POWER PLANT INSTRUMENTATION PDF
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EI POWER PLANT INSTRUMENTATION. UNIT I. (PART-A). 1. What are the thermal power plant (Steam) circuits? Coal and ash circuit, Air and gas circuit. Control & Instrumentation Principles Reference Manual Electrical Power & Machines Download Ebook: power plant control and instrumentation in PDF Format. Buy Power Plant Instrumentation by K. Krishnaswamy PDF Online. ISBN from PHI Learning. Download Free Sample and Get Upto 33% OFF on.
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The table is a rather basic form for board evaluation. It also has an on-board comparator that allows a square wave to be produced for clock generation. Chipsbuy The search system of electronic components for online warehouses and price lists of suppliers. The reason that the opamp is needed is that the output of the AD is about mV. View More. AD Breakout Board. Order today, ships today. Setting the standard TM. Department of Education for those involved in international credential evaluation and comparative education research.
Your score reflects the degree to which assessors were able to locate clear, consistent, and convincing evidence that you have met the National Board Standards specific to your certificate field. There are some codes from websites where they got the AD family running well with. What are the most common types of evaluation? There are several types of evaluations that can be conducted. Evaluation Boards. For an APRN verification form, please click the link below.
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This is exactly what i am currently doing! The State Board of Education will use its constitutional authority to lead and uphold the system of public education in North Carolina that guarantees every student in this state an opportunity to receive a sound basic education. The AD and AD produce high-performance sine and triangular outputs. In an overhead distribution system between 75 to 95 per cent of the faults are of a temporary nature and last, at the most, for a few cycles or seconds.
Order size and frequency. Second, in most designs, the same PDN interconnects that are used to trans- port the power supply are also used to carry the return currents for signal lines. The power flows only in one direction: from upper voltage levels down-to customers situated along the radial feeders. Contemporary distribution networks, therefore, often include types of activities, for example production, not traditionally considered distribution activities.
In this work, we focus primarily on the large num-ber of small-scale residential consumers in the grid, since they rep-resent the vast majority of end-points in the distribution network. When product arrives at a distribution center it is immediately prepared to be shipped. Distribution networks plays lot of activities in the form of procurement, logistics, warehouse, and inventory and customers order.
The current dynamics in distribution are thus assumed to affect, and be affected by, efforts aiming at enhanced sustainability. The astounding technological developments of our age are highly dependent upon a safe, reliable, and economic supply of electric power.
Understanding how these effects affect the network faults, as well as historical trends of when and where they are occurring for specific area can help in 1 developing more predictive network is optimal capacitor placement performed with numerical, analytical, heuristic, or, more recently, intelligent methods [1, ].
Service level requirements.
Power Plant Instrumentation
Proposed solutions to power ow control problems in the literature range from fully centralized to fully local ones. Analysis of Water Transmission Lines 46 3. Markov distribution URD systems are cleared by the blowing of the nearest sectionalizing fuse or fuses.
Certainly, a process that can enhance service levels, improve inventory control and reduce costs merits a closer look before investing in a full-scale optimization of a supply chain network.
Ebrahimi, A. The distributor Reason of existence p. Introduction Distribution refers to the steps taken to move and store a product from the supplier stage to a customer stage in the supply chain. Most earlier work in joint sizing and location are in a deterministic setting e.
Configuration of the network. Since the distribution network is composed of low and medium voltage networks, both are included in this procedure. Executive summary. Medium and low voltage cables systems as core technology in distribution networks as support of Smart Grids Underground Medium and Low Voltage Cable Systems up to 36 kV Medium voltage MV cables up to 36 kV are deployed for the connection of the LV network to the primary distribution network.
Distribution substation. With these improvements to the system, overall transportation costs distribution networks of future with high penetration of distributed inverter-based renewable generators. It is an intermediate point to get products from the manufacturer to the end customer, either directly or through a retail network. A fast and reliable distribution network is essential to a successful business because customers must be able to get products and services when they want them.
The objective of the Caution Ahead: U. Faults occurring on the feeder are cleared by tripping and lockout of the feeder breaker. The first section, The Legacy of the 20th Century Utility, provides an overview of the history and current state of the distribution New Technologies for Electric Power Distribution Systems It is in their vested Power distribution Network is a part of electric networks between transmission and consumer service point.
It is the method s in which you market and sell your tours and activities to customers from all over the world. But choosing the best distribution network design from the myriad of options is a challenge. Head Loss in a Lumped Equivalent 45 3.
As distribution centers are made for material movement, improvements in transportation logistics are a third advantage. Distribution and Logistics Managers Knowledge Areas and Technical Competencies represent the knowledge, skills, and abilities needed by distribution and logistics managers. In this paper a generalized classic model for the nodal analysis of complex looped systems with nonstandard network components is formulated and the solvability of new The goal is to optimally size and site BESS within a distribution network under a certain penetration of distributed PV.
Mahmoud and Imad H. Distribution channels are defined and classified. During the initial design of the control system, dialogue with the process engineer or boiler designer will show whether or not surge protection will be required. Other means of controlling flow are dampers, vanes or speed adjustment. Each of these devices has its own characteristics, advantages and disadvantages, and the selection of the controlling device which is to be used in a given application will be a trade-offbetween the technical features and the cost.
This provides a form of draught control but it is not very linear and it is most effective only near the closed position.
Another form of damper comprises a set of linked blades across the duct like a Venetian blind. Such muhibladed dampers are naturally more expensive and more complex to maintain than single-bladed versions, but they offer better linearity of control over a wider range of operation. The task of designing a control system for optimum pertbrmance over the widest dynamic range will be simplified if the relationship between the controller output signal and the resultant flow is linear.
Although it is possible to provide the required characterisation within the control system, this will usually only be effective under automatic control. Under manual control a severely nonlinear characteristic can make it difficult for the operator to achieve precise adjustment. It is possible to linearise the command-flow relationship under both manual and automatic control by the design of the mechanical linkage between the actuator and the damper.
However, this requires careful design of the mechanical assemblies and these days it is generally consid- ered simpler to build the required characterisation into the DCS. This approach provides a partial answer, but it should not be forgotten that such a solution is only effective under automatic control. These vanes are clearly visible near the centre of Figure 3. Such vanes are operated via a complex linkage which rotates all the vanes through the same angle in response to the command signal from the DCS.
These may involve the use of electronic control- lers which alter the speed of the driving motor in response to demand signals from the DCS or they can be hydraulic couplings or variable-speed gearboxes, either of which allows a fixed-speed motor to drive the fan at the desired speed. Variable speed drives offer significant advantages in that they allow the fan to operate at the optimum speed for the required throughput of air or gas, whereas dampers or vanes control the flow by restricting it, which means that the fan is attempting to deliver more flow than is required.
Naturally, the handling systems for these types of fuels differ widely. Moreover, the variety of fuels being burned is enormous. Solid fuels encompass a wide spectrum of coals as well as wood, the waste products of industrial processes, municipal and clinical waste and refuse-derived fuels. The last are produced by shredding or grinding domestic, commercial and industrial waste material.
Liquid fuels can be heavy or light oil, or the products of industrial plant. Gas can be natural or manufactured, or the by-product of refineries. Each of these fuels requires specialised handling and treatment, and the control and instrumentation has to be appropriate to the fuel and the plant that processes it. The treatment depends on the nature of the coal. Some coals lend themselves to being ground down to a very fine powder called pulverised fuel PF which is then carried to the burners by a stream of air.
Other coals are fed to impact mills which use flails or hammers to break up the material before it is propelled to the burners by an air stream. The type of mill to be used on a particular plant will be determined by the process engineers and it is the task of the control engineer to provide a system which is appropriate. To do this it is necessary to have some understanding of how the relevant type of mill operates.
Various types of pulverised-fuel mill will be encountered, but two are most commonly used: In this device, the coal that is discharged from the storage hoppers is fed down a central chute onto a table where it is crushed by rotating steel balls. Air is blown into the crushed coal and carries it, via adjustable classifier blades, to the PF pipes that transport it to the burners.
The air that carries the fine particles of coal to the burners is supplied from a fan called a 'primary-air fan'. Because of this and because of its other con- structional features, this type of mill is properly called a 'vertical-spindle, pressurised ball mill'. The air-supply system for this type of mill is discussed in more detail in Section 3.
This cage contains a charge of forged-steel or cast alloy balls each of which is between 25 mm and mm in diameter which are carried up the sides of the cage by the rotation, until they eventually cascade down to the bottom, only to be carried up again. The coal is pulverised by a combination of the impact with these balls, attrition of adjacent particles and crushing between the balls and the cage and between one ball and another. In this type of mill the crushed mixture is drawn out of the cage by a fan, which is called an exhauster.
It should be noted that the cooler of the two air streams is commonly referred to as 'tempering air' since, because it is obtained from the FD fan exhaust it may already be slightly warm, and its function is to temper the mixture. Here, hot air and cold air are again mixed to obtain the correct temperature for the air stream but, because the mill in this case operates under suction condi- tions a primary air fan is not needed, and the cold air is obtained directly from the atmosphere. The warmed air mixture is again fed to the mill as 'primary air' but in addition a stream of hot air is fed to the feeder for transportation and drying purposes.
O n the face of it, it would appear that all that is required is to extract the oil from its storage tank and pump it to the burners. But in reality life is more complicated than that!
Proper ignition of oil depends on the fuel being broken into small droplets atomised and mixed with air. The fuel oil itself may be light such as diesel oil or gas oil , or it may be extremely viscous and tar-like heavy fuel oil, commonly 'Bunker C'. The handling system must therefore be designed to be appropriate to the nature of the liquid.
With the heavier grades of oil, prewarming is necessary, and to prevent it cooling and thickening the fuel is continually circulated to the burners via a recirculation system shown schematically in Figure 3.
The latter process is sometimes referred to as 'spill-back'. When a burner is not firing, the oil circulates through the pipework right up to the shut-off valve, which is mounted as close to the oil gun as possible. Burner 4 Figure 3.
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Any escape of gas, particularly into confined areas, presents considerable hazards, and great care must therefore be taken to guard against leakage, for example, from flanges and through valves. But natural gas is colourless, and any escape will therefore be invisible. Also, it is not safe to rely on odour to detect leakages.
By the time an odour has been detected sufficient gas may have already escaped to present a hazard. It is therefore necessary for gas-leak detectors to be fitted along the inlet pipework wherever leakage could occur, and to connect these to a comprehensive, central alarm system. It is also necessary to prevent gas from seeping into the combustion chamber through leaking valves.
If gas does enter undetected into the furnace during a shut-down period, it could collect in sufficient quantities to be ignited either by an accidental spark or when a burner is ignited. The resulting explosion would almost certainly cause major damage and could endanger lives. It should be noted that this risk is present with propane igniters such as those used with fuels other than oil. The fuel, air andflue-gas circuits 41 Protection against leakage into the furnace through the fuel-supply valves is achieved by the use of 'double-block-and-bleed' valve assemblies which provide a secure seal between the gas inlet and the furnace.
The operation of this system see Figure 3. In this condition any gas which may occupy the volume between the two block valves is vented to a safe place and it can therefore never develop enough pressure to leak past the second block valve.
When start-up of the burner is required, a sequence of operations opens the block valves in such a way that gas is admitted to the burner and ignited safely. Double-block-and-bleed assembly Main safety shut-off valve inlet pressure - reducing system Main flow-.
Early units suffered from the unpredictable nature of the waste material and the severe corrosion resulting from the release of acidic compounds during the combustion process.
But the problems have been largely overcome through the application of improved combustion systems and by better knowledge of the materials used in the construction of the plant. Waste material may be obtained from any of several sources, including the following: The material may be burned after very basic treatment shredding etc.
Several types of waste-to-energy plant are in existence, and we shall look at one of them, so that its nature and characteristics can be appre- ciated. Other plants will differ in their construction or technology, but from an operational point of view their fundamental characteristics will probably be quite similar to those described below.
First, the incoming waste is sorted to remove oversized, bulky or dangerous material. The remainder is then carried by a system of conveyors to a h a m m e r mill where it is broken down until only manageable fragments remain.
After a separator has removed incombustible magnetic items, the waste is held in a storage building, from where it is removed as required by a screw conveyor and transferred via another conveyor to the boiler. Immediately before entering the boiler, nonferrous metals are removed by a separator.
The boiler itself comprises a volume of sand which is kept in a fluidised state by jets of air. A portion of dolomite is added to the sand to assist in the reduction of corrosion and to reduce any tendency of the sand and fuel to coalesce a process known as 'slagging'. The heat released is used to generate steam in a way that is similar to conventional boilers such as those described in Chapter 2.
A variety of igniters are used, but most modern systems comprise a means of generating a high-energy electric spark which lights a gas or light-oil supply which in turn lights the main fuel. In addition to igniting the fuel, the igniter may sometimes be used to ensure that the fuel remains alight under conditions where it may otherwise be extinguished. This is referred to as providing 'support' for the main burner.
Like many aspects of power-station burner operations, the requirements for igniters are defined in standards such as those developed by the National Fire Protection Association e. NFPA In these standards, igniters are divided into three categories each of which is defined in detail. In essence, the three classes have the following character- istics. Class 1: An igniter providing sufficient energy to raise the temperature of the fuel and air mixture above the minimum ignition temperature, and to support combustion, under any burner light-off or operating conditions.
This class is also referred to as a'Continuous Igniter'. Class 2: Capable of lighting the fuel only under a defined range of light-off conditions. Class 3: Small igniters, generally applied to gas or oil burners.
These igniters are capable of lighting the fuel only under a defined range of conditions and may not be usedfor support purposes. Two types of Class 3 igniter are defined: The fuel, air andflue-gas circuits 45 The type of igniter in use will define the methods of operation of the burner and the sequences that are to be employed in the associated burner- management system. In outline, these systems include a means of monitoring the presence of the flame and a reliable method and procedure for operating the associated fuel valves in a sequence that provides safe ignition at start-up and safe shut-down, either in the event of a fault or in response to an operator command.
The procedure for lighting a burner depends, first, on checking that it is safe to light it at all. This means that, if no other burner is firing, confir- mation has been received that any flammable mixtures have been exhausted from the furnace by means of a purge. Such a purge involves the operation of FD and ID fans for a defined time, so that a certain volume of air has passed through the furnace. Once confirmation has been received that the furnace purge is complete or if other burners are already firing , ignition of the burner will depend on the successful operation of some form of igniter or pilot and, once the main burner has been successfully lit, its operation must be con- tinuously monitored, because an extinguished flame may mean that unburned fuel is being injected into the combustion chamber.
If such fuel is subsequently ignited it may explode. Once a burner has ignited, the BMS must ensure that safe operation continues, and if any hazard arises the system must shut off the burner, and if necessary, trip the entire boiler.
On shut-down of a burner, steps must he taken to ensure that any unburned fuel is cleared from the pipework.
This procedure is known as scavenging, and in an oil burner it may involve blowing compressed air or steam through the pipework and burner passages.
Such procedures are defined in codes such as NFPA Each component of the BMS is vital to the safety of the plant and to the reliability of its operation, but the most onerous responsibility rests with the flame detector: The sighting of the flame may also be affected by changes in flame pattern over a wide range of operating conditions, and it may also be obscured by swirling smoke, steam or dust.
Safe operation of the boiler depends on proper design of the BMS, including the flame scanner, and on careful siting of the scanner so that it provides reliable and unambiguous detection of the relevant flame under all operational conditions.
After installation, the system can be expected to perform safely and reliably only if constant and meticulous attention is paid to maintenance. This important matter is all too often ignored, and the inevitable result is that the system malfunctions, leading to failure to ignite the fuel, which may in turn delay start-up of the boiler.
In the extreme, malfunctions could even endanger the safety of the plant if they result in fuel being admitted to the combustion chamber without being properly ignited.
A properly designed BMS will not allow this to happen, but if repeated malfunctions occur it is not unknown for operators to ignore the warning signs and even to override safety systems. This important subject is discussed in greater depth in Chapter 5. In such plant, unless supplementary firing is used, the combustion process occurs entirely in the gas turbine.
Where supple- mentary firing is used the relevant control systems take on many of the characteristics of the oil- or gas-firing systems discussed earlier in the present chapter. With this understanding we can now look at the control and instrumentation systems associated with the plant. This survey will be structured in much the same way as the preceding chapters, starting with an overview of an important fundamental: Afterwards, we shall see how this demand is transmitted to all the relevant sections of the plant so that the requirements are properly and safely addressed.
Alternatively, the primary purpose of the plant may be to incinerate indus- trial, domestic or clinical waste, with steam being generated as a valuable by-product, to drive a turbo-generator or to meet a heating demand.
In each case, the factor that primarily determines the operation of the plant is the amount of steam that is required.
Everything else is subsidiary to this, although it may be closely linked to it. The determinant that controls all the boiler's operations is called the 'master demand'.
In thermal power-plant the steam is generated by burning fuel, and the master demand sets the burners firing at a rate that is commensurate with the steam production. This in turn requires the FD fans to deliver adequate air for the combustion of the fuel.
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The air input requires the products of combustion to be expelled from the combustion chamber by the ID fans, whose throughput must be related to the steam flow. At the same time, water must be fed into the boiler to match the pro- duction of steam. As stated previously, a boiler is a complex, multivariable, interactive process.
Each of the above parameters affects and is affected by all of the others. The nature of the master demand system depends on the type of plant within which the boiler operates, and it is therefore necessary to examine it separately for each type of application.
In the following sections we shall deal with the master demand as used in the following classes of plant: We shall see that although all of these require the boiler to be operated to generate steam, each has its own requirements and constraints.
With a combined-cycle gas-turbine plant it is frequently the case that the power generated by the gas turbines is adjusted to meet the demand, with the steam turbine making use of all of the waste heat from the turbines. With all types of power-generating plant, however, the requirement for generation will be set, directly or indirectly, by the grid-control centre or the 'central dispatcher' , and the amount of power that is generated will be related to the local or national demand at that time.
In national networks, power stations are linked together to generate electrical power in concert with one another. Together they must meet a demand that is made up of the combined needs of all the users that are connected to the system domestic, commercial, agricultural, industrial etc. The overall demand will vary from minute to minute and day to day in a way that is systematic or random, dictated by economic, operational and environmental factors. This pattern of use relates to the entire network, and the fact that a large number of power generators and users are linked via the network has little bearing on the overall demand, although the extreme peaks and troughs may well be smoothed out.
The interlinking does, however, have operational implications. For example, a sudden failure of one generating plant will instantly throw an extra demand on the others. Setting the demandfor the steam generator 51 In a cold or temperate climate the demand will be based predominately on the need for light, heat and motive power.
In warmer climates and developed areas it will also be determined by the use of air-conditioning and, possibly, desalination plant for drinking-water production. Figure 4. Clearly, in addition to being affected by normal working patterns, the demand is determined by the level of daylight and the ambient temperature, both of which follow basic systematic patterns but which may also fluctuate in a very sudden and unpredictable manner.
Similar profiles can be developed for each country and will be determined by climate as well as the country's indus- trial and commercial infrastructure. These days, the demand for electricity in a developed nation is also affected quite dramatically by television broadcasts. During a major sporting event such as an international football match, sudden upsurges in demand will occur at half-time and full time, when viewers switch on their kettles.
In the U K this can impose a sudden rise in demand of as much as 2 GW, which is the equivalent to the total output of a reasonably large! Such a pattern of usage can be predicted to within a few minutes, and audience predictions are routinely fed to the power-genera- tion authorities on a daily basis to assist with the provision of adequate supplies.
But if the result of the match requires 'extra time' playing there will be two further peaks before the pattern of consumption returns to normal an hour or so after the end of the match. This type of demand is obviously not predictable. The Grid system has to be managed so that the demand for electricity is met within statutory limits at all times and under all conditions, and the available generating plant has to be used in the most economic manner.
Since the privatisation of the electricity supply industry in the UK, the generation of electricity is based on the demands of a trading system known as the Pool. This is briefly described below, because the operation of the Pool determines how each unit receives demand instructions. The subject is of critical importance because it governs the operation of the power plant and, ultimately, its demise. Although the following outline is based on the U K Pool, other countries use systems based on similar princi- ples.
These bids are then ranked nation- ally in a form of a league table, with the cheapest generator at the top and the most expensive at the bottom. This table is termed the 'merit order' for all the generating units that are capable of being connected to the system.
The details of each day's merit order are transmitted to the body respon- sible for operating the Grid system, the National Grid Control Centre, and to the body responsible for the trading systems. The National Grid Control Centre determines a notional schedule ol the generating plant that is available and, on the basis of this information, develops a system marginal price SMP for every minute period of the day in question.
The SMP is combined with a component which reflects the scarcity values of generating plant, and from these factors is deter- mined the selling price for power, the 'pool selling price'. This takes intc account the operational costs of the system, known as the 'uplift costs'. The uplift includes factors such as the cost of maintaining a security margin oJ available power above the demand, the cost of ancillary plant that L, required to maintain voltage and frequency, transmission constraints etc.
Setting the demandfor the steam generator 53 The National Grid Control Centre examines the security of the trans- mission network that links all the participating generating plant and consumers, and then plans its operations to ensure that the entire system operates securely and efficiently.
At the end of this process, the Centre issues instructions to all power stations connected to the network, setting the generation demand for their units on a minute-by-minute basis. It is these commands which determine the earnings of the plant, and within any given plant the boiler and turbine must respond to them in the most efficient and reliable manner possible. The load allocated to a unit will be based both on the cost of gen- eration and on the ability of the plant to respond to demand changes.
Under the right conditions, a unit whose operational costs are high but which responds quickly will be as likely to receive a load as one which generates very cheaply but is slow to respond to changes in demand.
These differ quite significantly from each other. The turbine, in very general terms, is capable of responding more quickly than the boiler to changes in demand. The response of the boiler is determined by the thermal inertia of its steam and water circuits and by the characteristics of the fuel system. For example, a coal-burning boiler, with its complex fuel-handling plant, will be much slower to respond to changes in demand than a gas-fired one. Also, the turndown of the plant the range of steam flows over which it will be capable of operating under automatic control will depend on the type of fuel being burned, with gas-fired units being inherently capable of operating over a wider dynamic range than their coal-fired equivalents.
The common factor in all these systems, however, is the master demand which, in addition to setting the firing rate, regulates the quantity of combustion air to match the fuel input, and the quantity of feed water to match the steam production.
In the present chapter we shall examine the master system. Chapters 3, 5, 6 and 7 look at how the commands from the master system are acted upon by the fuel, draught, feed-water and steam- temperature systems. The design of the master system is determined by the role which the plant is expected to play, and here three options are available.
Each of these results in a different performance of the unit, in a manner that will now be analysed. The principles of this system are illustrated in simplified form in Figure 4.
In such a system, the plant operates with the turbine throttle-valves partly closed. The action of opening or closing these valves provides the desired response to demand changes. Sudden load increases are met by opening the valves to release some of the stored energy within the boiler.
When the demand falls, closing the valves increases the stored energy in the boiler. In such a system the turbine is the first to respond to the changes. The boiler control system reacts after these changes have been made, increasing or reducing the firing to restore the steam pressure to the set value.
J Generator Figure 4. Particularly in the case of coal-fired plant, this method of operation offers slower response, because the turbine output is adjusted only after the boiler has reacted to the changed demand. However, the turbine-following system enables the unit to be operated in a more efficient manner and tuning for optimum performance is easier than with the boiler-foUowing system.
It is worth considering for large base-load power plant where the unit runs at a fixed load, usually a high one, for most of the time , or with gas-fired plant where the response is comparatively rapid. This is a sophisticated technique, which has come into its own with the development of powerful, fast, and versatile computer systems. It combines the best features of both the boiler-following and the 6 L Figure 4. However, its design demands considerable knowledge of the characteristics and limitations of the major plant items.
Also, commissioning of this type of system demands great skill and care if the full extent of the benefits is to be obtained. In particular, the rate-of- change of the demand signals, as well as the extent of their dynamic range, will need to be constrained to prevent undesirable effects such as the stressing of pipework because of excessively steep rates-of-change of tem- perature. Because of the nature of its operation, the details of a co-ordinated unit load control system have to be finely matched to the configuration and characteristics of the plant to which it is fitted.
Unless it is regularly readjusted, it can suffer from an inability to recognise and deal with the steady deterioration in performance that inevi- tably occurs in each item of plant as it ages. Unfortunately, for many practical reasons it is not universally used.
In older plant this type of master configuration may not be available or practical. Where the co-ordinated unit system is not available, the choice lies between using boiler-following or turbine-following control. Although they both orchestrate the operation of the boiler and the turbine to meet changes in demand, the performances of these configurations differ very considerably from each other, as is explained below.
Therefore, by using the turbine's ability to respond more rapidly, the boiler-following system provides a better response to load changes than the turbine-following system. After the turbine has responded to the change in demand, the boiler is commanded to follow on, correcting the steam- pressure error as quickly as it can.
However, such rapid response is only available for small-scale demand changes, that is, changes that are within the capacity of the allowable range of pressure-drop across the throttle valve. Also, the rapid response is obtained at a cost.
When operated in this way, the efficiency of the unit is inevitably reduced because of the pressure that is dropped across the throttle valves. The losses are reduced by decreasing this pressure drop, but this also reduces the scope for meeting sudden changes in demand.
Another problem is that it is not easy to tune the control parameters of a boiler-following system to obtain optimum overall performance, mainly because of the interaction between steam pressure and steam flow that occurs as the turbine and boiler respond to changes.
The first response is for the throttle valves to be opened. This increases the power generated by the machine, but it also results in the boiler pressure falling, and when this happens the boiler control system reacts by increasing the firing rate. This is all right as far as it goes since, quite correctly, it increases the boiler steaming rate to meet the increase in demand.
However, as the firing change comes into effect and the steam pressure rises, the amount of power that is being generated also increases. But as it has already been increased to meet the d e m a n d - - a n d in fact may have already done s o - - t h e power generated can overshoot the target, causing the throttle valves to start closing again, which raises the boiler p r e s s u r e. Various methods have been proposed to anticipate these effects, but these tend to increase the complexity of the system, and therefore its cost, with questionable long-term benefit.
However, when the fuel, air and water flows of a boiler are held at a constant value the amount of steam that is generated will not, in general, remain constant, mainly because of the inevitable var- iations that will occur in parameters such as the calorific value of the fuel, the temperature of the feed water etc. In the simple turbine-following system, these variations are corrected by modulation of the turbine throttle valve to maintain a constant steam pressure, but this results in var- iations in the power generated by the turbine.
Because the steam-generation rate of its boiler is not automatically adjusted to meet an external demand, a plant operating under the control of a simple turbine-following system will generate amounts of power that do not relate to the short-term needs of the grid system.
Such a plant is therefore incapable of operating in a frequency-support mode, although this mode of operation may be used where it is not easy, or desirable, to adjust the fuel input, for instance in industrial waste-incineration plants. Changes in this demand therefore change the boiler's firing rate, and a controller then modulates the turbine throttle valve to keep the steam pressure constant.
As might be expected, because of the slow reaction time of the boiler, this results in a slower response to load changes than that of the boiler-following system. This is a particular example of a 'co-generation' scheme: In the case of CCGT plant, the heat exhausted from a gas turbine is used to generate steam. In CHP plant, heat from a power station is used in another process. The heat may be taken from the power plant as steam extracted from the turbine, or it may be the heat abstracted from the condensate.
Co-generation plants are either 'topping' or 'bottoming' systems.
With the former, the first priority is to generate electricity, and as much use as possible is made of the heat that would otherwise be wasted in the process. With the latter, waste heat from some industrial process is used to generate electricity via a steam generator and turbine. A steam generator employed in a CHP plant has to serve two masters: In most cases the former predominates, because the entire raison d'etre of the plant was probably the need to serve a community or an industrial plant, and the plant's ability to generate electricity is of secondary importance even though, as a spin-off, it is extremely valuable.
For this reason, the development of a truly effective master-demand signal for a CHP plant is much more complex than it is with a plant whose only function is to generate electricity. The needs of all the users have to be taken into consideration, as must the cost of the steam, heat and electricity that is produced.
Furthermore, it is possible that the way in which the master demand is configured may need to be modified at some time over the life of the plant because of changes in fuel prices or alterations in the requirements of the industrial, commercial or domestic complexes which benefit from the process.This method provides a step-function type of control and fine adjustment of steam tem- perature is provided by spray-water attemporation.
A deaerator is a special case of the open feedwater heater which is specifically designed to remove noncondensable gases from the feedwater. Boiler stack economizers are simply heat exchangers with hot flue gas on one side and water on the other. The differential heating drives a global atmospheric convection system reaching from the Earth's surface to the stratosphere which acts as a virtual ceiling.
Also, it is not safe to rely on odour to detect leakages. There are two basic types of deaerators. Mechanical draft, which uses power driven fan, motors to force or draw air through the tower. It is caused by the density difference between the atmospheric air and the hot gas in the stack.
The type of mill to be used on a particular plant will be determined by the process engineers and it is the task of the control engineer to provide a system which is appropriate.
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