Practice of installing and operating the economic operation of compressed air system in large steel enterprises Zhang Li, Pan Hao, Liu Wei, Wang Bo (Wuhan Iron and Steel Group Gas Co., Ltd., Baiyushan 430083, Qingshan District, Wuhan, Hubei Province), leakage loss, air pipe network system operation The three main factors of pressure and operation mode are analyzed, and the optimization measures taken and the economic benefits obtained are described.
Engaged in the production and operation management of air separation equipment.
In other words, compressed air is mainly used in large steel enterprises for pneumatic transmission, pneumatic tools, cooling, instrument purging, drying, cutting and flame treatment. It is an important power source, so each steel company has a huge and complex compression. Air system. For example, the number of air compressor stations of Wuhan Iron and Steel Group (hereinafter referred to as: WISCO) has increased with the development of the Group. From the initial air compressor station and two sets of 150m3/h air separation equipment, it has gradually expanded to seven air pressures. Station, the total gas supply reached a scale of 340,000 m3 / h.
The total life cycle cost of a compressed air system includes three major components, namely, equipment capital construction cost, equipment daily operation and maintenance cost, and operating energy cost. According to statistics, the cost of equipment infrastructure construction and operation and maintenance only accounts for about 25% of the total cost of compressed air life cycle, while the annual energy consumption cost of compressed air system accounts for about 75% of the total system cost. Therefore, how to optimize compression The operation of the air system and the reduction of the energy cost of its operation are very important issues for steel companies.
1 Analysis of the reasons for the increase in energy consumption cost of compressed air system 1.1 Loss of compressed air system leakage Compressed air system leakage is a continuous source of lost power, resulting in a large amount of compressed air leaking to the outside of the system, resulting in loss of energy medium, is to increase compressed air The most important factor in the energy consumption of the system.
Air Pipe Leakage A large number of air pipes with a diameter of more than 50mm are more than 125km in the headquarters of WISCO. All of them are laid in the open air. Some pipes are still in a high temperature, humid and corrosive environment. Pipe rust and perforation often lead to leakage failure.
Automatic Traps Failure Leads to Continuous Leakage Automatic traps are the most inconspicuous and least valued accessory in compressed air systems, but they are the biggest cause of leaks in pipelines. Corrosives and granulated dust in the pipeline are deposited in the trap, which can easily cause the trap to become blocked or blocked and cannot be closed. Once the trap is blocked, it is difficult to repair. In order not to affect the drain of the pipeline, the bypass valve must be opened slightly to maintain continuous air release at any time. Therefore, a considerable amount of air leaks to the outside of the system while draining. The number of drain valves on the compressed air system pipe network of WISCO headquarters has reached more than 1,000. If the control is not good, the leakage point of this one alone will reach more than 1,000, and the leakage loss caused is very alarming.
Air dehydration unit regeneration gas consumption is large in large steel companies, air supply is divided into two modes: power air and instrument air. The air is compressed by the compressor and then enters the freezer to cool down to a dew point of -5 C. 60% of the air in the air gradually precipitates, and the lyophilized air is sent to the power air pipe network. After the freeze dryer, the adsorption drying device is connected in series to further reduce the air dew point to -40C, which is used as the instrument air of the system. The air dehydration and drying unit of the WISCO headquarters air compressor station adopts a non-thermal regeneration process. The main features of this process are low operating energy consumption but large loss of regeneration and dissipation. The consumption of regeneration gas required for desorbent desorption accounts for the amount of air compressors. More than 20%. With the continuous introduction of new technologies, the requirements for air quality are increasing. The ratio of instrument air to total air volume is increasing year by year. At present, it has reached 50%. The improvement of air quality will inevitably increase operating costs.
1.2 Air pipe network system has high operating pressure and poor operating economy. Air pressure is also an important indicator to measure air quality. The air compressor impeller changes the kinetic energy and pressure of air through high-speed rotation, and then the air flow rate is reduced by the static diffuser. To achieve the transformation of kinetic energy into pressure energy. The increase in air compressor exhaust pressure directly leads to an increase in the energy cost of air operation.
1.2.1 Defects in piping design in the user's building, the flow loss of the pipeline gas is large, and the output pressure of the air compressor is forced to rise. In the investigation of the user's use of air pressure, some users report that the pressure at the end of the plant is insufficient. The air ducts in these user buildings are laid in a tree-like manner, and the road from the pipeline access point to the farthest point of use is long, and the flow capacity of the air duct in the plant is insufficient, and the gas flow has a large loss along the path.
The preliminary design air consumption of WISCO's three hot rolling is 10000m3/h. The gas source comes from WISCO's six air compressor station. The design pipeline diameter is 250mm. The air pipeline from the access point of the three hot rolling plant to the end user is up to 500m, and the tree is adopted. Process layout as shown. After the production, due to changes in the three hot rolling processes, the actual air consumption reached 18000 m3 / h, the end air pressure was actually only 0.3 MPa, and the pipeline pressure loss from the access point to the end user exceeded 0.2 MPa, and production could not be performed. In order to meet the normal production requirements of the three hot rolling, it was forced to install a pressure regulating valve group on the outlet pipe of the six air compressor station, and the output pressure of the air compressor of the six air compressor station was increased to 0.75 MPa, so that the pressure at the end of the three hot rolling was reached. 0. Production. However, a large amount of air in the six air compressor stations must be depressurized by the pressure regulating valve and then sent to the compressed air system pipe network, resulting in energy loss. According to statistics, after installing the pressure regulating valve, under the same operating conditions, the power consumption of the six air compressor stations increased by 1.2.2. The air compressor station has many stations, wide distribution, long pipelines, and large pipeline circulation losses in large steel enterprises. In order to ensure the reliability of the production, the air delivery system is usually transported by a ring. The seven air compressor stations in the headquarters of WISCO are distributed in various areas of 25 km2 of WISCO, and each station has special communication. Limited by the range of gas supply at each air compressor station, when the gas consumption in a certain area is reduced, the output pressure of the air compressor station in the area will increase accordingly, and some of the surplus air products will be transported to other required stations through the communication pipe. In the region; conversely, when the gas consumption in a certain area increases, the surplus air products in other areas can also be replenished through the connecting pipe. However, due to the large difference in the production period of air compressor stations, and some air compressor stations have undergone expansion, the original connecting pipes have been unable to meet the demand for the deployment of surplus air products. Longer connecting lines and relatively small pipe diameters have caused Larger gas flow losses result in reduced economics of the compressed air system.
1.2.3 The air pressure demand of individual users is too high, which causes the air compressor exhaust pressure to be forced to increase the air ring network transmission. Although the reliability of the system production is improved, the pressure of the delivery pressure cannot meet the pressure requirements of individual users. Excluding the cause of pipeline design defects in the user's plant, only the partial pressure points of the three cold-rolled and silicon steels require a pressure of not less than 0.55 MPa. The use of these gas points is only 500 m3/h, which only accounts for the total air delivery. 0.16%, in order to ensure the normal production of three cold rolling and silicon steel, it is necessary to increase the pressure of the entire compressed air system pipe network by increasing the air compressor, and the exhaust pressure is increased, and the power consumption of the compressor is increased simultaneously. This mode of operation increases the operating cost of the entire compressed air system.
1.2.4 Different rated output pressure of air compressors led to the discharge of some units. There are 27 air compressors in the existing seven air compressor stations of WISCO headquarters, which were built in different periods in the past 50 years. Due to the increasing air conveying capacity, the pressure on the conveying system and other factors lead to an increase in the pressure demand of the user. The rated output pressure of the compressor is gradually increased from the earliest 0.65 MPa to 0.8 MPa. In order to meet the pressure of most users of the compressed air system. Demand, the pressure of compressed air sent to the system pipe network through the dehydration drying device is higher than 0. The output pressure of the compressor is to overcome the pressure loss of the dehydration drying device (usually higher than 0.1MPa), generally higher than 0.65MPa. In this case, the compressor with a low rated output pressure will be released and operated under anti-surge, which is not conducive to the safe and economic operation of the air compressor.
1.3 The operation mode is not optimized, resulting in the comprehensive energy consumption of the compressed air system. The energy consumption of the compressed air system is high. In addition to the defects of the system itself, human control and management are not one of the important factors. In large steel enterprises, the compressed air system is mainly based on the nature of production, and the operation mode, production scheduling organization optimization and economy are often neglected.
The output pressure of each air compressor station is controlled separately, with high and low, and poor deployment results in large pipeline circulation losses. The closer the user is to the air compressor station, the smaller the pipeline circulation loss. If the air usage of the user close to the air compressor station is limited, the air of the air compressor station must be transported to a remote area through the communication pipe, which will inevitably increase the circulation loss.
Unclear user usage makes it difficult to adjust the operating mode.
The air compressor station of WISCO headquarters is equipped with various types of air compressors from 3000m3/h to 30,000m3/h. When the pressure of the pipe network drops, because it does not know how much the user consumes, if the equipment that is larger than the user's increased consumption is started, This will increase the pressure on the entire air pipe network, resulting in wasted energy.
Air compressors vary greatly in energy consumption per unit because of the age of construction and the manufacturer. Long-term operation of a single machine with high energy consumption of air compressors results in increased energy consumption costs for compressed air systems.
2 Measures to reduce the energy consumption cost of compressed air system 2.1 Inspect the hidden dangers in time and reduce the air dissipation loss 2.1.1 Effectively reduce the leakage of pipelines Regularly conduct a comprehensive survey of the compressed air system pipe network to timely deal with pipeline leakage and reduce the leakage of pipelines. And according to the age and corrosion of the pipe network, plans are made and replaced gradually.
In 2009, WISCO compiled a management method for energy medium pipeline inspection, which clearly stipulated that on the basis of daily inspection, the automatic steam traps on the compressed air system and corrosion and rust on the compressed air system were divided twice in April and October each year. The gaskets, broken pipeline valves, etc. were investigated in a key place, and the leaks were ordered to be ordered to rectify within a time limit.
In 2010, WISCO introduced the electronic tube network system, and recorded all the information such as the construction date and maintenance status of pipelines, valves, flanges, pipe racks, etc. into the system, so that the system can promptly indicate the parts that should be focused on inspection and monitoring. Effectively manage the air pipe network online. Through the management of the electronic tube network system, firstly, the pipelines of the WISCO headquarters in the operating air pipelines with a service life of more than 30 years are 28.3km. After further on-site inspection of these pipelines, according to the corrosion of the pipelines and users The maintenance plan is arranged and the plan is changed one by one. As of December 2012, WISCO replaced a total of 11.74km of air pipelines, effectively reducing pipeline leakage caused by pipeline corrosion.
2.1.2 According to the use environment of different parts, choose different types of air automatic traps to install float traps in the air outlets with large water content, such as air compressor outlets. The biggest advantage of this trap is that it can effectively collect air. Water, emissions loss is small. However, impurities such as rust are easily accumulated in the middle of the discharge port and the float ball, resulting in blockage of the discharge port or jamming of the valve, resulting in no discharge or continuous discharge of the steam trap. Therefore, an inverted bucket trap with a filter is selected on the compressed air system pipe network, especially for pipes with a long life, and the filter in the steam trap is periodically cleaned. Since 2009, WISCO has selectively replaced more than 200 traps on the compressed air system. Although the one-time investment is large, the effect is very obvious. On the one hand, the ineffective discharge on the compressed air system is reduced, and on the other hand, the maintenance is reduced. cost.
2.1.3 Strengthen the maintenance and process management of air equipment to ensure the working effect of air compressor gas cooler, reduce the air compressor exhaust temperature, and reduce the load of air cooling unit and suction unit.
Reduce the failure rate of the freezer. The normal operation of the freezer is directly related to the water content in the air entering the dryer, thus affecting the size of the regeneration gas required for the regeneration of the adsorbent. It can be seen from the test that under the same intake air condition, the air temperature of the suction dryer is increased by 1C, and the consumption of the same dew point regeneration gas is increased by 1% 3%. The maintenance of the dryer is enhanced. Although the structure of the dryer is not complicated, due to the high switching frequency (switching once every 3 minutes), the valve moves frequently and the failure rate is high. In response to this situation, strengthen the point inspection management of the dryer, replace the faulty valve in time; monitor the use of the adsorbent and replace the failed adsorbent in time.
From the use situation of WISCO using the non-thermal regeneration air dehydration drying device, although the running energy consumption is low, the regeneration and dissipation loss is large, especially after the refrigerator is operated for 5 years, the failure rate is increased, and the equipment maintenance cost is increased. Therefore, WISCO is preparing to gradually replace the existing non-thermal regeneration air dehydration drying device into a micro-thermal regeneration process.
2.2 Optimize the pipe network structure to reduce the air compressor output pressure and comprehensive energy consumption 2.2.1 The end pressure is insufficient and the tree arrangement in the plant is changed to a ring arrangement. In March 2009, WISCO three hot rolled from the nearest compressed air. A new pipe is laid on the pipe and connected to the end pipe, and the air pipe inside the plant is changed to an annular arrangement (as shown). After the operation, the three hot rollings did not change the gas supply volume, and the end air pressure increased by 0.18 MPa, which satisfied the needs of the three hot rolling production and eliminated the pressure regulation of the outlet of the six air compressor stations. The valve function reduces the output pressure of the air compressor to 0.58 MPa. According to this method, the internal conveying pipelines of five users, such as three cold rolling and four steel making, are locally optimized.
Table 1 Comparison of comprehensive energy consumption of air compressor stations before and after implementation of optimization measures Air compressor station 2009 (before implementation) 2012 (after implementation) annual average export pressure / MPa comprehensive energy consumption / (kWh / m3) export pressure annual average Value/MPa comprehensive energy consumption 2.2.2 Optimize the compressed air system conveying pipeline, reduce the pipeline circulation loss DN600mm connecting pipe of the air compressor station, solve the problem of unbalanced output pressure of each air compressor station after the pipeline is put into use, reduce the gas circulation loss. At the same time, due to the increase of the communication capacity of the connecting pipe, the air compressor station that had no chance to stop all the time can also stop at any time to deal with hidden dangers of equipment, which greatly enhances the reliability of the production.
2.2.3 After the user with excessive air pressure demand and small amount is separated from the system pipe network and a large amount of preliminary communication work between the three cold rolling and silicon steel units, and approved by WISCO, in 2011, the two units respectively A small air compressor was added for the gas point with high demand pressure, and the original air duct was reserved as a backup air source. After the user with excessive pressure demand detaches from the compressed air system pipe network, the system pipe network pressure is stable at 0.53 MPa, and the output pressure of the air compressor is also reduced by the overall rated pressure of the air compressor. After taking a series of measures, the user is guaranteed. Under the premise of pressure demand, the output pressure of the air compressor can be controlled below 0.65 MPa, and the amount of air compressors originally limited by the rated output pressure is significantly reduced.
2.3 Reasonably organize production scheduling, optimize operation mode and strengthen dispatch management function, and change the original control pressure control mode of each air compressor station to be controlled by the dispatching room, and implement partition management. It is clear that each air compressor station is mainly responsible for the supply of user units. The dispatcher pays close attention to the outlet system pressure of each air compressor station, and optimizes the operation mode for air compressor stations with large pressure changes in order to reduce the pipeline pressure drop and various air pressures. The purpose of station pressure balance.
Make full use of the WISCO energy management system platform, keep abreast of the consumption of each user, adjust the operation mode in time, and open and stop the air compressor unit that matches the change of consumption. It is usually based on continuous operation of large equipment and peaking of small equipment. The auxiliary operation mode achieves the purpose of economic operation.
According to the performance of the air compressors of the air compressor stations, the equipment with relatively low energy consumption is selected for operation, and the equipment with high energy consumption is in the standby state. There are four air compressors in the second air compressor station. The air compressors in the first and second air compressors were put into operation in 1995. The unit consumption was 0.146 kW.h/m3, and the air compressors in 3 and 4 were put into operation in 2008. The unit consumption was 0. h/m3. According to this situation, it is stipulated that under normal conditions, the two air compressor stations are mainly operated by 3 and 4 air compressors. By adjusting, the energy consumption level of the second air compressor station decreased by 0.06 kW h/m3. The adjustment of the operation mode of the air compressor was successful, and then the method was applied to other air compressor stations, and the obvious effects were obtained.
3 Implementation effect WISCO took various measures to continuously optimize the compressed air system in three years. The outlet pressure and comprehensive energy consumption of each air compressor station decreased significantly (see Table 1), and achieved very good economic benefits.
After the implementation of the optimization measures, the comprehensive energy consumption of the compressed air system decreased by h/m3. According to the total air output of WISCO in 2009, it was calculated by 228.619x107m3/h. The total power consumption decreased: 10,000 kW in the case of constant air production, after the implementation of the optimization measures The annual power saving cost is 9.6 million yuan.
4 Summary At present, the structure of compressed air system of China's large steel enterprises is basically the same, mainly composed of air compression, air drying, pipeline transportation, user gas distribution, etc. According to the scale of steel enterprises and the consumption of users, reasonable optimization Pipe network, strengthen the control of leakage, reduce the pressure drop of pipe network, which improves the safety of production of compressed air system, saves a lot of energy consumption, and has strong practical application value. WISCO has also gone through many detours in the construction of compressed air systems. However, after continuous exploration, it has found an optimized transformation road suitable for its own characteristics and achieved very good economic benefits. It is hoped that it can be used for reference by peers. ,
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