Analysis and optimization of high pressure water descaling system for medium and heavy plate mill
1 Introduction
According to the principle of metal heating, when the billet, slab, etc. are heated in the heating furnace, oxidation will occur, and a thick iron oxide scale will be formed, which will adhere to the surface of the metal material. In addition, the metal will undergo secondary oxidation during the rolling process, forming a thin layer of secondary iron oxide scale. Under the rolling action of the roller, a part of the iron oxide scale is broken into small flakes and automatically falls off, while the other part is pressed into the metal surface to form inclusions, pits, scars, etc., which not only directly affects the surface quality and yield of the steel plate, but also Accelerates roll wear. Therefore, timely removal of scale before and during billet rolling has become an important process that affects the surface quality of steel sheets. With the continuous improvement of the plate-to-tube ratio of my country's iron and steel industry products, the increasing amount of the plate market, the increasingly fierce market competition, the requirements for the surface quality of steel are getting higher and higher. This paper analyzes and optimizes the problems existing in the high-pressure water descaling system of the medium and heavy plate rolling mill in our factory, and achieves good results.
2 Introduction of high pressure water descaling system
2.1 System Composition
There are 4 high-pressure descaling pumps in the high-pressure water descaling system of the medium and heavy plate rolling mill in our factory, which can meet the flow control requirements of the high-pressure water descaling pump by implementing variable frequency speed regulation on the motor. The pump inlet pipeline is equipped with a butterfly valve with proximity switch, a flow meter and a low pressure pressure sensor, and the pump outlet is equipped with a high pressure pressure sensor. At the same time, the high-pressure check valve and high-pressure electric gate valve, small flow valve, descaling jet valve and pipeline at the pump outlet ensure the pressure and flow of descaling in the descaling box and on the rolling mill. Equipped with a second-level computer model and a first-level automatic control, it can meet the automatic descaling requirements of the descaling system and the remote monitoring and operation control of the operator. There is no accumulator and high-pressure air compressor in the system, the system is easy to operate and saves energy, but a small-capacity pipeline buffer tank is installed in front of the high-pressure water main pipe at the pump outlet and the main rolling descaling valve to reduce the pressure of the high-pressure water system The water hammer force generated by the fluctuation improves the reliability and safety of the system.
2.1 System Features
The water source of the high-pressure water descaling system is provided by the power plant, the water supply pressure is 0.4-0.5MPa, the water supply volume is 3000L/min, and the water is turbid circulating water. After being filtered by the self-cleaning filter, it is used for the three-plunger high-pressure water pump. After the high-pressure water pump sucks the low-pressure water and pressurizes it to 15-23MPa, it is supplied to the descaling box and the four-roll mill for descaling. The descaling box is used for one-time descaling, which can remove most of the iron oxide scale on the slab. Four-high mill descaling is used to remove the regenerated scale produced during the rolling process. After descaling, the water flows into the cyclone well along the slag flushing ditch of the main rolling line. After precipitation, it is pumped into the 4 self-cleaning filters in the filter room for filtration and sent to 2 chemical degreasers to remove the water. The waste oil is then cooled down and enters the circulating water pump station for recycling.
3 Analysis and optimization of existing problems
3.1 System piping
The main problem of the high-pressure water descaling system pipeline is large vibration, which often leads to pipeline leakage accidents. The high-pressure water pump house is far away from the descaling header of the rolling mill, and there are many tees, elbows and valves in the middle of the pipeline, and the local resistance coefficient is large; there is water hammer during the water pumping process.
The main improvement measures we have taken in this regard are:
(1) Add heavy metal pipe clamps at the elbow and tee of the intermediate pipe; rebuild the foundation at the cracked position of the pipe clamp foundation.
(2) Formulate a reasonable inspection period, strengthen the inspection of pipe clamps, flanges and valves, and deal with problems such as looseness and water leakage in time; improve metal gaskets to prevent breakdown and water leakage.
(3) The pre-filling water valve of the supporting pipeline of the descaling jet valve is kept normally open to adjust the appropriate water volume; the accumulator tank of the pump house adjusts the appropriate water level according to the production demand to stabilize the water pressure of the system. Through the above measures, problems such as welding and water leakage caused by vibration have been solved.
3.2 Descaling water nozzle and its branch pipes
The service life of the descaling nozzle and its branch pipes is short, resulting in an increase in the cost of spare parts. The rolling mill in our factory is a four-high reversible rolling mill, which adopts the periodic intermittent water spraying and descaling method, and the frequency of descaling and watering is high. For each rolling pass, the high-pressure water descaling system has a short water spraying time of 1-2s ahead of the billet head, and a lag water stop time of 1-2s at the billet tail. During the period of advance and lag, the billet is not in the area hit by the high-pressure water nozzles, the nozzles of the lower and upper headers are washed with each other under high pressure, the branch pipes are impacted by high-pressure water for a long time, the pipe wall becomes thinner, and the high-pressure water blows and bursts, and the welded parts Cracking causes the nozzle to fall off, which not only increases downtime for maintenance, but also poses a safety hazard.
Optimization measures include:
(1) Design and install a protective sleeve for the nozzle branch pipe. The protective sleeve is made of thick-walled seamless steel pipe with a length of about 200mm and is only fitted on the nozzle of the lower header. The upper header is installed on the guide guard, and the nozzle branch pipe is protected by the lower base plate of the guide guard without installation.
(2) In order to reduce the cost of replacement and maintenance of the nozzle every year, the nozzle is localized. Choose domestic high-quality products, the performance reaches imported products, shorten the supply cycle, and save tens of thousands of dollars every year.
(3) Strengthen spot inspection, send special personnel to inspect nozzles and branch pipes during each roll change, and deal with hidden dangers in time.
3.3 Incomplete descaling
In order to prevent the collision between the nozzle and the strip steel, the descaling spray device used on site artificially increases the spray distance by a large margin, and the spray device is rigidly connected to the machine base. The spray distance cannot be adjusted with the change of slab thickness, which greatly reduces the jet flow. For the impact pressure of the iron oxide scale, there is still a certain amount of iron oxide scale on the surface of the strip after descaling. Due to the existence of the iron oxide scale, the surface quality of the hot strip steel is seriously affected. The inexhaustible oxide slag aggravates the wear of the rolls, increases the consumption of the rolls, reduces the service life of the rolls, and increases the production cost; at the same time, the frequent replacement of the rolls wastes a lot of time and reduces the effective operation rate of the equipment. The high-pressure water descaling removes the iron oxide scale, and at the same time cools the surface of the strip steel with high intensity. The temperature difference affects the rolling effect and the surface quality of the product.
A good descaling effect is to obtain a larger striking force per unit area with as little water consumption as possible. Under the premise of ensuring the descaling effect, following this requirement can not only directly reduce the consumption of high-pressure water, but also reduce the temperature drop of the strip, which brings benefits to the billet rolling. The effective way to achieve this requirement is to shorten the spray distance. . The high-pressure water pressure of the descaling system used by Meishan Iron and Steel is 18MPa, the vertical distance between the descaling nozzle and the roller table is 230mm, and the height cannot be adjusted with the thickness of the strip.
Improvement method: shorten the spray distance. When the thickness of the strip steel changes, the nozzle header adopts a follow-up mechanism to always keep the vertical distance between the nozzle and the upper surface of the strip steel at 160mm.
Another factor that affects the descaling effect is the incidence angle of the nozzle. If the incident angle is too small, the impact pressure of the jet on the iron oxide scale will be greatly reduced. If the incident angle is too large, the injection distance from the nozzle to the billet will be increased and the injection pressure of the nozzle will be reduced. Arrangement of descaling nozzles: The spraying direction of each flat nozzle is opposite to the rolling direction and forms an angle of 15° with the vertical surface of the roller table. Such an angle configuration can obtain a better descaling effect. Since the blowing force of the nozzle on the two sides of the spray width is smaller than that in the middle area, in order to avoid the result of poor descaling effect caused by the reduction of the blowing force on the side, it is necessary to maintain a certain amount of overlap on the side. Accurate control of the overlap of the jet width is crucial to improve the uniformity of the strip surface temperature after descaling. The best spray overlap is 5mm. If it is greater than 5mm, the overlap will cause excessive cooling and black stripes will appear on the surface of the strip; on the contrary, if it is less than 5mm, the overlap will cause insufficient cooling and red stripes will appear on the surface of the strip. In both cases, the temperature of the strip surface will be uneven. This is also the main reason for the bright and dark streaks clearly visible to the naked eye on the strip after descaling.
3.4 Descaler water supply system
The water supply problem is mainly aimed at the performance problem of the existing high pressure water pump. The high-pressure water pump is a horizontal three-piston high-pressure water pump, which is characterized by high pressure, strong anti-pollution ability, and continuous operation.
However, there are mainly the following problems:
(1) The noise is large and the working environment of the operator is poor.
(2) The water supply is low, the continuous water supply capacity is poor, and the descaling time is slightly longer, which will cause insufficient water supply, which directly affects the surface quality of the board.
(3) The failure rate is high, and frequent maintenance is required, and the maintenance personnel work intensively.
The optimization measures mainly include:
(1) Replace the original 4 three-piston high-pressure water pumps with 2 centrifugal pumps with a rated pressure of 23MPa and 5000L/min (open 1 and reserve 1). The centrifugal pump has the characteristics of high system pressure, large flow, low noise and low failure rate. However, there are higher requirements on the water quality of the water source (the content of suspended solids should be less than or equal to 10mg/L), so the water treatment system also needs to be transformed accordingly.
(2) Change the turbid circulating water to static ring water to reduce the content of suspended solids in the water, and at the same time transform the self-cleaning filter, choose a self-cleaning filter with good quality, good performance and high filtration precision, so as to improve the overall equipment of the descaling system. Use efficiency.
4 Conclusion
Aiming at the problems that occurred in the production and operation of the high-pressure water descaling system of the medium and heavy plate production line of our factory, we have found out the economical and effective solutions and improvement measures. The life and replacement cycle of the descaling header of the high-pressure water descaling system have been extended; the operation mode of the pump set has been optimized according to the production situation, and a good effect of energy saving and consumption reduction has been achieved. While the motor capacity is increased, the power consumption does not increase but decreases. , reducing the maintenance cost of the water pump; at the same time, the stability and safety of the descaling system operation are improved.
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