cmc boiler controls
TRANSCRIPT
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Without automatic controllers, all regulationtasks will have to be done manually. Forexample: To keep constant the temperatureof water discharged from an industrial gas-fired heater, an operator has to watch atemperature gauge and adjust a gas controlvalve accordingly (Figure 1). If the watertemperature becomes too high, the operator
has to close the gas control valve a bit - justenough to bring the temperature back tothe desired value.If the water becomes too cold, he has toopen the valve
To relieve our operator from the tedious task of
manual control, we automate the controls - i.e. we
install a PID controller (Figure 2). The controllerhas a Set Point (SP) that the operator can adjust to
the desired temperature. We also have to automate
the control valve by installing an actuator (and
perhaps a positioner) so that the Controller's
Output (CO) can change the valve's position. And
finally, we'll provide the controller with an
indication of the temperature or Process Variable
(PV) by installing a temperature transmitter. The
PV and CO are mostly transmitted via 4 - 20mAsignals . So, when everything is up and running,
our PID controller compares the process variable
to its set point and then calculates the difference
between the two signals, also called the Error (E).
Then, based on the error, a few adjustable settings
and its internal structure (described next), the
controller calculates an output that positions the
control valve.
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PROCESS
CONTROLLED QUANTITIES
(I/L WTR FLOW & O/L WTR FLOW)
MANIPULATED QUANTITIES
(I/L VLV & O/L VLV OPENING
SENSED VALUESPV
FEEDBACK CONTROL
SET VALUE
MANIPULATEDVALUES
MV
Tank level
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P
I
D
ERROR
INPUT
If P Controller is used , it will have fast response
but with steady State error.
If only I controller is used, its response is slow
compared to P , but no steady state error.
If D derivative controllers are used it gives an extremely fast
response as they give vast output for even moderate variations in
input, as their Gain is dependent on rate of change of input.Because of this they have the characteristic of ANTICIPATION.
These controllers anticipate What is going to happen and apply corrective
action. The greatest disadvantage is any slightest hunting in the processvalue will create severe disturbances in the process.
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The use of proportional control alone has a large drawback i.e. Offset. Offset is a
sustained error that cannot be eliminated by proportional control alone.For example, let'sconsider controlling the water level in the tank as shown in Figure above with a proportional-
only controller. As long as the flow out of the tank remains constant, the level (which isour process variable in this case) will remain at its set point.But, if the operator shouldincrease the flow out of the tank, the tank level will begin to decrease due to the imbalance
between inflow and outflow. While the tank level decreases the error increases and
proportional controller increases the controller output proportional to this error . Consequently,
the valve controlling the flow into the tank opens wider and more water flows into the tank. Asthe level continues to decrease, the valve continues to open until it gets to a point wherethe inflow matches the outflow. At this point the tank level remains constant, and sodoes the error. Then, because the error remains constant P-controller will keep itsoutput constant and the control valve will hold its position. The system now remains atbalance with the tank level remaining below its set point. This residual error is calledOffset.
P - CONTROLLER
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PI
P+I controller helps to have a finite output even though the
input (ERROR) is ZERO. as the integration of zero is some
constant value. Presence of P element helps in faster
response, and presence of I element makes the steadystate error to make zero.
PI - CONTROLLER
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PID
MV=100/PB (E(t) + 1/TE(t) dt + Td / t E(t) )
MV=100/PB(PV +t / T (E) + Td /(PV))MV=100/PB(E+t / T (E) + Td /(PV))
Note that the PID controller output is oscillating , this is due to theD controllers sensitive response to NOISE.
PID - CONTROLLER
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LT
Water I / l
Water o / l
MAIN steam.
EXAMPLE FOR FEED FORWARD CONTROL CONCEPT
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PROCESS
SENSED VALUESPV
FEEDBACK CONTROLHARDWARE.
SET VALUE
MANIPULATEDVALUES
MV
Disturbances
Controlled quantities
Sensed valuesof
Disturbances
FEED FORWARD & FEEDBACK CONTROL CONCEPT
Feedback control works to eliminate errors, but feed forward feedback controloperates to prevent errors from occurring in the first place.
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P+I_
+PV
SV X
COMPENSATED I/P VALUE
CS+
INPUT COMPENSATION METHOD
_+
PV
SV
P+I
X
COMPENSATED I/P VALUE
CS
OUTPUT COMPENSATION METHOD
P+I
ERROR DERIVING BLOCK
CONTROL ALGORITHAM
COMPENSATION BLOCK
P I D
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POWER PLANT INSTRUMENTATION
~
DRUM LEVEL MONITORING
FURNACE DRAFT
BOILER MASTER(FUEL CONTROL)
STEAM TEMP. MONITORING&CONTROL
AIR FLOW CONTROL
TURBINE LOAD CONTROLTURBO SUPERVISORY SYSTEM
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DRUM LEVEL MEASUREMENT
INTRODUCTION:Level measurement is very important aspect in thermal power plant.
We are having level measurements in the following areas
1. HOT WELL LEVEL2. DEARATOR LEVEL3. DRUM LEVEL4. LPH,&HPH LEVELS
Generally for measurement and control we use level transmitters . There areSeveral principles for level measurement.
1. Differential principle. : Drum level , Dearator level, HPH&LPH level
2. Displacer tube principle.: Hotwell level
3. Capacitive principle. : MOT oil tank level
4. Ultrasonic principle : ----
5. Air bubble principle. : proposed in BCW sump level.
Out of all, the Boiler Drum level measurement is crucial one. it uses
the differential method.
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HYDRAUSTEP DRUM LEVEL MONITORING
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PRINCIPLE OF OPERATION
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T
X
AIR
BUBBLER SYSTEM FOR LEVEL ME SUREMENT IN OPEN SUMPS
V CONST.
CURRENT
SOURCE
Bubbler type method is a cost effective way to measure level in open tanks only.In this method, a positive air/gas flow is passed through the liquid & back pressure
is sensed which is proportional to liquid head. Excess pressure is vented in the
form of bubbles .The bubble air through the tube must be maintained at a constant
Flow rate, A bubbler system typically consists of pressure regulator, needle valve,
pressure gauge, airflow meter, air line extending to the bottom of the wet well.
Transmitter
H= X.* Specific gravity.
6 bar1.2 barAir flow
50 l/h
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H L
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Now, assuming the transmitter has a reversed output, LRaw, calibrated with cold water, so that when the level
increases so does the analog signal, then it is possible toexpress the level, L, in % as a function of the densities ofthe water (both hot and cold) and of the steam:
L=100(100L raw) / K
K=(ws) / c
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DRUM LEVEL CONTROL
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DRUM LVL
(L) (R)
/2
DRUM PR
XCOMPENSATED
DRUM LVL
DRUM LVL SET
WITH VEL LIMITER
P+I
1ELEMENTDRUM LVL
CNTRLER
P+I
FEED FLOW
(BFPS DISCH)
TOT SH
SPR FLOW
RH SPR
FLOW
P+I
3ELEMENTDRUM LVL
CNTRLER
FEED FL TO DRUM
TOT STM FLOW
SEL SW
SEL SW
FEED FL CNTRLR
LOW LOAD
CVMAIN LOAD
CV
ST BY LOAD
CV
1 ELEMENT/ 3 ELEMENT
DRUM LEVEL Y FEED CVS
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P+I
3ELEMENTDRUM LVLCNTRLER
TOT STM FLOW
COMPENSATED DRUM LVL
DRUM LVL SET
P+I
SEL SW
SEL SW
FEED FL CNTRLR
MAIN LOAD
CV
ST BY LOAD
CV
1 ELEMENT/ 3 ELEMENT
FEED FL TO DRUM
P+I
DRUM LVL
CNTRL
BFP SCOOP
DP ACROSS FCV
(1) (2)
/2
P+I
DP ACROSS FCV
SV
SEL SW
BFP A,B,C SCOOPS
I
II
IF POS II selected
Force to manual
DRUM LVL BY BFP SCOOP
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COORDINATED MASTER CONTROL
Objective:To have a quick and stable load control thatmatches variable power demand.
CMC
TURBINE MASTER BOILER MASTER
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TURBINE MASTER
TURBINE MASTER
Power generation is controlled byturbine governor by varyingsteam flow to turbine.
CMND. FROM CMC
PR.DEVIATIONFROM
BLR. MASTER
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BOILER MASTER
BOILER MASTER
Air flowcontroller
Fire demandcontroller
THE BOILER HAS MANY SYSTEM VARIABLES AND AlSO HAS A GREAT DELAY INDYNAMIC RESPONSE. CMC SYSTEM ENABLES THE OPERATORS TO CONTOL THESTEAM FLOW RATE TO THE TURBINE. THIS IS DONE BY REGULATING THEBOILER INPUT IN ADVANCE TO MEET THE LOAD DEMAND..
FD-ABLADEPITCH.
FD-BBLADEPITCH.
PA-Damper.
Bypass PADamper.
CMND. FROM CMC
O/P FROMMW CNTRL(as feed forward comp.)
AFL CNTL
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Unit Operations Mode
There are four unit control operation modes,where in the boiler and turbine controllersboth operate simultaneously or any one ofthem in Auto.
They are as follows . . .
1. coordinated control mode.
2. Turbine follow mode. 3. Boiler follow mode.
4. Runback mode.
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Unit Master Control
This also called as coord inated mastercont ro l (CMC).
In this mode, both boiler master control
and turbine master control automaticallycoordinate the boiler and turbine to match
the load setting signal given by the
operator or load dispatch system.
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The fundamental of unit master controller is to control the steampressure at the inlet of governing valves of steam turbine. This controllerhas to give Load Setto Turbine Masterafter taking into consideration ofPressure Deviation, and similarly Pressure Set to the Boiler Master inadvance. Boiler master considers the following 2 points . . .
1. Normally if there is an imbalance between the boiler and turbine loads,the Steam pressure changes steadily (but not abruptly) because theboiler has a thermal capacity. It may be visualized as the time taken forthe steam pressure to change by a specified amount for a specifiedchange in load demand. For a large Boiler a change in demand equal to
full-load would cause the pressure to fall by 10 % in about 12 sec.
2. There is a lag of some 200 sec between a change in steam demand andthe response of the steam pressure action of a Master pressure control.
D
1 2TM 0003
LOAD SET FROMFREQ.
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TP0501
PR. SET TO HPBP
TP0503
PR. SET TO EHG
BM0509AIR FLOW CNTRL
BM0508FUEL FLOWCNTRL
F(x)
SS1861
Selector switch
A/M STNDS1889
DS1888
RATE LIMITER
MW VEL LMTVL1894
F(x)
FRQ. INFL OFF
TM0001
DS1890 SS1861A
LOWSELECT
TM0002
DS1891 SS1862
LOWSELECT
TSE UPPER MARGIN
UPPER MARGINSET FROM CRT
LOWER MARGINSET FROM CRT
TSE LOWER MARGIN
PI1992SUB
PI
PC1597
MWGENERATED
GW0001
PC1595-M
WCNTRL
PI1993SUB
SS19941=MW, 2=STM FLW
PI1994SUB
PI
PC1596-BL
R.MSTR
STEAMFLOW
MF0003
SS1867
SS1865
MP001
MP002
SS1596
SP.TOBM
SV1596
LoadVsPr
PI1997
BIASAS1596
PV1596
TP0507
PI1996
LOAD SET TO EHG
TP0505
PR. ACT TO EHG
TP0502
PR. ACT TO HPBP
LOAD SET FROMLDC
SCHEME TO ENSURE FUEL FOLLOWS AIR DURING LOAD INCREASE
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CL1620
TOT PA FLOW
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