Hebei Thermal Power Co., Ltd. has installed two CC200508.83/0.98/0.16 double-cylinder double-cylinder steam-fired reaction-type heating units manufactured by two ALSTOM companies for the eight-phase technological transformation project (hereinafter referred to as the project), and four (2) Furnace 1) Dongfang Boiler Co., Ltd. manufactured DG410/9. 811 CFB boiler and two ALSTOM company 50WX23Z10.9 static tape TEWAC system (ie, fully enclosed Feng Shui cooling system) generator.

Since the operation of 08, there have been many times the phenomenon of slow condenser vacuum reduction. Through regular vacuum system tightness test, the measured vacuum is reduced to 0.15 kPa/min DL/T6081996200 MW class turbine operation guide I provisions: vacuum reduction within 5min Less than 0.4 kPa/min means that the stringency is acceptable, and it is determined that there is no problem of air leakage. Each condenser vacuum reduction occurs shortly after the backwater of the feedwater pump is poured back into the condenser. After the seal water is returned to the trench, the vacuum of the condenser quickly returns to normal and no vacuum reduction occurs. It can be seen that the change of the operating mode of the feed water pump sealing water system affects the vacuum of the condenser. The following is a detailed analysis of the reasons for this phenomenon.

1 System Introduction 1.1 Feedwater Pump Sealing Water System Each unit water supply system of this project is composed of a DG-440-140 fixed speed pump and two DGT-440-140 type variable speed pumps. Both the drive end and the free end of the feed pump must be sealed with a water seal to prevent the high pressure and high temperature water from leaking out of the pump and leaking air from the low pressure side.

The main condensate is used as the sealed water source. In the start/stop, accident and abnormal conditions, the feedwater pump seal water is supplied by the seal pump. Sealed water backwater has 2 routes, 1 road is discharged into the trench, and 1 road is returned to the condenser through the water seal cylinder.

1.2 Water Sealed Water Pump Sealed water is returned to the condenser after being sealed by a 3-level water seal. The bottom of the second stage water seal tube is connected with a water seal tube with cut-off door, and the replenishment source is desalinated water.

1.3 Sealing of the shaft end of the feed pump The feed water pump used in this project is a horizontal multi-stage centrifugal pump. There are 9 impellers in the rotor and there is an induction wheel at the entrance. The balance device is an integral balance drum and balance plate structure. The shaft seal of the feed pump adopts a spiral seal structure. The outer circle of the shaft is processed with a double-head spiral groove, and the inner side of the seal bush is provided with a rectangular groove corresponding to the concentricity. The pump's drive end is a left-handed spiral groove and the free end is a right-handed spiral groove. When the shaft rotates, a pumping action is provided to send water into the pump to prevent leakage of the liquid in the pump. The clearance between the spiral groove on the shaft and the sealing sleeve can play a role in throttling and depressurizing, so that the water in the pump can be rapidly depressurized after entering the sealing sleeve, and is overcome by the sealing water entering the pump. Leakage to achieve a sealing effect. The sealing water is formed by a condensing jellyfish tube, which is depressurized to a pressure higher than the inlet pressure of the pump by at least 0. 10 MPa after the seal water is adjusted to enter the shaft seal. Most of the sealing water enters the pump through the spiral groove, except that a very small portion is throttled down along the spiral groove tip clearance and flows out of the pump-end drain hole, and the rest is returned to the condenser by the water-sealing cylinder.

2 Analysis of the reasons for vacuum reduction 2.1 Operating conditions of the water seal cylinder Before the sealing water of the feed water pump is returned to the condenser, the vacuum of the condenser does not drop, indicating that the water seal cylinder is tight. Since the pipeline from the water-sealed tube to the condenser is indirectly out of the top of the third stage of the water-sealing tube, there is space at the top of the water-sealing tube, and the air that leaks from the backwater of the sealing water accumulates at the top of the water-sealing tube, when the air accumulates to a certain degree. When the water seal inside the water seal was damaged, the air and water in the seal water return pipe were mixed into the condenser and the vacuum was lowered.

2.2 Change in sealing water pressure When the sealing water pressure is low, the sealing water entering the pump is stable due to the pumping action of the mechanical seal of the feed pump, and the amount of water leaking from the gap between the shaft and the sealing device is reduced, making the air Can enter the sealing water return pipe, the leakage air carried by the sealing water accumulates at the top of the water seal cylinder. When the sealing water pressure is too low, the amount of air carried by the sealing water backwater increases, so that the water seal in the water seal cylinder is quickly destroyed, causing the condenser to drop in vacuum. The high sealing water pressure will cause the feed water pump bearings to enter the water, so that the oil quality of the lubricant is deteriorated, which poses a threat to the safe operation of the pump. Therefore, the water pressure of the feed water pump cannot be kept too high.

2.3 Start/Stop Feedwater Pump and Variable Condition Operation When too much air accumulates on top of the water seal cartridge, the water seal in the water seal cartridge is quickly destroyed, causing the condenser to drop in vacuum.

When the sealing water pressure is constant, if the speed of the speed-regulating feed pump is increased, since the rotating speed cube is proportional to the flow rate, the pumping function of the mechanical sealing device of the pump is enhanced, thereby increasing the amount of sealing water entering the pump and reducing the amount of water leaking from the clearance. Sealed water return pipe leaks into the air.

2.4 Sealed Water Sources The feed water to the water pump seal has two sources of water. One is carried by the sealed pump before and during the start-up and the other is carried by the condensate pump after the start-up. Sealed water pump water supply from the demineralized water, due to more users of desalinated water pressure, many factors, it is extremely difficult to maintain stability, and at the same time because the seal pump flow is difficult to maintain a high seal water pressure, so when the seal water is brought by the seal pump Condenser vacuum drops.

2.5 Condenser vacuum changes found through field tests, when the condenser vacuum is high, the water pump seal water seal tube is evacuated, the air returns to the condenser through the feed pump to the condenser and affect the vacuum; On the other hand, when the condenser vacuum is poor, the water seal of the feed water pump can maintain the water level in the cylinder, thereby ensuring that the condenser vacuum is not affected by the backwater of the sealing water.

2.6 The system connection is not in accordance with the design of the water seal cylinder manufacturer ALSTOM designed the feed water pump backwater to the condenser inlet at the bottom of the condenser, and the water seal cylinder manufacturer designed the sealing water back to the condenser inlet and water. The difference between the bottom of the seal cylinder is 7m. See the design diagram of the water seal cylinder manufacturer.

The design of the water seal tube factory and the on-site connection method does not conform to the design of the water seal tube manufacturer, and the actual height difference is only 4m. In addition, the water seal tube make-up tube is not designed and installed according to the water seal tube manufacturer (the manufacturer's design is installed in the first-class water seal tube Top) Through field tests and observations, it was found that the first-stage water-sealed cylinder was basically empty, and the injection was not full of water. This was related to the installation position of the water-filled-tube refilling tube and the height of the water-sealing cylinder.

Measures taken at 3 meters In view of the above-mentioned reasons for the vacuum effect of the sealed water system, the following measures have been taken in the existing operating mode to better maintain the vacuum.

Increase the water pressure of the feedwater pump until the drainpipes drip, preventing air from leaking.

When the feed pump is in standby, the seal water pressure should be kept slightly higher than the pressure at which the pump operates, preventing the seal water pressure from decreasing when the pump is started.

Appropriately limit the total water return gate of the sealing water return and increase the backwater pressure of the sealing water on the pump side.

d Strengthen monitoring, especially when changing the working conditions, adjust the water pressure of the feed water pump in time.

e. Open the water fill door of the water seal when the vacuum is high.

4 Conclusions and recommendations Add U-shaped pipe to the condenser tube from the water seal tube to meet the height difference requirement in the design of the water seal tube manufacturer.

The water supply source of the water-sealed cylinder shall be changed to a two-way belt of condensate and demineralized water (adding throttling holes).

In accordance with the design of the manufacturer, the water fill port of the water seal tube is connected to the top of the water seal tube of the first stage.

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