Analysis and Improvement of Flash Tank Level Gauge Issues

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Author: Zhang Linmao (Zhongyuan Dahuahua Company, Henan Energy and Chemical Group). Please indicate the copyright when reproducing this content. Abstract: The melamine production plant of a certain company experienced multiple product quality incidents. One of the main causes of these incidents was inaccurate readings from the level gauge in the flash tank, which led to liquid being carried over into the gas phase. To prevent such incidents, the level gauge was modified several times, which resolved the issue of inaccurate readings and ensured the stable operation of the plant. 1 Process Flow: The post-EuTech melamine production process employs a catalyst-free high-pressure method. Over 99.8% of the urine is condensed at 8.0 MPa and 380°C to produce melamine. This product is then cooled rapidly to dissolve in the liquid phase, during which ammonia and the majority of carbon dioxide are vaporized. CO2 is further removed through stripping, and in the hydrolysis tower it reacts with 13%–14% ammonia added to the mixture, causing the polycondensate to hydrolyze into melamine. Impurities are removed through filtration and adsorption/decolorization in a purification system, followed by cooling in a crystallizer to induce crystallization of melamine at 45°C. Finally, centrifugal separation and drying yield the finished melamine product. The mother liquor containing ammonia and by-products OAT (monomeric cyanuric amide and dimeric cyanuric amide) has its ammonia recovered through an ammonia recovery system for reuse. The bottom liquid from the ammonia distillation tower enters a flash tank, where it flashes to produce low-pressure steam that is returned to the CO2 stripping tower as stripping vapor to recover some of the heat. Should the level gauge in the flash tank fail, it will cause the by-product OAT to enter the CO2 stripping tower, ultimately leading to an excessive level of OAT in the feed stream. This results in crystals ending up in the product, causing an increase in its turbidity. 2 Problems Existence: In the original design, in order to improve the accuracy of the instrument readings and enable mutual reference during operation, both radar-type and differential pressure-type level gauges were used in the flash tank. However, in practice, distortion in the level readings occurred in both types of gauges. The problems that arose were analyzed, and corresponding modifications were made, which effectively resolved the issues affecting product quality. 2.1 Radar level gauge: The level gauge used in this rear-mounted device is of the E+H Micropilot MFMR230 model. The system makes use of quartz crystals to control the frequency at which the carrier pulses are emitted, thereby ensuring frequency stability and improving measurement accuracy. The oscillation frequency of quartz crystals is hardly affected by temperature; therefore, over a wide range of temperature variations, pulse radar does not require high temperature stability, and the measurement accuracy is essentially unaffected by temperature. The measurement is carried out using non-contact methods, which provides a high safety factor and is not affected by factors such as liquid viscosity. It also features simple installation and easy maintenance; the equipment parameters are shown in Table 1. http://s2.sinaimg.cn/mw690/002sSSR6zy7csjEHPwZ91&690 The rear radar antenna periodically emits microwave pulses, which are reflected off the surface of the material being tested and then detected by the radar system. The antenna receives the reflected microwave pulses and transfers them to the electronic circuitry; the microprocessor processes these signals to identify the echoes generated by the microwave pulses on the surface of the material, and uses this information to calculate the liquid level, as shown in Figure 1. The distance H between the radar level gauge and the surface of the material (with the process connection point of the radar level gauge serving as the reference point for measurement) is proportional to the time it takes for the microwave pulses emitted by the antenna to travel that distance, i.e., H = ct/2; here, c is the speed of light. Based on the measurement principle, the factors affecting measurement accuracy are analyzed: (1) Impedance jumps inside the instrument and at the antenna connections. After (2), install an impedance step inside the short tube. After (3) Interfering reflections from obstacles inside the tank. After (4) multiple reflections caused by the tank walls, roof, and bottom. Later (5), the high-frequency head became contaminated with adhering substances. Afterward (6), there is too much oil, gas, or steam inside the tank, resulting in condensation or excessive reflection. http://s3.sinaimg.cn/mw690/002sSSR6zy7csjFh1*22&690 2.1.1 Fault symptoms and cause analysis: During subsequent use, it was found that the output of the radar level gauge LT-86005 was always at its maximum value, rendering it completely useless for reference purposes. Taking advantage of the parking opportunity, the level gauge was inspected, and obvious erosion marks were found on the wall opposite the liquid inlet, as well as some white crystalline deposits on the radar antenna of the level gauge. Subsequently, the problem was analyzed based on the measurement principles and the crystalline nature of the antenna. First, in order to reduce liquid in the gas phase, the liquid level in the flash tank was kept below the feed inlet; moreover, since there was no baffle at the feed inlet, the material coming from the bottom of the ammonia stripping tower struck directly against the walls of the flash tank, causing liquid droplets to splash about. This led to significant disturbances in the liquid surface, and such turbulence could result in multipath reflections. The surface of the wet foam would reflect microwave signals; when the surface of the medium is covered by thick foam, measurement errors occur or measurements become impossible. In the material of type (2), the easily crystallizable OAT medium has a high viscosity; the splashed material adheres to the antenna, creating false echoes that result in an overestimated liquid level reading. 2. 1.2 Problem resolution: Taking advantage of the major maintenance opportunity, the cover flange and the quartz window flange were removed. The surface of the quartz was cleaned using silk cloth dipped in solvents such as alcohol or gasoline; it is important to avoid using alkaline solvents for cleaning. Once the quartz glass was thoroughly cleaned, the instrument’s readings returned to normal. However, the problem of readings reaching full scale occurred again shortly after the instrument was put into operation. Therefore, in media environments that are prone to crystallization or have high viscosity, it is recommended to clean the antenna regularly. When using mechanical methods or water flushing, care should be taken to avoid damaging the antenna; when using cleaning agents, attention must be paid to material compatibility. To reduce the frequency of antenna cleaning and resolve the issue completely, improvements were made to the equipment and operating procedures: the liquid level control in the post-vaporization tank was increased from 30% to 50%, and baffles were added at the material inlet to minimize the impact of liquid level fluctuations on the level readings. This prevented crystallization of materials on the radar antenna, which would otherwise cause false echoes, thus resolving the problem of abnormal level indications. 2.2 Differential pressure level gauges: A differential pressure level gauge operates on the principle that as the liquid level in a container changes, the static pressure generated by the liquid column also changes accordingly. For a closed tank, if the pressure at the bottom is P, the pressure at the liquid surface is Ps, and the liquid level height is H, then it can be expressed as: P = Ps + ρgH. In this formula, ρ represents the density of the medium, while g is the acceleration due to gravity. The value △P = P – PS = ρgH. Usually, the density of the medium being measured is known. The pressure difference △P between the positive and negative phases is proportional to the liquid level height H; by measuring this pressure difference, it is possible to determine the liquid level height. Therefore, the pressure readings in the positive and negative directions are important factors that affect liquid level measurement. The differential pressure level gauge adopted by the company later features positive and negative pressure guide tubes connected to each other, with a design that involves introducing flushing water in the positive direction to prevent blockages, as shown in Figure 2. http://s3.sinaimg.cn/mw690/002sSSR6zy7csjKIVSq32&690 2.2.1 Fault symptoms and cause analysis: After operating for a period of time, the liquid level indication showed to be low, which led to overflow in the flash tank and resulted in quality issues with the product; this caused the company to suffer direct losses of nearly one million yuan. During subsequent maintenance, it was found that the gas-phase pressure transfer pipe was blocked by material crystallization. Subsequently, based on the measurement principle of the level gauge, it was determined that the reason was a low level control in the flash tank, which caused the inlet to be above the liquid surface; as a result, droplets splashed and gradually led to crystallization and blockage at the base of the gas-phase pressure guide tube, thereby isolating that part of the equipment from the gas phase. As the liquid level rose, the pressure increased, which reduced the measured pressure difference and caused the level indication to be low. 2.2.2 Problem resolution: Following modifications, the positive and negative sides of the level gauge’s pressure transfer tube were isolated from each other, and a negative-side flushing fluid was added to ensure simultaneous flushing on both sides (as shown in Figure 3), thereby preventing blockages in the negative side of the pressure transfer tube. Currently, the level gauge is operating stably. http://s15.sinaimg.cn/mw690/002sSSR6zy7csjF99Jcfe&690 3 Conclusion Level gauges are very important instruments in chemical production; it is necessary to select and design them based on the type of medium and operating conditions in order to ensure their proper functioning. In melamine plants, due to the crystallizing tendency of their medium, instruments need to be designed to prevent clogging and crystallization. Through the analysis and modification of the flash tank level gauge, not only was the problem caused by liquid overflow leading to quality defects in the products completely resolved, ensuring the stable operation of the facility and saving the company millions of yuan in direct economic losses, but it also provided a reference for the analysis and handling of similar issues with other instruments.