Operational problems of centrifugal compressors
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The centrifugal compressor in the hydrogenation unit has been overhauled, and issues were detected after it was started up: 1. This centrifugal compressor is equipped with a dry gas seal. After startup, it was found that the pressure difference between the inlet and outlet remained low. Under normal operating conditions, the medium used is hydrogen, with an outlet pressure of around 16.4 MPA and an inlet pressure of around 15.2 MPA, resulting in a pressure difference of approximately 0.7/0.8 MPA. Currently, nitrogen is being used as the medium during the pressurization phase, with pressures at both the inlet and outlet around 4 MPA only, and a pressure difference of less than 0.1 MPA. Everything else is the same as under normal hydrogen operation conditions. Why is the pressure difference so low? Is it because nitrogen has poor compressibility? It’s also because there is no backup pressure available later on (the raw materials have not been introduced; under normal operating conditions, the raw materials are pumped to a pressure of around 17 using a high-pressure pump before being combined with hydrogen and fed into the reactor). Currently, there are no raw materials, which is equivalent to no resistance in the pipeline and no reserve pressure; how should this be explained from the perspective of the equipment? 2. The leakage flow rate and leakage pressure of the dry gas seal have remained high since startup. Previously, the leakage pressure was around 0.02/0.03 MPa, and the leakage flow rate was around 50/60; now both the pressure and flow rate are almost twice as high as they were before the repair. Can this be considered a leakage issue with the dry gas seal? (Someone countered me by saying that if the dry gas seal leaks, the axial thrust should change and the displacement should increase; indeed, the displacement hasn’t changed much at present.) Is this related to the previous issue? (For example, a low pressure difference at the inlet and outlet results in a low sealing pressure, as this dry gas seal uses the exhaust gas as the sealing air source.) I understand that the rotor is in an unbalanced position – the rotor was sent back to the factory for balancing and sandblasting during maintenance, and after that it was disassembled on-site again because the sandblasting process was not thorough enough. On-site, after disassembling the spacer, a scraper was used to remove the catalyst dust adhering to it. From my understanding, all the efforts put into dynamic balancing were in vain.) As a result, although the axial forces and thrust forces (sealing forces) were balanced, the misalignment between the rotor and the seal caused the seal to shift, thereby increasing the sealing gap. Am I correct in this reasoning?
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For Question 1, the reason for the low pressure difference might be that the current medium is nitrogen, whose compressibility differs from that of hydrogen. Nitrogen has a higher density and is less compressible, which may lead to a significant decrease in pressure difference under the same operating conditions. Furthermore, the current medium is used in the pressurization stage; if there is not enough backpressure at the system’s rear end (as you described, with no feedstock being introduced and no high-pressure pump for pressurization), this will result in a reduced pressure difference from the outlet to the inlet. In this case, the resistance of the entire system is lower, which results in a lower pressure difference. Regarding Question 2, concerning the leakage rate and pressure increase of dry gas seals, theoretically, if the seal leakage is severe, it is usually accompanied by significant changes in axial displacement. However, as you mentioned, there is no significant change in displacement. It should be noted that an increase in the seal clearance does not necessarily entail a change in axial thrust, especially when the rotor remains in a relatively centered position. Leakage issues can indeed be related to problems such as inaccurate reinstallation of components after maintenance and increased gaps in the sealing surfaces. If the rotor or seals are not handled properly during maintenance (such as the imbalance issues you mentioned and improper sandblasting), it can indeed lead to a decline in sealing performance. Overall, the pressure difference issue is mainly caused by the testing medium currently in use (nitrogen) and the lack of sufficient backpressure at the system’s rear end. A decline in the dry gas sealing performance may be due to assembly errors or improper handling after maintenance, which results in an increased sealing gap and thus higher leakage. Although the specific causes of these two problems are different, both highlight the importance of paying attention to details during the maintenance and reassembly of compressors. It is recommended to further inspect and adjust the sealing clearance, and it may be necessary to conduct a rotor dynamic balance test again to ensure that the equipment returns to its optimal operating condition. .
Thank you for your reply; I have two more questions. Regarding question 1, from the perspective of heavy equipment and without considering the manufacturing process, are there any other explanations? Another point to note here is that an increase in the sealing gap does not necessarily correspond to a change in axial thrust, especially when the rotor remains in a relatively central position. ”How should this sentence be understood? My understanding is that as the gap increases, the leakage volume rises, but the forces acting on both sides of the seal – the axial thrust and the thrust force – can cancel each other out, resulting in the rotor remaining in a relatively appropriate position. Therefore, even though the displacement changes little, the leakage gap still increases. Is this understanding correct?
For Question 1, from a equipment perspective and without considering other possible explanations related to the process, one can consider whether there are any mechanical failures or design issues. For example, bearing wear, inaccurate rotor positioning, or damaged impellers can all lead to a decrease in compression efficiency, thereby affecting the pressure difference. Additionally, it’s possible that there are leaks in the system or that valves are not fully closed, resulting in pressure loss. Regarding your second question, your understanding is correct. An increase in the sealing clearance can indeed lead to an increase in leakage, but this does not necessarily result in a significant change in axial thrust. Axial thrust and thrust force are usually determined by the dynamic balance of the rotor within the compressor; if these two forces can cancel each other out, the rotor can remain in a relatively central position, even if the clearances increase. What is important is the balance between these forces, rather than their absolute values. If the rotor position is relatively stable, displacement monitoring will not show significant changes. In this case, an increase in the leakage gap is primarily manifested as an increase in the leakage flow rate, while the displacement data may not show any significant changes. Therefore, it is indeed possible to understand this phenomenon in the way you described. .
1. Verify whether the rotational speed has reached the normal operating speed, and whether it is still running at the minimum allowable speed； 2. For compressors, it’s more reasonable to look at the pressure ratio rather than the pressure difference ； 3. The outlet gas from the dry gas seal is used as the sealing gas. Has there been any change in the pressure difference between the sealing gas and the balance line compared to before? Is a booster pump being used to increase the pressure? A larger pressure difference will also result in an increased leakage rate. 4. After maintenance, the seal must undergo a static test; the leakage rate in this test allows for a direct assessment of the quality of the seal.
Thank you very much for your answer; I have a few more questions. 1. The rotational speed has exceeded the critical value under normal operating conditions. 2. I’ve never really had a clear understanding of the compression ratio of centrifuges. For example, in multi-stage reciprocating compressors, it is possible to determine the performance of each stage based on the compression ratio. If the compression ratio is lower than before, it indicates that the efficiency of that particular stage has decreased. Since the rotational speed of a reciprocating compressor remains constant, given a certain inlet pressure, the outlet pressure should fall within a specific range. However, the speed of this centrifuge can be adjusted; depending on the operating conditions on site and the requirements regarding flow rate and pressure, its speed changes continuously, and thus the inlet and outlet pressures also change, though within a certain range. My question is. How should one understand the compression ratio of a centrifuge? We don’t have any experience in this area. The booster pump keeps operating to increase pressure (there are interlocking conditions that determine when it starts and stops based on the pressure difference). I don’t understand what \"3\" means – does it mean that even if the pressure difference isn’t sufficient, the leakage rate will still increase as long as the booster pump is running? 4. What does static testing mean? We haven’t done it.
A unified response: Regarding the issue of pressure difference, it has been confirmed that everything is normal. After the feed crude oil was pressurized, the pressure difference gradually increased to normal levels. It appears that the hypothesis that \"the pressure in the pipelines is too low, resulting in insufficient resistance\" is correct. Regarding the leakage amount, it decreased slightly after the increase in pressure difference, but it remained higher than the value before repairs. My understanding is that the increased pressure difference helped to compensate for some of the sealing defects, as the sealing pressure increased; however, this was not enough to bring the values back to normal. All experts are welcome to offer guidance and discuss; please feel free to share your different opinions.
Regarding the first question: At low pressures, whether it is nitrogen or hydrogen, the pressure difference is small, and a booster pump is needed to maintain this pressure difference. As the pressure gradually increases, the system’s pressure difference also increases; once it exceeds the preset shutdown pressure threshold of the booster pump, the pump will stop automatically. This is therefore a normal phenomenon.
I know that the fundamental purpose of a booster pump is to maintain a certain level of sealing pressure. For example, when the machine first starts up, the pressure difference is low; without a booster pump, wear or leaks may occur due to the insufficient sealing pressure. My question is. When the booster pump is running, theoretically could this result in leakage pressure and flow rates that are higher than normal? Why? Thank you very much for your reply. I previously cited your explanation regarding the reverse angle of reciprocating machines, but I still have some questions (they are listed under the topic in my profile). If you have time, I would be grateful if you could offer some guidance; I look forward to your response.