Application of guided-wave radar level meters in drum level measurement

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Overview: Steam drums are important equipment in industrial processes such as petrochemicals and power generation. Maintaining a stable liquid level is a crucial condition for the safe operation of steam drums. The guided-wave radar level gauge with weather compensation overcomes the disadvantages of differential pressure level gauges, float level gauges, and electric contact level gauges; it requires minimal maintenance and offers accurate and reliable measurements. The current status of drum level measurement: At present, based on the fundamental principles of drum level measurement, the methods that are widely used are those based on the principle of continuity and the principle of pressure difference. The instruments used for measuring the level of water in a steam drum include differential pressure level gauges, float level gauges, and guided wave radar level gauges, among others. 1. Differential pressure drum level gauge. The measurement principle of the differential pressure drum level gauge is to convert changes in liquid level into changes in differential pressure in order to determine the level. This conversion is achieved through a balance chamber that creates a water column; the key to accurate level measurement with this gauge lies in the precise conversion between the liquid level and the differential pressure. The differential pressure drum level gauge offers high accuracy and stability, along with a low failure rate during operation and minimal maintenance requirements. However, the errors associated with this measurement method are related to the drum pressure and the temperature of the reference water; it is necessary to calibrate the drum regularly, and the compensation calculations are complex. In addition, the effects caused by changes in the temperature of the balance vessel must also be taken into account. 2. Float level gauge. A float level gauge operates on the principle of buoyancy. When the level gauge is at position 0, the torsion tube experiences the maximum torsional torque resulting from the neutral position of the float, and the angle of the torsion tube is 0°. As the liquid level gradually rises to its maximum, the torsion tube experiences a torque due to buoyancy, causing it to rotate by a certain angle. The transmitter converts this angle into a 4–20 MA DC signal, which is proportional to the measured liquid level. With this method of measurement, changes in the density of the medium can affect the accuracy of the readings, and mechanical vibrations can also lead to inaccurate results. 3. Electric contact level gauge. The electric contact level gauge belongs to the type of level gauge that uses a communicating tube; its principle involves measuring the liquid level by taking advantage of the fact that the impedance between the electrodes submerged in the liquid inside the boiler is low, while the impedance between the electrodes in the steam is high. The electrical conductivity of the water in high-pressure boilers is generally tens of thousands to hundreds of thousands of times higher than that of saturated steam; as a result, the indication values from the electric contact switches are less affected by changes in the pressure in the air vessel, allowing for easy transmission of liquid level signals. However, it has the disadvantages of poor reliability of the sampling sensors, easy leakage in the motor’s mechanical seal, short service life of the electrodes, discontinuous indication, and high maintenance requirements. In summary, due to the complexity of the object being measured in terms of drum liquid level, as well as the uncertainties in actual operation and significant measurement errors, the readings from drum level gauges often exhibit large deviations. The guided wave radar level gauge measurement is a completely new measurement technique that overcomes the shortcomings of differential pressure, float-type, and electric contact type level measuring instruments, fully meeting the requirements for measuring the level in steam drums. Measurement principle and characteristics of guided wave radar level gauges 1. Measurement principle. The guided wave radar level gauge is a type of radar level gauge that operates on the principle of reflection. Electromagnetic pulse signals travel along the steel cable at the speed of light; when they encounter the medium being measured, some of these pulses are reflected to form echoes, which then return along the same path to the pulse transmitter. The distance between the transmitter and the surface of the measured medium is proportional to the time it takes for the pulses to travel between them, and the level height can be determined through calculations. 2. Features. The advantages of guided-wave radar level gauges are stable signals, measurement that is not affected by the liquid’s density or electrical properties, high accuracy, ease of measurement and calibration, low installation costs, and easy maintenance. 3. Selection and installation requirements for guided-wave radar level gauges – Selection. A guided wave radar level gauge relies on sensors to emit electromagnetic waves; therefore, the selection of these sensors is an important aspect in choosing a guided wave radar level gauge. The sensors of guided-wave radar level gauges come in three types: rod-type, loop-type, and coaxial-type. Rod-type sensors are usually chosen. When the measurement range is large, and since transportation and installation remain unchanged, it is recommended to use a wrap-around sensor. Installation. The installation of guided-wave radar level gauges requires consideration of factors such as installation requirements, container characteristics, and process connections. The main installation methods are as follows: top mounting or side mounting. When installing a guided wave radar level gauge in one of the two possible ways, it is important to ensure that there is an appropriate distance between the guided wave radar and the surface being measured ； Avoid obvious obstacles beneath the instrument sensor, as these can prevent radar waves from reaching the surface of the medium being measured properly ； Do not install the waveguide rod near the feed inlet ； There should be a certain distance between the sensor and the bottom of the device; it must not come into contact with the bottom of the tank. 4. Gas phase compensation technology (GPC). Under high temperature and pressure conditions, the propagation speed of electromagnetic wave signals in the steam above the medium decreases, causing the radar-measured liquid level to drop. By using a guided-wave radar with vapor compensation, and applying the vapor compensation function to adjust the measured values, an accurate actual liquid level can be obtained. Application of guided wave radar level gauges in drum level measurement: On the drum of a certain boiler installation, which is a key component of the steam generation system, high-temperature heat from the flue gas section of the converter and the residual heat from the converted gas at the furnace outlet are utilized to produce steam at 10.5 MPa pressure. Part of this steam is used as process steam for reactions, while the rest is sent to the high-pressure steam network, thereby enabling comprehensive utilization of energy and improving the operational efficiency of the installation. Due to the importance of the drum in boiler systems, three different types of level measuring instruments have been used to measure the drum level: differential pressure level gauges, conventional guided wave radar level gauges, and guided wave radar level gauges equipped with GPC functionality. As can be seen from the figure below, actual measurements show that at high temperatures, the error of conventional guided wave radar is as high as 18%, while with GPC the measurement error is only 2%. Guided wave radar level gauges equipped with the GPC function provide relatively stable and accurate measurement results at high temperatures. Comparison of measurement data from three types of instruments: It can be concluded that the guided wave radar level gauge with GPC functionality exhibits significantly superior performance in high-temperature and high-pressure environments compared to other types of level gauges. It is not affected by process conditions, requires less maintenance, and offers reliable performance. It is the best choice for measuring the drum liquid level.