Water Meter Vibration Issue

[James08]

One of our clients wants to purchase a water meter from us. That clients wants to install this meter between two pumps operating at frequency of 200~400Hz. Coincidently, our meter's natural frequency is around 200Hz. Would this cause the meter be destroyed by vibration excited by two coincidental frequencies? If it would, how much should the natural frequency of the meter be increased to avoid such a problem?

 

[GregLocock]

That depends on a lot of things, 600 Hz would be my first guess for a safe resonance, but we often use margins as low as 15% in practice. You need to be aware that their pump probably creates more harmonics than just the fundamental as well. I'm guessing 400 Hz is blade passing frequency, expect to see 3x that as well

Can you get a spectrum of the pressure in the system when it is running?





Cheers

Greg Locock

 

[hacksaw]

you need to specify the meter and clarify what you are describing with the frequency range given

 

[James08]

A meter will be installed inline in the same pipe between two pumps. The meter is a pressure differential V-cone meter purchased from McCrometer Inc. (

http://www.mccrometer.com).

The meter's natural frequency is around 200Hz. The two pump strokes around 200~400Hz, which is considered to cause vibration by pumping the water columns in stroke motions. Since the force vibration caused by the water columns will be around 200~400Hz which coincides the natural frequency of the meter, it is likely to cause V-cone inside the meter to be damaged greatly. The fluid running through the pipe is a high temperature and high pressure water. So my question is “how much higher should the natural frequency of the meter be altered to?”

 

[franck]

Just a simple suggestion, why not trying to lower a little bit the frequency by mounting your water meter on a soft interface, which is I think the easiest fix.

Franck

 

[James08]

Even if the meter is mounted on some isolater stand, would the vibration caused by the water column strokes still destroys the V-cone inside the meter?

 

[GBor]

You design the "soft interface" to absorb the vibration. Basically, you assume your meter is a lumped mass and you're designing the spring that connects it to the base. If you suspect that the base is moving at 200Hz, design the spring to dampen, or absorb 200Hz. Look for information on "beams on an elastic foundation" or, more to the point, a 2 degree of freedom system where you are trying to minimize the vibration of the second mass.

Garland E. Borowski, PE

 

[James08]

It's not the pipe that is permeating the vibration, it's the fluid. Isolating the pipe will not reduce the vibration, right?

 

[GregLocock]

Correct. Now, if you can get hold of a spectrum then you will see where you need to put your resonance. If you don't have that information then you will just have to guess. If the maximum frequency of excitation is 400 Hz then a rule of thumb is 15% higher (depending partly on the damping characteristic of your system), ie 460 Hz

It is unlikely that you will be able to shift your natural frequency by this much.



Cheers

Greg Locock

 

[James08]

Unfortunately, the spectrum is not available. Would it be enough even if the meter's natural frequency can be increased 15% considering the unknown vibration forces caused by the water columns?

 

[hacksaw]

the likelihood and consequence of failure dictates that you need use an alternative meter. sounds like a good hazop item...

 

[GregLocock]

James08 - No, obviously not.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GBor]

Greg,

Do you think lowering the frequency of the meter would work?

James,

Since you are on the lower end of the expected excitation frequencies, the same principal in the opposite direction may be the way to go. If the pumps steadily operate at 200-400Hz (as opposed to a variable speed pump) and your meter operates at 200 Hz, you may be able to lower the operating frequency to 160-170Hz and avoid the excitation. Is this an option?

Garland

Garland E. Borowski, PE

 

[James08]

Even if the meter's natural frequency can be lowerd to 160~170Hz, the frequencies will still be crossed ever time the pumps are turned on. Wouldn't this still damage the meter depending on how long the pumps jump from 160Hz to 200~400Hz range?

 

[vanstoja]

James08's description of two pumps stroking a fluid column in the piping at frequencies of 200 to 400Hz sounds irrational for positive displacement pumps. I would tend to go with Greg Locock's initial guess that these are actually the fundamental blade passing (IBP) frequencies of centrifugal pumps possibly with 7 impeller blades that can run at speeds of 57.14Hz and 28.57 Hz which suggests that pump drivers are 2 pole/4 pole induction motors with a slip of 4.76%. Possibly both pumps have motor drivers that run at two speeds with either two windings or a variable frequency power supply. Dealing with impeller blade passing pulses in piping fluid columns downstream of the pumps is a far cry from dealing with surging columns of water pulsing at frequencies as high as 200-400Hz. Amplitudes of the fundamental and any higher harmonics of pump IBP sources are needed to assess resonant response coincidence with some unknown mode of 200Hz vibration of the cited water meter. Possibly the IBP excitation travels better in the pipewall than it does in the fluid within the pipe making other modes of watermeter vibration as critical as the 200Hz mode. There is no sane answer to James08's query without much more detail on the actual vibration sources, transmission characteristics and the structural and hydroacoustic reponse modes of the watermeter.

 

[James08]

Even if the first mode natural frequency of the meter can be lowered to 160~170Hz, the second or third mode frequencies will still be around 200~400Hz and coincide with the frequency of the water column pulses.

 

[GregLocock]

Yup. So, what are you going to do? Get some data? or do you feel lucky?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.