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Babcock & Wilcox 1

Development and Application of a Dynamic Simulation Model for aDrum Type Boiler with Turbine Bypass System

W.J. PeetBabcock & WilcoxCambridge, Ontario, Canada

Presented to:International Power Engineering ConferenceMarch, 1995Singapore

BR-1588

T.K.P. LeungBabcock & WilcoxCambridge, Ontario, Canada

AbstractDynamic simulation models using powerful personal

digital computers are a very cost effective tool for study-ing the operating characteristics of power plants to im-prove the design and control strategy to meet stringentoperational requirements. The required operating modesof large fossil fuelled generating plants call for continuedoperation after load rejection, and rapid, frequent and re-liable unit start-ups to achieve flexible and economic pro-duction of electricity. This paper discusses the develop-

ment of a dynamic simulation model and its applicationin the study and design of a new control philosophy andturbine bypass system to meet the operational require-ments.

Key WordsLoad rejection, temperature control.

IntroductionModern fossil fuel fired generating plants must be ca-

pable of sliding pressure operation, two-shifting, fast loadramps and continued operation under load rejection. Thereasons for this requirement vary among high nuclear baseload, limited interties within utility grids, flexibility, effi-

ciency and fuel cost aspects. The response and perfor-mance of main steam temperature control is one of theimportant criteria determining the capability of fast loadramps to avoid thermal stress-related fatigue of high pres-sure and temperature components. With the steam turbine,the critical areas in regard to thermal stressing are at theadmission of steam to the working parts, where high steamconditions necessitate the greatest wall and flange thick-ness for the casings and involve the highest temperaturedifferentials. For two-shift units, the basic requirement isto match progressively, within certain limits, the steamtemperatures with the turbine metal temperatures under

the various starting conditions to control the rate of metaltemperature change and thus to limit the thermal stresses.A turbine bypass system is one of the methods for im-proving start-up by allowing the boiler to build up match-ing steam temperature and pressure independent of theturbine. The turbine bypass system can also be used toallow the unit to continue operation after a load rejection.

Development and Validation of theDynamic Simulation Model

The advance of the personal computer and the devel-opment of dynamic simulation language have enabled thedevelopment of economic and versatile first principlesdynamic models. These models are essential for the studyof control strategies to be applied to either base loaded orcycling type plants.

The boiler simulated in the dynamic model is a Bab-cock & Wilcox Carolina style coal-fired drum type boilerutilizing a parallel downpass arrangement with biasingdampers for reheater steam temperature control. The tur-bine is a partial arc type high pressure cylinder with im-pulse stage. The maximum continuous rating of the plantis 650 MW, with the steam conditions of 170 kg/cm 2

throttle pressure, 538C superheater temperature and 538C

reheat temperature. The schematic of the numerical modelof the plant simulated is shown in Figures 1 and 2, for theboiler model and the balance of plant respectively, andidentifies the components modelled. The major controlloops modelled include the following:

Drum level and feedwater flow control Coordinated boiler turbine control for firing demand

and turbine governor valve demand Main steam temperature control Reheat steam temperature control HP turbine bypass system control LP turbine bypass system control


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The implementation of the plant model was performedon an IBM compatible 80486 66 MHz personal computerusing PC-TRAX simulation software.[4] The thermody-namic parameters for the numerical model are set at one

Figure 3 Fuel flow step change (100%-95%-100%) open loop response.

equilibrium condition and the physical parameters repre-sent all the storage terms (masses and volumes) to repli-cate the dynamics of the physical unit. The physical andperformance data for the plant modelled were obtainedfrom data information sheets, design data and heat bal-ance diagrams provided by the turbine and boiler manu-facturers.

Model ValidationThere are two levels of validation for a plant model:

static validation and dynamic validation.Static Validation: After proper tuning of the control

loops, the dynamic simulation model can be operated in astable steady state at various load points from 25% to 100%load. The steady state parameters at different load pointsobtained from the dynamic simulation model are comparedwith the design data from the turbine and boiler manufac-turers heat balances. The outputs from the model line upvery closely with the design data at 50%, 75% and 100%load with deviations of less than 1.3% in key parameters.

Dynamic Validation: The simulation software supplierhas performed dynamic validation for their simulationsoftware as detailed in their technical paper[1] in conjunc-tion with the Intermountain Power Project in Utah, USA.Agreement between the model and plant data during dis-turbances are generally very good. Despite the lack of fielddata available for this model, fuel step change and turbinevalve step change open loop tests were carried out andthe key parameters are shown in Figures 3 and 4. The re-sults are consistent with the characteristics obtained inthe Intermountain Power project. Tests run from 25% to100% load at constant and sliding pressure showed thatthe model was stable and repeatable over this range. Thisshows that the equations are suitably stable to provide validresults over the range of 25% to 100% load. The modelwas also run to a house load condition and cold start-up

condition and compared with the design data. The resultwas that it matched closely with the design data.Thus, the model can be used to study control system

design and analysis and to assist in the development ofoperating procedures under different disturbances.Figure 2 Schematic of drum boiler simulation model.

Figure 1 Schematic of turbine simulation model.


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Figure 5 Main steam temperature control. Figure 6 Unit coordinated control.

Steam Temperature ControlAn improved main steam temperature control philoso-

phy [2] was verified and refined with the dynamic simula-tion model. The control logic diagrams for this main steamtemperature control and its associated boiler/turbine co-ordinated control are shown in Figures 5 and 6. Steamtemperature control for drum-type boilers has been con-sidered one of the most difficult loops within the boiler

Figure 4 Turbine valve step change (100%-95%-100%) open loop response.

control system. The capability to maintain a small steamtemperature deviation over wide-load ranges or duringrapid load changes has limited the permissible rate of loadchange due to thermal stress limitations primarily associ-ated with the turbine. The mathematical derivation of thisphilosophy in the reference [3] has led to a different defi-nition of spray attemperation as follows:

MAIN STEAM PRESS

FLOW/PRESS%


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The spray attemperation is that mass flow which isequal to the difference between the actual saturatedsteam flow from the drum and the output steam flowderived from the total absorption in the unit.

For any given equilibrium condition, the steam flowrequired to the turbine is known from the unit load de-mand, as is the fuel input. With the correct mass flow leav-ing the boiler, the temperature, or more precisely, the en-thalpy, will also be correct. The only possible error wouldbe due to the boiler efficiency (actual vs. assumed), theeffect of which will be very small and can be compen-sated by controller action.

The major advantages of this concept may be summa-rized as follows:

Variations in firing rate due to load changes are auto-matically included in the control since any deficiencyin steam flow from the drum will provide the neces-sary increase in demand for spray.

Variations in excess air or gas recirculation to the fur-nace required for reheater steam temperature controlwill change the steam production from the drum, andthus the spray flow demand will be compensated au-tomatically.

Figure 7 Transient response for load ramp from 55% to 95% load at 5% per minute.

Figure 8 Transient response for load ramp from 75% to 95% at 20% per minute.


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Variations in furnace slagging conditions which re-duce steam production are automatically accountedfor in the spray water controls.

Upsets to the steam temperature control produced dur-ing sootblowing, or other auxiliary steam extractions,can be eliminated by providing a demand signal intothe steam temperature control to account for this ex-traction flow.

The performance of the new steam temperature con-trol based on the mass balance approach may be demon-strated by a 5% per minute ramp rate from 55% to 95%load and a 20% per minute ramp rate from 75% to 95%load. The results are shown in Figures 7 and 8.

With the dynamic simulation model, the conceptualcontrol circuitry has been able to be simplified and en-hanced so that the concept can be applied practically andreliably at site. Furthermore, the performance of the newsteam temperature control may be predicted by the model.In conjunction with the new steam temperature control, acorresponding unit turbine-boiler coordinating control isrequired to provide reliable and accurate feedforwardsteam flow signal for the new steam temperature control.In selecting a signal for the feedforward to the turbinecontrol valve, a number of possible signals were consid-ered and studied, including unit load demand, electricalload and fuel flow signals. It was found that only the fuelflow reflects the actual heat transfer available in the fur-nace, and thus can provide the best index for the turbinegovernor valve control.

HP/LP Turbine Bypass SystemThe arrangement of a typical turbine bypass system for

a drum-type boiler unit is included in Figure 2.As shown in Figure 2, the high pressure bypass draws

steam from the main steam lines, immediately before theturbine stop valves; passes it through a flow-control/pres-

sure-reducing valve; desuperheats it to a temperature simi-lar to that of normal HP turbine exhaust; and exhausts tothe cold reheat lines adjacent to the turbine. The low-pres-sure bypass takes steam from the hot reheat lines at theturbine interceptor valves; passes it through pressure-re-duction and desuperheating stages; and discharges to thecondenser. A small auxiliary bypass ventilates the HPcylinder to the condenser, normally from a tap at the HPexhaust, upstream of the exhaust non-return valves. Fastopening of the HP bypass is provided from a contact sig-nal, such as loss of export, load while normal opening ofthe HP bypass can be controlled by analog control to asteam pressure set-point. The HP bypass maintains a setupstream pressure. Similarly, the LP bypass opens on ei-

ther a contact signal or reheater overpressure, and main-tains a set reheater pressure. The auxiliary bypass open-ing is controlled by turbine speed and load to protect theHP turbine from overheating.

Operational requirements of the turbine bypass systeminclude:

a) start-up and run-up of boilerb) continued boiler operation in the event of sudden

load shedding or tripping of the turbine generatorc) continued operation of the turbine generator at house

load after sudden loss of export load without the need tolift a main steam or drum safety relief valve

d) a relief valve for partial turbine load rejections(power/load unbalance), eliminating the operation of theERV or spring loaded safety valves.

Sizing RequirementsThe sizing of a turbine bypass system depends on the

operational requirements of the plant and acceptability offluid loss from the cycle through safety valves or atmo-spheric dump valves. There are two main areas which haveto be considered to size the HP and LP turbine bypasssystem properly:

a) Start-up Requirements: The start-up of the boiler tur-bine is performed in a variable pressure mode and the tur-bine bypass system must handle the requirements, bear-ing in mind that the pressure drop in steam or velocitylimits for valves, piping, etc. are directly proportional tothe specific volume of the steam.

b) Load Rejection Requirements: The size of the by-pass system depends on the magnitude of the partial loadrejection which the boiler can sustain without tripping;the acceptability of fluid loss from the cycle during a loadrejection, either through high pressure or intermediatepressure relief devices; and the rate at which the firingcan be reduced in a stable manner.

Full Load Rejection AnalysisA full load rejection transient was performed on the

model based on the following procedures:1) The model was stabilized at the 100% load condi-

tion operating with the turbine bypass valves closed.2) The generator breaker was opened to simulate the

loss of export power. The house load of 25 MW remainsconnected to the generator.

3) The turbine governor and intercept valves closedfrom the power/load unbalance.

4) When the load rejection signal was detected, after a

time delay of one second the following equipment operated: The fuel demand was run back to the minimum millload for five mills (55% MCR input approximately)

The HP bypass valve was commanded fast open. The IP bypass valve was operated by pressure con-

trol with a set-point of 30 kg/cm2 abs at the cold re-heat inlet.

The deaerator steam supply was switched from tur-bine extraction (IP exhaust) to cold reheat supplythrough a control valve with a set point of 1 kg/cm2

abs deaerator pressure.5) The following automatic operations were executed: The steam supply to the boiler feedpump turbine drive

was switched from IP exhaust extraction to main

steam. The HP bypass and LP bypass spray attemperatorvalves operated to control the steam conditions down-stream of the valves to a temperature set-point.

6) The turbine valves remained closed until the turbine/generator speed returned to normal and then the interceptvalves reopened on speed control.

7) To prevent the HP cylinder from overheating, theventilation valve was opened to connect the HP exhaustto the condenser. This put the HP turbine under vacuum.

8) As firing rate was reduced by running back the de-mand of the pulverizers in service, the flow through the


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bypass system reduced permitting the HP and LP bypassvalves to modulate under pressure control.

9) Condenser cooling water was maintained constantthroughout the full load rejection.

At the end of the load rejection, the turbine/generatorwas operating in a stable equilibrium condition at houseload while the boiler was at 55% load with HP/LP bypasssystem controlling the main steam pressure to 170 kg/cm2

and the cold reheat pipe to 30 kg/cm2 as required. Theunit was then available for the operator to select any ofthe following:

a) Hold as is, with the boiler master on hand, houseload on the generator and bypass system in operation.

b) Reload the turbine and resume exporting load to thegrid.

c) Reduce boiler load by operator action to save fuelcosts.

Excerpts from the results are shown in Figure 9. Theresults of the simulation showed that while the model wasable to run to an equilibrium condition after the full loadrejection, the transient response of the condenser pres-sure (maximum 331 mm Hg abs) exceeded the low vacuumpressure trip point of 210 mm Hg abs. Figure 10 is a plotof the heat load to the condenser versus time during the

load rejection and shows that the maximum heat load dur-ing the transient is about two times the normal heat loadat the 100% load conditions. In order to maintain accept-able condenser vacuum during the transient, either the heatload to the condenser must be reduced, (i.e. a smaller tur-bine/bypass) or the heat removal from the condenser in-creased (i.e. larger condenser capacity). The dynamicmodel can be used to establish the criteria for either ofthe solutions outlined above. With the dynamic model, aproper full load rejection procedure has been developed.

Figure 10 Heat load to condenser during full load rejection.

ConclusionsThe examples of an improved main steam temperature

control and operation of a turbine bypass system are in-tended to demonstrate the value of dynamic simulationfor the investigation and development of new control phi-losophies and enhanced operational procedures. This is

particularly true for a simulation package which is userfriendly and PC-based. The ability to perform transientanalysis on the proposed equipment with the control al-gorithm prior to the finalization of the design phase as-sures the supplier and customer of satisfactory operationboth for the hardware and the controls.

References1. Application and Validation of Dynamic Model for

Intermountain Power Project by G.G. Doby, TRAXCorp.; W.R. Morgan, Intermountain Power Project.Presented in the International Conference on Power Plantand Power System Training, Simulators and Modeling,EPRI, 1991.

2. U.S. Patent No. 4,887,431 Superheater OutletSteam Temperature Control, Inventor: W.J. Peet, As-signee: The Babcock & Wilcox Company, New Orleans,Louisiana.

3. Improved Steam Temperature Control for DrumType Boilers in D.S.S. or Load Cycling Operation, byW.J. Peet/T. Leung, Babcock & Wilcox International,presented at the 9th CEPSI Conference, 1992.

4. Analyst Instruction Manual, TRAX Corporation.Figure 9b Transient response of full load rejection.

Figure 9a Transient response of full load rejection.

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