The demand for flow meters in the paper industry is particularly huge. Domestic large and medium-sized paper enterprises have gradually begun to complete the process of replacing traditional differential pressure flow meters with electromagnetic flow meters. At present, about 400 electromagnetic flow meters have been used in 350000-ton paper mills in China. The papermaking industry is one of the main industries that generate wastewater pollution. In addition to the main process, most paper-making enterprises have built wastewater treatment processes at the same time with increasingly strict environmental requirements in addition to the main process. The investigation found that an electromagnetic flowmeter is used in almost all sections of the paper mill. This paper gives a brief introduction to the application of electromagnetic flowmeter in the production process of paper-making enterprises and the related technical contents.

Electrode selection during material preparation

The main task of the material preparation section is to make a slurry from different raw materials. The electromagnetic flowmeter is mainly used to measure the flow of refining, water, acid, or alkali during pulping. Generally, the slurry temperature shall reach 80 ℃ after mechanical grinding. When wood or grass is used as raw material for pulping, a cooking process is often required. Since the cooking liquid is alkaline, such as Ca (OH) 2 or Na0H solution, and the slurry is alkaline, the electromagnetic flowmeter mostly adopts a high-temperature resistant PTFE lining and split structure. In the selection of electrode materials, materials with strong corrosion resistance shall be considered. The alkaline slurry is generally made of Hastelloy Hc, titanium Ti, and other materials, and a tantalum electrode is selected; If there are different requirements for measuring acid slurry, acid-resistant steel 316SS can also be selected.
Usually, many domestic enterprises also use waste paper for pulping, that is, washing and bleaching after beating waste paper. Here, an electromagnetic flowmeter is mainly used to measure the water-adding flow after beating. Most of the water is from the nearby rivers and streams that are operated after being slightly treated, so the water quality is relatively poor, but it is not corrosive. The electromagnetic flowmeter can use conventional rubber lining and stainless steel electrode; When measuring deinking agents involved in the deinking process (common deinking agents include Na0H, Na2SiO3, Na2O2, or H2O2), the electrode and lining of the electromagnetic flowmeter should be carefully selected. Taking a typical 35-ton paper mill as an example, the number of electromagnetic flowmeters used in the preparation process is relatively high, about 50-60 sets.

Application of electromagnetic flowmeter in screening and bleaching section

The screening and washing section shall be sorted according to the quantity used. Taking a typical 350000-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 to measure the inflow of each pulp tank from each preparation workshop to the paper-making workshop and the flow from each pulp tank to the pulp mixing tank. There are five kinds of pulp (chemical pulp, deinked pulp, CTMP, mechanical pulp, and liquid loss pulp) involved in the pulp mixing process.

The temperature of machine slurry and CTMP slurry here is as high as 50-80 ℃. PFA or PTEE shall be considered as the lining material of the flowmeter.

In the process of slurry proportioning, different manufacturers use different slurry proportioning concentrations to obtain different quality papers. However, due to the slight difference in the slurry, the finished product is often unqualified, resulting in large material and economic losses. Therefore, each manufacturer attaches great importance to this link. Each pipeline participating in the proportioning is equipped with an electromagnetic flowmeter to measure the instantaneous and total flow into the main slurry preparation pipe. At the same time, an online concentration meter is installed at the lower end of the slurry preparation pipe to detect the slurry preparation effect. If there is a deviation in the preset concentration, immediately adjust the opening of the valve to adjust the corresponding slurry flow. This city requires not only high accuracy of electromagnetic flowmeter but also good repeatability and dynamic response. In the above situations, the diameter of the electromagnetic flowmeter is generally between DN50-DN150.

The screening process after mixing is known as paper making process. In this super process, the pulp concentration often affects the flow measurement. We found that the pulp with a concentration greater than 3% was excited with a conventional low-frequency square wave excitation frequency (such as 25Hz) or dual frequency excitation to improve the sloshing phenomenon. Taking the corrugated paper with a concentration of 3.5% measured by DN300 electromagnetic flowmeter as an example, when the conventional 1/32 power frequency is used, the shaking amount of the instantaneous flow is as high as 10.7%. When the electromagnetic flowmeter with changeable frequency is selected to change its excitation frequency to 1/2 power frequency, the shaking amount is improved to 1.9%, and the effect is very obvious. In the paper-making section, since the flow of cooling water and alum is required to be monitored in the process of pulp preparation and paper making, the number of them used is more than that of paper mills in China, reaching about 200 sets in this case.

In order to ensure the whiteness of the paper is uniform and fine, different additives, dispersants, and bleaching agents should be added. Like the use of additives in the food industry, these additives have small flow and strong corrosivity, so most of them choose PTEE lining and PFA lining. Because of its strong oxidation, H2O2 is often selected as a bleaching agent by paper mills, but H2O2 is particularly corrosive to metal materials. In consideration of corrosion protection, engineers like to choose Pt as electrode material. However, the actual use shows that when the process pressure is less than 0.3MPa, the Pt electrode reacts with H2O2, forming a layer of gas mist on the electrode surface, which we call a catalyst reaction. 

At this time, the output signal will fluctuate greatly. However, using a tantalum (Ta) electrode will eliminate the above problems. If we choose the tantalum electrode when replacing the NaOH solution, the situation is not so optimistic, because the alkali solution will cause a surface effect on the tantalum electrode. The data from the SGAIC calibration laboratory shows that the characteristics of the tantalum electrode change dozens of times more than other electrodes when measuring general water. Special attention shall be paid to concentricity when installing a small caliber intelligent electromagnetic flowmeter.

After years of practice, PTFE or other fluoroplastic linings are widely used in the field of papermaking for electromagnetic flowmeters; electrodes are selected according to different liquid properties. According to the statistics of thousands of sets of intelligent electromagnetic flowmeters used by SGAIC in the paper industry, more than 85% of them are PTFE, and the actual effect is satisfactory. Although Delioli mass flowmeter is used in some important small flow metering parts of foreign paper mills, electromagnetism still plays a leading role in liquid metering in the paper industry. Most of the problems occurred in the initial selection and improper installation. For example, the mismatch of electrode or lining materials, insufficient length of straight pipe, and different concentricity during small diameter installation are often the main factors for the failure of the electromagnetic flowmeter to operate normally.