Briefly describe the selection essentials and related applications of intelligent electromagnetic flowmeters in the papermaking industry

With the rapid development of flow meter measurement technology, various intelligent flow meters have played an important role in many industrial production fields. As a domestic electromagnetic flow professional manufacturer, Runzhong Instrument has noticed in many inspection sites that among the many complete sets of equipment imported from abroad,   electromagnetic flow meters account for the vast majority of the liquid medium measurement. In recent years, the manufacturing technology of domestic electromagnetic flow meters has been continuously improving, and the proportion of domestic electromagnetic flow meter products in large and medium-sized production projects has also increased. The demand for flow meters in the papermaking industry is particularly huge. Domestic large and medium-sized papermaking enterprises have gradually begun to complete the process of replacing traditional differential pressure flow meters with electromagnetic flow meters. At present, a 350,000-ton paper mill has used about 400 electromagnetic flow meters. The papermaking industry is one of the main industries that produce wastewater pollution. In the papermaking renovation process, in addition to the main process, as environmental protection requirements become more stringent, most papermaking enterprises have also built waste liquid treatment processes. The survey found that electromagnetic flow meters are used in almost all sections of paper mills. This article briefly introduces the application of electromagnetic flow meters in the production process of papermaking enterprises and related technical content.
 



1. Material preparation process
           The main task of the material preparation section is to make different raw materials into pulp. In the pulping process, the electromagnetic flowmeter is mainly involved in the flow measurement of polished pulp, water and acid or alkali. Generally, the temperature of the pulp after mechanical polishing should reach 80℃. When pulping with materials such as wood or grass as raw materials, it often needs to go through a cooking process. Since the cooking liquid is an alkali such as Ca(OH)2 or Na0H solution, the pulp is alkaline, so the electromagnetic flowmeter mostly uses a high-temperature resistant polytetrafluoroethylene lining and a split structure. In the selection of electrode materials, it is necessary to consider the selection of materials with strong corrosion resistance. Alkaline pulp generally uses Hastelloy Hc, titanium Ti and other materials, and tantalum electrodes must not be used; and if the requirements are not high when measuring acidic pulp, acid-resistant steel 316SS can also be selected.
           
         Usually, many domestic companies use waste paper pulping, that is, the waste paper is pulped and then washed and bleached. Here, the electromagnetic flowmeter is mainly used to measure the water flow after pulping. Most of the water is from nearby rivers and is slightly treated before being used in the operation, so the water quality is poor, but it is not corrosive. The electromagnetic flowmeter can use conventional rubber lining and stainless steel electrodes; when measuring the deinking agent involved in the deinking process (common deinking agents are Na0H, Na2SiO3, Na2O2 or H2O2), the selection of the electrode and lining of the electromagnetic flowmeter should be cautious. Taking a typical 35-ton paper mill as an example, the number of electromagnetic flowmeters used in the preparation process is relatively large, roughly between 50-60 sets.
 
2. Screening and bleaching section
       In terms of the number of uses, the screening and washing section should be ranked first. Taking a typical 350,000-ton paper mill as an example, the number of electromagnetic flowmeters used in these two sections should reach about 250 sets.
       The electromagnetic flowmeter is used at the flow measurement points of the inflow of each pulp pool from each preparation workshop into the papermaking workshop and the flow out of the pulp pool from each pulp pool. A total of five types of pulp (chemical pulp, deinked pulp, CTMP, machine pulp, and liquid loss pulp) participate in the pulping process.
The temperature of the machine pulp and CTMP pulp here is as high as 50-80℃, and the flowmeter lining material should consider using PFA or PTEE.
         
         In the process of slurry proportioning, different manufacturers use different slurry proportioning concentrations to obtain different quality papers. However, the finished product is often unqualified due to the slight deviation of one slurry, resulting in greater material and economic losses. Therefore, each manufacturer attaches great importance to this link. An electromagnetic flowmeter is installed in each pipeline participating in the proportioning to measure the instantaneous and total flow rate flowing into the slurry main pipe. At the same time, an online concentration meter is installed at the lower end of the slurry pipe to detect the slurry proportioning effect. If there is a deviation from the predetermined concentration, the valve opening is immediately adjusted to adjust the corresponding slurry flow rate. This city requires not only high precision of the electromagnetic flowmeter, but also good repeatability and dynamic responsiveness. In the above occasions, the caliber of the electromagnetic flowmeter is generally between DN50-DN150.
         
           The screening process after slurry proportioning is the so-called papermaking process. In this process, the pulp concentration often affects the flow measurement. We found that the pulp with a concentration greater than 3% can improve the shaking phenomenon by using a conventional low-frequency square wave excitation frequency (such as 25Hz) or dual-frequency excitation. Taking the DN300 electromagnetic flowmeter to measure corrugated board with a concentration of 3.5% as an example, when using the conventional 1/32 power frequency, the shaking amount of the instantaneous flow is as high as 10.7%. When the excitation frequency of the electromagnetic flowmeter with adjustable frequency is changed to 1/2 power frequency, the shaking amount is improved to 1.9%, and the effect is very obvious. In the papermaking section, since the flow of cooling water, alum and talcum powder needs to be monitored during the pulping and papermaking process, the number of its use is the largest in the paper mill, reaching about 200 sets in this case.
       
         In order to ensure the uniform and fine whiteness of the paper, different additives, dispersants and bleaching agents should be added. Like the use of additives in the food industry, these additives have a small flow rate and strong corrosiveness, so most of them use PTEE lining and PFA lining. Because of its strong oxidizing property, H2O2 is often used as a bleaching agent by paper mills, but H2O2 is particularly corrosive to metal materials. For corrosion protection, engineers like to use Pt as the electrode material. However, actual use has shown that when the process pressure is less than 0.3MPa, the Pt electrode will react with H2O2 and form a layer of mist on the electrode surface, which we call a catalytic reaction. At this time, the output signal will fluctuate greatly. However, there is no such problem when using tantalum (Ta) electrodes. If we use tantalum electrodes when switching to NaOH solution, the situation is not so optimistic, because the alkali solution will cause the tantalum electrode to produce a surface effect. Data from the SGAIC calibration laboratory show that the zero point variation of tantalum electrodes when measuring ordinary water is also dozens of times greater than that of other electrodes. Pay special attention to concentricity when installing small-diameter electromagnetic flowmeters.
In the above example, using a DN10 small-diameter electromagnetic flowmeter to measure these additives can achieve relatively ideal results. In foreign paper production lines, there are also many examples of using Coriolis mass flowmeters in these occasions.
 
         After years of practice, electromagnetic flowmeters in the papermaking field generally use PTFE or other fluoroplastic linings: the electrodes are selected according to the different properties of the liquid. According to the statistics of SGAIC, more than 85% of the thousands of electromagnetic flowmeters used in the papermaking industry have chosen PTFE, and the actual effect is satisfactory. Although some important small flow measurement in foreign paper mills uses Deloris mass flowmeters, electromagnetics still dominate the liquid measurement in the papermaking industry. The vast majority of usage problems occur in the initial selection and improper installation. Such as mismatch of electrode or lining materials (such as the table) and such as incomplete pipes, insufficient straight pipe length, non-concentricity during small-diameter installation, etc., these are often the main factors for the failure of electromagnetic flowmeters to operate normally.