Hi,
Currently im working on vibration analysis for a pump equipment support.
Following is the step i did.
Step 1,
Modelling of beam supporting the pump with discrete joint where the interface bolt is attached to the structure. These interface bolt is set with retain degree of freedom (222000) point of interest only andthe whole topside hard point is not having any retained degree of freedom. No dummy modeling of pump with CoG is included.
Step 2
Run free vibration analysis to generate the mass and mode file.
Step 3
Create dynamic input file using engine vibration analysis which is available in dynamic reponse module.
Step 4
Define the min & max operating RPM. Interpolation power of 2 is used. User defined RPM to be analyzed by SACS.
Step5
Define the unbalance dynamic load in X, Y, & Z Direction at the interface bolt joint with no phase angle being input. All translation dynamic load is group in the same group load case. Define with non-linear interpolation to be used by SACS.
Step 6
Run engine vibration.
Used file is mass, mode and the engine dynamic input file.
Question.
1. In step 5, how SACS use the information. Will it be applied as per user input direction translation or rotation in combination for defined RSPEED and generate the respone base on it together with the dynamic mode natural frequency close to the defined RSPEED?
2. Is it accurate to read the interface bolt response using this module or time history analysis?
3. There is option card "Mode select" to select particular mode from free vibration analysis. If i use this option card and define the structural mode that it is close to operating RPM of equipment it is the correct way to represent the interface bolt response for a single operating RPM of a pump and ignore the modes from the free vibration analysis?
1. The UNBAL force response is essentially periodic vibration force which is used to generate a modal response of the structure. The loads will be generated for every speed defined in the USRSP line. If there is no RSPEED and UNBAL line which matches the speed in the USRSP line, SACS will interpolate the forces from the other UNBAL load lines using the non-linear interpolation factor you defined. The modal response will contain multiple responses, not just the closest frequency as the other modes will still contribute to the overall response.
2. Yes, this is an accurate approach. You may see slightly larger transient forces is if you perform a time history analysis as this is calculates the steady-state response.
3. The Mode select option is intended to remove unwanted modes instead of only selecting one mode close to the engine frequency. Typical unwanted modes are modes that are unrelated to the analysis in question or local vibrations that don't affect the global behavior of the structure. I would only use these if you are seeing odd behavior.
Hi Geoff, thank you for your explanations, i really appreciate it. Please be noted that i have limited knowledge on vibration analysis.The unwanted modes that unrelated to the structure i want to highlight in the vibration analysis i did, is the modes having small mass participation which is close to my pump operating frequency. In your opinion, can we agree that the mode can be eliminated from being considered. The fundamental frequency is having mass participation more that 80% which is away from the 80%< Fo (operating frequency)<120%.