Reasons and Treatment of Vacuum Degradation of High-pressure Unit

Reasons and Treatment of Vacuum Degradation of High-pressure Unit

Reasons and Treatment of Vacuum Degradation of High-pressure Unit
The core prompts: 1 The problem with the unit The Meishan Power Plant No. 6 turbine unit is a N8.83-535 unit produced by a factory in Beijing. Since the start of production in 1994, the unit vacuum has experienced various degrees of declines at various times during the peaking of the unit, and has even worsened. This problem has not been effectively solved for many years.

1 Problems existing in the unit The turbine unit No. 6 of the Meishan Power Plant is a N8.83-535 unit produced by a certain factory in Beijing. Since the commencement of production in 1994, the unit vacuum has experienced various degrees of declines at various times during the peaking of the unit, and has even experienced serious deterioration. This problem has not been effectively resolved for many years. Repeated leak detection and vacuum tightness testing of the unit failed to find out the reasons for this and laid a hidden danger for the safe operation of the equipment. During the peaking of the unit, the degree of vacuum declines to varying degrees, which brings the following effects: On the one hand, due to the increasing peak period of the electricity valley, the peaking capacity of the unit is restricted; on the other hand, the vacuum reduction of the unit will increase the steam consumption of the unit. Exhaust steam temperature rise, affect the unit's thermal expansion, and even change the unit's center caused the unit's vibration. For every 1kPa drop in the unit vacuum, the exhaust temperature will increase by 3*C, and the steam consumption will increase by 1.5%~1.6%. The effective enthalpy drop of the unit will be reduced and the steam will be weakened. According to the above, if the exhaust pressure is from 0.004MPa When the temperature rises to 0.006MPa, the thermal efficiency will be reduced by 2.2%. The changes in the parameters before and after the peak regulation of the unit in different periods are listed in Table 1. It can be seen from Table 1 that when the unit is at full load, the vacuum and the end-to-end difference remain good. However, with the reduction of the random load, the vacuum began to deteriorate. For example, the unit load decreased from 50 MW to 29 MW, the end-to-end difference increased from 4*C to 21*C, an increase of 17*C, and the lower the unit load, the more the vacuum decreased. Severely, the greater the increase in terminal difference. The increase of the terminal difference not only affects the heat exchange capacity of the condenser, but also increases the steam consumption of the unit and negatively affects the economy of the unit.

2 The internal cause of the problem and its treatment Table 1 Date Load vacuum exhaust temperature...

Condenser end differential main steam flow inlet temperature...

Water temperature...

It increased, but the vacuum decreased when the unit peaked. When the vacuum drops, the operator usually adopts the method of adjusting the pressure of the shaft seal, or carries out tentative plugging of some negative pressure systems, but the effect is not obvious.

Because the increase in condenser end difference occurs when the load of the unit decreases, the leakage of air is the main cause of the vacuum drop of the condenser. When the load of the unit decreases and steam flow changes, some of the unit The operating conditions of the system also change, and the air enters the condenser. Therefore, looking for air leakage, we must first understand the conditions of shaft seal steam conditions. The shaft seal steam of this unit is supplied by the steam balance of the deaerator. Under normal circumstances, the steam source has the characteristics of stable pressure and temperature. It is used for a long time in operation, and the heating steam of the deaerator mainly has a door rod leakage. Steam, high-gather hydrophobic, steam exhaust from continuous expansion tanks, and steam extraction from section 3 of the unit. When the unit is running at full capacity, the pressure of the deaerator has reached 0.6MPa~0.62MPa (absolute pressure) and exceeds its rated pressure. Therefore, during the operation, the third stage of the unit is out of steam. When the unit is peaking, the heating steam supplied to the deaerator will be rapidly reduced. Due to the sudden drop in heating steam, the deaerator is in accordance with common sense. When the load of the unit is reduced, the vacuum should be due to changes in steam flow.), M, Guangdong Heyuan, who runs squad leader and technician, is mainly engaged in the operation, maintenance and maintenance of steam turbine equipment.

When the unit load is greatly reduced, the steam leakage of the high pressure shaft seal will be reduced. Due to the action of the air extractor, the negative pressure of the shaft seal heater will change from the full load of the unit and the pressure in the deaerator will change. Non-saturated water (due to the steam extraction in the third stage, the water in the deoxygenation tank is insufficient for heating, the deaerator pressure is up to 6 MPa, and the water temperature of the deaeration tank is only 152*C). Boiling occurs due to a sudden drop in gauge pressure (eg, unit When the load is quickly reduced from 51MW to 43MW, the deaerator pressure will drop from 0.61MPa to 0.56MPa). Due to the boiling effect, a large amount of wet saturated steam is precipitated from it. The humidity of the wet saturated steam in the deaerator increases, which will reduce the specific volume ratio of the wet saturated steam supplied to the shaft seal, resulting in a sharp drop in the shaft seal and air entering. After the steam seal caused a vacuum drop. At this time, although the pressure of the shaft seal can be adjusted to increase the steam supply of the shaft seal, the throttling effect of the shaft sealing control valve is weakened, and the moisture content of the throttling steam is further aggravated, and the specific volume will continue to decrease. Steam seal with wet steam injection.

Therefore, during the peaking process of the unit, the stability of the deaerator pressure should be maintained, and as much as possible, load shedding should be avoided. At the same time, the third stage steam extraction should be opened in time to prevent rapid changes in the deaerator pressure.

With the above method, the unit can achieve better results when it is running at more than 70% load. However, if the unit peak load is lower than the above-mentioned rated load operation, it is still not enough according to the above method. At the same time, the following measures are taken: In the normal operation, the shaft seal gas comes from the steam balance gas source of the deaerator, and the pressure of the shaft seal After the valve is throttled, the dry saturated steam at pressure 106*C is supplied to the front and rear shaft seals of the steam turbine (as shown). The front steam seal has a total of 5 steam leaks: 1, 2, and 3 are high pressure steam leaks. The file is connected to the second, fifth, and seventh-stage extraction pipes of the main engine in turn. The fourth gear is the shaft-seal steam extraction file, and the remaining steam-swing shaft exhaust fans are exhausted. The fifth gear is the shaft-seal steam supply file. .

The new steam comes from -0.002 MPa of the I~X3-regulator valve to -0.(1)8MPa. Due to the fast flow rate and large flow of the vapor, the steam used by the front and rear steam seals of the unit is reduced, and the air enters the front and rear steam seals. In addition, the third-stage steam leakage from the shaft seal is connected to the seventh-stage steam extraction pipe of the main engine. The hot air in front of the steam turbine is entered by the fifth and fourth gears of the shaft seal under the action of the negative pressure of the steam condenser. , and then through the seventh paragraph of the extraction pipe into the exhaust cylinder, condenser vacuum dropped sharply, the end difference increased sharply. Therefore, when the unit load changes significantly (reduction of load), the steam leakage in the shaft seal should be promptly turned off in order to promptly shut off the leakage of steam in the 1st and 2nd gears of the high pressure shaft seal and timely remove the door rod for the deaerator. The role of this is: on the one hand, it can increase the third-stage leakage of steam supplied to the high-pressure shaft seal; secondly, it can heat the high-pressure front steam seal to prevent the negative difference from drastically increasing due to the rapid cooling of the front-side steam. If the load of the unit continues to decrease, the gate valves of the third to seventh-stage extraction pipes of the high-pressure shaft seal can be closed to prevent the hot air from entering the condenser. At the same time, the steam of the shaft seal heater can be switched to the shaft exhaust fan. go.

3 Conclusion The reason for the vacuum affecting the unit is that the steam supply system of the shaft seal system changes during the peaking of the unit, resulting in insufficient steam supply for the shaft seal and a drop in vacuum. At the same time, as the load of the unit continues to decrease, the leakage of the front high pressure shaft seal is reduced, and the hot air passes through the shaft seal into the 7th section of the extraction pipe, causing the vacuum of the condenser to deteriorate.

When the unit is peaking, depending on the vacuum change, the shaft seal can be used to convert to a new steam source. Because the new steam has the characteristics of high temperature and large specific volume, it is also possible to consider starting the shaft exhaust fan.

The hydrophobic water level of the high-pressure heater is strictly controlled during operation (it has been in a water-free running state). Maintaining a reasonable water level can avoid the problem of excessive pressure caused by the trapped steam entering the deaerator. At the same time, rationally adjust the opening of the continuous expansion vessel to the vapor balance valve, and try to maintain the fully open state (only 5% of the original opening is operated).

Ensure that the third stage extraction of the unit is used as the first heating steam for oxygen removal, so that the water in the oxygen removal tank is saturated.

When the unit is operating at full load, the No. 4 low pressure heater outlet temperature (152*C) has exceeded the rating (148*C). In order to ensure sufficient steam injection in the third stage of extraction, moderately reduce the amount of steam entering the No. 4 low pressure heater to ensure that the outlet temperature does not exceed the rated value.

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