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    Test and development trend of papermaking felt permeability and other properties Release time: 2009-07-10 Visited: 7004 Times

    Lu Xiangyang 1 Han Bangchun 2


    (1. Taicang Chang'e Industrial Co., Ltd. Taicang, Jiangsu 215437)


    (2. Xuzhou Golden Crown Industrial Co., Ltd. Xuzhou, Jiangsu 221009)


    Abstract: Introduces the purpose, principle and test method of performance test of papermaking felt permeability, analyzes the relationship between the permeability of papermaking felt and its impact on the running performance of the felt; development trend.


    Keywords: papermaking felt; permeability; performance; test; development trend


    Papermaking blankets are important dewatering equipment in paper mills, and are also valuable consumables. They play a role in conveying, dewatering, and cushioning in the press section of the paper machine. The permeability of papermaking blankets mainly includes air permeability, water permeability and water content. It is an important physical performance index for evaluating the quality of papermaking blankets during production and use. They directly affect the production efficiency, product quality and production of paper mills. Cost, in order to understand and grasp the changing trend of the performance of papermaking blankets, and make effective adjustments in its life cycle to maintain the best working conditions, it is necessary to test the performance indexes of papermaking blankets such as permeability.


     1. Purpose of permeability test of papermaking felt


    1. It helps to prevent accidental shutdowns. Periodically testing the permeability of the felt section of the paper machine is an effective way to find paper diseases in the early stage, otherwise it may cause the following economic losses, such as paper quality does not meet the standards, paper The width is often broken or various energy consumption is increased. Practical experience in paper mills shows that regular inspection and analysis of the use of blankets can not only further optimize the dryness of wet paper sheets from the press section to the dryer section, make the dryer section operate efficiently, but also greatly reduce the number of unexpected shutdowns .


    2. It is helpful to determine the working condition of the blanket. The test of the blanket is not only used for fault diagnosis, but can also be used to determine the working status of the blanket under normal operation. Testing blankets regularly and irregularly is not only able to monitor blanket trends and tendencies, but also easily detects errors and takes corrective measures in a timely manner. Practice has proved that the more often blankets are tested and analyzed, the longer the time interval between unexpected shutdowns and quality problems related to the operating conditions of the press section.


    3. Contribute to the trend analysis and optimization of the press section. The adjustment of the press felt, the distribution of the load in the nip, the wear of the felt and the degree of clogging of the felt all have an important influence on the permeability detection curve. Minimize the change of the curve to the greatest extent, which is directly related to the improvement of the performance of the paper machine. The use of these detection methods in the press section can further increase production output and quality. Through the correct evaluation of the chemical, press roll and press felt tests, the reason for the change can be correctly judged. Comparing the relationship between the dewatering efficiency of the press section and the curve changes (related to changes in blankets or equipment) can clearly illustrate the effects of these changes. For example, the benefits of new advanced technology in pressing tracks and shoe covers for improving the efficiency of pressing can be explained by the dehydration rate and the uniformity of the banner curve. These comparisons can be made during the entire life cycle of the press felt and when it is replaced. Trend analysis is also a good way to predict the useful life of press felts.


     4. It is helpful to analyze the source of the problem. Wet streaks of wet paper sheets and other banner problems have always been a headache. These problems almost always occur in the press section. By using a steamer after the drying section or adding moisture, a smaller distribution uniformity problem can be solved. However, by analyzing the problem, finding the cause of the problem, and eliminating it, it is always a better solution. Just testing the moisture content distribution or water permeability of the felt is not enough to explain the operation of the press section. These two methods must be used at the same time to obtain a comprehensive test result. Using the blanket moisture content tester to test the current moisture content distribution of the blanket, it is easy to obtain a balanced pressure distribution in the nip. The distribution of the moisture content can explain the dehydration situation in the nip. Using a felt permeability tester to test the permeability can reveal the running condition of the felt. The permeability is obviously related to the dewatering performance of the press area and is affected by the condition of the felt.


    5. Get quick returns The practical experience of paper mills shows that the correct inspection of the working condition of the felt in the press section can greatly reduce the number of unexpected shutdowns. In fact, under optimal conditions, blankets should be replaced during normal shutdown. According to the data provided by the L & W 04/05 manual, the cost per minute of a modern paper machine during an unexpected shutdown is between $ 1.000 and $ 2.500, and the shutdown will last 2 to 3 hours. In addition, there is a cost caused by the replacement of the blanket due to the inability to determine the condition of the blanket. The cost of downtime is between $ 100.000 and $ 300.000. The cost of blankets is between $ 10.000 and $ 40.000. This is closely related to the cost of owning the test equipment and the testing with the equipment. It takes 15-20 minutes to test all felts in the paper machine. For a paper mill of average size, this means about 1 hour of work per day (including analysis of test data). Instrument cost $ 25.000 / instrument man-hour cost $ 10.000 / year The paper mill can recover the investment in the felt moisture tester and felt permeability tester in a short time.


    2. The principle and method of permeability test of papermaking felt


    1. Air permeability test of paper making blankets The air permeability of paper making blankets is expressed by the air permeability (degrees). It is the air flow through the unit area of the sample per unit time while maintaining a certain pressure difference across the sample under steady flow . The unit of air permeability is ml / (cm2 · s), that is, in 1s time, a sample of 1cm2 area permeates 1ml of air, which is called a unit of air permeability (I note: the unit mm / s can also be used). The calculation formula is as follows:

    In the formula:-air permeability (degrees) ml / (cm2 · s);-air volume (ml) passing through the sample in T seconds;-sample area (cm2). It can be seen that the greater the air permeability (degree), the stronger the air permeability of the papermaking felt. The commonly used air permeability instrument is Y561 type fabric air permeability instrument. The test results of the air permeability of the needle punched weftless paper felt are shown in Table 1.

         The air permeability (degree) of papermaking felt depends on the size and amount of voids in the fabric, and these will depend on factors such as fiber shape, production method, thickness and finishing method. The finer the fibers used in the papermaking felt, the greater the resistance to air penetration and the smaller the air permeability (degrees). Most blankets made of special-shaped fibers have better air permeability than round fibers, and blankets made of fibers with good compression elasticity have better air permeability. Fibers with strong hygroscopicity, the diameter of the fiber expands significantly after moisture absorption, voids decrease, and air permeability decreases. The permeability of blankets made by different processing methods is very different. In general, the thicker the blankets of the same variety and the same weight, the greater the permeability. After finishing the blanket, its breathability will generally decline. 2. Test of water permeability of papermaking felt The water permeability of papermaking felt refers to the performance of liquid water infiltrating from one side of papermaking felt to the other. The water pocket test can be used to determine the water permeability of papermaking felts. During the test, under the condition that a certain water pressure is maintained on one side of the sample, the amount of water (ml / cm2.h) that passes through the unit area of the sample per unit time indicates the water permeability of the paper felt (author's note: the unit ml / s ml / min said).


    (1) Test principle: After water flows through the sand filter, flow meter and pressure controller, it reaches the felt through the nozzle.


    (2) The blanket permeability is defined as follows (according to Darcy):-blanket permeability-blanket constant-blanket thickness-nozzle area-water pressure-viscosity By detecting the permeability of the blanket, it is easy to detect whether the blanket is worn, compacted and blocked , Can be used to quickly troubleshoot, while improving the possibility of optimizing the important functions of the press section. During the test, the tester is pressed against the running blanket, and the test can be performed. The test results are immediately displayed on the built-in display in the form of horizontal and vertical distribution curves, so that the paper machine operator can quickly find possible problems (Such as blanket defects) to avoid downtime. By regularly comparing the permeability of the blanket, the paper machine operator can make predictions and schedule plans for blanket cleaning and replacement. Comparing the data obtained before and after cleaning can optimize the use of cleaning agents.


     3. Test of moisture content of papermaking felt


    The blanket moisture content tester is based on the unique microwave resonance and dual-frequency testing technology, which can test the blanket moisture content very accurately at a very high sampling rate. It is mainly used to collect, analyze, report and store data about the performance of the felt and wet paper sheets in the press section for further understanding and control of the press section.


    When in use, the test can be performed manually along the horizontal and vertical directions of the felt. The test results are immediately displayed on the built-in display in the form of horizontal and vertical distribution curves, so that the paper machine operator can quickly find possible moisture problems to avoid downtime. Using the test results, it is possible to determine the replacement of blankets and optimize the use of chemicals.


    Basic parameters of L & W Scanpro blanket moisture content tester:


    Test range: Max 3000gH2O / m2


    Resolution: 1gH2O


    Test method: microwave dual frequency resonance


    Test area: 90mmx145 mm


    Test speed: 0.6-320 test values / s (10 steps)


    Memory: 48 numbered distribution maps or 20,000 test values


    Memory time: The memory time is 1 hour at 5 values per second, and the highest rate when the memory time is 60 seconds


     3. The relationship between the permeability of papermaking felt and its effect on the running performance of the felt


    The air permeability of the blanket is tested in the dry state. It is one of the important quality indicators that the papermaking blanket production enterprises must strictly control during the production process. The selection of air permeability should be determined according to the comprehensive technical parameters of the paper machine, which is a relatively empirical value. In actual production, the air permeability of the new blanket, the air permeability of the best use of the blanket, and the air permeability of the machine when the blanket fails are different. With the extension of the use time, the air permeability of the blanket will become worse and worse in general . The air permeability of the blanket should be slightly larger than the air permeability of the blanket in the best state. If the air permeability is too large, the vacuum of the vacuum system will not rise, the moisture content of the blanket is too large, and the moisture of the paper sheet cannot be taken out. 1. Problems with blanket penetration, etc .; if the selected blanket has too little air permeability, it will cause the blanket to jump, the vacuum system load is too heavy, the blanket wear is increased, and the paper sheet is difficult to dehydrate.


    The air permeability of papermaking blankets is closely related to water permeability. The higher the air permeability, the higher the water permeability of the blankets. The relationship between them is not equal and non-linear. The chemical treatment methods are related.


    The water permeability and water content of the blanket are tested during the use of the blanket on the machine, that is, tested in the wet state, which is an important performance index that the blanket production and use unit, especially the latter, must strictly control and track. Their distribution curves give valuable information about the working state of the press section, the conditioning of the press felt, the distribution of the nip load, the felt weft and the wear of the felt have a significant effect on these curves, and the distribution curve has the smallest fluctuations It is directly linked to the improvement of the working performance of the paper machine. Generally speaking, the blanket moisture content distribution curve mainly explains the dehydration of the nip and the problem of the suction box, while the water permeability distribution curve indicates the condition of the blanket in operation. From the frequency analysis of the moisture content and water permeability of the press felt, it can be seen that there are signs of different types of problems. For example, fluctuations in longitudinal moisture may be due to vibrations of the vacuum dewatering element in the forming section, while fluctuations in water permeability may be due to mechanical vibrations in the press section. By often measuring these two features in parallel, you can make decisions ahead of time without having to wait for unplanned downtime, operational problems, and quality problems.


    In order to evaluate the maximum life of the press felt, not only can the comparison be made when the press felt is replaced, but also during the entire life of the press felt. Different blankets can be evaluated and compared individually, with a view to making them optimally applied at a certain pressing position.


    Those who get the most benefit from moisture content distribution and water permeability testing are paper mills that produce high-quality cultural paper and use recycled fiber paper, because impurities in the recycled fiber can block the pores of the felt. Testing is also important for new high-speed paper machines, especially ultra-high-speed tissue paper machines. The test results are also very important for the felt. It not only detects the dewatering process of the felt, but also directly determines whether the last wet paper web pressed in the press section has wet streaks. However, the reality is that blanket and chemical suppliers are testing blankets for most paper mills. This is not because the tests are not profitable for some paper mills. People have recognized the benefits of testing. Although most paper mills realize that they can save a lot of money, many paper mills do not focus on testing themselves. The reason is usually the lack of resources, that is, there is no staff who can test and analyze the blankets.


    One disadvantage of others conducting all tests is that it is often not the same person who is testing. Instead, different vendors take turns to test, which will produce different test results. Moreover, suppliers often have no time when they need to test blankets. Suppliers usually visit paper mills on a regular basis, but will not always be on site.


    The good news is that although most blanket and chemical suppliers carry out this test, more and more paper mills are starting to use blanket testing as an important production tool from a strategic perspective. Because testing is not just for tracking results, it is more important for preventive maintenance.


    4. The development trend of performance testing of papermaking felt permeability


    1. Comprehensive testing system for paper felts The traditional testing method is limited to testing a single indicator of the felt, and cannot comprehensively test the comprehensive performance indicators of the felt online. The Web1QF-1 blanket comprehensive performance testing system developed by Huazhang Electric Co., Ltd. is a portable testing device, which can simultaneously detect the permeability and surface flatness of the blanket, and can comprehensively judge the quality status and service life of the blanket based on the test results, reflecting sensitivity. . The detection system can display real-time curves, and conduct in-depth analysis of the detection results, including historical curve comparison, paper quality records at that time, expert analysis reports, etc. The following is a brief introduction.


    1) Working principle The working principle of the WebQF-I handheld air permeability detection system is to detect the air permeability and smoothness of the blanket online by detecting the relative changes of the air flow and air pressure of the blanket under a certain pressure and filtering area at the same time. Sexual performance indicators. The detection method is to set the probe along the longitudinal direction of the blanket into two detection zones with independent controllable air pressure; at least one of the independent detection zones provided by the probe is a closed loop detection zone and forms a seal with the other detection zone Surrounded by the plane, the enclosed detection area adopts the same or similar detection pressure as the closed annular detection area. Outer ring another

    The function is to provide an air cushion with the same pressure as the inner circle to ensure that the edge of the inner circle detection area does not leak air, thereby ensuring the accuracy of the inner circle air permeability detection. Air permeability detection: There is a special designed ring detection area in the detection probe, which forms a closed plane surrounding the air permeability detection area, providing it with a leak-free detection environment. When the air permeability detection area and the ring detection When the detection air pressure in the zone is the same, the air permeability can be determined by the gas flow of the felt under a certain pressure. Flatness detection: The flatness is judged by detecting the air leakage of the blanket under a certain pressure. The ratio of the detection value of the outer ring detection area to the inner ring detection area can reflect the proportion of air leakage at the edge of the blanket. The flatter the blanket surface and the better the density, the closer the gas flow per unit area of the two. The more uneven, the greater the amount of gas leakage, and the greater the difference in gas flow per unit area between the two.


    2) System characteristics ① Under a certain pressure and filtration area, two performance indicators of blanket air permeability and surface flatness are simultaneously detected. ②The test repeatability is very good. For 80-120 unit values, the mean square error is only 0.01-0.1. ③Reflect sensitive and high resolution. Response time is 0.5 seconds. The response speed is very fast, air pressure detection is more sensitive than hydraulic pressure, and it can clearly distinguish the subtle quality changes inside the blanket. ④In the harsh detection environment, the detection performance is also very stable, and truly realize online safety detection.


    3) Composition of the system The comprehensive detection system of papermaking blankets is mainly composed of probes, pressure sensors, gas flow meters, pressure regulating valves, manual valves, voltage stabilizing tanks, miniature compression pumps, portable batteries, controllers and notebook computers.


    2. Put forward higher requirements for testing


    With the rapid development of the paper industry, the requirements for accurate testing are constantly increasing. While the speed of the paper machine is increasing, the use time of the blanket is decreasing. In order to obtain high efficiency, it is very important to minimize downtime by optimizing blanket replacement and reducing the occurrence of wet paper breaks. In addition, the compact new press section makes manual testing more difficult, and personal safety is always the most important during the test. All these factors require automated testing. The fixed on-line sensor scanning device in the press section can realize the automation of testing. The system can quickly collect test results and then use the test results in a whole new way, not just to detect failures. Even, it is easier to optimize the felt and make it easier to adjust the press section. Test results can be electronically transmitted to outside experts, which means quick help is available. Last but not least, it fully meets the requirements for personal safety.


    3. A new concept of monitoring the state of the press felt. The method used to determine and determine the detailed full-scale condition of the press felt is to use a portable instrument for manual measurement, which is more used for the purpose of troubleshooting and the recording of its status. In most paper mills, the frequency of this measurement is not high enough to track development and changes, and therefore cannot optimize the dewatering of the press section. For this reason, it is necessary to use an automatic measurement felt system. The automatic measurement system of felt provides a new type of online scanning rack, which can measure the moisture content and water permeability of the press felt in parallel, and store the measurement results in the database. Trend graph over time. The distribution curve and trend graph can be compared with the data of the historical database and the data of the paper machine QCS system. At the same time, frequency analysis and statistical analysis are also required. All these techniques are used to determine the fluctuations in the moisture content and water permeability of the felt during the production process, and the impact of these fluctuations on the quality and running performance of the paper. The equipped online water content and water permeability system are directly connected to the paper mill's QCS system and DCS system, and can also transmit data through the Internet. This allows blanket and chemical suppliers to be involved in remote data analysis and process evaluation. This new blanket automatic measurement system can output the status report of the blanket. Based on these results, paper machine operators can improve and optimize the performance of felt conditioning systems. In this way, it is easier to decide when to shut down for maintenance or replace the blanket. The purpose is to prevent Yu Wei from responding passively, thereby improving the production efficiency of the paper machine, reducing production costs, and improving product quality. The unique results provided by the frequency analysis of the moisture content and water permeability of the press felt, as well as the display of a complete and detailed felt map, make the system a powerful tool for optimizing the operation of the press section. These latest tools are also very helpful for the development of new press felts. The last and important point is that it avoids manual measurements in the most dangerous parts of the paper machine.


     4. Conclusion


    The permeability of papermaking blankets mainly includes air permeability, water permeability and water content, etc. It is an important performance index that the production and use units of blankets, especially the latter, must strictly control and track. Testing can help prevent unplanned downtime, determine the working condition of the blanket, analyze and optimize the trend of the press section, analyze the source of the problem and quickly obtain returns, which has a great impact on the performance of the blanket. With the rapid development of the paper industry, the precise requirements for testing and personal safety are constantly improving. The use of a comprehensive inspection system and online automatic inspection system for paper blankets will be an inevitable trend and will also be a powerful way to optimize the operation of the press section. Tool of. (Senior Engineer Tang Zhichao of Huazhang Electric Co., Ltd. has given great support, and I would like to express my gratitude!)


    References: [1] Edited by Guo Bingchen, "Performance and Testing of Nonwovens", Beijing, China Textile Press, 1998 [2] L & W HANDBOOK 04 Sweden AB Lorentzen & Wettre, 2004 [3] L & W HANDBOOK 06/07 Sweden AB Lorentzen & Wettre, 2006


     About the author: Mr. Lu Xiangyang, senior engineer, master of engineering; currently working at Taicang Chang'e Industrial Co., Ltd .; main research direction: dewatering equipment for paper making.