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Control Valve Pressure Drop


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#1 CHEMSTRONG

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Posted 17 April 2022 - 03:45 AM

Dear Sir,

 

I have certain queries on Control valve Pressure Drop.

 

For  a control valve on 2 inches line going to compressor, There is a flow control valve FIC  which maintains the flow of oil to compressor. Supply of Oil is at 36 kg/cm2 a and but what will be the pressure after control valve on this line .??? How much pressure drop shall i consider for a control valve on this Oil line to compressor ?

 

Please note that Control valve is there on this line going to Compressor and supply pressure of oil is at 36 kg/cm2 a .There are control valves on all lines for all stages of compressor here. For each stage, separate line is given and with separate control valve .

 

Regards,

CHEMSTRONG 



#2 Pilesar

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Posted 17 April 2022 - 06:51 AM

For pressure drop calculations, first find the pressure of the oil at the compressor. Then determine the pressure needed at the control valve to flow the required rate through the line. At that point, you have a complete pressure profile and simple math gives the control valve pressure drop.



#3 latexman

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Posted 17 April 2022 - 08:57 AM

A rule of thumb from an old company is the CV should have at least 30% of the system total frictional losses to have “good control”. BUT, that is just a starting point. Good engineering work and judgement come after that.

#4 CHEMSTRONG

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Posted 17 April 2022 - 09:48 AM

Dear Pilesar,

Pressure of the oil is around 2 kg/cm2a

Then by your logic, Control valve will have pressure drop of 35 -2 equals 33 kg/cm2a . As supply pressure before control valve is 35 kg/cm2 a .?

Plz advice on this...

Regards

Regards,
CHEMstrong

#5 CHEMSTRONG

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Posted 17 April 2022 - 09:49 AM

So if FIC is auto control valve mode then valve opening will be very less aroumd 40 percent as inlet and outlet pressures are very less..???

I need your expert advice on this

#6 CHEMSTRONG

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Posted 17 April 2022 - 09:53 AM

Even if Supply pressure at Control valve inlet is around 35 kg/cm2 a but downstream pressure at compressor stage inlet is very low...around 2 kg/cm2 a ....so ther will be very large pressure drop here... Control valve opening will be reduced depending on flow setting of Set point of Flow is my understanding correct??

#7 latexman

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Posted 17 April 2022 - 12:59 PM

If CV is FIC, then the set point/flow rate should be known. 2” line is known. Pressures at start and end of line are known. Are fittings known too? If so, a complete pressure profile can be calculated from supply oil reservoir (35 kg/cm2 a) to CV and from oil depository (2 kg/cm2 a) to CV (working backwards). Then, CV dP is the difference.

#8 CHEMSTRONG

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Posted 17 April 2022 - 10:49 PM

Dear Sir,

 

Pressure diff will be 33 kg/cm2 a ???

Flow of Control valve is known , but i want to know how control valve opening will change here?



#9 latexman

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Posted 18 April 2022 - 06:07 AM

Pressure diff will be 33 kg/cm2 a ???

 

I don't know, nobody really knows at the moment, but dP will probably not be exactly 33 kg/cm2 a.  It depends on the pipe and fittings layout/isometric between the supply oil reservoir (35 kg/cm2 a) to CV, and from CV to the oil depository (2 kg/cm2 a), and the oil flow rate and oil properties.  You have to calculate the frictional losses, elevation changes, and velocity changes from the two known pressures to the CV inlet and outlet flanges.  Then, knowing the calculated pressures at the CV inlet and outlet flanges, simply take the difference.



#10 CHEMSTRONG

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Posted 18 April 2022 - 07:06 AM

Dear Sir,

 

I do not want to consider Frictional pressure losses and other things as you mentioned.

 

My question is very simple, there are some variables here, like Valve opening,  Control valve downstream pressure etc. My upstream pressure is fixed around 35 kg/cm2 a here, so how my downstream pressure will be calculated ? Whether it will be calculated based on FIC valve opening or it will be determined by downstream backpressure ??

 

Best Regards,

CHEMSTRONG 






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