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# Wake Frequency

08/02/2017 3:29 AM

Calculating wake frequency we must need fluid velocity.But in case of a thermowell to be installed at the bottom of a vessel, where total amount of fluid will come, it will be agitated,and then transfer to some other place via gravity force or pump.Now in this case i dont have flow rate/Velocity of fluid into the vessel.So how can i calculate wake frequency for the thermowell? Should i consider in late/out late flow rate of the vessel or the RPM of the agitator ?Any suggestion guys.

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#1

### Re: Wake frequency

08/02/2017 3:34 AM

A <...wake frequency...> is relevant to a vortex-shedding flowmeter. Thermowells contain temperature instruments, which are part of first-order systems that don't have a frequency in themselves; any frequency response is likely to be measured in ones and tens of seconds, and not milliseconds as would be the case with a flowmeter.

Any vessel with an agitator will achieve thermal equilibrium throughout the fluid at a speed far higher than a practical temperature instrument can detect. Frequency calculation for such an arrangement is something not previously encountered here, on the basis of a distinct lack of need.

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#2

### Re: Wake frequency

08/02/2017 7:30 AM

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phoenix911
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#3

### Re: Wake frequency

08/02/2017 8:08 AM

Dear phoenix911

Thank you for your response.Please let me know the information you required.I can give you.Normally for a thermowell to be installed in a pipeline required information are :Process data: Velocity,temp,density,Thermowell dimensions etc.In this case i have all of them except velocity of the fluid into the vessel.

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#4

### Re: Wake frequency

08/02/2017 8:28 AM

Thank you,... I feel their is something missing here, and you would have more information on your process that what a stranger would have where Wake Frequency is used to determine the size of the Thermowell.

Since you need the flow rate to determine this,.. you need to take an educated guess based on with what information you have and an educated guess with what's missing..

And far as calculating your thermal size,... there are plenty of suppliers that have calculators online that can assist you, with the information you have.

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#7

### Re: Wake frequency

08/02/2017 10:36 AM

To get the average velocity in a pipeline, divide the volumetric flow rate by the cross-sectional area of the pipe:

v = Q/A

Make sure you keep units consistent.

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#5

### Re: Wake frequency

08/02/2017 9:14 AM

Thermowells fitted to pipes are often installed in one end of a tee such that the tip of the thermowell impinges the static streamline just before the fluid changes direction into the side port of the tee. Provided the fluid velocity and the pipe diameter are compatible (<3m/s for liquids and <10m/s for gases, for example) there is rarely any need to consider vibrational frequencies as a result of the fluid's passage.

Consideration of the resonant frequency of a thermowell installed in a vessel in most process industries is somewhere between [extremely rare] and [practically unheard of].

Most thermowells are machined in such a way that the overall shape doesn't have a dominant resonant frequency. In any case, the presence of a temperature measuring element inside it is likely to lower it owing to the presence of more mass than for an empty thermowell and an increase in stiffness as a result. Thermowell manufacturers do not tend to test and publish figures as a result.

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#6

### Re: Wake Frequency

08/02/2017 9:30 AM

If it's your own wake, the frequency would usually be one/lifetime. if you're discussing attendance of the wakes of others, the frequency would be expected to increase with your age, up to about 75 yo.

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#8

### Re: Wake Frequency

08/02/2017 1:31 PM

Hilarious!

You do know why they call them "wakes"? Same reason they used to attach a bell to a pull string fed into the coffin...

It is like when all those contribution solicitations arrive in the mail, and you get buried alive in a mountain of junk mail.

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#9

### Re: Wake Frequency

08/03/2017 12:24 PM

I had to dig into this thermowell stuff a while back. If the TW vibrates it can break and let the stuff in the pipe out. ASME PTC 19.3 TW-2016 is used for solid drilled bar stock thermowells in tapered, straight or stepped designs to address if the wake frequency will be a problem or not. Google “thermowell wake frequency” and you can find information, examples of damage and on line calculators.

And some situations may not lend themselves to use the side port of a tee as PWSlack suggested. For 1 or 2 inch pipe, fairly easy and when done right gives the best results vs. other configurations. Might not work for a 42 inch pipe as well, e.g. insertion of the TW into the fluid straight in thru the wall.

Information on the standard: http://www2.emersonprocess.com/en-us/brands/rosemount/temperature/thermowells/calculations/pages/index.aspx

Just the top 3 hits from my Google search.

And yes, you do need to know the fluid velocity to do a wake frequency calculation. But the usual situation is flow in a pipe, like water in a river; not agitated in a vessel as you describe. And if you agitate it long enough, like with a propeller that spins in only one direction, you will have a fluid velocity; just not an easily (for me) determined one.

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#10

### Re: Wake Frequency

08/05/2017 2:52 AM

Thank you JL dowell for your nice answer, So i have to consider agitator rpm and in this case, i have my agitator rpm.Now i need fluid velocity.Is there any way?or if i convert this rpm into simply mean velocity by basic physics formula would that be okay or are there a lot of fluid mechanics inside it ? Pardon me if something is silly here.

Thank you again

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#11

### Re: Wake Frequency

08/07/2017 3:06 AM

It would benefit the forum to know why this calculation is necessary in this particular case.

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#12

### Re: Wake Frequency

08/07/2017 3:13 AM

Perhaps the whim of some local bureaucrat. Or regulatory org. Or some hotshot specifying engineer.

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#13

### Re: Wake Frequency

08/07/2017 3:15 AM

Based on the information received so far, the forum might never discover this and put it to good use. So much for Monday mornings...

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