Electromagnetic flowmeter installation, use and common faults

Foreword Xizhijiang Pumping Station is located in Laoershan, the left bank of Xizhijiang, Ma'an Town, Huicheng District, Huizhou City. It is one of the two water intakes for the Shenzhen Dongjiang Water Source Project. The pumping station passes the Xizhi River water through a water diversion pipe culvert, front pond, After the water pump is pressurized, it passes through a quantity of water to measure and measure the water pipeline of the Dongjiang Water Source Project that has been built directly. The pump station is built with one unit and two units, the rated flow rate of the single unit is 3.75m3/s, and the electromagnetic flowmeter is used for the water volume. Measured by computer group instantaneous pumping flow per second and total cumulative pumping volume. Electromagnetic flowmeter (EMF for short) is a measuring instrument for measuring the volumetric flow rate of conductive liquids using Faraday's law of electromagnetic induction. In the early 1950s, EMF realized industrial application of r. It has been widely used in the world in recent years. Electromagnetic flowmeters are a type of industrial flow measuring instrument. There are some tips and precautions in their installation and use. Here I combine some of them. Personal experience, talk about the installation, use, and common faults of electromagnetic flowmeters. I hope to help with the installation and use of peers.

1. Installation and Usage Precautions 1.1 General Considerations for Use The liquid should have the required conductivity for measurement and require a substantially uniform conductivity distribution. Therefore, the flow sensor should be installed so as to avoid places where conductivity is not easily generated. For example, liquid medicine is added near the upstream of the sensor. The liquid addition point is preferably located downstream of the sensor. When used, the sensor tube must be filled with fluid (except for non-full tube types). When liquids are mixed, their distribution should be generally uniform. The liquid should be at the same potential as ground. Must be grounded. If the process piping is made of plastics and other insulating materials, the transport liquid generates friction and static electricity and other causes, resulting in a potential difference between the liquid and the ground.

1.2 Flow sensor installation 1.2.1 Installation site Usually the electromagnetic flow sensor housing protection is extremely IP65 (dust and spray water level specified in GB4208), all the following requirements for the installation site.

(1) When measuring the mixed phase fluid, choose the place that will not cause phase separation; when measuring the two-component liquid, avoid installing in the downstream where the mixing is not uniform; when measuring the chemical reaction pipeline, install it in the reaction fully completed section;

(2) Avoid as much as possible negative pressure in the measuring tube;

(3) Choose a place with little vibration, especially for integrated type instruments;

(4) Avoid large electric motors and large transformers nearby so as to avoid electromagnetic field interference;

(5) It is easy to realize the place where the sensor is grounded separately;

(6) Avoid high concentrations of corrosive gases in the surrounding environment as much as possible;

(7) The ambient temperature is in the range of -25/0 to 50/600°C. The temperature of the integral structure is also limited by the electronic components and the range is narrow.

(8) The relative humidity of the environment is in the range of 10% to 90%;

(9) Avoid direct sunlight as much as possible;

(1O) Avoid rain soaking and will not be immersed in water.

1.2.2 Straight Pipe Length Requirements In order to obtain the normal measurement accuracy, the electromagnetic flow sensor should have a straight pipe section with a certain length, but its length is lower than most other flow meters. 9O. After the elbow, T-tube, concentric reducer, and fully open valve, it is generally considered that as long as the pipe section is 5 times the diameter (5D) from the center line of the electrode (not the sensor inlet end), the valve with different opening needs 1OD. The downstream straight section is 2~3D or no requirement. However, prevent the butterfly valve plate from reaching into the sensor measuring tube. The lengths of the upstream and downstream straight pipe sections proposed by various standards or verification regulations are also inconsistent. As summarized in Table 1, the requirements are higher than the usual requirements. This is due to the requirement to ensure that the current 0.5-level precision instrument is achieved.

1.2.3 Mounting position and flow direction The sensor can be installed horizontally, vertically or tilted without restriction. However, it is better to measure the solid and liquid two-phase fluids vertically and flow from bottom to top. This can avoid the disadvantages of local wear in the lower half of the liner during horizontal installation, solid phase sedimentation at low flow rates, and the like.

When installing horizontally, make the axis of the electrode parallel to the horizon, and do not be perpendicular to the horizon, because the electrode at the bottom is easy to be covered by the deposit, and the top electrode is easily wiped by the bubbles in the liquid to cover the surface of the electrode, causing the output signal to fluctuate. . In the piping system shown in Figure 1, c and d are suitable locations; a, b, and e are unsuitable locations; b may not be full of liquid a, and e may easily accumulate gas, and the sensor may not be fully filled at the e-segment. The discharge port is preferably shown in a Æ’ shape. For the solid and liquid two-phase flow C is not an appropriate location.

1.2.4 Grounding The sensor must be grounded separately (ground resistance 100Ω or less). In principle, the separation type should be grounded on the sensor side, and the converter grounding should be on the same grounding point. If the sensor is installed on the cathode corrosion protection pipeline, in addition to the sensor and the grounding ring together with the ground, but also use a thick copper wire (16mm2) to bypass the sensor across the connecting flange on the two flanges, so that the cathodic protection current between the sensor isolation.

1.3 Converter Installation and Connection Cables The integrated EMF does not have a separate converter; the discrete converter is installed near the sensor or in the instrument room, so there is a large selection of room, better environmental conditions than the sensor, and the distance between the converter and the sensor is limited by the measured The dielectric conductivity and signal cable type, ie the distributed capacitance of the cable, the conductor cross-section and the number of shielding layers. Use the signal cable attached to the instrument (or model number) supplied by the manufacturer. Low conductivity liquids and longer transmission distances are also stipulated with three-layer shielded cables. The general instrument “operating instructions” gives the corresponding transmission distance range for different conductivity liquids. Single-layer shielded cables are usually used for industrial water or acid-base solutions and can travel over a distance of 100m.

In order to avoid interference signals, the signal cable must be put in the protective grounding pipe separately. The signal cable and the power cable cannot be installed in the same steel pipe.

3. Faults during fault operation are faults that occur after the electromagnetic flowmeter has been commissioned and operated for a certain period of time. Common operational faults are caused by factors such as the adherence of the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions.

(1) The sensor inner wall adhesion layer. Since electromagnetic flowmeters are commonly used to measure dirty fluids, they often accumulate adhesive layers on the inner wall of the sensor after a certain period of operation and time. These faults are often caused by too much or too little conductivity of the adhesion layer. If the attachment is an insulating layer, the electrode circuit will be open circuit, the meter can not work properly; if the adhesion layer conductivity is significantly higher than the fluid conductivity, the electrode circuit will appear short circuit, the instrument can not work properly. Therefore, the adhered scale layer of the electromagnetic flowmeter measuring tube should be removed in time.

(2) Lightning strikes. Lightning strikes easily induces high voltage and inrush currents in the instrument line, causing damage to the meter. It is mainly introduced through the power line or the excitation coil or the flow signal line between the sensor and the converter. In particular, it is mostly introduced from the power cable of the control room.

(3) Changes in environmental conditions. During commissioning, the meter is working properly due to good environmental conditions (eg, no interference source), and it is often easy to neglect installation conditions (eg, grounding is not good). In this case, once the environmental conditions change, a new source of interference (such as welding on pipelines near the flowmeter, and large transformers installed near the flowmeter) will occur during operation. This will interfere with the normal operation of the meter and the output of the flowmeter. The signal will fluctuate.

4. Conclusion The correct installation and use of the Xizhijiang pump station flowmeter has been performed well so far and the measurement is accurate. As the company moves toward the market, the company's production and operation management will be further deepened, and the flow measurement will become more and more important. As one of the industrial flow measuring instruments, the electromagnetic flow meter must play its role and how to properly install and use it on the basis of good selection. Has become a key issue. Therefore, under the premise of having certain professional knowledge, automated instrumentation professionals must master certain skills and methods in how to correctly install and use them, and at the same time understand the common failures in the use of electromagnetic flowmeters. Satisfy the production requirements of the company and make the flow metering play an important role.

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