equal to the cross-sectional area at this position. The mathematics to show this are below. The seal between the piston and the barrel cannot be situated too far from the pilot end of the tee, or else this concept fails. The ideal situation would be for the piston to be in the center of the tee or nearer to the pilot. Placement of the seal toward the barrel end of the tee creates a number of problems. Among these would be a further opening distance for the piston, eliminating one of the coaxial piston's major benefits.
Minimum Required Opening Distance for a Coaxial Piston Valve
In a perpendicular valve, the smallest cross-sectional area of the valve is the opening between the chamber-side face of the piston and the opening in the tee to the barrel. This "half moon" shaped opening requires the piston to move further before the flow is unrestricted by the valve. (I'm working out the math of it, but it has been many moons since I took trigonometry. Maybe calculus would be a better method of solving the problem...)
Below are drawings of coaxial and perpendicular valves closed and opened to 1/4 of the barrel's diameter. It is clear in the drawing of the perpendicular valve that the airflow is restricted severely when the piston is in this position. (Issues regarding flow around the barrel in a coaxial valve are addressed in the barrel size section below.
A Drawing of the Flow of Air through a Coaxial Piston Valve
A Drawing of the Flow of Air through a Perpendicular Piston Valve
Dead Air Space:
The amount of dead air space between the seal in the valve and the projectile effects how strongly that projectile will move. Larger volumes can cause the air pressure to drop significantly before it interacts with the projectile, decreasing the launcher's performance.
Depending on how it is built, a coaxial valve can have as little as zero dead air space behind the projectile. This means that the instant that the valve opens, the projectile is exposed to highly pressurized air (the full effect would not be felt until the valve had opened completely, of course.) Even with interchangeable barrels, the dead air space can be kept to a minimum, as the barrel can pass through a bushing and come very close to the sealing face of the piston. There will still be dead air around the barrel, and some in between the barrel and the piston, but it is a relatively small amount.
Depending upon how the valve is built and connected, a perpendicular valve can have a relatively small volume of dead air space, or a quite large one. In the way that they are typically oriented in the launcher (as drawn, with the chamber and pilot along the horizontal axis and the outlet to the barrel along a vertical axis,) the dead air volume is inherently large. The top portion of the tee is all dead air, as is any elbow used to change the axis of the barrel from perpendicular to the chamber to parallel to it. This orientation of the valve is far inferior to a coaxial piston valve in this regard. If the valve were oriented so that the piston moved along a vertical path and the outlet to the barrel were horizontal, then pass through adapters could be made in a similar fashion to those used in coaxial valves, eliminating much of the dead air space. This could introduce difficulties, as the net force on the piston would be downward (due to the addition of the force of gravity.) While friction would likely overcome this force, it could make the valve less stable and more prone to being fired accidentally.
Clearly any sized barrel can be used with any sized valve. However, this is a discussion of which sizes of barrels can be fully supplied with air by these types of valves. At very small sizes compared to the diameter of the fittings in which they are made, there is little or no difference between the valves at all. For the purposes of this section, the size of the fitting will be assumed to be 4". The same logic would apply to any sized fitting, with appropriate scaling done for the barrels' dimensions.
The choice of barrels that can be fully fed by a coaxial piston valves is limited by a couple of factors. These are the maximum size that can be actuated and the maximum size that can be fully supplied air when the valve is open. While some of these factors can be relieved (or removed altogether) by altering the distance that the valve must move to be considered fully open, I will only consider the situation in which the valve is open to 1/4 of the diameter of the barrel. In fact, if the sealing face is situated in the center of the tee or more toward the barrel end of the valve, then a coaxial valve can feed a barrel that is close to its fitting's diameter and the rest of the problems discussed here are irrelevant. However, many of the benefits that I listed above for coaxial valves over perpendicular valves would be eliminated.
A maximum valve size in coaxial pistons occurs because the piston's chamber-side face must be exposed to at least some of the air from the chamber. This would limit the barrel's size to the largest diameter pipe that could fit inside of the valve's pipe or fitting (depending upon how the valve is made.) A slightly more complicated problem is the issue of fully supplying the valve once it has been opened. In a standard coaxial launcher, the area of the gap around the barrel must be greater than or equal to the area of the barrel for it to supply the barrel adequately. This would not be the case with a 3" barrel in a 4" pipe. The difference in the areas (12.55 square inches for the 4" - 9.62 square inches for the outer diameter of the 3" = 2.93 square inches) is less than the area of the barrel (7.27 square inches for the inner diameter of the 3".) 2.5" pipe would be the largest standard sized pipe that could be fully fed with this type of valve. In a piston valve built in a tee, this problem is ameliorated somewhat by the fact that a large portion of the barrel can be fed directly from the chamber without traveling around the outside of the barrel. The air supply for the chamber-side half of the barrel can come directly from the top of the tee, roughly speaking. This means that the air supply to the other half of the barrel is all that needs to travel around the pipe. The same issue still exists, with the 3" pipe being inadequately supplied over about half of it's area, but the difference is much less, and the performance would still be slightly superior to a 2.5" barrel's maximum flow. More importantly, the fittings and pipe are easier to find for 3" sizes. Below is a mathematical description of what I am discussing. The flow through this valve can be increased dramatically by either removing some of the outer surface of the barrel pipe or removing some of the inner surface of the tee in which the valve is made. However, this is beyond the consideration of this webpage. (Great care must be taken when altering ANY of the fittings used in a valve. The forces involved are enormous and plastic that has been excessively altered can fail, resulting in injury!)
How to Check to See if a Coaxial Piston in a Tee is Fully Supplying a Barrel with Air
(Example of 3" barrel with in a 4" tee)
Note: This is actual barrel ID, not the nominal dimensions used in piping.
This is about 0.1" larger than a 2.5" schedule 40 pipe.
This is the area where a perpendicular piston valve far surpasses the capabilities of a coaxial piston valve. A perpendicular valve, when fully open, can supply any sized barrel up to the diameter of the fitting it is made in. So, if you are using a 4" tee, you can feed a 4" barrel. There is nothing to restrict the flow once the valve is opened. The difference between the maximum barrel size for a perpendicular piston valve and a coaxial piston valve can be quite large. Using the 4" tee example, the perpendicular piston has the capability of supplying an extra 7.45 square inches of air to the barrel above the performance of the coaxial. The differences can be overcome by using larger fittings still (such as a 6" tee to build the coaxial piston valve,), or situating the valve in the tee so that further opening of the piston would result in greater airflow (more than enough to feed a 3" pipe, in fact), but this would not be an equal comparison, and the cost of larger fittings quickly grows to a frightening level.
Cost and Labor:
Building any type of piston valve is more labor intensive than simply buying a valve from off of the shelf. Some care must be taken to ensure that the valve will operate correctly once it is assembled, and the potential for mistakes is large for beginners. However, once you have built a piston valve, you will be able to craft others relatively quickly and easily, with little chance for error.
The cost of a piston valve can vary widely, depending upon the materials used and the scale on which it is made. For a first valve, building a smaller sized model in a 2" tee might be worth considering, as any mistakes will be far less expensive than they are with a full scale 4" valve.
Coaxial piston valves can be built extremely cheaply. The only requirements for the piston are that it fits the barrel snugly, is sufficiently strong to hold up against the force of the air sealing the valve shut, and that it has an airtight sealing face (usually a rubber gasket of some sort, often cut from a sheet of red rubber or neoprene.) The other requirements are a tee, an elbow, a male or female adapter (I use female adapters so that the piston can be removed if it does not seal properly), a threaded plug, clean out cap, or bushing, an exhaust valve, a fill valve, and a bushing or reducer to convert from the size of the tee to the size of the size of the barrel. The piston itself can be made from plumbing parts (such as a coupler with plugs), empty cans, wooden or plastic rods, or even a flat assembly of washers and gaskets.
Perpendicular piston valves require a higher level of precision in their construction due to the fact that they require a perfect seal around both ends of the piston. This typically requires o-rings on either end of the piston to seal it completely. Typically these pistons are made of either plumbing fittings or out of solid PVC rods that are machined to fit the tee in which they are built. If solid PVC rod is used, it can be quite expensive. Aside from this, the only difference in parts between the valves is the addition of o-rings and the removal of the gasket and associated hardware to connect it to the piston. If the design permits the use of plumbing supplies as a piston, and if the builder has a lathe or similar tool to cut grooves for the o-rings, the cost can be as low as that of a coaxial piston valve.
Building a piston valve can be a trying process. Often the first attempt will not work properly due to some basic oversight in construction. (When this occurs, you can get mountains of help from the friendly folks on the <A href="http://forums.spudtech.com/">Spud gun Technology Center Forum</A>.) However, once you have assembled one piston valve, you will be able to craft them much more quickly and easily than you would expect. Even with experience, however, building one of these valves takes time. The actual amount of time it takes to make a basic coaxial piston valve is probably going to be three or four hours when you first start, not including any of the time it will take to plan the valve or buy the components. This time will be spread over many days as time is required for PVC cement and epoxy to cure properly. It would certainly be possible to speed up the process significantly in an assembly line style of build, but for a normal builder, working alone in a garage or tool shed, it is not unreasonable to expect to spend the majority of a few evenings getting it all together. If a machined piston is desired, then the process can take much longer and require machine shop time to acquire the proper fit for the piston.
In many ways, a coaxial piston valve is superior to a perpendicular piston valve. It is easier to construct, it requires less travel to open fully, it opens at a faster rate (as far as reason and physics tell me anyways, the experimental evidence is lacking on this,) and it has an inherently small dead air space. Perpendicular pistons offer a wider range of barrel sizes and the potential for greater absolute performance when launchers are designed to take advantage of their power to the fullest extent. In most applications, there will be very little noticeable difference between these valves, but when the launcher is constructed to exploit the incredible power of these devices, some consideration must be taken to decide which is right for which application.
Hopefully this has been helpful. If anyone has constructive criticism or advice for other information to include, I'd be happy to listen.
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