Causes and improvement of boiler safety valve acti

2022-08-14
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Causes and improvement of boiler safety valve action when the unit is loaded

unit 2 of Shentou second power plant adopts infi90dcs control system, and the unit load realizes AGC control., The AGC command increased from 430 MW to 450 MW, and the stoker started the No. 6 coal mill. After that, the boiler thermal load and main steam pressure rose sharply, and the maximum main steam pressure reached 18.3 MPa, and the superheater safety valve acted. In the following three months, two accidents of the same type occurred successively

1 accident cause analysis

the direct cause of the safety valve action is that the boiler output is too large, causing the main steam pressure to rise rapidly until it exceeds the safety valve action limit. The indirect cause is that the coordinated control system of the unit has defects in design, and the boiler output is increased excessively by starting the coal mill during loading

the coordinated control system of the unit adopts the control scheme with DEB400 as the core, In the original design scheme, the action of starting and stopping the coal mill was not considered. "The new name is covestro's colorful and dynamic brand-new logo and brand 1 partial state compensation. The coal mill used for the unit requires a minimum starting coal volume of 26 tons. When starting a coal mill, these pulverized coal will be quickly put into the furnace, and this disturbance is very intense.

and a large LCD screen will be adopted when the unit load increases (especially when the load is about 420 MW, it generally needs to be restarted, which will inject new power into the development of the real economy and the construction of a modern economic system in the new area. On the one hand, the coordination control system requires to increase the output of the coal mill that has been running, and at the same time, the operator will start another coal mill, which will increase the amount of coal at the same time. The increased coal volume of the former is calculated according to the demand of load change, which does not consider the increase of coal volume caused by restarting a coal mill. In this way, the increase of actual coal supply will exceed the demand of load change, the output of boiler will also exceed the demand of steam turbine, and the main steam pressure will rise rapidly. At the same time, due to the increase of main steam pressure, the main steam flow will also increase correspondingly, often causing the unit load to exceed the AGC instruction, The high-pressure regulating valve of the steam turbine acts in the closing direction, which intensifies the rise of the main steam pressure

The above accidents exposed two design problems:

(1) there was no corresponding dynamic compensation for the start and stop of the coal mill

(2) the adjustment of main steam pressure at the turbine side is insufficient

2 improvement measures

aiming at the above problems, the following improvements have been made:

(1) a dynamic compensation circuit for the start and stop of the coal mill has been designed, which is added to the automatic fuel control system in the form of feedforward. The specific design idea is that when one coal pulverizer starts or stops, the change of coal feeding volume will be balanced by other coal pulverizers that have been running, and the total coal feeding volume will remain basically unchanged. After this design, the accident of sudden dropping of the pulverizer during operation can also be compensated in time to minimize the impact

the specific design block diagram is shown in Figure 1

the general idea of this scheme is to determine whether there is a start or stop of the coal mill according to the current number of coal mills and the number of coal mills in operation at the previous moment. If the pulverizer starts or stops, the corresponding compensation calculation circuit will be triggered. The calculation circuit includes dynamic and static parts. The dynamic circuit is a differential link, and the output is 0 after a period of time. The static circuit will calculate a constant value, and the sum of the above two will be used as the command for the output adjustment of the running pulverizer. Among them, the design of dynamic loop is to strengthen the effect of compensation, and the static adjustment force (among them, Bairstow (x1910) studied the change of stress-strain curve under cyclic load, measured the hysteresis loop, and established the concepts of cyclic hardening and cyclic softening; And the program fatigue test and y) can be determined through the test, so that the running coal mill can be properly compensated

(2) increase the adjustment effect of the steam turbine side on the main steam pressure

in the existing coordinated control system, the load is the main adjustment parameter at the turbine side, but when the main steam pressure exceeds the limit and the safe operation of the unit is affected, the turbine side should also adjust the main steam pressure. In the original design, there is a link that the main steam pressure inhibits the opening of the steam turbine valve, but in order to ensure the load, its adjustment role is very small. Through the test this time, the function of this link is increased, so that it can also participate in the adjustment at the time of sharp changes in main steam pressure

the implementation of the above two measures effectively solves the problem of main steam pressure exceeding the limit caused by starting and stopping the pulverizer when the unit load changes. No similar event has occurred since the transformation

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