How to resolve abnormal readings from the magnetic float level gauge at the bottom of the alkali scrubber tower
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Guys, I’ve run into a rather tricky problem and would like to ask for your advice. The factory where I work has an alkali scrubbing system that is primarily used to absorb the sulfur dioxide-containing exhaust gases coming from the sulfur unit. Normally, the liquid level inside this system is kept at around 50%, with a pH level of 7–9. Since this device operates intermittently, this anomaly was not detected during its previous operation. As a result, prolonged operation was required during the shutdown this month, and the following issues were identified: when the temperature inside the tower was high, around 78°C at the top of the tower, the liquid level gauge inside the tower began to show unstable readings. Therefore, an instrument technician was sent to the site to calibrate the gauge, and it was found that the magnetic flap type liquid level gauge on site was malfunctioning. Due to the high height of the tower, our magnetic flap level gauge is divided into 3 layers, with each layer measuring 2.8 meters; in total, this amounts to 8 meters. This means there is a 20-cm section where the measurement is repeated. When the remote level gauge indicated a liquid level of 45%, all three local level gauges also showed the liquid level, with fluctuations in the readings. Schematic diagram of instrument installation: Since there is no packing at the bottom of the tower and the inner diameter of the tower is 4 meters, in theory the liquid level at the bottom should be completely full; at this point the liquid level is above 33% as indicated by the lowest level gauge, the middle level is close to half full, while the top level is empty. However, both the magnetic flap gauges in the middle and at the bottom of the site only display a liquid level that corresponds to half of their respective ranges, while the gauge at the very top also shows a liquid level of nearly 40 cm. The lowest-level level gauge shows..., the middle-level level gauge shows..., and the top-level level gauge shows... The field instrument has stopped operation of the lowest magnetic flap level gauge to allow drainage; the magnetic flap level gauge can return to its lowest level normally ； After shutting off the waste discharge line and only turning on the exhaust line along with the lower valve, the liquid level can rise to its maximum point, with liquid escaping from the top. Then, by first shutting off the exhaust line and then opening the upper valve, it can be observed that the magnetic float of the level gauge drops automatically to the bottom before rising back to its original middle position and fluctuating there. The gauge at the middle level was stopped; after draining the liquid, the level dropped back to 0. After closing the drainage line, the upper and lower valves were opened simultaneously. Water vapor could be seen being discharged through the vent line, and the level fluctuated in the middle area. However, the gauge at the top indicated that the liquid level was much higher than that at the vent line. Since the internal absorption liquid is a saline, weakly alkaline solution at a relatively high temperature, close to boiling point, significant foaming was suspected; however, our instruments do not seem to provide an explanation for why the level gauge at the very bottom shows only half of the liquid level, namely around 16%. I hope the experts here can provide some guidance; I would be extremely grateful.
This situation may be caused by the float of the magnetic flap level gauge being affected by high temperatures and foam, resulting in inaccurate movement of the float. You can try the following methods to solve the problem: 1. Check and clean the interior of the level gauge to ensure that no impurities or dirt are interfering with the proper movement of the float. 2. Check the sealing of the level gauge to ensure there are no leaks that could allow liquid to enter and affect the density of the float. 3. If possible, reduce foam in the internal liquid, for example by adding antifoam agents or adjusting operating conditions. 4. Consider replacing it with a level gauge that is suitable for high temperatures and not easily affected by foam, such as a radar level gauge. 5. If the level gauges are installed in sections with overlap, ensure that each section’s level gauge is calibrated consistently to avoid misreadings caused by discrepancies in readings. .
Under dynamic operating conditions, the temperature inside the tower is close to the boiling point; therefore: 1. Inside the tower, there will be liquid in the upper and middle sections of the packing layer, and a dynamic equilibrium will be reached； 2. The temperature and pressure inside the tower are higher than those outside, so liquid will flow out when the level gauge is emptied ； After ruling out abnormalities such as impurities and crystallization in the absorption liquid phase and confirming that the level gauge is functioning properly, a comparison with the static condition (in theory) can be made to verify it. These are just my personal opinions for reference only.
From your description, it seems to me that the valve at the upper root of the magnetic flap might not be open, or it could be blocked. You can slightly open the upper discharge opening when the last magnetic flap is in operation, to see what is being discharged and whether the liquid level rises. If it does rise, check what happens after the valve is closed.
The valves at the base of the gauges were all open. At first, we suspected a blockage in the pipelines; therefore, we emptied and drained fluid from all three gauges. Whether we activated the valves at the upper part or those at the lower part, the result was the same: the liquid level fluctuated in the middle section of each gauge, which suggested stratification, even though there was no packing inside the tower. The exhaust into the air contains water vapor, similar to the foam that forms after boiling dumplings. Another point is that the tower is a constant-pressure tower, with a pressure at the top of less than 1 kPa, approximately 0.5 KPa
Just turn on the valve above and see if anything comes out; I still think it’s blocked there
We have taken into account the impact of high temperatures on the foam, and we checked points 1 and 2 you mentioned; there are no issues. Adding an antifoaming agent is something to consider next. Finally, point 5: During the water transportation process, we tested the high and low liquid levels in the tower, and they were all normal. This level gauge only shows abnormalities under high-temperature conditions; when the temperature at the top of the tower is below 65°C, the on-site level gauge functions properly
Thank you for your suggestion. We will check it after shutting down, but I don’t think it’s the case, as the probability of all three being blocked at the same time is too low
Thank you for your suggestion. However, our level gauge is installed at the bottom of the tower, where there is no packing. Moreover, this is a spray tower with only one layer of demister at the top; we do not think that liquid flooding could cause stratification. Secondly, this tower is a constant-pressure tower; the internal pressure is around 1 KPa. After the level gauge is emptied, the height of the liquid column should be only about 10 cm, so it shouldn’t spray out from the top of the middle section, which makes the situation quite tricky. Moreover, once the temperature of this circulating fluid drops to a certain level, the above phenomena disappear and reappear only at high temperatures. We all plan to request from our management to replace the magnetic flap level gauge with a glass tube level gauge
Double flange for long-distance transmission? I suggest you measure the specific gravity of the alkaline solution and check the density of the float. Perhaps changing to a float with a lower density will solve the problem. No thanks
Is it possible that the temperature affects the thermal expansion and contraction of the float, and that the float’s conduit becomes corroded or deformed/crystallized due to temperature effects, causing the float to get stuck in the middle position and unable to move upward?