Can anyone provide me with a a web link, book refrence, or any experience in how to choose an inherent valve characteristic?

I've looked everywhere for a praticle approach for choosing valve characteristics and so far I've only found "guidelines" that valve vendors provide.  Moreover, most valve vendors tell me to stick to =% since you can now characterize in the positioner.  My question is then "Why have different valve cages if characterization can be done in the positioner?"

Anyway, characterization in the positioner - based on my experience - is not too popular of an option.

So, what I'm looking for is a praticle method of determining which valve characteristic is best suited for a specific application and why.

If anyone is a guru in this area or knows where I can find good, regarded information on this topic I'd greatly appreciate it.

The relationship between the valve stem position and the flow is defined as the valve characteristic.  This relationship with constant (design value) pressure drop is termed the inherent characteristic, and the relationship in a specific process in which the pressure drop may vary with flow is termed the installed characteristic.  Two related units are used for the characteristic; one is flow in gallons of water per minute per stem percent that is used for sizing control valves.  The other is percent maximum flow per stem percent which is used to plot typical valve characteristics.

The charecterstic selection is explained in a reference book called "Control Valves" by Guy Borden and Paul G. Friedmann and is available through ISA.

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