Application of high-pressure magnetic flap level gauges in the ammonia scrubber tower of Lunan Chemical

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This post was last edited by admin2 on 2019-7-5 at 14:44. Authors: Yang Ren, San Zhang Yifei, Liu Dehai (Shandong Lunan Chemical Industry Group Company, Tengzhou City, Shandong Province). Please indicate the copyright of the article when reproducing it. In the past, our company used electric type III float level gauges for measuring the liquid level in the ammonia scrubber towers. Due to the significant vibrations present at the installation site, these gauges had poor shock resistance, which often led to inaccurate measurements. Furthermore, as there was no on-site indication of the liquid level, the calibration of these gauges relied entirely on experience – the lower limit was set against zero, and the upper limit against full scale. This posed many difficulties for chemical plant operators and instrument maintenance personnel, threatened safety both for people and equipment, and affected the safe and continuous operation of production processes. To address the aforementioned issues, after investigation and research, it was decided to use a high-pressure magnetic float flap level gauge that provides both local level indication and remote transmission capabilities. After comprehensive comparison, the UHC high-pressure remote transmission magnetic float flap level gauge developed and produced by Huai’an Sanchang Instrument Factory was ultimately selected. 1 Structure and working principle of the magnetic float level gauge 1.1 Gauge structure The structure of the UHC high-pressure magnetic float flap level gauge is shown in Figure 1. The level gauge body is a seamless stainless steel tube with an inner diameter of 50 mm and a wall thickness of 10 mm. On the sides of the gauge body there are two short tubes and flanges that are welded to the gas-phase and liquid-phase flanges of the equipment; adjacent to the outside of the gauge body, a flap box and a magnetoelectric conversion sensor are also installed. There is a flushing hole at the top of the gauge body, used to flush the tubes of the gauge itself and the float. The lower part consists of the flange connected to the drain valve and the drain valve itself; it is primarily used for removing and installing the float as well as for draining water from the gauge body. The lumen of the table body is equipped with a magnetic float. 1.2 Working Principle: The high-pressure magnetic float level gauge is connected to the container being measured, forming a communicating vessel; as a result, the liquid level inside the gauge is exactly the same as that in the container being measured. The magnetic float floats on the surface of the liquid inside the gauge. A flap box and a magnetoelectric conversion sensor are installed right on the outside of the gauge body. One side of the flap post inside the flap box is red, while the other side is green; a cylindrical magnet is placed inside the post. When the liquid level in the container under test changes, the float moves up and down as the liquid level rises and falls. A magnet ring is installed inside the thin-walled cylinder of the magnetic float. Due to magnetic coupling, like poles repel each other while opposite poles attract, causing the flaps inside the flip-box to turn synchronously driven by the ring magnet, thereby changing color. Red indicates the gas phase, and green indicates the liquid phase; thus, the flipping of the flaps in the flap box can be used for on-site level indication. At the same time, the magnetoelectric sensor also outputs a 4–20 mA DC standard signal by sensing changes in the position of the magnetic float, thereby enabling remote display of the liquid level, recording, alarm interlocking, and automatic level control. http://www.huoyumi.com/d/file/news/industry/2018-05-29/b001889afd804dbb9b9fcf70160a5c49.jpg The magnetic float is the core component of high-pressure magnetic float flap level gauges; the structure of the magnetic float is shown in Figure 2. http://www.huoyumi.com/d/file/news/industry/2018-05-29/e3a8ff48d6ce2146aed71bad5acc0c37.jpg A magnetic float is a float with a thin-walled cylindrical structure made of special materials; there is a ring of magnetic material in the upper middle part of the inside of the float, and small gaps or holes allow communication between the inside and outside of the float. It is a pressure-compensation type float that can withstand very high pressures, enabling automatic compensation and balance of pressures on both sides of the cylinder. During the pressure compensation phase, since the inside and outside of the float are connected, there is a pressure difference between them, but it is not significant. In the pressure equilibrium phase, the pressure difference between the inside and outside of the float is zero; therefore, the float can withstand any pressure. Situations where magnetic float level gauges cannot be used: (1) The density of the medium being measured is < 0.35 g/cm3; (2) The viscosity of the medium being measured is > 0.02 Pa·s; (3) The medium being measured is quite dirty. 2 Installation, commissioning, and calibration of instruments 2.1 The installation of instruments should be carried out strictly in accordance with the requirements specified in the instructions, while paying attention to the following points: (1) Since this instrument operates on the principle of magnetic induction, in order to ensure its measurement accuracy and eliminate external interference, there must be no magnetic materials within a range of 100 mm around the level gauge. (2) The axis of the instrument’s main body must be perpendicular to the ground, with an error of ≤ 3°; otherwise, the float will not be able to float properly with the liquid level, affecting the accuracy of the measurement. (3) A reliable stop valve must be installed at the interface between the gas phase and the liquid phase of the container under test. (4) Due to the heavy weight of the liquid level gauge, additional load-bearing supports need to be installed. 2.2 There are two scenarios for bringing the instrument into operation: operation at atmospheric pressure and operation under pressure. For operation at atmospheric pressure, it is sufficient to open the gas-liquid shut-off valves of the instrument; once a normal liquid level is established, the system can be pressurized and brought online for operation. Steps for putting it into operation under pressure: (1) Open the drain valve; (2) Slightly open the gas phase valve – at this point, a hissing sound can be heard from the drain outlet; (3) Close the drain valve – at this time, a ‘hissing’ sound can be heard inside the gauge body; (4) Once the sound disappears, slowly open the gas phase valve fully; (5) Slowly open the liquid phase valve – it can be seen that the flaps in the flap box turn one by one from bottom to top, changing from red to green. When the flap no longer moves, the boundary between red and green on the flap box indicates the actual liquid level inside the device. The display also shows the corresponding liquid level indication. 2.3 Calibration method: The magnetic flap level gauge can be calibrated on-site. The specific procedure is to close the gas-phase and liquid-phase stop valves, and open the flushing port at the top of the gauge. The level can be indicated using the flap box; alternatively, a tee can be connected to the outlet of the drain valve, with one end connected to a glass tube to serve as a level gauge and the other end used for drainage. During calibration, a rubber hose is inserted through the flushing port and clean water is slowly added against the inner wall of the gauge body; the liquid level is displayed on-site. After calibrating one full stroke (0–100%), the water is slowly drained through the drain valve, and then another stroke (100%–0) is calibrated. Then add water and continue with the calibration; usually, two or three repetitions are sufficient to complete it. 3 Instrument drainage and troubleshooting of common faults: The UHC series magnetic float flap level gauges are equipped with a drainage valve. If the medium being measured is dirty, it can affect the upward and downward movement of the float inside the gauge; in severe cases, the float may get stuck, resulting in inaccurate level measurements or even no measurement at all. Therefore, a regular waste discharge system should be established based on the actual circumstances. When draining waste during the operation of the instrument, it is necessary to first contact the operator. Only after the controller used in conjunction with the level gauge is switched to manual mode, or after the signal interlock device used with the level gauge is disabled, can drainage take place. The drain valve should be opened slowly; once the clear liquid has been drained, the valve should be closed and the instrument brought back into operation. Common faults and their troubleshooting methods are shown in Table 1: http://www.huoyumi.com/d/file/news/industry/2018-05-29/45017b6b9fb53b9200b687601c36566c.jpg. The operation status of the instruments: After a major system overhaul in April 1997, these instruments were put into use. Their performance is as follows: (1) The instrument measurements are relatively stable, with proper level regulation; the regulation records generally fluctuate within a range of ±2% of the set value. (2) In June 1997, because the operator failed to follow the operating procedures, the system was started up with increased pressure while the ammonia scrubber was at full liquid level; this caused water to enter the float, resulting in it being crushed. After replacing the float, it has been operating normally ever since. The local indication of the level gauge and the remote transmission unit are completely isolated from the medium being measured; therefore, the magnetic flap box and the magnetoelectric transduction sensor can be replaced on-site. However, since both the local display and the remote transmission of the level gauge rely on the magnetic float moving up and down in sync with the rise and fall of the liquid level, any failure of the float will cause both the local display and the remote transmission to stop working, resulting in uncontrolled liquid levels. The UHC series of high-pressure magnetic float level gauges feature a simple structure, easy maintenance, low cost, safety and reliability, as well as minimal requirements for upkeep. Once the problem of simultaneous failure of both local indication and remote indication due to float failures is resolved, these gauges have even broader prospects for widespread use.