Selection method of electromagnetic flowmeter
The correct selection of electromagnetic flowmeter is the premise to ensure the good use of electromagnetic flowmeter. What kind of electromagnetic flowmeter to choose should be determined according to the user's process flow, the physical and chemical properties of the measured fluid medium, and the installation and use environment. The structure, diameter, flow range, village and electrode material, installation environment, output signal and other parameters of the electromagnetic flowmeter meet the measurement requirements.In order to choose the flowmeter correctly and reasonably, the following aspects can be considered according to the specific use conditions.
First of all, the following process parameters must be specified
Selection of the diameter of the electromagnetic flowmeter
(1) The composition, density and conductivity of the measured fluid
(2) Maximum flow, common flow, minimum flow
(3) Maximum working pressure
(4) Maximum temperature, minimum temperature
The volume flow rate of the electromagnetic flowmeter is proportional to the flow rate of the fluid, so the flow rate of the electromagnetic flowmeter can be calculated by knowing the flow rate and limiting a flow rate range.
Example: When the sensor caliber is 'DN65mm', we can see from the flow range table of the electromagnetic flowmeter that when V=5m/s, the volume flow of the measured fluid is Qx60m3/h. At this time, we set 'by Measuring range 'is 60m3/h.Followed by the actual flow calibration of this FLDC series product with a caliber of 65mm and a verification report:
(1) According to Q60m3/h (DN65mm), take 20% (serial number A), 60% (serial number B) and 100% (serial number C) respectively as three percentage points, and calibrate the corresponding test flow rate.
(2) Each percentage point (serial number A, B, C) is calibrated three times in a row (the interval between each time is 20s).
(3) If serial number A has been calibrated three times, you can see the error value between the 'measured volume value' of the measured meter and the 'standard volume value' of the standard meter, such as: 66.9167-66.8333= 0.084,0.084/66.8333=0.12%.
(4) Therefore, the error of all measured calibration points of CKAS's FLDC electromagnetic flowmeter is <0.5%.So the accuracy is well within 0.5%.
Electromagnetic flowmeter flow range table
Select the corresponding electromagnetic flowmeter according to the flow rate in the above table 7-1. If the inner diameter of the selected electromagnetic flowmeter does not match the inner diameter of the current process pipeline, the pipe should be reduced or expanded.
If the pipe is shrinked, if some pressure loss caused by the shrinkage is considered, it can be calculated according to Appendix 2'Pressure loss of attached reducer'.
Considering the price and accuracy of the product, you can choose an electromagnetic flowmeter with an appropriate flow rate and caliber, while relatively reducing investment.
Choice of Electrode Material
The electrode material should be selected according to the corrosiveness of the measured medium, please refer to the relevant information on the corrosiveness of the medium.
Corrosion tests should be done for special media, as shown in Table 7-2.
Choice of lining material
The lining material should be selected according to the corrosiveness, abrasiveness and temperature of the measured medium, as shown in Table 7-3.
Selection of flowmeter structure
One-piece
The electromagnetic flowmeter completely eliminates the connection between the converter and the sensor, the price and installation cost are relatively economical, and the display is intuitive;
The integrated flowmeter is suitable for most applications, especially for flow measurement of low-conductivity fluid media, but not suitable for high-temperature fluid, severe vibration, severe dripping and water immersion;
If it is installed in an inaccessible place, it is inconvenient to maintain;
The one-piece default protection level is IP65;
Split type
Split type is selected when the following situations apply:
Ambient temperature or converter surface temperature is greater than 55°C;
The pipeline vibrates greatly;
It will seriously corrode the aluminum shell of the converter;
The site has high humidity or corrosive gas;
It is inconvenient to debug the flowmeter installed at high altitude or underground;
illustrate
The sensor of the split type electromagnetic flowmeter can be installed on the process pipeline or buried in the underground water accumulation place for a long time (IP68 type), while the intelligent converter is installed in the instrument room or near the sensor;
The sensor and the converter are installed separately in two different places, and they are connected by a two-core shielded cable and a two-core rubber-sheathed flexible cable. Generally, the length of the cable should not exceed 30m to ensure measurement performance;
The installation size of the sensor is the same as that of the integrated type, and the converter is wall-mounted.
Selection of rated pressure
The actual maximum working pressure must be less than the rated working pressure of the flowmeter.
Choice of working temperature
The maximum and minimum operating temperatures must comply with the temperature requirements specified by the flowmeter.
Output signal selection
(4~20) mA output: suitable for long-distance analog signal transmission occasions;
Frequency/pulse output: used for instrument calibration and cumulative flow output;
Modbus RTU communication protocol (RS485): It can support networking of multiple electromagnetic flowmeters and long-distance data transmission;
The user needs to select the output signal according to the actual situation and supporting equipment.
Choice of working power
110V/220V (AC): suitable for applications with AC power supply (commonly used);
24V (DC): 24V DC power supply is required in some special applications such as explosion-proof;
Choice of ground ring
The grounding ring is used to ground the measured medium to improve the stability of the electromagnetic flowmeter measurement.However, the grounding ring needs to be in contact with the measured medium, which may be corroded and worn, and needs to be replaced regularly.
The CKAS-FLDC series electromagnetic flowmeter can adopt the structure of the grounding electrode, and its function is to effectively ensure the grounding for a long time and ensure the measurement accuracy under normal circumstances, so the grounding ring is generally no longer needed.If the stray current is too large, such as the leakage current of the electrolytic cell along the electrolyte affects the normal measurement of the electromagnetic flowmeter, it is recommended to have a grounding ring.
Some small-diameter electromagnetic flowmeters only have 2 electrodes, and users can configure double grounding rings according to site needs.
When installing the grounding ring, the grounding ring needs to be installed in an accurate pipeline position so as not to affect the flow field of the measured medium.
Choice of protection class
The protection level should be selected according to the actual situation: if the sensor is installed below the ground, if it is often soaked in water, IP68 should be selected; if the sensor is installed above the ground in a non-exposed environment, IP65 should be selected.
