Clearance CV Calculation 1

[Dinu Stan]

Hi

i just wanted to calculate the clearance CV for control valves.
i have calculated the cross sectional area for plug OD and cage ID and K factor for the cage.

Herewith i have tried with actual CV calculated formula

CV = Area * K factor

But i couldn't get the exact clearance CV values since the declared sizing value is mismatched with the calculated value.

Could you please anyone help to find the exact formula for the clearance CV calculation?

Thanks

Dinu...

There are no secrets to success. It is the result of preparation, hard work, and learning from failure.

 

[casflo]

If Cv is the valve flow coefficient, you need to know the flow rate and the pressure drop in order to calculate it.
The equation to obtain the Cv is:
Cv = 0.037Q[1/vDP]**0.5
Q is the Flow rate in m3/h, v the specific volume in m3/kg and DP the valve pressure drop in kg/cm2

 

[Dinu Stan]

Hi casflo,

Thanks for the reply... My question is different and very general from the design point of view. it is not related to sizing...
It is irrespective of the flow rate and pressure drop to find out the clearance flow in the valve between cage and plug by their area with the help of k-factor.

Dinu...

There are no secrets to success. It is the result of preparation, hard work, and learning from failure.

 

[3DDave]

Since Cv in valves relate flow to pressure drop and you say that's not what you mean then there is some other factor you are applying Cv to that isn't normally used to describe valves.

Perhaps you could tell what the Cv you want is used for and then people could figure out what you are looking for.

 

[Dinu Stan]

hi 3DDave

From the design point of view, Specific Valve CV will be calculated based on the area for a whole which is available in the flow passage area in the valve with the help of some coefficient (normally called as K factor universal). here there will be a term clearance flow which is derived from the clearance area between the plug and cage.

for example:

My Plug OD - 1.618" and Cage Id - 1.626"
so the clearance area is - 0.0204 sq.in.

we need to calculate the CV for the clearance area = Area * K factor (here i will take k factor as 26 from the cage)(normally k factors is varied based on the design for each and every manufacturer.)

CV = 0.0204 * 26
CV will be 0.5297

but i need to confirm the formula which i have used. because we are using the same formula for finding the individual component CV calculation. but i am not sure about clearance CV. since my calculated value and defined value in our system was different. so i have made this post for clarification.

Hope it will clear what i have really asked?


Dinu...

There are no secrets to success. It is the result of preparation, hard work, and learning from failure.

 

[3DDave]

Is CV = Clearance Volume and then Clearance CV is Clearance Clearance Volume?

I have never heard of any part of a valve called a cage and you haven't said what the number tells the user of the valve about the valve characteristic.

 

[Dinu Stan]

3DDAVE

Please read the Emerson handbook for valves for your further understanding about valve components and clearance flow...

https://www.emerson.com/documents/automation/control-valve-handbook-en-3661206.pdf

here clearance flow will calculate irrespective of characteristics.



Dinu...

There are no secrets to success. It is the result of preparation, hard work, and learning from failure.

 

[SPDL310]

Dinu you are comparing your calculated calculate clearance flow Cv to you employers database Cv value, and seeing a discrepancy if I understand properly? Your formula seems to be correct for this type of sizing, but I believe that your K factor may have some issues. I am goin out on a limb here by making multiple assumptions, but I believe your employers data base is likely to come from test data. That test data is probably comes from flowing through seats with fixed orifice diameters in them. If that is the case then you would likely need to adjust your K factor down due to the fact that you are developing laminar flow at the wall interfaces, and the actual turbulent flow area is much smaller than what you would see from the roughly 10 - 11 1/64" bean that would have the same diameter as your clearance flow. You have a .004" wide annulus with a mean diameter of 1.622" Vs a 0.161" diameter hole. Your defined vale may also be linearly interpolated from larger diameters which is not an accurate assumption as the circumference to area ratio does not change linearly.

 

[3DDave]

Dinu - the term "clearance CV" does not appear in that document. Not as helpful as you thought and does not further my understanding of your question. However I suspect SPDL310 is right and you will not find a formula because flow characteristics are dependent on far more than an annular dimension.

 

[Dinu Stan]

SPDL310

Thank you so much for your help. it means a lot.
i got your points, Yes the data had come from the test values and even we are using the some error factors to find out non tested valve CV values. however i am just trying to confirm the theoretical method behind the fact of test values.

Thank You Once again...

Dinu...

There are no secrets to success. It is the result of preparation, hard work, and learning from failure.

 

[Dinu Stan]

3DDave (Aerospace)

Clearance flow and Clearance CV having the same definition as well the same meaning only and the changes will be in the units... it will helpful when we are in to valve application. the formula will be correct only as SPDL310 also mentioned. but it has to adjust the K factors due to some flow type variations and area ratio issues. hope i can do CFD and try to get some further more clear view....


Thank You so much once again.

Dinu...

There are no secrets to success. It is the result of preparation, hard work, and learning from failure.