Questions Answers on basics of instrumentation for Power plant Mechanical Engineers

1. What is an instrumentation?
Instrumentation is the art of measuring a value of plant parameters.
The technology which is used to measure and control the process system of plant is called instrumentation.
2. What is an instrument?
An instrument is a device that transforms a physical variable of interest (the measurand) into a form that is suitable for recording (the measurement).
3. What is measurand?
A physical quantity, property, or condition being measured. Often, it is referred to as a measured value.
4. What is a sensor?
The portion of a measurement system that responds directly to the physical variable being measured.
5. What are the various variables measured in a power plant?
Pressure, temperature, force, speed, frequency, current, voltage, resistance, time, displacement, acceleration, light intensity, density, pH, conductivity, flow etc.
6. What is calibration of a sensor?
The relationship between the physical measurement variable input and the signal variable (output) for a specific sensor is known as the calibration of a sensor.
7. Define true value.
The desired result of an experimental measurement.
8. Define error of an instrument.
The error of an instrument is defined as the difference between the true value and measured value.
 9. What is accuracy?
An expression of the maximum possible limit of error or the closeness of the agreement between the result of a measurement and a true value of the measurand.
10. What is an actuator?
An actuator is a device that is responsible for moving or controlling a mechanism or system.
11. Define the terms input and output of control system.
Input is an applied signal or an excitation signal applied to a control system from an external energy source in order to produce a specified output.
Output is the particular signal of interest or the actual response obtained from a control system when input is applied to it.
12. What are the various sub systems of instrumentation to control the power plant operation?
The efficient and reliable operation of the power plant requires following sub systems.
Monitoring Instruments: These include electromagnetic and electronic indicators, multi point/multi pen recorders, digital display units, recorders, drum level and flame indicators, ammeters, voltmeters etc. These are very essential for starting, running and shutdown of the units.
Transmitters and Sensors: These include, thermocouples, RTDs, pressure and level transmitters, differential transmitters. Generally transmitters are of two wire transmission principle and provide current signal of 4–20 mA.
Local Instruments or Field Instruments: These include pressure and temperature gauges, level switch, pressure switch, draft gauges, vibration switches etc.
Analytical Instruments: They include measurements required to ascertain chemical condition of process medium at various points. These include pH and conductivity analyzers, dissolved O2 analyzers, silica analyzers, smoke and density meters, SOx and NOx analyzers.
Analog and Digital Control System: The control systems in thermal power plant cover various closed loop control system. It employees sophisticated digital computers for process control. The control systems have
Interconnection with measurement systems, interlocks, protection, sequential control system annunciation and data acquisition system. Programmable Logic Control (PLC) and Digital Control System (DCS) find wide applications in modern power plants.
Control Modules: Control modules are provided to control the equipments.
Protection and Interlock Systems: The operation of the protection is accompanied by audible and visible annunciations to indicate primary causes for operation of protection. Both relay based and logic based methods are adopted.
Control Drives: Include pneumatic dampers, solenoid operated valves, control valves, motorized valves etc.
Power Supply Arrangement: The control and instrumentation devices are provided with power supply sources having back up/standby arrangements. Uninterrupted power supply modules (UPS), battery chargers, unit battery banks, station battery banks, normal/emergency switch gear supply power to the control and instrumentation.
13. What is a control system? What are the various controls used in boiler?
To control means to regulate, to direct or to command. Hence a control system is an arrangement of different physical elements connected in such a manner so as to regulate, direct or command itself or some other system.
  • Drum level control
  • Steam temperature control
  • CBD tank level control
  • Combustion control
  • Deaerator level and pressure control
  • Furnace draught control
  • Soot blower pressure control
14. What is an automatic control system?
Automatic control is to maintain a desired value of a parameter or the condition in a process system by measuring the existing value, comparing it to the desired value and employing the difference to initiate corrective action for reducing the difference.
15. What is control action? What are its different types?
Controller action is to maintain the controlled variables of the process at the desired value set by the operator.
Different Types of Control Actions:
Proportional control action
Integral control action
Derivative control action
16. What are the main components of DCS?
DCS system has following main units:
  • Main servers
  • Operation systems
  • Firewall
  • Controllers
  • Digital input and output modules
  • Analog input and output modules
  • Field terminal assembly
17. What is server?
Server is a master computer whose main purpose is to handle request for data from other peripheral computers.
18. What is redundancy of a system?
Redundancy means supporting simultaneously or back up for any online system. This term is often used in DCS, where redundant server is needed. Redundant server works as main server when main server fails. Similarly we have redundant controller also.
19. What is the difference between consoles and flex stations?
In server Architecture of DCS, there are two types of communication depending on the configuration of the client. There are 2 types of clients – Flex Station and Console station.
Flex station talks to server for data exchange. So, if the server fails, the flex stations do not receive anything and go blind.
The console stations directly talk to controller for IO status, set points, and alarm set points. So, if the server is down, still the plant can be visualized on these computers, dynamic data exchange is possible, you can start/stop the motors, and you can open/close the valves through console stations. (Motors can be started/stopped only if they are connected through hardwired IOs/connected to the controller serially).
20. What are fire wall in DCS?
A firewall is a security device that can be a software program or a dedicated network appliance. The main purpose of a firewall is to separate a secure area from a less secure area and to control communications between the two.
21. What do you mean by controller? What are its functions?
A controller is a comparative device that receives an input signal from a measured process variable, compares this value with that of a predetermined control point value (set point) and determines the appropriate amount of output signal required by the final control element to provide corrective action within a control loop. An Electronic Controller uses electrical signals and digital algorithms to perform its receptive, comparative and corrective functions.
An electronic sensor (thermocouple, RTD or transmitter) installed at the measurement location continuously sends an input signal to the controller. At set intervals the controller compares this signal to a predefined set point. If the input signal deviates from the set point, the controller sends a corrective output signal to the control element. This electric signal must be converted to a pneumatic signal when used with an air operated valve.
22. What are the different types of standard signals used in Industrial Instrumentation measurement?
Standard signals used in instrumentations are 4–20 mA, 0–5 VDC, 1–5 VDC, 24 VDC etc.
23. Why 4 mA and 20 mA are used instead of 0 mA and >20 mA in signal measurement?
For error detection 4 mA is good, If instrument is faulty, it can be easily identified. If the output is 0 mA it will be difficult to identify whether it is a faulty signal or a good signal.
20 mA is used as maximum because the human heart can withstand up to 30 mA of current only. For safety reason 20 mA is chosen as maximum value.
24. What is pressure?
It is the force per unit area.
Pressure = Force/Area
25. What is absolute pressure?
Absolute pressure is the sum of atmospheric pressure and gauge pressure.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
26. What are the various pressure detectors?
Bourdon tubes, bellows, capsules, diaphragms, strain gauges and manometers.

27. Explain the piezoelectric and capacitive methods of pressure measurements in transmitters.
Piezoelectric Method:
These are certain small crystalline materials which generate voltage when they are subjected to forces or stress. Crystalline materials when stressed or forced they develop electric charges, these charges are measured and converted.
Example: Naturally available Quartz, synthetic crystals, Rochelle salts and barium titrate.
Capacitive Method:
In this method the mechanical movement of mechanical material like diaphragm changes the capacitance of an electronic component.
28. What is pressure transmitter?
A pressure transducer, often called a pressure transmitter, is a transducer that converts pressure into an analog electrical signal.
29. What is the difference between pressures switch and pressure transmitter?
Pressure Switch is an electromechanical device which measures the pressure in a system and when the pressure reaches too high or too low of a given set point, the device will switch meaning it will open orclose the circuit that powers a certain devices like creating alarms, start or stop of pump, open or close of valve etc.
Pressure Transmitter is also an electromechanical device which senses the pressure and sends signal to remote device for further measurement and control. For an example continuous measurement of steam pressure, lube oil and control oil pressure measurement etc.
30. What is the difference between pressure transducers and pressure transmitters?
Transmitters are current output devices and transducers are voltage output devices.
31. What are the different types of temperature sensors used in power plant?
RTD, Thermocouples, thermometer, infrared sensors, bimetallic devices.
32. Which effect is used in thermocouple?
See back effect is used in thermocouple.
33. What are active and passive sensors?
Active sensors generate an electric current in response to an external stimulus which serves as the output signal without the need of an additional energy source. Such examples are a diode, piezoelectric sensor, and thermocouple.
Passive sensors require an external power source to operate, which is called an excitation signal. The signal is modulated by the sensor to produce an output signal. For example, a thermistor does not generate any electrical signal, but by passing an electric current through it, its resistance can be measured by detecting variations in the current or voltage across the thermistor.
34. What is the function of thermo-well in thermocouples or temperature sensors?
Thermo-well is used to protect the thermocouple from harmful atmosphere, corrosive fluid, and physical damage.
35. How do you identify J, K and S type thermocouples on their physical appearance?
  • J Type Thermocouple: Positive wire is white and negative wire is red.
  • K Type Thermocouple: Positive wire is yellow and negative wire is red.
  • S Type Thermocouple: Positive wire is red and negative wire is white.
36. What are the different types of RTDs used in power plant?
2 wire, 3 wire and 4 wire RTDs are generally used.
37. What is RTD PT-100?
RTD PT-100 has resistance of 100 ohms at temperature 0 °C.
38. What is the change in resistance at every 1 °C rise in temperature of RTD PT-100?
0.384 ohm resistance will rise/lower at every 1 °C rise/lower in temperature.
39. What are the different types of flow meters used in power plant?
Following are the Flow Meters used in Power Plant:
  • Venturimeter
  • Orifice meter
  • Flow nozzle
  • Turbine flow meter
  • Mass flow meter
  • Magnetic flow meters
  • Rotameters
 40. What are the different types of orifice plates? Explain.
Different Types of Orifice Plates are:
Concentric: These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates that’s why it is known as concentric orifice.
Segmental: This plate has hole in the form of segment of the circle. This plate is used for colloidal and slurry flow measurement.
Eccentric: This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.
41. Why negative voltage is used in turbine vibration probes?
The voltage found between the sensor tip and the target is a negative DC voltage. The negative bias voltage used by proximate may be due to its safety. The negative voltage may vary while adjusting the gap that can be checked by the signal conditioner used by the probe.The extreme end of the gap gives a negative DC voltage of 24 and its zero when the gap between sensor and target is almost lapping.
42. What is load cell?
Load cell is transducer which converts a force into an electrical signal. It comprises of strain gauge which is connected to Wheatstone bridge circuit in load cell.
43. What is a control valve? What are different types of control valves?
A control valve is a device capable of modulating flow at varying degrees between minimal flow and full capacity in response to a signal from an external control device. The control valve, often referred to as “the final control element,” is a critical part of any control loop, as it performs the physical work and is the element that directly affects the process.
44. What is the function of positioner?
A positioner is a device put into a valve to ensure that it is at a correct position of opening as per the control signal. An I/P converter only sends the opening/closing request to valve but cannot confirm its position.
                Positioner senses the valve opening through a position feedback link connected to valve stem which is its input signal. I/P converter output is its set point input. The difference between these two is the error signal based on which the positioner positions the valve to correct position to reduce error to zero. Hence positioner is nothing but a pneumatic feedback controller.
45. What do you mean by direct acting and reverse acting control valves?
If the controller output increases when the measurement value rises, it is a direct-acting controller, and if controller output decreases when the measurement rises, it is a reverse-acting controller.
     46. What is the function of an actuator in control valve?
 An actuator is a pneumatic powered device which supplies force and motion to open or close a control valve.
4   47.What is Solenoid Valve? What are its main types?A solenoid is electrically operated valve. It consists of solenoid coil in which magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve.
There are two types of solenoid valves:
                Normally Open
                Normally closed
Questions & Answers on batteries
48.What are the input and output signals of Turbine governor?
Governor Inputs:
  • Power supply 230 V AC/110 V AC
  • 4–20 mA MW signal
  • Main steam inlet pressure
  • 3rd Extraction pressure
  • 4–20 mA HP and LP governor valve demands
  • Turbine speed
Governor Outputs:
  • Control signals to HP and LP actuators
  • Light fault and heavy faults
  • Electrical and mechanical over speed
Also read basic questions & answers on power plant electrical system


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