Night shift at minus 30 degrees
Thread Content
Last night was another sleepless night; the temperature dropped as low as minus 30 degrees. There were continuous calls regarding equipment issues, and my colleagues and I worked non-stop throughout the night, starting our shift at 5 p.m. and continuing until 7:30 a.m. the next day. Apart from taking some meals in between, we spent the entire time answering calls and dealing with instrument failures. In summary, almost all of these issues are caused by freezing. During the installation of heat tracing for the instrument, its insulation performance was tested last night at nearly minus 30 degrees Celsius. The arrangement of the heating tubes, the temperature of the heating water, smooth circulation, and insulation – I’ll repeat, insulation. Last night, many instruments had heating systems, but they still froze; the insulation wasn’t adequate. I won’t go into too much detail on this, as it has little to do with instrument maintenance techniques. Everything was fixed last night. What I mainly want to discuss, and what I would like to ask the experts about, is a problem related to level gauges. This is the boiler drum liquid level, measured using double flanges. The process control instruments are experiencing significant fluctuations. I went to the steam drum to check; there are two double-flange instruments there that measure the same liquid level, one in the south and one in the north. The heating for these instruments is working properly. The pressure tapping points on the double flanges, as well as the diaphragms themselves, are insulated using the temperature of the boiler itself. They were quite hot to the touch. My first question is: does a high heating temperature for the double flanges have any impact on the measurement accuracy? I observed the comparison between the liquid level indicated by the glass plate and that shown by the instruments, and found a discrepancy; moreover, the glass plate itself was also shaking. I have a second question: during previous maintenance tasks, the differential pressure transmitter also showed problems in measuring the liquid level, with the instrument readings not matching those on the glass plate. As a result, there were doubts regarding the accuracy of these instruments. I noticed that in summer, the readings were more or less accurate. So, how does the density of the water inside this steam drum change? And if there is a change in density, what extent of discrepancy would there be compared to the reading on the glass plate? How do parameters such as water feeding to the drum, steam generation, steam temperature, and pressure affect liquid level measurement? I would appreciate some guidance from experienced professionals. I worked all night last evening, and most of the problems were resolved. It’s just this liquid level issue that’s a bit troublesome; in a profession based on technology, I think this shouldn’t happen. I claim that the gauge is accurate, but I can’t explain why there’s a deviation of up to 20 units compared to the reading on the glass plate. Another interesting incident: a double-flange level gauge suddenly returned to zero. I went to the site; there was heating in place but the temperature was low, and the insulation wasn’t sufficient. I improved the insulation around the gauge, and its reading started to rise, but it still didn’t reach normal levels. Since there was another double-flange gauge used for measuring the same liquid level, I continued to work on improving the insulation – I spent nearly 3 hours on this task. The standard liquid level was around 50, but this gauge’s reading stopped rising at around 30. I rarely give up, but this time I really felt exhausted. I called again tonight to inquire about the situation; I almost cried. It turned out that a flushing valve in the process system hadn’t been closed, which caused the differential pressure to drop and the gauge reading to decrease. I want to say to the people in charge of the process: it was extremely cold, and I spent nearly 3 hours working with insulation materials like glass fiber cloth and wire. At least let me know so I could work for one hour less. At first it was an insulation issue, but once the gauge started giving readings, it was no longer an insulation problem. Is it because they don’t know, or did they forget due to being busy? To put it another way, keeping things warm isn’t even my main job. I ran out of energy last night, but I’ve summarized all the tasks I completed that night in order to improve efficiency in my future work. There are many minor tasks to deal with, and the situations on site vary widely. It’s important not only to have solid technical skills but also to communicate well with colleagues. Ah, the work related to instrument maintenance is really complicated to describe; I’m just sharing my frustrations with you guys, sorry if it’s awkward
Working at extremely low temperatures is indeed very challenging, and your hard work is truly admirable.
Thank you for your hard work. Be careful to stay warm in the cold weather, pay attention to safety while working, and take care of yourself.
Dual-flange level measurement relies on the principle of liquid pressure difference, while the glass plate utilizes the principle of a communicating vessel. If the density of the liquid changes due to environmental factors, it will naturally not match the level indicated by the glass plate; this is a quite common issue. The glass plate level is determined based on the principle of a communicating vessel, and it is not affected by the density of the liquid being measured; therefore, it should serve as an accurate basis for adjusting the instrument. The dual-flange range can be adjusted according to the actual level indicated by the glass plate – there isn’t really any better way to do it.
Regarding the issue of heat retention, it’s definitely not acceptable if it’s too hot. I’ve encountered situations where differential pressure flow meters used for measuring heavy oil had their antifreeze evaporate due to excessive heat, which led to blockages in the pressure guiding tubes. At that time we used steam for heating; I think using hot water for heating would be better, or else the heating tube should be separated from the pressure guiding tubes.
Thanks for your hard work, brother in instrumentation. There may be discrepancies between the liquid level indicated by the glass panel in the drum and that indicated by the double-flange gauge. First, check the measurement range; the double-flange gauge calculates the level based on density, and if there is a large difference in density, the displayed liquid level will also be inaccurate. Second, consider whether the water vapor present in the drum could cause an incorrect reading on the glass panel. The magnetic float level gauges we use can also show inaccuracies, as the water vapor can push up the float and lead to errors in the measured level.
After reading your description from start to finish, I’m deeply moved; it’s been extremely difficult. Regarding your question, first, excessive temperature can cause fluctuations in the measured liquid level, leading to inaccurate readings; the heating tube needs to be separated from the double flanges using insulating material. Furthermore, many double-flange level gauges freeze in winter when the capillary silicone oil temperature drops below minus 20 degrees Celsius, due to issues with their selection. Another point is that communicating with the production process is a technical task; there are many matters that they can wait until daytime to handle, and this requires some knowledge of production processes as well as good interpersonal skills. Good luck.