CV Valve

[tankanghan]

Does the control valve CV change when the valve throttle ??
or is it the same Cv ?

 

[imok2]

As you can see it changes according to the gain

Gain is defined as: delta flow/delta stroke

Now, at three flowrates:
Q min = 25 gpm
Q op = 110 gpm
Q des = 150 gpm
we have corresponding Cv values of 6.5, 28, and 39. The corresponding stroke percentages are 35%, 73%, and 85% respectively. Now we construct the following table:

Formore go here:

http://www.cheresources.com/valvezz.shtml

 

[tankanghan]

Thanks for the response....is there a site somewhere where i can see the Cv at different valve opening for a generic globe valve 1-1/4" and below??? I spent some time but couldnt find any...not even from valve manufacturer...
I appreciate any response...

 

[DRWeig]

Tankanghan,

Can't say as I've see such before... Manufacturers typically only list wide-open Cv.

However, if you get hold of an ASHRAE Fundamentals book, there's a graph in there that describes characteristic curves for various generic types of valves, in terms of percent-of-flow versus valve stem position. You'd simply apply the wide-open Cv to the percentages.

Let us know if you find anything that works for you!

Good on ya,

Goober Dave

 

[DRWeig]

Tankanghan,

Try this:

http://www.spiraxsarco.com/resource...n-actuation/control-valve-characteristics.asp

Goober Dave

 

[tankanghan]

Thanks for the responses.

 

[tankanghan]

I have been looking at these sites you receommnded, but i have not found a Cv vs %valve open , or "pressure drop vs valve open"??? for a typical 3/4" to 1/1/4" globe control valve.... are there any more site you can recommend for this???
i did found the formula for the %flow vs %valve travel for equal percentage valve...but no Cv no pressure drop info...
Thanks

 

[DRWeig]

Tankanghan,

Again, these values are not typically published. What I was hoping to show from the curves in the link above is: there is not such thing as a "typical" globe valve. See how different the quick-opening, equal-percentage, and linear curves are? You'd need a table of Cv versus valve position for each type of valve to get anywhere close.

For your edification, here's a valve with such a table published:

http://www.kele.com/olcat/CV12/V-Series.pdf

Note that this is a modified-equal-percentage characteristic vee-ball valve.

For the globe valves you are interested in, you may actually have to measure performance to find your Cv versus stem travel chart.

Holler and let us know if you find a better answer!!

Good on ya,

Goober Dave

 

[tankanghan]

Thanks Goober Dave. I appreciate the input.
The Cv vs valve position is exactly what im looking for. The only thing i can find is wide open Cv.
The Ashrae characteristic curve show the % flow vs valve travel. But no Cv...i wonder if that %flow on the y-axis is also the %Cv???

 

[DRWeig]

Tankanghan,

Nope, the % flow isn't equal to % Cv. With constant inlet pressure, as the valve closes, flow decreases and pressure drop (i.e., differential pressure across the valve) increases. You need both numbers in order to compute part-closed Cv.

I don't think you'll find any relevant tables, it just doesn't come into play much. The Kele V-Series table of Cv values was determined in a lab.

Good on ya,

Goober Dave