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what is diffrence between cascade & three element system.
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Answer Posted / raza
First thing that when you need to have a cascad control
action??
The Answer is when there is an inherent disturbing variable
which you have to compensate in final Controlled Output.
Example: You have to controll level of pressurised steam
drum, now when there is open vessel you dont have any
disturbing element, just get level actual reading as PV
send it to your controller and give Sp for controlling.
But now when you heat the water in Drum as a result
temperature as well as pressure also increses, if you look
towards level reading (PV) only drum may blast due to
excessive pressure and offcourse temperature will be
controlled by flow of water toward inside.
Now water flow------direction In
Temperature-----Increasing and in term will increase
pressure
Pressure-------Increasing due to increament in drum
contents (steam+Water)
At this stage you must have to see all three elements for
controlling level in drum.
Now one final LCV-01 whose Out is LY-01 whose PV is LT-01
SP is out you get from PCV-01 whose PV is PT-01 out is PY-01
(SP to LCV) and SP is out you get from LIC-01 whose PV is
LT-01, SP from Operator.
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