How to Control the Rapid Decline of Steam Temperature after RB

At present, China's power station boiler fans, especially primary fans, have a high energy consumption and noise problem during operation. In North China Power Grid, a number of power plants have applied frequency conversion governors to the centrifugal primary fans of newly-built units and newly-built units. After fan speed adjustment, the fan's power consumption is reduced, the operating efficiency is improved, the power consumption rate of the plant is reduced, and the energy saving effect is significant. However, there are new problems in some renovation projects: such as “grabbing wind” phenomenon when the unit is heavily loaded; before the primary fan motor The side bearing is overheated and damaged; the overload protection action of the frequency converter caused by the primary fan RB causes the unit MFT to move, which seriously affects the safety and economical operation of the fan and the boiler.

Considering the primary fan RB: Why does the primary fan RB succeed in achieving the same primary fan and adopting the inlet baffle adjustment, but the primary fan RB fails after the frequency conversion and speed control reform? How to suppress a sharp drop in primary air pressure after RB? How to control the sharp drop in steam temperature after RB? Does the unit RB use constant pressure or sliding pressure?

First, the meaning of rapid load dumping

The meaning of rapid load dump (RB or RunBack) of the unit: The main auxiliary equipment of the unit, such as primary fan, blower, induced draft fan, air preheater, boiler steam feed pump, boiler water circulation pump, etc. The control system (CCS) quickly issued instructions to reduce the actual load of the unit by a certain amount. The boiler, steam turbine main controller separately adjusts the control system such as combustion, feed water, steam temperature and steam turbine DEH, so that the unit load and related parameters eventually reach the capacity of a single auxiliary machine to ensure safe operation.

One of the security functions of the RB genus is the unit. In order to implement the RB function, two control systems, CCS and BMS, are required to coordinate their actions. Except that the RB instruction of the primary blower is sent by the BMS itself, the rest of the RB instructions are issued by the CCS. The schematic diagram of the RB is shown in FIG.


The RB module operates according to its internally set load-reducing rate and target load command. The boiler load is reduced at a predetermined rate. The reduction in the fuel volume is controlled by the fuel regulator. The BMS system also stops the corresponding coal supply according to a certain logic. Powder machine, or cast the corresponding oil gun together to complete. In the RB process, the pre-press pressure is automatically controlled by the turbine. When the BM S receives the RB instruction, it first sends an alarm signal and sends a data record (DL) signal to the DAS. At the same time, stop the top mill running. Then, CCS is used to reduce the rotation speed of the coal feeders in each operation layer. After the F-layer coal powder stops for 10 seconds, if the RB command continues to exist, the BMS stops the E-layer coal mill while the CCS continues to reduce the coal feeder rotation speed. After 10s, if the RB command still exists, the BMS will stop the D-pulverizer and finally keep the A, B, and C three-pulverizers running.

After a single delivery and induced draft fan accident trips, the same side of the lead and blower will automatically trip and stop by interlocking.

If the RB command still exists after the coal mills D, E, and F are stopped, it means that another auxiliary machine with the same function also fails, and the result leads to the MFT action.

Second, the realization of RB function when a fan speed control

The following two factors must be taken into account when implementing the RB function of a fan.

1. Primary fan and system equipment characteristics

(1) Parameters and margins of a single primary fan

For large-scale units, a single primary fan is generally designed for 50% unit load. The larger the design capacity, the more advantageous it is to implement the primary fan RB function, but it is not good for energy saving. Excessive fan design margin will result in a single blower with large unit consumption. Especially when the baffle adjustment is adopted, a large amount of energy is wasted on the fan throttling loss; even if variable frequency speed control is adopted, excessive head pressure and flow margin are selected. When the load is low, the fan will run to the left of the highest point of the fan performance curve, causing the fans to be in parallel. The two fans will have a “wind rush” phenomenon. Single primary fan with load capacity should also reduce air leakage from the air preheater; improve primary air system piping and dampers; improve the thermal control interlock protection logic in several aspects to start, take countermeasures.

(2) System air leakage

The power plant adopting the positive pressure direct-fired pulverizing system generally reflects that the primary fan RB is not very successful, and the single primary fan has insufficient load capacity, which often causes all the coal mills to trip or MFT. The reason for this is often that it is not a small selection, but a serious system leak, which is the root cause of the problem. In the process of primary fan RB, when a single primary fan is running, the load gradually decreases, and the air preheater (hereinafter referred to as the air preheater) will continuously increase the air leakage; during the process of switching the number of coal mills, the primary air system will have a pipe network resistance. In the event of a change, a wind takes a short cut, bypassing a large amount of air through two air pre-equipments and a primary air contact door, and the entrance of the tripping fan counteracts a lot of air leakage.

1 primary air leakage

Check the leakage of manholes, flanges, etc. in the primary air duct to eliminate leaks and reduce the amount of air leakage. When necessary, press the powder system and smoke leaks.

2 air preheater air leakage

The factors affecting air leakage in air preheater include air pressure, temperature of flue gas, and manufacturing process.

The leakage rate of air preheater and primary air leakage are different concepts. The former refers to the total air leakage of primary air and secondary air. The three-position rotary air preheater has a design air leakage rate of 6% to 10%. The amount of primary air leakage accounts for most of the total air leakage, which is more than 80%. When the load is low, the primary wind leakage rate accounts for 30% to 40% of the total primary air volume, or higher.

The air leakage rate of the air preheater is a major assessment indicator for the unit to be put into operation. It can generally be achieved at the initial stage of production. In the long cycle operation of the unit, there is a widespread phenomenon that the air leakage rate exceeds the standard. Air preheater seal gap increase is closely related to air preheater low temperature corrosion, rotor deformation and seal wear.

With the continuous reduction of the load of the unit, the air leakage rate of the primary air system is increasing. The primary air leakage rate under the same load is related to the operating mode, such as the primary air pressure and the number of coal mills being operated.

The air precipitator will increase the resistance of the primary air system and limit the output of the fan.

(3) Not put into operation coal mill

The RB logic does not consider the ventilation of the unprimed coal mills. It only trips the upper coal mills and only keeps 2 to 3 coal mills in the middle and lower layers of the coal mills. When the coal mill is not put into operation, the air pressure of the system will also be reduced, which will affect the normal combustion of the primary air.

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