Monday, April 13, 2015

 Why out of so many Boiler controls, steam Temperature control is so critical???


Functional aspects of superheat temperature control types in utility boiler. by: Swapan Basu*

*Author:  Power plant Instrumentation and control handbook Elsevier (http://store.elsevier.com/Power-Plant-Instrumentation-and-Control-Handbook/Swapan-Basu/isbn-9780128011737/ )

*Systems & Controls (Consulting Engineers I&C) Kolkata India. (basu.swapan@gmail.com)

Abstract
Complexity, influence of various inter active parameters, non linear load dependent process response coupled with time delay make superheat temperature control in a thermal power plant the most challenging job. Philosophy behind controlling superheat temperature is to maintain correct balance between operational efficiency and life expectancy of Boiler and turbo-generator (BTG). There have been many approaches such as conventional PID with gain scheduling & feed forward signal, Direct Energy Balance, State Variable controller, Predictive controller based on system model, Fuzzy logic controller, Adaptive fuzzy  neural controller etc. In this paper functional aspects of various superheat temperature controller types have been discussed briefly to get some idea to compare various approaches and select the one most suitable for particular application.
Key word: Gain scheduling, State variable, Adaptive, Predictive, Fuzzy control, Neural network.

1.0    Introduction
To get better operational efficiency and also to prevent unnecessary material and thermal stress in  thick walled components of boiler and turbine, it is very much important to precisely control steam temperature on utility boiler. To get best possible heat rate to reduce fuel costs operators tries to maintain steam temperature at the rated value. By adjusting the amount of spray water in to the steam header after it pass through a stage of super heater (SH) it is possible theoretically to control superheat temperature control. However, on account of non linearity, load dependent  time constant (of the system response), gain, high dead time,   time lag/time delay & uncertainty, it is very difficult to maintain the temperature precisely with conventional PID controllers. In view of this, there have been many approaches to combat the situation. Process dead time changes with load, so, there will be requirement of feed forward signal requirement to take care of changes in  firing rate and/or feed flow. This is especially important for once through boilers. Therefore, for conventional PID with gain scheduling, Feed forward approach is necessary. Direct energy balance strategy is also effective especially in case of once through(OT) boilers, here fluid enthalpy is matched to the pressure levels over entire range to get optimum efficiency[1].  In state variable approach not only output variables of a process is used to control but also intermediate process conditions are considered. State controller with observer (SCO) uses intermediate states of mathematical model in place of impractical intermediate measurements[2]. Predictive adaptive process control technology utilizing Dynamic melleing technology (DMT)[3], and other mathematical model based systems are some other approaches to get precision control. Predictive control strategy is precise but may be  costlier, so people utilized human intelligence in developing fuzzy logic controller which in conjuction with PLC/DCS can give a better result. Application of neural network in steam temperature control could be another means to get better regulation. Later some engineers combine neural network with fuzzy to form fuzzy neural network control [4]. In order to assimilate various control strategies, it is fundamentally important to understand the functional requirements of the same  and associated  influencing parameters. Starting from process and operational requirements, in this paper functional aspects of all these approaches have been dealt with so that pros and cons of each of these systems could be well understood to select one technological approach which best suited for the plant.
2.0    The Process and functional requirements:
After saturated steam is formed, it is superheated to increase enthalpy i.e. to get better thermal efficiency. Steam temperature needs to be controlled at the rated value for the best result. Superheat steam temperature is normally controlled by spraying water in the intermediate stage(s). The place where water is sprayed is called de super-heater or attemperator. There could be numbers of desuperheater stages. Normally this spray water is taken from Feed water (FW) circuit at (BFP discharge)/ High Pressure Heater (HPH) outlet/ Economizer outlet depending on design and sizes of the boiler. In large modern plants high temperature at SH outlet is expected and this high temperature approaches creep point of the thick steel making up the super heater tubes. Also higher temperatures cause thermal stress in turbine metallurgy. So, if there is higher than rated temperature there is possibility of deterioration of life time of BTG. On the contrary if the temperature of superheater outlet is less then there will be fall in thermal efficiency of the system on account of less enthalpy. A few other process and operations aspects need special attention have been presented in Table 1 Process and Operational Aspects below:
Table 1 Process and Operational Aspects
Sl.
Factors
Discussions:
1
Main steam flow/Load
Process dead time and non linearity changes with load/MS flow
2
Fairing rate (FR), FW, Air flow or   FR: FW ratio

Fuel/coal flow has direct impact on the steam temperature. Higher coal flow will try to increase temperature. Similarly Feed flow and air flow has impact on SH temperature control. In case of OT boilers/Supercritical (SC) boilers FR/FW is very important for SH temperature control. Gain of DSH O/L will be highly affected by this.
3
Fuel (Coal) quality
This directly affects flue gas temperature and flow which in turn affects SH  temperature control.
4
Heat release at boiler
Observed as change in drum pressure for drum type boiler
5
Change in Spray water pressure /Temperature
This will affect the spray flow. Also at higher load, differential pressure between feed water and steam will be less- hence less spray flow (load change sl.1). Also Feed water temperature affects the quantity requirement of spray water.
6
Furnace cleanliness/ soot blowing
When the furnace is clean there will be more absorption of heat by the furnace hence less SH (or RH) temperature. Reverse will be the situation in case of dirty furnace.
7
Burner Tilt (BT) : Divided Back pass Damper (DBD)
Applicable for corner fired boilers: when Tilt mechanism is used for RH temperature control, it affects the furnace heat distribution (e.g. Up more heat for RH less for furnace) and finally affects SH temperature hence can be a feed Forward signal. In Wall fired boilers a common heat transfer section is located across SH & RH . mass flow through RH (or SH) can be adjusted through damper for RH temperature control. This will affect SH heating also.
8
Fouling of superheater tubing
Affects the control
9
Spray valve closure & directional blocking [4]:
To make sure no water reaches turbine as per TWDPS spray valve and associated block valves are closed following Master fuel trip. Also at low steam flow spray valve is closed as safety measure. Directional block is put to prevent from DSH outlet approaching saturation.
10
Spray at two stages
The Spray valve capacity & delay time in primary spray after primary SH will be more than  the same in Secondary spray before final SH. Therefore unbalance coupling between Primary & secondary spray need to be properly addressed ( ref cl.7.0)


3.0    Conventional PID loop with Gain scheduling and Feed forward signal:     
Conventional PID controllers can meet basic control requirements in many control loops but not in all types: such as Superheat temperature controls on account of various factors discussed above. Since inherently, temperature loop is slow, the total effect of spray flow change could not be responded by thermocouples for SH outlet temperature measurement up to several minutes. In view of the above cascade PID control loop with inner slave control loop as shown in fig 1 is used mostly. In master controller, set point is compared with the SH outlet temperature to generate the set point for the slave controller where it is compared with DSH outlet Temperature controller to generate control demand for the spray control valve opening. During start up to low load Main stream (MS) temperature set point is kept low or may be derived from air flow which is proportional to boiler load.  When steam flow is less naturally due to low flow the process dead time increases affecting adversely on the stability of the control loop [5]. Therefore it is necessary to schedule the controller gain at various loads. Also this calls for adjustment of I & D parameters of the controller. As DSH outlet temperature will be affected by steam flow rate (e.g.  Less cooling for same spray flow at higher steam flow) also at higher load DP between steam and FW will be less; hence there will be less spray flow. Therefore gain adjustment of the secondary controller is necessary. In addition to gain scheduling feed forward is also very help tool to get better response from the loop. During any load change there will be change in steam flow, which affects the steam temperature directly.  During turbine following mode firing rate will be affected immediately than steam flow, this will cause big deviation in steam temperature control. Also as shown in the table 1 burner tilt etc. will affect the control loop. Therefore, a number of parameters such as MS flow, Fuel flow, air flow, FR/FW ratio, and (depending on applicability): Burner tilt/ Drum pressure are used as feed forward signal to further enhance the response of the loop.  Although in most of the cases Spray water is used for SH temperature control, yet in some cases of corner fired boiler, Burner tilt controls has been selectively used between RH & SH control ( e.g. SGTPS 500MW in MP India). However PID controllers cannot ensure control quality for all process control loops:
4..0    Direct Energy Balance technique 

In large utility stations it is desirable to maintain the MS temperature near the rated value with a deviation of  +/- 5°C however in reality it is very difficult to maintain the temperature so precisely. Again in larger stations a change in the tune of 10°C could drop in efficiency as high as 0.3%.  D-E-B ® Co ordinate control of steam by Metso [1] is another way; where co ordinate control system has been directly linked with Superheat Temperature control.  In this method fluid enthalpy matches the pressure levels. As shown in Fig 2 enthalpy matches the pressure levels by adjusting the Fluid distribution (FD) and Heat distribution (HD). Fluid is distributed in the systems via water wall, superheater circuit and superheater attemperator circuit. On the other hand, heat is distributed via water all, superheater and reheater circuit. In this technique, from unit demand feed water demand is set through energy balance controls, but while setting the FW demand, the distribution is set as per superheat temperature control. With this technique about 10% of FW can be redistributed at full load [1].  Again, heat distribution is set from SH outlet temperature control error-coupled with DSH outlet differential to make the loop more responsive. In cases of OT boiler especially in cases of super critical boilers, Firing Rate (FR) and feed water (FW) ratio play very important role. So, this also sets the FR:FW ratio to ensure optimum system efficiency without sacrificing any stress to the system. Since various intermediate stage temperatures are used to set HD, the possibility of P.SH approaching saturation is eliminated. Since Re heat temperature control devices e.g. Burner tilt will have immediate effect of SH temperature control, yet on account of change in heat distribution in water wall, it will try to oppose this. This technique offers better temperature control when compared with conventional system also it can act fast for load changes offering better efficiency and lesser outage [1].

Tomorrow 15th April 2015  is Bengali NEW YEAR 1422 !!!
we meet again next week.
 “stay tuned for a new post next week…” 


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