Working Principle and Operation of Deaerator

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The last edit to this post was made by The wise are free from confusion on 2009-10-13 at 10:25. The main function of a deaerator is to remove oxygen and other non-condensable gases from the boiler feedwater, in order to ensure the quality of the feedwater. If dissolved oxygen is present in water, it will cause the metals in contact with the water to corrode. Meanwhile, if gases accumulate in heat exchangers, it will increase the thermal resistance to heat transfer, thereby reducing the efficiency of heat transfer in the equipment. Therefore, the presence of any gases dissolved in water is undesirable, especially oxygen, as it poses a direct threat to the safe operation of the equipment. Thermal deaeration is employed in thermal power plants, and the deaerator itself functions as a mixed heater within the feedwater reheating system. At the same time, the drain water from high-pressure heaters, chemical make-up water, as well as high-pressure drain water and exhaust steam from various parts of the plant that meet quality standards can all be directed to the deaerator for reuse, thereby reducing steam and water losses in the power plant. I. Working Principle of Deaerators Deaerators are divided into headed and headless types; generally, boilers are equipped with headed deaerators, while large-scale units with a capacity of over 1000 t/h use towerless deaerators. The principle of deaeration is as follows: the main condensate water from the low-pressure heater (along with make-up water) is regulated by an inlet control valve before entering the deaerator, where it mixes with other types of drain water. From there, it is sprayed through spiral-membrane porous tubes to form a skirt-shaped water film, which then undergoes mixed heat and mass transfer with the heating steam coming from below, allowing the feed water to quickly reach the saturation temperature at the operating pressure. At this point, most of the dissolved oxygen and other gases in the water are essentially removed, achieving the purpose of deoxygenation. The dissolved oxygen and other gases that precipitate from the water are continuously removed outside through the steam exhaust pipe at the top of the deaerator, along with the excess steam. Some of the water from the high-pressure heater drain that enters the deaerator will also flash vaporize to serve as a heating steam source. All of the heating steam, after releasing its heat, is condensed into condensate water, which then mixes with the deaerated water and flows downward through the outlet. To maintain the water temperature in the deaerator at the saturated temperature corresponding to the operating pressure, heating steam can be introduced into the deaerator via a reboiler tube. Deoxygenated water enters the high-pressure heater via the outlet pipe after being pressurized by the feed pump. III. Structure of deaeration equipment The structure of a deaerator consists of a deaeration head and a deaeration water tank. The deaeration head is a key component of the deaerator, consisting of a housing, a film-forming device, a water spraying grate, and a liquid-vapor network. ·Film former: The film former is a key component of the rotary-diaphragm deaerator; it consists of a water chamber, a steam chamber, a film-forming tube, a condensate water inlet pipe, a make-up water inlet pipe, and a primary steam inlet pipe. 　·The showering grate is composed of several layers of angular steel components arranged in an interlaced pattern; the feed water, which has undergone preliminary deoxygenation in a degassing section, is mixed with water-repellent substances here to enable secondary distribution, allowing the water to fall in a uniform shower-like pattern onto the liquid-vapor mesh located below it. 　·The liquid-vapor network is composed of spaced flat steel strips and a cylindrical chamber filled with a special stainless steel mesh; it allows the feed water to come into full contact with secondary steam. This process heats the water to its saturation temperature and enables thorough deoxygenation, ensuring that the concentration of oxygen in the deoxygenated water is below 10 ug/L in low-pressure atmospheric deoxidizers, and below 5 ug/L in high-pressure deoxidizers. 　·Water tank: Uncondensed steam and gases such as oxygen released from the feed water are discharged through the steam exhaust pipes installed on the deaeration head, while the oxygen-free feed water is collected in the water tank located at the bottom of the deaeration head. IV. Startup of deaeration equipment 1. Pre-startup checks (1) Verify that all conditions required for starting the vacuum pump are met, that the shaft seal steam of the turbine is in operation, and that the shaft seal pressure is normal. (2) Start the vacuum pump from the DCS screen; check that the negative pressure at the inlet of the vacuum pump should increase gradually, and the inlet pneumatic valve should open automatically. (3) Check that the starting current and return time of the vacuum pump motor are normal, as well as the bearing vibration, the water level in the air-water separator, and the exhaust conditions. (4) Check that the plate heat exchanger is operating properly, and that the temperature of the seal water at the vacuum pump inlet is normal. (5) Follow the same steps to start the other two vacuum pumps in sequence. (6) Once the unit’s vacuum level is normal, stop one of the vacuum pumps based on the situation and keep it as a spare. (7) The vacuum system can be started by activating the vacuum pump start function group. 2. Steps for putting the deaerator into operation: (1) Confirm that the deaerator’s start-up exhaust electric valve and the continuous exhaust bypass valve are in the open position. (2) Once the condensate system has passed the flushing test, open the deaerator flushing drain valve to flush the deaerator with water. (3) After the water quality in the deaerator meets the requirements, lower the water level to -900 mm and close the deaerator flushing drain valve. (4) Supply auxiliary steam to the deaerator for heating; open the isolation valves before and after the valve that regulates the flow of auxiliary steam to the deaerator, and slowly open the pressure control valve for auxiliary steam going to the deaerator. Ensure that the temperature rise rate of the water supplied to the deaerator does not exceed 4.26°C/min. During the heating process, pay attention to the vibration level of the deaerator; if vibration is severe, reduce the heating speed. (5) During the heating up of the deaerator, continue to use the condensate pump to fill the deaerator with water until it reaches the normal level. (6) Once the water temperature in the deaerator reaches 100°C, close the start-up exhaust electric valve, put the pressure control valve for the auxiliary steam to the deaerator in automatic mode, and ensure that the temperature rise rate of the deaerator does not exceed 4.26°C/min; meanwhile, the pressure in the deaerator should rise gradually to 0.147 MPa. (7) During the auxiliary steam heating process, the deaerator water level should be controlled; if a vacuum has not been established in the condenser, it is prohibited to open the overflow and drain valves leading to the condenser electric valve. (8) After the condensate water system is started, the deaerator water level control is switched to automatic mode as required. (9) When the pressure of the fourth extraction steam reaches 0.147 MPa, check that the pressure and water level in the deaerator are normal; then open the electric valve for feeding the fourth-stage extraction steam to the deaerator. The deaerator will be supplied with steam from the fourth extraction steam instead of from the auxiliary steam, and the pressure control valve for the auxiliary steam to the deaerator will be closed. As a result, the deaerator will operate in a variable-pressure mode rather than a constant-pressure mode. (10) Once the check valve behind the fourth-stage extraction steam electric valve is open, it should be checked that the pneumatic drain valve before the electric valve leading to the deaerator is closed. (11) Adjust the continuous exhaust electric valve of the deaerator based on the oxygen content in the feed water. 3. Deaerator shutdown: (1) When the load is less than 20% of the rated load, the deaerator switches from using four extraction stages to being heated by auxiliary steam, operating at a constant pressure of 0.147 MPa. (2) After the unit stops operating, decide whether to stop feeding water to the deaerator based on the specific circumstances. (3) If the deaerator is out of service for more than two months, it should be protected by nitrogen filling. All steam and water supplies must be cut off, the remaining water in the tank drained, and the drain valve closed. After complete isolation, the main nitrogen filling valve and the isolation valve should be opened to fill the deaerator with nitrogen and maintain a certain pressure. V. Normal operation of deaeration equipment (1) Once the unit is operating normally, close the two electric globe valves on the steam discharge pipeline at the top of the deaerator; the steam is then discharged through throttle orifices. (2) When the turbine loses load, and the extraction pressure at the deaeration unit drops below 0.15 MPa, the extraction valve should close automatically; the backup steam source should be activated urgently, and automatic pressure control should be engaged to keep the deaeration unit operating at a constant pressure of 0.15 MPa. When the feed water pump is shut down, the backup steam source is turned off, as well as the inlet and outlet valves; thus, the deaeration equipment enters a shutdown state. (3) If the oxygen content in the effluent is found to be below the required standard while the deoxygenation equipment is operating normally, the opening degree of the exhaust valve can be increased appropriately. (4) During operation, the water level should be monitored regularly to ensure it remains at the normal level; the automatic water level regulator should activate when the level is too high or too low, and any failures should be addressed promptly. (5) During normal operation, various valves, water level gauges, pressure gauges, thermometers, etc. should be available in full quantity, be sensitive and reliable, and should be regularly inspected. (6) Regularly monitor and record parameters such as the operating pressure, temperature, water level, oxygen content in the effluent, and output of the deaeration equipment as required by the operating procedures. VI. Deaerator interlock protection (1) An alarm is issued when the deaerator water level rises to the High I value. (2) When the deaerator level rises to the High II value, the interlock activates the deaerator drain valve to the condenser electric valve. (3) When the deaerator water level rises to the High III value, open the emergency drain control valve of #3 high-pressure heater simultaneously, and close the electric valves for the fourth-stage extraction steam to the deaerator, as well as check valves 1 and 2 for the fourth extraction and the main electric valve for the fourth extraction. VII. Regulation of the heating steam source: When the unit operates in sliding pressure mode, no control valve is installed on the pipeline leading from the fourth stage extraction turbine, which serves as the heating steam source, to the deaerator; as a result, the operating pressure inside the deaerator changes accordingly with the pressure of the fourth stage extraction steam. At this time, the control valve is installed on the pipeline from the backup steam source to the deaerator. If the pressure of the four-stage extraction steam drops to 0.147 MPa, the steam supply for the deaerator should automatically switch to an auxiliary steam source; at this point, the deaerator operates at a constant pressure. The pressure signal is generated by the signal tube installed on the deaerator, and it is used to control the steam inlet regulating valve through electronic instruments. When the unit’s load increases and the pressure of the fourth-stage extraction steam rises to 0.147 Mpa, the auxiliary steam source should also automatically switch over to the fourth-stage extraction steam. When the unit operates at constant pressure, the control valve is installed before the heating steam source; the pressure signal is generated by the deaerator, and then used to control the steam inlet valve through electronic instruments. The pressure signal is also sent to the pressure gauge in the control room for operators to monitor. VIII. Shutdown protection for deaeration equipment: If the deaeration equipment is to be shut down for less than a week, it is possible to slightly open the backup steam supply and close all other steam and water inlet and outlet valves to provide hot-state protection; the internal pressure can be maintained at 0.02 MPa. When the equipment is out of service for an extended period (more than a week), the water accumulated inside it should be drained to enable nitrogen protection; the nitrogen pressure should be maintained at 0.02 MPa, or other protective measures (such as the use of anti-corrosion agents) should be employed to prevent the inner walls of the deaerator from being corroded by oxygen or other harmful gases.