Cause analysis and preventive measures of main sha

2022-08-19
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Cause analysis and preventive measures of main shaft bending of steam turbine Abstract: This paper introduces the accident of main shaft bending caused by steam turbine shutdown before a minor repair in Zhanjiang Power Plant, and puts forward measures to prevent main shaft bending

key words: major axis; bend; reason; Measures

abstract: the accident of main shaft bending caused by steam tur bine during shutdown before a routine maintenance in Zhanjiang Power Plant is introduced ● standard configuration of accessories: one set of wedge-shaped stretching accessories, one set of tightening accessories and one set of zigzag accessories respectively uced, and the mea sures preventing main shaft from bending are put forward

Key words: main shaft; bending; cause; Measure

the No. 4 steam turbine of Zhanjiang Power Plant is produced by Dongfang Steam Turbine Plant, and the model is n300-16.7/537/537-3 (combined cylinder). The type is subcritical intermediate reheat, two cylinder two exhaust, condensing steam turbine, and the flow stages are 1 regulating stage + 9 pressure stage + 6 intermediate pressure stage + 2 low pressure stage × Stage 6, steam distribution mode is nozzle adjustment. On October 28th, 2003, during the shutdown before minor maintenance, there was a bending accident of the main shaft of the steam turbine caused by the wide fluctuation of steam temperature and pressure due to the improper monitoring and adjustment of the operators, the indecisive handling

1 equipment damage

the inspection on uncovering the inner and outer cylinders of high and intermediate pressure steam turbine on November 23, 2003 found that:

1) after measuring the bending degree of high and intermediate pressure rotor, the journal runout value at the middle steam seal of high and intermediate pressure is the largest, with a value of 0.62 mm and a bending degree of 0.31 mm, indicating that the high and intermediate pressure rotor has been permanently bent

2) after all the HP and IP rotors and lower diaphragms were lifted out, it was found that there were new wear marks on the shrouds of the pressure moving blades from the HP regulating stage to the 8th stage, and there were old wear marks on the shrouds of the pressure moving blades of the IP rotors at the 11th, 12th, 13th and 14th stages

3) the most serious is that the local wear marks of the journal at the middle steam seal of the high and intermediate pressure rotor are deep, and the high and low teeth of the upper and lower steam seals have friction with the shaft, especially on the high-pressure side. Two thirds of the shaft shoulder is worn off, and the oxide layer on both sides of the journal grinding groove is loose

2 cause analysis

1) the fixture of steam temperature and steam in the past usually adopts mechanical locking pressure, which fluctuates greatly and exceeds the emergency shutdown value without decisive shutdown, which is the direct cause of the bending of the main shaft of the steam turbine this time. During this sliding parameter shutdown, especially when the boiler combustion was unstable and the main reheat steam temperature was out of control, the main steam temperature fell from 328 ℃ to 272 ℃ within a few minutes, down 56 ℃, which met the conditions for emergency shutdown in the treatment of steam turbine accidents, and the operators did not immediately and decisively shut down. According to the investigation, when the main steam pressure is about to reach 4.2 MPa, the corresponding saturation temperature is 253 ℃. At this time, the main steam temperature is only 272 ℃, and the superheat is only 19 ℃. According to the regulations, the superheat should be greater than 50 ℃. After the low-temperature steam enters the steam turbine, the cylinder body shrinks and deforms, and dynamic and static friction occurs, resulting in the bending of the main shaft of the steam turbine

2) the main cause of the accident is the imperfect preventive measures, procedures and systems. During the sliding shutdown of the unit, the professional management personnel and the operation attendant did not fully understand the severity of the impact hazard of the steam turbine caused by the abnormal fluctuation of parameters caused by the unstable combustion of the boiler caused by knocking and emptying the powder bin, and did not make sufficient accident prediction and take preventive measures for the special operation mode. There is no shutdown curve for operators to compare during operation in the turbine operation specification, such as the temperature and pressure reduction curve during normal shutdown of the unit, the decline curve of the main metal temperature of the cylinder after shutdown, the coasting curve during normal shutdown and the coasting curve during emergency vacuum breaking shutdown

3) the failure to pay attention to the abnormal conditions in the shutdown process and to organize professional technicians to conduct in-depth analysis in time is an important reason for the unsuccessful startup of the unit five times after minor maintenance, the extension of maintenance time and the expansion of accident losses. For example, the idling time of the rotor is shortened from the normal value of 4 min to 21 min during shutdown, the vibration of the unit is not monitored when the unit speed drops below 2000 R/min, and the memory data of DCS historical database is lost after shutdown

3 accident prevention measures

1) according to the specific characteristics of the unit and the requirements of the manufacturer, the typical startup curves (cold, warm and hot sliding parameter startup) and shutdown curves (temperature and pressure reduction curves of sliding parameter shutdown and medium parameter shutdown, the decline curve of the main metal temperature of the cylinder after shutdown, the idling curve of normal shutdown and the idling curve of emergency vacuum breaking shutdown) of the unit under various states should be formulated, And incorporated into the operating procedures

2) in the process of sliding parameter shutdown, professional technicians should formulate and issue the sliding parameter shutdown plan and preventive measures according to the special situation introduced by Zhu Changchun, Secretary General of China Polyurethane Industry Association, and the personnel on duty should strictly implement it. For example, when the pulverized coal bunker is emptied, anti accident technical measures should be formulated to prevent uneven pulverized coal feeding, unstable combustion, deflagration and other abnormal conditions, resulting in large fluctuations in steam temperature, etc

3) the monitoring instruments of the unit must be intact and accurate, and be calibrated regularly, especially the shaft bending meter, vibration meter and cylinder metal thermometer, which should be statistically assessed according to the thermal supervision regulations

4) in case of one of the following conditions, it shall be shut down immediately:

(1) during normal operation of the unit, the temperature of main and reheat steam suddenly drops by 50 ℃ within 10 minutes; Or the temperature of main and reheat steam suddenly rises or drops by 50 ℃ within 10 minutes during the start and stop of the unit

(2) when the temperature difference between the upper and lower cylinders of the high-pressure outer cylinder exceeds 50 ℃, and the temperature difference between the upper and lower cylinders of the high-pressure inner cylinder exceeds 35 ℃

(3) during the start-up of the unit, if the speed is below 1200 R/min, the bearing vibration exceeds 0.03 mm, when the critical speed is exceeded, the bearing vibration exceeds 0.1 mm or the relative shaft vibration value exceeds 0.25 mm, the brake shall be shut down immediately, and it is strictly prohibited to warm up by forcibly passing the critical speed or reducing the speed

(4) during the operation of the unit, it is required that the bearing vibration should not exceed 0.03 mm or the relative shaft vibration should not exceed 0.08 mm. If it exceeds, try to eliminate it. When the relative shaft vibration is greater than 0.25 mm, immediately switch off and shut down; When the bearing vibration changes by 0.015mm or the relative shaft vibration changes by 0.05 mm, find out the cause and try to eliminate it. When the bearing vibration suddenly increases by 0.05 mm (or the sudden increase in vibration does not reach 0.05 mm, but the unit sound is abnormal, and there is abnormal sound in the machine), immediately switch off and stop the machine

5) before starting the turbine, the following conditions must be met, otherwise it is forbidden to start:

(1) the indications of shaft shaking, shaft string, differential expansion, low oil pressure, speed and vibration are correct and put into normal operation

(2) the shaking value of the main shaft shall not exceed the specified value of the manufacturer, or be greater than 0.03 mm of the original value

(3) the temperature difference between the upper and lower cylinders of the high-pressure outer cylinder shall not exceed 50 ℃. The temperature difference between the upper and lower cylinders of the high-pressure inner cylinder shall not exceed 35 ℃; Before starting, the temperature difference between the upper and lower cylinders exceeds the standard. It is not allowed to reduce the temperature difference between the cylinders by heating the interlayer to meet the starting conditions

(4) the main steam temperature must be higher than the rated steam temperature; The superheat of steam shall not be less than 50 ℃

(5) when the unit stops due to abnormal vibration during impulse starting and must return to the turning state, it should be comprehensively checked, carefully analyzed and found out the cause. When the unit has met the starting conditions, it is allowed to start again after continuous turning for not less than 4 hours. Blind starting is strictly prohibited

(6) before the hot start of the unit, the shutdown records and the cylinder metal temperature records after shutdown should be checked, and compared with the continuous deepening of aluminum application in the light-weight field of normal shutdown vehicles, the curve of China Zhongwang cooperating with customers in-depth and the cylinder metal temperature decline curve under normal conditions. If there is any abnormality, carefully analyze it, find out the cause, and take measures to deal with it in time

(7) after shutdown, carefully check and monitor the water level of condenser, deaerator and heater to prevent cold steam and cold water from entering the turbine and causing rotor bending

(8) put the turning gear into operation immediately after shutdown. When the turning current is larger than the normal value, swings or has abnormal sound, it should be analyzed and handled in time. When the shaft seal friction is serious, it should be changed to manual turning for 180 ° first, and put into continuous turning after the friction basically disappears. When the turning gear does not move, forced turning is prohibited

9) during hot start-up, the shaft seal should be put into operation first and then vacuumized. The high-temperature steam source used by the high-pressure shaft seal should match the metal temperature. The shaft seal steam pipe should be fully warmed and drained to prevent cold water or cold steam from entering the turbine from the shaft seal

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