Electromagnetic flowmeter main features and selection key

Case introduction

　　Electromagnetic flowmeter selection and installation of the law:

　　First, the first clear unit of measurement needs

　　Should be clear about the measurement of a number of needs, mainly: measurement medium, flow m3 / h (small, operating point, large), medium temperature ℃, medium pressure MPa, device (pipeline or piercing type) and so on.

　　Second, the choice (electromagnetic flowmeter) conditions conditions

　　1, the measured medium must be conductive liquid (i.e., the need to be measured by the fluid has di limit of conductivity).

　　2, the measured medium should not be rich in more ferromagnetic media or many bubbles.

　　Third, should know the electromagnetic flowmeter measurement principle

　　The principle of measurement based on Faraday's law of electromagnetic induction, that is, measuring the flow rate of fluid flow perpendicular to the direction of the magnetic field, conductive fluid activity and induced a uniform flow rate (also known as volumetric flow rate) is proportional to the voltage, the voltage signal through the two electrodes in direct contact with the fluid detected and transmitted through the cable to the amplifier, and then transformed into an output signal.

　　Fourth, should know the main features of electromagnetic flowmeter

　　Its main features are:

　　(1) measuring tube is basically no pressure loss, not easy to block, the slurry class measurement has a common adaptability;

　　(2) low demand for straight pipe section;

　　(3) low-frequency rectangular wave excitation, not subject to the influence of industrial frequency and the scene of collective disturbance, stable and reliable operation;

　　(4) Transmitter body can be all stainless steel, plus fabric materials with acid, alkali, corrosion resistance;

　　(5) field display type converter can be used in the intelligent chip, convenient parameter setting;

　　(6) transmitter internal self-calibration system can be set up at any time on the transmitter constants and factory calibration value of self-calibration, to facilitate debugging and maintenance;

　　(7) Measurement scale is wide, full-scale flow rate setting can be in the scale of 0.3m/s-12m/s;

　　(8) Its piercing type can be installed or dismantled under the condition of constant flow;

　　(9) Wide range of applications: can be used in the chemical industry, metallurgy, paper, food, petroleum, urban water supply and other areas.

　　Fifth, the type of selection key

　　1, the first clear is to select the pipeline ‘) electromagnetic flowmeter ’), or piercing electromagnetic flowmeter.

　　2, usually pick the field without showing type electromagnetic flowmeter, its output of 4-20mA (or 0-10mA) current signal to the control room of the secondary appearance and can show the flow rate and the total amount.

　　3, if you focus on the convenience of field operation to investigate the flow in the pipeline, you can select the field display type electromagnetic flowmeter.

　　4, in the environmental needs or measurement accuracy needs to be higher, you can select the safe voltage intelligent electromagnetic flowmeter.

　　5, in more than 200mm large diameter measurement of flow or non-constant flow conditions mounting and dismounting, can give priority to the selection of piercing or enhanced piercing electromagnetic flowmeter.

　　Sixth, the sensor calibre selection key

　　1, select the sensor calibre and articulation of the same calibre of the technical pipe, the advantage is that the device is convenient (no reducer); the condition is that the flow rate of the fluid in the pipe must be in the 0.3m / s-10m / s scale; its applicable conditions for the use of the project in the early stage and the flow rate of the fluid in the pipe is in the lower condition.

　　2, select the sensor calibre and articulation of the technical pipeline calibre is not the same, the applicable conditions: (1) the flow rate is low, the flow rate is stable: (2) to reduce the cost-effective:

　　Seven, the key to the selection of fabric materials

　　According to the company's measured media corrosive, abrasive and temperature, selected by the subscriber, can refer to the manufacturers to provide the ‘fabric material function and applicable scale table’.

　　Eight, the selection of electrode materials

　　According to the corrosive nature of the measured medium, selected by the orderer, see the manufacturers to provide the ‘electrode material corrosion-resistant function table’.

　　Nine, the selection of the level of protection

　　According to the national standard GB, shell protection level: if the appearance of the device below the ground and often subject to flooding, it is appropriate to select the submersible type; if the appearance of the device on the surface, it is appropriate to select the anti-spray type.

　　Ten, the upper limit of the selection of flow

　　1, usually select the transmitter diameter and the piping connected to the same diameter. Because of the diameter, flow rate and flow rate of the three there is a strict function, the selection of manufacturers can refer to the upper limit of the electromagnetic flowmeter flow rate table, in the corresponding diameter under the selection of the upper limit of the flow rate value.

　　2, because of the technical pipeline flow rate is low and can not reach ‘1’ in the corresponding diameter of the xiao upper limit of the flow rate value, you can select the transmitter diameter is smaller than the diameter of the technical pipeline, that is, before and after the addition of the transmitter connected to the reducer.

　　Eleven, the selection of the device

　　1, the whole type: line will flowmeter sensing some and change some installed in one, the advantage is easy to install.

　　2, separate type: the line will be the sensing part of the flowmeter is installed in the measured pipe, will change some of the devices in the room for easy operation. Applicable to the site environment of poor occasions (but the length of its separation should be less than 30m).

　　Twelve, the selection of grounding ring

　　If the pipeline connected to the exterior related to the measured medium is insulating, it is necessary to select the grounding ring.

　　Thirteen, the selection of ball valve conditions

　　On the technical needs of the pipeline fluid flow shall not stop, and the demand for media shall not overflow the occasion, the need to install ball valves. On the installation and dismantling of the flowmeter promised to cut off the flow (or installation and dismantling operations do not affect the pipeline operation), it is not necessary to install ball valves and the flowmeter will be installed directly in the flange on the short pipe.

　　Fourteen, the device focus on the key

　　1, the electrode axis is necessary to adhere to the approximate level.

　　2, to ensure that the measuring tube at all times necessary to fill.

　　3, in the pipe flange adjacent to ensure that there are enough bolts and nuts of the device space.

　　4, in the pipe section of the flowmeter to have pipeline support to reduce the pipeline operation oscillation.

　　5, the flowmeter should be prevented from strong electromagnetic fields in the vicinity.

　　6, long pipeline, the flow meter should be installed in the downstream control valves and shut-off valves.

　　7, in case of ‘open feed or discharge’ situation, should be in the pipeline of the low section of the device appearance.

　　8, to the electrode axis as a reference, the inlet straight line pipe section should be greater than or equal to 5 times the diameter of the measurement pipe, the outlet pipe should be greater than or equal to 2 times the diameter of the measurement pipe.

　　XV, (flowmeter) grounding requirements

　　Sensors should be outstanding alone grounding line (copper core cross-section ≥ 16mm2), the grounding resistance <10 ohms; if the pipeline connected to the sensor is coated with an insulating layer or non-metallic pipeline, the sensor should be set up on both sides of the grounding ring (grounding line copper core cross-section ≥ 16mm2).

　　Sixteen, cable and electrical connection focus on the key

　　Separate connection cable (signal transmission and excitation), from the point of view of cutting down on disturbances and saving money, the shorter the better. Usually to co-ordinate the conductivity of the medium, the cross-section of the excitation cable, the type of signal cable (core, layer, shielding) the three constraints.