Abstract: This paper introduces the working principle of the drainage system of the domestic 50MW steam turbine shaft seal heater, analyzes the reasons why the drainage system of the shaft seal heater of two units in Zhanjiang Biomass Power Plant runs without water level, expounds the harm caused by the vacuum drop of the steam turbine, and puts forward some attempts and ideas for reforming the drainage system of the shaft seal heater. After the successful reconstruction of drainage system, the axial compressor can run at normal water level, which effectively improves the safety and economy of unit operation.

Key words: shaft seal heater; Hydrophobic system; No water level; vacuum

China Library Classification Number: TK26 Document Identification Number: A

Overview of shaft-added hydrophobic system

The drain pipe from the shaft seal heater to the condenser in power plant usually adopts U-shaped water seal pipe. It relies on the pressure difference between the inlet and outlet of the U-shaped water seal pipe to drain water, which is divided into single-stage and multi-stage, and multi-stage water seal pipe is often used in practical application of power plants. Zhanjiang Biomass Power Plant 1 No.2 steam turbines are N50-8.83-5 high temperature and high pressure condensing steam turbines produced by Dongfang Steam Turbine Works. The supporting shaft seal heater model is JQ-50- 1, the heat exchange area is 50m2, and the working pressure on the steam side is 0.095 1MPa(a). The preliminary design scheme of shaft drainage is adopted. The steam turbine is arranged on the operation floor (8m), the shaft seal heater is arranged on the 4.3m floor, and the U-shaped tertiary water seal pipe is arranged on the 0m floor.

Analysis on the operation problems and causes of 1 shaft loading

1. 1 shaft plus no water level operation

Before the shaft seal heater is put into operation, water is injected into the U-shaped three-stage water seal pipe to drive out the air, and after water is continuously injected to the normal water level (240mm), water injection is stopped and the drainage system of the U-shaped water seal pipe is put into operation. During the start-up of the unit, with the increase of condenser vacuum, the pressure difference between the inlet and outlet of the shaft-added U-shaped water seal pipe gradually increased, and the water seal was destroyed. The water level of the shaft heater rapidly drops to 130mm (local water level gauge), and the shaft heater is in an anhydrous operation state.

1.2 Fault Cause Analysis

According to the manufacturer's data, the shaft water level is low I value 180mm, normal water level is 240mm, high I value is 300mm, and high water level alarm value is 340mm. As shown in Figure 2, the full scale of the local water level gauge is 560mm, the centerline of the nozzle on the water level gauge is consistent with the centerline of the shaft cylinder, and the inner diameter of the shaft cylinder is 400mm, that is, when the displayed value of the local water level gauge is 160mm, the shaft will run without water level.

In general, the height of each stage of the U-shaped multistage water seal pipe can be calculated by the following formula:

h =(Pin-Pout)/nγ+(0.5 ~ 1)/n( 1)

Where: H—— the height of each water seal pipe in multistage water seal, m; Pin, pout-inlet and outlet pressure of multistage water seal, Mpa；; Water seal series in N-class water seal; Severity of γ-water, n/m3; Coefficient (0.5 ~ 1)- multiplicity (negligible).

The working pressure on the steam side of the shaft is 0.095 1MPa(a), the designed back pressure of the condenser is 7.2KPa(a), and the U-shaped water seal is designed as Grade III. Substituting the corresponding data into the formula (1), it is calculated that H =2.93m, while the height of each stage of the original U-shaped multi-stage water seal is 2.69m (as shown in Figure 65438+). In addition, the reasons for the damage of the U-shaped water seal are: the resistance along the negative pressure side and local resistance are small, so it is difficult to offset the influence of vacuum, and the water seal cannot be established in the U-shaped bobbin; In the process of hydrophobic rising on the negative pressure side of U-shaped tube, the pressure decreases and vaporizes, and the average density decreases. The effective water seal height required to balance the pressure difference between the two sides of the U-shaped pipe is greater than the calculated value. Due to the characteristics of fuel, the load of biomass generator sets changes frequently, and the steam intake and internal pressure of shaft seal heater change frequently, which makes the shaft seal water level unable to maintain in a certain range, resulting in the constant change of water displacement in its U-shaped water seal pipe.

Influence of 1.3 shaft plus no water level operation on the unit

When there is no water level in important official, the U-shaped water seal breaks, and the uncondensed steam-water mixture in the shaft seal heater is directly discharged into the condenser. On the one hand, the steam entering the condenser increases the heat load of the condenser, and the vacuum of the condenser will inevitably decrease without increasing the circulating water; On the other hand, the increase of air leakage into the condenser will increase the partial pressure of gas and hinder the condensation of steam, thus reducing the vacuum of the condenser.

In order to quantitatively analyze the influence of shaft operation without water level on the vacuum of the unit, in February of 201kloc-0/65438, under the condition that the load of the unit is 50MW, the steam supply pressure of shaft seal, temperature, circulating water temperature, circulating water volume and other parameters are stable, relevant data are obtained through the following tests.

1.3. 1 Close the drain manual door at the outlet of U-shaped water seal.

Slowly close the manual water drain valve at the water seal outlet, and quickly open the water drain valve when the water level of the shaft increases to the normal water level (240mm). Within a short time after the door was closed, the vacuum degree of the unit increased from -92.88KPa to -94.28KPa, and the vacuum degree increased by 65,438 0.4kpa..

1.3.2 vacuum value comparison between single and two vacuum pumps.

When two vacuum pumps are running at the same time, the vacuum of the unit is -92.94KPa. Stop vacuum pump A, and when pump B is running alone, the vacuum of the unit is reduced to -90.25 kPa. Stop the vacuum pump B, and the vacuum of the unit will drop to-90.37 kPa when the pump A runs alone; The vacuum drop of single vacuum pump is larger than that of double vacuum pump, which shows that the vacuum tightness of the unit is poor and more air leaks into the condenser.

1.3.3 vacuum leak test

According to the vacuum tightness test steps, the test was completed, and the measured data and calculated vacuum drop value were about 1.35kPa/min, while the qualified value of vacuum tightness test was 0.67kPa/min, which once again verified the poor vacuum tightness of the unit.

Optimization and transformation of double-shaft drainage system

2. 1 Initially transformed into a four-stage water seal.

During the operation of the shaft seal heater, the actual working pressure on the steam side is-1 ~-0.5 kPa (gauge pressure), which leads to the pressure difference between the inlet and outlet of the U-shaped water seal being greater than the original design value. Considering the influence factors such as hydrophobic vaporization and load fluctuation, it is preliminarily decided to increase the effective height of shaft seal, transform the three-stage water seal into a four-stage water seal, and observe the application effect. After modification (as shown in figure 1), when the unit is started, the shaft is filled with water to the normal water level and put into operation. After a short period of stability, the water level of the shaft quickly dropped to 140mm, and it was still running without water level. Under the condition that the parameters of the unit are basically the same as before, the vacuum value of the unit is -93.37KPa, which is about 0.5KPa higher than that under the same working conditions before. The effective total water seal height after the U-shaped water seal is transformed into four-stage water seal is 10.76m, which has met the requirements of water seal height under design conditions in principle, and the vacuum of the unit has also been improved. However, due to the influence of hydrophobic vaporization rate and other factors, the shaft heater still cannot maintain normal water level operation. When one vacuum pump is stopped, the vacuum of the unit is reduced to -9 1. 17KPa, which is 2.2KPa lower than that when two vacuum pumps are running, and the water seal transformation effect is not obvious.

2.2 Using vapor-liquid two-phase water level regulating device to control the water level of the shaft.

2.2. 1 Reconstruction basis and scheme

The design of multi-stage water seal is a complicated calculation process, and the height of each stage water seal is usually determined by static calculation method. In the actual operation of the unit, affected by various dynamic factors, the import and export parameters of the multi-stage water seal with shaft are inconsistent with the field parameters. For example, when the vacuum of the unit is different in winter and summer, the steam quantity of the shaft seal increases due to the increase of the sealing gap, and the change of the cooling water (condensed water) flow rate of the shaft seal leads to the change of the pressure measurement and the evaporation rate of the shaft seal. These are all uncertain factors that affect the stable operation of water seal. Therefore, it is difficult to accurately calculate the height of each water seal by accurately and quantitatively analyzing the influence of various dynamic factors on the water seal.