How to operate a low-temperature variable-frequency pump for purging subsequent pipelines?
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This post was last edited by gader on 2011-8-23 at 20:13. The process requires nitrogen purging of the subsequent pipelines, and the amount of nitrogen used varies greatly – it can reach 3500 in some moments before dropping back to 0. It’s not clear how a low-temperature variable-frequency nitrogen pump should be operated in such situations in order to effectively prevent cavitation Do I need to turn on the pump return? Is the pump controlled via frequency conversion? Is it better for the pump to operate under high load or low load?
We don’t have liquid nitrogen pumps here; we use nitrogen compressors instead. In situations like those mentioned by Brother San, we adjust the load frequently. As a result, the operator of my centrifugal compressor has to keep an eye on the pressure – when it gets too high, we open the vent valve; once the air compressor can adjust itself back to normal, we slowly close the vent valve and let the compressor regulate on its own. Additionally, we have reciprocating nitrogen compressors here, and in such cases we use those compressors. Even if the load changes rapidly, opening the vent valve on a reciprocating compressor doesn’t cause any issues like surging. . Furthermore, for liquid pumps, frequent addition of alkali is not desirable as it can easily lead to cavitation. . Pay attention to the following points: 1. Inform the subsequent operator that when using or shutting down the valve, it should be done slowly to give the previous operator time to make adjustments. 2、Keep someone at the exhaust valve of the liquid nitrogen pump; once the load is reduced, open the reflux valve appropriately to discharge an appropriate amount of liquid. 3. I’m not sure if your nitrogen outlet pipeline is equipped with a vent valve; if it is, then the vent valve is used to regulate pressure and maintain it within the desired range. .
Thank you for your guidance, Brother Jiu Tian. This is the first time I’ve encountered such a situation; the pump suffered from cavitation several times. What I did was reduce the flow rate at the outlet valve and increase the flow rate at the vent valve, in an effort to minimize the impact on the pump. I also used manual control of the pump’s frequency, as operating it at too low a load seemed to help. I increased the reflux flow slightly as well, but this resulted in very low pressure in the pipeline system. It’s still necessary to adjust the vent flow to get things under control. I’m not sure what the most appropriate way to make these adjustments is
This post was last edited by fhwfhwfhwfhw on 2011-8-23 21:35. All centrifugal pumps have a minimum flow rate. The operating flow rate must not be lower than this value; otherwise, the pump is likely to be damaged. I’m not very familiar with the specific principles; I hope someone knowledgeable can give me some guidance. Generally, this value can be seen on the pump’s curve; I’m just making an educated guess – could it be that the flow rate is lower than this value? This value will not change significantly even with frequency conversion. If this value is still higher than what you need, it’s advisable to vent a small amount; if the amount is large, then recycle it
Reply to 4# fhwfhwfhwfhw: I understand what you mean. Cavitation occurs easily at low speeds, so I added a frequency converter, but it can’t handle large fluctuations in volume
The boss’s explanation isn’t very clear; I tried to understand it for a long time but still couldn’t figure out which part is being referred to. If it’s a user, then the ball tank should work.
Reply to 6#: What’s our ranking in the industry? We use liquid nitrogen, which is pressurized by a liquid nitrogen pump and then reheated using a plate-type reheater before being supplied to customers; there are no buffer tanks in between
This post was last edited by fhwfhwfhwfhw on 2011-8-24 00:33. Actually, 3500 is a very small amount; I guess the pipe is also quite thin? Without a buffer tank, it’s difficult to maintain the pressure in the pipeline; when the pump is running, the pressure is too high, and when it’s not running, the pressure is zero. I think what Third Brother said is correct – in such cases, the pump should be operated at its minimum capacity (I don’t think low flow rates cause cavitation). . . (Sweat) The most important thing is to ensure the pump runs smoothly; if the pipes aren’t thick enough or long enough, it’s impossible to maintain stable pressure. I guess fluctuations will be quite large in such cases
I’d like to say a few words: as Brother Watermark mentioned, the 3500’s capacity is really not that large. If, under normal conditions, the liquid pump operates smoothly and can meet the requirements of normal operation, I think it’s better if the low-temperature liquid pump doesn’t start at all ； We can appropriately reduce the integration time and proportionality of the vent valve; the same applies to the return valve as well. The specific settings depend on the interlock configuration! (And also, is the relief valve for pressure control or flow control?)
Reply to 9# yefeng13: When a single nitrogen pump is operated at full capacity, the pressure reaches 4500; pressure control is applied during venting
A volume of 3500 corresponds to 77.8% of full capacity, and it’s hard to say whether the pump used by the moderator is operating at full capacity at that point. If regulation is done via the pump’s backflow, I think it’s not as good as regulating it through the valve, as this allows for minimizing the backpressure on the pump. If choosing between more valve operations and large fluctuations in equipment load, I personally think it’s better to have more valve operations!