Dynamic pressure changes
Dynamic pressure changes
(OP)
Hello Forum-
I have a question concerning pressure changes. We are trying to make small pressure changes to a static pressure of 5psi by varying the volume in a cylinder. We have had no problem making these pressure changes, but an interesting phenomenon is occurring. At the leading and trailing edges of the pressure pulses there are very fast pulses that are approximately 2-3x the amplitude of the pressure pulse. I do not know what is causing these extra pulses, or even what to call them, but they are dependent on how fast the velocity changes. Does anyone know what I would call these extra pulses, or does anyone know how to attenuate, or eliminate these extra pulses?
Thanks,
ENGINEERRMECH
I have a question concerning pressure changes. We are trying to make small pressure changes to a static pressure of 5psi by varying the volume in a cylinder. We have had no problem making these pressure changes, but an interesting phenomenon is occurring. At the leading and trailing edges of the pressure pulses there are very fast pulses that are approximately 2-3x the amplitude of the pressure pulse. I do not know what is causing these extra pulses, or even what to call them, but they are dependent on how fast the velocity changes. Does anyone know what I would call these extra pulses, or does anyone know how to attenuate, or eliminate these extra pulses?
Thanks,
ENGINEERRMECH





RE: Dynamic pressure changes
RE: Dynamic pressure changes
RE: Dynamic pressure changes
and what fluid
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"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Dynamic pressure changes
I've attached a pdf to show my prototype setup and the fluid is air.
Thank you,
Engineerrmech
RE: Dynamic pressure changes
May be your pressure transmitter is too sensitive and it may be close to entry and exit points.
Engineerrmech: We do have a sensitive pressure sensor due to it's other uses in the future product. What do you mean "too close to entry and exit points"?
Are you continuously discharging the air from cylinder during the experimental set up?
Engineerrmech: The system is closed so we do not discharge air during the dynamic pulses.
Thanks,
Engineerrmech
RE: Dynamic pressure changes
It looks a lot like under shoot and overshoot, how are you varying the volume of the cylinder?
Engineerrmech: The volume is changed by moving a simple pneumatic cylinder by means of a stepper motor.
Thanks,
Engineerrmech
RE: Dynamic pressure changes
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"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Dynamic pressure changes
Calculate the leak down time consant
RE: Dynamic pressure changes
2. By water you try to avoid this by slower flow, using wider pipelines and slower movements and longer closing (opening) times, sometimes also water expansion tanks, pressurized with constant overpresuure air to expand /damp the shockwaves.
3. How to cope with your particular problem would be to try to balance your total system, depending on how exact and how fast you have to have your application to react.
Some suggestions:
a. Slow down all movements to maximum allowed time.
b. Check if any components (vessels, pipelines, all) or restrictions should be larger or smaller to constrict or even out the flow.
c. Look at check valves. Select spring loaded or soft closing types and 'correct' sizes (perhaps smaller?).-
d. Closing time and sizes for solenoid valves, throtteling of any ports in and out?
e. Set in needle valves some places to experiment with slowing /constricting the flow?
RE: Dynamic pressure changes
Egrmech, you might have some success with adding a volume accumulator on the pump discharge to smooth things out.
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"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Dynamic pressure changes
Ted
RE: Dynamic pressure changes
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"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Dynamic pressure changes
RE: Dynamic pressure changes
Thank you for the tip, when I moved the sensor to the 400mL volume the extra pressure pulses did magically disappear. How would you explain this? Is this because the large volume isn't affected as the easily as 1/16th tubing's volume? I have a couple other questions, when is air considered compressible vs noncompressible? Depending on how fast I move the pneumatic cylinder the extra pulse would grow substantially, why is this?
Thanks,
Engineerrmech
RE: Dynamic pressure changes
Air is considered compressible, whenever the compressibility factor does not affect your calculations,
http://en.wikipedia.org/wiki/Compressibility_chart
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"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Dynamic pressure changes
"How would you explain this? Is this because the large volume isn't affected as the easily as 1/16th tubing's volume?"
Yes. It takes a finite amount of time to move air into or out of the 400 ml volume, and a much smaller finite amount of time to fill/empty the tubing and components connected to it. FWIW, the same thing happens with any fluid...if you have instruments that can respond fast enough to see the pressure transients.
" I have a couple other questions, when is air considered compressible vs noncompressible? "
Mmm. Air is compressible. All (real) fluids are compressible. Incompressibility is a fictitious condition used to simplify analysis. You can sometimes ignore the compressibility, sometimes not. Aeronautics and fluid flow generally says to ignore compressibility when you are below Mach 0.3, or pressure ratios that will drive those speeds.
But, acoustics treats air as "linearly compressible", if you will, at zero speed (pressure waves travelling thru still air). That's not quite correct, but you get my meaning.
In your case, you are deliberately compressing the air to create a pressure change, so you should model the air as being compressible (and that multiply recursive statement oughta make the English teachers shudder). If you tried your experiment with water, you know what would happen - your stepper motor would stall, because the water is much less compressible than air, and the motor wouldn't have enough torque to move the piston.
"Depending on how fast I move the pneumatic cylinder the extra pulse would grow substantially, why is this?"
You are compressing the cylinder side volume, causing its pressure to rise. The pressure decays thru an outlet (to the 400ml bottle) that can pass only a fixed rate of air per sqrt(diff pressure). Smash down the small volume faster, and the pressure goes up.
A little spreadsheet model of the volumes of the cylinder and bottle, tracking the mass of air in each, and connected by the flow thru the tube (just model it as an orifice) will teach you a great deal. And, once you do this, you will have taken the first steps on the path to Computational Fluid Dynamics (CFD), using Finite Difference Methods (well, really finite volume methods, but hey).
Next thing you know, you'll be coloring in pretty pictures and drawing streamlines...and that way lies madness.
RE: Dynamic pressure changes