PROJECT REPORT ON80MW THERMAL POWER PLANT

AT WELSPUN CAPTIVE POWER GENERATION PLANT (WCPGL)ANJAR, GUJARAT – 370110

SUBMITTED BY:SHOVNA MOHAPATRA

2nd YEAR, KIIT UNIVERSITYBHUBANESWAR - 751024

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Welspun Captive Power Generation Limited (WCPGL)

Anjar, Gujarat – 370110

CERTIFICATE OF TRAINING

This is to certify that, Miss. Shovna Mohapatra; studying B.Tech (Electronics and Instrumentation) from Kalinga Institute of Industrial Technology (KIIT),

Bhubaneswar, Odisha has successfully completed training in our 80MW plant during period, 8th May, 2014 to 4th June, 2014.

Mr. Rajendrasinh Jadeja Mr. Jaydeepsing Jethwa

(General Manager, WCPGL) (Manager, WCPGL)

Mr. Devendrasinh Jadeja Miss. Dhara Thakkar

(Engineer, WCPGL) (Manager, HR, WCPGL)

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ACKNOWLEDGEMENT

With the deepest gratitude I wish to thank Welspun Captive Power Generation Limited (WCPGL) for allowing me to undergo my training in such an esteemed and reputed organisation.

I would also like to acknowledge and express my sincere gratitude to the following people for their magnificent support and contribution for the training to be successful and without whom my training wouldn't have been possible.

I also wish to thank Mr. Rajendrasinh Jadeja (Plant Head) for allowing me to complete my training at WCPGL.

First of all I would like to thank my guide Mr. Devendrasinh Jadeja for sharing is wisdom and experience with me. He has been an excellent motivator for me through this entire training period and it was an honour for me to work under such a person. So thank you very much sir for bearing with me and sharing your knowledge.

I would also like to thank Mr. Jaydeepsinh Jethwa (Department Head) for taking care of entire training procedure and thus making it smooth and hassle free.

I would like to express my sincere gratitude to Ms. Dhara Thakkar and her HR team for arranging our training requirement.

Finally I am greatly indebted to all the engineers and the technicians of WCPGL for teaching me many things regarding the actual processes in the field and also for sharing their knowledge and experience with me.

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INDEXSL.NO. TOPICS PAGE NO.

1. WELSPUN

2. WELSPUN CAPTIVE POWER GENERATION PLANT (WCPGL)

3. SAFETY GUIDELINES

4. BOILER, TURBINE, GENERATOR

5. PRESSURE MEASUREMENT TECHNIQUES

6. TEMPERATURE MEASUMENT TECHNIQUES

7. LEVEL MEASUREMENT TECHNIQUES

8. CONTROL VALVES

9. BOILER ANALYSER

10. FLOWMETER

11. HART COMMUNICATOR MODEL

12. DISTRIBUTED CONTROL SYSTEM

13. AUTOMATIC CONTROL

14. REFERENCES

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WELSPUNWelspun Group is a multinational company which was founded by Mr G. D. Goenka in the year, 1985, whose core industry is Steel, Energy, Oil & Gas, and textiles. Welspun is one of India's fastest growing conglomerates doing business in over 50 Countries with 24,000 employees and over 100,000 shareholders. Its clients include Fortune 500 Companies operating in Oil & Gas and retail sector such as Chevron, ExxonMobil, WalMart, and Target. Guided by the three ‘E's - Education, Empowerment and Health, each and every Welspunite contributes towards the betterment of the community at large.

Our business strategy revolves around getting things done the right the first time.

We value professionalism and focus highly on systems, process and execution.

We have forayed with power generation and endeavour to expand across the value chain to become fully integrated company. This will enhance our reliability and reduce cost for our consumers as well.

We believe in building alliances with preferred partners on the basis of trust and knowledge sharing.

Quality and cost consciousness is of paramount importance to us.

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WELSPUN ENERGYWelspun Energy Ltd. has ventured into setting up of Solar, Hydro, Biomass and Wind energy power generating facilities. It offers the full value chain from “Silicon to Power”, and has also entered into the development of oil and gas natural assets across the country.

With a focus on green technology, Welspun Energy is interweaving inclusive growth, social, economic and environmental sustenance in our projects to provide sustainable energy for all. India is experiencing rapid economic growth. However, for India to truly shine, she needs to bring light and opportunities to her citizens living in rural areas. Welspun Energy Pvt. Ltd. has demonstrated strong Engineering, Procurement and Construction capabilities in the renewable projects space.

Welspun energy has successfully commissioned 297MW capacity of solar PV projects on a turnkey basis well in advance of the stipulated timelines. Currently, another 140MW of solar PV projects are collectively under construction in 5 states in India.

More than 250MW solar power projects and 800MW wind power being developed on ground including 60MW operational solar power projects in Gujarat, Rajasthan and AP.

130MW solar PV power plant in Madhya Pradesh Asia’s largest such project.

350MW wind power project and 55MW solar power project in Rajasthan

100MW solar project in Chhattisgarh.

750 and 100MW wind projects and over 550MW solar capacities in Karnataka.

500MW wind capacity and 100 MW solar capacity in Andhra Pradesh.

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SAFETY GUIDELINESWelspun Captive Power Generation Ltd. has two plants of different capacity under its name. One

80MW plant and one 43MW plant.

The 80MW plant has the following working parts, where utmost safety precautions are required:

1. Boiler coal bunker2. Electrostatic precipitator3. Chimney 4. D.M. Plant5. Air cooling condenser6. Coal crusher house7. 220KV switch yard

RISK It is the product of the severity of the accident and the likelihood of that accident.

HAZARD It is defined as the potential cause of an accident. The 6 hazards that can kill are:

B – Burns

S – Striking objects

A – Asphyxiations

F – Falls

E – Electrocution

R – Road accidents

PERSONAL PROTECTIVE EQUIPMENT (PPE) This may not prevent an accident but it reduces the severity.

UNSAFE ACT – This arises due to lack of awareness about a hazardous area. And when we meet with an accident because of our own carelessness, it’s called an unsafe act.

UNSAFE CONDITIONS – Even after taking all the safety precautions, if we meet with an accident because of the surrounding or the faulty equipment, then the accident is said to be caused because of unsafe conditions.

Most of the accidents are caused because of UNSAFE ACTS. The Pie-chart shows the percentage of the causes of accidents.

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88

102

CAUSES OF ACCIDENTS

Unsafe ActsUnsafe ConditionsBeyond Human Control

An Emergency Assembly Point is needed in an industrial area because, whenever a crisis situation arises or there is a need to warn everyone working in the plant about the accident that might occur. This helps to evacuate the workers easily and within a short span of time, without the loss of any life and property.

Regular paramedical checkup of the workers is necessary to know about the problems they are facing and the problems they might face while working in the factory. This helps to lower the risk of accidents.

First-aid training is needed because workers with basic knowledge of medication can give the first treatment to their fellow workers, before the proper medical treatment arrives.

People need to discuss about their near-miss accidents experiences. This will create awareness among the others and they will be able to prevent accidents.

Safety is needed in a factory because:1. It cuts the cost of medication. 2. It improves the quality and the production rate.3. It takes care of the health issues of the workers.4. It provides job security.5. And it doesn’t hamper the reputation or market values of the factory.

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BOILER

The boiler is outdoor located, bi-drum, natural circulation, forced draft, front wall fuel/fuel gas fire type. The different parts of the boiler are:

1. UPPER DRUM (STEAM DRUM) The steam drum is of fusion welded construction, located at the upper section of boiler. Steam drum is bottom supported by bank tubes and water wall tubes. One man hole is provided at the dished end of steam drum.

2. LOWER DRUM (MUD DRUM) There is a water drum in distributed water from steam drum to the furnace bottom supply header, for front and rear walls. It also supplies water for side and dividing walls. This is connected with steam drum through bank tubes.

3. BANK TUBES There are 547 no. of bank tubes connecting water and steam drums. There are installed in the convection zone in four different zones and receive heat from fuel gases. These are segmented to provide approach for ease of maintenance and soot blowing. These ensure supply of water from steam drum to water drum and cool the flue gases before entering into the economizer by absorbing heat of the flue gas.

4. WATER WALL TUBES Water wall surround the furnace on all the four sides. These are of membrane wall type construction. Water and steam mixture from front and rear water walls is collected into upper frontal and rear headers. These headers are connected to steam drum through overflow tubes. There is no other connection between upper headers and steam drum.

5. FURNACE Furnace is of water cooled membrane wall construction with tube materials of carbon steel of seamless quality. Furnace is provided with 3 nos of oil/gas burners mounted in front wall. Furnace wall are stiffened at required level by buck stays which are attached to the filler blocks welded on the water walls to withstand the furnace pressure/force and prevent any vibration. The front and rear wall buck stays are hinged with side wall buck stays.

6. BURNERS Boiler is provided with 3 nos. of fuel oil / fuel gas burners along with igniters and flame scanners. The burner will be lighted on LPG. Burner can be started on Diesel, FO or fuel gas.

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Burner has following major parts:1. Air damper 2. Atomizer Fuel 3. Firing system 4. Scanner and Igniter.

Air dampers provide combustion air for the burner through diffusers. There are two air dampers, one for primary air adjustment and second for secondary air. Primary air damper is manually operated whereas secondary air damper is pneumatically operated.

Atomizer is provided to break the heavy fuel oil into fine particles for easy combustion because heavy fuel oil is very thick and viscous. Medium Pressure Steam is used for atomizing the heavy fuel oil by the fuel gun.

Burner can be started on by fuel oil, fuel gas or diesel oil. Fuel gas is supplied to the burner through 8” pipe. This gas is carried to tip of the gas burners in 8 nos. of tubes. The gas burner tips are situated in primary air zone. Solenoid trip valves are provided in fuel oil line, Atomizing steam/air lines, Diesel Oil line, Fuel Gas line, Combustion air and LPG line which are operated from control room through DCS.

Main burner is provided with scanner and igniter. The main flame is scanned by main flame scanner and pilot flame is scanned by ionization rod in the igniter. The pilot flame is generated by igniter using LPG as fuel. The air required for cooling of these flame scanners as well as ignition air required for igniter is supplied by cooling and ignition air fans.

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7. STEAM COILED AIR PRE-HEATER (SCAPH) Steam coil air preheater is located in air path to heat combustion air for proper combustion. LP steam flows in tube side and air flows over the tubes. LP steam is supplied from LP steam header. Its condensate from SCAPH flows through steam trap to deaerator.

8. ECONOMIZER In boilers, economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling point of that fluid. Economizers are so named because they can make use of the Enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. They are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the cold water used to fill it (the feed water).

9. SUPERHEATERS There is a super heater provided to super heat the saturated steam, which is coming out of steam drum. Surer heater is provided in tow stages, superheater -1 and superheater -2. Both the superheater banks are located in the convection zone of the boiler. The superheaters are arranged in counter flow, to the flue gases to achieve specified superheater outlet temperature. The superheater tubes are of horizontal drainable type construction.

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Superheater -1The superheater -1 is arranged at lower elevation in convection path of flue gas. This consists of 43 coils. Each coil consists of 3 no. of tubes. The size of this super heater is 38 mm in diameter and 4 mm in thickness.

DesuperheaterThis is also known as the Spray Attemparator. The spray attemparator is placed between the superheater-1 outlet and superheater-2 to the attemparator..

Superheater – 2 The superheater -2 tubes are arranged in convection zone at the top of superheater -1 and size of tubes is 38 X 4mm. The superheater -2 consists of 39 coils each consists 3 no of tubes.

10. FORCE DRAUGHT FAN (FD FAN)

Each boiler is provided with 2 no. of FD fans of 100% capacity, each. The type of the fan is centrifugal (radial). One FD fan is motor driven and one FD fan is turbo driven. Normally turbo driven fan will remain in service. The motor driven FD fan remains in standby.

11. DEAERATOR

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Deaerator is mainly used for storage purpose of water, or also to remove oxygen from water to protect boiler from corrosion. Deaerator is the mechanical removal of dissolved gasses such as oxygen and carbon dioxide from a fluid.

12. SOOT BLOWER Boiler is provided with 10 no. of soot blowers to clean heating surface when the boiler is in service. There are 6 no. of long retractable blowers in economizer and 4 no. of soot blower.

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BASIC PRINCIPLE OF A POWER PLANT

The power plants works on the basic principle of the Rankine Cycle.It is the Thermodynamic Cycle which converts heat into work. The heat is supplied externally to a closed loop which usually uses water as a working fluid. This cycle generates about 80%

of electric power used in throughout the work.

PROCESSES OF RANKINE CYCLE

There are four processes in the Rankine Cycle, each changing the state of the working fluid (Water). These states are identified by the number in the diagram.

Process 1-2:- The working fluid pumped from the low to high pressure, as the fluid is a liquid at this stages the pump requires little input energy.

Process 2-3:- The high pressure liquid enters a boiler where it is heated at constant pressure by and external heat sources to become a dry saturated vapour.

Process 3-4:- The dry saturated vapour expands through a turbine, generating power. This decreases the temperature and processes of the vapour and some condensation may occur.

Process 4-1:- The wet vapour then enters a condenser where it is cooled at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase – change.

In an ideal Rankine cycle the pump and turbine would be isentropic, i.e. the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the T-S diagram and more closely resemble that of the Carnot

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Cycle. The Rankine cycle shown her prevents the vapour ending up in the superheat region after the expansion in the turbine which reduces the energy removed by the condenser.

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TURBINE

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work.

An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft.

PRINCIPLE PARTS OF TURBINE

CASTING :The shape of the casing and their supporting methods are carefully designed to obtain free and symmetrical movement due to thermal expansions and thereby minimize the thermal stress. The casing of the high pressure part is made of cast steel, and the casing of the low pressure parts is fabricated with welded steel plates. When assembling the casings, high and low pressure casings are bolted together by vertical flange to be handled as a single casing.

ROTOR: The rotor is carefully machined from an ingot of alloy steel forging. A solid rotor is composed of shaft, wheels, bearing journals and thrust bearing collar. The turbine rotor is supported by two main bearings, and is connected to the generator coupling. The critical speed of the rotor is lower than the operating speed. However, as the rotor is subjected to accurate dynamic balancing test after the blades and shrouds are fitted, excess vibration due to unbalance will not develop at any speed.

BLADING:

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The blade path includes a control stage operating with partial admission control followed by impulse blading in the high pressure and intermediate pressure zone and single flow reaction blading in the low pressure.

BEARING:The main bearing ate of two lobe type journal beating. The material of sleeve is Cast Steel and that of the bush is also of Cast Steel lined with Babbitt metal and they are of forced circulated lubrication type.

THRUST BEARING:The thrust bearing is of the Kingsbury type suitable for taking operating thrust and is located at the governor end of the turbine.

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GENERATOR

The electric generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The reverse conversion of electrical energy in to mechanical energy is done by a motor, a d motor and generators have many similarities a generator forces electric charge to move through an external electrical circuit but is does not create electricity or change, which is already present in the wire of its windings.

The turbo-generator is indoor type; single end drive, stationary armature and revolving field with cylinder rotor and brushless excitation complete with permanent magnet pilot exciter and rotating rectifier assemble. It has totally enclosed water to air cooled (TEWAC) heat exchanger mounted on the top of the stator frame. The cooling air required for generator cooling is circulated by fans located at both the ends of the generator.

PRINCIPLE PARTS OF GENERTOR

ROTOR:The rotor consists of a solid forging of high quality alloy steel, the physical analysis of the material and also the forging and heat treatment process being carefully controlled by the steel makers, and subjected to expert metallurgical inspection at every stage.

MAIN EXCITER:The pilot exciter is a permanent magnet revolving field, three phase stationary armature AC generator to supply excitation power to the automatic voltage regulator. It is mounted at the extreme end of the generator shaft.

BEARING:The generator beating is of journal type. The bearing surface is of sleeve type lined with a white metal.

COOLONG AIR CIRCUIT:Cooling air is driven round the generator by axial fans provided at both sides of the generator rotor. Cool air is drawn into each end of the generator stator and will then pass through the air gap between the winding slots in the rotor body. The air then flows out of the rotor through vent slots machined in the rotor surface in the line with the outlet compartment behind the stator core. The hot exhaust air will then leave machine from the center portion of the stator.

SPECIFICATION: Rated Power 80 MV Rated Output 100 MVA Rated Voltage 11000 V Rated current 5248.6 A Rated Speed 3000 r/min

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Rated Frequency 50 Hz Power Factor 0.8 lagging Exciting Current 1016.4 A Connection Y Insulation Class 155(F) Application Class 130(B) Phase Number 3

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PRESSURE MEASUREMENT TECHNIQUES

PRESSURE

It is defined as the force acting per unit area. Pressure = Force/Area [Force = Mass x Acceleration] N (newton) is the SI unit of force and is gravity dependent. But,

Newton/m2 = Pa (Pascal), which is gravity dependent and is the SI unit of pressure. Other units of pressure are atu, atm, Torr, kg/cm2 , psi.

Some essentials of pressure measurement are:1. Pressure is strongly influenced by position in a static fluid but, at a given position, it

is independent of direction.2. Pressure is unaffected by the shape of confining boundaries.

CONVERSIONS

1 atu = 14.5 psi1 bar = 100KPa = 1 atu1 atm = 14.7 psi = 1.003 atu = 101.33KPa1 Torr = 133.1 Pa = 0.02psi

TYPES OF PRESSURE

1. GAUGE PRESSURE It is the difference between the actual and the atmospheric pressure. Its unit is kg/cm2.

It is the pressure above atmospheric pressure. It can be converted to absolute pressure by adding actual atmospheric pressure value.

2. ABSOLUTE PRESSURE It is the actual total pressure acting on a surface. Its unit is kg/cm2.

It is measured above total vacuum or zero. It represents total lack of pressure.3. VACUUM PRESSURE

It is the pressure having value below atmospheric pressure.

4. STATIC/LINE/WORKING PRESSURE It is the pressure at a particular point when the fluid is in equilibrium. Or, we can say, it is the pressure exerted on the surface of the pipe by the fluid, flowing parallel to the pipe wall.

5. ATMOSPHERIC/BAROMETRIC PRESSURE Pressure exerted by the earth’s atmosphere. The value of atmospheric pressure decreases with increasing altitude.

6. DIFFERENTIAL PRESSURE

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It is the difference in magnitude of some pressure vale and a certain reference value.

7. HYDROSTATIC PRESSURE It is the pressure below a liquid surface, exerted by the liquid above.

8. DYNAMIC PRESSURE Pressure exerted by a flowing material parallel to the directions of such flow.

9. COMPOUND PRESSURE Pressure measured from a base reference point, i.e., neither the atmosphere nor total vacuum.

Let’s take a cylinder filled with gas. When force is applied, the gas inside the cylinder compresses to its minimum volume and the gas exerts pressure on the inner walls of the cylinder. This is called static pressure exerted by the gas.

With the increase of the weight, the pressure increases. The increased downward push is directly proportional to height of accumulated weight. When a column of liquid or gas stacks up inside a cylinder, it produces a downward push.

This height of the stack represents the strength of the downward force called the head pressure. Units of heads are ‘length’ but they represent ‘force per unit area’.

The water’s downward push is represented in W.G. or W.C. (Water Gauge or Water Column) The higher above sea-level we go, the lighter is the object. The downward force is local

gravity dependent and latitude dependent.

SOME MORE CONVERSIONS

1” WG at 15o C = 248.3Pa1mmWG at 15o C = 9.8Pa1mmHg at 0oC = 1 Torr

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MANOMETERS

It is the device used to measure pressure.

PRINCIPLE OF OPERATIONManometers derive pressure by the combination of a height differential of a liquid column and the density of the fluid within the liquid column. The U – type manometer, which is considered as a primary pressure standard, derives pressure utilizing the following equations:

P = P2 – P1 = hw pgWhere, P = differential pressure.P1 = pressure applied to low pressure connection.P2 = pressure to the high pressure connection.Hw = height differential of the liquid columns between the two legs of the manometer. p = mass density of the fluid within the columns.g = acceleration due to gravity.

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ADVANTAGES

1. These are simple and they consume less time.2. They have accuracy and sensitivity.3. Its cost is reasonable.4. They are suitable for low differential pressure and its applications.

DISADVANTAGES1. They are large and bulky.2. They are not portable.3. They need leveling.4. In a manometer, the measured fluid must be compatible with manometer fluid.5. There is no over-range protection in a manometer.6. Condensation may create problems in manometer.

TYPES OF MANOMETERS

There are five types of manometers.

They are:

1. U – tube manometer. It is the easiest to manufacture and is most widely used.

2. Well resoirver manometer. It amplifies the smaller level movement in the larger resoirver by using a narrower scaled tube. This increases sensitivity of measurement in the ratio of the areas of tube.

3. Inclined manometer. Inclined leg amplifies the small level fluctuations of the larger well by the sine of the angle of inclination. Dry oil is used and detergents are added to reduce frictional effects within the glass.

4. Mercury – float manometer. This type of manometer uses a metallic displacer which floats in mercury.

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5. Bell – type manometer. This type of manometer is used in industries to measure low pressure with a range, generally from 2-20cms of water. Principle:When no pressure is there, the bell sits at the bottom. As pressure is applied to the bell, the difference between the external atm. pressure and the inside pressure becomes a lifting force.

MECHANICAL PRESSURE ELEMENTS

These devices convert fluid pressure into force. If unrestrained, the natural elastic properties of the element will produce a motion, proportional to the applied pressure.

PRINCIPLE

Pressure works on Hook’s law. Measuring the stress in an elastic medium.

There are different types of such measuring devices such as:

1. Bourdon tube gauge: It is a hollow tube with an elliptical cross section. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the ends move. The movement is usually coupled to a needle on a dial, to make a complete cage. It can also be connected to a secondary device such as an air nozzle to control air pressure or to a suitable transducer, to convert it into an electric signal. This type can be used for measuring pressure difference.

PARTS OF BOURDON TUBE GAUGE

C-type bourdon tube Connecting link Sector gear Pinion gear Hair spring Pointer Dial

Hair spring serves two purposes namely, 1. To avoid back-lash error (eliminate any play into linkages).2. It serves as a controlling torque.

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OPERATION: System pressure is applied to the inside of a slightly flattened arc-shaped tube. As pressure increases, the tube tends to restore to its round cross-section. This change in cross-section causes the tube to straighten.

Since, the tube is permanently fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center. The tip movement can be used to position a pointer or to develop an electrical signal.

TYPES OF BOURDON TUBE C – TYPE TWISTED HELICAL SPIRAL

Most pressure gauges use a Bourdon tube as their pressure sensing element. Bourdon tubes may be made in spiral or helical forms for greater motion and therefore greater gauge resolution.

2. Bellows

They consist of series of circular parts joined such that they can be expanded axially by pressure

By increasing the diameter of the bellows force , pressure becomes low.

3. Metallic Diaphragms

The deformed middle section of the diaphragm pushes against and deflects a pointer on a scale.

4. Force Balance

Pressure is easily converted into force by acting on the surface area of a sensing element such as the diaphragm or bellows. The amount of displacement is detected and the element is returned to its null or zero displacement positions. It works well under high pressure. They are big and sensitive to vibration and temperature.

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ADVANTAGE:

It has no spring characteristics at all. Balance with the force of the process fluid pressure is achieved by the application of either an adjustable air pressure or an adjustable electric current not by the natural tensing of the spring element.

This makes a force balance instrument far less susceptible to errors due to metal fatigue or any of the degradation of spring characteristics.

DISADVANTAGE:

Force balance mechanisms tend to be bulky and they translate external vibrations into inertial force which adds noise to the output signal.

The amount of electrical power necessary to provide adequate balancing force in an electronic force balance transmitter is such that it is nearly impossible to limit below the level necessary to ensure intrinsic safety.

STRAIN GAUGE

A strain gauge converts mechanical displacement caused by pressure into an electronic signal. Variable resistive strain gauge elements are configured into a Wheatstone bridge. A strain gauge is accurate but is non linear. They are temperature sensitive. Accuracy varies from 0.1% to 1.0% , depending on the amount of temperature compensation.

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DIFFERENTIAL CAPACITANCE SENSORS

Electrical pressure sensor design works on the principle of differential capacitance. In this design, the sensing element is a taut metal diaphragm located equidistant between two stationary metal surfaces, forming a complementary pair of capacitances. An electrically insulating fill fluid (usually a liquid silicone compound) transfers motion from the isolating diaphragms to the sensing diaphragm, and also doubles as an effective dielectric for the two capacitors.

Since capacitance between conductors is inversely proportional to the distance separating them, capacitance on the low-pressure side will increase while capacitance on the high-pressure side will decrease. These pressure sensors are highly accurate, stable, and rugged.

FORCE-BALANCE PRESSURE TRANSMITTER

Pressure is easily converted into force by acting on the surface area of a sensing element such as a diaphragm or a bellows. A balancing force may be generated to exactly cancel the process pressure’s force, making a force-balance pressure instrument.

Differential pressure is sensed by the same type of liquid-filled diaphragm capsule, which transmits force to the force bar. If the force bar moves out of position due to this applied force, a highly sensitive electromagnetic sensor detects it and causes an electronic amplifier to send a different amount of electric current to a force coil. The force coil presses against the range bar which pivots to counteract the initial motion of the force bar. When the system returns to equilibrium, the milliampere current through the force coil will be a direct, linear representation of the process fluid pressure applied to the diaphragm capsule.

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Advantages: A distinct advantage of force-balance pressure instruments (besides their inherent linearity) is

the constraining of sensing element motion. Unlike a modern diaphragm-based pressure transmitter which relies on the spring characteristics

of the diaphragm to convert pressure into force and then into motion (displacement) which is sensed and converted into an electronic signal,

A force-balance transmitter works best when the diaphragm is slack and has no spring characteristics at all. Balance with the force of the process fluid pressure is achieved by the application of either an adjustable air pressure or an adjustable electric current, not by the natural tensing of a spring element.

This makes a force-balance instrument far less susceptible to errors due to metal fatigue or any other degradation of spring characteristics.

Disadvantages: Force-balance mechanisms tend to be bulky, and they translate external vibration into inertial

force which adds “noise” to the output signal. The amount of electrical power necessary to provide adequate balancing force in an electronic

force-balance transmitter is such that it is nearly impossible to limit below the level necessary to ensure intrinsic safety (protection against the accidental ignition of explosive atmospheres by limiting the amount of energy the instrument could possibly discharge into a spark).

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DIFFERENTIAL PRESSURE TRANSMITTER

The DP transmitter use to measure an actual difference of pressure across a process vessel such as a filter, a heat exchanger, or a chemical reactor.

Measuring absolute pressureAbsolute pressure is defined as the difference between a given fluid pressure and a perfect vacuum. We may build an absolute pressure sensing instrument by taking a DP instrument and sealing the “low” side of its pressure-sensing element in connection to a vacuum chamber. This way, any pressure greater than a perfect vacuum will register as a positive difference.

Measuring vacuumThe same principle of connecting one port of a DP device to a process and venting the other works well as a means of measuring vacuum (pressures below that of atmosphere). All we need to do is connect the “low” side to the vacuum process and vent the “high” side to atmosphere.

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TEMPEARATURE MEASUREMENT

TEMPERATURE

It is the measure of average kinetic energy within a substance.

It is the degree of the relationship between the average kinetic energy(Ek) and temperature(T) for a monatomic (single-atom molecule) gas:

Ek =3kt/2

Where,

Ek=Average kinetic energy of the gas molecule (joules)

k=Boltzman's constant (13.8 X 10-23 joules/Kelvin)

T=Absolute temperature of gas (Kelvin)

Thermal Energy

It is a different concept: the quantity of total kinetic energy for this random molecular motion.

Ethermal = 3kNT/2

Where,

N=Total number of molecules in gas

Types :

BI-METAL TEMPERATURE SENSORS

Solids tend to expand when heated. The amount that a solid sample will expand with increase in temperature depends on the size of the sample, the material is made of, and the amount of temperature rise. The following formula relates linear expansion of temperature change:

l =l0(1+α∆T)Where,

l = length of the material after heating

l0 = original length of the material

α = coefficient of linear expansion

∆T = change in the temperature

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If a bi-metal strip is twisted over a long length and when it is heated, it will tend to untwist . This twisting may be used to directly drive the needle of the temperature gauge. The above diagram shows the working principle of the temperature gauge.

FILLED BULB TEMPERATURE SENSOR

Filled -bulb system exploit the principle of fluid expansion to measure the temperature . If the fluid is enclosed in a sealed system and then heated, the molecule in that fluid will exert a greater pressure on the walls of the enclosing vessel. By measuring the pressure , and/or by allowing the fluid to expand under constant pressure, we may infer the temperature of the fluid.

THERMISTORS AND RESISTANCE TEMPERATURE DETECTORS

One of the simplest classes of temperature sensor is one where the temperature effects a change in electrical resistance. With this type of primary sensing element , a simple ohmmeter is able to function as a thermometer , interpreting the resistance as temperature measurement.

Thermistors are the devices made up of metal oxide which either increase in resistance with increase in temperature (a positive temperature coefficient ) or decrease with resistance with decrease with temperature(a negative temperature coefficient)

RTDs are made up of pure metal (using Platinum, Nickel or Copper) which always increase in resistance with increase in temperature.

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The major difference between Thermistors and RTDs are that RTDs are relatively insensitive and linear and whereas Thermistors are sensitive and non linear .

TEMPERATURE COFFECIENT OF RESISTANCE (α)Resistive temperature detectors relate resistance to temperature by the following formula :

RT=Rref[1+α(T-Tref)]Where,RT = Resistance of RTD at given temperature T(ohms)Rref = Resistance of RTD at given reference temperature Tref(ohms) α = Temperature coefficient of resistance(ohms/degree)

Water's freezing and melting point is the reference temperature for most RTDs. Nickel - 0.00672 Ω/0C Tungsten - 0.0045 Ω/0C Silver - 0.0041 Ω/0C Gold - 0.0040 Ω/0C Platinum - 0.00392 Ω/0C Copper - 0.0038 Ω/0C

The types of RTDs are : Two wire RTDs Four wire RTDs Three wire RTDs

Accuracy increases with increase in the number of wires.

SELF - HEATING ERROROne problem inherent in both Thermistors and RTDs. In order to measure the resistance of the device, we must an electric current through it. Unfortunately this results in generation of heat at the resistance according to the Joule's Law :

P=I2R

This dissipated power causes the thermistor or RTD to increase in temperature beyond its surrounding environment, introducing a positive measurement error. The effect may be minimized by limiting excitation current to a bare minimum, but this results in less voltage dropped across the device. The smaller the developed voltage, the more sensitive the voltage-measuring instrument must be to accurately sense the condition of the resistive element.

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Thermocouples

Thermocouples, however, generate their own electric potential. In some ways, this makes thermocouple systems simpler because the device receiving the thermocouple's signal does not have to supply electric power to the thermocouple. The self-powering nature of thermocouples also means they do not suffer from the same “self-heating” effect as RTDs. Though typically not as accurate as RTDs, thermocouples are more rugged, have greater temperature measurement spans, and are easier to manufacture in different physical forms.

Dissimilar metal junctions: When two dissimilar metal wires are joined together at one end, a voltage is produced

at the other end that is approximately proportional to temperature. That is to say, the junction of two different metals behaves like a temperature-sensitive battery. This form of electrical temperature sensor is called a thermocouple:

The principle is called ‘SEE BACK effect’.

Junction J1 is a junction of iron and copper – two dissimilar metals – which will generate a voltage related to temperature. Note that junction J2, which is necessary for the simple fact that we must somehow connect our copper-wired voltmeter to the iron wire, is also a dissimilar-metal junction which will generate a voltage related to temperature. Note also how the polarity of junction J2 stands opposed to the polarity of junction J1 (iron = positive ; copper = negative). A third junction (J3) also exists between wires, but it is of no consequence because it is a junction of two identical metals which does not generate a temperature-dependent voltage at all. The presence of this second voltage-generating junction (J2) helps explain why the voltmeter registers 0 volts when the entire system is at room temperature: any voltage generated by the iron copper junctions will be equal in magnitude and opposite in polarity, resulting in a net

(series-total) voltage of zero. It is only when the two junctions J1 and J2 are at different temperatures that the voltmeter registers any voltage at all.

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We may express this relationship mathematically as follows:

Vmeter = VJ1 − VJ2

Thermocouple types: Thermocouples exist in many different types, each with its own colour codes for the

dissimilar-metal wires. Here is a table showing the more common thermocouple types and their standardized colours, along with some distinguishing characteristics of the metal types to aid in polarity identification when the wire colours are not clearly visible:

The negative (−) wire of every thermocouple type is colour-coded red.

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LEVEL MEASUREMENTThe height of the water column, liquid and powder etc., at the desired measurement of height between minimum level point to maximum level point is called level. The measurement principle is, head pressure method.

Many industrial processes require the accurate measurement of fluid or solid, a wide variety of technologies exist to measure the level of substances within vessel.

Units: Meters, mm, cm, percentage

Level gauges (sight glasses):

The level gauge or sight glass is to liquid level measurement as manometers are to pressure measurement: a very simple and effective technology for direct visual indication of process level. In its simplest form, a level gauge is nothing more than a clear tube through which process liquid may be seen.

Some level gauges called reflex gauges are equipped with special optics to facilitate the viewing of clear liquids, which is problematic for simple glass-tube sight glasses.

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Interface problems:

One such circumstance is in the presence of a lighter liquid layer existing between the connection ports of the gauge. If a lighter (less dense) liquid exists above a heavier (denser) liquid in the process vessel, the level gauge may not show the proper interface.

Temperature problems:

When the liquid inside the vessel is substantially hotter than the liquid in the gauge, causing the densities to be different. This is commonly seen on boiler level gauges, where the water inside the sight glass cools off substantially from its former temperature inside the boiler drum.

FLOAT:

A device that rides on the surface of the fluid or solid within the storage vessel. The float itself must be of substantially lesser density than the substance of interest and it must not corrode or otherwise react with the substance.

PRINCIPLE:

“Based on Buoyancy Principle”.

A person lowers a float down into a storage vessel using a flexible measuring tape, until the tape goes slack due to the float coming to rest on the material surface. At that point, the person notes the length indicated on the tape (reading off the lip of the vessel access hole). This distance is called the ullage, being the distance from the top of the vessel to the surface of the process material.

Fillage of the vessel may be determined by subtracting this “ullage” measurement from the known height of the vessel.

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The float’s position inside the tube may be readily detected by ultrasonic waves, magnetic sensors or any other applicable means. Locating the float inside a tube eliminates the need for guide wires or a sophisticated tape retraction or tensioning system. If no visual indication is necessary, the level gauge tube may be constructed out of metal instead of glass, greatly reducing the risk of tube breakage. All the problems inherit to sight glasses; however still apply to this form of float instrument.

Float level measurement is to use a principle called magnetostriction to detect the position of the float along a metal guide rod called a waveguide.

Bubbler Systems:

An interesting variation on this theme of direct hydrostatic pressure measurement is the use of a purge gas to measure hydrostatic pressure in a liquid-containing vessel. This eliminates the need for direct contact of the process liquid against the pressure-sensing element, which can be advantageous if the process liquid is corrosive.

Such systems are often called bubble tube or dip tube systems, the former name being appropriately descriptive for the way purge gas bubbles out the end of the tube as it is submerged in process liquid. A very simple bubbler system may be simulated by gently blowing air through a straw into a

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glass of water, maintaining a steady rate of bubbles exiting the straw while changing the depth of the straw’s end in the water.

Displacer:

Displacer level instruments based on Archimedes Principle to detect liquid level by continuously measuring the weight of a rod immersed in the process liquid. As liquid level increases, the displacer rod experiences a greater buoyant force, making it appear lighter to the sensing instrument, which interprets the loss of weight as an increase in level and transmits a proportional output signal.

The displacer itself is usually a sealed metal tube, weighted sufficiently so it cannot float in the process liquid. It hangs within a pipe called a “cage” connected to the process vessel through two block valves and nozzles. These two pipe connections ensure the liquid level inside the cage matches the liquid level inside the process vessel, much like a sight glass.

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Some displacer-type level sensors do not use a cage, but rather hang the displacer element directly in the process vessel. These are called “cageless” sensors. Cageless instruments are of course simpler than cage-style instruments, but they cannot be serviced without de-pressurizing (and perhaps even emptying) the process vessel in which they reside. They are also susceptible to measurement errors and “noise” if the liquid inside the vessel is agitated, either by high flow velocities in and out of the vessel, or by the action of motor-turned impellers installed in the vessel to provide thorough mixing of the process liquid(s).

Echo:

A completely different way of measuring liquid level in vessels is to bounce a traveling wave off the surface of the liquid – typically from a location at the top of the vessel – using the time-of-flight for the waves as an indicator of distance15, and therefore an indicator of liquid height inside the vessel .

Echo-based level instruments enjoy the distinct advantage of immunity to changes in liquid density, a factor crucial to the accurate calibration of hydrostatic and displacement level instruments.

Ultraviolet Level Measurement:

Ultrasonic level instruments measure the distance from the transmitter (located at some high point) to the surface of a process material located further below using reflected sound waves. The frequency of these waves extends beyond the range of human hearing, which is why they are called ultrasonic.

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The time-of-flight for a sound pulse indicates this distance, and is interpreted by the transmitter electronics as process level. These transmitters may output a signal corresponding either to the fullness of the vessel (fillage) or the amount of empty space remaining at the top of a vessel (ullage).

This arrangement makes fillage the natural measurement, and ullage a derived measurement (calculated by subtraction from total vessel height). Ullage = Total height – Fillage

Ultrasonic level instruments enjoy the advantage of being able to measure the height of solid materials such as powders and grains stored in vessels, not just liquids. Again, the fundamental criterion for detecting a level of material is that the densities above and below the interface must differ. Certain challenges unique to these level measurement applications include low material density and uneven profiles laterally instead of straight back to the ultrasonic instrument.

Radar level measurements:

Radar level instruments measure the distance from the transmitter (located at some high point) to the surface of a process material located further below in much the same way as ultrasonic transmitters – by measuring the time-of-flight of a traveling wave.

The fundamental difference between a radar instrument and an ultrasonic instrument is the type of wave used: radio waves instead of sound waves.

Radio waves are electromagnetic in nature and very high frequency. Sound waves are mechanical vibrations and of much lower frequency than radio waves.

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The instruments using waveguides are called guided-wave radar instruments, whereas the radar instruments relying on open space for signal propagation are called non-contact radar.

Weight:

Weight-based level instruments sense process level in a vessel by directly measuring the weight of the vessel. If the vessel’s empty weight (tare weight) is known, process weight becomes a simple calculation of total weight minus tare weight.

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Weight-based level sensors can measure both liquid and solid materials, and they have the benefit of providing inherently linear mass storage measurement.

Weight-based measurements are often employed where the true mass of a quantity must be ascertained, rather than the level. So long as the material’s density is a known constant, the relationship between weight and level for a vessel of constant cross-sectional area will be linear and predictable.

Capacitive:

Capacitive level instruments measure electrical capacitance of a conductive rod inserted vertically into a process vessel. As process level increases, capacitance increases between the rod and the vessel walls, causing the instrument to output a greater signal.

The basic principle behind capacitive level instruments is the capacitance equation:

Where, C = Capacitance € = Permittivity of dielectric (insulating) material between plates A = Overlapping area of plates d = Distance separating plates

The amount of capacitance exhibited between a metal rod inserted into the vessel and the metal walls of that vessel will vary only with changes in permittivity (ǫ), area (A), or distance (d). Since A is

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constant (the interior surface area of the vessel is fixed, as is the area of the rod once installed), only changes in or d can affect the probe’s capacitance.

Capacitive level probes come in two basic varieties: one for conductive liquids and one for nonconductive liquids. If the liquid in the vessel is conductive, it cannot be used as the dielectric (insulating) medium of a capacitor.

Radiation:

Certain types of nuclear radiation easily penetrates the walls of industrial vessels, but is attenuated by traveling through the bulk of material stored within those vessels. By placing a radioactive source on one side of the vessel and measuring the radiation making it through to the other side of the vessel, an approximate indication of level within that vessel may be obtained.Other types of radiation are scattered by process material in vessels, which means the level of process material may be sensed by sending radiation into the vessel through one wall and measuring back-scattered radiation returning through the same wall.The four most common forms of nuclear radiation are:

Alpha Particles (α) Beta Particles (β) Gamma Particles (ϒ) Neutrons (n)

Alpha particles are helium nuclei (2 protons bound together with 2 neutrons) ejected at high velocity from the nuclei of certain decaying atoms. They are easy to detect, but have very little penetrating power and so are not used for industrial level measurement.

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Radioactive sources naturally emit radiation, requiring no source of energy such as electricity to do their job. As such, they are “always-on” devices and may be locked out for testing and maintenance only by dropping a lead shutter over the “window” of the box. The lever actuating the shutter typically has provisions for lock-out/tag-out so a maintenance person may place a padlock on the lever and prevent anyone else from “turning on” the source during maintenance. For point-level (level switch) applications, the source shutter acts as a simple simulator for either a full vessel (in the case of a through-vessel installation) or an empty vessel (in the case of a backscatter installation).A full vessel may be simulated for neutron backscatter instruments by placing a sheet of plastic (or other hydrogen-rich substance) between the source box and the process vessel wall.

The detector for a radiation-based instrument is by far the most complex and expensive component of the system.

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CONTROL VALVEOne of the most common final control elements in industrial control systems is the control valve. A “control valve” works to restrict the flow of fluid through a pipe at the command of an automated signal, such as the signal from a loop controller or logic device (such as a PLC).Control valves are comprised of two major parts: the valve body, which contains all the mechanical components necessary to influence fluid flow; and the valve actuator, which provides the mechanical power necessary to move the components within the valve body.A control valve is the final control element, which directly changes the valve of the manipulated variable by changing the rate of flow of control agent.

Control Valve Parts: 1. Actuators 2. Valve Positioners 3. I/P converter 4. Limit switches 5. Hand wheel 6. Air seats 7. Position Sensor 8. Solenoid Valve 9. Air Block Relay etc

Control Valve Characteristics: 1. Quick Opening 2. Linear 3. Equal Percentage 4. Modified Parabolic

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Quick Opening: Used for on/off application 10% of all Industrial valve used

Linear: Used for liquid level control Used where load is small

Q = KX

Where, Q = Flow rate in % K=100X = Valve Travel(0.1, 0.2, 0.3…., 1)

Equal Percentage:

Used for pressure control and flow control where the pressure drops across the valve is small fraction of system drop.

Also used where little or no information available about the system.

Q = Q0 em X

Where, Q = Flow Rate in % Q0= Minimum controllable flow M = ln(R/T ¿R = Range ability T = 1” stroke (maximum travel) X = Valve Travel (0.1, 0.2, 0.3…….,1)

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Factors for Selection of characteristics: 1. Load Variation 2. Variable Pressure Drop 3. System stability 4. System non-linarites

Control Valve Actuator:

The purpose of a control valve actuator is to provide the motive force to operate a valve mechanism. Both sliding-stem and rotary control valves enjoy the same selection of actuators: pneumatic, hydraulic, electric motor, and hand (manual).

Pneumatic actuators:

Pneumatic actuators use air pressure pushing against either a flexible diaphragm or a piston to move a valve mechanism. The following photograph shows a cut-away control valve, with a pneumatic diaphragm actuator mounted above the valve body. The air pressure required to motivate a pneumatic actuator may come directly from the output of a pneumatic process controller, or from a signal transducer (or converter) translating an electrical signal into an air pressure signal. Such transducers are commonly known as I/P or “I to P” converters, since they typically translate an electric current signal (I) of 4 to 20 mA DC into an air pressure signal (P) of 3 to 15 PSI. Pneumatic valve actuators are equipped with hand jacks which are used to manually position the valve in the event of air pressure failure.The greatest disadvantage of piston actuators as applied to control valves is friction between the piston’s pressure-sealing ring and the cylinder wall.

Hydraulic actuators:

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Hydraulic actuators use liquid pressure rather than gas pressure to move the valve mechanism. Nearly all hydraulic actuator designs use a piston rather than a diaphragm to convert fluid pressure into mechanical force. The high pressure rating of piston actuators lends itself well to typical hydraulic system pressures, and the lubricating nature of hydraulic oil helps to overcome the characteristic friction of piston-type actuators.

Self-operated valve:

Although not a type of actuator itself, a form of actuation worthy of mention is where the process fluid pressure itself actuates a valve mechanism. This self-operating principle may be used in throttling applications or on/off applications, in gas or liquid services alike. The process fluid may be directly tubing to the actuating element (diaphragm or piston), or passed through a small mechanism called a pilot to modulate that pressure before reaching the valve actuator.

Electric actuators:

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Electric motors have long been used to actuate large valves, especially valves operated as on/off (“shutoff”) devices. Advances in motor design and motor control circuitry have brought motor operated valve (MOV) technology to the point where it now competes with legacy actuator technologies such as pneumatic in actuating throttling valves as well.

Most electric valve actuators use a worm gear set to reduce the high rotational speed of the electric motor to a slow rotation suitable for moving a large valve mechanism.

The worm screw looks much like a threaded fastener, with its “threads” properly pitched to engage with the teeth of the worm wheel gear. As the worm screw turns, it slowly pushes or pulls the circumference of the worm wheel, resulting in a large gear ratio.

A small lever to the left of the hand crank actuates a clutch mechanism to engage or disengage the valve mechanism from the electric motor and the hand wheel. This clutch “selects” either the motor or the hand wheel as the prime mover for the valve, to avoid having the hand wheel spin as the motor turns. Electric motors require no external fluid power system to function, unlike pneumatic or hydraulic actuators. All they require is a source of electrical power (often 480 volts AC, three-phase). Some electric valve actuators even have the capability of running off battery packs, for reliable operation in the event of a power system outage.

Hand (manual) actuators:

Valves may also be actuated by hand power alone. The threaded stem of the left-hand valve in the last photograph pair. This stem rises and falls with the handle’s turning, providing visual indication of the valve’s status. Such an actuator is called a rising-stem design.

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Valve Positioners:

A positioner is a motion-control device designed to actively compare stem position against the control signal, adjusting pressure to the actuator diaphragm or piston until the correct stem position is reached.Positioners essentially act as control systems within themselves, the valve’s stem position is the process variable (PV), the command signal to the positioner is the set point (SP), and the positioner’s signal to the valve actuator is the manipulated variable (MV) or output. Thus, when a process controller sends a command signal to a valve equipped with a positioner, the positioner receives that command signal and does its best to ensure the valve stem position follows along.

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Sliding-stem valves:A sliding-stem valve body is one that actuates with a linear motion.

Globe valves:Globe valves restrict the flow of fluid by altering the distance between a movable plug and a stationary seat (in some cases, a pair of plugs and matching seats). Fluid flows through a hole in the centre of the seat, and is more or less restricted by how close the plug is to that hole. The globe valve design is one of the most popular sliding-stem valve designs used in throttling service.

The globe valve design is the port-guided valve, where the plug has an unusual shape that projects into the seat. Thus, the seat ring acts as a guide for the plug to keep the centre lines of the plug and seat always aligned, minimizing guiding stresses that would otherwise be placed on the stem. This means that the stem may be made smaller in diameter than if the valve trim were stem-guided, minimizing sliding friction and improving control behavior.

Some globe valves use a pair of plugs (on the same stem) and a matching pair of seats to throttle fluid flow. These are called double-ported globe valves. The purpose of a double-ported globe valve is to minimize the force applied to the stem by process fluid pressure across the plugs.

Differential pressure of the process fluid (P1 − P2) across a valve plug will generate a force parallel to the stem as described by the formula F = PA, with A being the plug’s effective area presented for the pressure to act upon.

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If the plug areas are approximately equal, then the forces will likewise be approximately equal and therefore nearly cancel. This makes for a control valve that is easier to actuate (i.e. the stem position is less affected by process fluid pressures).

Gate valves:Gate valves work by inserting a dam (“gate”) into the path of the flow to restrict it, in a manner similar to the action of a sliding door. Gate valves are more often used for on/off control than for throttling.

Diaphragm valves: Diaphragm valves use a flexible sheet pressed close to the edge of a solid dam to narrow the flow path for fluid. These valves are well suited for flows containing solid particulate matter such as slurries, although precise throttling may be difficult to achieve due to the elasticity of the diaphragm. The next photograph shows a diaphragm valve actuated by an electric motor, used to control the flow of treated sewage.

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Rotary-stem valves: A different strategy for controlling the flow of fluid is to insert a rotary element into the flow path. Instead of sliding a stem into and out of the valve body to actuate a throttling mechanism, rotary valves rely on the rotation of a shaft to actuate the trim. An important advantage of rotary control valves over sliding-stem designs such as the globe valve and diaphragm valve is a virtually obstruction less path for fluid when the valve is wide-open.

Ball valves: In the ball valve design, a spherical ball with a passageway cut through the center rotates to allow fluid more or less access to the passageway. When the passageway is parallel to the direction of fluid motion, the valve is wide open; when the passageway is aligned perpendicular to the direction of fluid motion, the valve is fully shut (closed).

Simple ball valves with full-sized bores in the rotating ball are generally better suited for on/off service than for throttling (partially-open) service. A better design of ball valve for throttling service is the characterized or segmented ball valve, shown in various stages of opening in the following set of photographs:

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Butterfly valves: Butterfly valves are quite simple to understand: the “butterfly” element is a disk that rotates perpendicular to the path of fluid flow. When parallel to the axis of flow, the disk presents minimal obstruction; when perpendicular to the axis, the disk completely blocks any flow. Fluid-tight shutoff is difficult to obtain in the classic butterfly design unless the seating area is lined with a soft (elastic) material.

Disk valves: Disk valves (often referred to as eccentric disk valves or as high-performance butterfly valves) are a variation on the butterfly design intended to improve seat shut-off. The disk’s center is offset from the shaft center line, causing it to approach the seat with a “cam” action that results in high seating pressure. Thus, tight shut-off of flow is possible even when using metal seats and disks.

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Dampers and louvers:A damper (otherwise known as a louver) is a multi-element flow control device generally used to throttle large flows of air at low pressure. Dampers find common application in furnace and boiler draft control, and in HVAC (Heating, Ventilation, and Air Conditioning) systems.

BOILER ANALYZER

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pH Measurement:

pH is measured as a voltage or potential of the Hydrogen (H3O+) ion activity in an aqueous solution. In any event a pH scale of 0-14 was developed with pH 7, the midway point as neutrality where a pH of 7 is equal to zero milli-volts.

Theoretically, the voltage developed between the two internal electrode wires is 59.16mV per pH unit. The total pH scale is 0-14 with a neutral pH of 7.000 measured voltage of zero milli-volts. The pH scale is negative above seven and positive below seven. A pH of 4.000 would have a voltage

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which corresponds to three pH units from neutrality or 3 x 59.16mV= +177.48mV. A pH of 10.000 also has a value of three pH units from neutrality, but the resulting voltage would be negative (-3 x 59.16mV or - 177.48mV). By using the mV key or Range key on your pH meter, one can directly view the voltages produced. This can be a very desirable diagnostic feature. When buying a new meter, always make sure to get this feature even if you are not measuring ORP.

Conductivity Meter: The conductivity meter was first calibrated using the calibration constant solution. The probes from the various conductivity

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meters were dipped into the calibration solution. The units were calibrated by adjusting the value on the meter to read the value of the constant (0.1413 milli-siemens (mS)). This was done by using either increase/decrease buttons on the meter or using a small tool supplied with the meters to adjust a small potentiometer.

Care was taken in ensuring that the conductivity measurements were made when the water and foam solution were at the same temperature. Small differences in temperature can substantially change conductivity measurements. This method had a very minimal effect on the accuracies of the readings. The second method was chosen for this evaluation because of the consideration of the fact that these units would be being used in the field and not in a lab. The second method is the easiest method to use in a field application. As long as the same procedure is used in the field for each of the test samples the upward trend of the data as the concentrations increased would be maintained.

The conductivity meter readings were taken by dipping the conductivity probe into the sample and the digital scale read. It is important to ensure that any air bubbles are removed from the probe. To do this the probe was gently tapped on the sides of the graduates. The readings on the meter were then allowed to stabilize and the value recorded.

Dissolved OxygenIntroduction:Dissolved oxygen (DO) is the term commonly used in liquid analytical work for the measurement of the amount of oxygen dissolved in a unit volume of water. It is an important indicator of the degree of usefulness of a sample of water for a specific application. The requirements of a given application determine the level of DO that can be tolerated.

Application of DO: In water quality application, to maintain a fresh water stream fit for recreational purposes

such as swimming and fishing and as a source of potable water.

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In sewage treatment, solids are allowed to settle in large basins to which are added solutions rich in bacteria to speed the decomposition of the solids.

Another important application of DO is the control of the quality of boiler make up water. In this case, since the presence of oxygen in the water will enhance corrosion and cause the buildup of boiler scale that inhibits heat transfer, it is very desirable to hold the DO concentration to a minimum.

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FLOWAny fluid or liquid flowing from one place to another is called a flow and it is defined as volume per unit of time at specified temperature and pressure conditions.

Its units are kg/hour, litre/min, gallon/min, m3/hour, Nm3/hour (gases).

CONVERSIONS

1m3 = 264.2 US Gallons

1L = 10-3 m3

1 Barrel = 42 US Gallons oil = 35 Imp Gallons

FACTORS AFFECTING FLOW RATE IN PIPES

The velocity of the fluid. The friction of the fluid in contact with the pipe. The viscosity of the fluid. The specific gravity of the fluid.

PRESSURE BASED FLOWMETERSAll fluids possess mass and therefore require force to accelerate just like solid masses. If we consider a quantity of fluid confined inside a pipe, with that fluid quantity having a mass equal to its volume multiplied by its mass density (m = pV , where ρ is the fluid’s density), the force required to accelerate that fluid “plug” would be calculated just the same as for a solid mass.

Force = mass x accelerationF = pVaWhere, F = force acting on the fluid.p = density of the fluid.V = velocity of the fluid.a = acceleration of the fluid.

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VENTURI TUBESThese flowmeters are the largest and the most costly. They operate by gradually narrowing the diameter of the pipe and measuring the resultant drop in pressure. An expanding section of the meter then returns to the flow to very near its original pressure.

The Venturi tube, illustrated in the above figure, is the most accurate flow-sensing element when properly calibrated. The venturi tube has a converging conical inlet, a cylindrical throat, and a diverging recovery cone. The inlet section decreases the area of the fluid stream, causing the velocity to increase and the pressure to decrease. The low pressure is measured in the centre of the cylindrical throat since the pressure will be at its lowest value, and neither the pressure nor the velocity is changing. The recovery cone allows for the recovery of pressure such that total pressure loss is only 10% to 25%. The high pressure is measured upstream of the entrance cone. The major disadvantages of this type of flow detection are the high initial costs for installation and difficulty in installation and inspection.

ORIFICE PLATESOf all the pressure-based flow elements in existence, the most common is the orifice plate. This is simply a metal plate with a hole in the middle for fluid to flow through. Orifice plates are typically sandwiched between two flanges of a pipe joint, allowing for easy installation and removal.

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The simplest design of orifice plate is the square-edged, concentric orifice. Square-edged orifice plates may be installed in either direction, since the orifice plate “appears” exactly the same from either direction of fluid approach. In fact, this allows square-edged orifice plates to be used for measuring bidirectional flow rates (where the fluid flow direction reverses itself from time to time). A text label printed on the “paddle” of any orifice plate customarily identifies the upstream side of that plate, but in the case of the square-edged orifice plate it does not matter.

PITOT TUBE

The Pitot tube , for example, senses pressure as the fluid stagnates (comes to a complete stop) against the open end of a forward-facing tube. A shortcoming of the classic single-tube Pitot assembly is sensitivity to fluid velocity at just one point in the pipe, so a more common form of Pitot tube seen in industry is the averaging Pitot tube consisting of several stagnation holes sensing velocity at multiple points across the width of the flow.

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ANNUBARAn “Annubar” is an averaging pitot tube consolidating high and low pressure sensing ports in a single probe assembly.

FLOW NOZZLEAnother variation on the venturi theme is called a flow nozzle, designed to be clamped between the faces of two pipe flanges in a manner similar to an orifice plate. The goal here is to achieve simplicity of installation approximating that of an orifice plate while improving performance (less permanent pressure loss) over orifice plates.

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LAMINAR FLOWMETERS

Laminar flow is a condition of fluid motion where viscous (internal fluid friction) forces greatly overshadow inertial (kinetic) forces. A flow stream in a state of laminar flow exhibits no turbulence, with each fluid molecule traveling in its own path, with limited mixing and collisions with adjacent molecules. The dominant mechanism for resistance to fluid motion in a laminar flow regime is friction with the pipe or tube walls.

VARIABLE AREA FLOWMETERS

A variable-area flow meter is one where the fluid must pass through a restriction whose area increases with flow rate.

ROTAMETERS

The simplest example of a variable-area flow meter is the rotameter, which uses a solid object (called a plummet or float) as a flow indicator, suspended in the midst of a tapered tube. As fluid flows upward through the tube, a pressure differential develops across the plummet. This pressure differential, acting on the effective area of the plummet body, develops an upward force (F = P/A). If this force exceeds the weight of the plummet, the plummet moves up. As the plummet moves further up in the tapered tube, the area between the plummet and the tube walls (through which the fluid must travel) grows larger. This increased flowing area allows the fluid to make it past the plummet without having to accelerate as much, thereby developing less pressure drop across the plummet’s body. At some point, the flowing area reaches a point where the pressure-induced force on the plummet body exactly matches the weight of the plummet. This is the point in the tube where the plummet stops moving, indicating flow rate by it position relative to a scale mounted on the outside of the tube.

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WEIRS AND FLUMES

A very different style of variable-area flow meter is used extensively to measure flow rate through open channels, such as irrigation ditches. If an obstruction is placed within a channel, any liquid flowing through the channel must rise on the upstream side of the obstruction. By measuring this liquid level rise, it is possible to infer the rate of liquid flow past the obstruction.

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VELOCITY BASED FLOWMETERS

The Law of Continuity for fluids states that the product of mass density (ρ), cross-sectional pipe area (A) and average velocity (v) must remain constant through any continuous length of pipe: If the density of the fluid is not subject to change as it travels through the pipe (a very good assumption for liquids), we may simplify the Law of Continuity by eliminating the density terms from the equation:

A1v1 = A2v2

TURBINE FLOWMETERSTurbine flow meters use a free-spinning turbine wheel to measure fluid velocity, much like a miniature windmill installed in the flow stream.

VORTEX FLOWMETERS

When a fluid moves past a stationary object, there is a tendency for the fluid to form vortices on either side of the object. There is no moving element to “coast” as in a turbine flow meter if fluid

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flow suddenly stops, which means vortex flow meters are better suited to measure erratic flow. A significant disadvantage of vortex meters is a behaviour known as low flow cut-off, where the flow meter simply stops working below a certain flow rate. When the flow is laminar, fluid viscosity is sufficient to prevent vortices from forming, causing the vortex flow meter to register zero flow even when there may be some (laminar) flow through the pipe.

MAGNETIC FLOWMETERS

When an electrical conductor moves perpendicular to a magnetic field, a voltage is induced in that conductor perpendicular to both the magnetic flux lines and the direction of motion. This phenomenon is known as electromagnetic induction, and it is the basic principle upon which all electro-mechanical generators operate.

E = BlvWhere, E = Motional EMF (volts) B = Magnetic flux density (Tesla) l = Length of conductor passing through the magnetic field (meters) v = Velocity of conductor (meters per second) This type of flowmeter working based on electromagnetic induction. These flowmeters are commonly known as magnetic flowmeters or simply magflow meters.

ULTRASONIC FLOWMETERS

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Ultrasonic flow meters measure fluid velocity by passing high-frequency sound waves along the fluid flow path. Fluid motion influences the propagation of these sound waves, which may then be measured to infer fluid velocity.

CORIOLIS FLOWMETERS

A mass flow meter operating by the Coriolis principle comprises a mechanical oscillating system having two straight measuring tubes which are clamped at both ends. Oscillating system is arranged axially in a support tube. In the centre of the measuring tubes an oscillation exciter is disposed which sets the two measuring tubes into oppositely phased flexural oscillations. Oscillation sensors sensing the mechanical oscillations at equal distances on both sides of the oscillation exciter generate electrical oscillation sensor signals which are characteristic of the frequency and phase position of the sensed oscillations. A correction circuit receives the two temperature sensor signals and imparts to the measuring signal on the basis of the temperatures measured a correction for eliminating the temperature influence on the measurement result.

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HART COMUNICATOR MODEL

INTRODUCTION:

The hart(high way addressable remote transmitter)communicator is a hand held interface that provides a common communicator link to all hart compatible microprocessor based instrument.

The hart communicator interfaces with any hart compatible device from any wiring termination point using a 4-20ma loop provided a minimum load resistance of 250 ohms is present the communicator and power supply. Our hart communicator uses the bell202 frequency digital signal voltage added to the loop averages to zero, communication to and from a hart 0- compatible device does not disturb the 4-20 ma signal.

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DISTRIBUTED CONTROL SYSTEM

INTRODUCTION: Modern automation problems in the industry which need to be solved by using a process control computer. Supervision & control of small plant of production plant, to the integral control &management of a large plant. This in consequence implies need of different approaches foe design of hardware &software of the automation system.

The DCS provides display, operation & recording functions, implementing a superior man-machine interface that concentrates the spreads of the process and centralizes information and operation. The DCS supports process operation by gathering data from the field station assigned to individual field areas and enabling the operator to display on the CRT screens by a simple key operation.

The process variables, set points, control parameters and status information, such as alarm statuses, with a variety of texts and graphics etc. provide a smooth man-machine interface. It is equipped with printing function record, alarm, logging and other printouts.

DCS is abbreviation for Distributed Control System

As is apparent from the abbreviation, the word „Distributed‟ supports following functionality's

Physical Distribution- Nodes or Subsystems can be Distributed i.e. located physically apart .

Functional Distribution - Specific Functionality is imparted for a Node basing on the combination of hardware and software used.

Structural Distribution-Different Structural hardware platforms are used to achieve the required functionality.

DCS Is Required For :

• For Total Plant Automation. • For Higher Productivity. • For Optimal Process Control. • For Advance Process Control. • For Regulatory Compliance. • For Management Information System. • In Tune with Global Requirement.

ADVANTAGES OF DCS:

The cost of upgrading system will also lower. Each processor carries out a clearly defined set of functions & system growth could be

achieved by incorporating additional processor.

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A DCS is inherently more reliable than a centralized processing system. There is substantial reduction in the cost of interfacing the process of the computer system. DCS are more flexible than a centralized processing system. DCS allow the duplicate storage, if necessary, of critical data. The management of computer operation can be simplified. The cost of DCS is lower than the cost of a centralized system.

SYSTEM ARCHITECTURE:

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Communication:

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PROGRAMMABLE LOGIC CONTROLLER

OPERATION: Figure shows how the control action is achieved.

Input devices are, push buttons, selector switches, proximity switches, limit switches, timer contacts etc., and output devices are, control relays, solenoid valves, pilot light, horns, timer etc.

APPLICATION:

Timing, counting, data calculation, sequence control,

Batch or continuous process control, Open loop and feedback control, Precise motion/position control, Process data acquisition and display, Emergency Shut-down.

PLCS FOR EMERGENCY SHUT-DOWN:- Modern chemical plants are designed to perform very complex and critical operational functions. Fast, automatic and reliable instrumentation and control system is one of the prime requirements of any modern process plant. The instrumentation system shall provide safe and accurate control of various parameters during the normal operation of the plant and also shall bring the plant to a safe position, in case of any abnormalities. Controlling and monitoring of the plant parameters during normal operation of the plant is taken care of by the DCS while the safe shutdown of the plant during emergencies is taken care of by the Emergency Shutdown System (ESD). Thus the function of an Emergency shutdown system (ESD) is to shutdown the plant automatically and safety in case of any abnormality and thereby protecting the equipments as well as the personnel. An emergency shutdown system consists of basically three functional blocks:

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1. Sensing Devices: Sensing device senses any abnormalities in the process conditions and gives signals to the Logic operation device. Such condition changes from '1' to '0' in case of abnormality. 2. Logic Operation Devices : Logic operation device carry out all the logic operations involved and give outputs to the Final actuation devices (the output change from '1' to '01).

3. Final Actuation Devices: The final actuation devices take necessary action to bring the plant to a safe shutdown.

Figure shows an example of Trip or Start logic and its implementation in PLC.

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REFERENCES:

1. INSTRUMENTATION SYMBOLS AND IDENTIFICATION, AMERICAN NATIONAL STANDARD (ANSI/ISA-S5-1-1964)

2. LESSONS IN INDUSTRIAL INSTRUMENTATION, BY-TONY R. KUPHALDT

Websites:1. www.google.co.in 2. www.welspun.com 3. www.energynext.in 4. www.welspunenergy.com 5. www.welspunindia.com

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