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How To Measure Flow Through A Control Valve


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#1 muthukmaar

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Posted 20 May 2019 - 10:05 AM

Three instrument air compressor Operate in parallel supplying air at 10ksc pressure,it is reduced to 7ksc pressure by a control valve and is supplied as Instrument air after moisture removal.
Is it possible to measure the flow through control valve at different manipulated values.

#2 Bobby Strain

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Posted 20 May 2019 - 11:31 AM

If flow is important, use a flow meter.



#3 Art Montemayor

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Posted 20 May 2019 - 12:09 PM

Is this an academic question?  If not, then Bobby Strain’s response is the experienced and correct answer.

 

If it is an academic query inquiring as to your knowledge on control valves, then you can refer to such theory as found in our website regarding valves:

http://www.cheresour...-selection?pg=1

 

The basic relationship in sizing a control valve is:

Q = Cv (DP/G)1/2

where,

Q = valve capacity;
DP = pressure differential; and
G = fluid specific gravity.

 

Obviously, from a theoretical standpoint - if flow is steady and the Cv is constant and known with accuracy for the flow conditions and type of valve, you can solve for the flow rate knowing the specific gravity and pressure drop.

 

The practical problem is that the theoretical conditions don’t hold out in the real world and the identity of the Cv isn’t accurate.  I have never tried to calculate flow in this manner and know of no one else who has.  As Bobby states, if flow rate is important, than install a flow meter - but it better be an appropriate one for pulsating, compressible flow.



#4 muthukmaar

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Posted 20 May 2019 - 08:05 PM

We have been asked to find the efficiency of the three reciprocating compressors, but there are no flow meters available. So, only asked whether we could calculate the flow

#5 breizh

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Posted 20 May 2019 - 08:17 PM

Hi,

 You may consider to use or rent  a portable flow meter ,ultrasonic type to perform the task . An other option could be to contract a third party familiar with this activity .

https://www.asco.com...calculator.aspx

Good luck

Breizh


Edited by breizh, 22 May 2019 - 03:31 AM.


#6 astro

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Posted 20 May 2019 - 08:43 PM

If you've got a reciprocating compressor, one other way to estimate flow is to account for volumetric efficiency and the machine's speed. For this, you need the compressor's cylinder dimensional data and you'll need to translate machine speed to piston speed.

 

Ultimately, you'll get a quantitative handle on the volume swept through for a given time frame, i.e. volumetric flow.

 

In my past, I was involved with a manufacturing works that included an 1960s era urea plant. Plant rates through the main CO2/NH3 compressor (reciprocating type) were inferred via the method described above but in that case, the machine was built and its base operating data was defined. I'm not sure whether you have that data available but, even so, it's worth being aware of.

 

I'll leave you to trawl through the detail but from a quick glance, these references look like they should set you down the right path:

https://jensapardi.f...compressor4.pdf

https://pdfs.semanti...7824d88ba13.pdf

or at least give you some ideas.

 

I've used dP across a control valve to estimate flow before but it's a method of last resort. However, if you're stuck and have nothing else, it's a lot better than a wild guess. Depends on how critical the quality of the flow rate number is for the work you're doing. For many field based assessments, if you end up with an answer within +/- 5% you're doing really well.

 

Art points out the valid issue of pulsating flow, which I suspect is a big part of the reason for the method used that I refer to up above. However, again, if you've got nothing else then applying an averaging approach might work - just depends on how sharp a pencil you need for the work you're doing. The other issue for you is to understand what's happening with the valve position and the valve's trim characteristic (linear, eq%, quick open?).

 

The other point about control valve sizing equations is that for gas, the critical pressure drop ratio factor xT can significantly influence the results you get. The value of xT is dependent on the valve type and will also vary by make.

 

The Parker Instrumentation Engineering Performance Report EPR 4103.3, Modernising the Sizing of Valves for Determination of Flow, provides a useful summary with technical detail of the issues relevant to xT. Search the web. You should be able to find it.


Edited by astro, 20 May 2019 - 11:34 PM.


#7 Bobby Strain

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Posted 20 May 2019 - 10:30 PM

What do you mean by efficiency?

 

Bobby



#8 muthukmaar

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Posted 21 May 2019 - 08:08 AM

Efficiency means volumetric efficiency

#9 rikakose

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Posted 22 May 2019 - 10:47 AM

If you know the composition,temperature of the gas, and opening of the control valve, characteristic of the control valve.

The calculated value is quite reliable. 


Edited by rikakose, 22 May 2019 - 10:47 AM.


#10 muthukmaar

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Posted 22 May 2019 - 08:57 PM

Yes that's the way I need to know

#11 Bobby Strain

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Posted 22 May 2019 - 10:51 PM

Why do you care? Is there a deficiency? Maybe all you need is to compare operating data on each to the original performance datasheet. If you want to know the condition of each machine, enlist the service or a company who can provide performance data.

 

Bobby



#12 muthukmaar

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Posted 23 May 2019 - 08:50 AM

Okay Sir
Thank you




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