BOILER TUNING BASICS.docx

8/20/2019 BOILER TUNING BASICS.docx

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BOILER TUNING BASICS – PART I

On my frst project as a combustion control engineer, I was responsible or loop checks and orwatching the experts tune the system controls. The frst loop I tried to tune solo was the drum levecontrol. t that time the trend!tune program deaulted to a "!minute window, and no one bothered tomention to me that the proper time span to tune drum level control to is "# to $# minutes. I also%oomed in on the drum level, which has a normal range o &'( inches, though my trend range was

&$ inches. )inally, I did not know that drum level can be a very *noisy* signal, so the hours I spenttrying to tune out that noise were wasted.

+ventually, I got the bright idea to add a little derivative to the loop control. In the time it took toprogram #.#' as the derivative gain and then immediately remove it, the boiler tripped. Thus beganmy career in boiler tuning.

In the "#!plus years since my inauspicious debut, Ive had the opportunity to successully tunehundreds o boilers, new and old, that needed either a control loop tweak or a complete overhaul.

-any inexperienced engineers and technicians approach boiler tuning with a heavy hand and littleinsight into the inner workings o individual control loops, how highly interconnected they are withother loops in the boiler system, or what change should be expected rom the physical euipment theloops are to control. -y purpose in writing this article is to explore these undamentals and share my

experiences. I trust these insights will be o value to the power industry and specifcally to those whowant to tune boilers or rock!solid stability yet agility when responding to process changes.

What Constitutes Good Control?+very boiler ever built has its own set o peculiarities. +ven two boilers built at the same plant at thesame time to the same drawings will have uniue uirks and special tuning issues. I begin with adescription o the various boiler and subsystem control loops beore moving to good boiler!tuningpractices that are su/ciently robust to accommodate even minute di0erences between what shouldbe identical boilers.

)rom a pure controls perspective, the most important goal is to tune or repeatability o a value, not

the actual value itsel. 1e do not care that there are exactly $(",(23.( pph o uel going into theurnace4 we only care that, or a given uel master demand, we get the same amount every time. There will be process variation, o course, but the goal is to tune the controls to keep that variation assmall as possible and then tune or accuracy.

5oiler control processes are where I will begin. dditional control unctions outside the urnace will beexplored in 6art II in a uture issue o POWER.

Operator Controls The operators window into the control system is reerred to as a master or as a hand7auto station,control station, or operator station. The station is the operator interace to a given control loop and istypically a switch located on the control panel in older plants or accessible rom the operators

keyboard in those euipped with all!digital controls. Typically, the control station allows the operatorto move between manual and automatic modes o operation. ll o the control loops discussed in thisarticle combine to orm the set o controls that manage the key boiler operating unctions.

1hen a control loop is placed in manual mode, the operator will have direct control o the output. Inautomatic mode the output is modulated by the proportional!integral!derivative 86I9: controller. Inautomatic mode the operator usually has some control over the set point or operating point o theprocess, either directly or through the use o a bias signal. Occasionally, as in primary air;ow control,the set point is displayed either on the controller located on the control panel or on the computerscreen graphic display.


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is some digital logic that reuires the station to be interlocked to manual, as well as control outputtracking and set point tracking.

urna!e Pressure Control)urnace pressure control is a airly simple loop, but its also one that has important saetyimplications. The >ational )ire 6rotection ssociation 8>)6: codes, such as >)6 ?(@ 5oiler and)6 also reuires some additional logic or the urnace pressure control loop to ensure adeuateoperating saety margins. There should be high and low urnace pressure logic to block the I9 anrom increasing or decreasing speed, as is appropriate. )or example, because this an sucks ;ue gas

out o the urnace, on a high urnace pressure signal the an should be blocked rom decreasing speedand on a low urnace pressure signal it should be blocked rom increasing speed. On a very negativeurnace pressure signal, there should be an override that closes the I9 inlet damper or decreases I9an speed. The settings o these signals are determined by the boiler and an supplier during thedesign o the plant.

lso, on a main uel trip 8-)T: there should be -)T kicker logic. n -)T occurs when the burnermanagement system detects a dangerous condition and shuts down the boiler by securing the uelper >)6 and boiler manuacturer reuirements. 1hen uel is removed, the ;ame within the urnacecollapses violently, which can cause a lot o wear and tear on the boiler and related boiler euipment.It also presents the very real danger o an implosion. The -)T kicker should immediately reduce thecontrol output to the an8s: proportional to the load being carried at the time o the -)T and thenrelease the device back to normal operation.

I am constantly ama%ed at how well urnace pressure can be controlled, especially when you considerthe amount o uel and air being injected into a ball o fre many stories tall and the erocious andchaotic environment inside a boiler. The act that a well!tuned system can maintain urnace pressureto C #.( inches B"O is remarkable.

typical mistake made by boilers tuners is the use o very ast integral action to the urnace pressurecontroller. )urnace pressure changes uickly, but not instantaneously, so consider the si%e o yoururnace and the amount o duct work between the urnace and the ans as capacitance in the system,because air is compressible. I recommend restraint when tuning urnace pressure when it comes toadding integral gain. Interestingly, the eedorward or almost every boiler is on the order o #D to'##D in, and #D to ?#D out.

The trends in the ollowing fgures show what you should expect to see rom your urnace pressurecontrol. The plant rom which these data were taken uses both an inlet damper position and anspeed to control urnace pressure. )igure ' illustrates an I9 an tuning trend and the reaction o the I9ans and the urnace pressure to a change in set point.


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"# Blo$in% hot air# Induced drat ans are used to control urnace pressure and primary combustionair;ow. In this test, induced drat an and urnace pressure respond to a step increase in urnacepressure set point. Aource@ Tim Eeopold

Air&o$ and O'(%en Tri))orced drat ans are typically placed in automatic ater the I9 an master is placed in automatic.=sually, the )9 an master is only controlling air;ow4 however, some boilers are designed withsecondary air;ow dampers that control the air;ow. In this case the )9 an will control the secondaryair duct pressure to the dampers 8)igure ":.

*# a+orite trend# I typically monitor air;ow, O" content in the ;ue gas, and urnace pressure controwhen I tune air;ow. The particular response o those variables was observed ater a "#D loadincrease in coordinated control mode. Aource@ Tim Eeopold

ir and, conseuently, O" control are critical to the sae and e/cient operation o a boiler. The air;owsignal is normally measured in terms o a percentage and is usually not available in volumetric ormass ;ow units. The obvious uestion is, *6ercentage o whatF* The answer is the percentage oair;ow that is available rom a given an or system o ans. The actual measured pounds per hour oair does not matter, because air is ree, and the fnal arbiter o proper air;ow is the O " content in the;ue gas 8gases leaving the urnace:. 5ecause o variations in coal heat content, air temperature, and


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combustion conditions inside a boiler, we ensure proper burning by measuring the amount o oxygencontent in the ;ue gas, commonly reerred to simply as O".

6ulveri%ed coal has an interesting property@ =nder certain conditions o heat in a low!oxygenatmosphere, coal can sel!ignite or even explode. Thereore, personnel saety and euipmentprotection reuire boiler operators to maintain excess O" in the ;ue gas. The amount o excess O" isdetermined by the load on the plant and the type and design o boiler. Typically, the load signal usedis steam ;ow. In any coal!fred boiler, air;ow demand is a unction o the boiler fring rate or boilerdemand 8)igure $:. Gas! and oil!fred boilers have lower O" reuirements at higher loads.

,# E'tra air is a %ood thin%# typical O" set point curve or a coal!fred plant is a unction o boilerfring rate or boiler demand. -inimum levels o air are reuired so that reducing conditions in theurnace never occur. Aource@ Tim Eeopold

The term cross!limiting reers to the unction o uel ;ow that limits the decrease in air demand andthe unction o air;ow that limits the increase in uel demand. 1hen decreasing load, the air demandollows its lag unction and the uel demand ollows the boiler demand to ensure that there is alwaysmore air than uel going into a urnace so explosive conditions never develop inside the urnace1hen increasing load, the opposite is true. This is truly an elegant piece o logic.

The output rom the boiler master is the boiler demand. )6 codes:, and a unction o theactual uel ;ow. The cross!limited uel demand is selected rom the least o three signals@ boilerdemand unction, a lag o boiler demand, and a unction o actual air;ow. 1hen load is increased, airdemand ollows the unction o the boiler demand and the uel demand ollows its lag o the boilerdemand.

To develop the air demand or your boiler, hold your O" trim controller in manual at (#D output. t alow, medium, and high load, place your )9 an master, or secondary air;ow dampers 8i the boiler isso euipped:, and your uel master in manual. Then manipulate the air;ow until you fnd the amountthat satisfes your O" set point reuirement, using stack opacity as a reality check on the O " set point>ext, manipulate the air;ow characteri%ation curve as reuired to allow the air demand to eual or

slightly exceed the uel ;ow or boiler demand. Hecord the air;ow reuired or that uel ;ow and thenmove on to another uel ;ow setting. Three points should be su/cient or a good air;ow curve.

Typically, the air;ow measurement is a di0erential pressure taken in air ductwork and reuires asuare root in order to make it linear. +nsure that your signal is also temperature!compensated. +achboiler should have an air;ow characteri%ation curve that should be a virtual straight line. I it isnt, Iwould be concerned about unexplained *correction actors* or *magic numbers* that should not benecessary.

>ext, the characteri%ed air;ow is multiplied against a unction o the O " trim controller. The O" trimcontrol loop uses the set point curve, discussed above, plus an operator bias to calculate an O " setpoint or various loads. This set point is compared with the O " content o the ;ue gas used by the


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control system. It is best to have several O" measurements because o striations or variations otemperature and oxygen that are present across the stack cross!section.

9i0erent plants use di0erent measurement schemes, selecting the average, the median, or thelowest measurement to control. O" trim is designed to be a steady state trim o the air;ow. I you, oryour tuner, are trying to control air;ow with the trim controller, stop it. The O " trim controller shouldbe mostly integral action with very little proportional and no derivative gain. our time is better spentreworking your air demand curves or air;ow characteri%ation than attempting to tune the air;owusing the O" controls.

The output rom the O" trim control station then goes through a unction generator such that a #D to'##D input signal euals a #.? to '." output signal. This value is then multiplied against thecharacteri%ed air;ow. This means that the O" trim controller can adjust the air;ow &"#D. In someextreme cases this amount can be varied, but or most boilers &"#D is more than su/cient. The fnalresult is a signal reerred to as *O" trimmed air;ow.* This value is then used by the air;ow controllerto modulate the I9 ans or dampers.

5ecause O" trim control uses a primarily integral!only controller, it does not have the dynamiccapabilities o most controllers. s a result, there are times when the controller should not be allowedthe ull range o control. t low loads, typically less than $#D to $(D, output rom the O " trimcontroller should not be allowed to go below (#D but should be limited to some minimum setting sothat an air!rich atmosphere is always maintained in the urnace.

lso, when the lag unction in the cross!limited air demand is driving air demand, air;ow will lagbehind. That is, the air will remain elevated or a period o time as the load, and the uel ;ow,decreases. s a result, oxygen in the ;ue gas will spike up. I the O " trim controller is not limited, thecontrols would see the O" go higher than the set point and start cranking, cranking, cranking down.

Then, when the load gets to where the operators have set it and the uel ;ow is no longer decreasing,air;ow demand will catch up with the boiler demand, and the O " will uickly begin to all. Thecontroller will see the O " alling and begin to crank up. 5ut because there is very little, or no,proportional gain, it will take a long time to bring the air back. This can result in an unsae or, at theleast, a nerve!wracking condition.

The >)6 reuires some additional logic or the air;ow control loop. There should be high and lowurnace pressure logic to block the air;ow rom increasing or decreasing, as is appropriate. 5ecausethis an orces air into the urnace, on high urnace pressure, the an should be blocked rom

increasing speed4 on a low urnace pressure signal, it should be blocked rom decreasing.

lso, on an -)T there are >)6 and boiler manuacturer reuirements that must be considered. Oneimportant consideration is the need to hold the air in place or a time ater an -)T or i the air;owshould drop very low during or just ater a trip. The dampers should go to a ull open position shortlyater the loss o all )9 or I9 ans 8providing a natural drat air path:. -oreover, in the typical boiler aircontrol system, i the I9 an is placed in manual, then the )9 an is normally orced to manual. I the)9 an is in manual, then O" trim is orced to manual.

-ru) Le+el and eed$ater Control)eedwater is ed into the drum in a typical subcritical pulveri%ed coal C fred drum boiler via either a

series o valves in parallel with a series o constant!pressure eedwater pumps or a battery ovariable!speed eedwater pumps. I the eedwater level in the drum goes too high, water can becomeentrained in the steam going to the turbine and can cause catastrophic results. I the drum eedwaterlevel goes too low, the drum itsel can become overheated, possibly resulting in catastrophe.

)eedwater 8and drum level: control has two modes o automatic operation@ single! and three!elementcontrol. The drum level set point or both modes is set by the operator. In single!element control thedi0erence between the drum level and the drum level set point provides the error signal that is usedby the single!element controller to control the rate o water entering the drum by modulating theeedwater ;ow control valve. Three!element control governs the three variables, or elements, that areused in this control scheme@ drum level, steam ;ow, and eedwater ;ow.


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9rum level control uses a cascaded controller scheme consisting o an outer and an inner controller.Ateam ;ow is an indication o the rate at which water is being removed rom the drum. unction osteam ;ow is used as a eedorward to the outer controller. The drum level error is then operated onby the outer controller. The output o this controller is the eedwater ;ow set point. The di0erencebetween this set point and the eedwater ;ow is then operated on by the inner controller. The outputrom this controller is then used to modulate the eedwater ;ow control valve.

Three!element control is much more stable and robust than single!element control. The reason thatwe use single!element control at all has to do with the nature o the instrumentation. Typically

eedwater ;ow, and occasionally steam ;ow, is developed by using a ;ow!measuring device like anorifce plate or a ;ow no%%le, where ;ow rate is proportional to di0erential pressure. Bowever, aproblem occurs at low ;ow rates 8low boiler load:, where di0erential pressures are not as solidlyproportional as we would like and thereore untrustworthy or boiler control.


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.# Rapid responder# typical coal!fre boiler with a properly tuned drum level control will respondvery uickly to a substantial load increase 8top: or load decrease 8bottom:. The dynamic response oother key variables in boiler drum level control system is also illustrated. Aource@ Tim Eeopold

ou may notice that as the load decreases, the drum level sags downward, and as the load increases,the drum level is slightly elevated. This means that the steam ;ow eedorward is just a tad toostrong. minute adjustment to the eedorward signal can add stability to the control loop 8)igure (:.

/# S)all is 0i%# small increase in the eedorward signal added more stability to the drum leve

controls. Only very small incremental changes in eedorward should be made when tuning drum levelcontrols. Aource@ Tim Eeopold

Superheat Te)perature ControlAuperheated steam temperature control is very straightorward. >ormally, steam leaves the drumand travels through a primary superheater8s: beore entering the desuperheater, whereattemperating water is mixed with the steam to modulate its temperature beore it enters the nextsuperheater section. ter the steam passes through that superheater, the outlet temperature ismeasured.


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I the inlet temperature to the superheater is a measured variable, the preerred method o control isa cascaded loop. In this case the outer controller uses the superheater outlet temperature as theprocess variable. The output rom the outer controller is the inlet temperature set point. The outputrom the inner controller is spray water demand. I the superheater outlet temperature is the onlyavailable measurement, then we are orced to use a single!element control loop. In either case, it isimportant that the controls are euipped with a eedorward signal.

variety o signals can be used or the superheater temperature control eedorward. =sually, theboiler demand is a good starting point or the eedorward because this signal anticipates the

measured temperature signals. -y experience is that the boiler demand usually has a well!defnedrelationship with the superheater temperature.

Other measured variables are available to supply the eedorward signal. Throttle pressure is usuallyused in tandem with the throttle pressure set point as an indication o over! or underfring o theboiler, but throttle pressure is transient in nature. ir;ow versus uel ;ow or steam ;ow may be usedin the same way. The ratio o uel ;ow to the top mill versus the other mills is a good indicator o thechanging dynamics in the boiler, especially i the boiler is large and has many burner levels. In thiscase it is a good rule o thumb to think o the top elevations as a0ecting temperature more thanpressure, and the lower elevations as a0ecting steam pressure more than temperature. )inally, thereheater temperature control a0ects the superheater temperature to a greater or lesser degree,depending on the type o boiler manuacturer and its method o control.

The eedorward signal development may include both static and dynamic unctionality. The staticcases are basically a unction o the variable that you are using. 9ynamic eedorward reers to aderivative kick based on the movement o the chosen variable. )or example, the ratio o air;ow tosteam ;ow might be used as an indicator o the boilers movement up or down, and the eedorwardthen can be manipulated accordingly.

6atience is a virtue when tuning these eedorwards, because steam temperature processes mayhave long time constants.

-eaerator Le+el ControlIt is oten possible to use a three! element controller or deaerator level control. 1hereas the drumlevel controls use drum level, steam ;ow, and eedwater ;ow, the three!element controller or the

deaerator uses deaerator level, eedwater ;ow, and condensate ;ow.

It is usually not necessary to provide adaptive tuning or this control loop, but do add it i possible.

Reheat Te)perature ControlIt is an interesting act that superheater spray adds to the e/ciency o a unit but reheater spray ;owdecreases the units e/ciency. -aximum boiler e/ciency is always the goal, so boiler manuacturershave developed alternative approaches to control reheat steam temperature.

5abcock K 1ilcox uses a gas recirculation an to move ;ue gas rom the outlet o the boiler back intothe urnace, either directly or through the secondary air wind box. -ore recirculation yields higherurnace temperature and, thereore, higher steam temperatures.


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the spray controller. =sually, the spray set point is set higher than the primary reheat temperaturecontrol set point beore the sprays are enabled, to reduce the reheater spray ;ow.

6art II will look at uel ;ow control, pulveri%er air control, and overall plant control options such asboiler! and turbine!ollowing modes and plant coordinated control.

CTim Eeopold 8tim.leopoldLhotmail.com: is a feld service engineer with 55 and has more than "#years experience tuning controls on power plants around the world. Bis book You Can Tune a BoilerBut You Can’t Tuna Fish is slated or publication in -arch.

BOILER TUNING BASICS – PART II

Boilers have enormous thermal mass and are relatively slow to react. Turbines are nimble and

uic!ly answer an o"erator’s command. Coordinatin# an entire "lant reuires an intimate

!nowled#e o$ both systems and selectin# the ri#ht lo#ic tools to brin# them to#ether.

The ront end, in the jargon o the power plant controls engineer, consists o the boiler master

and turbine master. s explained in 6art I o this two!part series, the operators window into the

control system is reerred to as a station or master, and it provides the operator interace or a

given control loop. ccess to that loop is typically rom a switch or hand station located on the

control panel in older plants or, more commonly, the operators keyboard in plants ortunate

enough to be euipped with digital controls.

The best case is when both the turbine and boiler masters are in the distributed control system

89


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Ta0le "# Options or plant boiler control. %ource& Tim 'eo"old

In general, the boiler master will be either in auto or manual control mode. The turbine is another

matter. Turbine controls generally have a number o stand!alone loops M such as megawatt,

pressure, valve position, or speed M which are control loops that do not respond to the 9


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have any o your gains disproportionately high. 1e use the derivative because we are trying to

anticipate the steam pressure deviation.

The eedorward signal is an important part o this control loop and is oten reerred to as target

steam ;ow. Target steam ;ow is the measured steam ;ow multiplied by the ratio o throttle

pressure setpoint to throttle pressure. Typically, there is a unction generator designed such that

#D to '##D o the input signal is proportional to a #D to '## D output signal. The nicely

dynamic nature o the ratio helps the boiler master move in the right direction. dditiona

*kickers* may also be available. One option is a throttle pressure setpoint kicker that adds a littleto the eedorward signal i the setpoint is changed. The derivative action o the controller also

acts as a kicker.

Turbine)Followin# (ode. In many ways, this is my avorite plant operating mode, because it is

the easiest to tune. It also o0ers a good strong saety net to operators in times o crisis. In

turbine!ollowing mode the boiler master is in manual and the turbine master is in automatic

mode. The turbine master controls throttle pressure by modulating the turbine governor valves.

-egawatts are then produced in the generator and pushed to the grid as a unction o the boiler

load.


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*# Under !ontrol# Taming a control loop that switched out the integral control on a load ramp.

%ource& Tim 'eo"old

9uring a change in unit load demand, in coordinated control, it is common practice to decrease

the integral action o the boiler master controller to %ero until the load ramp is fnished. This

strategy was used in all o the turbine and boiler master controller modes. This is a case where

more is defnitely not better4 there was a touch o eedorward, based on boiler demand

substantial proportional gain, and no integral gain when I looked at the logic. Tuned as it was, the

error signal between throttle pressure and throttle pressure setpoint will never go away.

I tried to tune out the error without success. lthough the error decreased, as shown in )igure ",we soon discovered that the tuning was not robust under all operating conditions. 1e thendownloaded the necessary logic modifcations 8the second white vertical dotted line:, causing theunit to drop out o turbine!ollowing and into base load mode, and then back again. 1hen thelogic modifcations were made, rom that point on 8the third white vertical dotted line: you cansee good control o the throttle pressure. This is how a well!tuned turbine!ollowing mode shouldoperate.

Coordinated Control 2ode


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I there is no coordination between the boiler and turbine controls, they will fght each other tothe death. The boiler really cannot do much more than control throttle pressure, and even then itis slow because o its massive thermal capacitance.

The turbine valves are much aster and are capable o controlling both megawatts and pressure The valves tap into the boilers thermal capacitance when the plants load changes. These ratiosocus the turbine controls on megawatt production with the megawatt setpoint and throttlepressure are near the setpoint. 1hen deviations occur, the throttle pressure error becomes moreimportant and slows the turbine down, moving it in the opposite direction that a pure megawatt

controller would demand. ma%ingly, or all boilers 8drum or once!through, coal! or gas! or oil!fred: this rule o thumb will give you a good solid starting point to begin tuning the ront!endcoordinated mode controls.

>ext comes the tuning o the controllers. In general, the turbine master is the easier o the twocomponents to tune, so that is the one to attack frst. The gains will be less aggressive than wereused or the turbine!ollowing mode, but it is good practice to have the turbine master controlthe megawatts as tightly as possible at frst. I that response is too much or the boiler to handle,the tuning can be loosened up later. >ote that this will only be proportional and integral tuningwith no derivative action.

The key to tuning the boiler master is balancing the proportional, integral, and derivative actiono the controller so that the pressure is maintained with good control, moves toward the setpoint

in a timely manner, and correctly anticipates the movement o the error signal. In general, theproportional gain will be airly large, the integral action slow, and the derivative gain in thecontroller should be relatively small.

)inally, the controls that make up the coordinated ront end may use some eedorward and thevarious kickers that are part o it. The eedorward signals to both the turbine and the boilermaster controllers, in coordinated mode, is a unction o unit load demand.

Tunin% 3or Unit Response=nit load demand is the high! and low!limited and rate!limited version o the unit masterdemand. The operator enters in his target megawatt load into the 9


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is or the turbine to provide megawatts and the less swing will occur when the load change isfnished. Aome boilers are well behaved and very responsive, so this kick is minimal. Aomeboilers are not well behaved, and their kickers can be pretty substantial. There can be otherkickers, possibly based on the throttle pressure or the throttle pressure setpoint kicker, asdescribed or the boiler!ollowing mode.

Pra!ti!al Controls 2a%i!

The tuning process cant be rushed and does take some time to get right. Bere is an exampleHecently, I walked into the control room o an ?##!-1 unit just as the operators made a loadchange. s you can see, the response o the unit let something to be desired 8)igure $:.

,# Unresponsi+e. load change on this ?##!-1 unit showed poor response and controls in

need o a good tuning. Aource@ Tim Eeopold

5y the third day, the coordinated controls were responding well ater I slightly decreased theintegral and proportional gain and increased the derivative action o the controller by about "(DI also modifed the eedorward signal slightly. )igure J illustrates the unit response to a $($!-1load increase test. bout halway through, the operator was unable to start an induced!drat 8I9:an, so he changed to base mode and then to boiler!ollowing mode. 1hen the I9 an was fnallystarted, he returned to coordinated control mode. s you can see in )igure J, a reuest wasreceived by the ront end to increase load just ater the operator decided to raise his throttlepressure. This well!tuned boiler sailed through each test with rock!solid perormance.


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.# Ne$ lease on li3e. The same ?##!-1 unit as in )igure $ showed much better response to aload change ater tuning the proportional and integral gain and increasing the derivative actiono the controller by "(D. Aource@ Tim Eeopold

Run0a!4s and Rundo$ns The fnal phase o tuning is runback testing. Turbine ollowing is a nice sae place to retreat towhen the operator has the time to take action. Bowever, what happens when there is no time toreactF

)or these situations two control strategies are used@ runbacks and rundowns. runback is anaction taken on a loss o a major piece o euipment. Typical runbacks include coal eeders,boiler eed pumps, or any plant an M induced drat, orced drat 8)9:, or primary air.

rundown is a reaction to a large process error that does not go away, such as a major boilertube rupture. In this incident, the eedwater pumps pick up the increased eedwater demand or

the eedwater valve goes completely open, but the drum level keeps dropping. +ventually, theplant must initiate a rundown or reduction in steam generation rather than trip the boiler. Typicalrundowns are associated with air ;ow, urnace pressure, uel ;ow, eedwater ;ow, or drum level.

Hundowns are seldom tested, on purpose, and thats not because they are overlooked. Hather,the logic decides i the boiler or the turbine can or should respond. I the uel master is in autoand looking at the boiler master or its output, then the boiler is capable o responding, and thereis no need or the turbine to respond. I the turbine is not looking at the ront!end controls or itsoutput and the uel master is not in auto, then the only device that can respond is the turbine,and so it does. This last scenario has a very high potential or tripping the unit.

=sually, the uel master will be in auto. The boiler demand is then reduced by the rundown logicrom where it was to some value that allows the error that is driving the rundown to all below

some preset limit. I the error does not go down, the rundown will continue to reduce boiler loadto a set minimum value.

The frst runback logic that I ever came into contact with was very severe. On a loss oeuipment, the boiler controls would attempt to stay in coordinated mode. The unit load demandwould run down, at some preset, ast, rate. This would decrease the boiler demand and thedemand to the turbine governor valves. That worked all right or some boilers, but the rate thatwas necessary or the boiler to get to a sae operating load was very ast. The di/culty is thatthe turbine governor valve would close down at the same rate. 1hen these valves close, themain steam pressure must climb and may eventually lit the boiler pressure saety relie valves.

This is very hard on the drum level and your ears, and oten results in a master uel trip. Granted


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it was a trip rom a lower boiler load, rather than i we had otherwise simply tripped the boiler,but it was a trip nonetheless.

s a result, what I like to call a kinder, gentler runback was developed. Aome call it the turbine!ollowing runback, where the boiler switches to manual on the loss o a piece o euipment. Iyou are in coordinated mode, the boiler should go to manual control and turbine!ollowing modeor the steam turbine. t this time, the runback logic reduces the boiler demand to apredetermined level at a preset rate. In the meantime, the turbine is ree to control the mainsteam pressure. The megawatt load is then gently reduced, and the plant experiences a sot

landing. Turbine!ollowing is the best mode to select in an emergency.

urther goal o a runback is to recover automatically so the operators can fgure out whathappened to the euipment and fx it while the unit is still online and avoid a master uel trip.

The data shown in )igure ( were collected during an actual runback test on a N(!-1 plant thatoperated with three pulveri%ers. The runback occurred when an I9 an was tripped, which hadthe e0ect o tripping one o the )9 ans. The runback o the boiler was set to a point that wasbelow the three!mill minimum load or sae and stable operation. s a result, automatic milltripping on a runback was developed.

/# A+oidin% unit trips. runback test is necessary when any changes are made to boiler gas

pass, ans, or mills. In this test o a N(!-1 unit, the runback occurred when an I9 an was

tripped. Aource@ Tim Eeopold

ou can see the boiler demand dropping, and the uel ;ow percentage dropping even urther as

one o the three mills is shut down by the runback logic. The pulveri%er master 8coal masterdemand: picks up momentarily as the mill is stopped, then ramps back down, eventually gettingthe uel percentage down to the boiler demand. utomatic mill tripping is generally a good idea,especially on larger units with a lot o mill capacity. lso, notice how the turbine pushes thethrottle pressure back to the setpoint. 9rum level also dropped slightly beore it recovered. Theentire runback occurred in just over two minutes. )igure 3 is a longer view o the entire episode.


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5# 2an( )o+in% parts. The same runback test 8)igure (: o a N(!-1 unit but with a longertime!span is illustrated. Bere you can see the pulveri%er master ramping back and the loweringo the turbine operating pressure setpoint. Aource@ Tim Eeopold

In this test, as is true or most o the tests I have run over the years, the an and uel runbacksare easily handled by the turbine!ollowing runback logic. Bowever, the boiler eedwater pumprunback can be another matter. The turbine valves are relatively slow to respond and tend tosuck steam rom the drum. Though some boilers are able to survive this without tripping on lowdrum level, many cannot.

s a result, new logic was developed. I like to call this special type o runback the separatedrunback. On the loss o a boiler eed pump, the boiler master goes to manual, coal mills are

tripped, and the boiler demand is driven to minimum. The turbine master remains in auto to stayin turbine!ollowing mode. t this point, we add a special high!limit override enabled during thisrunback that overrides the turbine!ollowing controller and marches the governor valves to apredetermined position. The rate at which the valves are closed is variable and depends on thethrottle pressure. Bigher pressures tend to depress the drum level, which we do not want, andreally high pressures lit saeties, which started us on this runback logic journey in the frst place.

I you plan to test your runback logic, its a good idea to elevate the drum level a ew inchesbeore your test. t this same N(!-1 plant, we tested the boiler eedwater pump runback usingseparated runback logic rom 2(D load with the drum level rundown initiated when the runbackwas complete. )igure 2 data illustrate this successul test rom the eedwater perspective. >oticethe action o the eedwater control valve. The drum level dropped about 3.( inches. The lowdrum level trip was set at 2.2 inches. That was successul, but a little too close or comort.


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6# -i7erent perspe!ti+e# The same runback test 8see )igure (: o a N(!-1 unit but rom theperspective o the eedwater system. >ote the drum level response. %ource& Tim 'eo"old

When Enou%h Is Enou%hOne o the big challenges aced by a boiler and turbine controls tuner is to know when to stop. Its a

job that has no defned stopping point, and there are always ways to urther improve perormance.

Ao how do we know when boiler tuning is fnishedF Typically, I call it uits when the operators are

satisfed and, based on my experience, the plant is as good as other units Ive worked on over theyears. Or, in the words o Aupreme

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BOILER TUNING BASICS.docx