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CONTROLS OF SUPER CRITICAL BOILERS. Presentation By AJAY SHUKLA Sr.Faculty PMI,. Boiler Turbine Control. Boiler Following Mode. Turbine Following Mode. Coordinate Mode Control. Coordinate Mode Control. Drum and OT Control Comparison. Drum and OT Control Comparison. OT Control Overview.
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CONTROLS OF SUPER CRITICAL BOILERS Presentation By AJAY SHUKLA Sr.Faculty PMI,
Start-Up System with Recirculation Start-Up System Recirculation Pump in Main Bypass Line SH Separator Flash Tank WW C ECO HWL To Condenser C Deaerator HPH C BFP
START-UP If the water system of the boiler is empty (economizer, furnace walls, separators), then the system is filled with approximately 10% TMCR feed water flow. When the level in the separator reaches set-point, the WR valve will begin to open. When the WR valve reaches >30% open for approximately one minute, then increase feed water flow set-point to 30% TMCR. As the flow increases, WR valve will reach full open and ZR valve will begin to open. The water system is considered full when: The separator water level remains stable for two(2) minutes and The WR valve is fully opened and ZR valve is >15% open for two(2) minutes.
Separator level is maintained by the combined action of a separator storage tank level feed water demand and the positioning of WR and ZR drain valves. Feed water demand is developed in response to separator storage tank level error and total fuel flow so as to prevent tank level from dropping too low. The WR and ZR valves are controlled in a split range manner to maintain the liquid level once the level reaches a high limit. The WR valve will respond first and then the ZR when the WR exceeds its linear operating range. Tank geometry is such that fluctuations in tank level are very dynamic, for this reason, only proportional control action established through the WR/ZR function curves is used to position these valves in response to level error. SEPARATOR STORAGE TANK LEVEL CONTROL
Control objective: Maintain minimum economizer inlet flow. Control action: The boiler circulating pump is started following the start of a turbine-driven feed water pump and the final clean-up cycle. This pump circulates feed water from the evaporator outlet back to the economizer inlet. Located at the outlet of this pump is the UG valve which controls economizer inlet flow during the start-up phase of operation. Demand for this recirculation control valve is established based on measured economizer inlet flow compared to a minimum boiler flow set point. UG VALVE CONTROL
Control objective: Develop total unit feed water demand as required to support unit load. Adjust feed water demand to maintain desired separator outlet temperature. Adjust separator outlet temperature set point as required to maintain acceptable platen superheat spray control range. Incorporate separator storage tank level (wet mode) feed water demand. Maintain minimum required economizer inlet flow. . FEEDWATER CONTROL LOOP
MODEL d dt X Feed Forward with model Process Setpoint Feed Forward f(x) S P +/- C -/+ D K C +/- P +/-
f(x) d dt d dt Feed Forward FF FF PROCESS process setpoint f(x) P +/- C -/+ P +/- C -/+ D D K K C +/- P +/- C +/- P +/- S X
Model predictive control + MW _ Fuel with model Without model Turbine setpoint Water wall Temperature C t[sec] 0 60 180 240 300 120
START-UP contd.. Water flows through the economizer and evaporator, and discharges the boiler through the WR valve to the flash tank and via connecting pipe to the condenser. Start BCP and open the UG valve to establish minimum water wall flow at 30% TMCR. As the pressure is raised, first the WR and then the ZR valves will open when separator water level increases due to boiler water swell. As pressure further increases, the WR and ZR valves will start to close as the water level decreases. The steam temperature at the separator inlet will reach a stable superheated condition at app. 30% TMCR, causing the level in the separator to decrease and eventually disappear. The boiler is now in once-through mode (dry mode). The BCP (Boiler Circulating Pump) will be stopped automatically. It is extremely important that minimum water wall flow be maintained at all times when firing the boiler to prevent tube damage due to overheating.
Demand for feed water is established predominately by the Boiler Master demand. This signal, processed though a “boiler transfer function” provides the feed forward component of the total feed water demand. The “boiler transfer function” is a tunable dynamic element providing a means to dynamically match the feed water feed forward demand to actual evaporator heat transfer. Optimization of the feed forward in this manner minimizes temperature fluctuations that may otherwise result from varying dynamic response between the firing and feed water control systems (as they relate to evaporator heat transfer) thereby lessening the dependence on feedback correction. FEEDWATER CONTROL LOOP contd..
The first controller acts on a load dependent average platen spray differential temperature. Its output represents the required adjustment to evaporator heat transfer/steam generation to maintain both the steam conditions and flue gas temperatures entering the platen superheat section so as to ensure adequate platen spray control range. A second controller acts on a load dependent separator outlet temperature set point corrected by the platen spray differential temperature controllers output. This controller acts to adjust feed water in response to firing system disturbances and the relatively fast effect they have on separator outlet steam temperatures. The overall combined feed water feedback control action is such that feed water demand is responsive to changes in the overall unit heat transfer profile. FEEDWATER CONTROL LOOP contd..
The combined feed forward/feedback demand signal is subject to a minimum economizer inlet flow set point (wet mode) activated if the boiler circulation pump is not in service and the unit is being fired. This ensures the minimum economizer inlet cooling flow is maintained by the feed water supply system in the event the start-up system is not available. The feed forward/feedback demand signal is subject to a second “wet mode” feed water demand developed to support separator storage tank level control. The resulting demand provides the set point to a feed water master controller. The fuel/feed water ratio protection logic provides overriding control of individual feeder speed demands in the event of an excessively high fuel to feed water ratio. FEEDWATER CONTROL LOOP contd..
