Composition and logic analysis of the hottest rehe

Composition and logic analysis of reheat steam temperature control system

overview

steam reheater is an important part of boiler. Its function is to reheat the steam after the high-pressure cylinder of the steam turbine works to the temperature of fresh steam again, and then send it to the medium pressure cylinder of the steam turbine to continue to work. After reheating cycle, on the one hand, it can further improve the thermal efficiency of the cycle, On the other hand, the steam humidity of the last stage blade of the steam turbine can be controlled within the allowable range. The temperature measuring points for regulation and the installation position of the actuator are shown in Figure 1

Figure 1 boiler reheat system P & ID

reheat steam cycle can reduce the steam humidity of the blade at the end of the steam turbine, reduce consumption, and improve the thermal efficiency of the power plant. The exhaust steam after the high-pressure cylinder works. After returning to the boiler, it mainly improves the dryness of the steam and enters the steam turbine to do work, which will not cause damage to the steam turbine. The purpose of controlling reheat temperature is to improve the thermal efficiency of the unit and reduce the steam with water in the low-pressure part of the steam turbine, Heze Power Plant controls the Reheater Temperature in two ways: one is to control the position of the hot air baffle to control the reheat steam temperature; the other is to record the required value and lift the pen to control the reheat water spray volume to control the reheat steam temperature

1. System composition

peripheral equipment of reheat steam temperature control system includes reheat desuperheating outlet temperature te0601a and te0601b temperature elements, reheater pressure signal pt0501 transmitter, reheater spray water flow signal pdt2123 differential pressure transmitter, reheater outlet temperature signals are te0691a and te0691b temperature elements respectively, hot air injection signals on side a are pdt7107aa and pdt7107ab respectively, and hot air injection signals on side B are pdt7107ba and pdt7107ba respectively Pdt7107bb differential pressure transmitter should ensure proper range, correct measurement, normal CRT display, reheat desuperheating actuator bn0601, hot air injection baffle cd7107a, cd7107b, actuator should ensure correct action direction, proper travel time and travel error, as shown in Figure 2

Figure 2 block diagram of reheat steam temperature control system

2, functional analysis

2.1 spray desuperheating control

spray desuperheating control is shown in Figure 3

Figure 3 spray desuperheating control

in the spray desuperheating control circuit, the adjusted main parameter is the signal with the second largest reheater outlet temperature, the adjusted auxiliary parameter 7. With the customer setting function, it is the signal with the second smallest reheater desuperheating spray outlet temperature, and the fine adjustment parameter is the pdt2123 differential pressure signal and the reheater spray flow signal processed by square cutting

the reheat steam temperature spray water regulating valve adopts a cascade three impulse regulating system. The reheat steam temperature is the main signal, the spray water outlet temperature is the auxiliary signal, and the spray water flow is the fine adjustment signal. The inner loop regulator acts the spray water regulating valve according to the changes of the desuperheating water flow and the desuperheater outlet temperature to reduce the impact of these signal disturbances on the reheat steam temperature main signal; When the main signal of reheat steam temperature deviates from the given value, the main regulator of the external circuit sends a correction signal to operate the water spray valve, appropriately change the water spray volume, and maintain the main signal of reheat steam temperature basically equal to the given value

2.2 hot air injection control

in the hot air injection control circuit, the adjusted main parameter is the signal with the second largest reheater outlet temperature, and the adjusted auxiliary parameter is the single side hot air injection differential pressure signal. After square root, the second average is taken as the single side hot air injection flow signal, as shown in Figure 4

Figure 4 hot air injection control

the hot air injection control circuit is a cascade three impulse regulation system. In the main circuit, when the reheat steam outlet temperature rises due to some disturbance in the system, the measurement signal at the inlet of the main regulator will rise. Because the regulator C-1 module is a reaction, under the condition of constant set value, the output of the controller C-1 module will decrease, and the set value of its sub regulator will decrease, so the regulator C-2 The inlet deviation signal of C-3 module will rise. Due to the reaction of C-2 and C-3 modules of regulators, the position of hot air baffles on both sides will finally act in the direction of turning down, so as to reduce the hot air injection at the bottom of the furnace and try to reduce the reheat steam temperature, so as to ensure that the reheat steam temperature is near the given value and within the allowable range

in the secondary circuit, when the hot air input of the system increases or decreases due to some disturbance, the measurement signal of the secondary regulator C-2 and C-3 modules will rise or fall. Because the secondary regulator C-2 and C-3 modules adopt reaction, the output of the secondary regulator will reduce or increase the opening of the baffle, and also try to reduce or increase the hot air injection, so as to make the system approach balance

3. Logic analysis

override signal logic is shown in Figure 5

Figure 5 reheat steam temperature control logic

when the conditions of the first stage of purging are met, the change-over switches T5 and T6 will act at the same time. At this time, the opening of side a and B baffles will be determined by the value of SG-4 module or SG-5 module. According to the different values of the conditions of the second stage of purging, the value of SG-4 module or SG-5 module can be selected

under the automatic state of water spray regulation bn0601, if the outlet temperature of reheater is high, the water spray desuperheating valve mv2123 will override and close

under the sliding pressure operation mode, if the reheater outlet temperature is higher than 546 ℃ and the steam flow is higher than 160kg/s, or under the constant pressure operation mode, if the reheater outlet temperature is higher than 546 ℃ and the steam flow is higher than 211kg/s, the high reheat steam temperature alarm will be sent; Similarly, under the sliding pressure operation mode, if the reheater outlet temperature is lower than 536 ℃ and the steam flow is higher than 160kg/s, or under the constant pressure operation mode, if the reheater outlet temperature is lower than 536 ℃ and the steam flow is higher than 211kg/s, the low reheat steam temperature alarm will be sent

the composition of reheat steam temperature actuator is shown in Figure 6

Fig. 6 reheat steam temperature actuator

after the compressed air passes through the filter pressure reducing valve, the pressure range is (1.6 ~ 6) bar, that is (20 ~ 90) psig, and then it is divided into two ways. One way passes through the pressure switch and holding valve to the pneumatic actuator, and the other way is sent to the electrical converter. According to the 4 ~ command signal, the electrical converter converts it into the air pressure signal corresponding to the position of the pneumatic actuator. The action process of the holding valve is: when there is air source, turn on, When there is no air source, connect it. If 4 is blocked, a sealing system is formed and the position is maintained. If 4 is not blocked, the cylinder is vented through 4. According to the structure type shown in Figure 6, the actuator is fully open

tzid-c positioner operation panel is shown in Figure 7

Figure 7 positioner operation panel

5. Quality index

when the boiler operates stably, the reheat steam temperature should be kept within the range of ± 2 ℃ of the given value, and the actuator should not act frequently. In the process of internal disturbance, when the desuperheating water is disturbed by 10%, the transition time of reheat steam temperature from automatic start to disturbance elimination should not be more than 2min. In the process of constant value disturbance, when the given value of steam temperature changes by ± 4 ℃, The control system shall be stabilized within 4min

requirements for static and dynamic quality indexes (70% - 100% load range) under the coordinated control mode of turbine and boiler: in static state (unit load change rate ＜ 1% rated load/min), the allowable deviation of reheat steam temperature is ± 5 ℃; When the load change rate of the unit is 3% of the rated load/min, the allowable deviation of reheat steam temperature is ± 10 ℃. After disturbance test, the system meets the requirements of regulation index

references

[1] Xie Huawen Sama diagram of boiler island of Heze Power Plant Phase II project [Z] Shandong: Shandong Electric Power Engineering Consulting Institute, 1999

[2] Heze Ⅱ power station builder control logic diagrams [Z] English:mitsui Babcock Energy Limited, 2001

[3] Hou dianlai# 3 boiler steam temperature control system improvement [eb/ol] Shanghai: ie industrial engineering, 2004:

about the author: Hou dianlai (1963-), male, born in Liangshan, Shandong Province, Han nationality, graduated from Shandong University of technology in 1985, with a bachelor's degree in engineering, senior engineer, engaged in infrastructure installation, commissioning and production maintenance of thermal power plants for 20 years, and published 72 papers. E-mail: hdlacc@. (end)