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Capacitive Liquid Level Meter In Chemical Urea System

Jan 22, 2019


Application and Improvement of Capacitive Liquid Level Meter in Chemical Urea System



【Abstract】This paper analyzes the cause of the damage of the capacitive liquid level meter. According to its working principle and composition, it has been modified accordingly. After operation, it is stable, accurate and effective.


The urea system of Luoyang ** Chemical Industry Co., Ltd. currently uses four sets of capacitive liquid level meters supplied by Huaheng Instruments to measure the liquid level of important equipment. The level meter has the advantages of stable performance, reliable operation and accurate indication. Since the launch, the level meter has also had some problems that have affected normal process operations. This paper analyzes the faults that occur in the use of capacitive level meters and describes their improvements.

Application and Improvement of Capacitive Liquid Level Meter in Chemical Urea System

1 Basic structure and working principle of Capacitive Liquid Level Meter


Capacitive Liquid Level Meter

    As shown in Figure 1, the capacitive level meter consists of a capacitive sensor (electrode) and a measuring circuit for measuring capacitance (EC11Z and FMC420). The electrode converts the level of the liquid level into a change in capacitance, and then the size of the capacitor is detected by EC11Z and FMC420, and converted into a DC current signal output, thereby achieving continuous measurement of the liquid level, remote transmission indication, recording or automatic control. . Basic structure of capacitive level meter.

2 Original structural defects of Capacitive Liquid Level Meter

    The original electrode is made of stainless steel spring piece and coated with 2 mm enamel insulation. This structure has the following disadvantages:

    (1) The enamel insulation layer coated on the outside of the electrode is relatively brittle, not resistant to high temperature and high pressure, and cannot withstand scouring. From the damaged three electrodes, the insulation layer has different degrees of scratches, and the most severe one has two insulation layers that have been severely damaged.

    (2) The electrode is used once and cannot be repaired.

    (3) The flange structure is connected with the equipment. Due to the high temperature and large temperature difference of the tested equipment, the sealing gasket is excessively compressed and deformed, which is easy to cause leakage. After heating, tighten the bolt again to prevent leakage. In addition, due to the action of high temperature and high pressure for a long time, it is difficult to remove the inspection.

    (4) The FMC420 adopts 220 V (AC), the power supply voltage is high, and the on-site installation is not safe.

    (5) The output 0 to 20 mA (DC) is not an international standard signal, and DCS control cannot be realized.


3 Improvement measures of Capacitive Liquid Level Meter


    (1) The electrode is made of stainless steel rod, and the outer part is made of Teflon plastic tube as insulation. After the PTFE insulation layer is damaged, the electrode can be re-insulated and used after repair.

    (2) It is connected by internal and external threads and is easy to disassemble.

    (3) Sealing gasket (expanded graphite metal winding mat) with better rebound ability is used to ensure the sealing effect.

    (4) Change 220 V (AC) to 24 V (AC) for on-site installation and eliminate danger.

    (5) Changed to 4 to 20 mA (DC) international standard signal output, automatic control in DCS system.


4 Electrode improvementof Capacitive Liquid Level Meter


        The 600 mm long stainless steel electrode rod is covered with a PTFE insulated tube, and the electrode rod is thermoplastically insulated by an alcohol burner, and the electrode and the externally threaded joint are sealed with a high pressure resistant and high temperature resistant graphite filler. The fabricated electrode was subjected to a pressure test of 9.81 MPa for 24 h in a closed container and was used without leakage.

Application and Improvement of Capacitive Liquid Level Meter in Chemical Urea System

5 Adjustment  of Capacitive Liquid Level Meter


        Calibration is performed on the FMC420 with simple, reliable, accurate and fast settings. Set the zero point with an empty container, set the range with the full container, zero check and span calibration do not affect each other.


5 .1 basic position

    (1) Turn the adjustment knob for zero adjustment and range adjustment counterclockwise to the left end point.

    (2) On the range switch board, transfer the 3 lower switches to the right and the 3 upper switches to the left.

    (3) Turn the zero switch knob to F.

5 .2 zero calibration

    (1) Make the measured container liquid level zero.

    (2) Monitor with a digital multimeter and adjust the zero coarse adjustment so that the output is close to 0 V.

    (3) Adjust with the fine adjustment of the zero position so that the universal value is exactly 0 V.

5 .3 range check

    (1) Fill the container under test.

    (2) Adjust the range coarsely to make the universal value close to 10V.

    (3) Fine-tune the range so that the output is exactly 10 V.

6 Improved advantages

    (1) Easy to overhaul.

    (2) The seal is firm and there is no leakage.

    (3) It is cheap and easy to make.

    (4) The indication is accurate.


7 Faults and handling of Capacitive Liquid Level Meter


    During the application of the capacitive level meter, the fault mainly occurs in the following aspects:

    (1) The electrode is damaged. This type of failure is the main failure. The urea system was stopped because of 3 electrode failures. Therefore, how to improve the service life of the electrode is the key. After the transformation, the electrode has been connected for 4 years, indicating that the transformation effect is better.

    (2) The indication is malfunctioning. This type of failure mainly occurs on the wiring and damage to the EC11Z and FMC420. As long as the line is carefully checked, the EC11Z or FMC420 is found to be faulty and replaced immediately, and the fault can be eliminated.