1.
What is an instrumentation?
Instrumentation is the art
of measuring a value of plant parameters.
Or
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
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
