Analysis and attention of extreme hot start proces

2022-10-03
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220MW steam turbine extreme hot start process analysis and precautions

steam turbine extreme hot start refers to the start-up in which the metal temperature of the upper cylinder inner wall of the high-pressure inner cylinder of the steam turbine is greater than 400 ℃. It is a start-up method that poses the greatest threat to the steam turbine. If the temperature difference of the cylinder wall is large or the operation is improper, it will produce large thermal stress, cause dynamic and static friction of the steam turbine, and even bend the main shaft

the starting mode of steam turbine is divided into cold start (wall temperature 150 ℃), warm start (150 ℃ ~ 300 ℃), hot start (300 ℃ ~ 400 ℃) and extremely hot start (wall temperature greater than 400 ℃) according to the metal temperature of the inner wall of the upper cylinder of the high-pressure inner cylinder of the steam turbine. The centralized control operation procedure of Hebei Xingtai Power Generation Co., Ltd. (hereinafter referred to as the procedure) stipulates that the temperature difference between the upper and lower cylinders of the inner (outer) wall of the high-pressure inner cylinder is greater than 35 ℃, Or when the temperature difference between the upper and lower cylinders on the inner (outer) wall of the high-pressure outer cylinder or the intermediate pressure front cylinder is greater than 50 ℃, it is forbidden to start the turbine

the company's No. 5 unit is a 220MW

steam turbine generator unit produced by Dongfang Steam Turbine Plant. It was put into operation in October 1986. In December 1999, the DCS thermal control and DEH system were transformed, and the flow path was transformed at the same time. Now the rated power is 220MW. During the normal operation of the unit, it was found that the valve rod of No. 1 fuel injection test valve was automatically jacked up to eliminate the defects at low ebb. It was found that the connecting nut of the valve rod of No. 1 fuel injection test valve was loose. After eliminating the abnormal conditions, the unit was ready to start

1 starting process

at present, there are various "haze" weather in major cities. 3t03:10, boiler 5 was ignited. At that time, the temperature of the upper wall of the high-pressure inner cylinder was 420 ℃, which was an extremely hot start. In the process of raising parameters, when the main steam temperature is higher than the upper wall temperature in the high-pressure inner cylinder, open the drain valves of the flange, mezzanine heating header and sub header to warm the pipes of the flange and mezzanine header. The drain valve of steam seal high-temperature standby steam pipe is opened, and the pipe is warmed in front of the valve. At 04:20, the temperature difference between the upper and lower walls of the high-pressure outer cylinder suddenly increased from 21 ℃ to 132 ℃, and the inspection found that the temperature of the lower wall in the high-pressure outer cylinder suddenly decreased from 390 ℃ to 280 ℃, and the temperature of other cylinder walls was normal. Local inspection showed that the first and second steam inlet valves of the mezzanine header were in the closed position, and at 05:50, the temperature difference returned to normal (less than 50 ℃). When the unit is locked and warmed up, the turning gear is jacked off, the speed reaches 20r/min, the brake is opened, and after the rotor is stationary, the continuous turning gear is put into operation, and the brake is locked again. At 05:55, when the conditions are met, rush, and at 06:10, set the speed. The fuel injection test was normal. At 06:20, No. 5 steam turbine generator unit was connected with the system

2 process analysis

(1) the operating regulations stipulate that when the main steam temperature is higher than the upper wall temperature in the high-pressure inner cylinder, check that the steam inlet valve of the high-temperature standby steam source of the steam seal and the three steam supply valves are in the closed position, and the steam supply valves of the flange and sandwich heating header are in the closed position. Open the drain valve of header and sub header, and warm the pipe of flange and sandwich header. The drain valve of steam seal high-temperature standby steam pipe is opened, and the pipe is warmed in front of the valve. Pay attention to that the unit should not be impulsive during pipe warming. During the process of warming the steam seal and draining water to the flash tank, a special person should be assigned to monitor the temperature change of the cylinder wall, monitor the water level of each heater, and take measures to deal with abnormalities in time. Since the drain pipe of the mezzanine heating header of unit 5 and the drain pipe of the high-temperature standby steam source of the steam seal are collected into a drain main pipe and then discharged to the high-pressure drain flash tank, the drain of the high-temperature standby steam source of the steam seal flows back to the mezzanine header, and because the steam inlet valve of the upper header leaks, the drain directly enters the jacket, so the temperature difference between the upper and lower inner walls of the high-pressure outer cylinder rises linearly to

132 ℃. At present, the temperature of mezzanine heating header is about

290 ℃ during normal operation of the unit. Properly opening and closing the steam supply valve of the upper header of the movable mezzanine during operation is invalid, and there is still leakage

(2) during this extremely hot start-up, the opening of the desuperheating water valve of the drain flash tank was too small. At that time, the temperature in the drain flash tank was about 300 ℃, and no high vacuum was formed, resulting in poor drainage. It was cascaded back to the mezzanine heating header, causing the lower cylinder temperature to decrease. The procedure stipulates that when the unit was started in a warm and hot state, the unit can be drained to the drain flash tank only when the condenser vacuum was more than 40kpa. Before changing the drain, fully open the desuperheating water valve of the high-pressure drain flash tank, and master the appropriate opening of the drain valve according to the pressure in front of the turbine and the pressure of the flash tank. The operator on duty did not strictly follow the regulations

(3) when the unit is locked and warmed up, after a short delay, the turbine rotor is impulsive, and the turning gear automatically trips. The main reasons are that the opening of the steam regulating valve of the stage II bypass is small, the reheat steam can be postponed after being approved by the professional engineer in charge under special circumstances, the pressure is high (at that time it was

2.0mpa), the check valve of the high-pressure exhaust is not tight, the steam after the stage I bypass flows into the high-pressure cylinder in reverse from the exhaust steam of the high-pressure cylinder, and the steam turbine rotor is impulsive. The regulation stipulates that when the bypass system is put into operation, the level II bypass shall be put into operation first, and then the level I bypass shall be put into operation to prevent the reheater from overpressure. During warm start-up, pay attention to adjusting the opening of class I and II bypass steam regulating valves (the opening of class II bypass steam regulating valve should be greater than that of class I bypass steam regulating valve), and keep the reheat steam pressure not greater than 0.4MPa. After the bypass system is put into operation, when the steam temperature after the stage I bypass (the opening of the steam regulating valve should be greater than 10%) reaches 360 ℃, slightly open the desuperheating water valve of the stage I bypass to maintain the steam temperature at 300 ~ 360 ℃. When the steam temperature after stage II bypass reaches 160 ℃, open the desuperheating water valve of stage II bypass to prevent overtemperature after stage I and II bypass. Before shutting down the bypass system, the desuperheating water valve should be closed first, and the bypass system should be shut down according to the principle of shutting down the level I bypass first and then the level II bypass, which will be increased to 43% of the bypass system in 2015. The operator on duty did not strictly follow the regulations, and the opening of level I and II bypass steam regulating valves did not match, resulting in the rise of reheat steam pressure

3 precautions for extreme hot start

(1) high vacuum shall be maintained in the condenser for extreme hot start, so as to facilitate the smooth discharge of all drains. According to the regulations: drain water to the front of the flash tank, and fully open the spray desuperheating water valve of the flash tank

(2) in the process of warm start-up and lifting parameters, the bypass system should be put into operation, and the reheat steam pressure should not be greater than 0.4MPa. Before running, open the automatic main steam valve 40mm (full stroke 80mm) with the start valve, warm up the pipe for 10 ~ 15min, and the reheat steam pressure should not exceed 0.2MPa during this period, so as to prevent the high-pressure exhaust check valve from being loose and the turbine rotor from being impulsive

(3) start in extremely hot state, turn on the drain preheating of flange and sandwich header, and check whether the rotor is impulsive in time. At this time, the temperature of the cylinder is very high, and the flange and sandwich heating device can not be put into operation according to the specific situation. According to the regulations, the service conditions for putting the heating device into operation are: when the unit is started, put the cylinder interlayer and flange heating device into operation according to the cylinder temperature, flange temperature, differential expansion and other conditions, that is, when the temperature difference between the upper and lower walls of the high-pressure outer cylinder or the temperature difference between the inner wall of the high-pressure outer cylinder and the outer wall of the high-pressure inner cylinder is 20 ~ 40 ℃, put the cylinder interlayer heating device into operation; The temperature difference between the inner wall of the high-pressure outer cylinder and the inner wall of the flange reaches 25 ℃, and the flange is put into heating

according to the cylinder temperature, the unit quickly rises to the starting load point after merging. When the upper wall temperature in the HP inner cylinder is 420 ℃, the corresponding starting load point is about 150MW. The regulation stipulates that when the load rises to 180MW, the flange and sandwich heating devices shall be stopped

(4) make the change trend chart of cylinder temperature and temperature difference in the DCS screen and closely monitor it. If abnormal conditions are found, immediately analyze the causes to prevent further expansion of the situation

4 improvement measures

(1) it is recommended to separate the drain pipe of mezzanine heating header from the drain pipe of steam seal high-temperature standby steam source, and connect them to the high-pressure drain flash tank respectively

(2) based on the principle of required maintenance and good maintenance, improve the maintenance process, ensure the maintenance quality of the valve, and create good conditions for the safe and economic operation of the unit

(3) strengthen technical training and improve the professional quality of operators on duty

(4) strictly implement the procedures, strengthen the tracking and inspection in the key process, and enable the duty officer to develop a good working habit of consciously observing the rules and regulations

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