Gas Fired-Boiler Control Scheme
Ⅰ. Boilercombustion control system
This device contains three 70T/H gasfired-boilers (A, B, C) fueled by natural gas. They produce 3.8Mpa steamrespectively and the steam assembles in the main tube to drive steam turbineand process steam.
Boiler combustion control system is one of themost important control systems in boiler systems, and this system is a cascadestage made of major tone of main tube steam pressure and assistant tune of fuelgas quantity and combustion air quantity—ratio regulating system, toguarantee appropriate combustion under various combustion loads—air ratio.
(1) Guarantee the stability of the main tubesteam pressure by controlling fuel of the gas fired-boiler.
(2) When the gas fired-boiler loads up, firstadd air then fuel gas; while first reduce fuel gas then air when it unloads.
(3)Increase the warm-pressing additions and corrections for fuel gas flow.
Control function design:
Main loop to stabilize main tube steam which ismade of main tube steam pressure PIC001, works out how to change each presentboiler’s fuel gas quantity and air quantity. The secondary loop consists offuel airflow regulator FIC811 and air mass flow regulator FIC812.
Precedent and hysteretic functions should berequired technically to conquer steam load perturbation, namely first add airthen fuel gas when loading up while first reduce fuel gas then air whenunloading. In order to achieve this goal, high signal selector FX0812 and lowsignal selector FX0811 are set. When the system is in stability, input modelsof FX0812 and FX0811 are the same; once perturbation occurs, output of PIC01A changes. If it is a change ofincrease, only high signal selector FX0812 can be gotten through, and thissignal stands for fuel gas flow, which can turn into air dose required as FIC812’s external set point by RL0812 plusair-fuel ratio coefficient. FIC812 indeed increases air mass flow, namely whenmeasured value PV from FIC812 increases, this value, which turns into fuel gasflow by RL0811 divider to divide air-fuel ratio coefficient, is sent to lowsignal selector FX0811; low signal selector’s input signal from the other wayis the increased PIC01Aoutput, then the set value of fuel gas regulator rises to open wide fuel gasvalve which leads to increase of fuel gas flow and achieves the goal of firstadding air then fuel gas as loading up. When perturbation occurs, output of PIC01A decreases, then this signal will leadto decrease of fuel gas flow by means of low signal selector FX0811 to FIC811’s external set value, namely when PVvalue of FIC811 decreases, this signal is sent to high signal selector FX0812,while FX0812’s input signalfrom the other way is the decreased PIC01A’soutput, so PV value of FIC811, which is transformed into air dose as airregulator FIC812’sexternal set value first by FX0812 then multiplying by air-fuel ratiocoefficient, leads to decrease of air dose and achieves the goal of firstreducing fuel gas flow then air as unloading.
Gas fired-boiler has three furnaces A\B\C, butonly one main tube manometer indicates PIC001. Therefore, we set inside threePID regulation meters for PIC01A,PIC01Band PIC01C respectively asregulators for three furnaces A\B\C. The measured values PV are still main tubepressure and the same with indications from three inner meters, while underautomatic status, these three set values SV are all equivalent to SV valuesfrom main tube pressure PIC001 which separately have two states AUT/MAN andrely on SEQ to realize by operators’ setting PIC001’s value instead of switchover by manual operation.
Air-fuel ratio control may be achieved
Under some circumstances, like when fuel gasvaries, air-fuel ratio can be changed by air-fuel ratio given controller FL0811to attain air dose-fuel gas flow ratio in line with production operationrequirements.
Particular self-protection capacity is included
If air dose is not enough, fuel gas willaccumulate inside combustor to endanger safety, which is not allowed. Thisregulating system can decrease fuel gas flow timely by divider FL0811 andlow-pressure selector FX0811 when air dose drops, while it will add air dosetimely by high-pressure FX0812 and multiplier RL0812 when fuel increases. So whensituation mentioned above happens, fuel gas into combustor won’t be excessiveas a safeguard.
Fuel gas’s temperature TI0880 and pressureLPS816 conduct T.P additions and corrections on fuel gas flow FI0811. Whenmeasured signals of pressure and temperature are normal, data is collectedevery 5 seconds to participate to warm-pressing compensation; once measuredsignals of pressure and temperature are abnormal, data collection stops, andwarm-pressing compensation adopts data before abnormal. So it won’t affectwarm-pressing compensation when inspecting and verifying pressure andtemperature.
Safety precautions
(1) FI0811andFIC0811 simultaneously start calibration with linkage
Function: one starts calibration while theother starts it as well automatically
One gets rid of calibration while the otherdoes it as well automatically.
(2) WhenPIC01A is in automaticstate, sequence control arithmetic expression will be executed:
Make SV (PIC01A) =SV (PIC001)
(3) When FIC811and FIC812 are not in thecascade stage, manual operation will be started by PIC01A, and air-fuel ratio meter will follow thepractical air-fuel ratio; PIC01Aoutput will follow the set SV of FIC811.
(4) WhenFIC811and FIC812 are in the cascade stage, automatic operation will be startedby PIC01A, and make SV (PIC01A) =SV (PIC001)
(5) WhenFIC811 is in the cascade stage while FIC812 inputs open circuit, automatic
operation will be started by FIC811 and manualoperation will be started by FIC812;
When FIC812is in the cascade stage while FIC811 inputs open circuit, automatic
operation will be started by FIC812 and manualoperation will be started by FIC811.
(6) WhenPIC01A is in automaticstate, if either of FIC811 and FIC812 is cut from cascade stage, the other willbe cut from it as well automatically, then, PIC01A will start automatic operation while CRT will givean alarm.
(7) Whenpositive and negative deviations of PIC01Aare abnormal, automatic operation will be started by PIC01A while alarm will be given by CRT.
(8) WhenFIC811or FIC812 is in the cascade stage and the practical air-fuel ratio PV (FL0811)<=1.0S, FIC811 and FIC812 will get rid of the cascade stage to operateautomatically while CRT will give an alarm.
(9) Whenall PIC01A, PIC01B and PIC01C are not in automatic operation, make SV(PIC001) =PV (PIC001) —main tube pressure set value follows actual value.
(10)After the compressor shuts down, decrease the loads from these three furnacesto minimum and do not stop the compressor.
Namely FIC811 and FIC812 all start manualoperation, and MV (FIC811) =10%, MV (FIC812) =15%. Furnaces B, C are in thesame way.
(11)When CO2 compressor shuts down, make these three furnaces stop and fueled oil.
Namely FIC815, FIC825 and FIC835 all startmanual operation and MV=0.0%.
II. Gas Fired-Boiler Three Pulses ControlSystem
Gas Fired-Boiler Three Pulses Control System isalso one of the most important control systems in boiler system. This is atypical three pulses liquid level control system with the drum liquid level asmain tune as well as steam production volume and feed water flow as reference.Here below is the picture for reference.
When making the three-parameter liquid leveladjustments, provided the steam production volume or feed water flow changes,it is required to adjust the feed water flow so as to maintain the stability ofliquid level before any change happening in it, especially to avoid the falseliquid level resulted from the sudden rise in steam production volume.
Under the circumstances of shut-down &start-up or systematic instabilities, the Parameter (Liquid Level) Adjustmentcould be used instead. Because of the special treatments made on the computer,the single parameter LIC81A andthree-parameter LIC810 will have the SV which equals the PV when not checked;moreover, the unchecked output will track on the checked output, which allowsus to change the control parameters by only changing the switching positions ofSW0811, yet with no other operations.
In order to avoid the affect of false liquidlevel, this system adopts the three pulses control, which employs the liquidlevel regulator LIC810 as the major regulator and BFW081 as the minorregulator. It deducts the steam flow FI0814 from water flow FI0813 in FX0810, and the result will thenbe used as the measured value for BFW081, which then would be sent to controlthe feed water flow of Boiler A. SW0810 serves as the shift switch for threepulses and single pulse, when the switch is at the position 1 it would besingle pulse and at 3 the other. LIC81Aserves as the single-parameter regulator for liquid level, which has the PVsourcing from the same converter with PV of LIC810. Auto selector SW0811 couldautomatically use the BFW 081 output signals that were not checked by SW081 totrack the checked ones.
To make the operation convenient, it should bethat the BFW081 always be maintained at CAS state and LIC 810 always be at AUTstate. In addition, the SV of LIC 810 should equal its PV when the singleparameter takes control, and SV of LIC 81Ashould equal PV when three-parameter takes control. So:
(1)When itis switched between “1”and ”3”,it is not necessary to balance SV and PV.
(2)It couldonly be operated on LIC81Awhen it is manually necessary.
III. Gas Fired-Boiler Mixing Fuel and otherInterlocks
Gas-Fired Boiler Mixing Fuel Interlocks:
Generally speaking, mixing diesel is used whengas-fired boiler is in operation in order to conserve natural gas as well as toincrease the raw gas of process installations. It is required of the gas-firedboiler to be of regular flame, regular oil pressure and utilization of fuelswitch, and only thus could the boiler be mixed with diesel.
In putting into use of fuel interlocks,provided the following happens:
Compressor Shut-down;
Oil Pressure above 3.5 MPA or below 1.2 MPA;
RF relay contacts disconnect (boiler flameblackout)
Fuel switch disconnects;
Then under these circumstances the fuel wouldstop burning.
When the mixing fuel interlocks are puttinginto use, if any of the conditions above from 2-4 takes place, fuel magnetic valueOSSV will lose electricity and the fuel stop will burning and there will be afuel-stop-burning alarm. In the maintenance of the instruments under the secondand third conditions above, it is required to cut off the fuel interlocks. Whencompressor shuts down, the three boilers A、B、C will all stop burning fuel, and the outputautomatic switch of the diesel flow regulator will be at 0.0%. At the sametime, the combustion regulating systems of the three boilers will be controlledmanually, the fuel regulator output will drop to 10% and air regulator outputdrops to 15%, the boilers will keep a low load operation and there will be acompressor-shut-down alarm. Then open or lose the fuel value FIC815 (Boiler B:FIC825, Boiler B: FIC 835) so as to regulate the fuel volume.
Feed Pump Low Outlet Pressure Interlock
When the feed pump has a low outlet pressure (LPS 841<=4.2MPA), delay it for 5 seconds, if the pressure remains low, thenstart up the spare pump 2006-JA in order to ensure the water-feed of boilers.
De-aerator Low Liquid Level Interlock
When the de-aerator has a low liquid level,start and shut the LLA 2009, and start up the 2207-JA, to make sure the watersupply in de-aerator. Normally the flow variation of FI2010 will reflect thestart situation of 2207-JA.
The realization of gas-fired boiler interlockprotection system by adopting HOLLYSYS, has changed the composition andoperation mode of the original design. It has made full utilization of thepowerful logical sequence control function of DCS system, and has connectedwith field experience of many years, which has improved substantially theprotection function of interlock systems, and provides convenience foroperations.
Fourth. Gas-Fired Boiler Ignition Interlock System
We use the Jingyi Huawen computer system torealize the interlock protections of water treatment and gas-fired boilerintellectualization. The software finishes all the interlock and alarmfunctions, accident memories and logical judgments and intellectualization. Itcan take the place of the operations and emergency treatments originally takenby the operating personnel; moreover, it also realizes the identification ofrelevant shut-downs by introducing intellectualization and identifying theauthenticity of accidents, avoids the mistaken shut-downs, shut-downenlargement and disturbances and mistakes during the shut-downs, and clears upthe mistaken actions and shut-downs caused by instrument problems.
The main characters of transforms:
To alter the spot important interlock switchsignals into simulating signals, adopt the double-factor means to set up thefunctions of authenticity identification, trend records, data and informationmemories of the signals, introduce circuit- measuring controlling signals,connect the analog quantity with discrete quantity, could raise the reliabilityand authenticity of conditional signals, improve the functions of accidentsauthenticity identification, condition delay, multiplexed output, multi-level alarmsand centralized alarms, and substantially enhances the reliability, accuracyand safety of interlock system.
By canceling the power relay, timer, switchesand other components, the DCS will provide all the functions of interlocks,alarms and circuit control.
Utilizing fully the computer’s display functionof color flow diagram, in displaying of CRT, the design has realized thefunctions of cutting/throwing, display, operation, alarming and memorizing ofthe interlock system, and solves the concealing in regular interlocks.
The realization of memorizing the firstaccident of the important interlocks has provided convenience to find thecauses and could help avoid the same accidents happening once again.
To enhance the intellectualization ofinterlocks, narrow the shut-down area, and adopt a local shut-down method inback breakdowns, could shorten the start-up time.
It could reduce operating personnel’soperations and inspects in emergencies, and by connecting with automaticcontrol system, when accidents happen, a number of regulating systems willautomatically cut the cascade and restore automatic control or manual control,and automatically open and close the valves from a safety concern and set up theindex value, to narrow down the shut-down area.
There are three quick loading boilers, whichcould provide steams for synthetization in start-ups, and provide 3.8MPAmedium-pressure for urea. Here introduce Boiler A as the example, and Boiler Band Boiler C are similar with it.
The purpose to set up interlocks:
(1) To ensure the fuel and gas cut off beforeIgnition and make full replacement.
(2) To make a normal operation and ensure theoperational safety of the boilers.
一)、吹扫程序:
I) PurgingProcedure:
Referring to the operation principle diagram, the conditions for purginginclude:
Steam drum liquid level is normal:
RLLWRelay contacts closed (probes);
B.LWCOcontacts closed (floating balls)
Then the spot liquid level indicating lamp willlight. In particular situations, the liquid level interlock bypass switchNSW081 could also be used.
2. Steam drum gas pressure is normal: PV(HPS815) <4.8Mpa
3. Fan oil pressure is normal: PV(LPS818)>=0.025Mpa
4. The manual cock of natural gas is off, andGC contact disconnects.
5. The big burner supervising cock magneticvalve GVB is not charged, and the contact disconnects.
5. If all the conditions above are available,turn on and off the NSW083 (Purging) and then there will be the followingactions:
The spot dampers are all open, MV(FIC812)=100%
If the all the spot dampers are openautomatically, namely:
Blow down air volume switch FD closes;
Air volume PV(FIC812)>60%
Thus in order to make it at the blow downstate:
a. The blow down lamp LD on CRT lights;
b. Meanwhile the timer begins to reckon.
After reckoning to 3 minutes, the blow down isfinished.
a. The blow down lamp on CRT goes off.
b. The blow down-finished lamp (LP)lights;
C. The fan dampers automatically adjust to thesmall fire position
MV(FIC812)=0%;
MV(FIC812)=0%
d. the natural gas valve automatically adjuststo the small fire position.
MV(FIC811)=5%。
MV(FIC811)=5%
Small fire procedure:
Necessary conditions:
1. The blow down-finished lamp (LP) lights;
2. The natural gas pressure is normal
PV(LPS816)≥0.05MPA;
PV(LPS816)≥0.05MPA
3. The natural gas flow PV(FIC811)<15%;
4. Theair flow PV(FIC812)≤20%;
5.the spot switch LFC closes, ( when MV(FIC811)≤10%, LFG closes )