Steam turbine generator set vacuum system leakage directly affects the thermal economy and safety of the turbine unit, one is to affect the thermal economy of the unit, the general vacuum value is reduced by 1, the steam consumption increases by about 1.5%--2.5%, the heat transfer terminal difference increases by about 1 °C, the power supply coal consumption increases by about 1.5%--2.5%, so the vacuum value has a great impact on the thermal economy of the turbine; the second is to affect the secondary deaeration effect, aggravate the corrosion of low-pressure equipment pipelines, which is very unfavorable to the safe operation of the unit The third is to affect the steam condensation and heat exchange performance, increase the supercooling degree and the difference between the heat exchange end, and increase the burden of the vacuum pump.
There are many reasons for the vacuum drop of condensing or suction condensing steam turbines, which are difficult to identify or deal with in a short period of time, which is a difficult problem to solve. Combined with his 20 years of work experience, he classifies the influencing factors step by step, gradually narrows the scope, and basically can accurately judge common problems. Although it is for small and medium-sized units, large units can also learn from it.
The rough judgment process is to determine the large category through the change of end difference and supercooling degree, and then determine the cause by temperature, pressure, liquid level, load and vacuum fluctuations.
First, when only the vacuum drops, the supercooling degree and the end difference are basically unchanged, it is generally a circulating water system failure.
(1) The condenser inlet pipe plate is dirty or the gas storage in the outlet water chamber will increase the flow resistance of the equipment, so that the pressure difference between the inlet and outlet of the circulating water increases, the amount of water decreases, the liquid heat transfer coefficient decreases, the total thermal resistance increases, the heat transfer temperature difference (the average temperature difference between saturated water vapor and circulating water) increases, the exhaust steam temperature rises, and the vacuum decreases: At the same time, the total heat transfer is basically unchanged, the water volume decreases, the import and export temperature difference increases, and when the import is unchanged, the outlet temperature increases.
(2) The condenser inlet pipe is blocked, which will increase the difference between the outlet pressure of the circulating water pump and the inlet pressure of the condenser, reduce the amount of circulating water, reduce the vacuum, increase the outlet water temperature, and reduce the pressure difference between the inlet and outlet water of the condenser.
(3) The condenser outlet pipeline is blocked or the valve is not fully opened, which will reduce the amount of water, reduce the vacuum, increase the outlet water temperature, increase the overall pressure, and reduce the pressure difference between the inlet and outlet of the condenser.
(4) Circulating water pump failure (low water temperature in the pool, clogged inlet filter, suction air, wear of the water wheel guide blade, etc.), will reduce the overall pressure of the pipeline, reduce the pump current, reduce the vacuum, and increase the temperature of the outlet water. The tripping of the partial circulation pump will cause the water pressure and exhaust vacuum to drop rapidly, and the pump current will disappear.
(5) The cooling fan is cut off, which will cause the inlet water temperature of the condenser to continue to rise and the vacuum to continue to decline. Circulating water failures will reduce the vacuum, but not the vacuum fluctuations.
Second, when accompanied by vacuum decline, only the end difference increases, and the supercooling degree does not change; this phenomenon can basically be judged as scaling of the condenser copper pipe.
Scaling (as shown in the picture) increases the heat transfer heat resistance, the heat transfer temperature difference increases, while the total heat transfer is basically unchanged, and the temperature difference between the circulating water in and out of the water is unchanged, so the outlet water temperature is unchanged, the exhaust steam temperature is increased, the end difference is increased, and the vacuum is reduced.
Third, when the vacuum decreases with the increase of the heat load, it can basically be judged that the heat load of the condenser is too high.
Due to the adjustment of the steam valve hydrophobic of the unit, the hydrophobic of the suction reverse stop door at all levels, the hydrophobic of the shaft seal heater and the hydrophobic of the vapor seal at both ends all enter the condenser through the body hydrophobic expansion container, which increases the heat exchange strength of the condenser, when the unit steam pumping volume increases or the circulating cooling water volume is insufficient or although the cooling water volume is certain but because of its high water temperature, it will lead to a decrease in the vacuum of the condenser.
Fourth, the electric load of the vacuum random group increases and increases.
Basically, it can be determined that the air valve of the low-pressure heater tube, the valve leak or the airgate of the low-pressure heater and its hydrophobic system leak or the air leakage of the rear shaft seal of the steam engine is caused. When the end of the unit is slightly negative pressure when the electric load is low, if the extraction system or the low-pressure heater connected to it has a leakage point, the vacuum of the unit will be reduced. When the electrical load of the unit increases, the end steam will gradually form a positive pressure, which can seal the above leakage points, and the vacuum will gradually increase.
The air leakage of the rear shaft seal affects the vacuum, and the rear shaft seal is generally caused by serious wear or the steam pressure of the rear shaft seal is low. However, there is a blind spot; the steam engine sewage pipe, some unit sewage pipe is drawn from the end of the rear axle seal, whether the pipe leaks or plugs will affect the vacuum.
Fifth, when the end difference and supercooling degree are increased, in addition to the condenser liquid level is too high, it can be judged as condenser gas collection.
The condenser liquid level is too high, submerging the copper tube, so that the condensate is supercooled, the supercooling degree is increased; at the same time, the steam-water heat exchange area is reduced, the same heat transfer, the heat transfer temperature difference increases, the heat transfer temperature difference increases, the exhaust steam temperature increases, the vacuum decreases, the outlet water temperature is basically unchanged, and the end difference increases. Condenser gas collection makes the steam partial pressure on the steam side of the condenser decrease, lower than the partial pressure in the exhaust steam, and its condensation temperature is naturally less than the exhaust steam temperature, and the supercooling degree increases; at the same time, the steam heat transfer coefficient is reduced, the total thermal resistance is increased, the heat transfer temperature difference is increased, and the end difference is increased; the cooling water also bears the steam cooling heat, and the outlet water temperature also increases.
The reasons for condenser gas collection are mainly divided into two categories, one is air leakage, and the other is the failure of the extractor. It can be judged by the tightness test of the vacuum system, and the test pass is the problem of the extractor, and there is a leakage point if it is unqualified.
The failure of the pump is divided into the following three aspects:
(1) The nozzle is blocked. Debris plugging the nozzle will make the mixing chamber form a vacuum low, the pumping capacity decreases, the vapor pump, the performance of the first and second stage steam pressure are increased, the blocking stage close in front of the shut-off valve vapor pressure drops slowly; the water injection pump is also manifested as the front water pressure increases. Can be flushed with buck pressure.
(2) Insufficient steam and water sources. The steam pump due to boiler maintenance and other reasons, the steam pipeline has impurities, blocking the filter or throttle, it will make the steam volume through the reduction, the suction ability to decrease, the performance of the first and second stage steam pressure are reduced, and fluctuations, the pressure is high when the nozzle works, the low time does not work, vacuum fluctuations, exhaust port has asthma phenomenon, only the secondary operation effect is sometimes better. The low water pressure of the water pump is mainly caused by the abnormal operation of the water pump, and the pump pipe is too high and too long, which affects the vacuum.
(3) The water temperature in the shooting tank is too high. The water temperature in the water injection tank is too high, which will cause vaporization at the nozzle of the water injection pump, forming a steam plug, which will affect the ability to extract the non-condensing gas inside the condenser, so that the efficiency of the water injection pump is low, the condenser is collected, and the vacuum is reduced.
Sixth, the water level on the vapor side is too high or too low, which will also reduce the pumping ability.
Too high submerged heat exchange tube, expansion pipe discharge mixed steam channel reduced, condensation heat exchange space reduced, steam in the mixture can not be completely condensed, affecting the operation of the extractor, uncondensed steam is discharged from the exhaust port, the exhaust port emits white vapor, and even water. When the hydrophobic water seal is done low, the trap opens widely, the water level of the first level is too low, the pressure of the condensate chamber is normal, the pressure difference with the condenser of the unit is greater than the pressure of the water seal water column, the water seal is destroyed, the steam exhaust condenser of the condenser chamber is reduced, the vacuum of the condenser chamber rises, the pressure difference is reduced, and the water seal is formed, which is manifested as the fluctuation of the vacuum regularity of the condenser and the condensing chamber.
Seventh, the gas storage in the cooling water chamber will also be reduced in the heat exchange area, the heat exchange will decrease, and the pumping capacity will decrease.
However, it generally only occurs in the opening and stopping stage, because the normal operation is already unsaturated water after being pressurized by the condensate pump, and the solubility increases, even if the pump is slightly leaky, it will dissolve, and the heating will not precipitate. Therefore, there will be no gas in the water chamber. The media on both sides of the condenser heat exchanger tube is clean and non-corrosive, and there is no clogging and corrosion. In some places, the condensate recirculation door is always open, so that the water flow through the pump and vapor is increased, and the negative pressure is formed higher, but it also increases the thermal load of the condenser, but when the exhaust steam volume is low, it is advantageous to protect the condensate pump. The air leakage of the vacuum system will collect gas in the condenser, reducing the vacuum and increasing the end difference and supercooling. However, the location of the leakage point is different, and the impact on the end difference and supercooling degree is not the same. Air is relative to the vapour density, flows downward in the condenser, when the air leakage point is in the lower part, the air is easy to accumulate, not easy to be pumped out. There is less upper air, no supercooling, the heat exchange coefficient is basically unchanged, the end difference increases less, and the temperature of the cooling water outlet increases with the exhaust steam temperature; while the steam partial pressure on the lower water surface is low, corresponding to the low temperature of the condensate, and the supercooling degree increases.
Eighth, when the load changes, the adjustment of the pressure equalizing box is not timely or improper, which will make the rear steam seal lack of steam, so that the air leaks in, and the exhaust steam vacuum is rapidly reduced.
When the rear vapor seal vapor seal tooth and vapor seal blade are not well combined or the vapor seal is damaged, the air is easy to leak in, when starting, the vapor seal pressure is difficult to meet the requirements, the vacuum pumping can not reach the specified value, the temperature of the rear cylinder rises fast; in normal operation, the vacuum is formed by steam condensation, the impact of air leakage becomes smaller, at the same time, the steam supply is borne by the valve stem leakage of the front steam seal and the steam system, the steam volume is guaranteed, the vacuum is improved, but it is easy to fluctuate. This phenomenon is also observed when the shaft seal of the condensate pump is not strict. The leakage of the heat exchanger tube will cause the cooling water to leak in, bringing in the air to affect the vacuum, but generally not very large, the supercooling degree increases, the condensate water quality changes, and the end difference does not change much.
Nine, desalinate water replenishment will also bring into the air, affecting the vacuum.
However, if the water replenishment port is located in the upper part of the condenser, the air leakage is easy to be pumped out, which has little impact on the supercooling degree, at the same time, the water replenishment and steam heat exchange, recover part of the heat, save the cooling amount, so that the vacuum rises, the exhaust steam temperature decreases, and the end difference should not be obvious. The water replenishment port is located in the lower part, which is not good, and the opposite of the above will definitely affect the vacuum.
Ten, finally
The reduction of the vacuum of the condenser is often the result of the joint action of a variety of factors, and because the vacuum system is relatively large, the strict control is also more difficult. However, as long as we do not blindly take the instrument to conduct the census, we need to calmly and carefully analyze, check section by section, and narrow the scope of leakage, it is not difficult to determine one by one, and take corresponding measures to maintain the normal operation of the unit.
Poor vacuum tightness? Vacuum system governance knowledge dry goods
First, the importance of vacuum rigor
Condenser vacuum is one of the important monitoring parameters of power plants, and condenser vacuum changes have a greater impact on the safe and economic operation of steam turbines.
- The operation test shows that every 1KPa reduction in condenser vacuum will increase the steam consumption of the steam turbine by 1.5% to 2.5%, and the coal consumption of the generator will increase by 0.25%, which will reduce the cycle efficiency.
- The increase in the exhaust temperature of the steam turbine will cause the center of the turbine bearing to be offset, and in severe cases, it will cause the vibration of the steam turbine.
- In addition, when the condenser vacuum is reduced, the steam flow must be increased when the output of the unit is unchanged, resulting in an increase in axial thrust and affecting the safe operation of the steam turbine.
- On the other hand, the leakage of air into the condensate will make the dissolved oxygen of the condensate unqualified, corrode the steam turbine, boiler equipment, and affect the safe operation of the unit.
Therefore, in the operation of the steam turbine, vacuum is a very important parameter, and the level of vacuum value directly affects the economy and safety of the unit.
Second, the reason for the low vacuum pressure of the condenser
(1) The heater or deaerator accident trap is mistakenly opened, causing a large amount of soda from the heater or deaerator to flow to the condenser, resulting in a vacuum drop.
(2) The shaft sealing system of large or small machines fails or is improperly adjusted, and a large amount of air is sucked in from the rear steam seal of the turbine to cause a rapid decline in vacuum.
(3) The circulating cooling water of the condenser is interrupted or the amount of water is insufficient. The interruption of circulating water will cause the pressure loss of the condenser circulating water inlet, resulting in a sharp increase in the exhaust temperature of the steam turbine and a rapid reduction in the vacuum. The amount of circulating cooling water is insufficient, which cannot meet the requirements of the full exhaust steam capacity of the cooling steam turbine, which will lead to a gradual decrease in vacuum.
(4) The high temperature of the circulating cooling water inlet will inevitably affect the cooling of the exhaust steam of the steam turbine, and then affect the vacuum of the unit.
(5) The water level of the condenser is too high. The condensate level in the condenser exceeds the upper limit of the hot well water level gauge, and when submerging part of the cooling water pipe, the vacuum slowly decreases due to the reduction of the cooling area of the vapor side of the condenser, and in severe cases, if the water level rises, the condensate enters the suction pipe, and the vacuum drops rapidly.
(6) The vacuum pump does not work normally, which will inevitably cause the insoluble gas of the condensation system to increase continuously, resulting in the reduction of the vacuum of the condensation system.
(7) The vacuum system is not tight, the amount of air leaking into the steam side of the condenser increases, and the vacuum drop caused by the overloaded work of the extractor.
If there is a problem of vacuum drop during the operation of the unit, excluding the more common faults, the leakage of the vacuum system is the main reason for the vacuum drop. The main phenomena are vacuum drop, vacuum pump current increase and so on.
Third, helium mass spectrometry leak detector vacuum leak detection method
The system principle is shown in Figure 1.
- The low pressure of the condenser vacuum system is mainly due to the loose pipe or instrumentation interface in the system, the gasket is broken or the weld crack causes the air to be sucked in.
- The air leaking into the vacuum system is pumped through a vacuum pump and discharged into the atmosphere.
In the test, the suction gun holder of the high-sensitivity helium mass spectrometry leak detector is at the exhaust port of the vacuum pump; the helium is sprayed into the various areas where leakage may occur in the vacuum system, if there is a leakage point, the helium will be sucked into the condenser by negative pressure, pumped out by the vacuum pump, discharged into the atmosphere through the exhaust port, the sniffer will suck in part of the gas with helium molecules, and the leak detector will show the detected helium molecular weight.
The larger the leak point, the more molecular weight of helium is inhaled, and thus the more helium molecules are captured by the leak detector, so that the location and size of the leak point can be accurately locked.
Fourth, the estimation of economic improvement
The No. 210 steam turbine model of a power plant in Hubei is N330-16.67/538/538 unit, and the test found that its vacuum rigor was 0.57kPa/min, and the leakage point was found by helium mass spectrometry leak detector, and the vacuum rigor was reduced to 0.06kPa/min after the defect was eliminated. At 300MW load, vacuum -95.2kPa changes to -97.2kPa.
For every 1kPa increase in vacuum in a 300MW unit, the impact on heat consumption is 1.0502%, and the impact on coal consumption for power generation is 3.099g/(kW·h). After the vacuum leakage and elimination of the unit described in this article, the vacuum of the unit is increased by about 2kPa, and the coal consumption decreases by 6.198g/(kW•h), according to the calculation of the annual utilization of the unit described in this article, it can save 9628.48 tons of coal per year, and can save 4.814 million yuan in power generation costs per year.