Overview of Lining Material Selection and Electrode Processing Technology for Electromagnetic Flowmeter

Lining materials of electromagnetic flowmeter
: In the production and manufacturing of electromagnetic flowmeter, in order to meet the roughness requirements of the measuring tube for future measurement, it is often necessary to select excellent lining materials according to the type of fluid. Different lining materials need to adopt excellent processing methods. At present, the commonly used lining materials are: chloroprene rubber, EPDM rubber, polyurethane, fluoroplastic PTFE and PFA.

For different pipe linings, the processing technology is different. Here are the key points of the process of different linings for the reference of flowmeter manufacturers. According to the measurement specifications, the media used for measuring linings are roughly the following:

1. Chloroprene rubber  : Chloroprene rubber is suitable for large-caliber sensors above DN300, and is mostly used to measure water, sewage, weak acid and weak alkali medium fluids. It is generally directly bonded to the inner wall of the stainless steel pipe with film and made by vulcanization. In this process, the surface roughness of the rubber lining is generally high, and special attention should be paid to the flatness of the lap joint during operation, but the relative roughness should be low. For small-caliber chloroprene rubber and EPDM rubber linings, it is best to use mold pressurization, adhere to the inner wall of the catheter and then vulcanize. Reducing the surface roughness of the lining depends on the surface roughness of the mold core rod and the pressurization and vulcanization process.

2. Polyurethane: At present, domestic polyurethane linings mostly use soft materials and adopt the pouring process. The lining roughness depends not only on the surface roughness of the core rod mold, but also on pouring, exhaust, heating, cooling and material composition and proportion. Foreign hard polyurethane mostly uses polyurethane rubber. The important process point of its forming is to remove the bubbles involved in the injection process and allow the chemical reaction (hardening, cross-linking) to proceed stably; the centrifugal pouring process method is used: ensure that the raw materials are in a dry state during storage, mix and stir the raw materials evenly and smoothly, set the appropriate rotation speed of the catheter to remove the bubbles involved in the raw materials, and well control the temperature of raw material processing, hardening and cross-linking. Fluoroplastic should be used for lining materials with low roughness. PTFE fluoroplastic linings used for piping, tanks and other containers usually line a metal pipe with a thin-walled polytetrafluoroethylene tube, or insert a polytetrafluoroethylene tube and then bond it. The main disadvantages of this lining are that it is not very resistant to negative pressure, is greatly affected by temperature, and bonding is often unreliable.

3. Fluoroplastic: For electromagnetic flowmeters, the best fluoroplastic lining is PFA. The main method used for PFA is to inject molten resin and then injection mold (injection molding). Injection molding is used for seamless one-piece molding. The quality of PFA lining has good chemical resistance, heat resistance, and adhesion resistance (surface finish). Especially in terms of chemical resistance and heat resistance, the use of unique manufacturing technology can reduce internal stress and internal bubbles to avoid cracks, so that the flowmeter still has high reliability when used in harsh environments.
For this reason, in the manufacturing process of PFA lining, the important management points are injection molding temperature (resin viscosity, metal mold temperature), metal mold cooling control (cooling time, temperature), and resin pressure control. The injection molding temperature should be set as low as possible to reduce the thermal degradation of PFA resin.
During injection molding, the temperature of the metal mold should be uniformly maintained above the melting point of the resin. Since high-precision cooling control is required, multiple cooling circuits should be set in the metal mold and independent cooling control operations should be performed. While performing cooling control, the resin pressure should also be controlled. Electrode processing technology of
 
electromagnetic flowmeter
: Electrode processing includes polishing and passivation processes. There are three methods of polishing. Mechanical polishing is the first process of the three types of polishing (i.e. mechanical polishing, chemical polishing and electrochemical polishing) of stainless steel polishing. Next, the two are combined, such as mechanical polishing-chemical polishing or mechanical polishing-electrochemical polishing. Mechanical polishing is used for primary polishing, and the unevenness of the electrode surface is processed to a certain roughness, and then chemical polishing or electrochemical polishing is performed. Chemical polishing and electrochemical polishing can remove the microscopic unevenness of the electrode surface, thereby improving it to a mirror finish. At the same time, they can complete the purpose of polishing and passivation, increase the surface chromium content, and form a good passivation layer.

Due to the macroscopic unevenness of the rough surface, mechanical polishing should be used to achieve a roughness of Ra≤0.8μm, and then chemical polishing or electrochemical polishing should be used to increase the roughness to Ra=0.05μm or above, in order to obtain the final brightness, mirror gloss and good passivation layer. The electrode after polishing and passivation can form a stable passivation layer. However, during storage, transportation and assembly, it is important to keep the surface passivation layer intact.

Compared with the traditional and simple production method of electromagnetic flowmeter sensors that immerse in water (sometimes this method takes several days and nights) to naturally form a passivation layer, the electrode after polishing and passivation process can obtain an electrode with stable corrosion resistance, which greatly improves production efficiency and is an advanced production process.