to upgrade an 109 to BA

[rsingler]

Does the movement of air across the seat into the air barrel create a low pressure area which draws more air out of the poppet and balance chamber, resulting in a pressure drop in excess of the IP drop?

OR...(just thinking off the top of my head here) does the shift from static IP to dynamic flow result in an increase in pressure inside the balance chamber???
The hole in the poppet seat is staring directly at air coming at high speed from the first stage. Although dynamic IP measured upstream of the poppet drops, I wonder if there's any dynamic compression of the air along the axis of the poppet due to gas velocity?

Lots of fun stuff to think about!
@Luis H

 

[Angelo Farina]

Yes, the air hitting on the hole at the center of the seat will cause a local increase of pressure. It is called dynamic pressure, and it is what is measured by a Pitot tube.
However, this local pressure increase will only compensate partially for the pressure drop already occurring in the air flow due to acceleration inside the orifice.
The pressure increase would equate entirely the pressure drop only if the flow was perfectly lossless (not viscous fluid, no friction).
As air has relatively high viscosity, instead, so there are definitely some losses. So, despite the dynamic pressure increase, the pressure inside the balance chamber will always be lower than the static pressure when the valve is closed.
In the end, it is always the principle of energy conservation working. At zero flow all the energy is potential energy (static pressure).
When there is flow, some of this energy is converted to kinetic energy (velocity).
Whe the air flow is slowed down hitting the seat, in the stagnation point the kinetic energy is converted back to potential energy, but the viscous losses impede that the pressure recovery is complete.

 

[halocline]

However, this local pressure increase will only compensate partially for the pressure drop already occurring in the air flow due to acceleration inside the orifice.

That's a completely different issue. You're talking about the expansion of air from IP to ambient, I'm talking about the drop in IP.

 

[halocline]

OR...(just thinking off the top of my head here) does the shift from static IP to dynamic flow result in an increase in pressure inside the balance chamber???

I don't see how that is possible. The air inside the balance chamber is at IP by design. The air moving across the seat is at less than IP, it must be or it would not be flowing. Therefore, the air in the balance chamber is at higher pressure than the air moving around the seat. Air in the balance chamber would expand (depressurize) to match the surrounding air. Not the other way around.

That does not take into consideration any possible venturi effect of air moving laterally around the edge of the seat. That effect, if it does exist, would be to further lower pressure in the balance chamber as the moving air produces a localized drop in pressure. I just don't know how much of an effect there would be.

 

[Angelo Farina]

That's a completely different issue. You're talking about the expansion of air from IP to ambient, I'm talking about the drop in IP.

Both factors concur, causing the pressure in the balance chamber to reduce below the value which was present with the valve closed. Hence the force acting in parallel with the spring will also be reduced, requiring less negative pressure for keeping the valve open, in comparison with the unbalanced reg.

 

[Open Ocean Diver]

Here’s an excellent video of what you guys are explaining.

 

[Open Ocean Diver]

Here’s an excellent video of what you guys are explaining.

Including the Pitot tube....