Control and instrumentation in a thermal power plant

Control and instrumentation is considered as the heart of any major power plant, steel factories or space stations as the major problems can be avoided using the control and instrumentation equipment. Few advantages we get with control and instrumentation in a thermal power plant are below:

1. We can avert major damages caused due to raising temperatures by putting a setpoint for temperature which is measured using RTD  or TC
2. The opening and closing of the control valves, actuators or dampers is done through commands. The position of these valves can be known using the position feedback installed in respective areas.
3. There are open and close feedback control systems which take decisions based on the feedbacks received.

The equipments as part of the control and instrumentation are

1. RTD
2. THERMOCOUPLE
3. PRESSURE TRANSMITTERS
4. LEVEL TRANSMITTERS
5. PRESSURE SWITCHES 
6. LEVEL SWITCHES
7. TEMPERATURE SWITCHES
8. SOLENOIDS
9. AIR FILTER REGULATORS.
10. I TO P CONVERTERS
11. CONTROL VALVES
12. ACTUATORS
13. CONDUCTIVITY , PH AND OTHER TRANSMITTERS ANS ANALYZERS
14. RELAYS
15. COILS
16. MCB 
17. DIODE CIRCUITS
18. DP TRANSMITTERS
19. DP SWITCHES

RTD stands for resistance temperature detector which detects the temperature of the location where these RTD is installed by varying it's resistance. There are 2,3,4 wire RTD which improve the accuracy of temperature detected. 4 wire RTD is most accurate one.

The TC stands for thermocouple is used to generate millivolts voltage based on the temperature at the junction of the thermocouple. There will be temperature converters for thermocouples which are used to convert these millivolts into temperatures. Generally thermocouples are used to detect temperatures from 200-600 deg c temperatures in a thermal power plant.

Level transmitters are used to find the level of liquid in a particular tank or vessel. For example level of the boiler drum indicates the amount of water present in the drum, if sufficient level is not maintained in the drum it should be properly filled using the boiler feed pumps. If level transmitters are not present then it will be difficult to maintain the drum level with in specified range.

Conductivity and PH transmitters are used to find the conductivity and PH value of the liquid respectively. Conductivity/PH transmitters are mainly used in the primary water system of the generator to find the conductivity of the primary water and proper NAOH dosing of the primary water needs to be done if the conductivity moves into acidic region. Where as the conductivity/Ph analysers are used in the swas system.

Control valves are one of the main areas of the thermal power plant which will be operated frequently depending on the conditions set by the operator and in auto mode. There are two two types of control valves. They are Hydraulic and pneumatic control valves. The pneumatic control valve is operated using the air pressure and it contains air filter regulators, I to P CONVERTERS, positioners, booster relays, air lock relays and feedback unifs. The air filter regulators is is used to filter the air and regulate the pressure of the air supplied for operation. The air lock relay is used to hold the control valve in the same position if the power supply goes off. The I to P converter is used to provide air supply to the valve based on the current signal or operator signal.

The hydraulic control valves operate based on the amount of oil pressure supplied to the valve. These contains oil pumps and are very fast in response compared to the pneumatic control valves and are very costly and used in applications where high precision and accuracy are needed such as hpbypass and lpbypass unit and hydraulic governor. For other applications pneumatic control valves are used.