Override control to protect a boiler system

Report
Advanced Control Systems
Selective Control Systems
These are control systems that involve one
manipulated variable and several controlled
outputs.
Since with one manipulated variable we can
control only one output, the selective control
systems transfer control action from one
controlled output to another according to need.
Override Control
During the normal operation of a plant or during its startup or
shutdown it is possible that dangerous situations may arise
which may lead to destruction of equipment and operating
personnel. In such cases it is necessary to change from the
normal control action and attempt to prevent a process variable
from exceeding an allowable upper or lower limit. This can be
achieved through the use of special types of switches:
• HSS (high selector switch):- whenever a variable should not
exceed an upper limit.
• LSS (low selector switch); to prevent a process variable from
exceeding a lower limit.
Override Control
Override control to protect a boiler system
Steam
Discharge Line
Loop 1
Loop 2
PT
Boiler
LT
LC
LSS
PC
Water
Steam pressure in the boiler is controlled through the use of a pressure
control loop on the discharge line (Loop 1).
Water level in the boiler should not fall below a lower limit necessary to
keep the heating coil immersed in water thus preventing its burning out.
Therefore an override control system using an LSS is used. If liquid level
falls below the allowable limit, the LSS switches the control action from
pressure control to level control (Loop 2) whenever discharge pressure
reaches the upper limit.
An override control with HSS is used to prevent discharge pressure from
exceeding an upper limit. It transfers control action from the flow control to
the pressure control loop (Loop 2).
F
C
HSS
S
Loop 2
C
P
C
PT
Loop 1
FT
Gas in
Gas out
Motor
Compressor
Override control to protect a compressor
ii- Auctioneering Control
Hot Spot
Reactants
Temperature
Products
TT
TT
TT
TT
TT
Auctioneering system
0
L
T
C
Coolant
Auctioneering control system for a tubular catalytic reactor
Split Range Control
The split range configuration has one measurement only (controlled
output) and more than one manipulated variable.
Valve stem
position
Open
P
C
V2
Closed
PT
Reactants
3
Reactor
V1
V1
Products
V2
Reaction system with split-range control
15
6
9
Controller’s output signal
psi
Controller’s output
signal
Valve, V1
Stem position
Valve, V2
Stem position
3 psig
Open
Closed
9 psig
Open
Open
15 psig
Closed
Open
Adaptive Control
It is a control system which can
adjust its parameters automatically
to compensate for variations in the
characteristics of the process it
controls
Why do we need adaptive controller in
Chemical Processes?


Most chemical processes are nonlinear,  linearized models
that are used to design linear controllers depend on the
particular steady state around which the process is
linearized. As desired steady state operation of a process
changes the best values of the controller parameters
change.
Most of the chemical processes are non-stationary i.e. their
characteristics change with time e.g. decay of catalyst
activity in a reactor and the decrease of the overall heat
transfer coefficient in a heat exchanger due to the fouling.
This change leads to deterioration in the performance of the
linear controller which was designed using some nominal
values for the process parameters.
. Programmed or Scheduled Adaptive Control
New values of controller
parameters
Adjustment
mechanism
Auxiliary
measurements
Outer Loop
Set Point
Controller
Process
Controlled Output
Inner Loop
Suppose the process is well known i.e. an adequate mathematical model
for it is available. If there is an auxiliary process variable which correlates
well with the changes in process dynamics, by measuring the value of the
auxiliary variable we can program the adaptation of a programmed
adaptive control system. The figure shows the block diagram of a
programmed adaptive control system.
Example : Programmed Adaptive Control of a
Combustion System
Consider a burner where the fuel/air ratio is kept at its optimal value to
achieve highest combustion efficiency. This is maintained through a
ratio control mechanism shown in the figure below.
Fuel
Fuel
FT
FT
Desired ratio
Ratio
Adjustment
Desired ratio
FC
FC
TT
FT
Air
FT
Air
Optimal value of F/A depends on the process conditions (e.g. air
temperature). If we know how (F/A)op changes with Tair for max. efficiency
 we can use programmed adaptive control system as shown in figure
below.

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