Inherent Control Valve Characteristics

[cornejoi]

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.

I'm gonna post this in the measurement and control forum as well.

Thanks guys.

--Igor

 

[Fzob]

Some basic stuff

Use a control valve designed for throttling service.
Size the valve trim for present operating conditions; avoid oversizing.
Select the inherent flow characteristic by calculating the installed static characteristic.
The valve should provide steady flow when the trim position is held fixed.
Minimize friction in the valve and actuator (seals, bearings, packing).
Eliminate lost motion in linkages and maximize stiffness of rotary shafts.
Select the actuator type and size for best control with the positioner.
Select high proportional gain in the positioner.
Test with the appropriate amplitude and shape of input signal for the control loop.
Judge installed control valve response by change in the process variable.


The gains of the process and transmitter are usually fixed by the plant design. Some controllers
provide either gain scheduling or an output characterizer block; a static equivalent to the latter is
an input characterizer in the positioner (signal-to-position characterizing).1 However, because the
position-to-flow gain of the valve occurs after the signal-to-position dead band, the most important
task is selecting the best valve size and best shape of the inherent flow characteristic. High position-to-
flow gain multiplies the effect of dead band; for example, if this gain is 4.0 and the dead band is 0.5%,
the smallest process variable change that can be made reliably is 2%, regardless of characterization
in upstream blocks.

From

Process/Industrial Instruments and Controls Handbook (5th Edition)

I hope this helps.

 

[Guidoo]

This has been discussed in: thread698-79132