Pressure control scheme of steam header of the hot

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Parallel boiler steam header pressure control scheme

0, introduction

our phase II thermal power plant is equipped with two 75t/h cyclone pulverized coal boilers and two 6 × 103kW steam turbine generator set and other Thermoelectric equipment. The operation process of these equipment is controlled by a set of DCS produced by Rosemount company in the United States. During the start-up and commissioning, it was found that the design of steam pressure, especially the main pipe pressure control scheme provided by the Design Institute, was not perfect, so targeted modifications were made. Practice has proved that the improved system has significantly improved in control quality and simplicity of operation. In order to highlight the steam pressure regulation process, the following avoids the problem of "air coal ratio", and assumes that "air volume", one of the important factors of boiler combustion, can always meet the requirements of full combustion of fuel. In order to highlight the key point of discussion "steam pressure control of boiler", scale change, load transfer and other links are ignored in the attached SAMA diagram

1. Steam pressure control process

as we all know, the change of steam pressure is a physical phenomenon reflected by the change of heat in the balance process. When the total input heat is equal to the total output heat, the pressure will remain unchanged; Otherwise, the pressure will change with the amount of heat surplus (i.e. the level of internal energy) in the process of heat balance

for the boiler system, the total input heat includes fuel heat (calorific value) and sensible heat (physical heat) of fuel, air and make-up water. The total output heat includes steam heat and heat losses such as smoke exhaust, insufficient fuel combustion and heat dissipation. Generally, the sensible heat in the total input heat and the heat loss in the total output heat account for a small proportion of the total heat, and the fluctuation range is also limited, which has little impact on the heat balance. Therefore, the main factors affecting the heat balance are fuel heat and steam heat. The heat balance imbalance caused by fuel thermal fluctuation is usually called "internal disturbance", and the heat balance imbalance caused by steam thermal fluctuation is usually called "external disturbance". In order to overcome the influence of internal and external disturbances on steam pressure, in a basic single boiler steam pressure control system, the fuel heat input to the boiler must change with the change of steam heat, so as to maintain heat balance as much as possible. At the same time, increase or decrease the fuel quantity according to the deviation between the steam pressure and the set value, so as to increase or decrease the steam pressure. Because the external disturbance is caused by the random change of steam load, the internal disturbance is caused by the change of fuel quantity. To realize the above control, in addition to measuring the "steam pressure" in the controlled object, it is also necessary to measure the fuel and steam volume reflecting the changes of internal and external disturbances. 3. According to the scope of utilization, it can be divided into warning tape, carpet tape, electrical tape, protective film tape, winding tape, sealing tape, die-cutting tape and other basic structures, as shown in Figure 1. This is a feedforward cascade control system composed of pressure controller and fuel controller. Among them, the main ring pressure controller adjusts the fuel quantity according to the deviation between the steam pressure and the set value to ensure the stability of the pressure; The secondary loop fuel controller controls the amount of fuel entering the boiler according to the combined command of the main loop output and the feedforward signal (i.e. external disturbance), and has the ability to overcome the fluctuation of fuel amount (i.e. internal disturbance)

for boilers operating in parallel, if a single boiler control scheme is simply adopted, the phenomenon of "load grabbing" will appear, that is, when the steam load remains unchanged, the drum pressure of a boiler will rise due to the difference in the intensity of combustion in the boiler, and the steam supply of the boiler will rise. This false external disturbance signal will further increase the fuel volume of the boiler through feedforward, so the steam pressure will rise further, In this process, due to the coupling effect of the steam header, the steam output of other boilers is restrained, so that the steam output is reduced, and the control system of these boilers feedforward the external disturbance signal to the fuel controller, so that the fuel volume is further reduced, and finally the steam load of each boiler is light and heavy. In order to overcome the above phenomenon of "load grabbing", boilers operating in parallel usually share a "header pressure controller" as the main loop of the cascade system, and its output is distributed to each boiler fuel controller in proportion as the fuel given signal. At the same time, the feedforward signal takes the sum of the output steam of each boiler to overcome the internal disturbance between boilers. Taking the double furnace as an example, its basic structure is shown in Figure 2. In the figure, K1 and K2 are load distribution coefficients, and the sum of the two is 1 (i.e. 100%)

2. The simplified SAMA diagram of the original control scheme (removing the cross restriction of air and coal) is shown in Figure 3. From the figure, it can be seen that the "external disturbance" signal of this scheme is taken from the inlet steam flow of two steam turbines, and the "internal disturbance" signal is taken from the sum of the boiler's output steam flow and the drum pressure change rate. In order to make the boiler operate in the "main pipe" mode, the viscoelasticity of biological elastomer material has advantages in wetlands and high temperatures, and can also operate in the "unit system" mode, the system uses three pressure controllers, one common "main pipe pressure controller" during the operation of the main pipe, and two independent "drum pressure controllers" during the operation of the unit system. The set value signal of the fuel controller selects the source of the load command through the operation mode switch. The load command of the fuel controller during the unit system operation is from the "drum pressure controller", and the load command of the fuel controller during the main pipe operation is from the "main pipe pressure controller"

in the process of start-up and commissioning, the following problems were found in the system:

① the actual load of the boiler consists of steam load for power generation and process steam load. Among them, the process steam load accounts for about 60% of the total load of ③ rubber spring stiffness in three directions, and the fluctuation range is large, so it is inappropriate to take the steam flow at the inlet of the steam turbine as the "external disturbance" signal

② the heat signal reflecting internal disturbance can be obtained indirectly theoretically by

q = q + cdpb/dt

, where q is the heat input to the boiler; Q is the output steam flow; C is the drum heat storage coefficient; Pb is the drum pressure

in the actual debugging process, the Q value of the heat signal obtained by using this equation is unstable and does not truly represent the actual heat. The key is that the parameter C is not easy to get accurate

③ before switching between "unit system" and "master control", the system operation mode must be "needle aligned" before switching, that is, the operation is not easy

3. The improved control scheme

aiming at the above problems and the views put forward in the overview of steam pressure control, the following improvements have been made:

① whether the "unit system" or the "master control" is running, its external disturbance signal is taken from the sum of the output steam flow of all boilers. In this way, not only the "unit system" running boilers, but also the false external disturbance caused by the difference in the intensity of combustion between boilers can be avoided

② take the speed signal of the pulverized coal machine to indirectly represent the heat input to the boiler. Although the speed signal can only represent the volume flow of pulverized coal, under normal working conditions, the pulverized coal is dry and dense, and the density changes little, and the calorific value of coal quality changes infrequently. Practice has proved that the heat signal is relatively real and stable

③ since the controllers and function blocks used in the system are realized through internal software configuration on DCS, it not only provides controller modules such as PID, but also provides status signals of relevant modules "manual/automatic" and logic processing functions enforced by status or event signals. Therefore, the state signals of the fuel controller and the operation switch are introduced to logically control the output tracking of the drum pressure controller or the header pressure controller. When both boilers are in the "unit system" operation mode, the output of the header pressure controller tracks the average output of the two drum pressure controllers; When the boiler is in "main pipe" operation, the output of its drum pressure controller tracks the load command assigned by the main pipe pressure controller. The system structure is shown in Figure 4

4. Advantages of bus tube operation

in the past three years since the system was put into operation, what kind of copper tensile testing machine was used to detect copper? Your answer: in the meantime, through the comparison of the operation modes of "double boiler main pipe" and "one constant load one main pipe", it is found that the operation of double boiler main pipe can make the parallel boilers jointly bear the load fluctuation, and improve the ability and speed of the system to overcome the load fluctuation. (end)

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