finite element analysis for structural modification and control resonance of a vertical pump

in the final motor setup, a part of metal has four webs of 15 mm thickness and each is added by welding to the lower motor Motor upper bearing Motor lower bearing Figure 2 Vibration spect

ORIGINAL ARTICLE

Finite element analysis for structural modification

and control resonance of a vertical pump

Dalia M El-Gazzar

Mechanical & Electrical Research Institute, National Water Research Centre, Ministry of Water Resources & Irrigation,

Delta Barrage, Egypt

Received 23 October 2016; revised 22 January 2017; accepted 13 February 2017

KEYWORDS

Vibration;

Vertical pump;

Modal analysis

Abstract The main objective of this research was to evaluate and enhance dynamic performance for a vertical pumping unit The original electric motor of the pump unit had been replaced by another one different in design and weights Vibration has been increased greatly after installing the new motor Consequently, it is necessary to estimate the change in the vibration characteristics owing to the difference in the boundary conditions of the new motor Measured vibration levels and frequency analysis were dangerous at 1 due to resonance problem Finite Element Analysis was used to model the motor structure in order to find its natural frequencies and mode shapes The results confirm that the third natural frequency is very close to 1 operating speed with deviation about 1% To solve the resonance problem, it was recommended to increase the structure stiffness The results after modifications confirmed that the overall vibration level decreases by 89%

Ó 2017 Faculty of Engineering, Alexandria University Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

1 Introduction

The pumping system is the most important water system

sup-plying water for irrigation and removing subsurface water for

drainage purposes There are more than 2000 large scale

irriga-tion and drainage pumping stairriga-tions in Egypt operating under

different conditions Pumping stations in Egypt are subjected

to many problems

Podugu[1]indicated that the sources of vibration in pumps

can be categorized into three types such as mechanical,

hydraulic and peripheral causes Imbalance and misalignment are the major reasons for mechanical problems Peripheral causes of vibration include harmonic vibration from nearby equipment or drivers, operating the pump at critical speed Problems with any of these issues will show up as symptoms, showing higher than normal vibration at certain key frequencies

Redmond and Hussain[2]analyzed the vibration resulting from a simple linear rotor model on isotropic supports and showed the dominant response to be similar to that resulting from a shaft bow The predicted vibration response did not contain any second-harmonic content The reliability and per-formance of any pump system can be directly affected by its dynamic characteristics Sinha and Rao[3]conducted Modal E-mail address: dalia_engdalia@yahoo.com

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analysis on the complete assembly of pumps and piping layout

and identified resonance as the root cause for pump failure

DeMatteo [4] stated that modal techniques are powerful

tools that enhance an analyst’s ability to understand the

sources of vibration A case history of the vertical pump was

investigated Testing progression from problem identification

in route vibration measurements to resonance testing was

pre-sented Resonance problems are difficult to solve Modal

Anal-ysis gives a clear picture of the machine’s motion; however,

neither tool has the capability to solve resonance problems

Marenco[5], has experimentally investigated the effect of

bearing housing design will influence the dynamic

characteris-tics of the system In this paper, an attempt was to study the

effect of the base plate stiffness on improving the dynamic

characteristics of the pump assembly Scheffer[6], conducted

an experiment to monitor pump condition through vibration analysis This research illustrates the typical steps required to solve resonance problems This paper describes the use of operational deflection shape (ODS) and modal analysis testing for problem-solving

Kumatkar and Panchwadkar[7], carried out a modal anal-ysis of a vertical turbine pump to determine its dynamic char-acteristics such as its natural frequencies and corresponding mode shapes They have analyzed the rotor assembly of VT Pump theoretically, numerically and experimentally The sys-tem is modeled as a lumped mass structure to theoretically determine its torsional natural frequencies and as a continuous system to determine its transverse natural frequencies The numerical model is validated with the results of the theoretical analysis

Nikumbe et al.[8]discussed the modal analysis of vertical turbine pump Natural frequencies of a vertical turbine pump are calculated by performing a modal analysis using the Finite Element Method (FEM) They founded total six modes of vibration for this analysis Experimental analysis is defined

as the study of dynamic characteristics of a mechanical struc-ture Experimental analysis is done by using Fast Fourier Transform (FFT) analyzer During this analysis, exciter anism is done by using an instrumental hammer, as this mech-anism requires a minimum amount of hardware and provides shorter measurement times Comparison between natural fre-quencies with an operational frequency of vertical turbine pump ensured the safe working of the pump

de Souza[9], has used Operating Deflection Shape (ODS) technique to analyze the dynamic behavior of the machine or structure, by determining the existing strains and their proba-ble causes Measurements of phase and of the amplitude of vibration at predetermined points were carried The cause of high levels of vibration of the centrifugal pump was deter-mined and the recommendations for correcting the problem were achieved

Dupac and Rahman [10], have developed and used ODS procedure to monitor relative in-service planar or orbital dis-placements of vertical pump for any signs of excess or incom-patible displacements In such cases, the system is taken out of Figure 1 Measurement locations at the pump unit

Table 1 Overall vibration measurements on the pump unit

Measurement locations Overall velocity before supporting Overall velocity after supporting

Full load test

No load test without coupling

Full load test

No load test without coupling

service for full dynamics analysis and subsequent design

mod-ification A combination of ODS, Modal analysis and FEA

has been used to verify any design improvements The

undesir-able structural dynamics is the root cause of poor reliability

Prajapati[11], found out the solution for reducing vibration

in vertical turbine centrifugal pump He enumerated some

methods of identifying vibration in pump and some possible

cause of vibration He found out that the possible cause of

vibration in pump is due to its structure and it is due to either

weight of the motor placed at higher cause maximum vibration

or due to improper misalignment between upper and lower

base part of pump

2 Problem description & task

In this research, dynamic measurements are evaluated for an

axial pumping unit at El-Shabab Pumping Station in the area

of El-Salhiya This pumping station is used to irrigate

9000.5 feddans and consists of 6 pump units; each pump unit

is of discharge 1.5 m3/s, head 11.6 m, 992 rpm, and motor power 1000 kW The vibration problem on Pump unit (1) began after its original motor (a 1000 kW, BROWN BOVERI Type: SOV560wb) was replaced with a new Hungarian motor

as shown inFig 1 The old motor was in service for many years The old motor is 987 RPM and weighs 6200 kg The new motor is 992 RPM and weighs 8600 kg Since the new motor was installed, the vibration on the machine has been extremely rough Therefore, the decision was to modify the motor setup by supporting its base plate with reinforcing con-crete supports at the sides of the base After supporting, the measured vibration level had been duplicated especially on the motor upper and lower bearings and reached a danger value So the task was to determine the source of the high vibrations and the method to overcome this problem in the final motor setup, a part of metal has four webs of 15 mm thickness and each is added by welding to the lower motor

Motor upper bearing

Motor lower bearing

Figure 2 Vibration spectrum measured during full load at motor upper and lower bearings before supporting

base to increase the stiffness to overcome the problem of high

vibration due to resonance

3 Results of vibration measurements tests

Overall vibration levels and vibration spectra are measured at

the motor running speed (16.53 Hz) at different locations on

the whole pumping unit parts to determine the dynamic

per-formance for such huge machines installed at the heavy base

plate and located at classI according to ISO 1-10861 This

class defines that up to 2.8 mm/s is a good level, up to

4.5 mm/s is an allowable level, up to 11.2 mm/s is just tolerable level and what exceeds this value is not permissible and dan-gerous Measurements were done during no load condition where the motor was disconnected completely from the pump via the coupling and full load condition Vibration measure-ments were done on 9 locations on the motor upper and lower bearings and pump bearing in the axial, vertical, and horizon-tal directions as shown inFig 1

Vibration measurements were taken before and after sup-porting the motor base at full load and no load conditions

as shown in Table 1 Firstly the measurement results before

Motor upper bearing

Motor lower bearing

Figure 3 Vibration spectrum measured during full load at motor upper and lower bearings after supporting

supporting during full load condition indicated that the overall

vibration levels are extremely rough On the motor upper

bear-ing, the overall vibration level reached 6.91 and 9.48 mm/s in

vertical and horizontal directions On the other hand, the

over-all vibration level reached 5.27 and 4.72 mm/s vertical and

hor-izontal directions at the motor lower bearing These levels of

vibration are a danger to the machines and structures where

it transmits to the foundations and structures After that, the

motor was disconnected completely from the pump via the

coupling to check and confirm that the main source of high

vibration level is from the motor The overall vibration levels

are decreased on the motor upper bearing about 24% in the

vertical direction and 50% in the horizontal direction, while they decreased on the motor lower bearing about 84% in the vertical direction and 79% in the horizontal direction Frequency analysis is used to define the exciting frequencies and determine the level of vibration at each specific frequency Also, it is used to determine the sources of vibration, to control vibration levels and solve vibration problems Results of fre-quency analysis shown inFig 2indicated that, there is a high vibration occurs from the mechanical defect at 1 rotational speed Structural modifications were done by adding rein-forced supports to the motor base in order to decrease and control the vibration level

Motor upper bearing

Motor lower bearing

Figure 4 Vibration spectrum measured during no load at motor upper and lower bearings after supporting

Results of the overall vibration level after supporting are

extremely increased and reached a danger level according to

ISO 10816 In the case of full load, the overall vibration level

at the motor upper bearing increased about 400% in the

verti-cal direction and 100% in the horizontal direction On the

other hand, the overall vibration level at the motor lower

bear-ing increased about 43% in the vertical direction and 74% in

the horizontal direction Results of frequency analyses in the

case of full load indicated that the vibration amplitude reached

a danger value about 26.3 and 15.7 mm/s in the motor upper

bearing in the vertical and horizontal directions respectively

as shown inFig 3 In the case of no load, the overall vibration

level at the motor upper bearing increased about 180% in the

vertical direction and 65% in the horizontal direction as shown

inFig 4 On the other hand, the overall vibration level at the

motor lower bearing increased about 300% in the vertical

direction and 500% in the horizontal direction Results of

fre-quency analyses in the case of no load indicated that the

vibra-tion amplitude reached a danger value about 11.32 and

6.75 mm/s in the motor upper bearing in the vertical and

hor-izontal directions respectively Vibration level change during

full load and no load conditions before and after supporting

is shown inFig 5

4 Run-up test

A common Way to identify resonances empirically is to oper-ate the equipment across its range of operating speeds while measuring the vibration it exhibits Run-up or coast-down tests, which monitor vibration from standstill to maximum speed and back down, are a quick way to see whether trouble-some resonances are present in the system As shown inFig 6,

a waterfall plot of the spectral data is used to identify a peak vibration level at a certain speed during the run-up a test of the pump unit This plot consists of the 1 vibration ampli-tude being collected simultaneously with a 1 rpm phase read-ing as the machine coasts to a stop runnread-ing speed It could be seen that during the pump starting up, there is a discrete peak

in the magnitude at 1 and it’s a good indication that a reso-nance exists there The speed range includes 10 Hz, 12.5 Hz, 13.75 Hz, 15 Hz, 16.5 Hz, and 17 Hz, 18 Hz and the amplitude

at this speed frequency is 0.0479 mm/s, 0.2077 mm/s, Figure 5 Vibration level during full load and no load conditions

0.488 mm/s, 0.659 mm/s, 0.17 mm/s, 0.301 mm/s, 0.15 mm/s It

could be seen that the amplitude increases until the rotor

reaches its critical speed (16.5 Hz) and then decreases to the

normal level as the speed continues to change So the speed

at 16.5 Hz which the amplitudes decrease is a possible natural frequency The run-up data identified a natural frequency 1 Figure 6 Run-up test data

at 16.5 Hz The resonance is coincident with 1 rotational

speed Vibration from this mechanical defect at 1 rotational

speed is exciting a natural frequency of the pump structure

As a result of operating the pump with its 1 at a resonant

frequency, there was an experience excessive vibration and it

will wear prematurely Corrective actions should be taken to

move the resonant frequency away from the 1 frequency,

through the addition of stiffness (to increase the resonant

fre-quency) or mass (to lower the resonant frefre-quency) to the

struc-ture Another solution is to use a ‘‘tuned absorber” or a ‘‘tuned

mass damper” These devices greatly reduce the amount of

vibration observed at the natural frequency

All the previous results confirmed that characteristics of the

problem are listed below:

 The foundation is not rigid enough (weak) to support the new motors

 The new motors are heavier than the old one affected the natural frequency of the foundation leading to high vibra-tion levels due to resonance

 Vibration from a mechanical defect at 1 rotational speed

is exciting a natural frequency of the pump structure Therefore it is mandatory to perform dynamic structure study to determine the exact stiffness needed The study determines the type and the method of re-enforcing the structure The study includes the

following:- build a numerical model of the structure,

 determine its dynamic characteristics,

 Design of the re-enforcement to ensure safe and normal operation of the pumps

5 Finite element analysis Finite element analysis (FEA) was used to model the motor structure to estimate the dynamic characteristics using ANSYS WORKBENCH 14.5 The FEA model was built for the orig-inal motor structure and simulation is made to find its natural frequencies and mode shapes The model consists of a motor weighing 8700 kg, a steel base, and a concrete foundation Concrete is assumed to be a homogeneous and isotropic mate-rial and to behave in a linear elastic manner The mechanical properties of the concrete are assumed to be modulus of elas-ticity: 32 GPa, Poisson’s ratio: 0.2, and weight density:

2400 kg/m3 Steel is assumed to be a homogeneous and isotro-pic material behaving in a linear elastic manner The mechan-ical properties are assumed to be modulus of elasticity:

200 GPa, Poisson’s ratio: 0.3, and weight density: 7800 kg/

m3 A simple geometrical structure was designed using SOLID WORKS 2010 to simulate the motor structure as shown in

Fig 7 When a 3D model of solid volumes is generated, solid modeling is generally more convenient compared to direct gen-eration Solid modeling is tedious and too much time consum-ing The boundary conditions assumed that the concrete foundation is fully clamped in the two cases including modified and final motor structure setup

6 Model results

Natural frequencies of a symmetric structure occur in orthog-onal pairs as shown inTable 2 The physical significance is that the motor can actually vibrate (bend) in any direction based on the direction of the applied excitation So it is recommended to increase stiffness results in the high natural frequencies The results of the model indicated that the low stiffness of the motor base contributed to the relatively high oscillatory motion of the motor Add stiffeners as proposed will decrease the amplitude of vibration; however, it will not eliminate the source of vibrations The predicted mode shapes are shown

in Fig 8 The possible cause of this problem is due to that, all these motor applications have a high thrust bearing design consisting of 3 angular contact bearings (2 down and 1 up)

Figure 7 Geometrical dimensions of motor and base plate

structure

Table 2 Predicted natural frequencies

Mode 1 Mode 2

Figure 8 Normal mode shapes of motor and base plate structure

Lack of precision of fits at upper thrust bearing could create

high unbalance due to eccentricity, besides that insufficient

down thrust causing improper loading of the bearing and

affecting stiffness

7 Modified model

Since the natural frequency at 16.5 Hz is at the upper end of

the motor at 1 operating speed, it is recommended increasing

the machine stiffness Increasing stiffness results in a higher

natural frequency To accomplish the change, a part of metal

has four webs of 15 mm thickness and each is added by

weld-ing to the lower motor base as shown inFig 9 The modified

model produced acceptable results Increasing stiffness by

add-ing the four web model leadadd-ing to alters the natural frequencies

of the motor This modification increases the frequency of the

first pair and second bending modes The change at each

nat-ural frequency based on adding the four webs model is shown

inTable 3 Normal mode shapes of motor and base plate

struc-ture after modifications are shown inFig 10

8 Results of vibration measurements after modifications

After achieving the new modifications to the pump unit, a new

measurement was done to indicate the effect of this

modifica-tion on the performance of the pump unit Overall vibramodifica-tion levels and vibration spectra are shown in Table 4 indicating that the vibration level on the pump unit is extremely reduced The overall vibration level reached 3.6 and 2.631 mm/s on the motor non-drive end in the horizontal and vertical directions The overall level decreases about 89% and 86% on the motor drive end in the horizontal and vertical directions respectively The frequency spectrum showed that the measured vibration amplitude is obviously decreased The maximum vibration amplitude reached 0.227 and 0.294 mm/s on the motor non-drive end in horizontal and vertical directions respectively as shown inFig 11 The results after achieving the new modifica-tions indicated that the overall vibration level is in the permis-sible zone of ISO 10816-1

9 Conclusion Dynamic behavior of the pumping system is affected greatly by the changing in the motor weight The low stiffness of the motor base contributed to the relatively high oscillatory motion of the motor Adding stiffeners as proposed decreased the amplitude of vibration The results confirmed that how the dynamic characteristics of the pump structure are improved after applying modifications The increase in the stiffness of the motor base moves the natural frequencies away from

Figure 9 A four web model added to the lower motor base

Table 3 Normal mode shapes of motor and base plate structure

Modes Description Frequency (Hz) before adding stiffness Frequency (Hz) after adding stiffness