CV 1

[jboy]

If a control valve has a CV of say 300, with the valve fully open the flow is 800m3/hr. Does this mean that if the valve was only open 50%, the CV would be 150 and the flow would be 400m3/hr. At 25 % would the cv be 75 and the flow 200m3/hr and so on.
Thankyou.

 

[ghillis]

It is not likely in a real-world system. It depends on the pressure supply and drop in the piping. If there is no drop in the piping and the supply head is constant, then yes. Since as you open the valve the flow goes up, the effect of the pressure drop in the piping becomes more significant as the valve moves to the open position. In a pumped system, the supply head will drop as the valve opens. Gravitational affects may even enter into some systems. Also percent open may not correlate well to Cv open.

 

[way]

jboy,

I think that ghillis is probably correct. I don't think that it works that way.

Cv is defined as the flow of water (60 F) in gallons per minute with a pressure drop of 1 psi across the valve.

Cv=Q*(Density/(PressDrop*62.4))^.5

and using this equation

Q=Cv*(PressDrop*62.4/Density)^.5

How valve position fits into this, is a question that must be answered by the valve manufacturer. I'm sure that they test each valve and know the relationship between CV, Q, and PressDrop.

More Importantly.....
Control valve manufacturers typically know the max Cv of a valve, and using your max flow conditions they calculate the Cv required. Then they match the Cv required with a valve with the appropriate Cv. That way you will get a control valve that is best suited for your needs and should have adequate control. Now if they really mess up, and oversize the Cv....then you probably will have extremely erratic and poor control at low flow conditions because small movements in the valve will result in large changes in flow.

Hope this helps.

 

[DaleE]

Ask the valve mgfr. for a cv curve.
It will show you the stroke vs cv relationship.

 

Realistically you also need to know what type of flow characteristic that you are looking for in order to apply the valve for it to be useful as a control element. The three type of characteristics are linear, equal percentage, and quick opening. I suggest that you try and find a volume put out by Fisher Controls called the "Control Valve Handbook".

 

[Robg]

Cv varies with position based on the trim characteristics. For example, a butterfly valve will have poor control at 10% open compared to a cage guided globe with anti-cav trim at 10% open. The full open Cv will also vary greatly between valve styles. Globe valve are available with characterized trim allowing very small Cv change with stroke at the low end and large changes at the high end. Bottom line is, you need to use a position vs. Cv to size the correct valve.

 

[whylie]

Hi Jboy,

Very good question..I can furnish real data for equal percentage globe valves..

Valve size %opening Cv

1" 10 0.79
1" 100 13.2

4" 10 4.9
4" 100 190

For a linear valve ,the Cv(x)=Cvmax [x/100]
where x is % opening (stem travel )
and Q=flowrate = Cv (x) square root of {[p1-p2]/s.g}

delta P ,will depend on hydraulic data of system(Q,P1..etc)

Regards, Whylie

 

[MortenA]

Two types of control valve trims are commonly seen:

1) Equal%
2) Linear

Equal % means that if you close the valve 10% you reduce the flow by 10% if upstram/downstream pressure remains constant (or at least thats what the manuf. tried to obtain).

Linear means that the Cv changes approx. linear when closing. But since the flow isnt a linear function of Cv (see previous postings) then the flow wont change as a linear function.

Best Regards

Morten

 

[marklobo]

The term "Equal Percent" means, by MortenA's example, for each reduction in stroke, an "equal percent" reduction of capacity will be achieved. For the small control valves we manufacture, it is 50%. It could be other values, including, though inappropriately, 10%.

The formula for capacity at travel x (CsubVx),is (from Lyons): CsubVx = CsubV * R ^((x/T)-1), where CsubV is valve capacity at full travel, R is "rangeability", T is full travel.

 

[MortenA]

Q to marklobo

Have i misunderstood equal%? Maybe its because im not certain what stroke menas. Could you explain that? I checked with Perry that seems to agree with my definition. Again it seems to be because i dont know what a "stroke" is. I was refering to steam travel in % of full open.

The way i read what you write then the opening could then valve position can only be changed in steps? And your valves have only 3 positions: full open, halfway closed (one "stroke equals 50% reduction in capacity) and then fully closed?

Best Regards

Morten