The magnetic plate level gauge in the iron and steel plant heat transfer equipment problems and solutions

At present, some large-scale heat exchange equipment of heat exchange stations take tubular heat exchangers as an example. Generally, they are centrally arranged. The upper layer is the main body of the heat exchange equipment, and the lower layer is the condensate recovery device. This will not only save space but also facilitate condensation. Recycling. However, in recent years, some heat exchanging stations have arranged heat exchange equipment and condensate recovery devices on a horizontal surface. At this time, how to make these condensate waters that can normally be recovered under the condition of continuous steam changes in these heat exchange equipments? It faces new challenges. This article focuses on the solution to this problem.

In order to meet the normal production needs of steel mills, Shougang Jingtang Steel Company has set up 5 centralized heating stations in the plant, which are 1#, 2#, 3#, 4#, and 5# heat exchange stations, distributed throughout the plant. It is mainly responsible for the supply of high and low temperature hot water needed for the heating of all production and living and management facilities throughout the company, taking into account the balance of the steam consumption of the pipe network. The main heat exchange equipment of each heat exchange station is a steam-water tube heat exchanger. The condensate generated after heat is released from the heat exchangers is all recovered and sent to the plant's various process equipment as production water. However, when external sources such as pipe network and users cause less steam entering the tube heat exchanger and the pressure is lower, the condensed water formed during the heat exchange process cannot be recovered and gradually increases in the tube heat exchanger. The water level continues to be high, and the heat exchange efficiency is drastically reduced, especially in large heat exchangers. For this reason, in this paper, Shougang Jingtang Iron & Steel Company 4# heat exchange station is taken as an example to analyze the two QTQH-WN-32MW high-temperature steam-water heat exchangers in the field, and a new solution to the problem of heat exchanger hydrophobicity is proposed. The new method to adapt to the constant change of outside steam.
1. Introduction to the water system 1.1 A replenishing tank is set in the replenishing tank station. The replenishing tank volume is 20m3 and the specific size is 3000×3000×2500mm. The replenishing tank is provided with an inlet, a sewage outlet, an overflow port, a tank top hole, and a liquid level. The necessary interfaces such as the meter interface, the water pump inlet, and the inner and outer ladders are provided, and the bottom is provided with a barrier. The water inlet is equipped with an automatic float valve, and the float valve can be opened and closed automatically according to the level of the tank. The bottom partition can effectively filter the hard impurities in the tank. The water tank is used to buffer the change of the water flow of the system and prevent the normal operation of the heating system when the fluctuation of the water flow is large.
1.2 Automatic constant pressure constant pressure water supply device This station is equipped with an automatic frequency conversion constant pressure constant pressure water supply device, including 2 frequency conversion water pumps, 1 frequency control box, the main parameters of a single pump are as follows: (1) flow: 20m3/ h; (2) Lift: 58.8m; (3) Motor power: 11kW/380V; (4) Delivery water temperature: ≤ 40°C; (5) Ambient temperature: 5°C to 40°C; (6) Working pressure: ≤1000kPa . There are two kinds of manual and automatic operation modes for the device, and the relevant parameters can be set automatically according to the actual operation conditions at the scene. During the stable operation of the heating system, it is generally set to automatically replenish the water, the pump is used one by one, the two pumps are switched automatically according to the set time, and the device detects the system pressure through the pressure sensor installed on the pipeline connected to the system and the data Uploaded to the internal CPU, through the comparison of the preset parameters, to control the start and stop of the frequency conversion pump. When the system pressure equals to the already set pressure parameter, the water pump stops running and enters the dormancy period. When the system pressure decreases, the frequency conversion starts again. At the same time, the device is equipped with an automatic pressure relief valve. When the system pressure exceeds the preset parameters, the device will release pressure through the automatic pressure relief valve to maintain the system's constant pressure.
1.3 Circulation Pumps There are 3 sets of ISR200-150-440 horizontal high-temperature hot water circulation pumps in this station. They are used for dual use and one for normal operation. The main parameters of each pump are as follows: (1) Flow: 550m3/h; (2 ) Head: 54.5m; (3) Motor power: 132kW/380V; (4) Operating temperature: ≤ 100°C; (5) Working pressure: ≤ 1200kPa. The pump is connected with a flexible coupling. With standard bearings, pump impeller bronze material, corrosion resistance; pump resin sand casting, pump shaft made of stainless steel. The pump's rotor and its main rotating parts have been tested for static and dynamic balance. The static balance accuracy is not lower than G6.3 in GB9239, and the dynamic balance accuracy is not lower than G2.5 in GB9239. The vibration of the pump was measured under cavitation-free operation conditions. The vibration value at the bearing complied with JB/T8097. The starting mode of the water pump is divided into two ways: local start and remote start.
2. Introduction to Steam System 2.1 Steam Pipeline The main route of the steam pipe at this station consists of five parts: electric shut-off valve, Y-type filter, pressure reducing valve, electric control valve, and secondary manual valve; the bypass is only equipped with a manual valve. Under normal circumstances, the plant pipe network steam enters the pipe heat exchanger through the main road. The electric shut-off valve is used to cut off the steam source quickly and reliably in an emergency. Y-type filters are used to filter hard impurities carried by steam. Impurities can be periodically discharged through the drain outlet provided by the equipment. The three-way pressure reducing valve regulates the steam pressure entering the tube heat exchanger by adjusting its own pressure regulating bolt to ensure that the steam entering the tube heat exchanger fluctuates within the allowable normal pressure range. The electric control valve is generally set for remote operation. The center of the plant dispatches the opening of the remote control valve to control the amount of steam entering the tube heat exchanger to timely adjust the heating temperature. The secondary manual valve is in the fully open state when it is working normally, and it is mainly used to adjust the amount of steam after the electric control valve exits operation. The bypass valve is normally in the normally closed standby state. When the pipe network pressure is too low, the steam cannot pass through the pressure reducing valve, or when the main road is inspected for isolation, steam is provided by opening the bypass.
2.2 Condensate Recovery Unit A total of 3 sets of condensate recovery units are provided at this station. The treatment capacity of a single unit is 40t/h. Each device is composed of seven main parts: backwater main body tank, decontamination device, cavitation eliminator, pressure regulating device, condensate feed pump, level sensor, and electric control cabinet. The condensed water formed during the operation of the tube heat exchanger passes through the steam trap and passes through the ascending pipeline and the horizontal pipeline successively into the main tank of the recovery device. At this time, first, through the precision filter and decontamination device, the oil and hard impurities in the entire pipeline are treated by mechanical physics, so that the contaminants are periodically discharged through the drain valve opened in the recovery device. The condensed water in the tank separates the secondary steam and the saturated condensate through the steam-water separator, so that the secondary steam maintains a certain space in the closed tank and the condensate maintains a relatively stable state, and then the condensed water is eliminated through cavitation. Device, enter the supporting use of the feed pump to the plant pipe network.
3. Introduction to Heat Exchanger System 3.1 Tube Heat Exchanger The heat exchanger is a device that transfers part of the heat of a hot fluid to a cold fluid. It is also called a heat exchanger. The heat exchanger is an indispensable device for heat exchange and transmission in the production process. In the heat exchange, there are often some corrosive and highly oxidizing materials. Therefore, the material for manufacturing the heat exchanger is required to have strong corrosion resistance. The classification of heat exchangers is relatively extensive, mainly divided into spiral plate heat exchangers, bellows heat exchangers, column heat exchangers, plate heat exchangers, shell and tube heat exchangers, positive displacement heat exchangers, floating head type Heat exchangers, etc. In view of the strong corrosion resistance of materials required for the manufacture of heat exchangers, it can be made of non-metallic materials such as graphite, ceramics, glass, and stainless steel, titanium, tantalum, zirconium, and other metal materials. However, heat exchangers made of materials such as graphite, ceramics, and glass have drawbacks such as fragility, bulkiness, and poor thermal conductivity. Heat exchangers made of rare metals such as titanium, tantalum, and zirconium are too expensive, but are made of stainless steel. The resulting heat exchanger is poor in corrosion resistance and prone to intergranular corrosion.
There are 2 sets of high-temperature steam-water heat exchangers in this station. The main parameters of the single equipment are as follows: (1) heat transfer load: 32000 kW; (2) hot water circulation: 550 m3/h; (3) water supply temperature: 130 °C; (4) Return water temperature: 80°C; (5) Steam calculation pressure: 0.2-0.4MPa; (6) Steam calculation temperature: 200°C, temperature of equipment considered at 300°C; (7) Condensate outlet water temperature: ≤85°C; (8) Steam consumption: 48t/h; (9) Water side working pressure: ≤1200kPa. The heat exchanger inner tube material is: stainless steel.
Tubular heat exchanger works:
As shown in the figure below, the saturated steam with a pressure of 0.4~0.6 MPa enters the tube heat exchanger from the steam inlet and flows up and down through the baffle plate and the partition plate. Finally, most of the heat is absorbed by the hot water to form condensed water from the condensed water. Outflow. One hot water (temperature about 20°C) flows in from the hot water inlet and flows through the left-to-right heat exchange tube to the head. It flows out of the hot water outlet after passing through the right-to-left heat exchange tube and becomes two. Secondary hot water, secondary hot water flow to the system, after returning heat to the circulating pump inlet of the heat exchange station after exothermic, flows again to the hot water inlet after being pressurized, so that repeated heating is repeated, and finally the user is provided with high temperature hot water of 80~130°C. .
4. Hydrophobic problem 4.1 Heat transfer The 4 # heat exchange station of Jingtang Iron & Steel Co., Ltd. of Shougang was put into operation in the winter of 2008. At that time, most external users did not have heating conditions, and some heating pipes outside the station were not installed in place. The heat exchange station cannot bring all the high temperature users together. It needs only a small amount of steam to meet the user's requirements. Therefore, the heat exchanger cannot operate normally at full load. In this case, the steam enters the tube heat exchanger. The amount is less, and at the same time, there is not enough pressure inside, which causes a small amount of condensate water to pass through the steam trap and cannot return to the condensate tank through the ascending pipe behind the trap. Finally, the condensed water in the heat exchanger gradually increases. The magnetic flip plate liquid level gauge shows high liquid level for a long period of time, and the heat exchange efficiency of the heat exchanger also begins to drop sharply. In the later period, even a serious crash phenomenon occurs, directly threatening the service life of the equipment.
4.2 Mid-term Heat Exchange Shougang Jingtang Iron & Steel Co., Ltd. #4 Heat Exchange Station In the winter of 2010, with the completion of various projects in the plant, the high-temperature tube heat exchanger in the station brings almost all users, and the heat exchanger is also close to full. In the load operation, almost all of the condensed water formed during heat exchange is recovered. However, the heat exchange station is an important facility for balancing steam in the whole plant network, and it is often necessary to make timely adjustments based on the amount of steam in the pipe network. In the case of a shortage of steam in the pipe network, the heat exchanger only uses a small amount of steam intermittently. Through long-term on-site observation, this situation is often gradually increased in the condensed water in the heat exchanger, three magnetic level gauge shows high liquid level, heat transfer efficiency is gradually reduced, there are crashes and other phenomena, threatening the service life of equipment .
4.3 Late heat exchange With the long-term operation of the heat exchange equipment and its ancillary equipment, the steam trap of the heat exchanger sometimes appears to be unstable, resulting in the condensate formed during the heat exchange process of the heat exchanger cannot be completely recovered. It is gradually increasing in the heat exchanger. Eventually, it also causes the heat exchange efficiency to decline, and the occurrence of collisions and other phenomena threatens the service life of the equipment.
5. Solution 5.1 As shown in the above figure, two loops are added between the original ascent pipe and the steam trap, including the main route check valve and the pipeline pump. The pipeline pump and the heat exchanger are linked by the three-level magnetic flap level gauge. The high and low level pipeline pumps are automatically started and stopped according to the actual operation conditions on the site. The value of the magnetic level gauge is uploaded to the plant command center for real-time remote control. Monitoring, at the same time, increase the emergency stop button of the pipeline pump. The bypass only has a check valve. Under normal conditions, the condensed water can be smoothly recovered to the condensate recovery device through the bypass check valve; under abnormal conditions, it can be smoothly recovered after being pressurized by the pipeline pump.
5.2 As shown in the above figure, the electric valve is added on the lower side of the manual valve of the original sewage pipeline. During normal operation, the manual valve is in the normally open state, and the electric valve and the heat exchanger have three magnetic plate level gauges, which are set to automatically start and stop the high and low liquid levels. The upper limit of the high liquid level should be higher than the upper limit of the pipeline pump when it is started, as a backup of the pipeline pump or when the condensate is too much, so as to ensure the normal operation of the heat exchange equipment; the upper limit of the low level can be slightly higher than the pipeline The upper limit of the pump stop ensures that there is a small amount of condensed water in the tube heat exchanger, reducing the direct impact of steam on the equipment.
6. Summary Although some of the condensed water can be discharged by adjusting the manual drain valve, the high liquid level of the tubular heat exchanger is appropriately relieved, but this method wastes the condensed water and has limitations in the field application. The size of the valve opening is directly related to the level of the condensate in the tube heat exchanger. If the vapor is not fully exothermic after the assembly, the direct overflow will occur. If it is too small, the level of the equipment will gradually increase, so it is often necessary. A person's repeated adjustment at the scene not only consumes a lot of manpower and material resources, but the actual effect is not ideal. Shougang Jingtang Iron & Steel Company's No. 4 heat exchange station combines the above two methods with each other, adds some equipment on the original basis, and passes the winter 2010 operation inspection, not only fundamentally solving the problem of the hydrophobicity of large-scale heat exchange equipment. The manpower is released, and the heat exchange efficiency of the tubular heat exchanger is improved, so that the equipment can operate stably for a long period of time. -- Ren Jingyu of Shougang Jingtang Iron & Steel Co., Ltd., and more exciting related level gauge articles are on the website of the three-level liquid level meter. Http://

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