The main features and selection key of electromagnetic flowmeter

Rules for selecting and installing electromagnetic flowmeters:

1. First, clarify the measurement requirements of the unit

There are several metering requirements that should be clarified, mainly including: measuring medium, flow rate m3/h (minimum, operating point, maximum), medium temperature ℃, medium pressure MPa, installation method (pipeline or insertion), etc.

2. Conditions for selecting (electromagnetic flowmeter)

1. The measured medium must be a conductive liquid (that is, the measured fluid must have a minimum conductivity limit).

2. The measured medium should not contain too much ferromagnetic medium or many bubbles.

3. Know the measurement principle of electromagnetic flowmeter

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

4. Know the main features of electromagnetic flowmeter

Its main features are:

⑴ There is basically no pressure loss in the measuring tube, which is not easy to be blocked and has unique adaptability to slurry measurement;

⑵ Low demand for straight pipe sections;

⑶ Low-frequency rectangular wave excitation is not affected by power frequency and on-site distribution interference, and the operation is stable and reliable;

⑷ The transmitter body can be made of all stainless steel, and after adding fabric materials, it has the ability to resist acid, alkali and corrosion;

⑸ On-site display converter can use dedicated intelligent chip, which is convenient for parameter setting;

⑹ The transmitter can be equipped with a self-calibration system to calibrate the transmitter constants and factory calibration values ​​at any time, which is convenient for debugging and maintenance;

⑺The measurement range is wide, and the full-scale flow rate setting can be within the range of 0.3m/s-12m/s;

⑻The insertion type can be installed or disassembled under continuous flow;

⑼Wide application range: can be used in chemical industry, metallurgy, papermaking, food, petroleum, urban water supply and other fields.

5. Key points for type selection

1. The first thing to know is whether to choose a pipeline type electromagnetic flowmeter or an insertion type electromagnetic flowmeter.

2. Usually, a non-display electromagnetic flowmeter is selected, which outputs a 4-20mA (or 0-10mA) current signal to the secondary surface of the control room and can display the flow rate and total amount.

3. If you want to observe the flow rate in the pipeline during on-site operation, you can choose an on-site display electromagnetic flowmeter.

4. When the environment or measurement accuracy requirements are high, you can choose a safe voltage intelligent electromagnetic flowmeter.

5. When measuring flow in a pipe with a large diameter of more than 200 mm or installing in a continuous flow state, an insertion type or enhanced insertion type electromagnetic flowmeter may be preferred.

6. Key points for selecting sensor caliber

1. The diameter of the selected sensor should be the same as the diameter of the connected technical pipeline. The advantage is that it is easy to install (no reducer is required); the condition is that the flow rate of the fluid in the pipe must be within the range of 0.3m/s-10m/s; it is applicable to the early stage of the project and the flow rate of the fluid in the pipe is at a low level.

2. The caliber of the selected sensor is different from the caliber of the connected technical pipeline. Its applicable conditions: ⑴ Low flow rate and stable flow rate; ⑵ Reduced cost performance:

7. Key points for selecting fabric materials

The order is selected by the customer based on the corrosiveness, abrasiveness and temperature of the medium being measured by our company. Please refer to the "Fabric Material Function and Applicable Scope Table" provided by each manufacturer.

8. Selection of electrode materials

The order is selected by the customer based on the corrosiveness of the medium being measured by our company. Please refer to the "Electrode Material Corrosion Resistance Function Table" provided by each manufacturer.

9. Selection of protection level

According to the national standard GB4208-84, the enclosure protection levels are: if the surface is installed below the ground and is often flooded, a submersible type should be selected; if the surface is installed on the surface, a water-spray-proof type should be selected.

10. Selection of upper limit flow

1. Usually, the diameter of the transmitter should be the same as the diameter of the connected pipe. Because there is a strict relationship between the diameter, flow velocity and flow rate, you can refer to the upper limit flow table of the electromagnetic flowmeter provided by the manufacturer when selecting, and select the upper limit flow value under the corresponding diameter.

2. When the flow velocity in the technical piping is too low to reach the minimum upper limit flow value under the corresponding diameter in "1", the diameter of the transmitter can be selected to be smaller than the diameter of the technical piping, that is, a reducer can be added before and after the transmitter.

11. Selection of installation method

1. Integral type: The sensing part and the conversion part of the flow meter are installed in one body, which has the advantage of being easy to install.

2. Separation type: The sensing part of the flow meter is installed on the measured pipe, and the conversion part is installed in a room that is convenient for operation. It is suitable for occasions with poor on-site environment (but its separation length should be less than 30m).

12. Selection of grounding ring

If the pipe connected to the surface is insulating relative to the measured medium, a grounding ring must be used.

13. Selection conditions of ball valve

For situations where the technical requirements require that the flow of fluid in the pipeline must not be interrupted and the medium must not overflow, a ball valve must be installed. For situations where the flow meter is allowed to be interrupted during installation (or the installation and disassembly operation does not affect the pipeline operation), a ball valve is not required and the flow meter can be directly installed on the flange short pipe.

14. Installation focuses on key points

1. The electrode axis must remain approximately horizontal.

2. Ensure that the measuring tube is filled at all times.

3. Ensure that there is enough space near the pipe flange for installing bolts and nuts.

4. There should be pipeline support in the pipe section where the flow meter is installed to reduce pipeline operation vibration.

5. Avoid strong electromagnetic fields near the flow meter.

6. For long pipelines, control valves and cut-off valves should be installed downstream of the flow meter.

7. In case of "open feed or discharge", the surface should be installed in the lower section of the pipeline.

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

15. Grounding requirements for (flow meter)

The sensor should have an excellent independent grounding wire (copper core section ≥ 16mm2), and its grounding resistance is less than 10 ohms; if the pipe connected to the sensor is coated with an insulating layer or is a non-metallic pipe, grounding rings should be installed on both sides of the sensor (grounding wire copper core section ≥ 16mm2).

16. Pay attention to the key points of cable and electrical connection

Separate connection cables (signal transmission and excitation) should be as short as possible from the perspective of reducing interference and saving money. Usually, the conductivity of the medium, the cross-section of the excitation cable, and the type of signal cable (core, layer, shielding) should be considered.