Abstract : To minimize nuclear radiation exposure for maintenance personnel and ensure accurate measurement of production material levels, this paper briefly describes the measurement principle and configuration of a nuclear level gauge, details the online calibration methods for the nuclear level switch and continuous nuclear level gauge, provides relevant calibration data and curves, and analyzes fault diagnosis methods. Furthermore, the rationality and accuracy of this calibration method have been effectively verified during three years of continuous production. Keywords : radioactive source, detector, lead plate, calibration, online, offline Abstract : To reduce nuclear radiation for maintenance workers and for precise measurement in production surveys, this article explains the measurement theory and disposes of unclear level meters. It shows the nuclear level switch and nuclear level meter online calibration in detail. It lists relevant calibration data and curves and analyzes the way of checking obstacles. The calibration method has proven reasonable and accurate in production for three years. 0 Introduction : Zhanjiang Dongxing Petroleum Enterprise Co., Ltd. put into operation a 500,000-ton/year continuous reforming and regeneration system in March 2005. All six level meters used are nuclear level meters. The quality of the level meters determines the operation of the unit, whether it is running or shutting down. Therefore, the calibration of the level meters is crucial. Thus, in order to improve the safe and long-term operation of the unit, the online calibration of the level meters is of great significance. 1. Measurement principle and composition of nuclear level meters 1.1 Measurement principle The measurement principle of nuclear level meters is based on the fact that when rays pass through matter, the rays are absorbed by the material, thus weakening in intensity. This process follows a physical law. A narrow beam of radiation with an initial intensity of I0 passes through a substance of thickness d and density ρ. The radiation intensity I is then: I = I₀×e^-μ×ρ×d, where I₀ is the radiation intensity before passing through the substance, I is the radiation intensity after passing through the material of density ρ and path d, and μ is the absorption coefficient. When the type of radiation source and the measured material are determined, μ can be considered a constant. With the density of the measured material remaining constant, the remaining radiation intensity depends only on d. Thus, when the material level rises, the number of radiation rays detected by the probe decreases; conversely, when the material level falls, the number of radiation rays detected by the probe increases. The height of the material level is deduced from the intensity of the detected radiation; this process is the calibration of the nuclear level gauge. Figure 1 shows the measurement principle diagram. 1.2 LB440 Main Unit: The main unit CPU uses a 32-bit processor, offering high computing performance and precision. It features an illuminated LCD display showing four lines of characters and numbers, and all operations are completed via six membrane keys. Multiple languages ​​and instructional dialogue "soft keys" are available. The circuit continuously self-tests, and calibration data is stored in flash memory, eliminating the need for a backup battery. Its function is to convert the detector's output signal into a calibration signal corresponding to the object level. 1.3 Shielded Radioactive Source: The radioactive source is encapsulated in a stainless steel sleeve and placed in a shielded lead container, ensuring no contact between the radioactive material and the measured object. Depending on the measurement task, a point source or a rod source (Co-60/Cs-137) can be selected. For more complex measurement geometries, the Co-60 rod source provides better linearity. Co-60 has relatively high energy, with two main energies: 1.17 MeV and 1.33 MeV. It is used for devices with thicker walls. Its half-life is approximately 5.27 years. Cs-137 has a main energy of 0.660 MeV and is used for devices with thinner walls. Because of its low energy and ease of shielding, its half-life is approximately 30 years. The shielding container is specifically designed according to the specific radiation source and its intensity, ensuring that operators are not exposed to radiation exceeding the radiation protection regulations. The lead container is vertically mounted on the bracket, eliminating the need to consider angle issues and not affecting measurement accuracy. 1.4 Detector Continuous level measurement typically uses point or rod-shaped scintillation detectors. The detector material is usually composed of NaI(T1) [Note: NaI(T1) is sodium iodide (thallium)] crystal or plastic scintillator (rod detector) and photomultiplier tube. Scintillation detectors have high sensitivity to gamma rays and a long service life under normal use. The signal from the detector to the host is transmitted through a two-core cable with FSK modulation technology, providing very strong anti-interference performance. 2. System Configuration Different measurement tasks require different system configurations. Selecting the optimal configuration means selecting the most suitable radiation source and the most suitable detector. The main basis for selecting the system configuration is the measurement range, the geometry of the measurement location, etc. Our factory uses the following two configurations: 2.1 Point Source/Point Detector Configuration: When the measurement range is very small, a point source/point detector configuration can be selected, as shown in Figure 2. In this case, the measurement nonlinearity is caused by an exponential law, which can be corrected by the software within the host computer. 2.2 Rod Detector/Point Source Configuration: Figure 3 shows the basic configuration of a rod detector/point source. The length of the rod detector depends on the required measurement range. If the required measurement range is too large, two or more rod detectors are needed. If one point source is not suitable, two or more point sources can be used. The measurement nonlinearity is compensated by the electronic circuitry within the host computer. Linear correction data for a specific measurement system is provided by EG&G Berthold. 3. Calibration of Level Switches Since accurately obtaining the material level in the container is difficult, lead plates are used to simulate the ideal catalyst level. Because the density of lead plates is much greater than the thickness of the catalyst, several lead plates can be used to simulate the actual catalyst level. The density ratio of the catalyst to the lead plate is as follows: Density of reforming catalyst: 0.5609 g/ml; Density of lead plate: 11.34 g/ml. The calculation steps are as follows: 1) Convert the catalyst thickness to lead plate thickness by multiplying the density ratio 0.0495 by the specified catalyst thickness. 2) Calculate the number of lead plates: Divide the calculated lead plate thickness by the thickness of a single lead plate (3 mm). 3) Determine external calibration: The level switch is calibrated based on the rise or fall of the material level. For example, the level switch in the reduction section is calibrated when the material level decreases. Because the switching point is between 6 and 5 lead plates, when the material level drops, 6 plates should still indicate a high level, and 5 plates should indicate a low level. A certain dead zone can be set at the switching point, which is also beneficial for calibration. Calculate the calibrated lead plate: Taking LSLL-3502 (the level switch in the reduction section as an example) [align=center][table][tr][td=1,1,147]Parameters[/td][td=1,1,112]LSLL-3502[/td][/tr][tr][td=1,1,147]Specified catalyst thickness (mm)[/td][td=1,1,112]300[/td][/tr][tr][td=1,1,147]Equivalent lead plate thickness (mm)[/td][td=1,1,112]14.85[/td][/tr][tr][td=1,1,147]Number of lead plates[/td][td=1,1,112]4.95 [td=1,1,147]Switch Points[/td][td=1,1,112]6 to 5[/td][/tr][tr][td=1,1,147]Switch Direction[/td][td=1,1,112]Descending[/td][/tr][/table][/align] Calibration Data: [align=center][table][tr][td=4,1,287]LSLL-3502 (Reduction Zone Level Switch)[/td][/tr][tr][td=1,1,105]Item[/td][td=1,1,49]CPS[/td][td=1,1,42]%[/td][td=1,1,91]Note[/td][/tr][tr][td=1,1,105]Closed Source [/td][td=1,1,49]42 [/td][td=1,1,42]100 [/td][td=1,1,91] [/td][/tr][tr][td=1,1,105]Open Source [/td][td=1,1,49]566 [/td][td=1,1,42]0 [/td][td=1,1,91] [/td][/tr][tr][td=1,1,105]15mm Lead [/td][td=1,1,49]122 [/td][td=1,1,42]84.7 [/td][td=1,1,91] [/td][/tr][tr][td=1,1,105]16mm Lead [/td][td=1,1,49]106 [td=1,1,42]85.2 [td=1,1,91]Switch Setpoint[/td][/tr][tr][td=1,1,105]18mm Lead[/td][td=1,1,49]92 [/td][td=1,1,42]90.2 [/td][td=1,1,91] [/td][/tr][/table][/align] 4. Calibration of Continuous Level Gauge The continuous level gauge can be calibrated by adding a fixed amount of catalyst, measuring the height of the material level, and setting the corresponding percentage height on the LB-440. When adding catalyst, the height of the material level is measured on-site with a plumb bob. The corresponding calibration values ​​are as follows. Taking the LT-3507 level gauge in the closed hopper buffer zone as an example, the measurement target is: [table][tr][td=1,1,63][align=center]0%[/td][td=1,1,56]2240[/td][td=1,1,63]mm[/td][/tr][tr][td=1,1,63]Length[/td][td=1,1,56]1600[/td][td=1,1,63]mm[/td][/tr][tr][td=1,1,63]100%[/td][td=1,1,56]640[/td][td=1,1,63]mm[/td][/tr][tr][td=3,1,182]Note: Measurement starts from the bottom of the middle nozzle of the locked hopper[/td][/tr][/table][/align] Berthold manufacturer's design data and curves: [align=center][table][tr][td=1,1,47]cps [/td][td=1,1,54]1000 [/td][td=1,1,47]964 [/td][td=1,1,47]903 [/td][td=1,1,47]809 [/td][td=1,1,47]673 [/td][td=1,1,47]587 [/td][td=1,1,47]539 [/td][td=1,1,47]460 [/td][td=1,1,47]353 [/td][td=1,1,47]245 [/td][/tr][tr][td=1,1,47]% [/td][td=1,1,54]0 [td=1,1,47]11 [/td][td=1,1,47]22 [/td][td=1,1,47]33 [/td][td=1,1,47]44 [/td][td=1,1,47]56 [/td][td=1,1,47]67 [/td][td=1,1,47]78 [/td][td=1,1,47]89 [/td][td=1,1,47]100 [/td][/tr][/table][/align] Actual field calibration data and curves: [align=center] [/align] If the actual calibration curve differs significantly from the manufacturer's design curve after calibration, it indicates a problem. Consider the following aspects: ①. Is the intensity and location of the radiation source correct? ②. Is the radiation source turned on? ③. Was the catalyst actually added? (Confirmed by on-site plumb bob measurement) ④. Are there any other undesigned metal objects outside the radiation source? ⑤. Is the support installed correctly (does it block the radiation source)? ⑥. Is the instrument insulation good? ⑦. If the output count rate is low at low material levels, this can be confirmed by rotating the radiation source. ⑧. If the count rate increases significantly when rotated, it indicates that the radiation source is not properly aligned with the receiver. Readjust the actual calibration curve based on the design calibration curve and the pulse count rate. 5. Conclusion This article lists two calibration methods for the online calibration of nuclear level gauges, along with on-site calibration data and corresponding calibration curves. It also analyzes several potential problems encountered during the calibration process. The successful start-up at our plant and the accuracy of the level gauge over two years prove that the pre-startup online calibration is ideal, ensuring the long-term operation of the unit. References 1 Zhang Binghai, Wang Zhi. Working principle and related knowledge of nuclear level gauges. Petrochemical Automation, 2007, (5): 84-85 2 Le Jiaqian. Chemical Industry Press, 2004. 259-260 3 Tai Xiufeng. Application of radioactive level gauges in level measurement. Petrochemical Automation, 2007, (6): 66-68 4 Moe H J. Radiation Safety Tutorial, Beijing: Atomic Energy Press, 1976 5 Lan Xiuying. Protection issues in the use of isotope instruments. Petrochemical Automation, 1999, (5): 75-76 6 Dong Jianjun. Application of isotope level gauges in high-pressure vessels. Petrochemical Automation, 2002, (6): 86-87 First author resume: Name: Shi Jianqing, male, born in 1975, attended Fushun Petroleum Institute, Department of Automation, 1996, graduated in 2000 with a Bachelor's degree; has worked at Zhanjiang Dongxing Petroleum Enterprise Co., Ltd., Instrument Workshop since 2000, currently holding the title of Instrument Engineer and positions as DCS/SIS Technician and Safety Technician; primarily engaged in instrument design, construction, and maintenance. Contact: Shi Jianqing, Address: No. 15, Huguang Road, Xiashan District, Zhanjiang City, Guangdong Province, 524012, China. E-mail: [email protected], Tel: +86 13724758190, 0759-2606983 (Office)