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Product Description
Drum is the most important equipment in boiler equipment, and it is the connecting hub of three processes of boiler heating, vaporization and overheating, which plays a role of connecting the preceding and the next. Water becomes qualified superheated steam in boiler, which needs to be heated, vaporized and overheated. It is the heating process that the water is heated to saturated water; The process of vaporization is that saturated water vaporizes into saturated steam; It is the process of superheating that saturated steam is heated to superheated steam. The above three processes are respectively completed by economizer, evaporation heating surface and superheater. The steam drum is related to the above three processes, and it needs to accept the water from the economizer; The circulation circuit is formed with the evaporation heating surface; Saturated steam shall be distributed to superheater by drum. The drum is not only the intersection of heating, vaporization and overheating, but also the boundary point of heating, vaporization and overheating. Therefore, the drum is the connecting hub of boiler heating, vaporization and overheating.
1. Drum function
1.1. Energy storage and buffering: the steam and water are stored in the drum, which has the function of energy storage. When the load changes, it can buffer the imbalance between evaporation and water supply and the change of steam pressure speed too fast. If the load is increased, the steam pressure will decrease. At this time, the water is in saturated state, and a part of the steam can be vaporized by itself, so that the steam pressure will decrease slowly.
1.2. Guarantee steam quality: steam water separation device, steam cleaning device and other equipment are installed in the drum, which can effectively carry out steam water separation, steam cleaning, dosing, sewage discharge, etc. to ensure the steam quality and boiler quality.
1.3. Ensure the safe operation of boiler: various water level gauges, pressure gauges, emergency water discharge valves, safety valves and other auxiliary equipment are installed on the drum to monitor the pressure and water level of the drum and protect the safe operation of the boiler.
2. Introduction to drum
2.1. The steam drum and heat exchanger are connected by the rising pipe and the descending pipe to form the water circulation of the drum itself. The steam drum water cycle is a convection heat cycle. The hot water (steam) produced by the boiler enters the drum from the rising pipe. The heat is exchanged during the process of the light water quality going up, and some water is converted into gas state; The cold water of the drum is supplemented to the boiler by the downcomer under the action of gravity. The water in siphon is the self circulation between the drum and the boiler. The steam rise is positively related to the water drop. If the boiler water is directly hit to the drum by means of a pump, the liquid level of the drum is maintained, this is called forced circulation. At the same time, the drum receives the feed water of the circulating water pump, and delivers saturated steam to the superheater or directly outputs steam. Therefore, the drum is the connecting hub of three processes, i.e. working medium heating, evaporation and overheating, so as to ensure the normal water circulation of the boiler.
2.2. Steam water separation device and continuous blowdown device are installed inside the drum to ensure the steam quality of boiler.
2.3. It has certain water quantity and certain heat storage capacity. When the boiler working condition changes, it can slow down the change speed of steam pressure and play a role of stabilizing the steam pressure.
2.4. Pressure gauge, water level gauge, emergency water discharge, safety valve and other equipment are on the drum to ensure the safe operation of the boiler.
2.5. The drum is a balance vessel that provides the pressure required for the flow of water-cooled wall steam water mixture.
3. Drum structure
The steam drum mainly includes three parts: steam water separation device, steam cleaning device, sewage, dosing, accident water discharge and other devices.
3.1. Steam water separation device
3.1.1. Principle of steam water separation device:
3.1.1.1. Gravity separation is carried out by using the density difference of steam water;
3.1.1.2. The inertia separation is carried out by using the inertia force when the steam flow changes direction;
3.1.1.3. The centrifugal force of steam flow rotation is used to separate the water and water and make the water adhere to the metal wall to form the adsorption separation formed by the water film flowing down.
The steam water separation device mainly includes cyclone separator, shutter separator and other devices. The cyclone separator is a cylinder made of 2-3mm steel plate. The steam water mixture enters the cyclone in tangent direction from the inlet volute, and the water drops are thrown to the wall by centrifugal force to separate the steam from water, and then it is subject to gravity in the cyclone, Steam enters the steam space of the drum through the top cap of the wave plate, and the water enters the water space from the lower part to complete the primary separation; Then, the secondary separation of the shutter separator can achieve the purpose of steam water separation. The center line of the inlet pipe of the cyclone is higher than the maximum water level in the drum, so that the water level in the drum is lower than the edge of the separator inlet pipe. The bottom of the drum of the separator shall not enter the normal water level for 200mm, so as to avoid the steam from passing through the bottom of the drum. In order to prevent the steam from entering the downcomer in the bottom drainage, tray is installed at the bottom of the cylinder. A porous plate is installed in front of the steam outlet pipe at the top of the drum, which is aimed at making use of throttling to make the saturated steam in the steam space evenly distributed along the length and width of the drum, so as to improve the separation efficiency. The porous plate is made of 3-4mm thick steel plate with a diameter of 10 mm.
3.1.2. Steam water separation process
3.1.2.1. The steam water mixture from the boiler enters into the inner part of the drum through the upper rising pipe of the drum, and flows down the narrow circular channel formed by the inner wall of the drum and the arc liner plate, so that the steam water mixture is uniformly heat transferred to the inner wall of the drum at the appropriate flow rate, which overcomes the difficulty of excessive temperature difference between the upper and lower walls of the drum when the boiler starts and stops, and can be started quickly.
3.1.2.2. The steam water mixture entering the steam drum enters the steam water cyclone separator respectively, and the inertia separation is carried out by using the inertia when changing the flow direction, which is the first separation of the steam water mixture.
3.1.2.3. The separated steam still has a lot of water, and enters the wave plate separator from the top of the separator. It is installed on the top of the cyclone separator. The steam with some water drops flows in the gap between the wave plates, and the water is bonded to the metal wall to form a water film to flow downward.
3.1.2.4. The steam after secondary separation is finally cleaned by steam, and gravity separation is carried out by using the density difference of water, which is the third separation.
3.1.2.5. After three separation, the steam quality standard is met, and then the saturated steam pipe at the top of the drum is led to the platen superheater.
3.2. Steam cleaning device
The basic principle of steam cleaning: it is to make the cleaning water with low salt content contact with the steam with high salt content, transfer the dissolved salt in the steam to the clean feed water, thus reducing the steam salt dissolution, and at the same time, it can transfer the salt content of the steam carrying boiler water to the clean water supply, thus reducing the salt content carried by the steam machinery and improving the quality of the steam. The harm of steam pollution: the steam with salt content exceeding the standard will cause salt scaling of steam turbine, boiler and other thermal equipment, and hinder the heat exchange of superheater, over temperature of pipe wall, increase of exhaust gas temperature and decrease of boiler efficiency; The steam pipeline valve fails to operate and leaks; The steam flow cross-section of the turbine will be reduced, the roughness of the nozzle and blade will increase, and even the shape of the nozzle and blade will be changed, so that the resistance of the turbine will increase, and the output and efficiency will be reduced; If the salt accumulation is not uniform, it will cause vibration of shaft system and even cause accidents. It can be seen that the steam quality is important to the safety of the unit.
3.3. Blowdown of drum
Continuous and regular blowdown is used for the blowdown of steam drum.
3.3.1. Continuous blowdown is mainly used to discharge concentrated water at the upper part of the drum, mainly to prevent the salt content and sulfur content of boiler water from being too high. The blowdown parts are mostly located in the most obvious place of drum water concentration, namely 200-300mm below the water level of the drum. The continuous discharge is usually adjusted according to the water quality analysis index of the drum.
3.3.2. Regular discharge is also called intermittent discharge, that is, the water slag, dirt and intermittent discharge deposited from the bottom of the boiler at a certain interval are generally discharged once every 8-24 hours, and the discharge rate is not less than 1% for 0.5-1 minutes each time. The intermittent discharge is good in frequent and short term, which can make the steam package water uniformly concentrated, which is conducive to improving the steam quality.
3.4. Why the steam drum is charged with medicine
Although the boiler feed water has been strictly treated, it is impossible to completely remove impurities, and the feed water will bring some impurities into the boiler. With the continuous evaporation and concentration of the water, the salt concentration of the water increases gradually, which may cause internal scaling. In order to prevent scaling, the medicine should be added to the boiler water continuously during operation. The chemical and physical effects of the drugs and calcium and magnesium salts in the water will produce non cohesive loose water slag, and deposit it to the lower part. The drugs shall be discharged outside the boiler regularly. The medicine added to the boiler water is usually Na3PO4 (trisodium phosphate), which is diluted and then pumped into the boiler water of boiler drum by the dosing pump. Adding trisodium phosphate into the water of the pot can not only make the calcium and magnesium salts in the water produce non cohesive loose water slag, but also can correct the alkalinity of the water, so that the pH value of the water in the pot can be kept within the scope specified in the regulations.
3.5. Accident water release
It is impossible for the accident drain pipe to drain the water from the drum. The function of the accident drain pipe is to discharge the boiler water quickly and restore the water level quickly when there is a full water accident or a co bubble of foam and water. The upper end of the accident drain pipe is in the drum, and the upper port is flush with the normal water level of the drum. Once the above conditions occur, open the emergency water discharge valve quickly to discharge the excess water and restore the normal water level. Because of the phenomenon that the boiler water rises in the accident water discharge hole, the water level can be placed slightly lower than the normal water level, but the water in the boiler will not be drained. Although the water in the drum will not be drained, the water level must be closely monitored after opening the emergency water discharge valve. Once the normal water level appears, the emergency water discharge valve shall be closed immediately. Otherwise, a large amount of saturated steam will be released through the emergency water discharge pipe, which will not only cause unnecessary work medium and heat loss, but also reduce the steam quantity entering the superheater, which will bring a threat to the safety of the superheater.