Implementation and Analysis of Vacuum Water Supply Scheme for Small Thermal Power Units

1 The Shaanxi Lueyang Iron & Steel Plant Power Plant has two 20t/h pure-fired blast furnace gas boilers and two 3MW condensing steam turbine generators. The atmospheric oxygen-free thermal spray deaerator is used to remove oxygen from the water supply. Since the start of production in March 1999, the feed water temperature has been low, resulting in a series of problems such as severe oxygen corrosion of the boiler economizer and the upper drum. In view of this situation, the method of transforming the hydration system from the deaerator pump to the condenser vacuum suction method has achieved satisfactory results and benefits.

2 Power Plant Thermal Cycles The Luen Steel Power Plant is a small thermal power plant whose thermal cycle is typically a Rankine 'ankine' shield ring. Its thermal system diagram is like this.

The heat cycle process is: water absorbs heat in the boiler and changes from unsaturated water to superheated steam; superheated steam expands in the steam turbine to drive generators to generate electricity; superheated steam turns into wet steam and then detonates). The exhaust gas in the device is cooled by the cooling water and condenses into the hot water well where the condensed water is collected in the lower part of the condenser. After being pressurized by the condensate pump, the gas is ejected into the deaerator by the steam evacuator and is heated by the saturated steam to remove oxygen. Saturated water is pumped back to the boiler by the feed pump to complete the closed circuit cycle.

3 problems lead to the loss of working fluid in the thermal cycle due to factors such as leakage of steam and water pipelines, boiler exhaust, steam turbine exhaust steam, steam turbine shaft exhaust, external heating and other factors, slightly steel power plant Single unit steam water loss rate is about 12! . If the rated evaporation of the boiler is 20 t/h, 2.4 t/h of water must be supplied to the system to ensure normal thermal cycling.

The make-up water is demineralized water. The original design was to send a deaerator to the human body by a soft water pump.

The deaerator is a hybrid heating system. The supplementary water and the condensate enter the upper part of the deaerator, and are divided into several fine strands. In the downstream process, they are mixed with the saturated steam from the boiler. The mixed water is called the water supply, and it is required to The temperature reaches 104 "saturation temperature". The feedwater is collected in the feedwater tank in the lower part of the deaerator. Here, the saturated steam continues to heat up and maintains the continuous boiling state. This will remove the dissolved oxygen in the water and ensure that the oxygen content of the feedwater entering the boiler is lower than 50!g/L.h, which is much higher than the actual need for replenishment, will inevitably result in the interruption of soft water replenishment, with the use of a soft pump replenishing water every 4h for 0.5h. During the replenishment period, due to a large amount of soft water temperature of 2030 ") The deaerator removes the temperature of the feed water to 70" or less. After the end of the replenishment, the temperature rises gradually, and only one hour can reach the specified value of 104". The low feedwater temperature causes the dissolved oxygen content in the feed water to greatly exceed the standard. In 3000! g/L or more. The dissolved oxygen in the feed water is separated from the boiler after it is heated and has a strong corrosive effect on the metal, which will lead to oxygen corrosion of the boiler heating surface and part of the steam flow through the turbine, shortening the service life of the equipment. At the same time, after the temperature of the feed water is reduced, the figure of the thermodynamic circulation system of the boiler evaporative power plant is reduced and the temperature of the steam is increased excessively, which not only reduces the output of the boiler and affects the power generation, but also is not conducive to the safe operation of the boiler heating surface and the steam turbine.

4 The determination of the transformation plan In response to the above problems, the direct solution is to install a throttle plate at the outlet of the soft water pump or replace the pump with a small flow rate. For the IS type clear water pump under the premise of satisfying the lift of 50m, the minimum flow rate is 12.5m3/h, which is much larger than the actual need for water supply of 2.4m3/h. There is still a phenomenon of “big horse car”. After the small flow pump reduces the soft water flow, it can not meet the requirements when the boiler starts to require a large amount of water, and will prolong the boiler watering time.Obviously, this scheme has no practical application value.

Analyze the steam cycle process, the pressure in the condenser is 10 kPa, the local atmospheric pressure is Pa=0.095 MPa, then the vacuum in the condenser is 85 kPa, and the vacuum suction height is 8.5 m. Can the vacuum be used directly from the soft water pool? Water absorption, to achieve the purpose of continuous balanced water, condenser and soft pool at the same level of ground, water absorption capacity is wealthy. The question to be considered is: After the supplementary water is changed by the condenser, the amount of condensed water is increased, and the flow of the condenser pump and the ejector of the steam ejector meet the requirements; the change of the working condition to the condenser and the deaerator Whether the operation has influence; whether the operation adjustment is convenient. The feasibility analysis of these aspects is as follows: On the supplementary water volume of 2.4t/h, the total amount of condensed water is 20m3/h, which is less than the design flow of the condensate pump and does not make the condensation pump head lower than the design value. So the condensate pump will not affect the vacuum replenishment. ) The ejector steam ejector is a surface heater. Its condensate flow passage is 28 016x1 brass pipes, and the flow area is larger than the flow area of ​​the condensate pipe (+N65). Therefore, the flow area of ​​the ejector steam ejector does not restrict the increase in the amount of condensed water. ) The diameter of the condensate pipeline is DN65. When the condensate flow rate increases from 17.6m3/h to 20m3/h, the resistance loss increases by 25% 6.75 squares of resistance area. The original actual resistance loss is 200kPa (20mH2), which will become 250kPa (25mH2), plus the deaerator is 10m higher than the condensate pump, the total required head is 35m, and the design of the condenser pump head is 0.5MPa (50m), and there is still plenty of money to deduct the negative side suction. Therefore, there is no need to change the condensate pipeline to meet the need for increased condensate water.

6) The soft water temperature is lower than the temperature of the exhaust gas. After entering the condenser, it will play a role in mixing and desuperheating the spent steam, which is conducive to the condensation of the spent steam and will not affect the vacuum degree of the condenser. The deaerator's design capacity is 25t/h, and the replacement of soft water is provided by condensate and there are also plenty of people. ) Adjustment of water supply shall be based on the water level of the deaerator feed water tank. The significant changes in the amount of replenishment occur when boilers regularly discharge pollutants and the external heating of the boiler changes. Under normal operating conditions, the amount of replenishment is almost constant. Therefore, the adjustment system can be simplified, and manual throttle regulation with the cut-off valve can meet the requirements.

Through the above analysis shows that: the replacement of the water system from the direct dehumidifier pump to condenser vacuum suction method is completely feasible, but also has good operability.

5 The implementation of the transformation plan Considering that the heating load for heating will increase in the future, the amount of replenishment will be calculated according to “=3m3/h. After the shutoff valve and elbow on the suction pipe are converted into straight pipe sections, the total hydraulic length of the suction pipe is !=100m. 038x3 without soft pool water surface 1-1 and suction pipe outlet section 2-2, Lebourne equation: surface pressure equal to atmospheric pressure: sediment.

The original soft water is added to the deaerator system to be retained for use when the boiler needs to be flooded and the steam turbine is not running.

6 Effect, Benefits and Precautions after the Transformation The transformation of the hydration system was completed after it was completed in May 2000.

Practice shows that the effect, benefits and precautions of the make-up water system after changing from the deaerator intermittent pumping method to condenser continuous vacuum suction method are as follows: h) During the continuous operation of the unit, the deaerator supply water temperature can be The pressure at the outlet of the steady suction pipe is equal to the pressure in the condenser: p2=10kPa"1mH2. The data is replaced by the formula 1 and is obtained after finishing: This shows that the suction pipe outlet (with the condenser interface) and the soft water surface are high. When the difference is less than 2.2m, the condenser vacuum can reliably suck the amount of water required by the person.

In order for the suction pipe to work properly, it must be ensured that no vaporization takes place anywhere on the entire pipe, ie the pressure at any point must not be lower than the saturation pressure at the corresponding temperature of the liquid at that point. Take the summer maximum water temperature = 35%, the corresponding saturation pressure p- = 5.6kPa "0.56mH2. The lowest pressure at the highest point near the outlet of the suction pipe, cross-section 3-3 at the site and soft pool surface 1-1 In the equation: the data generation 2 formula, so that the description, the highest point of the suction pipe and the soft pool surface height difference is less than 2.64m, the suction pipe will not vaporize, can normally suck the amount of water required.

According to the height of the bottom of soft pool, the highest point of the suction pipe is 3.5m, the height of the suction pipe is 2.0m, the soft water level is 1.8'3m. Check the lowest water level, the highest point of the suction pipe and the height of the outlet All meet the requirements. According to this construction, a 038x3 seamless steel pipe 75m is installed from the soft water pool to the condenser, and the J41H-1.6DN32 globe valve is installed on the pipe at the condenser. No soft pool water surface section, its end is higher than the bottom of the pool 0.5m, to prevent the suction pool is set at more than 102%, the oxygen content of the water supply is below 10%g/L, effectively guarantee the oxygen removal effect of water supply .

h) After the soft water supplements the condenser, it is heated when mixed with the spent steam, and the vacuum deoxidation is performed under a pressure saturation pressure of 0.01 MPa (=46%), and the separated gas is extracted by the ejector steam extractor. This is equivalent to adding a primary oxygen-removal system on the original basis, thereby increasing the oxygen removal effect and reducing the deaerator saturated steam consumption.

h) The operation is easy to adjust. When the water level of the deaerator feed water tank reaches the upper and lower limits, the opening of the stop valve on the suction pipe can be adjusted to adjust the amount of suction and replenishment in a sensitive manner, and the water level of the deaerator supply tank is maintained in the normal range. In the period when the external heat supply is not changed and the boiler is not polluted periodically, the opening of the stop valve is basically not adjusted.

Calculating each day of intermittent work for 3 hours can save electricity consumption by 22kWh per day and save 7,000kWh of electricity per year.) When the vacuum replenishment system malfunctions or the turbine shuts down, the shutoff valve should be closed and the vacuum replenishment system must be detached.

6) Note that the soft pool water level should not be less than 1m to prevent the increase of the water absorption height resulting in vaporization of the pipeline.

7 Conclusion In summary, after the replenishment system was transformed, it has significant benefits and effects. The program has a good promotion value in thermal power plants. For medium-sized or large-sized generator sets, in order to make the vacuum replenishing system more perfect, the following two points may be considered in the application: an automatic adjustment device for replenishing water volume. The water level of the deaerator feed water tank is used as a pulse signal to automatically adjust the opening of the valve on the water suction pipe; 2) After the make-up water enters the condenser and is atomized, the lack of steam can be cooled better, thereby improving condenser vacuum. At the same time, the supplemental water is more likely to reach the saturation temperature, so that the primary oxygen removal effect is better.

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