Structural Vibration Analysis, Pump

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Vertical Pump, FEM, API 610, Modal Analysis, Harmonic Response

Structure vibration analysis was performed for vertical pump in accordance with API 610 & HI 9.6.8-2014. This Vertical pump is driven by induction motor with 3100 kW capacity and 425 rpm at full load. This analysis computes finite element analysis for structure vibration assessment. INERTANCE performed 3D structure modelling, modal analysis and harmonic response analysis for Structural Vibration Analysis.

Contents

1. Scope of this analysis

2. Structure Model

3. Design Criteria

4. Modal Analysis

5. Harmonic Response Analysis

6. Major Keywords

7. Related Services

1. Scope of this Structural Vibration Analysis  

The scope of structural vibration analysis for vertical pump includes below activities;

  • 3D Structure modelling
  • Structural vibration analysis
  • As per API 610, HI 9.6.8-2014
  • Vertical centrifugal pump
  • Modal Analysis
  • Natural Frequency
  • Case Study for Dry, Wet condition
  • Case Study for High liquid level, Low liquid level
  • Harmonic response analysis

2. Structure Model

Structural vibration analysis of this pump structure was performed using commercial software named ‘ANSYS Mechanical Enterprise’, and Numerical model for the pump structure (including all important pump elements; shaft, impeller, coupling, bearings, wear-rings) was set except motor body. See below section drawing for pump structure model.

Pump Structure Model,
[Pump Structure Model]

Dry & wet running analysis needs to be performed for two (2) pumping liquid elevation level. In addition, Nozzle force & moment has been considered as per fixed nozzle and project nozzle condition respectively as per HI 9.6.8-2014. Altogether, below kinds of separate structure model was analyzed.

  • Dry condition
  • Wet condition, High liquid level
  • Wet condition, Low liquid level

Below figure shows FEM modelling status for wet condition with high liquid level. It should be noted that the pumping liquid inside of vertical column pipe is included in analysis model, which is hiding in below figure for clear visualization for structure model.

FEM Structure Model, Wet Condition. API
[ FEM Structure Model, Wet Condition ]

3. Design Criteria

Structural vibration analysis for this pump is carried out as per API 610 11th and HI 9.6.8-2014, in which the design criteria such as separation margin, etc. are mentioned. And, this analysis procedure has been applied as per Level 3 analysis of HI 9.6.8-2014.

  • Separation Margin

A separation margin (+/-15%) by this analysis will be used for all modes in order to design robust pump mechanical design between the natural frequency of the motor support structure and the operating speed.

  • Harmonic Analysis

In case the interference points as structure resonance are estimated to fall within the separation margin (+/-15%) of operating speed (425 rpm), the harmonic response analysis shall be performed to demonstrate that the potential resonance has no adverse effect.

4. Modal Analysis

Structure reed frequency is the first bending mode lateral frequency of a vertical structure in a particular direction of interest. This reed frequencies of a vertical pump structure are the primary structure frequencies of interest because of the occasional proximity of such frequencies to the operating speed and other excitation frequencies that can result in resonance. Structure reed frequency has been evaluated using the motor center of gravity (CG) and weight information supplied by the motor manufacturer.

Mode Shape, 1st & 3rd
[ Structure Mode Shape, 1st & 3rd ]
Mode Shape, 6th & 7th
[ Structure Mode Shape, 6th & 7th ]

Campbell diagrams and mode shapes for natural frequencies are estimated for the pump structure with dry and wet condition respectively. Excitation frequencies of concern are defined with running speed (1x), two (2) times of running speed (2x) and number (five, 5) of impeller vane (5x), which order excitations are marked in related campbell diagram.

  • One times of running speed (1x)
  • Two times of running speed (2x)
  • Number (five, 5) of impeller vane (5x)

Modal analysis results are shown for wet condition with fixed nozzle and high liquid level on this page. It is found some interference points on vibration mode 3, 4, 6, 7 and 8. These structure natural frequency and mode shape will be carefully reviewed and used into harmonic response analysis.

Campbell Diagram for Wet Condition, Fixed Nozzle, HLL, API
[ Campbell Diagram for Wet Condition, Fixed Nozzle, HLL ]

5. Harmonic Response Analysis

Harmonic analysis was performed for pump structure, because it was found some interference points as results of campball diagram analysis, which could be considered as potential resonance. Calculation input for amount of vibration amplitude was sourced from transient analysis of lateral vibration analysis. This vibration amplitude and phase as per discrete frequency is applied into each bearing housing location, which has narrow clearance between rotor and stationary element. See below example for vibration displacement source at thrust bearing.

Vibration Displacement, Thrust Bearing. bearing housing
[ Vibration Displacement, Thrust Bearing ]

The vibration response was calculated for high liquid level case, and below figures show the responses computed throughout the entire structure body. There is vibration displacement (0-Peak, mm) with phase information, and it is found that the highest vibration was observed around 1x running frequency (6 ~ 8Hz) at impeller location.

Vibration Displacement, 0-P (mm) and Phase (deg), HLL. harmonic.
[ Vibration Displacement, 0-P (mm) and Phase (deg), HLL ]
Deflected shape, Displacement for whole structure body, HLL
[ Deflected shape, Displacement for whole structure body, HLL ]

Below figure shows the vibration velocity with RMS, mm/s unit at bearing housing location. As results of harmonic analysis, the vibration velocities at all excitation sources are meet the vibration limit of API 610 11th.

Total Velocity at Bearing Housing, HLL. API
[ Total Velocity at Bearing Housing, HLL ]

6. Major Keywords – Structure Vibration

  • Structure Vibration
  • Vertical Pump
  • Mode Shape
  • Bearing Housing Vibration
  • Deflected Shape

7. Related Services  – Structure Vibration

INERTANCE has a capability regarding noise and vibration engineering, and our experts are familiar with assessment of Lateral Vibration (Rotordynamics) Analysis.

For further information, contact us at (mountain@inertance.com).

INERTANCE’s related services include;