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Answer Posted / warren holden
Yes, a control valve can be used without a positioner.
Some of the previous responses contain correct information
about certain elements of what a positioner actually does
however they do not address the fundamental issue from a
control perspective.
The positioner on control valve is essentially a cascade
(inner) loop acting as a high gain proportional controller
within the external (outer or master) loop (Shinskey, G.F.
1988). The positioner will be beneficial if its response
time constant is faster than the master loop element(s)
i.e. in the case of a temperature control (master) loop the
control valve positioner dynamics will undoubtedly be
faster than the temperature measurement change dynamics
(this being a fundamental requirement of cascade control).
However, the control valve positioner is of no use, and
actually a hindrance, on fast acting master loops i.e. flow
control with fast change dynamics. Hence control valve
positioners are often not used on flow control loops where
the outer (master) loop flow measurement and associated
controller can handle the control valve positioning quite
well themselves.
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