boiler turbine control basis

1

BOILER / TURBINE CONTROL BASICS

I. TYPE OF CONTROL

1. BOILER – A. ANALOG CONTROL INTENSIVE AND COMPLICATED (FOR ALL TYPES)

B. DIGITAL CONTROLS

GAS OIL FIRED

SOLID FUEL FIRED

FLUIDIZED COMB.

VERY CRITICAL CRITICAL CRITICAL

C. MONITERINGI. SWASII. STACK GAS MONITORING

2. TURBINE – A. ANALOG CONTROLS – LESS COMPLICATED & FEW IN NOS

B. DIGITAL CONTROLS – VERY CRITICAL & FOR TRIP & DRIVE CONTROLC. TSI & GOVERNOR CONTROLS – VERY CRITICAL WITH RESPECT TO CONTROL AND SPEED OF RESPONSE

II. FAIL SAFE PHILOSOPHY OF CONTROL (FOR BOILER + TURBINE)

FAIL SAFE:1. INTERLOCKS / SHUTDOWN2. TRANSMITTERS / SENSORS3. FINAL CONTROL ELEMENTS

TYPE OF FAILSAFE POSITIONS

1. FAIL OPEN2. FAIL CLOSE3. STAYPUT

FAIL OPEN / FAIL CLOSE IN CONTROL VALVES

1. ON SIGNAL FAILURE2. ON POWER AIR FAILURE

STAYPUT:ON POWER AIR FAILURE

REDUNDANCY REQUIREMENTS

MOST OF THE EMERGENCY CONTROL SYSTEMS ARE STATIC.

i.e. THEY JUST REMAIN WITHOUT OPERATING FOR QUITE SAME TIME.

HOW ARE WE SURE THAT THE SYSTEM OPERATES WHEN IT IS REQUIRED TO OPERATE?

I. IN NORMAL LIFE FOLLOWING ARE FEW EXAMPLES

1. UPS / diesel engine operation on failure of EB supply. 2. A fuse to blow on high current or short circuit. 3. An umbrella to unfold on sudden rain. 4. Kerosene stove to light on when gas cylinder getting emptied at home. 5. Torchlight to light while power is off. 6. Fire fighting co2 cylinder to open while fire catches up in a public place like cinema theatre. 7. Alarm clock to work for a critical early morning wakeup. 8. A relay to energize and trip the equipment on faulty condition.

II. TO IMPROVE ON THE DEPENDABILITY OF

THE CONTROL SYSTEM IT IS NORMALLY

HELD IN ENERGIZED CONDITION AND DE-

ENERGIZED DURING A FALLTY CONDITION.

A. SAFE FAILURES

These are called safe failuresOR

FALSE TRIPSOR

NUISANCE TRIPSSince they stop the process unnecessarilyThese faults REDUCE THE AVAILABILITY AND

CAUSE PRODUCTION LOSS

A wire opens

Output De-energizes

Processshutdown

I.

Fault in system

Output De-energizes

Processshutdown

II.

B. DANGEROUS FAILURESFaults that cause the outputs to remain Energized are generally not detected and are Considered “DANGEROUS FAILURES” Because the system cannot tell the difference Between ‘Normal Operation’ and ‘Failure’And therefore cannot take the process to aSafe state when required to do so.

FAULT TOLERENCE“Ability to tolerate a single failure and Continue to operate”.

Can be implemented using various redundancySchemes.

CHARACTERISTICS OF SYSTEM ARCHTECTURES FOR SAFETY SHUTDOWN

A 10011 Out Of 1

S

Process

WHEN A FAILS, PLANT TRIPS:

SAFETY - LOW

AVAILABILITY - LOW

B

1 Out Of 2S

Process

WHEN ‘A’ OR ‘B’ FAILS, PLANT TRIPS:

SAFETY - V.HIGH

AVAILABILITY - LOW

A

1002

B2 Out Of 2

S

Process

WHEN A & B FAIL THE PLANT TRIPS:

SAFETY - LOW

AVAILABILITY – V.HIGH

A 2002

Process

A

S

B

C

WHEN 2 (OR 3) OUT OF THE THREE (A,B,C)

FAIL THEN ONLY PLANT TRIPS:

SAFETY - HIGH AVAILABILITY - HIGH

2 Out Of 3

2003

SAFETY AVAILABILITY

1001 SINGLE TRANSMITTER LOW LOW

1002 2 OUT OF 1 TRANSMITTER AVERAGE HIGH

2003 2 OUT OF 3 TRANSMITTER HIGH HIGH

REDUNDANCYAT SENSOR LEVEL FOR ANALOG SIGNALS

1002

FT1

FT2

> OUTPUT

HIGH

SELECTION

2003

FT1

FT2

MID VALUE

SELECTIONOUTPUT

FT3

REDUNDANCY IN CONTROL SYSTEM

1. REDUNDANCY IN DCS / PLC

a) AT PROCESSER LEVEL

b) AT POWER SUPPLY LEVEL

c) AT COMMUNICATION LEVEL

d) AT I/O LEVEL

e) AT OPERATOR STATION LEVEL

CONTROLLER AND DATA

ACQUISITION SYSTEM

I/O MODULES

OPERATING & ENGINEERING STATION(BOP)

OPERATING STATION(BOILER)

CONTROLLER AND DATA

ACQUISITION SYSTEM

LOG PRINTERALARM &EVENT PRINTER

OPERATING STATION(TURBINE)

BASICS OF CLOSE LOOP CONTROLS

In order to achieve proper automatic

control of a process, it is necessary to know the

characteristics of the process material itself,

as well all the devices used for process control.

A given process might require the control

of temperature, pressure, density and level,etc,.

Also regulating one variable can affect

other variables.

For example, regulating temperature in a

process might affect the pressure and the

density.

Regulating the flow rate of material to a

process may also affect the level of material in

storage vessel.

It is important, therefore, to select a

controlling device which most accurately affects

the variable to be controlled.

ON – OFF CONTROL

In this control, when the value of the

measured or controlled variable is at or above

the SP, the final control element is closed. When

the range is below the SP, the final control

element is open.

A common example of ON – OFF control action

is a thermostat. Consider the thermostat is set

for 68.F. If the temperature drops below 68

.F,

the heating unit is actuated.

0%

100%

FE CLOSED

FE OPEN

RANGE OF

MEASUREMENT SP

OFF

ON

ON-OFF CONTROL

ON-OFF CONTROL

PROPORTIONAL ACTION

This action provides the control valve with

variant positions between ON and OFF. The

position of the FCE is not simply open or closed,

but varies, depending on how much the value of

the measured variable is above or below the SP.

The amount of energy to the process varies

accordingly.

Its actuating output is proportional to the

error signal.

c(t) = Kce(t) + Cs

where, c(t) - controller output

Kc – proportional gain of the controller

e(t) - error signal

Cs – controller’s bias signal (i.e, its

actuating signal when e(t)=0)

A proportional controller is described by

the value of its proportional gain Kc or

equivalent by its proportional band PB, where

PB=100/Kc.

The proportional band characterizes the range

over which the error must change in order to

drive the actuating signal of the controller over

its full range .

usually,

1 < PB < 500

0%

RANGE OF

MEASUREMENT

100%FINAL ELEMENT

FULLY CLOSED

FINAL ELEMENT

FULLY OPEN

INTERMEDIATE

POSITIONING OF FCESP

PROPORTIONAL CONTROL

PROPORTIONAL CONTROL ACTION

LT

CONTROLLER

TANKSP

P CONTROLLER

e

+-

PROPORTIONAL ACTION WITH RESET

This action enables the FCE to assume

intermediate position. In addition, it can shift

the relationship between the FCE and the value of

the measured or controlled variable. The

controller continues the corrective positioning

of the FCE until the measured variable returns to

the desired valve.

Its actuating signal is related to the error

by the equation

c(t) = kce(t) + e(t)dt + Cs

where, TI – Integral time constant or reset time,

in min.

The reset time is an adjustable parameter and is

sometimes referred to as minutes per repeat.

Usually it varies in the range

0 < TI < 50 min

0

t kc

TI

Some manufacturers do not calibrate their

controllers in terms of TI but in terms of its

reciprocal, 1/TI (repeats/min), which is known as

reset rate.

Consider the error changes by a step of magnitude

e.Initially the controller output is Kce (the

contribution of the integral term is zero). After

a period of TI min the contribution of the

integral term is

kc

TI

e(t)dt0

TI kc

TI

= eTI = kce

It is clear from the above equation that

the integral control has repeated the response of

the proportional action. This repetition takes

place every TI min and has lent its name to the

reset time.

PROPORTIONAL-PLUS-RESET ACTION

Cs

Cs + kce

Cs + 2kce

Cs + 3kce

0 TI 2TITime

c(t)

PI CONTROLLER

CONTROLLER

DPCELL

FLOW

SPe

FLOW ELEMENT

PROPORTIONAL ACTION WITH RESET

AND DERIVATIVE

Derivative, or rate action, adds even more

flexibility to the movement of the FCE. Response

is slow in same processes because a long period

of time is needed to detect and correct changes

in the measured variable. Derivative action

provides correct positioning of the FCE related

to the rate at which the value of the manipulated

variable is changing.

The output of this controller is given by

where, TD – Derivative time constant,in min. With

the presence of derivative term(de/dt), the PID

controller anticipates what the error will be in

the immediate future and applies a control action

which is proportional to the current rate of

change of error.

c(t) = kc e(t) + e(t)dt + kcTD + Cs

kc

TI0

t de

dt

Due to this property, the derivative control

action is sometimes referred to as anticipatory

control.

DRAW BACKS OF DERIVATIVE CONTROL ACTION:

For a response with constant non zero error

it gives no control action since de/dt=0.

For a noisy response with almost zero error

it can compute control action, although it is not

needed.

PID CONTROLLER

TO

PROCESS

STEAM HDR

SPRAY

WATER

TE

TT

PT

DSHPRV

STEAM FROM BOILER

FEATURES OF CONTROLLERS

PROPORTIONAL CONTROLLER

• Accelerates the response of a controlled

process.

• Produces an offset (i.e, non zero steady

state error) as a result of sustained load

change.

• Eliminates any offset.

• Produces sluggish, long oscillating

responses.

• If we increase the gain Kc to produce faster

response, the system becomes more

oscillatory and may lead to instability.

INTEGRAL CONTROLLER

DERIVATIVE CONTROLLER

• Anticipates future errors and introduces

appropriate action.

• Introduces a stabilizing effect on the

closed loop response of a process.

SELECTING THE TYPE OF CONTROLLER

Following rules can be adopted in selecting the

most appropriate controller for a process.

1. If possible, use proportional controller.

Proportional controller can be used if

we can achieve

acceptable offset with moderate values of

Kc

P Controller is recommended for liquid level

control where there is not sustained load change.

2. If a simple P controller is unacceptable,

use PI controller.

A PI controller should be used when

proportional control alone cannot provide

sufficiently small steady state errors

(offsets).

Therefore, PI will be generally used in

liquid level systems where there is

sustained load change and also for flow

control.

The response of a flow control system is

rather fast.

Consequently, the speed of the closed loop

system remains satisfactory despite the

slowdown caused by the integral control

mode.

3. Use a PID controller to increase the speed

of the closed loop response.

The PI eliminates the offset but reduces

the speed of the closed-loop response. For

a multicapacity process whose response is

very sluggish, the addition of a PI

controller makes it even more sluggish.

In such cases the addition of the

derivative control action with its

stabilizing effect allows the use of

higher gains which produce faster

responses without excessive oscillations.

Derivative action is recommended for

temperature and composition control.

ANALOGY OF PID CONTROLLER

Consider 2 cars ‘A’ and ‘B’ running on a main

road.

OBJECTIVE : Driver of car A aims at running his

car along with car B ( i.e., car B’s position is

the Set Point) say, speed of B is 50 km/h.

A

B

50 km/h

50 km/h

PROPORTIONAL ACTION

Consider driver A is a proportional

controller.Therefore, whenever speed of B

changes A’s speed will change according to

the distance between A and B. Assume B’s

speed changes to 60 km/h(Load change)

which means in 1 hr B will be ahead of A

by 10 km. A’s speed changes according to

the distance between A and B say 1 km/h

increase per 1 km of distance.

So A’s speed goes on increasing and B goes

more and more away from A. At one

point(when distance between A and B is 10

km) A’s speed will be equal to B’s speed

and A and B will have a constant distance

between them. But as per our objective A

cannot be along with B.This constant

distance between them is offset.

INTEGRAL ACTION

Now consider driver of A is a integral

controller.Here, after each km the driver of

car A senses the distance between A and B

and increases its speed accordingly.As it

approaches B, A slows down its speed and

when along with B, A drives in a constant

speed.

Consider A is a derivative controller.Assume

A and B are together.When B accelerates his

speed A senses the rate of change of B’s

speed by sensing the rate of change of

distance between A and B.A accelerates its

speed in proportion to the rate of change of

distance between them.

Thus, combination of proportional + integral

+ derivative action shall keep cars A and B

together.

DERIVATIVE ACTION

GAIN (P)

KC

X+_INPUT

SET PT

e (t)

TI

(A/B)

B

X

INTEGRATOR

SCALE 100 COUNTS / SEC INPUT – 100 – 0 - +100

DEAD TIME 1 SEC +_

X

to

KC

XDERIVATIVE TIME TD

IN SECS. CONSTANT

d (e) dt

I

+OUTPUT

CD (CONTROLLER’S B, AS SIGNAL

D

INTEGRAL TIME CONST IN SECS

A

KC. e(t) PROP. OUTPUT (P)