Error and Application of Liquid Level Transmitter

Abstract: This paper introduces the working principle and operation method of the common liquid level transmitter of generator set, analyzes the measurement error appearing in the actual operation of the unit and makes a solution.

1 Introduction

        In the country's major power plants, the degree of automation is getting higher and higher, and the use of liquid level gauge to measure and monitor the water level of the container is reasonable and accurate, which is very important for the automatic operation and safe operation of the unit operation. Such as condenser water level, boiler drum liquid level, heater water level, deaerator water level, etc. In the newly started plant, the magnitude and frequency of changes in various liquid level measurement values ​​are relatively large, which will mislead the operation of the operating personnel and affect the automation input rate. Therefore, for thermal commissioning personnel, it is very important to properly commission and put into operation equipment. In large power plants, commonly used liquid level gauges include capacitive, differential pressure, input, float, ultrasonic level transmitters, etc. The following are some typical measurement methods and principles, for example, The problems encountered in the actual application and the introduction of the operation process.

2. Differential pressure level transmitter

2.1 Working principle
        The working principle of the differential pressure transmitter is to convert the height difference of the liquid level into a pressure difference, and then through the secondary conversion, the 4-20 mA analog signal is transmitted to the CRT for monitoring by the operating personnel.
According to the pressure calculation formula, the following calculation formula can be obtained:
        P+=ρgL P- =ρ2gH+ρ1(L-H)g
Therefore, the calculation formula of the positive and negative pressure side differential pressure is as follows:
        P= P+ - P- =ρgL-[ρ2gH+ρ1(L-H)g]=gL ( ρ-ρ1 ) - gH ( ρ2-ρ1 )
        L: The distance between the pressure points on the positive and negative pressure side. ρ: The positive pressure side measures the density of condensed water in the tube.
        Ρ2: Density of water in the container being measured (when the unit is in normal operation).
        Ρ1: Density of steam in the vessel being measured (when the unit is in normal operation).
        H: Height of the measured water level.

        Therefore, using the above differential pressure transmitter to measure the water level, the relative accuracy is higher, which is conducive to the water level adjustment of the unit under normal working conditions, which is conducive to the automatic control of the thermal control. Generally used for boiler drum water level, deaerator water level measurement. However, there are certain factors that affect the accuracy of its measurements.

2.2 Influencing factors and solutions

2.2.1 Pressure effects and solutions
        When the unit starts or stops, the water level changes greatly, and the pressure in the measured container also changes relatively. When the water level is high, the pressure inside the container is relatively high, especially the negative pressure, the P-pressure value changes. It is very large, and it will cause △ P to change, but the rate of change of ρ1 and ρ2 is different, and the direction of change is opposite. When the water level H rises, P- increases with the decrease of the working pressure of the container, and Δ P decreases. Small, the water level measurement has a positive error. However, when the water level H is low, the impact is not great. The solution is as follows:
        (1) The DCS function is used for automatic compensation. According to the relationship between the working pressure and (ρ2-ρ1), the differential pressure value of the output is corrected, which is compensated by digital calculation.
        (2) In the positive pressure, the expansion container is connected with the equilibrium container to be tested, so that the positive pressure side head is always filled with saturated steam, and the vaporization heat energy released when the saturated steam is condensed is continuously absorbed, thereby ensuring the positive pressure side measuring tube. The temperature is consistent with the temperature of the negative pressure side, which does not cause the temperature difference to be too large, so that the relative change rate of the soda water density can be ensured.

2.2.2 Temperature effects and solutions
        In most power plants, the positive and negative measuring pipelines of the differential pressure transmitter are long. At normal temperature, the measuring pipeline radiates outwards quickly, and the farther away from the pressure point, the lower the temperature, causing the temperature to rise from above. The unevenness of the sub-segment leads to the deviation of the differential pressure, and the water level display is basically low. Depending on the environment, the impact is different. The relative impact is relatively small in southern China, but in the north, especially in winter, the impact is particularly obvious. Therefore, only the measuring pipeline can be insulated to solve the problem. Pay attention to the following points when keeping warm:
        (1) In southern China, such as Guangdong, basically no heat preservation is required. The temperature is relatively high all the year round. Just install the differential pressure transmitter in the incubator, set the heating device, and encounter the weather. When the temperature is lowered, it is put into heating and decoration; (2) In the central part of China, the instrument pipeline insulation should be carried out. However, the measuring cylinder and the expansion vessel must not be insulated, otherwise the steam cannot be condensed into water, resulting in unstable measurement of water level output. , CRT shows jump; (3) In the north, such as Beijing, Ningxia and other places, the instrument pipeline must be insulated, and the heating cable should be installed, and the transmitter should be installed in the incubator, and the incubator should be added. Install the heating thermostat.

2.3 Investment methods and precautions in cold state

2.3.1 Before the application, confirm that the positive and negative pressure pipelines are not crossed, the primary valve, the secondary valve and the blowdown valve are all closed, and the balance valve is opened.
2.3.2 Compare the two-color water level gauge or magnetic flap to find the relative position of the water level center point, mark and record.
2.3.3 Measure the actual range with a tape measure to ensure that the transmitter range setting is consistent with the CRT.
2.3.4 It is necessary to wait for the container to finish rinsing before starting the first operation to avoid water and block the pipeline.
2.3.5 After the container is full of water, first open the positive and negative pressure side primary valve, then open the drain valve. After the discharged water is clean, close the drain valve, open the secondary valve, and fill the measuring tube. At this time, it is best to put The vent hole of the positive pressure side transmitter is slightly opened, and the pipe is tapped with a tool to reduce the presence of bubbles in the pipeline. After the water injection is finished, use a multimeter to measure the output current of the transmitter. If the difference is 4 mA, the water in the measuring tube is not filled, and the water should be injected continuously.

2.3.6 Measuring cylinder If there is an exhaust valve or hole, water can be injected manually.
2.3.7 Connected from the condensate pipe to the water injection port of the measuring cylinder, it can stabilize the automatic water injection (suitable for the deaerator, condenser, low-pressure heater used when measuring with differential pressure level transmitter).
2.3.8 After the water is filled, close the balancing valve and the transmitter is ready for operation. Let the operator discharge water to the container under test. If the current output drops, the transmitter is successfully put into operation.
2.3.9 During the operation of the unit, pay attention to the inspection to ensure that there is no leakage at the connection of the drain valve and each joint.

2.4 Application methods and precautions in hot state
        When the unit is running, especially under high temperature and high pressure, the liquid level will often jump and be inaccurate. At this time, the transmitter needs to be re-operated. First, the water level logic protection should be removed.
        2.4.1 Liquid level display jump, generally can be eliminated by the following methods:
Use a small live-mouth wrench to slowly open the positive and negative vents of the differential pressure transmitter, and exhaust the steam. You can see white and milky white bubbles flowing out. The sewage is discharged to the positive and negative pressure sides, and the sewage discharge time should not be too long to avoid damage to the valve core. When re-commissioning, first apply negative pressure and then positive pressure, but the interval should not be too long, so as to avoid damage to the transmitter pressure sensitive component or balance valve spool.
        2.4.2 Liquid level display deviation, possible causes:
        (1) There are leakages on the positive and negative pressure sides of the measuring tube or the drain valve; (2) the pressure sensitive unit of the transmitter is damaged; (3) the valve balance valve is damaged; (4) the transmitter range Inconsistent with CRT; (5) zero drift; (6) signal loop insulation degradation.

2.5 Actual operation case
        The high, medium and low pressure steam drum liquid level of Beijing Caoqiao Phase II boiler is measured by 4 differential pressure transmitters and a two-color water level gauge. The transmitter is installed in the lower layer of the steam cooler, when the boiler water is rinsed clean. After the first application (the positive and negative pressure tubes are not insulated), the transmitter is put into normal use. (Before this, the positive and negative pressure pipelines confirmed that there is no crossover. Compared with the two-color water level gauge, the water level center point and the range have been Pull the level, measure and record). But slowly, the water level begins to deviate. When it is missing, it is found that some of the sewage valves are not dead or leaky, and the problem is solved after the treatment.

        However, after the unit is ignited and warmed up, the water level begins to change and re-use, and the problem remains. Re-examine the pipeline and find that all the pipelines from the pressure point to the drain valve are all insulated (the temperature in the north is low in winter, the owner requires the pipeline to be insulated), the water vapor in the positive pressure tube cannot be condensed smoothly, causing the water level to jump, and the positive pressure measuring cylinder is removed. The problem is solved after the insulation.

3. Capacitive level gauge

3.1 Working principle
        Capacitive level gauges measure the change in capacitance to measure the level of the liquid level. It is a metal rod inserted into a liquid container, the metal rod acts as a pole of the capacitor, and the wall of the container acts as the other pole of the capacitor. The medium between the two electrodes is the liquid and the gas above it. Since the dielectric constant ε1 of the liquid and the dielectric constant ε2 on the liquid surface are different, for example, ε1>ε2, the total dielectric constant value between the two electrodes of the capacitive liquid level gauge increases as the liquid level rises. Therefore, the capacitance is increased. Conversely, as the liquid level drops, the ε value decreases and the capacitance decreases.

3.2 Checking and Dosing Precautions
        The capacitive level gauge is mainly used to calibrate the zero and full scale of the liquid level. When the liquid level of the container is zero, the output signal of the capacitive liquid level gauge is set to 4 mA, corresponding to zero liquid level. Put
The measuring container is filled with the liquid level, and the output signal is set to 20 mA, corresponding to the full liquid level. Capacitive liquid level gauges vary according to the manufacturer and model. The specific parameters are set differently. As long as they are strictly set according to the manufacturer's data, the measurement is more accurate when it is used normally.

3.3 Actual operation case
        In the second phase of Beijing Caoqiao Project, the oil level of the main fuel tank was measured by E+H and FMI51 capacitive liquid level meter produced by Siemens. It was not calibrated according to requirements. Because the main fuel tank was oily and drained, the oil level was difficult to grasp. Only when the fuel tank is not zero, the measurement error is large. Finally, we made a small container filled with the oil from the oil tank. According to the instructions, the liquid level gauge is placed in the container. The problem is solved after the simulated fuel tank oil level is calibrated to zero and full scale.

4 Conclusion
        Through the above analysis of the application of the liquid level meter and the solution of the problem, it is proved that the analysis and the solution to the problem are correct, and the future work has been used for reference, so that the future work will be less detoured.
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