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Causes of steam erosion of water ring vacuum unit

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Causes of steam erosion of water ring vacuum unit

Date of release:2019-07-26 Author: Click:

Because of the long high load period every year, the steam turbine condenser water ring vacuum unit runs under high vacuum conditions for a long time, especially the summer conditions are more severe, and the vacuum pump rotor blade material reaches the limit in anti-cavitation, making it The service life is inevitably affected. After investigation, the installation of atmospheric injectors effectively avoids the influence of cavitation on the vacuum pump rotor and prolongs the service life of the vacuum pump.


1. Analysis of the cause of cavitation of the water ring vacuum unit The analysis of the size of the steam turbine condenser water ring vacuum unit of the Wuzhanshan Power Plant is determined according to the planned operation conditions of the generator set of the plant, when the operation is within the planned scale. When the vacuum pump can operate normally, for example, the unit is running at full load for a long time; if the temperature of the cooling water is high in summer or the flow rate of the cooling water is too small, the cooling effect of the vacuum pump heat exchanger is poor, and the vacuum pump will operate under high vacuum. It is equivalent to running close to or in a state of sudden death. Because of the inherent physical phenomenon, under the condition that the water temperature must be determined, the higher the vacuum, the higher the water temperature, the water in the pump is close to boiling and a large number of bubbles will occur, and the bubble will occur and burst. The process will cause cavitation damage to the impeller, damage the dynamic balance of the impeller, cause strong oscillation of the pump body, vibrate the auxiliary equipment of the vacuum pump (pressure vacuum gauge, pressure switch, feedback device of the imported pneumatic door, etc.), and will declare very large The cavitation noise, these conditions seriously affect the safe operation of the vacuum pump set and the safe operation of the steam turbine generator set.


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2. The basis of the technical transformation plan and the detailed clarification. If the problem of cavitation of the vacuum pump is to be solved, the cost of lowering the water temperature is larger, and the degree of vacuum is also more difficult to increase the back pressure of the condenser, which will seriously affect the belt of the unit. The ideal method of load capacity is to solve the occurrence of cavitation under the condition of the original water temperature and the system vacuum. The best choice now is to install the atmospheric injector for the vacuum pump, and the extraction function of the whole vacuum pump set will be better. Therefore, the vacuum of the condenser can still be guaranteed to the current level or higher, but the vacuum inside the vacuum pump body is only 12Kpa (the system vacuum is within the range of 3-8Kpa), which can effectively avoid the occurrence of cavitation.


The atmospheric jet pump consists of a nozzle, an inhalation chamber, and a diffuser, and its exhaust port is connected to the inlet of the water ring pump.


Introduction to the principle of atmospheric ejector: Firstly start the water ring vacuum pump to make a pressure difference between the nozzle inlet and the exhaust port. The atmosphere can enter the pump from the nozzle. When the pressure difference is 1/2 of the atmospheric pressure, the air medium passes through. The nozzle shortening section is accelerated, and the speed of sound can be reached to the throat, and the expansion section can be further accelerated to the speed of sound, and the jet is directed to the diffuser to form a high-speed jet, and the pressure in the suction chamber is lower than the pressure in the pumping chamber, and thus the gas is pumped. Inhaled into the room, because the two airflows are mixed in the suction chamber, the loss of momentum exchange causes the airflow rate to gradually slow down. When entering the throat of the diffuser, it falls below the speed of sound, and the expansion section of the diffuser further decreases, and the pressure rises continuously. Finally, When the exhaust pressure of the atmospheric ejector is reached, that is, the suction pressure of the water ring pump, the water ring pump sucks the gas and discharges it outside the pump, that is, the process of inhaling and exhausting is completed.


3. Detailed steps and detailed elaboration of technical transformation


3.1, mechanical transformation part


It can be known that the modified intake line is about 700mm longer than the original pump line. The added parts are now marked on the map. If the position is not enough, the gate valve on the line can be installed vertically on the line. This leaves room for cutting off 700mm pipes.

 

3.2. The pump unit control part is modified (refer to FIG. 3). The pressure switch 47a (double contacts 47a1, 47a2) is used to control the pneumatic butterfly valve (16b) and the injector between the intake tee and the injector tee. The upper pneumatic butterfly valve (19a) is opened and closed to control the injection and withdrawal of the injector.

 

3.2.1. When a main pump is running, when the pressure switch detects a pressure less than 10kpa (detailtable setting), the atmospheric injector switching valve [16b] is closed, and the atmospheric injector driving gas valve [19a] is open. After the above two valves are completed, the atmospheric injector is in the input state.


3.2.2. When a main pump is running, when the pressure switch [47a2] measures a pressure greater than 14Kpa (this value can be set by itself), the atmospheric injector drives the gas valve [19a] to close, together with the atmospheric injector switching valve [ 16b] Open, when the above two valves are completed, the atmospheric injector is in the exit state.


3.2.3. The set values of the unit pressure switch and differential pressure switch are not required to be changed.


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4. Advantages after the transformation and the function and economy before and after the transformation 4.1, the advantages after the transformation:


1 The main guarantee is that the vacuum pump will not cavitation, the noise and oscillation will be greatly reduced, and the service life of the vacuum pump will be improved. The noise of the modified No. 3 A vacuum pump since the installation of the atmospheric injector in August 2008 And the oscillation will be greatly reduced, and the phenomenon that the impeller is cracked and the oscillation is excessively damaged to damage the thermal control equipment has never been exhibited.


2 Because the condenser of this unit is tight, the pumping function of the pump set with atmospheric injector in the high vacuum stage is better than that of the single pump. The condenser vacuum is always maintained at an ideal level of about 5-6Kpa. The load factor of the progressive unit invented outstanding conditions.


4.2. Functional and economic comparison before and after the transformation


1 Before installing the atmospheric ejector, the vacuum in the vacuum pump is kept at about 6Kpa. After installing the atmospheric ejector, the vacuum in the vacuum pump is kept at about 12Kpa. Referring to the vacuum pump power curve, the shaft power is 113KW at 12Kpa, because condensation The vacuum of the boiler is constantly changing, so the vacuum of the vacuum pump used now is also changed, so it is impossible to explain in detail how much the shaft power is improved. Generally, we compare 6Kpa (power is 99KW) with 12Kpa (power is 113KW). That is to say, the power will be added to about 14KW after the installation of the atmospheric injector.


2 After installing the atmospheric ejector, the pumping function of the vacuum pump has been improved. After our investigation and the history curve of the DCS host computer, the condenser vacuum of the unit is more stable, and the vacuum energy can be improved by about 1Kpa compared with the original year-on-year conditions. And there is a trend towards higher vacuum.

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