Seeking assistance with a question regarding modal analysis on a vertical pump. The analysis will be conducted with no water in the columns or bowl assembly when the pump is not operational. The pump will be monitored both above and below ground level, with the water drained from the sump during the analysis. This absence of water may impact the stiffness and damping of components, potentially shifting resonance frequencies. Conducting an Operating Deflection Shape (ODS) analysis while the pump is running and filled with water will allow for visualization of mode shapes at resonance, aiding in identifying changes. Analyzing spectrums from the ODS in log scale can assist in determining frequency shifts and damping changes caused by water presence. In ME' Scope, consideration must be given to accounting for unknown changes in resonance values and determining the extent of frequency shifts. How have others addressed these challenges in similar projects? Any insights would be greatly appreciated as I prepare for an upcoming project. Thank you, KB.
It appears that Vibrant Technologies offers a version of MeScope which allows users to input variables, such as the mass of water, to make assessments based on the information provided. It might be beneficial to reach out to them directly for more information. As for the modal test, have you tried impacting above ground and collecting response data from the support column below ground? I have not personally conducted this test, but I am curious to see how the data turns out. When performing the Operational Deflection Shape (ODS) test with the added mass of water in the wetted piping, you may anticipate a change in natural frequency (Fn). It is crucial to consider the location of Fn in relation to the running speed, as this can determine whether or not the mode(s) will be excited or visible. One approach to potentially energize the modes is to "jog" the pump to provide an impulse to the entire structure. If you discover a solution to this testing challenge, please feel free to share. Thank you and best regards, Jim Powers.
Hello Jim, I appreciate you taking the time to review and respond. I am determined to investigate the issue with the mass of water and ME' Scope. I am intrigued by this and am eager to delve into it further. The individual we have enlisted for testing is highly skilled in Modal/ODS analysis, possessing extensive experience in the field. This assures me that the results we obtain will be highly accurate. As you mentioned, we will utilize a combination of coast down testing, ODS, and Modal data to pinpoint the frequencies and assess how they are affected by the presence of water. I will explore the possibility of using ME'Scope and report back to you. Additionally, I will share the findings on how the natural frequencies shift with and without water. Thank you once again for your assistance. KB
I have conducted testing on large vertical pumps in the past, although not specifically with ME-Scope. The presence of water both inside and outside the pump column increases the mass. As a general guideline, it is recommended to have twice the amount of water inside the column compared to the sump level in order to account for the external mass of water. Additionally, there may be variations in natural frequencies depending on the impeller support conditions in wet versus dry settings. It is crucial to ensure that the stop logs are secure when working in potentially hazardous conditions, such as standing knee-deep in water, debris, and sludge while working on the pump inlet bell. Walt.
Hi KB, Interesting dilemma you've got there. When I was working on a similar project, I utilized a combination of heuristic models and empirical data to account for these influence factors. Essentially, I measured the pump’s modal behavior with water at various levels and then established a mathematical model using regression analysis. By comparing the difference between the operational situations with the model predictions, I was able to deduce how the modal parameters change when the water is drained out. I must point out, it is prudent to be cautious of the influence of fluid-structure interaction on the stiffness, which can be dependent on the operational condition, geometry, and material properties. Experimentation will help you validate your theoretical assumptions in a real-world scenario. So, a phased approach where you assess, model, and then test might be beneficial. Best of luck in finding your solution!
While I haven't done exactly the same project, I have some experience with modal analysis on different structures. Considering fluid-structure interaction, involving water in your case, can indeed impact the dynamics significantly. I would suggest considering finite element analysis to simulate the behavior of the pump with and without water, as it could give you an idea of the potential shifts in modal frequencies. Additionally, comparing experiments in dry and wet conditions, if feasible, can provide valuable insights. There might be a need to implement updated physical parameters into the ME' Scope based on your findings. Remember, it's essential not only to focus on the changes in resonance values, but also on the associated mode shapes, as they can provide crucial details about structural behavior under varying conditions. I hope this perspective helps!
Hello KB, your approach to tackling this project seems largely on the right track. In previous similar projects, I've found it useful to carry out two sets of analysis: one without water and the other with water. Comparing the modal frequency between these two scenarios provides valuable insights. However, remember that water adds additional dampening effects which could skew your results if not properly accounted for. As for ME' Scope, using the "Modal Parameter Estimation" feature proved quite helpful in compensating the unknown changes in resonance. A little bit of trial and error goes a long way in these projects. Looking forward to hearing about your progress!
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Answer: - The absence of water in the columns or bowl assembly can impact the stiffness and damping of components, potentially shifting resonance frequencies. This change may affect the mode shapes observed during the analysis.
Answer: - Conducting an ODS analysis while the pump is running and filled with water allows for visualization of mode shapes at resonance, aiding in identifying changes that may occur when water is absent. This comparison can help in understanding the impact of water presence on the pump's behavior.
Answer: - Analyzing spectrums from the ODS in log scale can assist in determining frequency shifts and damping changes caused by the presence or absence of water in the pump. This analysis helps in quantifying the differences in resonance values and damping characteristics under different operating conditions.
Answer: - In ME' Scope, it is important to consider accounting for unknown changes in resonance values and determining the extent of frequency shifts that may occur due to the absence of
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