Answer / bharathi

Cascade loop is the general terminology in control system.
It consist one master and slave controller. In that one
controller output is given as set point to the other
controller. The secondary controller output will go to final
control element.
Three element controls is technique which is used in boiler
drum level control. In that, by measuring steam flow
(demand) we are controlling the feed water flow to maintain
the drum level. In this technique we are using cascade loop.

Answer / senthil.doosan

cascade control - it is a combination of two controllers in
which one act as a master and other as a slave.i.e the
output of one controller is fed as a setpoint to other
controller.
we use this cascade control in three element system(drum
level control)
three element-level,feed water flow and steam flow.
theory:
single element:here the feedback of the drum level is given
to feedwater flow controller to control the level in the
tank.the output of this controller controls the 30% valve
three element:the output of this controller will be summed
with the mass steam flow(combination of density and volume
flow rate) and given to the other controller(input is
feedwater flow) as a setpoint.the output of this controller
controls the 100% valve.

Answer / 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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