What Are the Application and Selection Guidelines of Electromagnetic Flow Meters in the Papermaking Industry

With the flow measurement technology rapidly developing, various intelligent flow meters play a crucial role in many industrial production sectors. The demand for flow meters in the papermaking industry is particularly significant. And large and medium-sized papermaking enterprises in China have gradually replaced traditional differential pressure flow meters with electromagnetic flow meters. Currently, in a paper mill with a production capacity of 350,000 tons, the usage of electromagnetic flow meters has reached around 400 units. As one of the major industries causing water pollution, the papermaking industry has integrated electromagnetic flow meters into almost all sections. It reflects their versatile applications in the production process.

What Are the Application and Selection Guidelines of Electromagnetic Flow meters in the Papermaking Industry

Stock Preparation Process

The primary task of the stock preparation section is to turn different raw materials into pulp. In the pulping process, electromagnetic flow meters are mainly used for measuring the flow of ground pulp, water, and acid or alkali. The temperature of the pulp generally needs to reach 80°C after mechanical grinding. When materials such as wood or straw are used for pulping, a cooking process is often required. As the cooking solution is an alkaline solution, such as Ca(OH)2 or NaOH, the pulp becomes alkaline. Therefore, electromagnetic flow meters often use high-temperature-resistant PTFE lining and a split structure. The selection of electrode materials considers strong corrosion-resistant materials for alkaline pulp, such as Hastelloy alloy (Hc) or titanium (Ti), while acid-resistant stainless steel (316SS) can be used for acidic pulp if the requirements are not high.

In China, many enterprises also employ the pulping process using waste paper, relying primarily on electromagnetic flow meters to measure the water flow added post pulping. The water, mostly sourced from nearby rivers, undergoes minimal treatment before use, resulting in poor quality but non-corrosive characteristics. In this case, electromagnetic flow meters can use conventional rubber lining and stainless steel electrodes. However, when measuring deinking agents (commonly NaOH, Na2SiO3, Na2O2, or H2O2) involved in the deinking process, one needs to carefully select electrodes and linings. In a typical 35-ton paper mill, the number of electromagnetic flow meters used in the stock preparation process is relatively high, ranging from 50 to 60 units.

Screening and Bleaching Section

Screening and Washing Section

In terms of quantity used, the screening and washing section should be the first. In a typical 350,000-ton paper mill, the number of electromagnetic flow meters used in these two sections reaches approximately 250 units.

During the blending process, electromagnetic flow meters actively measure the flow into and out of various pulp tanks, encompassing five types of pulp (chemical pulp, deinked pulp, CTMP, machine pulp, and reject pulp). The temperatures of machine pulp and CTMP pulp can reach 50-80°C, and flow meter lining materials should consider using PFA or PTEE.

Various manufacturers use different pulp concentrations in the process of pulp blending to achieve different paper qualities. However, even slight variations in one type of pulp can lead to substandard finished products, causing significant material and economic losses. To measure the instantaneous and total flow into the pulp mixing pipe during the blending process, operators install electromagnetic flow meters in every participating pipeline. At the lower end of the pulp mixing pipe, we install online concentration meters to actively detect the blending effect. If a deviation in the predetermined concentration occurs, operators immediately adjust the valve opening to regulate the corresponding pulp flow. This requires high accuracy, good repeatability, and dynamic responsiveness of electromagnetic flow meters. In these situations, the diameter of the electromagnetic flow meter is generally between DN50 and DN150.

3 electromagnetic flow meter

Papermaking Process

The screening process after blending is commonly referred to as the papermaking process.

In this process, the pulp concentration often affects flow measurement. The pulp with a concentration greater than 3% can cause fluctuation in instantaneous flow when using conventional low-frequency square wave excitation frequencies (such as 25Hz) or dual-frequency excitation to improve shaking phenomena. Measuring 3.5% corrugated board paper concentration with DN300 electromagnetic flow meter at 1/32 power frequency shows 10.7% flow fluctuation. Yet, with a variable excitation frequency set to 1/2 power frequency, the fluctuation significantly reduces to 1.9%, showcasing notable improvement. In the papermaking section, the paper mill employs the highest number of electromagnetic flow meters to monitor the flow of cooling water, alum, and talcum powder during the blending and papermaking processes.

To achieve the even and fine whiteness of the paper, it’s necessary to add various additives, dispersants, and bleaching agents. These additive liquids have small flow rates and strong corrosiveness. Therefore, most of them use PTFE lining and PFA lining. The strong oxidizing nature of H2O2 often leads to its selection as a bleaching agent, but it significantly corrodes metal materials. For corrosion prevention, engineers prefer to use Pt as the electrode material. However, practical use has shown that when the process pressure is less than 0.3MPa, Pt electrodes react with H2O2, forming a mist on the electrode surface, known as catalytic reaction. This leads to significant signal fluctuations. However, using tantalum (Ta) electrodes eliminates this problem. Switching to NaOH solution while using tantalum electrodes does not yield an optimistic situation due to surface effects caused by the alkaline solution on tantalum electrodes. It is important to particularly note the concentricity when installing small-diameter electromagnetic flow meters.

Electromagnetic flowmeter (1)

Conclusion

In the above example, using a DN10 small-diameter electromagnetic flow meter to measure these additives can achieve ideal results. In foreign paper production lines, there are also instances of using Coriolis mass flow meters in these situations. After many years of practice, PTFE or other fluoroplastic linings are commonly used in electromagnetic flow meters in the papermaking industry. Electrode selection depends on the nature of different liquids. Although some important small flow measurements in foreign paper mills use Coriolis mass flow meters, electromagnetic flow meters still dominate liquid measurement in the papermaking industry. Most operational issues arise from initial selection and improper installation, such as mismatched electrode or lining materials and issues like insufficient straight pipe length or lack of concentricity during small-diameter installations. These are often the main factors preventing electromagnetic flow meters from functioning properly.

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