Electromagnetic flowmeter process method and the effect of electrode PTFE liner on fluid noise
Abstract
Raising the roughness level of the electromagnetic flowmeter lining and electrodes not only improves the appearance of the product, but more importantly reduces the probability and magnitude of fluid noise generation, thereby improving the sensitivity and stability of the flowmeter measurement. . In this paper, the fluid noise of the dynamic zero point of the electromagnetic flowmeter is classified and generated from the roughness of the sensor lining and the electrode, and an important measure to reduce the noise of the fluid and improve the signal to noise ratio is introduced. Then introduce some key technological measures in the manufacture of electromagnetic flowmeter sensor, and hope to improve the manufacturing level of electromagnetic flowmeter and product competitiveness in China.
Key words fluid noise polarization voltage electrode passivation film
Preface
In the electromagnetic flowmeter measurement, the electrochemical reaction of the electrolyte fluid to the metal electrode generates a DC polarization voltage. This voltage independent of the flow rate is called fluid noise. From Faraday's first application of geomagnetic field and electromagnetic induction method to measure the failure of the Thames flow rate to the widespread use of electromagnetic flowmeters to measure the flow of conductive liquid today, fluid noise has always been one of the important technical problems to be solved by electromagnetic flowmeters. Especially in the era of low frequency rectangular wave excitation, the influence of fluid noise has become more prominent. Often some newly installed flowmeters are affected by the polarization of the electrodes, and output swings need to be soaked in water for a long time to eliminate them. The size of the fluid noise directly affects the sensitivity, linearity, and stability of the flowmeter measurement. Therefore, study fluid noise, investigate its causes, find ways to reduce fluid noise, and improve signal-to-noise ratio of sensors, especially for the development of weak excitation currents (electromagnetic water meters, two-wire electromagnetic flowmeters) and low flow rates ( 0.1 The expansion of the flow measurement range with high flow rates ( above m/s ) and high flow rates ( above 15 m/s ) is of great significance.
Fluid noise
In addition to the influence of ambient environmental conditions, electromagnetic fields, and electrostatic fields on the application of electromagnetic flowmeters, the magnetic noise of the measured medium is also a very important factor. Fluid noise is a type of DC polarization voltage that is particularly prominent in low-frequency rectangular-wave excitation methods, often including: slurry noise, flow noise, and high-end flow noise.
The causes of fluid noise are the following:
(1) The corrosion resistance of the stainless steel electrode has a very thin passivation layer on its surface, so that the electrochemical reaction reaches an equilibrium state. As shown in FIG. 1 , the solid matter in the fluid hits the electrode so that the electrode surface passivation layer is destroyed and the electrochemical equilibrium is lost. The contact of the metallic material with the fluid medium has the ability to regenerate the surface passivation layer to maintain electrochemical equilibrium. During the point chemical equilibrium is reached, free ions in the metal and fluid undergo electrochemical reactions continuously under the action of the signal electric field. The solid particles hit the electrode and continuously destroy the protective passivation layer; the electrochemical reaction repeatedly generates a passivation layer, so that the potential between the electrodes is continuously and drastically changed, and this changed potential causes fluid noise in the flow signal. This situation is also known as slurry noise in electromagnetic flowmeters. The theory and practice show that the increase of the frequency of the electric field that influences the electrochemical reaction signal increases the amplitude of the fluid noise rapidly. This is why the high frequency excitation and the dual frequency excitation can solve the slurry measurement.
(2) Fluid friction linings and electrodes, the positive and negative ions occurring in the fluid are separated from the electrolyte fluid. The rougher the lining and electrode surface, the higher the free ion concentration. As shown in Fig. 2 , due to the effect of the electric field of the electrode signal, a part of the ions will move toward the electrode to form a noise voltage. Such noise is called flow noise. Flow noise is more prominent at low conductivity measurements. The flow noise is related to the strength of the external electric field. When the flow rate is high, the larger the inductive signal, the greater the noise amplitude and the unstable output.
(3) Rapid changes in fluid conductivity and pH can also cause flow noise, and measurement instability at the upstream of the meter is a typical example. The reason is that when different media are mixed unevenly, the positive and negative ions can be easily separated in the fluid. Due to the electric field of the signal from the electrode, part of the ions will move toward the electrode, forming a flow noise voltage and causing instability of the output.
(4) The thickness of the laminar boundary layer near the lining and the electrode becomes very thin due to the high flow velocity of the flowing fluid. As shown in Fig. 3 , the roughness of the lining and the electrode highly breaks through the thickness of the laminar boundary layer of the flow velocity, and the fluid hits this part of the roughness. At altitudes, divergence and abrupt changes in velocity occur. Some of the velocity components that are the same (or opposite) to the central axis of the measuring tube are affected by the signal weight function, which has a great influence on the electrode signal and forms a large positive error. This is the high-end flow noise.
It can be seen that the flow noise and the high-end flow noise in the above fluid noise are directly related to the roughness of the lining and the electrode surface of the measuring tube, and the slurry noise generated by the polarization voltage is also greatly related to the surface roughness of the electrode.
2. Metal electrode anti-fluid corrosion passive film formation
The corrosion resistance of stainless steel electrodes is mainly due to the surface covered with a very thin ( about 1 nm thick ) dense passivation film. This layer of passivation film isolates corrosive fluid media and is the basic barrier for stainless steel electrode protection. Stainless steel electrode passivation has a dynamic character and should not be seen as complete stoppage of corrosion, but rather as a diffusion barrier, which greatly reduces the rate of anode reaction. For stainless steel electrodes, the passivation film tends to be destroyed in the presence of a reducing agent such as chloride , while the passive film can be maintained or repaired in the presence of oxidants such as air and water . Stainless steel electrodes placed in the air and water will form an oxide film, but the protective film is not perfect, the speed is very slow. FIG. 4 is a surface chemistry analysis of a stainless steel electrode using an X-ray photoelectron spectroscopy (XPS) photoelectron spectroscopy apparatus. The figure on the left shows the content ratio of chromium ( Cr ), oxygen ( O ), and iron ( Fe ) in different depths inward of the untreated electrode surface . It can be seen that the chromium content is about 20% at a depth of about 1 nm , which means that the passivation film is a chromium-depleted layer. The figure on the right shows passivation treatment measures such as mechanical grinding, polishing, pickling and chemical polishing. The oxides of iron and iron are preferentially dissolved compared to the oxides of chromium and chromium. The chromium-depleted layer is removed, resulting in chromium on the surface of stainless steel. With an enrichment, the chromium content is approximately 30% at a depth of about 1 nm . This Cr-rich passivation film has a polarization potential (SCE) of +1.0 V , which is close to the noble metal gold and platinum polarization potential. Therefore, stainless steel can improve the stability of corrosion resistance. Different methods of passivation also affect the composition and structure of the film, which affects the stainless steel. For example, through electrochemical modification, the passivation film can have a multilayer structure, CrO3 or Cr2O3 is formed in the barrier layer , or a glassy oxide film is formed, so that the stainless steel can exert maximum corrosion resistance. Guangzhou Mingbai Meter Factory is exclusively for
The corrosion resistance of the stainless steel electrode mainly depends on the surface passivation film. If the passivation film is incomplete or defective, the stainless steel will still be corroded, and fluid noise will still occur. In the process of forming, assembling and installing the marks, the electrodes will bring surface oil, rust, non-metallic dirt, low-melting-point metal contaminants, paint, welding slag and spatter, etc. These substances affect the surface quality of stainless steel electrodes. The oxide film on the surface was destroyed, the anti-corrosion performance of stainless steel was reduced, and the generation of fluid noise was also formed, which affected the stability of the flowmeter measurement. Therefore, it is one of the important technologies in the manufacture of electromagnetic flowmeters to improve the process and storage and assembly process of the electrode assembly and to protect the passivation film.
3. Effect of Surface Roughness of Lining and Electrodes on Fluid Noise
The level of fluid noise is related to the roughness of the lining and the electrode surface. Regardless of the relationship between slurry noise, flow noise, and flow velocity high-end noise, this relationship is very close.
Obviously, the rough lining and the electrode surface will increase the friction force on the fluid, which will easily lead to the increase of ion separation in the fluid and create conditions for the flow noise. The smooth lining and electrode surfaces allow fluids to flow smoothly, reducing the friction between the fluid and the lining and the electrodes, so the chance of ion separation will be greatly reduced, and the flow noise will also be reduced. It is conceivable that the flow rate of the fluid is accelerated, the friction between the liner and the electrode on the fluid is also increased, and the ion separation in the fluid is also increased. Coupled with the acceleration of the fluid flow rate, the induced potential increases, and the force of the electric field on the movement of ions increases, so the flow noise increases. Therefore, in the case of flow noise, the flow rate of the flowmeter should not be too high.
References have discussed the influence of the roughness of an electromagnetic flowmeter measuring tube at high Reynolds number (ie, high flow rate) on the measurement, and FIG. 5 shows the difference in the error of different liner materials (primary surface roughness is different). It can be seen that the PFA lining of Yokogawa Plus stainless steel mesh has the lowest roughness and good stiffness. There is no error in the formation of high-end flow velocity noise under test conditions; the rubber lining has the highest roughness, and the high-end flow rate noise error group is the earliest; polyurethane linings appear despite The high-end flow noise is later, but due to its low intensity, the resulting margin of error is greatest. This shows that the lining and electrode roughness are important causes of high-end flow noise.
For serous noise, since the passivation film covered by the electrode surface is only about 1 nm thick, if the electrode itself has a high roughness and the surface is uneven, the passivation film is difficult to achieve a dense and uniform thickness, which will stabilize the film. Sex is affected, which in turn affects the maintenance and repair of the membrane. The electrochemical reaction between the fluid and the electrode will continue and it will be difficult to achieve a stable measurement of the slurry fluid. In other words, the roughness of the electrode surface directly affects the generation and elimination of slurry noise.
6 taken from well-known electromagnetic flowmeters manufacturers manual flow measurement E + H's book, which is a sectional view of a production process for the food industry flow measurement with PTFE or PFA fluoroplastic lining electromagnetic flowmeter. Since foods are generally low in conductivity and high in viscosity, there is a high probability of flow noise during measurement. It can be seen that the roughness requirement of the lining and electrodes of E+H company is Ra0.3 , and the roughness requirement of the inlet and outlet connecting metal pipes is Ra0.8 . Ra0.3 and Ra0.8 represent the average height of the absolute roughness of 0.3 μm and 0.8 μm, respectively . Ra0.3 is already a specular roughness grade reached by mechanical or electrolytic polishing. Here, the metal tube acts as a liquid ring and is used to electrically connect the measuring fluid to a reference reference potential of the signal. The liquid-to-liquid ring (connected to a liquid electrode or upstream and downstream of a flow meter) is treated like an electrode and is also subjected to fluid noise due to the electrochemical action of the fluid to be measured. Therefore, it is also necessary to attach great importance to reducing the roughness height.
The two-frequency two-wire electromagnetic flowmeter researched and developed by Yokogawa of Japan in 2009 reduced the roughness of the lining and the electrode, and improved the electrode passivation film as one of the key technologies to improve the signal-to-noise ratio of the sensor. The specular requirements of Ra in the range of 0.05 to 0.15 μm have been proposed for the lining and electrode roughness . This measure increases the signal-to-noise ratio of the sensor sensing signal and the sensor by more than one time. Therefore, the two-wire dual-frequency excitation electromagnetic flowmeter can obtain the same excellent measurement accuracy as the four-wire system under the condition that the excitation current is greatly reduced. .
4. Lining and electrode processing method discussion
4.1 Lining
In order to meet the roughness requirements of the measuring tube, it is necessary to select an excellent lining material according to the fluid type. Different lining materials need to use good processing methods. Currently used lining materials are: neoprene, EPDM rubber, polyurethane, fluorine plastic PTFE and PFA . Here is a brief description of the process points for different liners and for reference by the flow meter manufacturer.
Neoprene is suitable for large-diameter sensors above DN300 . It is mostly used to measure fluids of water, sewage, weak acid and weak alkali. Generally, it is directly adhered to the inner wall of a stainless steel pipe by film adhesive and is made by vulcanization. In this process, the surface roughness of the rubber lining is generally high, and special attention should be paid to the smoothness of the lap seam at the time of operation, but the relative roughness should be low. For small diameter neoprene and EPDM rubber linings, it is best to pressurize the mold, attach it to the inner wall of the pipe and then vulcanize it. Decreasing the surface roughness of the lining depends on the surface roughness of the mold mandrel and the pressure and vulcanization process.
At present, domestic polyurethane linings are mostly made of soft materials, and they are watered. The lining roughness is not only determined by the surface roughness of the mandrel mold, but also affected by watering, venting, heating, cooling, and material composition and proportions. . In overseas, rigid polyurethanes mostly use polyurethane rubber. The important process of forming is to remove the entrapped air bubbles and stabilize the chemical reaction (hardening, cross-linking). The centrifugal casting method is used: The raw materials are kept in a dry state during storage, and the raw materials are uniformly and smoothly mixed and stirred. In order to remove the air bubbles entrained in the raw materials, the appropriate rotation speed of the conduit is set, and the temperature of the raw material treatment, curing, and cross-linking is well controlled.
Fluoroplastics should be used for lining materials that require low roughness. PTFE fluoroplastic linings for pipes, tanks, etc. , are usually lined with a thin-walled Teflon tube in a metal tube or a PTFE tube inserted and then bonded. The main disadvantage of this type of lining is that the negative pressure resistance is not high, and the temperature is greatly affected, and the adhesion is often unreliable. For electromagnetic flowmeters, an excellent fluoroplastic lining is PFA . The main method used by PFA is to inject molten resin and then injection (injection molding). The injection molding method is used to form one piece without joints. PFA linings have good chemical resistance, heat resistance, and adhesion (surface finish). Especially in terms of chemical resistance and heat resistance, the unique manufacturing technology can reduce internal stress and internal air bubbles to avoid cracks. This makes the flowmeter highly reliable even when used in harsh environments. Sex. For this reason, in the manufacturing process of PFA linings, 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 temperature setting should be as low as possible to reduce the thermal degradation of the PFA resin. In injection molding, the temperature of the metal mold should be uniformly maintained above the melting point of the resin. Due to the need for high-precision cooling control, multiple cooling circuits should be set up in the metal mold, and independent cooling control operations should be performed. While controlling the cooling, the resin pressure should also be controlled.
4.2 Electrodes
Electrode treatment includes polishing and passivation processes. There are three methods for polishing, and mechanical polishing is the first procedure for three types of polishing (ie, mechanical polishing, chemical polishing, and electrochemical polishing) of stainless steel polishing. The next combination of the two, such as mechanical polishing - chemical polishing or mechanical polishing - electrochemical polishing. Mechanical polishing is used for primary polishing. The irregularities on the surface of the electrode are machined to a certain roughness, and then chemical polishing or electrochemical polishing is performed. Chemical polishing and electrochemical polishing can remove the microscopic unevenness on the surface of the electrode, thereby improving the mirror surface brightness. At the same time, it can accomplish the purpose of polishing and passivating the two processes, increase the surface chromium content, and form a good passivation layer. For the rough surface, due to macroscopic unevenness, it is necessary to use a mechanical polishing method to achieve a roughness of Ra ≤ 0.8μm , and then use a chemical polishing or electrochemical polishing method to raise the roughness to Ra = 0.05μm or more to obtain the final brightness. Mirror gloss and a good passivation layer.
The polished and passivated electrodes can form a stable passivation layer. However, during storage, transfer, and assembly, care must be taken to keep the surface passivation layer intact. Compared with electromagnetic flowmeter sensors traditionally and simply soaked in water (sometimes this method takes several days and several nights), the passivation layer production method is naturally formed, and the electrode treated by the polishing and passivation process can obtain stability. The electrode with anti-corrosion properties, which considerably improves the production efficiency, is an advanced production process.
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