In thermal power stations, mechanical power is produced by a heat engine that transforms thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. Not all thermal energy can be transformed into mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a co generation power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses by-product heat for the desalination of water.
The efficiency of a steam turbine is limited by the maximum temperature of the steam produced and is not directly a function of the fuel used. For the same steam conditions, coal, nuclear and gas power plants all have the same theoretical efficiency. Overall, if a system is on constantly (base load) it will be more efficient than one that is used intermittently(peak load)
Besides use of reject heat for process or district heating, one way to improve overall efficiency of a power plant is to combine two different thermodynamic cycles. Most commonly, exhaust gases from a gas turbine are used to generate steam for a boiler and steam turbine. The combination of a "top" cycle and a "bottom" cycle produces higher overall efficiency than either cycle can attain alone.
Thermal Power Plants
Thermocouples
A thermocouple is an electrical apparatus that can generate a current proportional to the amount of heat it is exposed to. In electrical engineering and industry, thermocouples are a widely used type of temperature sensor[1] and can also be used as a means to convert thermal potential difference into electric potential difference.They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy; System errors of less than one kelvin (K) can be difficult to achieve.
Open Loop Controller
An open-loop controller, also called a non-feedback controller, is a type of controller which computes its input into a system using only the current state and its model of the system.
A characteristic of the open-loop controller is that it does not use feedback to determine if its input has achieved the desired goal. This means that the system does not observe the output of the processes that it is controlling. Consequently, a true open-loop system can not engage in machine learning and also cannot correct any errors that it could make. It also may not compensate for disturbances in the system.
For example, an irrigation sprinkler system, programmed to turn on at set times could be an example of an open-loop system if it does not measure soil moisture as a form of feedback. Even if rain is pouring down on the lawn, the sprinkler system would activate on schedule, wasting water.
Open-loop control is useful for well-defined systems where the relationship between input and the resultant state can be modeled by a mathematical formula. For example determining the voltage to be fed to an electric motor that drives a constant load, in order to achieve a desired speed would be a good application of open-loop control. If the load were not predictable, on the other hand, the motor's speed might vary as a function of the load as well as of the voltage, and an open-loop controller would therefore not be sufficient to ensure repeatable control of the velocity.
An example of this is a conveyor system that is required to travel at a constant speed. For a constant voltage, the conveyor will move at a different speed depending on the load on the motor (represented here by the weight of objects on the conveyor). In order for the conveyor to run at a constant speed, the voltage of the motor must be adjusted depending on the load. In this case, a closed-loop control system would be necessary.
An open-loop controller is often used in simple processes because of its simplicity and low-cost, especially in systems where feedback is not critical. A typical example would be a conventional washing machine, for which the length of machine wash time is entirely dependent on the judgment and estimation of the human operator. Generally, to obtain a more accurate or more adaptive control, it is necessary to feed the output of the system back to the inputs of the controller. This type of system is called a closed-loop system
Control Loop Basics
A familiar example of a control loop is the action taken to keep one's shower water at the ideal temperature, which typically involves the mixing of two process streams, cold and hot water. The person feels the water to estimate its temperature. Based on this measurement they perform a control action: use the cold water tap to adjust the process. The person would repeat this input-output control loop, adjusting the hot water flow until the process temperature stabilized at the desired value.
Feeling the water temperature is taking a measurement of the process value or process variable (PV). The desired temperature is called the setpoint (SP). The output from the controller and input to the process (the tap position) is called the manipulated variable (MV). The difference between the measurement and the setpoint is the error (e), too hot or too cold and by how much.
As a controller, one decides roughly how much to change the tap position (MV) after one determines the temperature (PV), and therefore the error. This first estimate is the equivalent of the proportional action of a PID controller. The integral action of a PID controller can be thought of as gradually adjusting the temperature when it is almost right. Derivative action can be thought of as noticing the water temperature is getting hotter or colder, and how fast, anticipating further change and tempering adjustments for a soft landing at the desired temperature (SP).
Making a change that is too large when the error is small is equivalent to a high gain controller and will lead to overshoot. If the controller were to repeatedly make changes that were too large and repeatedly overshoot the target, this control loop would be termed unstable and the output would oscillate around the setpoint in either a constant, growing, or decaying sinusoid. A human would not do this because we are adaptive controllers, learning from the process history, but PID controllers do not have the ability to learn and must be set up correctly. Selecting the correct gains for effective control is known as tuning the controller.
If a controller starts from a stable state at zero error (PV = SP), then further changes by the controller will be in response to changes in other measured or unmeasured inputs to the process that impact on the process, and hence on the PV. Variables that impact on the process other than the MV are known as disturbances. Generally controllers are used to reject disturbances and/or implement setpoint changes. Changes in feed water temperature constitute a disturbance to the shower process.
In theory, a controller can be used to control any process which has a measurable output (PV), a known ideal value for that output (SP) and an input to the process (MV) that will affect the relevant PV. Controllers are used in industry to regulate temperature, pressure, flow rate, chemical composition, speed and practically every other variable for which a measurement exists. Automobile cruise control is an example of a process which utilizes automated control.
Due to their long history, simplicity, well grounded theory and simple setup and maintenance requirements, PID controllers are the controllers of choice for many of these applications.
Proximity Sensors
Proximity sensors are sensors able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic or electrostatic field, or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. The object being sensed is often referred to as the proximity sensor's target. Different proximity sensor targets demand different sensors. For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor requires a metal target.
The maximum distance that this sensor can detect is defined "nominal range". Some sensors have adjustments of the nominal range or means to report a graduated detection distance.
Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between sensor and the sensed object.
Conditioning the output of a proximity sensor is frequently difficult. Proximity sensor designers must confront linearity, hysteresis, excitation voltage instability, and voltage offset.
Types of sensors
-Capacitive
-magnetic
-inductive
-photocell (reflective)
-laser rangefinders
-sonar (typically active or passive)
-radar
-doppler effect
-passive thermal infrared
-passive optical (such as CCDs)
-reflection of ionising radiation
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.
Boilers can be classified into the following configurations:
"Pot boiler" or "Haycock boiler": a primitive "kettle" where a fire heats a partially-filled water container from below. 18th Century Haycock boilers generally produced and stored large volumes of very low-pressure steam, often hardly above that of the atmosphere. These could burn wood or most often, coal. Efficiency was very low.
Fire-tube boiler. Here, water partially fills a boiler barrel with a small volume left above to accommodate the steam (steam space). The heat source is inside a furnace or firebox that has to be kept permanently surrounded by the water in order to maintain the temperature of the heating surface just below boiling point. The furnace can be situated at one end of a fire-tube which lengthens the path of the hot gases, thus augmenting the heating surface which can be further increased by making the gases reverse direction through a second parallel tube or a bundle of multiple tubes (two-pass or return flue boiler); alternatively the gases may be taken along the sides and then beneath the boiler through flues (3-pass boiler). In the case of a locomotive-type boiler, a boiler barrel extends from the firebox and the hot gases pass through a bundle of fire tubes inside the barrel which greatly increase the heating surface compared to a single tube and further improve heat transfer. Fire-tube boilers usually have a comparatively low rate of steam production, but high steam storage capacity. Fire-tube boilers mostly burn solid fuels, but are readily adaptable to those of the liquid or gas variety.
Water-tube boiler. 
Diagram of a water-tube boiler.In this type,the water tubes are arranged inside a furnace in a number of possible configurations: often the water tubes connect large drums, the lower ones containing water and the upper ones, steam; in other cases, such as a monotube boiler, water is circulated by a pump through a succession of coils. This type generally gives high steam production rates, but less storage capacity than the above. Water tube boilers can be be designed to exploit any heat source including nuclear fission and are generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can withstand the pressure with a thinner wall.
Fire-tube boiler with Water-tube firebox. Sometimes the two above types have been combined in the following manner: the firebox contains an assembly of water tubes, the gases then pass through a conventional firetube boiler. Water-tube fireboxes were were installed in many Hungarian locomotives, but have met with little success in other countries.
In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are assembled on site to create the finished boiler.
Superheated steam boilers
Most boilers heat water until it boils, and then the steam is used at saturation temperature (i.e., saturated steam). Superheated steam boilers boil the water and then further heat the steam in a superheater. This provides steam at much higher temperature, and can decrease the overall thermal efficiency of the steam plant due to the fact that the higher steam temperature requires a higher flue gas exhaust temperature. However, there are advantages to superheated steam. For example, useful heat can be extracted from the steam without causing condensation, which could damage piping and turbine blades.
Superheated steam presents unique safety concerns because, if there is a leak in the steam piping, steam at such high pressure/temperature can cause serious, instantaneous harm to anyone entering its flow. Since the escaping steam will initially be completely superheated vapor, it is not easy to see the leak, although the intense heat and sound from such a leak clearly indicates its presence.
The superheater works like coils on an air conditioning unit, however to a different end. The steam piping (with steam flowing through it) is directed through the flue gas path in the boiler furnace. This area typically is between 1300-1600 degrees Celsius (2500-3000 degrees Fahrenheit). Some superheaters are radiant type (absorb heat by radiation), others are convection type (absorb heat via a fluid i.e. gas) and some are a combination of the two. So whether by convection or radiation the extreme heat in the boiler furnace/flue gas path will also heat the superheater steam piping and the steam within as well. It is important to note that while the temperature of the steam in the superheater is raised, the pressure of the steam is not: the turbine or moving pistons offer a "continuously expanding space" and the pressure remains the same as that of the boiler.[3]The process of superheating steam is most importantly designed to remove all moisture content from the steam to prevent damage to the turbine blading and/or associated piping.
Boiler Accessories
1.Boiler fittings
Safety valve: used to relieve pressure and prevent possible explosion of a boiler
Water level indicators: to show the operator the level of fluid in the boiler, a water gauge or water column is provided
Bottom blowdown valves
Surface blowdown line
Circulating pump
Feedwater check valve or clack valve: a nonreturn stop valve in the feedwater line
Steam accessories
Main steam stop valve
Steam traps
Main steam stop/Check valve used on multiple boiler installation
Combustion accessories
Fuel oil system
Gas system
Coal system
Automatic combustion systems
Other essential items
Pressure gauges
Feed pumps
Fusible plug
Inspectors test pressure gauge attachment
Name plate
Registration plate
Steam
In physical chemistry, and in engineering, steam refers to vaporized water. It is a pure, completely invisible gas (for mist see below). At standard atmospheric pressure, pure steam (unmixed with air, but in equilibrium with liquid water) occupies about 1,600 times the volume of liquid water. In the atmosphere, the partial pressure of water is much lower than 1 atm, therefore gaseous water can exist at temperatures much lower than 100 C (see water vapor and humidity).
In common speech, steam most often refers to the white mist that condenses above boiling water as the hot vapor ("steam" in the first sense) mixes with the cooler air. This mist is made of tiny droplets of liquid water, not gaseous water, so it is no longer technically steam. In the spout of a steaming kettle, the spot where there is no condensed water vapor, where there appears to be nothing there, is steam.
Switch
A switch is a mechanical device used to connect and disconnect a circuit at will. Switches cover a wide range of types such as limit, pressure, temperature and else, from subminiature up to industrial plant switching megawatts of power on high voltage distribution lines.
In applications where multiple switching options are required (e.g., a telephone service), mechanical switches have long been replaced by electronic switching devices which can be automated and intelligently controlled.
The prototypical model is perhaps a mechanical device (for example a railroad switch) which can be disconnected from one course and connected to another.
The switch is referred to as a "gate" when abstracted to mathematical form. In the philosophy of logic, operational arguments are represented as logic gates. The use of electronic gates to function as a system of logical gates is the fundamental basis for the computer—i.e. a computer is a system of electronic switches which function as logical gates.
Transmitter
A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. In other applications signals can also be transmitted using an analog 0/4-20 mA current loop signal, analog 0-5Vdc/1-5Vdc/0-10Vdc voltage signal.
Flow
In mathematics, a flow formalizes, in mathematical terms, the general idea of "a variable that depends on time" that occurs very frequently in engineering, physics and the study of ordinary differential equations. Informally, if x(t) is some coordinate of some system that behaves continuously as a function of t, then x(t) is a flow. More formally, a flow is the group action of a one-parameter group on a set.
The idea of a vector flow, that is, the flow determined by a vector field, occurs in the areas of differential topology, Riemannian geometry and Lie groups. Specific examples of vector flows include the geodesic flow, the Hamiltonian flow, the Ricci flow, the mean curvature flow, and the Anosov flow.
Pressure
Pressure (symbol: 'P') is the force per unit area applied on a surface in a direction perpendicular to that surface.
Gauge pressure (G) is the pressure relative to the local atmospheric or ambient pressure.
by Mathematically:
where:
p=F/A or p=dF/dA
p is the pressure,
F is the normal force,
A is the area.
Pressure is a scalar quantity, and has SI units of pascals; 1 Pa = 1 N/m2.
Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a fundamental parameter in thermodynamics and it is conjugate to volume.
Engineering Units
Mercury columnThe SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·m-1·s-2). This special name for the unit was added in 1971; before that, pressure in SI was expressed simply as N/m2.
Non-SI measures such as pound per square inch (psi) and bar are used in some parts of the world. The cgs unit of pressure is the barye (ba), equal to 1 dyn·cm-2. Pressure is sometimes expressed in grams-force/cm2, or as [[kg/cm2]] and the like without properly identifying the force units. But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as units of force is expressly forbidden in SI. The technical atmosphere (symbol: at) is 1 kgf/cm2.
Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar). Similar pressures are given in kilopascals (kPa) in most other fields, where the hecto prefix is rarely used. The unit inch of mercury (inHg, see below) is still used in the United States. Oceanographers usually measure underwater pressure in decibars (dbar) because an increase in pressure of 1 dbar is approximately equal to an increase in depth of 1 metre. Scuba divers often use a manometric rule of thumb: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere.
The standard atmosphere (atm) is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows:
standard atmosphere = 101325 Pa = 101.325 kPa = 1013.25 hPa.
Because pressure is commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g., inches of water). The most common choices are mercury (Hg) and water; water is nontoxic and readily available, while mercury's high density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The pressure exerted by a column of liquid of height h and density ρ is given by the hydrostatic pressure equation p = ρgh. Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. When millimetres of mercury or inches of mercury are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. The water-based units still depend on the density of water, a measured, rather than defined, quantity. These manometric units are still encountered in many fields. Blood pressure is measured in millimetres of mercury in most of the world, and lung pressures in centimetres of water are still common.
Pressure convertion unit table
Thermometer
Thermometer is a device that measures temperature or temperature gradient, by using a variety of different principles. A thermometer has two important elements, the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a value (e.g. the scale on a mercury thermometer). Industrial thermometers commonly use electronic means to provide a digital display or input to a computer.
It can be divided into two groups according to the level of knowledge about the physical basis of the underlying thermodynamic laws and quantities.
1. Primary
thermometers the measured property of matter is known so well that temperature can be calculated without any unknown quantities. Examples of these are thermometers based on the equation of state of a gas, on the velocity of sound in a gas, on the thermal noise (see Johnson–Nyquist noise) voltage or current of an electrical resistor, and on the angular anisotropy of gamma ray emission of certain radioactive nuclei in a magnetic field.
Secondary
Thermometers are most widely used because of their convenience. Also, they are often much more sensitive than primary. Secondary thermometers knowledge of the measured property is not sufficient to allow direct calculation of temperature. They have to be calibrated against a primary thermometer at least at one temperature or at a number of fixed temperatures. Such fixed points, for example, triple points and superconducting transitions, occur reproducibly at the same temperature.
Internationally had agreed temperature scales are based on fixed points and interpolating thermometers. The most recent official temperature scale is the International Temperature Scale of 1990. It extends from 0.65 K to approximately 1358 K (−272.5 °C to 1085 °C).
References: About - Thermometer - Thermometers - Early History, Anders Celsius, Gabriel Fahrenheit and Thomson Kelvin.
Measurement in I&C
Basiccally Instrument as a device to measure non visual measure value such as pressure, level, flow, temperature, speed and others.
Let us take an example:-
Do you know your enviroment temperature surounding you know?
The answer you can give is only hot or cold only. ( Hot and cold is not a measure value)
You need a device to measure the temperature is't?
Yes, you need a temperature monitoring device. It will giving a numericle value with unit degree C or Farenheit. For examplewe use thermometer to measure it (refer to pictur below).
Is't simple?
What is I&C?
Not everybody know this.... I&C is a short form for Instrumentation and Control.
Instrumentation
An electrical device placed in the field to provide measurement and/or control capabilities for the system was called instrumentation.The simplest measurement instrumentation device is a thermistor. A thermistor is very similar to a typical resistor, except that it greatly varies its resistance depending on its temperature. Therefore this device can easily be used for measurement of temperature in the field.
Control instrumentation includes devices such as solenoids, Electrically Operated Valves, breakers, relays, etc. These devices are able to change a field parameter, and provide remote control capabilities.
Instrumentation plays a significant role in both gathering information from the field and changing the field parameters, and as such are a key part of control loops.
