Basics of Instrumentation & Control systems

in a TPS

Role of Instrumentation & ControlCase-I

To generate 210 MW we have to introduce steam of 640 TPH at 130 Kg/cm2 and 540 oC

4% of oxygen in the flue gas indicates proper combustion

Furnace pressure shall be maintained at

-10 mmWcl Water level in the drum shall be maintained to

ensure flow of water through boiler tubes

Role of Instrumentation & ControlCase-II

Consider Boiler Feed Pump

To Start the BFP Suction pressure should be adequate Suction valve fully open Recirculation valve fully open Lube oil pressure should be adequate

During Pump Run

Start AOP, if Lube oil pressure falls below 1.5 Kg/cm2

Stop AOP, if Lobe oil pressure reaches 3.5 Kg/cm2

Close the Recirculation valve if the suction flow is >220 TPH

Open the Recirculation valve if suction flow is < 110 TPH

BFP should be Tripped if

Suction pressure < 2.5 Kg/cm2

Discharge pressure < 40 Kg/cm2

Lube oil pressure < 0.8 Kg/cm2

Working oil temperature > 130oC Motor bearing temperature > 80oC Discharge water temperature > 175oC Deaerator level < 1520 mm

Role of Instrumentation & ControlCase-III

Drum Level Super heated steam temperature Main steam pressure Deaerator level control Reheated steam temperature Furnace pressure Mill outlet temperature Oxygen percentage in the flue gas Hotwell level Heater levels

Role of Instrumentation & Controls

Monitoring the parameters Permissives and Interlocks & Protections Controlling the Parameters

MonitoringMost common parameters that are to be

monitored are PressureTemperature LevelFlow

Local Remote

Local Monitoring

Pressure - Gauges

Temperature - Gauges

Level - Gauge glass

Flow - Flow meter

Pressure Gauge

Bourdon Type

Pressure Gauge

Bourdon Type

Temperature Gauge

Mercury Gauge

Gauge Glass

See through glass

Remote

Pressure/Flow/Level

- Transducers Temperature

- Thermocouple, RTD

Transducer

-Converts quantity of physical variable from one form to another

-Pressure transducer converts quantity of process pressure in to Electrical quantity

-Voltage Transducer (0-5, 0-10, 1-5 etc.,)

-Current TransducerTransmitter=Transduce + Transmit

Pressure Transmitter

-4-wire system of measurement

-2-wire system of measurement

-True Zero (0-20 mA) measurement

-Live Zero (4-20 mA) measurement

4-wire Transmitter (0-20/4-20 mA)

+ Tx-

Load Control / Monitoring mA

Wiring schematic of 4-wireTransmitter

+24 V DC

-Ve

2-wire Transmitter (4-20 mA)

+ Tx

-Load

Control / Monitoring

4-20 mA

Wiring schematic of 2-wireTransmitter

+24 V DC

-Ve

- Tx drives constant current up to a Load of 600 Ω

Transmitter Power supply Vs Load

400 600 800 1000 1200

40

30

20

Transmitter

- Capacitance based

Thermocouple- Junction of two dissimilar metals in contact, produce voltage

between the two (unjoined) wire ends

- Any pair of dissimilar metals will produce a measurable voltage when their junction is heated

Voltage is directly proportional to the temperature difference between joined and open ends

Commonly used thermocouples generate

5 µV/oC to 50 µV/oC Relationship between temperature and

voltage is very much Non-linear

Thermocouple

CJC – Cold Junction Compensation

Selection of Thermocouple

Which combination gives more µV/oC Which combination gives better linear

relationship between voltage and temperature

Thermocouple Temp Vs volt

K-Type Thermocouple K type thermocouple has 41 µV/oC

Chromel (Nickel-Chromium alloy) Alumel(Ni-aluminium alloy)

It can measure in the range of -200 to +1200oC

RTD – Resistance Temperature Detector

Electrical resistance of metals changing with temperature

temperature coefficient of metals is of the order - 0.003 to 0.007 ohms/ohm/°C

Most common metals used are

platinum (Pt-100), nickel, copper and molybdenum

2-wire configuration

Wheatstone Bridge

employed to measure

the RTD resistance Lead Resistance causes

error in the

measurement

3-wire configuration

Flow Measurement

Steam Flow

P1 P2

DP across orifice = P1-P2

Level measurement

P1

P2

DP Transmitter

CONTROL SYSTEMS

Control systems

Control systemControl system

Open Loop ControlOpen Loop Control Closed Loop ControlClosed Loop Control

Concentrated controlConcentrated control Distributed controlDistributed control

Distributed Analog

controlDistributed Analog

control

Distributed Digital

Control (DCS)Distributed Digital

Control (DCS)

Open Loop control system

SYSTEMInput Output

Closed Loop Control system

CONTROLLER Final control element

Measurement system

Setpoint

Feedback path

Output

PID controller

Proportional controllerController output α Error

= Kp * e(t)

Where, Kp =Proportional Gain

PID controller

Integral controllerController output α ∫ e(t) dt

= Ki ∫ e(t) dt

Where, Ki = Integral time constant

PID controller

Derivative controllerController output α de(t)/dt

= Kd de(t)/dt

Where, Kd = Derivative time constant

PID controller

DCS Features Single Input can be utilized for monitoring/I&P/Control Modifications in the I&P/control can be done easily in

software Redundancy Isolated Input / Output modules Remove and insert modules while powered Data Highway speed is 10/100 Mbps Centralized Engineering Station for programming,

configuration Point database (Global Data) accessible at any station

across the Network Facility for simulation of control logic schemes with

virtual processor

Points to Remember Monitoring Permissives and Interlocks & Protections Controlling Local and Remote monitoring Current transducers/Live-zero system (4-20 mA)/2-wire Thermocouples require Cold junction

compensation 3-wire configuration of RTDs eliminate error due

to lead wires resistance Distributed control system

Thank You