For measuring oil flow, rotating vortex type of flow meter is used. It involves a vortex pool which...

Suggested Answer Keys & Marking Scheme :: Oct 2019 :: QPCode: 482 :: Page No: 1

BOARD EXAMINATIONS :: OCTOBER 2019

ANSWER KEYS & MARKING SCHEME

Branch 1042 Instrumentation & Control Engineering

Semester & Regulation VI (M Scheme)

Subject 34283 Power Plant Instrumentation

Question Paper Code 482

PART - A

1 What is air pre-heater? It is a device which increases the temperature of air before it is supplied to the furnace using heat from flue gases. The main advantage of using air pre-heater is that it increases the thermal efficiency of the system.

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2 What is yawing? It is the mechanism used to turn the wind turbine rotor against the wind (i.e. perpendicular to the wind). Due to yawing, the maximum wind will be running through the rotor area.

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3 Why is it necessary to remove smoke? The emission of smoke from industrial premises has been brought under control of the “Clean Air Act”. Hence it is necessary to remove smoke to reduce environmental pollution and to comply with this Act.

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4 Mention any two inferential methods to measure combustion air flow.

Measurement of combustion air flow with orifice segment in duct

Measurement of air flow with venture section in duct

Measurement of air flow with air foil in duct

Inferred measurement of combustion air flow by pressure drop across baffles in flue gas stream

(any two)

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5 Mention any two methods used for measurement of oxygen in flue gas.

In situ flue gas analysis

Extractive ex situ method

Light or infrared beam across the stack

Paramagnetic oxygen analyser (any two)

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6 What is the need to control pH of feed water? To reduce corrosion and scaling in boiler.

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7 What do you mean by superheat temperature control? Any increase in temperature of the steam above its boiling point is called “superheat”. It is a feed forward cascade system in which air flow, tilt position and throttle pressure error are combined in summer to adjust the set point in controller for controlling temperature.

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8 What is known as lubrication? It is the process of applying a substance such as oil or grease to minimize friction in main bearings, thrust bearings and reduction gears in turbines. It helps to cool the journals and bearing surfaces.

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PART – B

9 Draw the piping and instrumentation diagram of a boiler.

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10 Write short notes on photovoltaic power generation. Photo voltaic power generation is a method of producing electricity out of a semi-conductor by means of a quantum effect which converts optical energy into electrical energy. If an n-type semiconductor is brought into contact with a p-type semiconductor a contact potential difference will be established at the interface by the diffusion of electrons. With the p-type semiconductor exposed to light, its electrons absorbs photons of light and pass it to n-type region thus generating an electric current in a closed circuit.

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11 Write short notes on non-inferential method of air flow measurement. Hot wire anemometer is a popular instrument used as non-inferential method of air flow measurement. It consists of an electrically heated wire suitably mounted in the flowing fluids. Due to the change in the velocity of flow fluid, the heat lost by the hot wire will also change. A relation between this loss of heat and the fluid flow can be desired. This heat loss changes the temperature and as a consequence resistance of the hot wire changes. Measuring the change of resistance or current the flow can be found out.

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12 Write short notes on electrical conductivity meter. It works on the principle that the electrical conductivity of water also depends on the type and amount of dissolved solids (i.e. impurities). It is used to check the existence NaCl, Na2SO4, CaSO4 and MgSO4 in feed water. The unit of measurement is micro mho per centimeter at 20OC. It has been recommended that the conductivity of water should not exceed 0.3 micro mho/cm. The most commonly used instrument for determining the purity of water based on the electrical conductivity principle is the “Dionic Water Purity Meter”. This apparatus measures the level of impurity in terms of resistance of standard water column or tube through which the sample is passed.

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13 Describe about any one method of measurement of oil flow. For measuring oil flow, rotating vortex type of flow meter is used. It involves a vortex pool which has a circular motion and drives a multi-bladed rotor. The oil flow is determined by counting the number of revolutions of this rotor. The instrument can be periodically calibrated using prover loop.

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14 Write short notes on flame photometer. It is used to measure the purity of steam supplied to turbines, which is measured in terms of conductivity. It has three parts: atomizer, combustion chamber and optical system. Air enters the tube at a controllable pressure into the atomizer nozzle, while gas enters another tube. The even mixture of gas, air and sample is passed to the burner where it is ignited to burn. Light from the flame is collected by the reflector and focused by lens through the filter of the photocell. The emitted light as detected by the photocell represents the sodium content which can be calibrated using solutions of known sodium concentration.

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15 Write short notes on drum level control system. The main objectives of drum level measurement are:

Control the drum level to a set point

Minimise the interaction with the combustion control system

Make smooth changes in the boiler water inventory as boiler load changes

Properly balance the boiler steam output with the feedwater input

Compensate for feedwater pressure variation without process upset or set point

The following are the important types of drum level control: 1. Single-element feedwater control (i.e. Simple feedback control) 2. Two-element feedwater control (i.e. feedforward-plus-feedback

control) 3. Three-element feedwater control (i.e. feedforward, feedback, plus

cascade)

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16 How is vibration in turbine system monitored? Vibration in turbines is measured using electrical pickup or electro-dynamic vibration pickup. It measures absolute rectilinear vibrations. In this system, the seismic mass comprises a pair of coils centered by two diaphragm springs within two magnetic fields. The right hand coil is the measuring coil, in which a voltage is developed electro dynamically, this being proportional to the velocity.

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PART - C

17 (a) With a neat block diagram, explain the generation of electric power in a thermal power plant.

Thermal power generation plant or thermal power station is the most conventional source of electric power. Thermal power plant is also referred as coal thermal power plant and steam turbine power plant.

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A power generation plant mainly consists of alternator runs with help of steam turbine. The steam is obtained from high pressure boilers. In coal thermal power plant, the steam is obtained in high pressure in the steam boiler due to burning of fuel (pulverized coal) in boiler furnaces. This steam is further super heated in a super heater. This turbine is mechanically so coupled with alternator that its rotor will rotate with the rotation of turbine blades. After imparting energy to the turbine rotor, the steam passes out of the turbine blades into the condenser. In the condenser, the cold water is circulated with the help of a pump which condenses the low-pressure wet steam. This condensed water is further supplied to a low-pressure water heater where the low-pressure steam increases the temperature of this feed water; it is again heated in high pressure. A typical thermal power station operates on a cycle which is shown below:

The general layout of thermal power plant consists the following four circuits:

1. Coal and ash circuit 2. Air and gas circuit 3. Feed water and steam flow circuit 4. Cooling water circuit

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17 (b) Explain the power generation method using the following types of renewable energy sources: (i) Solar energy (ii) Wind energy Power Generation using Solar Energy Solar energy is the energy received by the earth from the Sun. This energy is in the form of solar radiation, which makes the production of solar electricity possible. The solar power generation is mainly classified into the following:

1. Solar thermal 2. Solar photovoltaic

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Solar Thermal Solar thermal energy is the heat energy derived from the incident solar energy (sunlight). This heat is used to drive steam turbine which is coupled with generator, which converts thermal energy into mechanical energy and then into electrical energy. Solar Photovoltaic The energy generated by incident solar energy (light) into electricity is termed as photovoltaic solar power. This is done using photovoltaic solar cells. Power Generation using Wind Energy Aero turbine converts energy in moving air to rotary mechanical energy. In general, they require pitch control and yaw control for proper operation. A mechanical interface consisting of a step up gear and a suitable coupling transmits the rotary mechanical energy to an electrical generator. The output of the generator is connected to load or power grid. Advantages of Renewable Energy

Renewable

Unlimited supply

Free of cost

Free from pollution

No negative impact

Clean and pure energy

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18 (a) Which device is used for high pressure measurement in power plant? Explain the operation of the same.

The pressures in the range of 650 kg/cm2 to 16500 kg/cm2 are considered to be as high pressures. The conventional pressure measuring devices, such as strain pressure cells, Bourdon tubes etc. are not suitable for this pressure range.

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Hence very high pressure of the above order is measured using electrical resistance pressure gauge. In such gauges, the bulk compression effect is utilized to change the electrical resistance. The sensing element consists of loosely wound coil of relatively fine wire and does not actually contact the process medium, but is separated there from by a kerosene-filled bellows. Thus the bulk compression effect of the pressure results in an electrical resistance change which is calibrated in terms of applied pressure.

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18 (b) Explain in brief about radiation detectors. Radiation detectors are used to measure the amount of radioactivity encountered in nuclear power plant and for tracking of radiated alpha, beta, gamma particles and neutrons. The following are the types of radiation detections: Geiger Muller Tube Ionisation Chamber Scintilliation Counter Geiger Muller Tube

It is used for detection and measurement of alpha, beta and gamma rays. This tube is a diode, consisting of a cathode which is a long metal cylinder and an anode which is a fine wire running through the centre of the cylinder. Both of them are mounted in a thin walled, air tight, glass envelope, sealed by an extremely thin window through which radiations may pass at one end. The air is evacuated from the envelope and a small amount of an inert gas such as Argon, is added. Due to penetration of radiation, some of the gas atoms gets ionized. The resulting ions move towards the anode and positive ions towards cathode. In their passage, the ions collide with some of the gas atoms, causing them to be ionized in turn.

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A pulse of current thus flows through the tube. Whenever there is radiation, due to the above process, a series of current pulses pass through the anode circuit. By counting the number of pulses, we can estimate the intensity of radiation. Ionisation Chamber

It is a metal cylinder (the outer electrode) sealed at one end by a window. The other end of the cylinder is closed, and a metal rod (the centre electrode) is inserted from outside. The radiation creates ionization in the cylinder. As the positive ions collect on the central electrode (which is at ground potential), the d.c. potential of this electrode is increased. By measing this potential, we can determine the intensity of radiation. Scintillation Counter

Certain crystals such as zinc sulphide, sodium iodide, anthracene and naphthalene etc. produce a brief flash of light each time they are struck by an alpha, beta or gamma particles. The production of a flash of light by striking the crystals mentioned above, with alpha, beta or gamma rays is called scintillation. This principle is employed in this instrument. The flash of light is amplified by a photomultiplier and the output is applied to a high speed electronic counter. From the output of the counter, the intensity of radiation can be measured.

(any one type)


19 (a) Explain with a neat sketch a flue gas oxygen analyser.

Measurement of oxygen and carbondioxide content of flue gas are used as a guide to combustion. The change in oxygen percentage is proportional to the change in excess air, so that this measurement is the most appropriate one to apply to the combustion control process. Most gases are diamagnetic i.e. gas will seek the weakest part of a magnetic field, but oxygen is one of the few exceptions The oxygen analysers employs the paramagnetic property of oxygen and dependence of this property on temperature. The analysis cell contains a horizontal bypass tube which supports on its outer surface two adjacent identical platinum windings. These are connected to adjacent arms of Wheatstone bridge and are heated when a voltage is applied across the bridge. One of these windings is traversed diametrically by an intense magnetic field provided by a large permanent magnet. When the flue gas sample passes, the windings gets heated up due to magnetic susceptibility. This causes differential cooling of two windings and as the resistance of platinum windings changes the temperature, the bridge becomes unbalanced. The resulting emf is measured by a potentiometerand it is propotional to the oxygen content.

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19 (b) Write in detail about pollution monitoring system.

Monitoring air pollution is done to keep a track on quality of air with a view to collect information and improve it. The best indicators are SO2, smoke and suspended particles. The development of derivative gas-phase spectroscopy has opened a whole new dimension in gas-phase analysis. As a result, highly specific and accurate gas measurements may now be made in complex mixtures such as urban air. These instruments combine a number of performance oriented features to provide highly accurate, specific, reliable and continuous measurements of the concentrations of NO2, NO, SO2 or NH3 in the atmosphere. The light radiation emitted by an ultraviolet or visible source is spectrally dispersed by a grating monochromator. The wavelength of this monochromatic light is varied with respect to time by a literally oscillating monochromator entrance slit (“Wobbler”). The radiation entering the sample cell has a centre wavelength established by the positioning of the grating. A coil fixed to the oscillating wobbler and moving in a magnetic field provides a reference signal having the exact frequency and phase of the wavelength modulation. Air pollution monitoring systems tuned to a preciously idenfied peak that is characteristic of and unique to the compound being analysed. The height of that peak is then a direct measure of the compound’s concentration.

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20 (a) With a neat sketch, explain combustion control system.

Combustion is the rapid oxidation of fuel in a mixture of fuel and air with heat produced and carried by the mass of flue gas generated. When fuel and air are well mixed and all the fuel is burned, the flame temperature will be high and the combustion time will be shorter. When the fuel and air are not well mixed, complete combustion may not occur, the flame temperature will be lower and the fuel will take longer to burn. The following are the popular methods of combustion control:

1. Single-point positioning control 2. Parallel positioning control

Single-point positioning control

A single-point positioning or “jackshaft” system is a mechanical system. The position of the fuel control valve and the combustion air flow damper are connected together in a fixed relationship move in uniform to the demands of master regulator. Parallel positioning control The functions shown in the mechanical single-point positioning arrangement are performed here using instrumentation control.

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Such systems called parallel positioning control systems, link the functions pneumatically or electrically. The control logic diagram is given below

This system is useless without the use of a flue gas analyser or some other form of combustion guide. The advantage of the parallel system is that the timing of a fuel flow change or an air flow change can be modified by inserting a time constant in to either of the two control signals to improve matching of actual fuel and air flow to the furnace. This makes possible improved dynamic operation.

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20 (b) Discuss in brief about soot blowing. Soot built-up on the boiler tubes is a normal occurrence for liquid or solid fuel boilers. This soot accumulation can be reduced to some extent by maintaining the correct combustion conditions. In any event it must be removed periodically in order to avoid a severe loss in a heat transfer.

The above figure illustrate the loss in heat conductivity due to soot accumulation. Soat is an excellent insulator. But the negative effect is an increase in boiler draft loss and flue gas temperature with a resultant loss in boiler efficiency.

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Soot is normally removed from the tubes with devices called soot blowers. These are devices mounted along the sides of the boiler from which jets of steam or compressed air are used to blast the soot from the tubes. The normal practice is to start the soot blowing at the furnace and sequentially blow soot in to the flue gas stream.

21 (a) Explain with suitable diagrams about vibration monitoring and control in power plants.

A cross-section diagram of an electro-dynamic vibration pickup for the measurement of absolute rectilinear vibration is shown above. In this system, a seismic mass comprises of a pair of coils centered by two diaphragm springs within two magnetic fields. The right hand coil is the measuring coil, in which a voltage is developed electro dynamically, this being proportional to the velocity. The other coil is a hollow copper cylinder in which eddy currents are induced which bring the damping of the system upto g=1.

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As there are several bearings in the turbo-generator set and vibrations in all the three directions have to be monitored on each bearings, several pick ups have to be employed. The movement of the coil in the pick ups induces a voltage which is at any moment proportional to the velocity of the coil which is in turn proportion to vibration of the system. With the aid of an automatic selector switch measuring system, upto 12 pick ups can be connected for sequential recording.

21 (b) Explain in detail about shell temperature monitoring and control. The most important operational requirement is that excessive temperature differences within the steel casings of turbines especially at the h.p. and i.p. inlet must be avoided during starting and shutting down. In addition, accurate control to give a steady desired value of temperature at any instant will minimize thermal cycling, because excessive variationsin temperature tend to cause high gradients in thick pipe-walls and casings.

Themocouples were used for this measurement of temperature by fixing in various positions to enable stresses to be calculated from temperature distribution using simplifying assumptions.

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The platinum resistance temperature detectors with 100 ohms resistance is now-a-days used in place of thermocouples. In this transmitter, the emf generated by the thermocouple is converted into a DC current signal. The output signals obtained are fed to the receiving equipment for indication. Input is electrically separated from output by an isolation transformer permitting connection to any receiver without grounding techniques and operation with an input signal.

Solution prepared by:

K Thiyagarajan

Lecturer / Instrumentation & Control Engineering 224 Annamalai Polytechnic College

CHETTINAD – 630 102

For measuring oil flow, rotating vortex type of flow meter is used. It involves a vortex pool which...

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