Wednesday, November 4, 2020

HIGH VIBRATION OF A VERTICAL TURBINE PUMP MOTOR DUE TO STRUCTURAL RESONANCE

INTRODUCTION

High 1x vibration observed at motor NDE. The vibration was predominant in horizontal direction compared to that in vertical. Structural resonance was suspected. Cascaded plot data were collected during coast-down and bump test done on the structural components and motor body to confirm the NF. The level of the motor stand also checked and was found tilted by 6 mm/meter in the horizontal direction.

HISTORY

The pump had a history of high vibration and was mostly kept as standby. The usual vibrations were around 8-11 mm/s at motor NDE. During service, the flow rate of the pump dropped suddenly due to foreign object entrapment inside the casing. The pump had to be pulled out for the overhaul. At the same time, the motor NDE bearing which was having abnormality was replaced. But, after the overhaul activities, the vibrations found very high, which is explained below.

High directional vibration, predominant in horizontal (less stiff) direction was observed during the trial run. Even during the decoupled solo run, the motor vibrated above 11 mm/s. Resonance was suspected and coast-down test was done during the solo run to confirm the existence of resonance. The plots are provided below for reference.


Radial horizontal (less stiffness direction) vibrations during load trial (21 mm/s)

Radial vertical (greater stiffness direction) vibrations during load trial (1.3 mm/s)

Coast-down test data during solo run

Trend plot of 1x during coast-down

From the plots, it is clear that within a 20 rpm speed change, the vibrations dropped from 9 mm/s to 2.6 mm/s, during the solo run.

The load-trial vibrations were 21 mm/s, which could have damaged the motor bearings.

Bump test was done to identify the resonating structural component. The motor stand natural frequency in horizontal and axial mounting faces found coinciding with running speed frequency.


Bump test plot - Motor stand horizontal indicating peaks at running speed


Bump test plot - Motor stand mounting face (axial) also indicating peak close to running speed

Based on the above test results, it was concluded that the motor stand was weak in the horizontal direction and the mounting faces required additional support or rigidity. Hence, braces and gussets were added to improve the rigidity of mounting flanges and in the horizontal direction. Also, the motor stand level was corrected to within 0.5 mm/meter as per the equipment manual.


Suggested locations for braces and gussets on motor stand

Suggested locations for braces and gussets on motor stand


After improving the rigidity and correcting the level, the vibrations came down to 2 mm/s RMS in the horizontal direction.


Vibrations spectrum after implementing the corrective actions


SUMMARY

Improvement: Vibrations reduced from 21 mm/s to 2 mm/s at motor NDE horizontal.

Impact: Will improve motor and pump bearing life, thus saving the maintenance cost.

Activities performed:

1.       Improved structural rigidity by providing braces and gussets.

2.       Maintained the motor stand level as per the OEM standards.

Abbreviations used:

NDE: Non-Drive End/ Outboard

NF: Natural Frequency

RMS: Root Mean Square (value of vibration)

OEM: Original Equipment Manufacturer


1 comment:

  1. Good case study KV, do you have photo of gussets and braces added onto the motor stand

    ReplyDelete