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It is important to ensure correct measurement accuracy of the electromagnetic flowmeter
2012-3-16 11:38:37


的衬里应根据被测介质的腐蚀性,磨损性和温度来选择内衬材料:一、天然橡胶(软橡胶) 1、较好的弹性,耐磨性和扯断力 2、耐一般的弱酸、弱碱的腐蚀 测水、污水二、耐酸橡胶(硬橡胶) 可耐常温下的盐酸、醋酸、草酸、氨水、磷酸及50%的硫酸、氢氧化钠、氢氧化钾的腐蚀,但不耐强氧化剂的腐蚀 测一般的酸、碱、盐溶液三、氯丁橡胶(Neoprene) 1、极好的弹性,高度的扯断力,耐磨性能好 2、耐一般低浓度的酸碱、盐溶液的腐蚀,但不耐氧化性介质的腐 <80℃; 测水、污水、泥浆和矿浆四、聚胺脂橡胶(Polyurethane) 1、极好的耐磨性能 2、耐酸、碱性能差 <40℃ 测中性强磨损的煤浆、泥浆和矿浆五、聚四氟乙烯(PTFE) 1、耐沸腾的盐酸、硫酸、硝酸、王水、浓碱和各种有机溶剂 2、耐磨性能好,粘接性能差 -80~+180℃ The correct selection of the lining of the electromagnetic flowmeter should choose the lining material according to the corrosiveness, abrasion and temperature of the measured medium: 1. Natural rubber (soft rubber) 1. Good elasticity, wear resistance and breaking force 2. Resistance to general weak acid and alkali corrosion water measurement, sewage two, acid resistant rubber (hard rubber) resistant to hydrochloric acid, acetic acid, oxalic acid, ammonia, phosphoric acid and 50% sulfuric acid, sodium hydroxide, potassium hydroxide at room temperature , But not resistant to corrosion of strong oxidants General acid, alkali, salt solution Three, neoprene (Neoprene) 1, excellent elasticity, high breaking force, good wear resistance 2, resistance to low acid in general Corrosion of alkali and salt solution, but not resistant to oxidizing medium <80 ℃; Measurement of water, sewage, mud and mineral pulp 4. Polyurethane 1. Excellent abrasion resistance 2. Acid and alkali resistance The difference is less than 40 ℃. Testing neutral and strong abrasion coal slurry, mud and ore slurry 5. Polytetrafluoroethylene (PTFE) 1. Boiling hydrochloric acid, sulfuric acid, nitric acid, aqua regia, concentrated alkali and various organic solvents 2. Wear resistance Good performance, poor bonding performance -80 ~ + 180 ℃ Measure the concentration, strong alkali strong corrosive solution and sanitation

Reasonable selection and correct installation of the electromagnetic flowmeter are important to ensure the accuracy of the measurement and extend the service life of the instrument. The following is a brief introduction to the selection principles, installation conditions and precautions for use of electromagnetic flowmeters.

(1) Selection principle of electromagnetic flowmeter The selection of electromagnetic flowmeter is mainly the correct selection of the transmitter, and the converter only needs to be matched with it.

1. Choice of caliber and range The diameter of the transmitter is usually the same as that of the piping system. If the pipeline system is to be designed, the caliber can be selected according to the flow range and velocity. For electromagnetic flowmeters, a flow velocity of 2-4m / s is more appropriate. In special cases, such as solid particles in the liquid, considering the wear conditions, the commonly used flow rate ≤ 3m / s can be selected. For fluids that are easy to manage. Optional flow velocity ≥2m / s.

The range of the transmitter can be selected according to two principles: one is that the full scale of the meter is greater than the expected maximum flow value; the other is that the normal flow is greater than 50% of the full scale of the meter to ensure a certain measurement accuracy.

2. Selection of temperature and pressure There are certain restrictions on the fluid pressure and temperature that the electromagnetic flowmeter can measure. When selecting, the operating pressure must be lower than the working pressure specified by the flowmeter. At present, the working pressure specifications of domestically produced electromagnetic flowmeters are:

Caliber less than 50mm, working pressure is 1.6MPa;

900mm caliber, working pressure is 1MPa;

Larger than 1000mm, working pressure is 0.6MPa.

If there are special requirements for the pressure resistance of the transmitter, you can consult the manufacturer specifically. Some manufacturers have been able to manufacture electromagnetic flow transmitters with a pressure resistance of 32MPa.

的工作温度取决于所用的衬里材料,一般为5—70℃. The working temperature of the electromagnetic flowmeter depends on the lining material used, which is generally 5-70 ° C. Such as special treatment, it can exceed the above range, such as the wear-resistant and corrosion-resistant electromagnetic flowmeter produced by Tianjin Automation Instrument Factory. The transmitter allows the measured medium temperature to be -40 to 130 ° C.

3. Selection of lining material and electrode tree material The lining material and electrode material of the transmitter must be properly selected according to the physical and chemical properties of the medium, otherwise the meter will be quickly damaged due to the corrosion of the lining and the electrode, and the corrosive Once the medium leaks, it is easy to cause an accident. Therefore, the materials of electrodes and linings must be carefully selected according to the specific measurement medium in the production process.

(B) the installation of electromagnetic flowmeters To ensure the measurement accuracy of electromagnetic flowmeters, correct installation is very important.

① The transmitter should be installed in a dry and ventilated place indoors. Avoid installing in places where the ambient temperature is too high, should not be subject to strong vibration, and try to avoid equipment with strong magnetic fields, such as large motors, transformers, etc. Avoid installing in places with corrosive gas. Installation location is convenient for maintenance. This is the environmental condition to ensure the normal operation of the transmitter.

② In order to ensure that the measuring tube of the transmitter is filled with the measured medium, it is best to install the changer vertically and flow from bottom to top. Especially for liquid-solid two-phase flow, it must be installed vertically. If only horizontal installation is allowed on site, it must be ensured that the two electrodes are at the same level. ③ Valves and bypasses should be installed at both ends of the transmitter.

变送器的电极所测出的几毫伏交流电势,是以变送器内液体电位为基础的. ④ A few millivolts of AC potential measured by the electrodes of the electromagnetic flowmeter transmitter is based on the liquid potential in the transmitter. In order to stabilize the liquid potential and to maintain the equipotential between the transmitter and the fluid to ensure stable measurement, the transmitter housing and the metal tube should be well grounded at both ends, and the converter housing should also be grounded. The grounding resistance cannot be greater than 10 and cannot be shared with the grounding wire of other electrical equipment. If it is not possible to ensure good contact between the transmitter housing and the metal pipe, connect them with metal wires. Reliable grounding.

⑤ In order to avoid interference signals, the signal between the transmitter and the converter must be transmitted with a shielded wire. Signal cables and power cables are not allowed to be placed in the same cable steel tube in parallel. The length of the signal cable should not exceed 30m.

⑥ The installation site of the converter should avoid strong AC and DC magnetic fields and vibration. The ambient temperature is -20 to 50 ° C, does not contain corrosive gases, and the relative humidity is not greater than 80%.

⑦In order to avoid the influence of relative measurement of flow rate, the flow regulating valve should be set downstream of the transmitter. For the small-diameter transmitter, because the distance from the center of the electrode to the inlet of the flowmeter is equivalent to several times the diameter D, the upstream straight pipe may not be specified. But for larger caliber flowmeters, generally there should be more than 5D straight pipe section in the upstream, and generally no straight pipe section is required in the downstream.