Thread Content
Abstract: This paper introduces the working principle and operation methods of commonly used level transmitters in generator sets, analyzes the measurement errors that occur during the actual operation of the units, and proposes solutions. 1. Introduction In power plants across the country, the level of automation is increasing steadily. It is therefore very important to use level gauges to measure and monitor the water level in containers accurately and reliably, as this is crucial for the automated operation and safe functioning of the units. Such as the condenser water level, boiler drum water level, heater water level, deaerator water level, etc. During the initial startup phase of the unit, the amplitude and frequency of changes in various liquid level measurements are relatively large, which can mislead the operators and affect the level of automation in operation. Therefore, for thermal commissioning personnel, properly commissioning and putting the equipment into operation is of great importance. In large power plants, common level gauges include capacitive, differential pressure, submersible, float-type, and ultrasonic level transmitters. Below, some typical measurement methods and principles are analyzed, illustrated with examples, and the problems encountered in practical use as well as the commissioning process are described. 2. Differential pressure level transmitter 2.1 Working principle The working principle of a differential pressure transmitter is to convert the changing height difference of the liquid level into a pressure difference; through further conversion, this pressure difference is transformed into a 4-20mA analog signal, which is then transmitted to a CRT for operators to monitor. Based on the figure below, conduct a detailed analysis and write down the formulas. http://www.tx7878.cn/d/file/hynews/2017-12-18/c564b2e19aec49b76bc33192819429de.jpg Based on the pressure calculation formulas, the following equations can be derived: P+ = ρgL and P- = ρ2gH + ρ1(L-H)g. Therefore, the formula for calculating the differential pressure between the positive and negative pressure sides is as follows: P = P+ – P- = ρgL – [ρ2gH + ρ1(L-H)g] = gL(ρ – ρ1) – gH(ρ2 – ρ1). Here, L represents the distance between the pressure measurement points on the positive and negative pressure sides. ρ: Density of condensate water in the measuring tube on the positive pressure side. ρ2: Density of water inside the container under measurement (when the unit is operating normally). ρ1: Density of the steam inside the container under measurement (when the unit is operating normally). H: The height of the measured water level. Therefore, using the above differential pressure transmitter to measure water level provides relatively high accuracy, which facilitates water level adjustment of the unit under normal operating conditions and supports the automation of thermal control systems. It is generally used for measuring the water level in boiler drums and deaerators. However, there are also certain factors that affect the accuracy of its measurement. 2.2 Influencing Factors and Solutions 2.2.1 Pressure Effects and Solutions When the unit starts up or shuts down, the water level changes significantly, and the pressure inside the container being measured also changes considerably. When the water level is high, the pressure inside the container is relatively high as well; in particular, in the case of negative pressure, the P-value changes greatly, which in turn causes △P to change as well. However, the rates of change for ρ1 and ρ2 are different, and they change in opposite directions. As the water level H increases, P increases as the operating pressure of the container decreases, resulting in a decrease in △P, and this leads to a positive error in water level measurement. However, when the water level H is low, the impact is not significant. The solutions are as follows: (1) Utilize the DCS function for automatic compensation; by considering the relationship between the operating pressure and ﹙ρ2-ρ1﹚, the output differential pressure value is corrected through digital calculations, thereby achieving compensation. (2) Under positive pressure, the expansion vessel is connected to the balance vessel under test, so that the head section on the positive pressure side remains always filled with saturated steam. This allows it to continuously absorb the heat of vaporization released when the saturated steam condenses, thereby ensuring that the temperature inside the measuring tube on the positive pressure side remains relatively consistent with that on the negative pressure side, preventing large temperature differences. In this way, the rate of change in the density of the steam and water can be kept relatively balanced. 2.2.2 Effects of temperature and solutions In most power plants, the positive and negative measurement pipelines of differential pressure transmitters are relatively long. At normal temperatures, heat dissipates rapidly from these measurement pipelines, and the farther away they are from the pressure sampling point, the lower the temperature becomes. This results in uneven temperature distribution from top to bottom, leading to deviations in pressure difference conversion; as a result, the water level reading is generally on the low side. The impact varies depending on the environment; it is relatively minor in southern China, but it is particularly noticeable in the north, especially during winter. Therefore, insulating the measurement pipelines is the only way to resolve this issue. The following points should be noted when providing insulation: (1) In southern China, such as Guangdong, insulation is generally not necessary as the temperatures remain relatively high throughout the year. It is sufficient to install the differential pressure transmitter inside an insulated box, along with a heating unit that can be activated in case of sudden temperature drops; (2) In central China, it is necessary to insulate the instrument pipelines, but it is crucial not to insulate the measurement cylinders and expansion tanks, as this will prevent steam from condensing into water, resulting in unstable readings of the water level and fluctuations on the CRT display; (3) In the north, such as Beijing and Ningxia, the instrument pipelines must be insulated, and heating strips should also be installed. The transmitter should be placed inside an insulated box, which in turn should be equipped with a heating control device. 2.3 Methods of commissioning in the cold state and precautions 2.3.1 Before commissioning, ensure that there are no intersections between the positive and negative pressure pipelines; all primary valves, secondary valves, and drain valves must be closed, while the balance valve should be open. 2.3.2 Using a reference dual-color water level gauge or magnetic flip plate, determine the relative position of the water level center point, and make markings and records. 2.3.3 Use a tape measure to determine the actual range, ensuring that the transmitter’s range setting matches that of the CRT. 2.3.4 It is essential to wait until the container has been rinsed completely before starting its first operation, in order to avoid dirty water that could clog the pipelines. 2.3.5 Once the container is filled with water, first open the primary and secondary valves on the positive and negative pressure sides, then open the drain valve. Once the water discharged is clean, close the drain valve and open the secondary valve to fill the measurement tube with water. At this point, it is advisable to slightly open the vent hole of the transmitter on the positive pressure side, and simultaneously tap the pipeline gently using a tool to reduce the accumulation of bubbles in the pipeline. After the water injection is complete, use a multimeter to measure the output current of the transmitter; if it differs significantly from 4mA, it indicates that the measuring tube is not fully filled with water, and further injection is required. 2.3.6 If the measuring cylinder has an exhaust valve or hole, water can be added manually. 2.3.7 A pipe is led from the condensate water to the water injection port of the measuring cylinder, enabling stable automatic water injection (suitable for use when differential pressure level transmitters are employed for measuring in deaerators, condensers, and low-pressure heaters). 2.3.8 Once the water is filled, close the balance valve; at this point the transmitter is in operation. Have the operator drain water from the container under test; if the current output decreases, it indicates that the transmitter has been successfully activated. 2.3.9 During the operation of the unit, conduct regular inspections to ensure that there are no leaks in the drain valves and at all connection points. 2.4 Method of putting into operation in hot condition and precautions: When the unit is operating, especially under high temperature and pressure, the liquid level often experiences fluctuations and inaccuracies; in such cases, it is necessary to restart the transmitter. First, the water level logic protection must be disabled. 2.4.1 Level display jumps can generally be eliminated using the following method: Use a small adjustable wrench to slowly open the positive and negative vent holes of the differential pressure transmitter slightly, allowing steam to escape; white or milky bubbles will be visible flowing out. Drain the pressure and vacuum sides again; the drainage time should not be too long to avoid damaging the valve core. When restarting operation, apply negative pressure first and then positive pressure, but the interval should not be too long to prevent damage to the pressure-sensing elements of the transmitter or the spool of the balance valve. 2.4.2 Deviation in liquid level display; possible causes: (1) Leaks at the connections on the positive and negative pressure sides of the measuring tube, or at the drain valves; (2) Damage to the pressure-sensing element of the transmitter; (3) Damage to the spool of the transmitter’s balance valve; (4) Mismatch between the transmitter’s range and that of the CRT; (5) Zero-point drift; (6) Decreased insulation in the signal circuit. 2.5 Actual Operation Example: In the second phase of the Caqiao boiler in Beijing, the liquid levels in the high-, medium-, and low-pressure drum levels were measured using 4 differential pressure transmitters and one dual-color level gauge. The transmitters were installed inside insulation boxes located below the drums. When the boiler was cleaned out and put into operation for the first time (with no insulation on either the pressure-taking pipes), the transmitters functioned properly (prior to this, it was confirmed that the pressure pipes did not intersect; the center point of the water level as well as the measurement range were adjusted to be horizontal, and relevant measurements were recorded). But gradually, the water level began to deviate significantly. During troubleshooting, it was found that some sewage valves could not be closed properly or were leaking internally; the issue was resolved after corrective action was taken. However, after the unit was ignited and the temperature and pressure were increased, the water level began to fluctuate; upon restarting the unit, the problem persisted. Upon rechecking the pipelines, it was found that all pipes from the pressure sampling point to the drain valve were insulated (due to the low temperatures in winter in the north, the client required insulation for the pipes). As a result, the water vapor in the positive-pressure pipe could not condense properly, causing fluctuations in the water level. The problem was resolved after removing the insulation from the positive-pressure measurement tube. 3. Capacitive level gauge 3.1 Working principle The capacitive level gauge measures the height of the liquid level by detecting changes in capacitance. It is a metal rod inserted into a container filled with liquid; the metal rod serves as one pole of the capacitor, while the container wall acts as the other pole. The dielectric between the two electrodes is the liquid and the gas above it. Since the dielectric constant ε1 of the liquid is different from that ε2 at the liquid surface – for example, ε1 > ε2 – as the liquid level rises, the overall dielectric constant between the two electrodes of the capacitive level gauge increases, thereby increasing the capacitance. Conversely, as the liquid level drops, the ε value decreases, and the capacitance also decreases. 3.2 Verification and Operational Considerations The capacitive level gauge primarily requires calibration of its zero and full-scale points according to industry standards; when the liquid level in the container is at zero, the output signal of the capacitive level gauge should be set to 4mA, which corresponds to a zero liquid level. Fill the container under test to the full liquid level, and set the output signal to 20mA, which corresponds to the full liquid level. Depending on the manufacturer and model, capacitive level gauges have different specific parameters; as long as the settings are made in strict accordance with the manufacturer’s specifications, accurate measurements can be obtained during normal operation. 3.3 Actual Application Case: For the second phase of the Beijing Caoqiao project, the oil level in the main tank was measured using a capacitive level gauge manufactured by E+H, model FMI51, produced by Siemens. Initially, calibration was not carried out as required; since it was difficult to fill and empty the main tank and it was hard to determine the oil level accurately, calibration was done only when the tank was not at zero level. As a result, significant measurement errors occurred. Eventually, we created a small container filled with lubricating oil taken from the tank, and following the instructions, we placed the level gauge inside the container to simulate the oil level in the tank, thereby resolving the issue by calibrating the zero and full scale points. 4. Conclusion Through the above analysis of the operation of the level gauge and the methods used to solve problems, it has been proven that these approaches are correct. They serve as a reference for future work, helping to avoid unnecessary detours in subsequent tasks.