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This post was last edited by kareale88 on 2019-7-2 23:02. Summary: Accurate measurement, stable control, and reliable operation of the boiler drum level are important guarantees for the safe operation of boilers. Through a comparison of the advantages and disadvantages of several drum level gauges, it is concluded that magnetic flap level gauges, differential pressure level gauges, and electric contact level gauges should be given priority. Based on the actual conditions, it is then determined whether compensation for the drum level is necessary, and whether complex control methods such as three-transmission control should be used. Accurate measurement, stable control, and reliable operation of the boiler drum level are important guarantees for the safe operation of the boiler. Improper selection and installation of level gauges can lead to their explosion, leakage of high-temperature steam, inaccurate level measurement, or even complete failure. An excessively high or rapidly fluctuating drum liquid level can lead to a deterioration in steam quality and the presence of water in the steam, resulting in salt deposition on the heating surfaces. In severe cases, this can cause water hammer vibrations in the turbine and damage to its blades. A too low liquid level can result in ineffective slag removal, with chemicals added to the furnace ending up in the steam; it can also cause vapor to enter the downcomers, affecting the circulation of furnace water and leading to widespread damage to the furnace tubes. Severe accidents caused by the drum liquid level still occur from time to time, or there are ongoing hazards. 1 Measurement of the boiler drum liquid level 1.1 Drum level gauges For the design of drum level gauges, magnetic flap level gauges and differential pressure remote transmission level gauges are generally used. However, some people believe that: ① Although magnetic float level gauges indirectly measure the liquid level, they can withstand high temperatures and pressures, have a fully metallic structure, a long service life, good sealing properties, and are less prone to leakage; ② High-pressure glass plate level gauges should be phased out in medium-pressure boilers due to their high cost, numerous leakage points, and difficulty in maintenance; ③ Electrical contact level gauges become unreliable over time as they are susceptible to corrosion and scaling; ④ Differential pressure transmitters are particularly affected by changes in pressure and steam volume within the boiler drum, requiring frequent calibration of their zero point and range. They also entail a lot of maintenance work, and it is even argued that the uncertainties and workload associated with this measurement method do not meet the requirements of production and safety; ⑤ In capacitive level gauges, the capacitance measurement plates are vulnerable to scaling caused by water quality. Moreover, during boiler operation, steam volume, temperature, and pressure change rapidly depending on operating conditions, and the dielectric constant of the water-steam mixture also changes continuously. Manual compensation methods cannot eliminate these issues completely, leading to inaccurate level readings [1]. It has been suggested to use the on-site magnetic flap level gauge as a basis, and to add video monitoring and remote display functions in order to enable accurate monitoring of the boiler drum water level from the control room. The electric contact level gauge should be replaced with a magnetic expansion type gauge for measuring the boiler drum level. The differential pressure gauge used for this purpose should also be replaced with an intelligent guided-wave radar level gauge. An intelligent capacitive level gauge can provide precise compensation for changes in the dielectric constant of both the liquid and vapor phases, as well as for temperature changes. It offers accurate and continuous level measurement under all operating conditions (boiler startup, shutdown, blowdown, emergency situations, etc.). Such gauges are capable of withstanding high temperatures and pressures, have a long service life, and do not result in inaccurate \"false level\" readings. 1. Papers on drum level measurement and control: Papers related to drum level measurement and control continue to be published to this day. For example, \"A Brief Analysis of the Application of Fuzzy PID Control for Boiler Drum Level Control\" indicates that fuzzy PID control features rapid response times, lower overshoot, and higher tracking accuracy [2]; \"Selection and Calculation of Instruments for Boiler Drum Level Control in Chemical Plants\" presents two methods of level compensation: structural compensation and software compensation [3]; \"Drum Level Control Based on Single-Neuron Adaptive PSD\" enables dynamic adjustment of control parameters according to the operating conditions of the drum, thereby improving both the dynamic and static characteristics of the drum level [4]. For the control of the drum liquid level, the most common approach is three-pulse control; advanced control systems are also used. 1.3 Relevant regulations stipulate that the \"Safety Technical Inspection Regulations for Steam Boilers\" require that each boiler be equipped with at least 2 independently operating level gauges. The regulations do not specify what type of on-site level gauge should be used for the boiler drum, nor do they provide explicit requirements regarding control strategies. However, each steam boiler’s drum (shell) must be equipped with at least two independent direct-reading water level gauges; under certain conditions, the boiler may be fitted with only one direct-reading water level gauge. The regulations specify the inner diameter, length, slope, etc., of the steam and water connection pipes, and require that water level gauges in subcritical boilers correct for measurement errors caused by differences in liquid density. The \"Technical Specifications for the Water Level Measurement System of Boilers in Thermal Power Plants\" are specific regulations for boiler level gauges. Although they are intended for boilers operating at high pressures, their content is detailed and practical, with a strong emphasis on safety; therefore, they can serve as a reference for boilers operating at medium pressures and below as well. The regulations specify clearly that the steam drum must be equipped with local level gauges, differential pressure type level measurement devices, and electrode type level measurement devices, with a preference given to differential pressure type level gauges. The explanatory section stipulated further states that significant advances have been made in the technology of electrode-based water level measurement devices; extensive long-term use has proven their safety and reliability, as well as their ability to eliminate the influence of drum pressure and enable high-precision measurement of water levels at all times. Advances in balance vessel technology have ensured the accuracy and stability of differential pressure water level gauges, as well as the provision of water level protection during startup. The regulations place sufficient emphasis on safety and provide specific details, such as separating the water level protection and control functions of the drum, ensuring independent channels, and independent power supplies. No description is given for the control strategy. 2 Practical Applications of Drum Level Gauges Meiant Company has 2 sets of 400 kt/a sulfuric acid production plants. Among them, there are 2 HRS 1.0 MPa steam drums, each equipped with 1 magnetic flap level gauge, 1 high level alarm contact, and 1 differential pressure level gauge. Waste heat boiler: 2 steam drums at 4.0 MPa, each equipped with 2 quartz tube dual-color level gauges and 2 differential pressure level gauges for two-chamber balance vessels. The operation and renovation status of the drum level gauge are shown in Table 1. http://www.517mat.com/d/file/gsnews/2019-02-19/9b2f8f3ec5f505ca975200e0d3f97bea.jpg http://www.517mat.com/d/file/gsnews/2019-02-19/ae440b6896652465c92c5885808454e5.jpg Figure 1 shows a comparison of two differential pressure level gauges used in the same 4.0 MPa steam drum, one of which has been modified and the other not. Analysis of trend tracking shows that the level measured by the traditional differential pressure method may fail to function properly during the boiler startup phase due to shortcomings in design, installation, etc. (the 100% level shown in Figure 1 is actually an incorrect level); this issue usually disappears automatically as the drum pressure increases. The overlap point of the two level measurements in the instance starts at \"Point A\", with a corresponding pressure value of approximately 0.135 MPa. Due to the deceptive nature of this phenomenon, the issue of liquid level measurement during the pressure increase in the drum has not received the attention and resolution it deserves for a long time, especially with regard to the necessary adjustments in relevant design specifications. http://www.517mat.com/d/file/gsnews/2019-02-19/7d4b304884ab9ab7db8913336a34e9b6.jpg The effect of steam pressure fluctuations on the drum liquid level is shown in Table 1 and Figure 2. Testing method: First increase the drum pressure, then suddenly fully open the main steam line control valve (with the feedwater header still under pressure); switch level control from automatic to manual, and then observe the changes in relevant parameters. Test results show that fluctuations in steam flow within a certain range do not have an immediate and significant impact on liquid level measurement; the liquid level variation in the example was within 38 mm. There was also no significant time lag in level measurement, which may be attributed to the improvements made to the level gauge. http://www.517mat.com/d/file/gsnews/2019-02-19/b656d41b5c52b37698ae97d35f8f1c59.jpghttp://www.517mat.com/d/file/gsnews/2019-02-19/d2563b36eb474304071852182a37b3b1.jpg3 Discussion of issues1) Magnetic flap level gauges have a long service life and require no maintenance; they are far safer and more reliable than quartz tube level gauges. Although it is expensive, from a safety perspective, it is still advisable to choose it. 2) The differential pressure level gauge is also a long-lasting, maintenance-free, safe and reliable instrument for measuring the level of water in steam drums. Due to its broad scope, it is prone to being misunderstood as a result of design and installation factors. It is recommended that the pressure guide tube be designed in such a way that no heating is required. If steam or electric heating is used, it is advisable to first insert a layer of insulating material such as asbestos rope between the heating tube and the pressure guiding tube. If steam tracing is used, it is advisable to install PTFE electric tracing at the same time as a backup in case of emergencies. Since the discharge valve may not close properly after being used a few times, it is recommended to equip the end fitting with a plug that is easy to remove and install. 3) For the drum level gauge, it is recommended to give priority to magnetic flap level gauges, differential pressure level gauges, and electric contact level gauges. These are all cost-effective level measurement methods that have been verified through extensive use by numerous users and long-term practical operation. From a measurement principle perspective, magnetic flap level gauges, capacitive level gauges, float level gauges, and radar level gauges are less capable of achieving long-term, stable, reliable, and maintenance-free operation compared to others; if such devices are already in use, it is recommended to treat them as backups first. The Technical Specifications for the Water Level Measurement System of Boilers in Thermal Power Plants state that the selected level gauge “should have advanced adaptability to the medium.” Since its commissioning, the process equipment has been operating well, and all parameters of the wastewater discharged meet the **discharge standards. The pH value monitoring during the tailings neutralization process is shown in Table 4, while the monitoring of heavy metals in the products generated during this process is shown in Table 5. As can be seen from Table 4, sampling through the sampling port at the top of the reactor and using pH test strips revealed a significant increase in the pH value of the material inside the reactor, and this value was consistent with those measured by the pH meter installed on the connecting pipe for online monitoring. As can be seen from Table 5, after testing the materials resulting from the reaction at the testing center, the levels of heavy metal ions were all below the **allowed emission standards, thus meeting the requirements for industrial wastewater discharge. http://www.517mat.com/d/file/gsnews/2019-02-19/f5685828e471e33541e3d60f5eb0b4c2.jpghttp://www.517mat.com/d/file/gsnews/2019-02-19/d6c40505d87a6aa5c181c1dc1c81a7cd.jpg3 Conclusion: Compared with traditional limestone and alkali solution methods, the treatment technology using alkaline slurry to neutralize acidic wastewater has certain advantages in terms of equipment investment and operating costs, and it does not cause secondary environmental pollution. Practical application has proven that this technical solution is reliable, yields satisfactory treatment results, and holds potential for wider adoption.