ELECTRICAL SAFETY

Electrical PPE:

Personal Protective Equipment (PPE)These are the equipments used by workers for the protection of their body from any type of hazard whether electrical, mechanical or chemical.Employees working in areas where there are potential electrical hazards must use electrical protective equipment that is appropriate for the electrical work to be performed.

A person working in a plant should have following important points to be considered on Electrical PPE:- Use, store and maintain your electrical PPE in a safe, reliable condition. - Wear nonconductive head protection wherever there is a danger of head injury from electric shock

or burns due to contact with exposed energized parts. - Wear protective equipment for the eyes or face wherever there is danger of injury to the eyes

or face from electric arcs or flashes or from flying objects resulting from electrical explosion.

PPEs Used to Protect Workers from Electric Hazards:

Area of body to be protected PPE used Torso, arms, legs Thermal work uniforms, flash suits Eyes Face shields, goggles, safety glasses Head Insulating hard helmets, Hands Rubber gloves with leather protectors, thermally resistance gloves.

PPE Inspection:Electrical PPE with any of the following defects may not be used:

• A hole, tear, puncture, or cut • Texture changes: Swelling, softening, hardening, or becoming sticky or inelastic • An embedded foreign object • Any other defect that damages the insulating properties

Don’t use Defective Electrical PPE!

Safe Work Practices/procedures:

Before Starting a Work, followings should be practiced:De-energize, lock, tag and test all circuits of 50 volts or moreDe-energize all power sources. Disconnect from all electric energy sources

Control circuit devices such as push buttons, selector switches, and interlocks may NOT be used as the sole means for de-energizing circuits or equipment.

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Locks and Tags:

A person should always Lock and Tag all sources: • Place Lock and Tag on each disconnecting means used to de-energize circuits • Attach lock so as to prevent operating the disconnecting means. • Place tag with each lock.

If a Lock cannot be applied:A tag used without a lock must be supplemented by at least one additional safety measure that provides a level of safety equal to that of a lock.

For examples:Removal of an isolating circuit element such as a fuseBlocking of a controlling switch Opening of an extra disconnecting device

Release Stored Energy:Stored electric energy must be released before starting work.

Short-circuit and ground all high capacitance elements Discharge all capacitors.

Is it “Dead”?

Verify System is De-energized:Operate the equipment controls to check that equipment cannot be restarted. Use test equipment to test the circuits and electrical parts for voltage and current.

Check Your Tester: Check test equipment (e.g. Amp-Volt-Ohm Meter) on a known live source of same rating to ensure it works before and after checking the circuit on which you will be working.

Re-energizing Equipment: While re-energizing an equipment, following procedure should be followed:

• Conduct tests and visual inspections to ensure all tools, electrical jumpers, shorts, grounds, and other such devices have been removed. • Warn others to stay clear of circuits and equipment. • Each lock and tag must be removed by the person who applied it. • Visually check that all employees are clear of the circuits and equipment.

Working With Energized Parts:

Persons working on energized equipment must be familiar with the proper use of special precautionary techniques, personal protective equipment, insulating and shielding materials, and insulated tools.

Working on Energized Circuits:• Isolate the area from all traffic. • Post signs and barricades. • Use an attendant if necessary. • Use insulated tools, mats and sheeting. • Use electrical rubber sheeting to cover nearby exposed circuits.

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Conductive Materials: Conductive materials and equipment must be handled in a manner to prevent them from contacting exposed energized conductors or circuit parts.

Conductive Apparel: Remove all conductive articles of jewelry and clothing, such a watch bands, bracelets, rings, key chains, necklaces, metalized aprons, cloth with conductive thread, or metal headgear.

Lockout, Tagout and Try/Test-out (LOTOTO):

Definition and Description

Tags are used to identify equipment that has been removed from service for maintenance or other purposes. They are uniquely designed and have clear warnings printed on them instructing personnel not to operate the equipment.

Locks are applied to de-energized equipment to prevent accidental or unauthorized operation. Locks and tags are normally applied together; however, some special circumstances may require the use of a tag without a lock and/or a lock without a tag.

After locking and Tagging, the worker should test by all means that the equipment is not running or a system is not getting energized any way.

When to Use Locks and Tags

Locks and tags should be applied to open circuit breakers, switches, or contactors whenever personnel will be exposed to the conductors which are normally fed by those devices. The application of the tags will warn and inform other employees that the equipment is not available for service, who applied the tag, and why the tag was applied. The lock will prevent the operation of the breaker, switch, or contactor so that the circuit cannot be accidently reenergized.

Minor inspections, adjustments, measurements, and other such servicing activities which are routine, repetitive, and integral to the use of the equipment do not require the placement of locks and tags unless one of the following conditions exists:

• Guard, insulation, or other safety devices are required to be removed or bypassed.• An employee is required to place his or her body into close proximity with an exposed,

energized electric conductor.

Locks without Tags or Tags without Locks:Tags may be used without locks under both of the following conditions:

• The interrupting device is not designed to accept a lock.• An extra means of isolation is employed to provide one additional level of protection. Such

an extra procedure might take the form of an additional open point such as removing a fuse or disconnecting a wire or the placement of safety grounds to provide an equipotential work area.

Locks may be used without tags under both of the following conditions:

• The de-energization is limited to only one circuit or piece of equipment.• The lockout lasts only as long as the employee who places the lock is on site and in the immediate area.

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Sequence:

The following steps should be followed when shutting down an energized electric system:

• Motors and other operational equipment should be shut down using normal or emergency procedures as required. During the shutdown process personnel safety must be the prime consideration.

• All isolating equipment (circuit breakers, switches, and/or contactors) should be opened.

• Isolating equipment that is capable of being racked out should be racked to the discon-nected position.

• Stored energy, such as closing/tripping springs, hydraulic pressure, pneumatic pressure, or other such mechanisms should be discharged and released.

• Discharge and ground capacitors.

Removal of Locks and Tags

Normal Removal. When the work is finished and an employee is ready to remove his or her locks and tags, the following general approaches should be used:

• The work area should be inspected to ensure that nonessential items have been removed and all components are operationally intact.

• The work area should be inspected to ensure that all employees have been safely removed or positioned.

• Remove any specialized equipment such as safety grounds or spring tension blocks.• Notify all affected employees that locks and tags are to be removed.• Locks and tags should be removed by the personnel that placed them.

Control Bypass. If the employee who placed the locks and tags is absent and not avail able to remove them, and if the locks and tags absolutely must be removed, another authorized employee should assume control of the equipment and remove the absent employee's locks and tags.

Construction of Safety Locks and Tags:

Safety TagsSafety tags are applied to equipment to indicate that the equipment is not available for service. They should be of standardized construction and include a warning that says Do Not Start, Do Not Open, Do Not Close, Do Not Operate, or other such warning. The tag must also indicate who placed it on the equipment and the nature of the problem with the equipment.

Tags are to be applied using strong fasteners. Nylon cable wraps are suitable for such an application.

Fig. Tags

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Locks and Multiple-Lock Devices

Locks are used to prevent operation of equipment that has been de-energized. They must be strong enough to withstand all but the most forceful attempts to remove them without the proper key.

Multiple-Lock Devices. Sometimes several workers will need to place a lock on one piece of equipment. This often happens when several crafts are working in the area secured by the lock. In these circumstances, a multiple-lock device is used.

Fig. Multiple Lock Devices

Electrical Hazards in Cement Industry: Electrical hazards in Cement Industry could be due to Faulty Cable, Unearthed Equipments, Welding arc, Blasts in Switchgears etc. These electrical hazards are in the form of shock, burns, fall of electrically blasted materials on human body. The details of the type of shocks are given below:

Shock:

Electric shock is the physical stimulation that occurs when electric current flows through the human body. The symptoms may include a mild tingling sensation, violent muscle contractions, and heart or tissue damage.The severity of electric shock depends upon several factors such as Physical condition and physical response, current duration, frequency of current, voltage magnitude and current magnitude. This shock could be due to faulty cables, unearthed equipments, contacts with live wires etc.

SHOCK PREVENTION METHODS:

Shock can be prevented by:• Isolating the area where electrical equiment is installed• Ensuring double earthing on electrical equipment• Standing on rubber mat while working on electrical m/c• Wearing rubber sole shoes & rubber handgloves • Using jointless double insulation cables • Laying of cable beyond reach of individual • Using reduced voltage equipment (in confined work place)• Ensuring dry surface where electrical equipment is installed & on which work is to

be carried out• Shifting of welding accessories to shed when not in use• Switching off power to equipment when not in use

Arc/Fire/Blast:

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A discharge of electricity through a gas in the form of heat and light energy as a result of an electrical breakdown is called electric Arc.

Arc is initiated in several ways:when the voltage between two points exceeds the dielectric strength of the air.when the air becomes superheated with the passage of current through some conductor.when two contacts part while carrying a very high current. (e.g. in Ckt breakers.)

Causes of Electric fire:

Selection of improper/substandard equipment and materials.Overloading of equipment.Maintenance negligence.Failure of insulation level.Damage due to rodents, termites and pests.Lightning.Water seepage.

Blast:When an electric arc occurs, it superheats the air instantaneously. This causes a rapid expansion of the air with a wave front that can reach pressures of 100 to 200 lb/ft 2 (4.79 to 9.58 kPa). Such pressures are sufficient to explode switchgear, turn sheet metal into shrapnel, turn hardware into bullets, pushover concrete walls, and propel molten metal at extremely high velocities. Such blasts may cause serious injuries to plant personnels. One should wear protective equipment for the eyes or face to remain uninjured.

Burn:

It is a most common shock-related injury. It Occurs when you touch electrical wiring or equipment that is improperly used or maintained. Typically occurs on hands Very serious injury that needs immediate attentionVery serious injuries can result from electrical burnsMay involve internal bleeding, destruction of tissues and nerves

CURE (In case of Electrical accidents)

If the casualty is not breathing normally or heart beat has stopped, give artificial respiration and external cardiac massage.- Treat for burns if any.- Seek the help of a medical practitioner & if required shift to hospital.

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Generation, Transmission and Distribution of Electric Power

Introduction

In this lesson a brief idea of a modern power system is outlined. Emphasis is given to create a clear mental picture of a power system to a beginner of the course Electrical Technology. As consumers, we use electricity for various purposes such as:

1. Lighting, heating, cooling and other domestic electrical appliances used in home.

2. Street lighting, flood lighting of sporting arena, office building lighting, powering PCs etc.

3. Irrigating vast agricultural lands using pumps and operating cold storages for various agricultural products.

4. Running motors, furnaces of various kinds, in industries.

5. Running locomotives (electric trains) of railways.

The list above is obviously not exhaustive and could be expanded and categorized in detail further. The point is, without electricity, modern day life will simply come to a stop. In fact, the advancement of a country is measured by the index per capita consumption of electricity – more it is more advanced the country is.

Basic idea of generation

Prior to the discovery of Faraday’s Laws of electromagnetic discussion, electrical power was available from batteries with limited voltage and current levels. Although complicated in construction, D.C generators were developed first to generate power in bulk. However, due to limitation of the D.C machine to generate voltage beyond few hundred volts, it was not economical to transmit large amount of power over a long distance. For a given amount of power, the current magnitude (I = P/V), hence section of the copper conductor will be large. Thus generation, transmission and distribution of d.c. power were restricted to area of few kilometer radius with no interconnections between generating plants. Therefore, area specific generating stations along with its distribution networks had to be used.

Changeover from D.C to A.C

In later half of eighties, in nineteenth century, it was proposed to have a power system with 3-phase, 50 Hz A.C generation, transmission and distribution networks. Once a.c system was adopted, transmission of large power (MW) at higher transmission voltage become a reality by using transformers. Level of voltage could be changed virtually to any other desired level with transformers – which was impossible with D.C system. Nicola Tesla suggested that constructionally simpler electrical motors (induction motors, without the complexity of commutator segments of D.C motors) operating from 3-phase a.c supply could be manufactured. In fact, his arguments in favor of A.C supply system own the debate on switching over from D.C to A.C system.

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A.C generator

A.C power can be generated as a single phase or as a balanced poly-phase system. However, it was found that 3-phase power generation at 50 Hz will be economical and most suitable. Present day three phase generators, used to generate 3-phase power are called alternators (synchronous generators). An alternator has a balanced three phase winding on the stator and called the armature. The three coils are so placed in space that there axes are mutually 120° apart as shown in figure 2.1. From the terminals of the armature, 3-phase power is obtained. Rotor houses a field coil and excited by D.C. The field coil produces flux and electromagnetic poles on the rotor surface. If the rotor is driven by an external agency, the flux linkages with three stator coils becomes sinusoidal function of time and sinusoidal voltage is induced in them. However, the induced voltages in the three coils (or phases) will differ in phase by 120° because the present value of flux linkage with R-phase coil will take place after 120° with Y-phase coil and further 120° after, with B-phase coil. A salient pole alternator has projected poles as shown in figure 2.1(a). It has non uniform air gap and is generally used where speed is low. On the other hand a non salient pole alternator has uniform air gap (figure 2.1(b)) and used when speed is high.

Frequency, voltage & interconnected system

The frequency of the generated emf for a p polar generator is given by f =pn/2where n is speed of the generator in rps or f = pn/120 when n is in rpm. Frequency of the generated voltage is standardized to 50 HZ in our country and several European countries. In USA and Canada it is 60 Hz. The following table gives the rpm at which the generators with different number of poles are to be driven in order to generate 50 Hz voltage.

A modern power station has more than one generator and these generators are connected in parallel. Also there exist a large number of power stations spread over a region or a country. A regional power grid is created by interconnecting these stations through

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transmission lines. In other words, all the generators of different power stations, in a grid are in effect connected in parallel. One of the advantages of interconnection is obvious; suppose due to technical problem the generation of a plant becomes nil or less then, a portion of the demand of power in that area still can be made from the other power stations connected to the grid. One can thus avoid complete shut down of power in an area in case of technical problem in a particular station. It can be shown that in an interconnected system, with more number of generators connected in parallel, the system voltage and frequency tend to fixed values irrespective of degree of loading present in the system. This is another welcome advantage of inter connected system. Inter connected system however, is to be controlled and monitored carefully as they may give rise to instability leading to collapse of the system.

All electrical appliances (fans, refrigerator, TV etc.) to be connected to A.C supply are therefore designed for a supply frequency of 50 Hz. Frequency is one of the parameters which decides the quality of the supply. It is the responsibility of electric supply company to see that frequency is maintained close to 50 Hz at the consumer premises. It was pointed out earlier that a maximum of few hundreds of volts (say about 600 to 700 V) could be developed in a D.C generator, the limitation is imposed primarily due to presence of commutator segments. In absence of commutators, present day generated voltage in alternator is much higher, typically around 10 kV to 15 kV. It can be shown that rms voltage induced in a coil is proportional to φ and n i.e., Ecoil φn where φ is the flux

per pole and n is speed of the alternator. This can be justified by intuition as well: we know that mere rotating a coil in absence of magnetic flux (φ) is not going to induce any voltage. Also presence of flux without any rotation will fail to induce any voltage as you require rate of change of flux linkage in a coil. To control the induced voltage one has to control the d.c field current as speed of the alternator gets fixed by frequency constrain.

Thermal, Hydel & Nuclear Power Stations

In this section we briefly outline the basics of the three most widely found generating stations – thermal, hydel and nuclear plants in our country and elsewhere.

Thermal plant We have seen in the previous section that to generate voltage at 50 Hz we have to run the generator at some fixed rpm by some external agency. A turbine is used to rotate the generator. Turbine may be of two types, namely steam turbine and water turbine. In a thermal power station coal is burnt to produce steam which in turn, drives the steam turbine hence the generator (turbo set). In figure 2.2 the elementary features of a thermal power plant is shown. In a thermal power plant coil is burnt to produce high temperature and high pressure steam in a boiler. The steam is passed through a steam turbine to produce rotational motion. The generator, mechanically coupled to the turbine, thus rotates producing electricity. Chemical energy stored in coal after a couple of transformations produces electrical energy at the generator terminals as depicted in the figure. Thus proximity of a generating station nearer to a coal reserve and water sources will be most economical as the cost of transporting coal gets reduced. In our country coal is available in abundance and naturally thermal power plants are most popular. However, these plants pollute the atmosphere because of burning of coals.

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Stringent conditions (such as use of more chimney heights along with the compulsory use of electrostatic precipitator) are put by regulatory authorities to see that the effects of pollution is minimized. A large amount of ash is produced every day in a thermal plant and effective handling of the ash adds to the running cost of the plant. Nonetheless 57% of the generation in out country is from thermal plants. The speed of alternator used in thermal plants is 3000 rpm which means 2-pole alternators are used in such plants.

Hydel plants

In a hydel power station, water head is used to drive water turbine coupled to the generator. Water head may be available in hilly region naturally in the form of water reservoir (lakes etc.) at the hill tops. The potential energy of water can be used to drive the turbo generator set installed at the base of the hills through piping called pen stock. Water head may also be created artificially by constructing dams on a suitable river. In contrast to a thermal plant, hydel power plants are eco-friendly, neat and clean as no fuel is to be burnt to produce electricity. While running cost of such plants are low, the initial installation cost is rather high compared to a thermal plants due to massive civil construction necessary. Also sites to be selected for such plants depend upon natural availability of water reservoirs at hill tops or availability of suitable rivers for constructing dams. Water turbines generally operate at low rpm, so number of poles of the alternator are high. For example a 20-pole alternator the rpm of the turbine is only 300 rpm.

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Nuclear plants

As coal reserve is not unlimited, there is natural threat to thermal power plants based on coal. It is estimated that within next 30 to 40 years, coal reserve will exhaust if it is consumed at the present rate. Nuclear power plants are thought to be the solution for bulk power generation. At present the installed capacity of unclear power plant is about 4300 MW and expected to expand further in our country. The present day atomic power plants

work on the principle of nuclear fission of 235U. In the natural uranium, 235U constitutes

only 0.72% and remaining parts is constituted by 99.27% of 238U and only about 0.05% of 234U. The concentration of 235U may be increased to 90% by gas diffusion process to

obtain enriched 235U. When 235U is bombarded by neutrons a lot of heat energy along

with additional neutrons are produced. These new neutrons further bombard 235U producing more heat and more neutrons. Thus a chain reaction sets up. However this reaction is allowed to take place in a controlled manner inside a closed chamber called nuclear reactor. To ensure sustainable chain reaction, moderator and control rods are used.

Moderators such as heavy water (deuterium) or very pure carbon 12C are used to reduce the speed of neutrons. To control the number neutrons, control rods made of cadmium or boron steel are inserted inside the reactor. The control rods can absorb neutrons. If we want to decrease the number neutrons, the control rods are lowered down further and vice versa. The heat generated inside the reactor is taken out of the chamber with the help of a coolant such as liquid sodium or some gaseous fluids. The coolant gives up the heat to water in heat exchanger to convert it to steam as shown in figure 2.4. The steam then drives the turbo set and the exhaust steam from the turbine is cooled and fed back to the heat exchanger with the help of water feed pump. Calculation shows that to produce 1000 MW

of electrical power in coal based thermal plant, about 6 × 106 Kg of coal is to be burnt

daily while for the same amount of power, only about 2.5 Kg of 235U is to be used per day in a nuclear power stations.

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The initial investment required to install a nuclear power station is quite high but running cost is low. Although, nuclear plants produce electricity without causing air pollution, it remains a dormant source of radiation hazards due to leakage in the reactor. Also the used fuel rods are to be carefully handled and disposed off as they still remain radioactive.

The reserve of 235U is also limited and can not last longer if its consumption continues at the present rate. Naturally search for alternative fissionable material continues. For

example, plutonium (239Pu) and (233U) are fissionable. Although they are not directly

available. Absorbing neutrons, 238U gets converted to fissionable plutonium 239Pu in the atomic reactor described above. The used fuel rods can be further processed to extract 239Pu from it indirectly increasing the availability of fissionable fuel. Effort is also on to

convert thorium into fissionable 233U. Incidentally, India has very large reserve of thorium in the world. Total approximate generation capacity and Contribution by thermal, hydel and nuclear generation in our country are given below.

Transmission of power The huge amount of power generated in a power station (hundreds of MW) is to be transported over a long distance (hundreds of kilometers) to load centers to cater power to consumers with the help of transmission line and transmission towers as shown in figure 2.5.

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To give an idea, let us consider a generating station producing 120 MW power and we want to transmit it over a large distance. Let the voltage generated (line to line) at the alternator be 10 kV. Then to transmit 120 MW of power at 10 kV, current in the transmission line can be easily calculated by using power formula circuit (which you will learn in the lesson on A.C circuit analysis) for 3-phases follows:

Instead of choosing 10 kV transmission voltage, if transmission voltage were chosen to be 400 kV, current value in the line would have been only 261.5 A. So sectional area of the transmission line (copper conductor) will now be much smaller compared to 10 kV transmission voltage. In other words the cost of conductor will be greatly reduced if power is transmitted at higher and higher transmission voltage. The use of higher voltage (hence lower current in the line) reduces voltage drop in the line resistance and reactance. Also transmission losses is reduced. Standard transmission voltages used are 132 kV or 220 kV or 400 kV or 765 kV depending upon how long the transmission lines are. Therefore, after the generator we must have a step up transformer to change the generated voltage (say 10 kV) to desired transmission voltage (say 400 kV) before transmitting it over a long distance with the help of transmission lines supported at regular intervals by transmission towers. It should be noted that while magnitude of current decides the cost of

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copper, level of voltage decides the cost of insulators. The idea is, in a spree to reduce the cost of copper one can not indefinitely increase the level of transmission voltage as cost of insulators will offset the reduction copper cost. At the load centers voltage level should be brought down at suitable values for supplying different types of consumers. Consumers may be (1) big industries, such as steel plants, (2) medium and small industries and (3) offices and domestic consumers. Electricity is purchased by different consumers at different voltage level. For example big industries may purchase power at 132 kV, medium and big industries purchase power at 33 kV or 11 kV and domestic consumers at rather low voltage of 230V, single phase. Thus we see that 400 kV transmission voltage is to be brought down to different voltage levels before finally delivering power to different consumers. To do this we require obviously step down transformers.

Substations Substations are the places where the level of voltage undergoes change with the help of transformers. Apart from transformers a substation will house switches (called circuit breakers), meters, relays for protection and other control equipment. Broadly speaking, a big substation will receive power through incoming lines at some voltage (say 400 kV) changes level of voltage (say to 132 kV) using a transformer and then directs it out wards through outgoing lines. Pictorially such a typical power system is shown in figure 2.6 in a short of block diagram. At the lowest voltage level of 400 V, generally 3-phase, 4-wire system is adopted for domestic connections. The fourth wire is called the neutral wire (N) which is taken out from the common point of the star connected secondary of the 6 kV/400 V distribution transformer.

Some important components/equipments in substation As told earlier, the function of a substation is to receive power at some voltage through incoming lines and transmit it at some other voltage through outgoing lines. So the most important equipment in a substation is transformer(s). However, for flexibility of operation and protection transformer and lines additional equipments are necessary.

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Suppose the transformer goes out of order and maintenance work is to be carried out. Naturally the transformer must be isolated from the incoming as well as from the outgoing lines by using special type of heavy duty (high voltage, high current) switches called circuit breakers. Thus a circuit breaker may be closed or opened manually (functionally somewhat similar to switching on or off a fan or a light whenever desired with the help of a ordinary switch in your house) in substation whenever desired. However unlike a ordinary switch, a circuit breaker must also operate (i.e., become opened) automatically whenever a fault occurs or overloading takes place in a feeder or line. To achieve this, we must have a current sensing device called CT (current transformer) in each line. A CT simply steps down the large current to a proportional small secondary current. Primary of the CT is connected in series with the line. A 1000 A/5 A CT will step down the current by a factor of 200. So if primary current happens to be 800 A, secondary current of the CT will be 4 A. Suppose the rated current of the line is 1000 A, and due to any reason if current in the line exceeds this limit we want to operate the circuit breaker automatically for disconnection. In figure 2.7 the basic scheme is presented to achieve this. The secondary current of the CT is fed to the relay coil of an overcurrent relay. Here we are not going into constructional and operational details of a over current relay but try to tell how it functions. Depending upon the strength of the current in the coil, an ultimately an electromagnetic torque acts on an aluminum disc restrained by a spring. Spring tension is so adjusted that for normal current, the disc does not move. However, if current exceeds the normal value, torque produced will overcome the spring tension to rotate the disc about a vertical spindle to which a long arm is attached. To the arm a copper strip is attached as shown figure 2.8. Thus the arm too will move whenever the disk moves.

The relay has a pair of normally opened (NO) contacts 1 & 2. Thus, there will exist open circuit between 1 & 2 with normal current in the power line. However, during fault condition in the line or overloading, the arm moves in the anticlockwise direction till it closes the terminals 1 & 2 with the help of the copper strip attached to the arm as explained pictorially in the figure 2.8. This short circuit between 1 & 2 completes a circuit comprising of a battery and the trip coil of the circuit breaker. The opening and closing of the main contacts of the circuit breaker depends on whether its trip coil is energized or not. It is interesting to note that trip circuit supply is to be made independent of the A.C supply

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derived from the power system we want to protect. For this reason, we expect batteries along with battery charger to be present in a substation. Apart from above there will be other types of protective relays and various meters indicating current, voltage, power etc. To measure and indicate the high voltage (say 6 kV) of the line, the voltage is stepped down to a safe value (say 110V) by transformer called potential transformer (PT). Across the secondary of the PT, MI type indicating voltmeter is connected. For example a voltage rating of a PT could be 6000 V/110 V. Similarly, Across the secondary we can connect a low range ammeter to indicate the line current.

Single line representation of power system

Trying to represent a practical power system where a lot of interconnections between several generating stations involving a large number of transformers using three lines corresponding to R, Y and B phase will become unnecessary clumsy and complicated. To avoid this, a single line along with some symbolical representations for generator, transformers substation buses are used to represent a power system rather neatly. For example, the system shown in 2.6 with three lines will be simplified to figure 2.9 using single line.

As another example, an interconnected power system is represented in the self explained figure 2.10 – it is hoped that you understand the important features communicated about the system through this figure.

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Distribution system Till now we have learnt how power at somewhat high voltage (say 33 kV) is received in a substation situated near load center (a big city). The loads of a big city are primarily residential complexes, offices, schools, hotels, street lighting etc. These types of consumers are called LT (low tension) consumers. Apart from this there may be medium and small scale industries located in the outskirts of the city. LT consumers are to be supplied with single phase, 220 V, 50 Hz. We shall discuss here how this is achieved in the substation receiving power at 33 kV. The scheme is shown in figure 2.11.

Power receive at a 33 kV substation is first stepped down to 6 kV and with the help of under ground cables (called feeder lines), power flow is directed to different directions of the city. At the last level, step down transformers are used to step down the voltage form 6 kV to 400 V. These transformers are called distribution transformers with 400 V, star connected secondary. You must have noticed such transformers mounted on poles in cities beside the roads. These are called pole mounted substations. From the secondary of these transformers 4 terminals (R, Y, B and N) come out. N is called the neutral and taken out from the common point of star connected secondary. Voltage between any two phases (i.e.,

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R-Y, Y-B and B-R) is 400 V and between any phase and neutral is 230 V (=400/√3). Residential buildings are supplied with single phase 230V, 50Hz. So individual are to be supplied with any one of the phases and neutral. Supply authority tries to see that the loads remain evenly balanced among the phases as far as possible. Which means roughly one third of the consumers will be supplied from R-N, next one third from Y-N and the remaining one third from B-N. The distribution of power from the pole mounted substation can be done either by (1) overhead lines (bare conductors) or by (2) underground cables. Use of overhead lines although cheap, is often accident prone and also theft of power by hooking from the lines take place. Although costly, in big cities and thickly populated areas underground cables for distribution of power, are used.

Some basic and important points, in relation to a modern power system, are summarized below:

1. Generation, transmission and distribution of electric power in our country is carried out as 3-phase system at 50 Hz.

2. Three most important conventional methods of power generation in out country are: coal based thermal plants, Hydel plants and nuclear plants.

3. Load centers (where the power will be actually consumed) are in general situated far away from the generating station. So to transmit the large amount of power (hundreds of MW) efficiently and economically over long distance, high transmission voltage (such as 400 kV, 220 kV) is used.

4. Material used for transmission lines is bare is bare copper conductors which are supported at regular intervals by steel towers. Stack of disk type ceramic insulators are used between the HV line and the steel tower.

5. Level of current decides the section of the line conductor and the level of voltage decides the amount of insulation required.

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POWER FACTOR AND ITS IMPORTANCE

INTODUCTION:

Power capacitors have been employed in many and varied ways in industry over the past 40 years, but their increasing use is often limited by the apparent lack of practical information on their application. Here information and guidance on the selection, and use, of capacitors for power factor correction is provided.

With increasing electricity charges, and the need to save energy, it is of para mount importance to both industrial and commercial users of electricity to ensure that their plant operates at maximum efficiency. This implies that the plant power factor must be at an economic level.

IMPORTANCE OF POWER FACTOR

Most a.c. electrical machines draw from the supply apparent power in terms of kilovolt amperes (kVA) which is in excess of the useful power, measured in kilowatts (kW), required by the machine. The ratio of these quantities is known as the power factor of the load, and is dependent upon the type of machine in use. Assuming a constant supply voltage this implies that more current is drawn from the electricity authority than is actually required.

Power factor = true power (KW)/ apparent power (kVA)A large proportion of the electrical machinery used in industry has an inherently low power

factor, which means that the supply authorities have to generate much more current than is theoretically required. This excess current flows through generators, cables, and transformers in the same manner as the useful current. It is understood that there are resistive loads such as lighting and heating but these are generally outweighed by the motive power requirements.

If steps are not taken to improve the power factor of the load all the equipment from the power station to the factory sub-circuit wiring, has to be larger than necessary. This results in increased capital expenditure and higher transmission and distribution losses throughout the whole supply network.To overcome this problem, and at the same time ensure that generators and cables are not overloaded with wattless current (as this excess current is termed), the supply authorities often offer reduced terms to consumers whose power factor is high, or impose penalties for those with low power factor.

THEORY OF POWER FACTOR CORRECTION:

The kVA in an a.c. circuit can be resolved into two components, the in-phase component which supplies the useful power (kW), and the wattless component (kvar) which does no useful work. The phasor sum of the two is the kVA drawn from the supply. The cosine of the phase angle Φ1 between the kVA and the kW components represents the power factor of the load.The phasor diagram for this is shown in Fig.22.1 The load current is in phase with the kVA so that it lags the supply voltage by the same phase angle.

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To improve the power factor, equipment drawing kvar of approximately the same magnitude as the load kvar, but in phase opposition (leading) is connected in parallel with the load. The resultant kVA is now smaller and the new power factor, cosΦ2 is increased. Thus any value of cosΦ2 can be obtained by controlling the magnitude of the leading kvar added. This is shown in Fig. 22.2.

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METHODS FOR POWER FACTOR IMPROVEMENT:

In practice two types of equipments are available to produce leading kvar:

• Rotary equipment. Phase advancers, synchronous motors and synchronous condensers. Where auto-synchronous motors are employed the power factor correction may be a secondary function.

• Static equipment. Capacitors.

When installing equipment, the following points are normally considered;• Reliability of the equipment to be installed; • Probable life of such equipment; • Capital cost; • Maintenance cost; • Running costs and space required; and• Ease of installation.

Generally, the capital cost of rotating machinery, both synchronous and phase advancing, makes its use uneconomical, except where one is using rotating plant for a dual function-drive and power factor correction. In addition the wear and tear inherent in all rotary machines involves additional expense for upkeep and maintenance.

Capacitors have none of these disadvantages. Compared with other forms of correction, the initial cost is very low, upkeep costs are minimal and they can be used with the same high efficiency on all sizes of installation. They are compact, reliable, highly efficient, convenient to install and lend themselves to individual, group or automatic methods of correction. These facts, indicate that generally speaking, power factor correction by means of capacitors is the most satisfactory and economical method.

Voltage Classification

The classification of various voltage levels has been done in the following way:

Low Voltage (LV) : < 600VMedium Voltage (MV) : 601 - 69kVHigh Voltage (HV) : 69kV - 230kVExtra High Voltage (EHV) : 230kV - 800kVUltra High Voltage (UHV) : >800kV.

In cement plant generally, the voltage below 1000V is LV (generally 415V) and above 1000V is called HV voltage (generally 3.3KV, 6.6KV and 11KV).

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Measurements in Electrical Circuits

(1) Digital Clamp meter (or Tong Tester):

An electrical meter with integral current clamp is known as a clamp meter, clamp-on ammeter or tong tester.In order to use a clamp meter, only one conductor is passed through the probe; if more than one conductor were to be passed through then the measurement would be a vector sum of the currents flowing in the conductors and could be very misleading depending on the phase relationship of the currents. In particular if the clamp is closed around a 2-conductor cable carrying power to equipment the same current flows down one conductor and up the other, with a net current of zero. So, this point should be kept in mind when we use Clampmeter.

(2) Digital Multimeter, Voltmeter and Ammeter:

The ammeter symbol (A) and Voltmeter symbol (V) is shown in the circuit diagram below.The voltmeter is always connected in parallel to the element terminals across which the voltage has to be measured.The ammeter is always connected in series to the branch through which the current has to be measured.

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Digital Multimeter (DMM) Diagram

Function/Range Switch: selects the function (voltmeter, ammeter, or ohmmeter) and the range for the measurement.

COM Input Terminal: Common ground, used in ALL measurements.

V Input Terminal: For voltage or resistance measurements.

200 mA Input Terminal: For small current measurements.

10 A Input Terminal: For large current measurements.

Low Battery LCD: Appears when the battery needs replacement.

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Zero-Center Analog Ammeter Analog Voltmeter

(3)Megger:The Megger is used to measure insulation resistance in various electrical devices.

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The Megger consists of a DC generator and a special moving coil instrument contained within a single case. The generator has a permanent magnetic field system, which also serves the moving coil assembly. The generator which is usually of 250 V or 500 V is driven through gearing by the turning of the instrument handle or, in some models, by means of an electric motor. The generator is rotated by means of handle with constant speed of more than 100 rpm. In the megger, the connections are such that the instrument readings are independent of the voltage of the generator. The moving coil element consists essentially of two coils, one is pressure coil and the other is current coil. The current in the pressure coil depends only upon the generator voltage, and it decides the restoring torque on pointer. The current coil carries the current which depends upon the value of resistor under test connected between Line and the Earth terminals. This current coil current decides the position of the pointer on the resistance scale. The reading of the instrument should not be less than 1MΩ. It shows that the insulation has failed.Use of Guard terminal: In some circuimstances, false insulation resistance reading may be obtained due to surface leakage currents. The Megger is fitted with a Guard terminal to avoid such errors. The Guard terminal wound tightly around the insulation is directly connected to the negative terminal of the generator and any leakage current flowing in this circuit does not pass through the current coil. Thus any surface leakage currents return through the Guard lead and hence the reading obtained is true resistance of the dielectric.

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FIELD INSTRUMENTATION

Introduction to Industrial Instrumentation

Instrumentation is the science of measurement and control.The first step is measurement of a quantity e.g. temperature, pressure, flow etc. and the second step is to control the value of it in order to get the desired one.Without measurement, it can’t be controlled and without control there is no meaning of measurement.Measurement usually takes the following form in an Industry:

• Fluid pressure• Fluid flow rate• The temperature of a solid object, liquid or gas• Liquid/Solid Level • Machine position or motion• Electrical voltage, current, or resistance

Once the measurement is done, a signal is transmitted to a Computing Device which further sends a signal to Final Controlling Element which then influences the quantity being measured.

This final control device usually takes one of the following forms:

• Control valve (for throttling the flow rate of a fluid) • Electric motor • Electric heater

Both the measurement device and the final control device connect to some physical system which we call the process. To show this as a general block diagram:

Block Diagram of a Feedback Control System:

• Controlled Variable- output quantity of system (Level, Temperature, etc.)• Manipulated Variable- means of maintaining controlled variable at the set point• Error signal- equals the difference between the set point and the measurement.(e = SP − M)• Setpoint - desired process level. (SP)• Measurement- actual process level. (M)• Closed Loop- automatic control• Open Loop-manual control• Feedback control is error correction following a disturbance• Feed forward control is control of disturbances, which could cause a process error

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An Example: Boiler Water Level Control System:

Here the water level in boiler steam drum is being measured and transmitted by Level Transmitter in the form of pneumatic signal to the Level Indicating Device. The LIC compares this signal with a set value and generates an error which then in the form of a control signal is sent to the Air operated valve. This valve changes its position to vary the flow rate of water and maintain the water level at desired/set value.

The sensing of parameter values is done by various Sensors and the transmission of these values to a Controller is done through Transmitters which either send electrical signals as 4-20mA DC current or pneumatic signals as 3-15 psi air signal.

Various Sensors Involved are:Temperature Sensors - Thermocouple, RTD, Thermister, Pyrometer etc.,

Pressure Sensors - Diaphragm, Bellows, Bourdons Tube etc., Flowmeters – Venturi meter, Orifice Plate, Rotameter etc.,

Proximity Sensors – Inductive, Capacitive & Optical,Weighing Sensors - Load CellsPosition Sensors – Limit Switches etc.Level Sensors - Capacitive type, Load Cells, Radiation type etc.

There are various Transmitters for various parameters like Temperature Transmitter, Pressure Transmitter, Level Transmitters etc. All these Transmitters generate an electrical or a pneumatic signal in the range of 4-20mA or 3-15 psi (or 20-100kPa) respectively.

From the Controller side, majorly PLCs (Programmable Logic Controllers) are used for generation of control signals to Final Control Elements (like electric Motor, Control Valve etc.). Continuous control of parameters like temperature, flow etc. is done with PID controllers.

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Various Field Instruments: Digital Instruments Analog Instruments

1. Proximity Switch 1. Temperature Sensor

2. Limit Switch 2. Flow Sensor

3. Photo Sensor 3. Pressure Sensor

4. Pressure Switch 4. Level Sensor

5. Temperature Switch 5. Different types of Transducers

6. Level Switch

Measurement of Various System Parameters:Temperature MeasurementPressure MeasurementFlow MeasurementLevel MeasurementSpeed MeasurementVibration Measurement

Temperature Measurement

Various Devices used for the measurement of temperature are:

Resistance Temperature Detector (RTD)/ ThermistersThermocouplesPyrometersTemperature Scanner( Infrared)

Resistance Temperature Detector (RTD)Construction:

• The RTD incorporates pure metals or certain alloys that increase in resistance as temperature increases and, conversely.

• RTD elements are normally constructed of platinum, copper, or nickel.• These metals are best suited for RTD applications because of their linear resistance-temperature

characteristics.• RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a

sheath of metal.• The insulator prevents a short circuit between the wire and the metal sheath.• Inconel, a nickel-iron-chromium alloy, is normally used in manufacturing the • RTD sheath because of its inherent corrosion resistance.

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RTD works on the principle of change in resistance due to change in temperature. This change in resistance is then measured by a precision resistance measuring device normally a bridge circuit.The well protects the RTD from damage by the gas or liquid being measured. Protecting wells are normally made of stainless steel, carbon steel, Inconel, or cast iron.

METHODS OF CONNECTION:Three circuit types are commonly used: 2-wire, 3-wire and 4-wire circuits.To detect the small variations of resistance of the RTD, a temperature transmitter in the form of a Wheatstone bridge is generally used.

AREAS OF APPLICATION:Resistance thermometers can be used over a temperature range of -220 °C to +1100 °C.

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Thermocouples:

Construction: The leads of the thermocouple are encased in a rigid metal sheath.The measuring junction is normally

formed at the bottom of the thermocouple housing.Magnesium oxide surrounds the thermocouple wires to prevent vibration that could damage the fine wires and to enhance heat transfer between the measuring junction and the medium surrounding the thermocouple.

DESCRIPTION AND MEASUREMENT PRINCIPLE:A thermocouple consists of two electrical conductors of different materials connected to one another at one end (measuring junction).The two free ends build a reference junction.

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Extension cables are manufactured of the same material as the corresponding Thermocouple.Depending upon the difference in temperature of measuring junction and reference junction, an emf is generated at the reference junction. To convert the emf generated by a thermocouple to the standard 4-20 mA signal, a transmitter is needed. This kind of transmitter is called a temperature transmitter.

Thermal Wells:To facilitate removal of the temperature sensors (RTD and TC), for examination or replacement and to provide mechanical protection, the sensors are usually mounted inside thermal wells.

A thermal well is basically a hollow metal tube with one end sealed. It is usually mounted permanently in the pipe work. The sensor is inserted into it and makes contact with the sealed end.

Drawback of Thermal Well: Response time gets reduced

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Pyrometers

Virtually any mass above absolute zero temperature emits electromagnetic radiation (photons, or light) as a function of that temperature.This basic fact makes possible the measurement of temperature by analyzing the light emitted by an object.The Stefan-Boltzmann Law of radiated energy quantifies this fact, declaring that the rate of heat lost by radiant emission from a hot object is proportional to the fourth power of the absolute temperature:

Advantage of non-contact thermometry (or pyrometry as high-temperature measurement is often referred) is rather obvious:With no need to place a sensor in direct contact with the process, a wide variety of temperature measurements may be made that are either impractical or impossible to make using any other technology.A time-honored design for non-contact pyrometers is to concentrate incident light from a heated object onto a small temperature-sensing element.

Thermocouples were the first type of sensor used in non-contact pyrometers.Instrument manufacturers employ a series-connected array of thermocouples called a thermopile to generate a stronger electrical signal.

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In non-contact pyrometer, the thermopile is oriented so that all the concentrated light falls on the hot junctions, while the cold junctions face away from the focal point to a region of ambient temperature.

A table of values showing the approximate relationship between target temperature and millivolt output for one model of Radiamatic (Manufactured by Honeywell Company) sensing unit.

Their calibration does not depend on the distance separating the sensor from the target object

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Temperature Scanner:

Radiation thermometers need no contact with the measured object, which means no contamination, no interference with processes. A radiation thermometer detects infrared energy emitted from a target surface and converts it into an electrical signal for transmission to a signal processor. A signal processor takes the electrical signal and produces an output suitable for use with any indicating, recording or control equipment. The early detection of ‘hot’ or ‘cold’ spots is vital to avoid costly maintenance or an unplanned shutdown. Continuous monitoring of the kiln shell along its length, day and night will provide the earliest possible indication of potential problem spots.

Kiln Scanning System:

Various Kiln System Temperatures Scanning:

1. Rotary Kiln Shell temperature Scanning 2. Rotory Kiln Product Temperature3. Kin Burning Zone Temperature 4.Cement Clinker System Temperature

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Pressure Measurement

Pressure is generally measured as Gauge Pressure, Absolute Pressure and Vacuum Pressure.

Absolute Pressure:The absolute pressure (pa) is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum.

Gauge Pressure:A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called the gauge pressure and can be expressed as

pg = ps - pa         Where pg = gauge pressure, ps = system pressurepa = atmospheric pressure

Vacuum Pressure:Any pressure less than atmospheric pressure is called vacuum pressure. If it is absolute zero pressure then the point pressure is absolute vacuum.

Atmospheric PressureAtmospheric pressure is pressure in the surrounding air at - or "close" to - the surface of the earth. The atmospheric pressure varies with temperature and altitude above sea level.

Absolute pressure, Gauge pressure and Atmospheric pressure have got following relationship as seen from the above figure:

Pabs = Pgauge+ Patm

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Pressure Measuring Devices:

Manometers:Manometers more commonly known as Liquid Column Manometers are used for low range pressure measurement. Generally used within a range of 2 kg/cm2 i.e. 0.2 MPa.

In U-tube manometers, there is no fixed zero reference. So, it makes the accurate measure-ment difficult. When a manometer is used to measure low pressure then water is used as liquid. For high pressures, Mercury is used.

Well type Manometer are widely used and highly accurate due to almost stable level of liquid in well.

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Inclined Manometers are used to measure very low pressure differences (in hundredth of inch of water). These manometers have higher sensitivity due to the inclined tube.

Pressure Sensing Elements (Elastic Pressure Transducers):

Bourdon Tube:It has simple and rugged construction. It covers ranges from 0-15 psig to 0-100000 psig.A C-type Bourdon tube consists of a long thin-walled tube of non-circular cross section, sealed at one end, made from materials such as phosphor bronze, steel and beryllium copper and attached by a mechanical linkage which operates a pointer.

Advantages and Disadvantages of Bourdon tubes:Advantages:

Their cost is low.They have simple construction.They have been time tested in applications.These tubes are available in high variety of ranges including very high ranges.They are adaptable to transducer designs for electronic instruments.

Disadvantages:They are susceptible to shock and vibrations.They are susceptible to hysteresis.

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Bellows:The Bellows type gauges are more sensitive than Bourdon tubes. These gauges are generally used for the range down to 155.1 mmHg (3 psi). The Bellows are made of an alloy which is ductile, has high strength and very little hysteresis. The pressure is applied at one end and the result is displacement at the other end.

Advantages of Bellows:Its cost is moderate.It is able to deliver high force.It is adaptable for absolute and differential pressure.It is good in the low to moderate pressure range.

Disadvantages of Bellows:

It needs ambient temperature compensation.The availability of metals and work hardening of some of them is limited.

Diaphragm:A diaphragm is a thin disk of material which bows outward under the influence of a fluid pressure. The pressure medium is on one side and the indication medium is on the other. The deflection that is created by pressure in the vessel would be in the direction of the arrow indicated. They can detect pressure differential even in the range of 0-4 mm.

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Advantages:Their cost is moderate.They are adaptable to absolute and differential pressure measurement.They have good linearity.They are available in several materials for corrosion resistance.They are small in size.

Disadvantages:They lack good vibration and shock resistance.They are difficult to repair.They are limited to relatively low pressures.

Capsules:The capsule consists of two circular shaped, convoluted membranes (usually stainless steel) sealed tight around the circumference. The pressure is applied to the inside of the capsule and if it is fixed only at the air inlet, it can expand like a balloon. This arrangement is not much different from the diaphragm except that it expands both ways.

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Differential Pressure (DP) Transmitter:

One of the most common, and most useful, pressure measuring instruments in industry is the differential pressure transmitter. This device senses the difference in pressure between two ports and outputs a signal representing that pressure in relation to a calibrated range. Differential pressure transmitters may be based on any of the previously discussed pressure-sensing technologies, so this section focuses on application rather than theory.

DP Transmitters Photographs:

Regardless of make or model, every differential pressure (“DP”, “d/p”, or ΔP) transmitter hastwo pressure ports to sense different process fluid pressures.. One of these ports is labeled “high” and the other is labeled “low”. This labeling does not necessarily mean that the “high” port must always be at a greater pressure than the “low” port. What these labels represent is the effect that a pressure at that point will have on the output signal.

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Signal output of DP Transmitter:The most common sensing element used by modern DP transmitters is the diaphragm. One side of this diaphragm receives process fluid pressure from the “high” port, while the other receives process fluid pressure from the “low” port. Any difference of pressure between the two ports causes the diaphragm to flex from its normal resting (center) position. This flexing is then translated into an output signal by any number of different technologies, depending on the manufacturer and model of the transmitter:

An increasing pressure applied to the “high” port of a DP transmitter will drive the output signal to a greater level (up), while an increasing pressure applied to the “low” port of a DP transmitter will drive the output signal to a lesser level (down):

Pressure Measurement Applications:

Measuring process vessel clogging:We may use the DP transmitter to measure an actual difference of pressure across a process vessel such as a filter, a heat exchanger, or a chemical reactor. The following illustration shows how a differential pressure transmitter may be used to measure clogging of a water filter:

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Note the high side of the DP transmitter connects to the upstream side of the filter, and the low side of the transmitter to the downstream side of the filter. This way, increased filter clogging will result in an increased transmitter output.

Measuring positive gauge pressure: DP instruments may also serve as simple gauge pressure instruments if needed, responding topressures in excess of atmosphere. If we simply connect the “high” side of a DP instrument toa process vessel using an impulse tube, while leaving the “low” side vented to atmosphere, theinstrument will interpret any positive pressure in the vessel as a positive difference between thevessel and atmosphere:

For gauge pressure, the “low” side of the sensing element is capped off with a special vented flange.

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A closer look at this flange reveals a vent near the bottom, ensuring the “low” side of the pressure-sensing capsule always senses ambient (atmospheric) pressure.

Measuring Absolute Pressure:Absolute 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 vacuum:The 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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Gas and Liquid Flow Measurement Application of DP Transmitters:Another common measurement using DP transmitters is the measurement of fluid flowthrough a pipe. Pressure dropped across a constriction in the pipe varies in relation to flow rate (Q) and fluid density (ρ). So long as fluid density remains fairly constant, we may measure pressure drop across a piping constriction and use that measurement to get flow rate.

The most common form of constriction used for this purpose is called an orifice plate, being nothing more than a metal plate with a precisely machined hole in the center. As fluid passes through this hole, its velocity changes, causing a pressure drop to form:

Level Measurement Application:Liquids generate pressure proportional to height (depth) due to their weight. The pressure generated by a vertical column of liquid is proportional to the column height (h), and liquid’s mass density (ρ), and the acceleration of gravity (g):

P = ρghKnowing this, we may use a DP transmitter as a liquid level-sensing device if we know the density

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of the liquid remains fairly constant:

As liquid level in the vessel increases, the amount of hydrostatic pressure applied to the transmitter’s “high” port increases in direct proportion. Thus, the transmitter’s increasing signal represents the height of liquid inside the vessel:

This simple technique works even if the vessel is under pressure from a gas or a vapor (ratherthan being vented as was the case in the previous example). All we need to do to compensate forthis other pressure is to connect the DP transmitter’s “low” port to the top of the vessel so it sensesnothing but the gas pressure:

Calibration of DP Transmitter:For pressure range of 25-125psig and output signal of 4-20 mA.

• Turn on the transmitter and allow the internal components to reach normal operating temperature.

• Use a portable regulated air supply to apply 25 psig pressure to the instrument.• Adjust the zero to set exactly 4mA output, indicated on a precision milli -ammeter.• Supply 125psig pressure to the instrument.• Adjust the span to get a reading of exactly 20mA.

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• Apply 75psig pressure (half scale input pressure)• Adjust the linearity to bring the output signal within the specified tolerance of

12mA (half scale output current).

Flow Measurement

The methods of flow measurement are following:

Differential Flow metersVariable Area Flow metersMagnetic Flow metersTurbine MetersUltrasonic flow meters

Differential flow Meters:As discussed in earlier DP transmitter’s applications section, Differential flow meters operate on the principle of creation of pressure difference by inserting a restriction in the pipe of a flowing fluid.The differential pressure created is proportional to the flow rate. The proportionality is not linear but has a square root relationship. The flow rate is proportional to square root of pressure difference.

Here ρ is Density of fluid flowing

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Advantages of Differential Flow meters:Its cost is low.It offers widest application coverage of any type of meter.It is accurate and reliable.It can be easily removed without shutting down the process.It is adaptable to any pipe size and flow rate.

Disadvantages of Differential Flow meters:There is relatively high permanent pressure loss.It is difficult to use for slurry services.It does not give linear relationship between flow rate and DP.Low flow rates are not easily measured with these meters.Its accuracy depends upon fluid characteristics like pressure, temperature etc.It is difficult to measure pulsating flow rates.

Primary Parts of Differential Flow meters:

Orifice Plates:Of 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. Orifice Plates are made from stainless steel, nickel, phosphor bronze etc.

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Types of Orifice Plates:

Concentric Accentric

Concentric: Most widely used. Eccentric: Used for fluids containing solids, oils containing water.

Segmental: Used for fluids containing solids. Segmental

Quadrant edge Quadrant edge: Used for viscous flows.

Several standards exist for pressure tap locations. Ideally, the upstream pressure tap will detect fluid pressure at a point of minimum velocity, and the downstream tap will detect pressure at the vena contracta (maximum velocity). In reality, this ideal is never perfectly achieved. An overview of the most popular tap locations for orifice plates is shown in the following illustration:

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Advantages of Orifice Plates:-Its cost is low.-Can be used in a wide range of pipe sizes.-Can be used with differential pressure devices.-They are available in many materials.

Disadvantages of orifice Plates:

-They cause relatively high pressure loss.-They tend to clog, thus reducing use in slurry services.-Their accuracy is dependent upon care in installation.-They have changing characteristics because of corrosion and scaling.

Venturi tubes:Used where permanent pressure loss is of prime importance i.e. where the permanent pressure loss is not accepted. It is used where high accuracy is desired in the measurement of viscous fluid.Usually made of cast iron or steel. These are available in sizes 100mm to 813mm length.Venturi tubes can handle slurries, dirty liquids also.

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Classical type

Accentric type Rectangular type

Advantages of venturi Tubes:- It causes low pressure loss.- Widely used for high flow rates.- Available in large pipe sizes.- It is more accurate than Orifice plates or nozzles.

Disadvantages of Venturi tubes:Its cost is high.Generally not useful below 76.2 mm pipe size.It is more difficult to inspect.

Flow Nozzles:Used for flow measurements at high velocities. These are more resistant to erosion than orifice plates.It consists of a convergent inlet and a cylindrical throat. Differential pressure taps are normally located one pipe diameter upstream and ½ pipe diameter down stream.

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Advantages of Flow Nozzles:- Its permanent pressure loss is lower than that for an orifice plate.- It is available in numerous materials.- It is useful for liquids containing solids that settle.- It is widely accepted for high pressure and temperature steam flow.

Disadvantages of Flow Nozzles;- Its cost is higher than orifice plate.- It is limited to moderate pipe sizes.- It requires more maintenance (It is necessary to remove a section of pipe to inspect or install it)

Pitot Tubes:Pitot tubes are generally used for measurement of fluid velocity. It measures the difference in pressures at normal flow line and stagnation point. A pitot tube consists of a tube with an opening of 3.125 to 6.35mm which is placed directly in the line of flow.

Advantages of Pitot Tubes:-Have no process loss.-Economical to install.-Can be easily removed from pipes.

Disadvantages:

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-Poor accuracy.-Unsuitable for dirty or sticky fluids.-Sensitive to upstream disturbances.

Variable Area Flow meter (Rota meter):

The simplest example of a variable-area flowmeter 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. The tube is tapered to get linear relationship between flow and position of float. The tube is mounted vertically with a small end at the bottom. The fluid enters at bottom and exits at top. The tube material may be glass or metal. 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 = PA). If this force exceeds the weight of the plummet, the plummet moves up.After some movement plummet stops moving, indicating flow rate by its position relative to a scale mounted (or etched) on the outside of the tube.

Advantages of Rota meters:Its cost is relatively low.Rota meters have good rangeability.It is good for metering small flows.Its is easily equipped with alarm switches, or transmitting devices.It can be used in light slurry services.

Disadvantages:The glass tube is easily subject to breakage.It must be mounted vertically.It is not good in pulsating services.Limited to generally small pipe sizes.

Electromagnetic Flow meters: - It is used for high corrosive applications. - Used for the flow measurement of conductive liquids - Works on the principle of Faradays Law of Electromagnetic Induction

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Consists of a non-conductive pipe such as fiber glass. It has a pair of electrodes flush with the inside wall of the pipe. Induced emf is generated across the two electrodes.

Flow Rate Q = K.E

Advantages:It can handle slurries and greasy materials.It can handle corrosive fluids.It has very low pressure drop.It is totally obstruction less.It is available in large pipe sizes and capacities.It can be used as bi-directional meter.Measurements are unaffected by viscosity, density, temperature and pressure.

Disadvantages:It is relatively expensive.It works with fluids which are electrical conductors.It is relatively heavy, especially in larger sizes.It must be full at all times.

Turbine Flow Meters:Used for measurement of liquid, gas.Used for low flow rates.It consists of a multi-bladed rotor which is mounted at right angles to the axis of flowing fluid.

The rotor is supported by ball or sleeve bearings. The rotor is free to rotate about its axis. The flowing fluid impinges on the turbine blades (rotor), imparting a force to the blade surface which causes the rotation. The speed of rotor is directly proportional to fluid velocity.The speed is measured by a

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magnetic pick-up coil, which is fitted outside to the meter housing. As each rotor blade passes by coil, it generates a voltage pulse in the coil. The generated voltage is a measure of flow rate of the fluid.

Advantages:Its accuracy is good.It provides excellent repeatability and rangeability.It allows fairly low pressure drop.Easy to install and maintain.

Disadvantages:Its cost is high.Its use is limited for slurry applications.It faces problems by non-lubricating fluids.

Ultrasonic Flow meters:

- Ultrasonic flow meters measure fluid velocity by passing high-frequency sound waves along the fluid flow path.

- They work by transmitting a high-frequency sound wave into the fluid stream (the incident pulse) and analyzing the received pulse.

Two Types:

1. Doppler Flow meter - In this, an ultrasonic wave is projected at an angle through the pipe wall into the liquid by a transmitting crystal in transducer mounted outside the pipe. - a part of ultrasonic wave is reflected by bubbles or particles in the liquid and received by a receiving crystal - The reflected wave’s frequency gets changed according to the velocity of fluid.

The velocity of fluid is directly proportional to change in frequency of ultrasonic wave.

V = Δf Ct / 2f0cosθ = K.Δf Here Δf –difference between transmitted and received frequency

Ct = velocity of sound in the transducer f0 – frequency of transmission θ – angle of crystal with respect to pipe K - a constant

2. Time Difference Type Flow meter:- These devices measure flow by measuring the time taken by ultrasonic wave to transverse a

pipe section, both with upstream and downstream of flow.- The time taken in upstream is more than that in downstream.

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The velocity of fluid in pipe is proportional to this time difference.V = ΔT.C/2Lcosθ

Here V- Velocity of fluid in pipeC- Velocity of sound in fluid θ- angle of path with respect to pipe axis.

Advantages:It does not impose external resistance to flow or disturb the flow.Its velocity/output relationship is linear.It has no moving parts.Its repeatability is in the order of 0.01%.

Disadvantages:An electronic oscillator is needed to generate ultrasonic waves.A detector is required to measure time difference.

Weight Measurement

To produce a good cement quality, accurate weigh feeding of the different components is important.

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To bill the customer agreeable again, weighing plays the key role.For weighing two different ways are possible: the static weighing (weigh bridge) and the dynamic weighing (weigh feeder).

Static Weighing through Weigh Bridges:The weigh bridge at the factory entrance is regarded as the most important and accurate unit. For weigh bridges the measuring principle applied is usually one or several load cells.The entire steel framework is mounted on strain gauge double ended shear beam / Compression Type Load Cells (4, 6, & 8 No's. Load Cells), depending on length of the platform. The shear stress produced by the load is detected by a strain gauge, full bridge circuit and converted into an analog electrical signal. Signal from each load cell is fed to Junction Box. Output from Junction Box is fed to Micro processor based weighbridge terminal.

Circuit Arrangement for Weight Measurement:In Figure, if R1, R2, R3, and R4 are equal, and a voltage, VIN, is applied between points A and C, then the output between points B and D will show no potential difference. However, if R4 (gauge resistance) is changed to some value which does not equal R1, R2, and R3, the bridge will become unbalanced and a voltage will exist at the output terminals. This voltage is measured as an indication of weight.

Weigh Bridge

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Dynamic Weighing through Weigh Feeders:The most common measuring principle applied in the cement industry is the belt weigher. A section of the belt runs over idlers supported by a frame section placed onto load cells.The purpose of the Weigh Feeder is to:

• Accurately measure the mass flow of materials.• Control the flow rate of material into downstream equipment.

The weight over this belt section multiplied with the speed represents the feed rate Q = P * v whereas:

Q = feed rate [t/h]P = weight per width [t/m]v = speed [m/h]

This principle is well-known and if maintained properly very accurate (error < 1%).The load cell measures the weight of a fixed-length belt section. A tachometer (speed sensor) measures the speed of the belt.

Weigh Feeder Arrangement:

In this arrangement, the weight per meter (P) is measured through load cells and the belt speed (v) through a techogenerator. These two are multiplied by a multiplier and the flow rate Q = P* v is found. A totalizer gives the total material fed through the feeder in a particular time. A feed rate indicator is used to indicate the present flow rate. To control the feed rate, it is compared with the set point value and an error is sent to the controller. The controller generates a control signal which is sent to the variable speed drive (VSD). The VSD changes the DC motor speed in order to achieve the required feed rate i.e. set point value. Now a days, DC motor is avoided since 3-phase AC motor is better cost and maintenance and life point of view. Also its speed change is done through VFD.

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Thus when selecting the weigh feeder the following points must be taken into consideration:

♦ Accuracy required.♦ Mechanical suitability for the material and the environment.♦ Space availability (height and area). Especially building height can be reduced (cost saving) with certain weighing arrangements and weighing principles.♦ Signal availability and signal transmission. (4-20 mA and digital signals or communication via a bus system).♦ Maintenance that is time interval between calibration, access for calibration e.g. rerouting of material onto a lorry, cleaning required, complexity of the control, spare parts etc.

Level Measurement

The various methods of liquid/solid level measurement are explained ahead.

Level Gauge (Sight glasses):The level gauge or sightglass is to liquid level measurement as manometers are to pressuremeasurement: 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 liquidmay be seen. The following photograph shows a simple example of a sightglass:

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Advantages:- Direct reading is possible.- Numerous designs are available in numerous materials for corrosion resistance.

Disadvantages:- It is read only where the tank is located.- The liquid in the glass may freeze in cold weather.- Heavy, viscous fluids cannot be measured.- Accuracy and readability depends on the cleanliness of glass and fluid.

Level Measurement using DP Transmitters:Differential pressure transmitters are the most common pressure-sensing device used in this capacity to know liquid level within a vessel. In the hypothetical case of the oil vessel just considered, the transmitter would connect to the vessel in this manner (with the high side toward the process and the low side vented to atmosphere):

Connected as such, the differential pressure transmitter functions as a gauge pressure transmitter, responding to hydrostatic pressure exceeding ambient (atmospheric) pressure. As liquid level increases, the hydrostatic pressure applied to the “high” side of the differential pressure transmitter also increases, driving the transmitter’s output signal higher.

Any gas or vapor pressure accumulation in an enclosed vessel will add to the hydrostatic pressure at the bottom, causing any pressure-sensing instrument to falsely register a high level.

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A pressure transmitter has no way of “knowing” how much of the sensed pressure is due to liquid elevation and how much of it is due to pressure existing in the vapor space above the liquid. Unless a way can be found to compensate for any non-hydrostatic pressure in the vessel, this extra pressure will be interpreted by the transmitter as additional liquid level.

The only way to hydrostatically measure liquid level inside an enclosed (non-vented) vessel is to continuously compensate for gas pressure.

Fortunately, the capabilities of a differential pressure transmitter make this a simple task. All we need to do is connect a second impulse line (called a compensating leg), from the “Low” port of the transmitter to the top of the vessel, so the “Low” side of the transmitter experiences nothing but the gas pressure enclosed by the vessel, while the “High” side experiences the sum of gas and hydrostatic pressures. Since a differential pressure transmitter responds only to differences in pressure between “High” and “Low” sides, it will naturally subtract the gas pressure (Pgas) to yield a measurement based solely on hydrostatic pressure (γh).

The amount of gas pressure inside the vessel now becomes completely irrelevant to the transmitter’s indication, because its effect is canceled at the differential pressure instrument’s sensing element.

The calibration table for a transmitter close-coupled to the bottom of an oil storage tank would be as follows, assuming a zero to twelve foot measurement range for oil height, an oil density of 40pounds per cubic foot, and a 4-20 mA transmitter output signal range:

Ultrasonic 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. 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).

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Ullage is the “natural” mode of measurement for this sort of level instrument, because the sound wave’s time-of-flight is a direct function of how much empty space exists between the liquid surface and the top of the vessel. Total tank height will always be the sum of fillage and ullage, though. If the ultrasonic level transmitter is programmed with the vessel’s total height, it may calculate fillage via simple subtraction: Fillage = Total height − Ullage

The instrument itself consists of an electronics module containing all the power, computation, and signal processing circuits; plus an ultrasonic transducer to send and receive the sound waves. This transducer is typically piezoelectric in nature, being the equivalent of a very high-frequency audio speaker. The following photographs show a typical electronics module (left) and sonic transducer (right).

Electronic Module Ultrasonic Transducer

Radar level measurement:

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 (comprised of alternating electric and magnetic fields), and very high frequency (in the microwave frequency range – GHz). Sound waves are mechanical vibrations (transmitted from molecule to molecule in a fluid or solid substance) and of much lower frequency (tens or hundreds of kilohertz – still too high for a human being to detect as a tone) than radio waves.

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Non-contact radar transmitters are always mounted on the top side of a storage vessel. Modern radar transmitters are quite compact, as this photograph shows:

Level Measurement Using Load Cells:

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. Obviously, weight-based level sensors can measure both liquid and solid materials, and they have the benefit of providing inherently linear mass storage measurement. Load cells (strain gauges bonded to a steel element of precisely known modulus) are typically the primary sensing element of choice for detecting vessel weight. As the vessel’s weight changes, the load cells compress or relax on a microscopic scale, causing the strain gauges inside to change resistance. These small changes in electrical resistance become a direct indication of vessel weight.

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The following photograph shows three bins used to store powdered milk, each one supported by pillars equipped with load cells near their bases:

Each bin is supported by a pillar equipped with Load Cells near their bases.

When multiple load cells are used to measure the weight of a storage vessel, the signals from all load cell units must be added together (“summed”) to produce a signal representative of the vessel’s total weight. Simply measuring the weight at one suspension point is insufficient, because one can never be sure the vessel’s weight is distributed equally amongst all the supports.

Capacitive Method of Level Measurement:

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:

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

Capacitive level probes come in two basic varieties: one for conductive liquids and one for non conductive liquids. If the liquid in the vessel is conductive, it cannot be used as the dielectric (insulating) medium of a capacitor. Consequently, capacitive level probes designed for conductive liquids are coated with plastic or some other dielectric substance, so the metal probe forms one plate of the capacitor and the conductive liquid forms the other:

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In this style of capacitive level probe, the variables are permittivity (ǫ) and distance (d), since a rising liquid level displaces low-permittivity gas and essentially acts to bring the vessel wall electrically closer to the probe. This means total capacitance will be greatest when the vessel is full (ǫ is greatest and effective distance d is at a minimum), and least when the vessel is empty (ǫ of the gas is in effect, and over a much greater distance).If the liquid is non-conductive, it may be used as the dielectric itself, with the metal wall of the storage vessel forming the second capacitor plate:

In this style of capacitive level probe, the variable is permittivity (ǫ), provided the liquid has a substantially greater permittivity than the vapor space above the liquid. This means total capacitance will be greatest when the vessel is full (average permittivity ǫ is at a maximum), and least when the vessel is empty.

Advantages:- It is very useful in a small system.- It is very sensitive.- It is suitable for continuous indication and control.- It is good to use with slurries.- Remote adjustment of span and zero is possible in this type of level indicator.

Disadvantages:- The performance of this method gets affected by dirt and other contaminants.- Its sensitivity is adversely affected by temperature.- Measured fluid must have proper dielectric properties.

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- They usually require recalibration if measured fluid changes in composition or moisture content.

Radiation Type Level Detector: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 (gamma rays) 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.

These “sources” may be locked out for testing and maintenance by moving a lever that hinges a lead shutter over the “window” of the box. This lead shutter acts as an on/off switch for the radioactive source. 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.

Advantages:There is no physical contact with the liquid.They are useful at very high temperatures.They have good accuracy and response.They have no moving parts.

Disadvantages:The reading is affected by density change of liquid.Radiation source holders may be heavy.Their cost is relatively high.

Proximity Switches

A proximity switch is one detecting the proximity (closeness) of some object. By definition, these switches are non-contact sensors, using magnetic, electric, or optical means to sense the proximity of objects.A proximity switch will be in its “normal” status when it is distant from any actuating object. Being non-contact in nature, proximity switches are often used instead of direct-contact limit switches for the same purpose of detecting the position of a machine part, with the advantage of never wearing out over time due to repeated physical contact. However, the greater complexity (and cost) of a proximity switch

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over a mechanical limit switch relegates their use to applications where lack of physical contact yields tangible benefits.A proximity sensor detects an object when the object approaches within the detection boundary of the sensor. Depending on the principle of operation, each type of sensor will have different performance levels for sensing different types of objects. Common types of non-contact proximity sensors include inductive proximity sensors, capacitive proximity sensors and optical proximity sensors.

Most proximity switches are active in design. That is, they incorporate a powered electronic circuit to sense the proximity of an object. Inductive proximity switches sense the presence of metallic objects through the use of a high-frequency magnetic field. Capacitive proximity switches sense the presence of non-metallic objects through the use of a high-frequency electric field. Optical switches detect the interruption of a light beam by an object.

The following schematic diagrams contrast the two modes of switch operation

The next photograph shows a proximity switch detecting the passing of teeth on a chain sprocket, generating a slow square-wave electrical signal as the sprocket rotates. Such a switch may be used as a rotational speed sensor (sprocket speed proportional to signal frequency) or as a broken chain sensor (when sensing the rotation of the driven sprocket instead of the drive sprocket):

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Inductive Proximity Sensors:Metallic objects are sensed by such sensors. These sensors have up to 50mm switching distance. As soon as an object comes in the range of high frequency magnetic field generated, the current sensor senses the change in current and generates a DC signal.

The DC output signal generated is used for an NC or NO contact to move.

Capacitive Proximity Sensors:Metallic & Non Metallic objects, solids & liquids are sensed. These sensors have up to 20mm switching distance. The object to be sensed acts as the second plate of parallel plate capacitor. The first plate is inside the sensor itself. The current sensor generates DC signal after sensing of object.

Optical Proximity Sensors:

Thru-Beam Type:In this type of sensor, if any abject comes in the path of light rays, the current sensor does not receive light and this change is outputted in terms of DC signal output.

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Reflective Type:This works on the reflection of light wave principle. If any object comes in the path of light wave, the wave gets reflected and reaches the current sensor. The current sensor generates a DC signal for further switching purpose.

Speed Measurement

Two methods of speed measurement are discussed below:

Speed Measurement by Optical Proximity Sensor:An opaque disc with perforations or transparent windows at regular intervals is mounted on the shaft whose speed is to be measured.An LED source is aligned on one side of the disc in such a way that as the disc rotates the light will alternately passed through the transparent windows and blocked by the opaque sections. A photo detector fixed on the other side of the disc detects the variation of light. The output of the detector after signal conditioning would be a square wave (as shown) whose frequency is decided by the speed and the number of holes (transparent windows) on the disc. If frequency of pulses (i.e. number of pulses per second) increases, its means that the disc is rotating faster. Thus, by measuring frequency of pulses, the speed is measured.

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Schematic arrangement of optical speed sensing arrangement

Variable Reluctance type Speed Sensor:

A wheel with projected teethes made of a ferromagnetic material is mounted on the shaft whose speed is to be measured. The static sensor consists of a permanent magnet and a search coil mounted on the same assembly and fixed at a closed distance from the wheel. The flux through the permanent magnet completes the path through the teeth of the wheel and cut the search coil.As the wheel rotates there would be change in flux cut and a voltage will be induced in the search coil. This voltage is converted in to constant amplitude pulses by a comparator circuit. The pulses’ frequency is counted by a counter in a time interval and displays speed.

Variable reluctance type Speed Sensor

Metal Detector

Operating Principle:An electromagnetic field is generated across the conveyor line. When a metallic object comes with the material flow, eddy currents are induced in the metal. These eddy currents reduce the main magnetic field strength and a change is detected by a current sensor.

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If metal is detected, a signal is sent to the process controller which generates a required signal to generate alarm or trip a circuit.

If a metallic object is detected, the conveyor belt gets stopped with an alarm generation. A worker goes to find that object. After finding the object, it is removed from the material and the belt is restarted for normal operation.

Actuators

Pneumatic Actuator:It operates by a combination of force created by air and spring force. The actuator positions a control valve by transmitting its motion through the stem.

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Hydraulic Actuators:When a large amount of force is required to operate a valve (for example, the main steam system valves), hydraulic actuators are normally used. It consists of a cylinder, piston, spring, hydraulic supply & return line, and stem.

Solenoid Actuators:

It consists of a coil, armature, spring, and stem. Whenever the coil is energized, the armature moves vertically inside the coil and transmits its motion through the stem to the valve. Its response is quick and also it is easy to install.

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Only two valve positions, fully open or fully closed are there in such actuators. It is used for small valves due to low force creation.

Electric Motor Actuators:

Its major parts include an electric motor, clutch and gear box assembly, manual handwheel, and stem connected to a valve. Most electric motor actuators are equipped with limit switches, torque limiters, or both.

Whenever the electric motor gets supply, it rotates and after rotating by some angle it gets stopped. The motor gets stopped after a limit switch senses the position of valve and sends a signal to stop the motor automatically. Hand wheel is used to manually set the valve position.

Limit SwitchesLimit Switches are position sensors. The standard limit switch is a mechanical device that uses physical contact to detect the presence of an object (target). Limit switches use a mechanical actuator input, requiring the sensor to change its output when an object is physically touching the switch. They may be normally open (NO), normally closed (NC), or a combination of normally open and normally closed contacts.

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Circuit Example:

NC Limit Switch LS1 closedCoil M energizedContacts M ClosedMotor running

LS1 energized, gets open Coil M de-enerzised Contacts M open Motor stops.

A typical limit switch consists of a switch body and an operating head. The switch body includes electrical contacts to energize and de-energize a circuit. The operating head incorporates some type of lever arm or plunger, referred to as an actuator. When the target comes in contact with the actuator, the actuator is rotated from its normal position to the operating position. This mechanical operation activates contacts within the switch body.

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Momentary Operation:

The operation is called momentary when object comes in contact with lever and then leaves it. An spring returns the lever in free state after the object leaves.

Maintained Operation:

With maintained operation the actuator lever and contacts return to their free position when a force is applied to the actuator in the opposite direction. A fork style actuator is typically used for this application.

Magnetic SeparatorsPurpose

The Magnetic Separator removes any type of ferrous contaminants from a process material stream.Components

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Magnet: Powerful electrical magnet installed over the material of a running conveyor to attract tramp iron.

Cleaning belt: Conveyor with rubber or steel blades mounted transversely to the direction of the material flow to carry pieces of metal out of the magnetic field.Head pulley: A pulley located at the discharge end of the conveyor that is commonly used to transmit the force generated from the drive to the belt conveyorTail pulley: A pulley mounted at the tail end of a conveyor, its purpose is to return the beltTake-Up: An assembly used to maintain the desired tension of a belt conveyor. This adjustment is necessary to compensate for stretch, shrinkage or wear. Zero speed switch (ZSS): Belt conveyor protection device used to indicate motion.Conveyor frame: The support structure for the conveyor and the magnet assembly.Conveyor drive: An assembly of the necessary structural, mechanical and electrical parts which provide the motive power for a conveyor. Usually consisting of motor/reducer, chain, sprockets, guards, mounting base and hardware, assembly to operate the cleaning belt conveyor.Tramp iron hopper: Ferrous material collection hopper.Electrical control station: Used to remotely operate the magnet separator

Separator Arrangement:

Method of Operation of MS:Depending on the location, quantity and collection methods, three types of separators can be installed:

Manual-cleaning SMS.Inline self-cleaning SMSCross belt self-cleaning SMS

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The magnet is continuously energized when the main conveyor is running, so all pieces of metal are attracted by the magnet as they pass under it.With the manual cleaning type, the metal pieces stay attracted to the magnet.From time to time, the system has to be stopped and the magnet de-energized to remove the tramp iron from the magnet. With the self-cleaning type, the cleaning belt conveyor is continuously running at high speed. When metal pieces are attracted, they are held against underside of cleaning belt by the strong magnet field. The pieces are then evacuated from the magnetic field by the cleaning belt & propelled into a collection hopper. As the generation of a magnetic field produces heat, the magnet has to be cooled. Most of the MS are air-cooled. Some are oil-cooled.

Applications:

• The MS is used wherever there is a risk of getting tramp iron within the flow of material• In cement manufacturing, common sources of tramp metal include loader bucket teeth, drill

heads, crusher chains, wear rails, welding rods, wear materials, and common fasteners.

Gas Analyzers

Continuous monitoring and analysis of gases has become essential in many areas of modern life, particularly in industrial processes.

The aim of such analysis or monitoring may be one or more of the following.

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To improve product quality, To avoid loss of valuable materials, To optimize energy utilization, and To ensure safety of personnel and equipment.

Gas Sampling System:The object of a sampling system is to provide a clean (dry if necessary) representative sample of the gas or liquid which is to be analyzed, at an adequate and stead rate, and conduct it without change to the analyzing instrument.

System includes the sampling probe and the accessories to condition the sample gas so that its physical properties are acceptable to the analyzer.The gas sampling system transports the gas to the analyzer without changing the composition and at the correct temperature and flow rate. The sample gas should be free of dust particles and excessive water vapour.

PROBES:Specially constructed probes are used for extracting the sample gas from closed vessels or wide pipes/ducts. The probe must be installed so that the sampling point is away from the wall of the duct/pipe/vessel, where local effects may make the gas sample. A heated ceramic filter is used inside the probe to filter the gas before it gets condensed.

TWIN PROBES

Water cooled, twin probes are used for extracting sample of gas at high temperature, as at the inlet of rotary kiln. Here one probe is used for extracting sample gas, while at that time; the other probe is cleaned by air-purging at regular intervals.

SAMPLE GAS COOLER

An electric gas cooler is installed after the probe to cool the sample gas. This is done to avoid water vapour condensation along the lines causing several problems like blockages of the sampling lines and unstable flow of gas, damage to the analyzer cells etc.

DIAPHRAGM PUMPA pump is used to suck the sample gas into the analyzer, in case the pressure in the sampling is low (lower than 20 mbar). The output of the analyzer should be freely let into atmosphere so that the analyzer does not operate at an overpressure of more than 20 mm WG +, otherwise the readings of the analyzer may be affected.

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BASIC PRINCIPLES OF GAS ANALYSIS SYSTEMS:

• Gas Analyzers are utilized for quantitative determination of carbon monoxide (CO), Carbon Dioxide (CO2), Oxygen (O2), Hydrogen (H2), Nitrogen Oxides (NO2), Sulphur Dioxide (SO2), various Organic compounds etc. in process industries.

• The concentrations of the gases being analyzed are expressed either in Weight Units (mg/M3) or in volume units (% Vol.)

PARAMAGNETIC GAS ANALYSER FOR O2 ANALYSIS

Out of all industrial gases, Oxygen (O2) has the highest paramagnetic property and this property reduces as the temperature of the gas increases. Paramagnetic substances are pulled into an inhomogeneous magnetic field, in the direction of increasing field strength.

Construction & Principle of Operation:A metallic chamber ‘A’ is divided in two sub chambers named measuring chamber (B) and Reference chamber(C). Chamber B has permanent magnet (N-S poles) and chamber C has nonmagnetic dummy blocks D to maintain the heat balance between chamber B & C. Resistance R1 in chamber B and R2 in chamber C make a bridge circuit along with outside resistors R3 and R4. An SPS electric supply is used to pass the current through all the four resistors. This current in R1 and R2 generates heat. The heat generated in R2 is absorbed by dummy blocks D where as the heat generated in R1 is transferred to oxygen contained in the gas to be analysed. The gas enters the analyzer chamber A from gas inlet. The oxygen molecules present in the gas get attracted towards the magnetic field present in chamber B.

O2 concentrations are converted into equivalent DC Current signals which may be fed to an indicator, or recorder, directly calibrated in O2 %.

INFRARED ABSORPTION GAS ANALYSERSAll gases whose molecules have two or more different atoms - called Hetero-atomic gases, such as Carbon Monoxide (CO), Carbon Dioxide (CO2), Methane (CH4) etc., are analyzed by these gas analyzers.These gases exhibit selective absorption of infrared radiation.

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Each of these heteroatomic gases has a peak response of absorption for a particular narrow band of wave lengths in the infrared spectrum of Electro Magnetic Radiation.Monoatomic noble gases (He, Ne, Ar, Kr, Xe and Rn) and gases made up of atoms of the same kind like O2, H2 and N2 called "Symmetrical Diatomic" gases do not absorb infrared radiations.

Construction and Principle of Working:

The DC output of the amplifier (4-20mA) corresponds to the variations of carbon monoxide (CO) in the sample gas.

GAS ANALYSERS -APPLICATIONS IN CEMENT INDUSTRY:

OBJECTIVES OF GAS ANALYSIS:

• To obtain process information in order to control the combustion and calcining processes in the kiln and preheaters, respectively.

• To actuate interlocks and warning systems for eliminating or reducing explosion hazards; and • Pollution control. • O2 and CO are the main components of interest in flue gas analysis for the purposes of process

information and interlocks.• SOx and NOx are primarily determined for pollution control.• NOx is also used to control burning zone temperature.

INTERLOCK AND WARNING SYSTEMS: An electrostatic precipitator in the cement making process is a potential igniter (due to corona discharges and /or flashover between the electrodes) and has to be switched off if CO concentration in the flue gas exceed a certain limit.If the hot gases from the kiln are used for drying purposes in the raw mill circuit, precautions should also be taken to isolate the circuit and thus avoid the distribution of dangerous gases.

ALARM AND TRIP VALUES OF CARBON MONOXIDE:• Measurements of carbon monoxide are generally taken after the preheater or just before the

ESP.ALARM-at 0.4% CO (volume)TRIP (ESP)- at 0.8% CO (volume)

SAMPLING POINTS AND PROBES FOR GAS ANALYSIS:The kiln inletPre- Calciner OutletAfter the preheater towerESP Inlet

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In the Stack 80