Water or cold steam entering the steam turbine may cause serious damage to the equipment. Water impact will cause damage to the blades, rubbing of dynamic and static parts, cracks in the cylinder or permanent deformation, damage to the thrust bearings, etc. In this regard, the design and operation departments must attach great importance. Regarding steam turbine water intake accidents, prevention should be the mainstay. If an accident occurs during operation, prompt and decisive measures must be taken to deal with it. The following discussion is based on the source of water or cold steam:
1. From boiler and main steam system
Due to misoperation or failure of the automatic adjustment device, the boiler steam temperature or drum water level is out of control, which may cause water or cold steam to enter the steam turbine from the boiler through the main steam pipeline. In severe cases, water impulses can occur in the steam turbine.
When the steam turbine enters the water, it must quickly break the vacuum, emergency stop, and open the steam trap on the steam turbine body and the main steam pipe to conduct the drain. Where the shutdown is caused by water impact, the rotor idling time and vacuum change during idling should be correctly recorded. Listen carefully to the internal sound of the steam turbine during the idling process, check the indicator of the shaft shift meter, the thrust pad and the temperature of the same oil. For the intermediate reheating unit, when the main steam temperature drops and a water hammer occurs, the high-pressure cylinder feeds water, which increases the negative axial thrust, so it is important to monitor the non-working pad metal temperature. During the start and stop of the sliding parameters, for some reason, the speed control steam valve suddenly closes, causing the steam pressure to rise, which may cause water accumulation in the steam pipe. When the sliding parameters are stopped, if the cooling rate is too fast and the steam pressure is not reduced accordingly, so that the superheat of the steam is very low, condensate may be generated in the pipeline.
2. From the reheat steam system
The hot steam system is usually equipped with a desuperheating water device to adjust the temperature of the reheated steam. Water may flow back from the cold section of the reheated steam to the high-pressure cylinder or accumulate in the cold section tube. The phenomenon is that the flange of the check valve in the cold section emits white steam and the metal temperature of the lower cylinder of the high-pressure outer cylinder decreases. When the above phenomenon occurs, the boiler personnel should be notified immediately to close the desuperheating water door. 1 The cooling water for the bypass is not strictly controlled, which will cause the same consequences as the above situation. For the hot section of reheated steam, if the diameter of the drain pipe is too small, the drain will not be smooth when starting, and it will also cause water inflow for the turbine.
3. From the steam extraction system
Water or cold steam enters the steam turbine from the extraction pipe, most of them are caused by heater tube leakage or heater system failure. The phenomenon is: the water level of a certain heater rises, the pressure on the steam side of the heater is higher than the extraction pressure, there is a water impact sound in the casing or pipeline, the extraction check valve stem emits white steam or splashes water droplets, and the expansion difference is positive development of. When discovering the above situation, first open the large heater drain regulating valve. If a heater leak is confirmed, stop it immediately. In addition, if the deaerator leaks water, the water may be poured into the cylinder from the steam extraction and door stem leakage.
4. From the shaft seal system
When the steam turbine starts, if the steam seal system is not warm enough, the drain will be taken into the steam seal. In the event of an accident, when the backup steam source is switched, the shaft seal may also enter the water. In normal operation, the steam supply of the shaft seal comes from the unit of the deaerator. If the deaerator is full of water, the shaft seal will bring water. If the shaft seal heater is full of water, the water may be poured into the shaft seal. When the shaft seal enters the water, the steam valve of the shaft seal steam supply pipe should be opened immediately, the steam intake should be properly controlled, the operation status of the water level of the deaerator, the water level of the shaft seal extractor and the shaft seal exhaust fan should be checked separately.
5. From the condenser
The accident of entering water turbines and entering the steam turbine has occurred many times. During normal operation of the steam turbine, the water level of the condenser is taken seriously, and the rise of the water level will seriously affect the vacuum. Therefore, during normal operation of the steam turbine, the water level of the condenser generally does not fill the cylinder. But after shutdown, the monitoring of the condenser water level is often ignored. If the water supply valve entering the condenser is not closed tightly, water will be poured into the cylinder, causing water hammer.
6. From the steam turbine's own drainage system
Return water from the drain system to the cylinder is mostly due to design reasons. If different pressure drains are connected to a header, and the size of the pressure relief pipe is too small, such a high pressure water leak may enter the cylinder from the low pressure pipe. At this time, the accident phenomenon first showed that the temperature difference between the upper and lower cylinders increased, and then the cylinder was deformed, and the dynamic and static parts were rubbed.
The possibility of steam turbine inlet water and cold steam is multi-faceted. According to the thermal system of different units, there will be the possibility of other water sources entering the steam turbine, so the operator should analyze according to the specific situation. In order to prevent the occurrence of water shock, the following measures are emphasized in operation and maintenance:
1) When the main steam temperature and pressure are unstable, pay special attention to monitoring. Once the steam temperature drops sharply to the specified value, usually when it drops linearly by 50 ℃, it should be treated as an emergency shutdown.
2) Pay attention to monitor the metal temperature change of the cylinder and the water level of the heater and condenser, even after shutdown, it can not be ignored. If you find that there is a risk of water ingress, you should immediately identify the cause and quickly cut off the water source that may enter the water.
3) Before the hot start, the main steam and reheated steam should be fully warmed to ensure smooth drainage.
4) When the high-pressure heater protection device fails, the heater cannot be put into operation. Regularly check the heater water level adjustment device and high water level alarm device during operation to ensure that it is always in good condition. When the heater tube bundle is broken, the corresponding steam inlet valve and check valve on the steam extraction pipe should be closed quickly.
5) When the steam parameters cannot be guaranteed after the boiler is turned off, steam should not be supplied to the steam turbine.
6) Strengthen the supervision of the water level of the deaerator to prevent accidents.
7) When the slip parameter is stopped, the steam temperature and steam pressure are gradually reduced according to the prescribed change rate to maintain the necessary superheat.
8) Regularly check the tightness of the reheated steam and the desuperheating water valve of the bypass of Class I and Class II. If a leak is found, it should be repaired in time.
9) As long as the steam turbine is in operation, all kinds of protection must be put in and no exit is allowed.
10) The operating personnel should be clear that the water inlet of the steam turbine at a low speed is more threatening to the equipment. In this case, special attention should be paid to monitor the water inlet of the steam turbine.