8 replies to this topic

[Brightonnk]

Hi everyone, Ive spent a couple months studying ball valve (on/off) and have been looking at the pressure drop and cv value for when the valve is 100% open. Ive derived a method to calculate the rpessure loss and cv value and have had that validated from physical testing.

 

Im now on a new project where I need to produce cv values for some control valves... as my knowledge so far has been limited to on/off i need some help... I have a 4 valves with cv values for 100% open, as they were designed for on/off use this is the only data lying around, I know have been asked to find cv values for controlling the valve at equal percentage at different degrees. is this something that can be done by a simple formulae maniuplation? or is it a long process. Are there any control valve equations that I can use to find the cv value at different stages of the ball valve being open?

 

Thank You

 

I have found this website..

http://profmaster.bl...ng-vs-flow.html

 

but am unsure what 'lift' actually refers too...

 

 

[markymaark]

"Lift" is as you probably suspected,  is the % of the valve open as compared to the pipe DIAMETER.  Most pipes are cylindrical, so remember, opening (lifting) a valve up 1" on a 4" pipe does not mean 25% open.  Hence, why that curve is not a 1:1 relationship.  However, the relationship between % open and % total flow is 1:1, hence the "equal percentage".

 

The terms do sound identical, which is where your confusion is coming in.

Edited by markymaark, 18 April 2013 - 10:39 AM.

[Lai.CY]

Hello Bringtonnk,

 

I trust you already understand that control valves will throttles the shaft connecting the ball and the

actuator to meet the desire set point.

So, the flow coefficient, Cv value will constantly change depending on the percent lifting.

Liquid flows for non-vaporizing and vaporizing liquids have its own equation for calculating Cv.

 

Unlike on/off valve, that will opens fully and closes fully typically when it receives a signal.

Let's say open when getting a signal for high liquid level, and closes at a signal for low liquid level, etc.

 

As far as I know, the Cv equation is the same.

 

You can run a physical test at 25%, 50%, 75% and 100% opening, and you will see from the

Cv vs % lifting curve, it is a curve in between linear and equal percentage.

 

I think what you need is a positioner...

 

Hope I understand your question correctly...

[Brightonnk]

Hi, thanks for youre replies, i understnad the lift bit now...

 

I could do phyiscal testing however to order the valves and send them off for testing has a really long lead time... so I have the cv value of a 3" flanged ball valve of 513 US GPM

 

... If i wanted to find out the cv of this valve at 10° increments, I would use the same equation of ...

 

CV= Q*SQR(density of liquid/(density of water*Pressure differential))

 

But i know that my flowrate (Q) will change as only a percentage of it is flowing... so using this website

http://www.spiraxsar...acteristics.asp

 

I've worked out my different flow rates at 10% open - 100% open...

 

but now how do I implement this into the cv equation? Does the pressure differential stay the same?

 

I don't know if im just missing something easy

[Brightonnk]

Please see my attached spread sheet

 

Thanks

Attached Files

•  30 F55 lift v flowrate.xlsx   12.64KB   49 downloads

[Ajay S. Satpute]

Dear Brightonnk,

 

Please refer below link. Check the attachment in post # 24. This might help you confirm your calculation of Cv with vendor values.

http://www.cheresour...alve-selection/

 

Regards.

 

Ajay

[Brightonnk]

Hi Ajay,

 

Thanks for that information.. I have 2 question:

1.) what is meant by 'Trim'

2.) my valve is 3" and has a cv value of 500 at 100% open, so the table doesnt really relate to my valves? or am i understanding the meaning of the link wrong?

 

Thank You

[Ajay S. Satpute]

Dear Brightonnk,

 

1. The internal elements of a valve is called valve's trim. The valve may have different trim size (basically flow area). So the same size valve shall have different Cv value depending upon its trim size. PFA some info. regarding the trim.

 

2. The attachment (in the link) provides Cv values for globe valve meant for flow control. And you needed to control flow using ball valve. I guess I miss-read your post, may be because I have not seen ball valves used for flow control. Sorry for that. Nevertheless, the Cv values are different as we talked about different type of valve. Please find attached file for the difference in Cv values for globe and ball valves for the same size.

 

3. As I didn't have Cv values (to check) at different openings for ball valve, I had to use the attachment mentioned in sr. no. 2. First page of the same, mentions 3" valve with trim size 2.62".

 

Cv at % opening is as below;

% opening: 10%   50%    100%

Cv:              5.8     38       104

 

Now for 100% opening, Cv is 104, which means if the flow is 104 USGPM, delta P shall be 1 psi.

If I thottle the valve to 50%, pressure drop shall increase for the same flow rate i.e. 104 USGPM.

Cv = Q *Sqrt(SG/delta P)

Considering SG as unity,

Cv= Q*sqrt(1/delta P)

 

For 50% opening, Cv is 38 (as per valve vendor)

38 = 104 *sqrt(1/delta P)

Delta P = 7.5 psi

 

For 10% opening, Cv is 5.8 (as per valve vendor)

5.8 = 104 *sqrt(1/delta P)

Delta P = 322 psi

 

You may get reduction in flow rate if you wanna keep the pressure drop same, as you go on throttling the valve.

 

4. Is this exercise is for academic purpose or is it a real problem?

 

 

Regards.

 

Ajay

Attached Files

•  trim.pdf   449.19KB   35 downloads
•  Cv values for ball_globe_etc.pdf   100.46KB   40 downloads

Edited by Ajay S. Satpute, 24 April 2013 - 06:14 AM.

[Brightonnk]

Hi, Thank you! this definitely make a lot more sense!!

 

And I'm currently at University doing a final project with a valve manufacturers

 

Thank You