Unit 2 of a power plant is N660-25The new unit 00 600 600 is under construction and is currently in the start-up phase of the commissioning process. The unit has been idling for a long time in the previous shutdown process. Before this start, the rotor was abnormally impulsed in the vacuum state of the crank. It is understood and on-site information feedback that the phenomenon appeared after the second set of starts.
1.Unit Overview
Unit 2 of the power plant has a rated load of 660MW, ultra-supercritical, primary intermediate reheat, single-shaft, three-cylinder four-exhaust steam, double back pressure, condensing steam turbine, using high-pressure and low-pressure two-stage tandem steam turbine bypass, with a bypass capacity of 40 bmcr. The unit is operated in the mode of sliding-sliding and constant pressure.
2.Description of the fault
The idling time of the unit during the previous shutdown was seriously long (about 25h), and the normal idling time of the general unit should be about 55 60min (about 75 minutes in our factory). When the crank was put into use and vacuumed before the start-up, the rotor speed was rushed to 5 10 r min, which caused the low-speed cranking device to jump automatically.
3.Failure analysis
It was understood at the scene that there was no abnormal impulse when the No. 2 machine was vacuumed during the first start, but the abnormal impulse phenomenon has been present since the second start, indicating that steam entered the steam turbine during the first impulse process, and the impulsive rotor made the rotor accelerate abnormally. The reason for the long idle time of the shutdown is generally due to the existence of incomplete cut-off channels during the shutdown process, which causes the steam with the ability to do work to enter the inside of the steam turbine, the friction resistance of the rotor is partially canceled, and the duration of the speed drop is seriously prolonged.
The starting mode of the unit is medium pressure cylinder starting: according to the characteristics of the field equipment and systemto analyze and find the fault point from 7 aspects:
1) Data analysis of the wear of the shaft seal during the first start, the pressure and temperature of the shaft seal, etc.
2) Whether the tightness of the high and medium pressure main valves and adjusting valves is qualified, and whether the tightness test parameters are up to standard.
3) Whether the tightness of the high and low voltage bypass doors is qualified.
(4) Whether the tightness of the high-row backstop door is qualified, and if necessary, the high-row backstop door is dismantled and inspected.
5) Whether the trap of the high-pressure inner cylinder is tight.
6) Whether the high-pressure steam guide trap is tight.
7) Comparison of changes in the amount of steam leakage from shaft seals.
Combined with the above 7 problems, it is important to find out that there is no abnormal impulse in the first crank vacuuming, and it is judged that there may be wear and tear on the shaft seal of the steam turbine during the first impulse and operation, which leads to the steam supply of the shaft seal entering the steam turbine to do work in the second and subsequent start-up process and promote the rotor to rush, so it is recommended to analyze the vibration and friction in the process of the first incrimination, and check the steam supply pressure of the shaft seal during the start and shutdown process.
Since the unit is started by a medium-pressure cylinder, the fault point is that the shaft seal leaks large steam after medium pressure and the flushing vacuum is high. Comparative analysis of the temperature parameters of the shaft seal in the field shows that the temperature rise of the medium-pressure connecting pipe is faster than that of the metal temperature of the medium-pressure cylinder, and the metal temperature is higher (the temperature difference between the upper and lower cylinders of the medium-pressure cylinder is large). The main reason is that the medium pressure rear shaft seal leaks a lot of steam, which directly leaks to the rear section of the medium pressure cylinder, and does work through the medium pressure communication pipe to the low pressure cylinder, resulting in the automatic jump of the low-speed crank device.
In addition, the high and medium pressure main valve, adjusting the valve is not tight, will also lead to abnormal steam intake of the cylinder, because the valve and valve tightness test has been completed before starting, it is recommended to analyze the test process and results, to see whether the main and reheat steam pressure (reheat steam pressure is more than 50% of the rated steam pressure) in the test process meets the requirements of the test standard.
Because the high-pressure steam takes the bypass in the early stage of the start-up and shutdown process, it is necessary to check whether the high-pressure bypass door is tight, analyze the pressure and temperature before and after the high-side closure and the middle side closure in the shutdown process, the pressure and temperature before and after the high discharge pipe and the metal temperature of the inner wall of the high and medium pressure cylinders, etc., and comprehensively judge whether there is a poor closure of the bypass door. If necessary, the high-row backstop door can be dismantled and inspected in the cold state (the high-row backstop door of the unit is closed to meet the requirements of the test standard when it is cold, but it is closed when it is hot, and it does not meet the standard requirements, causing the unit to overspeed.) The main reason is that the valve plate and the shaft expand unevenly when it is hot, and the corner position is stuck. ）
If the traps of the high and medium pressure inner cylinders are not closed tightly, the pressurized steam may enter the high-pressure cylinder through the trap to do work. The unit does not have a high and medium pressure internal cylinder trap, and it is a medium pressure cylinder starting mode, which can be excluded. Poor closure of the trap of the high-pressure guide pipe may lead to abnormal steam intake of the high-pressure cylinder during shutdown, which can be ruled out according to the analysis of on-site parameters.
4.Conclusions and recommendations
According to the above analysis, it is judged that there is abnormal steam leaking into the cylinder to push the rotor to do work during the start and shutdown of the steam turbine of unit 2 of the power plant
1) The failure point is that the shaft seal leaks large steam after medium pressure, and the flushing vacuum is high. The comparative analysis of the temperature parameters of the shaft seal in the field shows that the temperature of the medium-pressure connecting pipe rises faster than the metal temperature of the medium-pressure cylinder, and the metal temperature is higher. The main reason is that the medium pressure rear shaft seal leaks a lot of steam, which directly leaks to the rear section of the medium pressure cylinder, and does work through the medium pressure communication pipe to the low pressure cylinder, resulting in the automatic jump of the low-speed crank device.
2) The unit does not have high and medium pressure internal cylinder traps, such as when the instantaneous steam temperature of the main steam is low, only open the steam pipe or valve body drainage, otherwise only the brake is stopped, it is recommended to do a good job of water impact accident prediction to prevent the accident from expanding.