Role of Valve Authority in Valve Selection

[PS]

Hi all,

"By definition valve authority: it is the ratio of full flow pressure drop across the valve to the total pressure drop in the circuit (incl the valve). I also read that valve authority signifies the difference between inherent valve characteristics and installed (actual) valve characteristics. Valve authority of N=0.5 indicates a good control over the flow."

I'm in need of clarification of the valve authority value. The tender in which I'm working now calls for a valve authority of 50% or high. And, the frictional pressure drop in the index circuit is 15m WC (1.5 bar), exclusive of control valve. In order to allow 50% valve authority, do I need to select a valve (for a particular flow and corresponding Cv) in such a way that it creates a pressure drop of 1.5 bar? If so, This will make select secondary pumps with 30m head. Frankly speaking, this really doesn't sound good. I'm spending half of my energy to make fluid cross the control valve!

I even thought of a better option. Why should I select the index circuit control valve for this purpose? Is it justifiable to do with a valve some where in the middle?

I also read in another article, some random stuff in Honeywell I guess NOT sure though, that Control Valve pressure drop should be equal to the sum of coil pressure drop, strainer pressure drop, and balancing valve pressure drop. This will provide a good valve authority and hence a better control. The article never mentions the valve authority value anywhere. Only relative terms were used, good, bad, fair, etc.. I also read in an article, a value of 4 psi pressure drop is common while selecting control valves.

Now, "which procedure to implement while selecting valves?" is the obvious question in my mind. I'm totally confused. Please do advice/suggest some article related to this. Please do ask in case any information required, if any.

Thanks in advance for the support.

 

[ChasBean1]

No, no, no. Just. No. I don't know where to start correcting this GCE. I started with a response then deleted it. Too many words...

 

[SAK9]

Control Valve pressure drop should be equal to the sum of coil pressure drop, strainer pressure drop, and balancing valve pressure drop.

This is all you need to know the rest is noise

 

[Drazen]

something is wrong with your pressure drop calculation or the circuit you are considering.

 

[quark]

VA can be anywhere from 30 to 70% and not exactly at 50%. Less than 30% indicates a bigger valve and you experience on/off rather than proportionate control. More than 70%, the valve may be in open condition, continuously.

 

[PS]

Thanks a lot @quark, @Drazen, @SAK9, @ChasBean1.