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Curran919

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I'm troubleshooting some high-criticality nuclear centrifugal safety pumps with high pulsation leading to (hopefully) high pipe/pump structural vibe. We are very limited by what we can do here, and we are inclined to install a Helmholtz resonator (fixed speed pumps). I've never spec'd or designed one of these though. My theoretical understanding is that they are the acoustic analog of an undamped passive dynamic absorber, that I have a lot more experience with. So theoretically, it should be easy, but practically, I am probably gonna screw up the design somewhere. Anyone had good luck with Helmholtz resonators?

and before someone orifices... yes, this is also an option, but when this needs to be designed for superheated water up to 180C, we are looking at a 10% range of speed of sound in water and tuning the acoustical system won't be easy. The Helmholtz resonator on the other hand (if it really works like a PDA), cares little about the acoustic eigenfrequencies, only the excitation frequency.
John from PA

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See if you have access by some means to Reducing low-frequency vibration in hydraulic systems by means of Helmholtz resonators by by S. V. Gorin and M. V. Kuklin.

 

electricpete

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We’ve never used one of those. I can’t say it’s ever even been on my radar when talking about pressure pulsations.    Let us know how it works out. 

 
I googled to figure out what a helmholtz resonator is. Some links to free articles in the unlikely event that you haven’t already stumbled across them:


Acoustics in piping systems by SWRI

https://pdfs.semanticscholar.org/ec5c/da92fa242b88d8ce745de26780c4db9f246e.pdf

 

Practical Design against Pump Pulsations by Corbo and Stearns, No Bull Engineering, presented at Turbo TAMU conference

https://www.911metallurgist.com/blog/wp-content/uploads/2016/01/Practical-Design-Against-Pump-Pulsations.pdf

John from PA

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Quote:
Originally Posted by electricpete
I googled to figure out what a helmholtz resonator is. 


I'm surprised you didn't know what a Helmholtz resonator was.  In its simplest form, when one blows across the opening of a Coke bottle and makes a whistle; the bottle is a form of a Helmholtz resonator.

Another modern day example are many of our cars with a sunroof.  Open the roof and on many a car you get a low frequency buffeting type of noise.  The car interior has formed a  Helmholtz resonator.  Simply crack a rear window and the noise often disappears.
fburgos

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ohhh thats a helmholtz resonator... I hate when someone open one of the rear windows.

for what i could read, the silencer of the exaust is also an example of a helmholtz resonator.

time to read.
Vibe-Rater

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me three I'm thingking rthe transmafrogromulater thingme do da.... 


Curran919

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Quote:
Originally Posted by electricpete

We’ve never used one of those. I can’t say it’s ever even been on my radar when talking about pressure pulsations.    Let us know how it works out. 

Practical Design against Pump Pulsations by Corbo and Stearns, No Bull Engineering, presented at Turbo TAMU conference



I've gotta say Pete, I'm also surprised that you have no experience with Helmholtz resonators. I remember reading this Corbo article in my last job when I was just out of school and being so confused. It made a lot more sense this time around. Seriously, turbo TAMU has the best articles...
electricpete

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I must have done a good job fooling you guys to think I know more than I do (haha).  I’m electrical by education, and picked up whatever else came along in my job as "motor engineer" for our plant since 2000.  I work on motor vibrations a lot, get dragged into working on the pump occasionally, turbine occasionally, fluid systems rarely.

I can remember a grand total of only three times I was  involved (at least peripherally) when acoustic vib came up in connection with reviewing piping vibration at our plant.

1 – large vertical motor with Kinsbury thrust bearings and oil lift system.  Positive displacement oil lift pump caused unusual noise in the system under certain conditions. In the end we determined it was a peculiarity of a regulating valve in that system.  There's a thread on this one somewhere on maintenanceforums.com

2 – vendor performed survey of main turbine lube oil system piping vibration and provided report which considered acoustic resonance (it ended up being mechanical resonance).

3 – positive displacement (gear type) shaft driven oil pump on a large pump causes high 300hz (5*3600rpm) vibrations on associated oil piping.    It caused a gage to break twice, causing oil leak and loss of pump.  The second time we added flexible tubing between piping and gage to eliminate the stub where the break had occurred. But the piping still vibrates 1 – 2 ips which concerns me given history of problems (even though I know it's only a small displacement at 300hz). I'm still not 100% sure what is causing that high piping vibraiton.  If I get time I’ll post more about it in another thread, but it’s not on the top of my pile right now.

While we’re talking piping vibration, fwiw attached is a spreadsheet (for those who use English units) that I built to figure out what lengths of piping would be acoustically resonant at particular frequency (inputs are in green, equations listed).  It uses equation 21 from the TAMU paper to adjust the calculated acoustic wave speed  (which as a first approximation depends only on fluid density and bulk modulus) for the piping flexibility.  

I saw in the SWRI paper that pipe stubs and Helmholtz resonators can sometimes be easily tuned if a volume of trapped gas is used above the fluid (vent or add gas for tuning).

 
Attached Files
xls AcousticPipeResonance.xls (47.50 KB, 11 views)

Curran919

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Quote:
I saw in the SWRI paper that pipe stubs and Helmholtz resonators can sometimes be easily tuned if a volume of trapped gas is used above the fluid (vent or add gas for tuning).


Yeah! When comparing the Helmholtz resonator to a mass-spring, the volume is the spring and the neck is the mass. Adding gas greatly increases the compliance of the spring, meaning you can have a smaller volume to achieve the same natural frequency. You generally want to contain that air, though, so you put it in a flexible balloon/bladder.

What you get is an accumulator, or a charged-bladder pulsation damper.
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