cmc boiler controls

8/12/2019 CMC Boiler Controls

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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

>

cmc boiler controls

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