flow of air inside a PCP

[rumbleUU]

I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm  on mine PR900W 5.5mm
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...

and where did you put the manometer? with 6mm hole threw there will be no material left to stop the manometer to tighten it..

Also, did you enlarged the valve-seat hole? cause it is only the same effective area as a 3.5mm hole when you subtract the area of the poppet...... otherwise this will be the place where airflow will choke....

[subscriber]

Also, did you enlarged the valve-seat hole? cause it is only the same effective area as a 3.5mm hole when you subtract the area of the poppet...... otherwise this will be the place where airflow will choke....

OK.  It is not only length to area ratio of a flow passage that matters.  If the air flow reaches the speed of sound through the narrowest section, no further increase in flow can take place, despite higher input pressure.   This is known as choked flow.  It can certainly happen at an orifice plate.  Or a nozzle shaped section.

Perhaps that is what you have been alluding to all along.  In which case you are correct.

As for the idea that an air flow path must always be parallel or at least constant area section; this is clearly not the case with the inlet and exhaust valves of internal combustion engines.  Here, pressure in the duct is converted to flow, and flow converted to pressure, via nozzle shaped paths leading to or from the valves.

The same rule applies for avoiding supersonic flow in the convergent section or throat of the nozzle; else the nozzle is flow limited due to chocking:

https://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.html

https://aeronotes.weebly.com/nozzle-choking.html

https://www.sciencedirect.com/topics/engineering/sonic-flow

https://en.wikipedia.org/wiki/Choked_flow

[rsterne]

I disagree that very little air can flow through the gauge block to refill the "plenum" during the shot.... It all depends on the diameter of the passage through it....

A Disco has almost no "plenum", the space between the gauge block and valve is about 1/16" long, or less than 1/2 cc.... The valve is roughly about 3 cc inside.... The hole through the gauge block is 1/8" and through the front of the valve it is 1/4".... The valve throat, stock, is 13/64" with a 5/32" stem in the way (throat area is the equivalent of a hole 0.130"), and the barrel port is 0.134".... The hole through the gauge port is almost as large as the throat area, and likely flows just as much air, so on a stock gun it makes little difference it is so small....

When you start hogging out the ports, that is no longer the case.... Go to a 3/16" barrel and transfer port and a 1/4" throat, and if you don't increase the hole in the gauge port to 1/4" or a bit larger, you will be giving up a significant amount of power.... I know, because I hogged out the ports, and initially ended up disappointed in the FPE.... Increasing the hole in the gauge port to 5/16" made a huge difference in the FPE.... because it makes the entire 135 cc reservoir act like a plenum.... The drastically increases the pressure available at the valve seat during the shot....

If you are dealing with a regulated PCP, where the regulator is in between the HPA reservoir and the plenum.... and the passage between the plenum and valve are large, the pressure in the valve and plenum remain essentially the same, all the time throughout the shot.... The regulator can't keep up with the flow requirements of the shot, so the plenum doesn't get topped up during the shot.... That is why larger plenums increase FPE, because they keep the pressure higher during the shot.... Take the regulator out, and the reservoir becomes part of the plenum, unless you have a gauge port (or tank block) in the way.... If the hole in the gauge port is smaller than the ports leading out of the valve, the pressure inside the valve drops.... If the hole is larger than the throat and valve inlet, then the air in the reservoir passes easily through the gauge port, keeping the pressure in the valve topped up....

While the length of the hole through the gauge port may make some difference to how quickly the air can get from the reservoir in to the valve, I think the area is the key.... I make sure that the inlet into the valve, and the passage through the gauge port, are at least as large in diameter as the valve throat diameter.... Since the actual throat area is reduced by the area of the stem, that insures that the entire HPA reservoir acts as plenum volume in an unregulated PCP.... I even do this on my unregulated bottle guns, by using a large passage through the tank block, to make sure that the bottle keeps the pressure in the main tube topped up during the shot.... If all I was adding to the plenum was (part of) the volume of the larger passage through the tank block, I would only be increasing the plenum volume by 1% or less.... In reality, by having large enough passages between the two through the tank block, my plenum is increasing by the entire volume of the bottle (up to 400%).... and it shows in performance, compared to having a small passage between them....

Bob

[rsterne]

Note, choked flow can only occur in the very last part of the shot cycle, when the pellet velocity is high.... At the beginning of the shot cycle, the pellet is stationary (or nearly so), and even though the valve is cracked just a few thou the flow velocity is very low, and no choking can occur.... Only once the pellet gets up a good head of steam (and with it the column of air behind it), and the valve starts to close, can the flow at the seat choke, and in fact it WILL do that just before the valve closes, because the curtain area under the poppet is so small.... The only effect that has is to make the flow think the valve has already closed, a few micro-Sec earlier than when the poppet actually hits the seat....

Also note, you can't just use a speed of sound of 1126 fps to calculate the choking velocity, you must use the speed of sound at the pressure involved.... At 3000 psi in air, Mach 1 is about 1370 fps.... The larger a passage, and therefore the slower the flow, the less likely it is to choke, so using large diameter passages on the inlet side of the valve, so that the smallest area is the valve throat, is the simplest way to insure that the flow can never choke at the valve inlet, gauge port, or tank block passages....

The column of air inside a barrel is expanding and accelerating as the pellet travels down the bore.... If the velocity of the column was the same everywhere, then shots of over 2000 fps would be impossible, because the flow out of the valve would choke long before that velocity was reached.... Far from being the stuff of legend, I have produced repeatable velocities of 2092 fps by using a 1.8 gr. airsoft BB in my 6mm PCP instead of the 75 gr. lead bullet at just over 1000 fps.... Interestingly, the mass of air expelled in the shot is much greater than even that 75 gr. bullet, over 100 gr. of air at 4200 psi....

Bob

[subscriber]

Thanks Bob.

The stock Disco is severely power limited because it has to flow so much air through the gauge body passage.  If it had a large plenum at the valve, the gauge block passage could be tiny and it would work fine.  The shot cycle would be more like a regulated plenum refilling after the shot.

Increasing the passage diameter leading up to the valve, such that air flow velocity that occurs while the valve is open, remains well below super sonic at the narrowest section, is key.  Else, any narrow portion, no matter how short risks choking due to supersonic flow. 

Certainly, increasing the passage diameter to over bore diameter should reduce flow velocity to a very useful degree.  The OP is talking about increasing a 3 mm flow path to 4 mm.  As area increases with the square of the diameter, he is going from minuscule to tiny .  Not sure what caliber he is considering.  I did suggest he open the flow path as wide as he could...

Now, to put things into perspective about long narrow flow paths being "bad"; consider the flow path down the barrel once the pellet is almost half way to the muzzle.    That long "flow path" does not stop the airgun from working (it can't be avoided).  And neither does a long skinny flow path leading up to the valve prevent an airgun from working.  It just limits maximum power and reduces efficiency at high power.

[subscriber]

Very good points about choked flow only being a "risk" once the pellet is moving fast, Bob.  Also, the reminder that the speed of sound in air is higher at high pressure.

[rsterne]

In fact, with typical airgun velocities of under 1000 fps, if you build a gun with near bore-size porting (which you would normally do if in search of high power)…. there is virtually no risk of the flow choking except at the seat, just as the valve shuts.... In reality, if the valve is closing before the pellet gets half way down the barrel (typical if you are looking for even reasonable efficiency)…. then the 80% barrel ports we drill (if matched to similar size transfer ports) are unlikely to choke either.... Any calculations predicting the choking of the flow at the transfer port must be done using the pellet velocity at valve close, NOT the muzzle velocity....

Bob

[subscriber]

Thanks, Bob

I was hoping you would weigh in sooner (hence the PM this morning ).  Certainly, you have a deep understanding, based extensive experience tuning and making from scratch, all manner of airguns.  This, while I can easily get bogged down in over emphasizing one theoretical aspect at the expense of more important ones.

This reminds me of my first year Strength of Materials class:  The professor asked the class, "Now that you understand the concept of yield strength, would you trust a product where the material yield strength had been exceeded?"

3/4" class raised their hands to indicate "yes".  The others smelled a trick question.

The prof then said, "For those of you who said "yes"; the chairs you are sitting on were formed by exceeding the yield strength the steel tubing.  And there is nothing unsafe about them.  The devil is in the details".

The point of that story, is the mistaken idea that choked flow is always bad.  As you pointed out, the flow always chokes in a PCP as the valve closes.  But reducing the flow is kinda the point of closing the valve, so no harm done

[rumbleUU]

I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm  on mine PR900W 5.5mm
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...

I didnt see yesterday when I looked quick, but it is a tapered hole right threw the gauge now where the manometer is going in, how do you make that leakfree, teflontape?

[rumbleUU]

I disagree that very little air can flow through the gauge block to refill the "plenum" during the shot.... It all depends on the diameter of the passage through it....

A Disco has almost no "plenum", the space between the gauge block and valve is about 1/16" long, or less than 1/2 cc.... The valve is roughly about 3 cc inside.... The hole through the gauge block is 1/8" and through the front of the valve it is 1/4".... The valve throat, stock, is 13/64" with a 5/32" stem in the way (throat area is the equivalent of a hole 0.130"), and the barrel port is 0.134".... The hole through the gauge port is almost as large as the throat area, and likely flows just as much air, so on a stock gun it makes little difference it is so small....

When you start hogging out the ports, that is no longer the case.... Go to a 3/16" barrel and transfer port and a 1/4" throat, and if you don't increase the hole in the gauge port to 1/4" or a bit larger, you will be giving up a significant amount of power.... I know, because I hogged out the ports, and initially ended up disappointed in the FPE.... Increasing the hole in the gauge port to 5/16" made a huge difference in the FPE.... because it makes the entire 135 cc reservoir act like a plenum.... The drastically increases the pressure available at the valve seat during the shot....

If you are dealing with a regulated PCP, where the regulator is in between the HPA reservoir and the plenum.... and the passage between the plenum and valve are large, the pressure in the valve and plenum remain essentially the same, all the time throughout the shot.... The regulator can't keep up with the flow requirements of the shot, so the plenum doesn't get topped up during the shot.... That is why larger plenums increase FPE, because they keep the pressure higher during the shot.... Take the regulator out, and the reservoir becomes part of the plenum, unless you have a gauge port (or tank block) in the way.... If the hole in the gauge port is smaller than the ports leading out of the valve, the pressure inside the valve drops.... If the hole is larger than the throat and valve inlet, then the air in the reservoir passes easily through the gauge port, keeping the pressure in the valve topped up....

While the length of the hole through the gauge port may make some difference to how quickly the air can get from the reservoir in to the valve, I think the area is the key.... I make sure that the inlet into the valve, and the passage through the gauge port, are at least as large in diameter as the valve throat diameter.... Since the actual throat area is reduced by the area of the stem, that insures that the entire HPA reservoir acts as plenum volume in an unregulated PCP.... I even do this on my unregulated bottle guns, by using a large passage through the tank block, to make sure that the bottle keeps the pressure in the main tube topped up during the shot.... If all I was adding to the plenum was (part of) the volume of the larger passage through the tank block, I would only be increasing the plenum volume by 1% or less.... In reality, by having large enough passages between the two through the tank block, my plenum is increasing by the entire volume of the bottle (up to 400%).... and it shows in performance, compared to having a small passage between them....

Bob

Thanks Bob, in other words: the hole threw gauge should be at least the same size as the inlet to the valve to prevent powerlose, so for any typical pcp gun with a gauge inbetween and a small "plenum": Gauge hole=inlet valve, the rest including valvethroat-stem should match for the power you want....   ?

[rumbleUU]

Im having hard time understanding this: If you have a gauge inside a tube, 3mm hole threw and a small tiny hole(1mm) that goes into the threaded manometer, there is also a white delrin washer in between the manometer and the gauge-block with a tiny 1mm hole in the middle...  How much force is acting on the manometer? is the force only applied by the tiny 1mm hole, or by the whole M8 threaded area? cant understand that.....

[subscriber]

Looking at your diagram suggests the delrin washer is shielded from air pressure; except inside the 1 mm diameter hole.  So, really very little force on the washer from air pressure.  Essentially none on the 8 mm faces.  The only forces of consequence acting on the washer faces are due to tightening the thread on the valve stem.

If the end face of the valve stem has a narrow raised sealing ring machined into it, then the delrin washer will see air pressure on one face, up to inside edge of that sealing ring.

The 1 mm channel is of no consequence here, as the pressure gauge does not flow air.  The hole could be even smaller, except that punching or drilling very small holes is difficult and expensive.

[rumbleUU]

Looking at your diagram suggests the delrin washer is shielded from air pressure; except inside the 1 mm diameter hole.  So, really very little force on the washer from air pressure.  Essentially none on the 8 mm faces.  The only forces of consequence acting on the washer faces are due to tightening the thread on the valve stem.

The 1 mm channel is of no consequence here, as the pressure gauge does not flow air.  The hole could be even smaller, except that punching or drilling very small holes is difficult and expensive.

ok, look at Reply #19 by Atom, how much force will be acting on the manometer the way he did,
Will the forces pushing the manometer be the same in his case?

[subscriber]

I looked at post 19"  https://www.gatewaytoairguns.org/GTA/index.php?topic=162493.msg155811577#msg155811577

I assume the manometer will screw into the threaded hole at 90 degrees to the air tube axis.  I am not sure where the sealing surfaces are, but if I understand correctly, then the air pressure will act on nearly the thread outside diameter.  That would potentially increase the force pulling on the threads.  Somewhere on this forum Bob has a calculator for checking thread strength; if that is your concern. 

Definitely good to calculate that the margin of safety has not been compromised when modifying parts.

An 8 mm diameter plug that sees 3000 PSI will only have 230 lb acting on it.  That should easily be handled by an 8 mm thread of sufficient depth. 

[rumbleUU]

I looked at post 19"  https://www.gatewaytoairguns.org/GTA/index.php?topic=162493.msg155811577#msg155811577

I assume the manometer will screw into the threaded hole at 90 degrees to the air tube axis.  I am not sure where the sealing surfaces are, but if I understand correctly, then the air pressure will act on nearly the thread outside diameter.  That would potentially increase the force pulling on the threads.  Somewhere on this forum Bob has a calculator for checking thread strength; if that is your concern. 

Definitely good to calculate that the margin of safety has not been compromised when modifying parts.

An 8 mm diameter plug that sees 3000 PSI will only have 230 lb acting on it.  That should easily be handled by an 8 mm thread of sufficient depth.

Im just concerned if Atom dont know that the forces will be much more in his case then the stock one was, We will see when he replies to my questions how he seals the manometer the way he did it...

[subscriber]

Pertinent posts:
https://www.gatewaytoairguns.org/GTA/index.php?topic=138842.msg1395806#msg1395806
https://www.gatewaytoairguns.org/GTA/index.php?topic=136973.msg1373003#msg1373003

https://www.gatewaytoairguns.org/GTA/index.php?topic=127914.0

[rumbleUU]

Pertinent posts:
https://www.gatewaytoairguns.org/GTA/index.php?topic=138842.msg1395806#msg1395806
https://www.gatewaytoairguns.org/GTA/index.php?topic=136973.msg1373003#msg1373003

https://www.gatewaytoairguns.org/GTA/index.php?topic=127914.0

dont have problems calculating forces mechanicly, the hard part as I see it to know wich material all the stuff is from, the manometer is brass, thats easy, the gauge is some kind of aluminum.....

yes I like also to calculate the forces so you are in the margin at least 3 times before the material starts to get weaker.. even better 4-5 times the margin, but that cant be expected for all pcp:s.....

Sometimes when someone else is doing a mod, im not sure they understand that the forces could get 20-30 times higher, not all people understand this.... Im very carefull of knowing before doing anything..

[rumbleUU]

in cross-section the manometer looks this way....
The H is the meat that is between the 3mm hole and the delrin washer, its 1.7mm thick at the lowest point.
Would it be safe to make the hole threw gauge like 5mm only leaving H=0.7mm

[subscriber]

Im very carefull of knowing before doing anything..

Good.  Lazy people say pounds when they mean pounds per square inch; or the other way around.  Then, effective area can make huge difference.

Bob and Lloyd  have a gate on this forum.  Lloyd runs Airgun Lab.  He has a bunch of interesting videos testing airgun parts to destruction.  Note that pressure tests use liquid, not gas. This minimizes the energy released when parts fail:  https://www.youtube.com/user/1227air500/videos

[subscriber]

in cross-section the manometer looks this way....
The H is the meat that is between the 3mm hole and the delrin washer, its 1.7mm thick at the lowest point.
Would it be safe to make the hole threw gauge like 5mm only leaving H=0.7mm

I don't want to answer that without calculating a few things.  Calculation is impossible without knowing the max pressure and the material type and state.  I assume 3000 PSI and 6061 T6...

EDIT:  I see 200 BAR pressure; or 2940 PSI.

You could make a test piece of similar material and destroy it with a hydraulic pump.  This is ultimately better evidence than any calculation.

Remember, the passage feeding the manometer is a cylindrical hole, even though it appears as a cut in your image.  As such, the loss in strength due to the 1 mm wall at the hole edge is not nearly as weak as it looks in your diagram.