boiler instrumentation and control present ti on

8/2/2019 Boiler Instrumentation and Control Present Ti On

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Presented byMANOJ KUMAR MANDAL

Roll No-802056


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Instrumentation and controls in a boiler plantencompass an enormous range of equipmentfrom simple industrial plant to the complex inthe large utility station

The boiler control system is the means by whichthe balance of energy & mass into and out of theboiler are achieved. Inputs are fuel, combustionair, atomizing air or steam &feed water. Of

these, fuel is the major energy input. Combustionair is the major mass input, outputs are steam,flue gas, blow down, radiation & soot blowing.


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CONTROL LOOPS

Boiler control systems contain several variable with interactionoccurring among the control loops for fuel, combustion air, & feed water

. The overall system generally can be treated as a series of basic controlloops connected together. for safety purposes, fuel addition shouldbe limited by the amount of combustion air and it may need minimumlimiting for flame stability

Combustion controlsAmounts of fuel and air must be carefully regulated to keep excess airwithin close tolerances-especially over the loads. This is critical toefficient boiler operation no matter what the unit size, type of fuel firedor control system used.

Feed water controlsIndustrial boilers are subject to wide load variations and require quickresponding control to maintain constant drum level. Multiple elementfeed water control can help faster and more accurate control response.


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BOILER DRUM

TRANSMISSION

CONTROLLER

CONVERTER

VALVE

FIG .1 General Block Diagram of Boiler Drum


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BLOCK DIAGRAM DESCRIPTIONThe block diagram of boiler control is shown in figure

1.the output from the boiler i.e., the steam outputsand the level of water is given to transmitters. Theoutput of transmitter is given to the controller whichact as level indicator controller and flow indicatorcontroller. If there is any error corresponding todesired set point, the signal from controller is given

to the converter which will open or close the valveand the water will be drained out or filled accordingto required steam.

The major loops in boiler control are

1) Combustion control

2) Feed water control


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COMBUSTION CONTROLA combustion control system is broken down

into (a) fuel control and (b) combustion aircontrol subsystems. The interrelationshipbetween these two subsystems necessitate theuse of fuel air ration controls.

The primary boiler fuels are coal, oil and gas. Thecontrol of gas and oil fuels requires simplestcontrols- i.e., a control valve in the fuel line.

The steam drum pressure is an indication ofbalance between the inflow and outflow of heat.Therefore by controlling the steam supply onecan establish balance between the demand forsteam (process load ) and supply of water.


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FIG. 2 Combustion controls


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There are three general types of combustion control schemes used

today: They are series, parallel & series-parallel controls.

In series control, variations in steam header pressure(the mastercontrol signal) cause a change in combustion air flow which in turnresults in a sequential change in fuel flow. This type of control islimited to small boilers having relatively constant steam load &

burning fuel.

In parallel control, variation in steam pressure simultaneouslyadjusts both fuel & air flows. This method is common to any sizeboilers.

In series-parallel, variation in steam pressure set points are used toadjust the fuel. Flow to the above boiler since steam flow is directlyrelated to heat release of the fuel and hence the air flow, the steamflow can be used as an index of the required combustion air.


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The control hardware used to carryout the above schemes includeON/OFF controls, positioning & metering systems.

(a) ON/OFF controls:Are still used in many industries but are generally used in small watertube boilers. When the pressure drops to a present value, fuel & airare automatically fed into the boiler at predetermined rate untilpressure has risen to its upper limit.

(b) Positioning systems:Respond to changes in header pressure by simultaneously

positioning the forced draft damper and fuel valve to a predeterminedalignment. This is not used in liquid , gaseous fuel fired boilers.

(c) Metering control system:In this system control is regulated in accordance with the measured

fuel and air flows. This maintains combustion efficiency over a wideload ranges & over long period of time.

Both metering & positioning control systems use steam headerpressure as their primary measured variable & as a basis for firingrate demand. A master pressure controller responds to changes

on header pressure & positions the dampers to control air flow andfuel valve to regulate fuel supply.


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Feed water control is the regulation of water to the boilerdrum. It provide a mass accounting system for steam leadingand feed water entering the boiler. The water is admitted tothe steam drum and after absorbing the heat from furnacegenerates the steam produced by the boiler.

Proper boiler operation requires that the level of water in the

steam drum should be maintained within certain band. Adecrease in this level may uncover boiler tubes, allowingthem to become overheated. An increase in the level ofwater may interfere with the internal operation of internaldevices in the boiler drum. It is important to made that thewater level in the boiler drum must be above 50% all the time.

The water level in the boiler drum is related to, but is not adirect indicator of , the quantity of water in the drum. At eachboiler load, there is different volume in the water that isoccupied by steam bubbles. So if load is increased there aremore steam bubbles and this cause water to swell or rise,rather than fall because of added water usage.


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FIG 3. PARTIAL VAPORISATION IN EVAPORATING TUBES


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As system for feed water control must bedesigned to maintain the mass balance overexpected boiler load changes so that the levelin the steam drum remains within therequired limits for safe and efficientoperation. Control system complexity isbased on number of measured variablesused to initiate control action and include

single element ,two element,3

element andadvanced control schemes to improveaccuracy of final control action.

Contd:-


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SINGLE AND TWO ELEMENT CONTROL SYSTEMFor small boilers having relatively high storage volumes and slowchanging loads ,single element control system is used. It controlsfeed water flow based on drum level. Response is very slow because

a change in feed water flow takes a long time to show up the levelchange. As a result the steam drum causes water to increase anddecrease in volume, resulting in false measurements.The two element system overcome these inadequacies by

using steam flow changes as a feed forward signal. Thiscontrol is used in intermediate boilers as well as large boilers.Here the flow and level transmitters are summed by a computingrelay and will be the set point for feed water. Here the response isfaster.

THREE ELEMENT CONTROLBoilers that experiences wide and rapid load changes require threeelement control. Three element control is similar to two elementsystem except that the water flow loop is closed rather than open.Control action, the third element based on feed water flow. Thelevel and steam flow signals are summed and used as an index orset point to the feed water flow. The feed water flow measurementprovides corrective action for variation in feed water pressure.


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Three Element Boiler Control


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Types of steam

Here we are using two types of steam i.e.,saturated steam and super heated steam.Steam obtained while heating is calledsaturated steam. It is called wet steam since

moisture content is also present. Superheated steam is obtained when saturatedsteam is further heated above 220 degrees. Itis also called dry steam.


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ADVANTAGES1. Multiple element feed water control can help:

I. Faster response of systems.

ii. More accurate control.iii. Maximum system stability.

2. Metering control system maintains combustion efficiencyover wide. load changes and over long period of time.

3. Parallel combustion control can be used in any size ofboilers. DISADVANTAGES1. Boilers require quick responding controls.

2. Level of the water in the boiler must be kept above 50%of height.


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FUTURE DIRECTIONSMicrocontrollers & PLC are used as controllers.

CONCLUSIONSThe various goals of boiler control includes:

1. To minimize excess air2. To minimize blow down

3. To minimize steam pressure

4. To measure efficiency5. To find when to perform maintenance

boiler instrumentation and control present ti on

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