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The lining material of the electromagnetic flowmeter should be selected according to the corrosivity, abrasion and temperature of the measured medium:

Author:vacorda    Source:www.vacorda.com    Date: 2020-06-08 15:36:56

The lining material of the electromagnetic flowmeter should be selected according to the corrosivity, abrasion and temperature of the measured medium:

 电磁流量计类型

1. Natural rubber (soft rubber)

  • Good elasticity, wear resistance and tearing force
  • Resistance to general weak acid and weak alkali corrosion
  • Water measurement, sewage


 

2. acid-resistant rubber (hard rubber)

It can withstand the corrosion of hydrochloric acid, acetic acid, oxalic acid, ammonia, phosphoric acid and 50% sulfuric acid, sodium hydroxide, potassium hydroxide at room temperature, but it is not resistant to the corrosion of strong oxidants. It can measure general acids, alkalis, and salt solutions.

 

3. Neoprene

  • Excellent elasticity, high breaking force, good wear resistance
  • Resistant to the corrosion of general low concentration acid and alkali, salt solution, but not resistant to the corrosion of oxidizing medium
  • Temperature <80
  • Water measurement, sewage, mud and slurry


 

4. polyurethane rubber (Polyurethane)

  • Excellent wear resistance
  • Poor acid and alkali resistance
  • Temperature <40
  • Measuring neutral and strong abrasion of coal slurry, mud and ore slurry


5. polytetrafluoroethylene (PTFE)


  • Boiling hydrochloric acid, sulfuric acid, nitric acid, aqua regia, concentrated alkali and various organic solvents
  • Good wear resistance, poor adhesion
  • temperature -80+180
  • Concentration measurement, concentrated alkali strong corrosive solution and hygienic medium


 

Common use failure

 

Typical fault diagnosis and treatment

 

1. No flow output.

Check whether there is a fault in the power supply section, test whether the power supply voltage is normal; test whether the fuse is on or off; check whether the arrow of the sensor is consistent with the flow direction of the fluid, if it is inconsistent, change the installation direction of the sensor; check whether the sensor is full of fluid, if not, replace the pipe or install vertically .

2. The signal is getting smaller or lower suddenly.

Test whether the insulation between the two electrodes is damaged or short-circuited, the resistance between the two electrodes is normally between (70 ~ 100) Ω; dirt may be deposited on the inner wall of the measuring tube, the electrode should be cleaned and wiped, do not scratch the lining. Check whether the lining of the measuring pipe is damaged, and replace it if damaged.

3. The zero point is unstable. 

Check whether the medium is full of the measuring tube and whether there are bubbles in the medium, if there are bubbles, you can install a deaerator upstream, if the horizontal installation can be changed to vertical installation; check whether the instrument grounding is intact, if not good, three Grade grounding (grounding resistance 100Ω); check the conductivity of the medium should not be less than 5μs/cm; check whether the medium is deposited in the measuring tube, take care not to scratch the lining when removing.

4. The flow indication value does not match the actual value. 

Check whether the fluid in the sensor is full of the tube and whether there are bubbles, if there are bubbles, you can install a deaerator upstream; check whether the grounding is good; check whether there is a valve upstream of the flowmeter, if it is, move it to the downstream or make it fully open ; Check whether the range setting of the converter is correct. If it is not correct, set the correct range again.

5. The indicated value fluctuates in a certain range. 

Check whether the environmental conditions have changed, such as new interference sources and other magnetic sources or vibrations that affect the normal operation of the instrument, you should clear the interference or shift the flowmeter in time; check the test signal cable and use insulating tape for end treatment to make The wires, inner shielding layer, outer shielding layer, and housing are not in contact with each other.

 

The fluid selected by the electromagnetic flowmeter to measure the flow must be conductive, so non-conductive gas, steam, oil, copper and other substances cannot be measured by the electromagnetic flowmeter.

 

Failure during operation

 

After initial commissioning and normal operation for a period of time, the failures that occur during operation, the common causes of failure are: the adhesion layer of the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions.

 

1. Inner wall adhesion layer

Because the electromagnetic flowmeter has far more opportunities to measure suspended solids or dirt than other flow meters, the probability of failure caused by the inner wall adhesion layer is relatively high. If the conductivity of the adhesion layer is close to the conductivity of the liquid, the instrument can still output signals normally, but the flow area is changed to form a hidden fault of measurement error; if it is a high conductivity adhesion layer, the electromotive force between the electrodes will be short-circuited; if it is an insulating adhesion layer , The electrode surface is insulated and disconnects the measuring circuit. Both of the latter two phenomena will make the instrument inoperable.

 

2. Lightning strike

Lightning strikes induce instantaneous high voltage and inrush current in the line, and entering the instrument will damage the instrument. There are three ways to introduce the lightning damage instrument: power line, flow signal line and excitation line between the sensor spoon converter. However, from the analysis of the damaged parts in the lightning fault, most of the induced high voltage and inrush current caused by the fault are introduced from the power supply line of the control room, and the other two ways are less. It was also learned from the scene of lightning strike accidents that not only the electromagnetic flowmeter failed, but also other instruments in the control room often experienced lightning strike accidents at the same time. Therefore, the user should recognize the importance of setting up lightning protection facilities for the instrument power cord of the control room.

 

3. Changes in environmental conditions

The main reason is the same as the fault environment during the commissioning period in the previous section, except that the interference source does not appear during the commissioning period but is intervened during operation. For example, an electromagnetic flowmeter with grounding protection is not ideal. During the commissioning period, there is no factory disturbance source, and the instrument operates normally. However, during the operation period, a new interference source (such as pipeline welding near the measurement point or a remote pipe is welded) interferes with the instrument. During operation, the output signal fluctuates greatly.

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