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CHAPTER 7
FAULT DIAGNOSIS OF CENTRIFUGAL PUMP AND
IMPLEMENTATION OF ACTIVELY TUNED DYNAMIC
VIBRATION ABSORBER IN PIPING APPLICATION
7.1 INTRODUCTION
Vibration due to defective parts in a pump can be an annoying
problem resulting in unnecessary maintenance and can affect the pumping
system performance and endurance. This chapter focuses on diagnosing of
faults of centrifugal pump by vibration analysis. Also, to control the amplitude
of vibration in a pipe line due to hydraulic pulsation frequency, this is the speed
result of operating condition of the pump. The developed SMA based actively
tuned dynamic vibration absorber was used to control the amplitude of
vibration for varying excitation frequency.
7.2 CENTRIFUGAL PUMP
Centrifugal pumps are one of the most important elements in almost
all industries. The pumps are the key elements in food industry, waste water
treatment plants, agriculture, oil and gas industry, paper and pulp industry, etc.
Its purpose is to convert energy of a prime mover (an electric motor or turbine)
first into velocity or kinetic energy and then into pressure energy of a fluid that
is being pumped. The energy changes occur by virtue of two main parts of the
pump, the impeller and the volute or diffuser. The impeller is the rotating part
that converts driver energy into the kinetic energy. The volute or diffuser is the
stationary part that converts the kinetic energy into pressure energy. The cut
section model and fluid path through centrifugal pump is shown in Figure 7.1.
126
Figure 7.1 Fluid path through the centrifugal pump (Suhane A, 2012)
7.3 SIGNIFICANCE OF FAULT DIAGNOSIS USING
VIBRATION ANALYSIS
The most revealing information on the condition of rotating
machinery is a vibration signature. Vibration parameters provide the needed
frequencies due to the flow and recirculation. When analyzing the vibration
data, an FFT vibration spectrum may be broken down into several frequency
ranges to help to determine the machine problem. Vibrations externally
measured on a pump have been used to monitor the operating condition of the
pump and diagnose the fault, if there is any, without interfering with the normal
operation. The most common method employed for examining mechanical
vibration is spectral analysis. Condition monitoring and fault diagnostics are
useful to ensure the safe running of machines. Vibration signals are often used
for fault diagnosis in mechanical systems because they carry dynamic
information from mechanical elements. These mechanical signals normally
consist of a combination of the fundamental frequency with a narrowband
frequency component and the harmonics. Most of these are related to the
127
revolutions of the rotating system since the energy of vibration is increased
when a mechanical element is damaged or worn. Some of the conventional
techniques used for fault signal diagnosis include power spectra in time domain
or frequency domain. These can provide an effective technique for machinery
diagnosis provided the signals are stationary.
7.4 CAUSES OF VIBRATION IN CENTRIFUGAL PUMP
Vibration due to unbalance
Dynamic imbalance in centrifugal impeller or shaft can cause heavy
vibration and transmits it to piping which can be cured by balancing the fan
with shaft on balancing machine. Simple unbalance, uncomplicated by other
problems can be identified by the following characteristics (Brain PG 2011):
a) Amplitude occurs at 1 X RPM of the shaft
b) The radial vibration is reasonably uniform and not highly directional
c) If the specific component such as impeller or fan is the source of
unbalance, it will have high amplitude at 1 X RPM frequency
Vibration due to misalignment
Misalignment of direct coupled machines is the most common cause
of machinery vibration. In spite of self-aligning bearings and flexible
couplings, it is difficult to align two shafts and their bearings, which will cause
vibration.
Although machines may be well aligned initially, several factors can
affect alignment, namely, operating temperature, setting up of the base or
foundation and deterioration or shrinkage of grounding. Misalignment can be
clearly identified from the following characteristics:
128
a) Predominantly occurs at 2 X RPM
b) Amplitude is high in axial direction when compared to horizontal
direction
Vibration due to hydraulic pulsation
The problems caused by hydraulic pulsation in pumps are easy to
recognize because the resultant vibration will occur at frequency which is the
product of number of impeller vanes and machine speed (rpm). The amplitude
of vibration due to hydraulic pulsation in a pipe line is inevitable because of the
working of the pump. It is not unusual to detect some vibration at the vane or
blade passing frequency on nearly every pump. It would be impossible to build
a machine where no hydrodynamic forces are present. However, when the
amplitude of hydraulic pulsation is excessive, a problem is indicated.
Vibration due to cavitation
Pumps are designed to operate at certain flow conditions including
suction and discharge pressures, flow rates, head pressures, product density or
specific gravity, etc. If operated beyond or outside these designed parameters, a
high amplitude of vibration generally results. Pumps that are forced to operate
amount of fluid enters the pump is insufficient. This creates vacuum pockets in
the fluid that are unstable and can even collapse or explode.
Cavitation can be identified by the following characteristics:
a) Cavitation occurs between 20000 CPM (333.33 Hz) and 150000 CPM
(2500 Hz)
b) Vibration can be detected at any location in the pump
129
c) A hydraulic problem and it may be due to the design of vanes
d) Causes haystack of vibration
Vibration due to bearing defects
When a rolling-element bearing develops flaws on the raceways and/
or on rolling elements, there are actually a number of vibration frequency
characteristics that can result, depending on the extent of deterioration.
Thus, identifying these characteristic frequencies can not only help to verify that a
bearing is definitely failing, but it can also give some indication on the extent of
deterioration. Bearing defects can be identified by the following characteristics:
a) Defects on bearing occurs between 20000 CPM (333.33 Hz) to 150000
CPM (2500 Hz)
b) The vibration can only be detected at the place of bearing
c) The haystack of vibration will increase and spread out as the day progresses
Apart from the above stated reasons, piping system might
experience vibrations due to improper supports, fittings and water
hammer. In case of gas pipe lines, the vibration can also come from
the pulsations generated by reciprocating compressors.
7.5 INFLUENCING PARTS OF PIPING SYSTEM
The following parts are subjected to vibration and defect due to the
working of the pump.
Pipe
All piping supports
Hangers
Snubbers
130
Pipe to pipe interfaces
Machinery or devices attached to the pipe
All these items can influence the pipe vibration patterns. The vibrations
produced in the pipelines contain various risks concerned with the industry as well
as the domestic applications. A pipe will not vibrate if it is prevented from moving.
However, this does not necessarily help the piping system design
from the standpoint of its ability to absorb differential thermal expansion.
Therefore, when addressing a vibration problem, the flexibility design of the
piping system must also be considered. Restraints that are added to reduce
vibration must not increase the pipe thermal expansion stresses or end-point
reaction loads to unacceptable levels.
7.6 SPECIFICATION OF PUMP
It is necessary to obtain amplitude vs frequency spectrums or FFTs
absolutely no value to the vibration analyst unless some specific details about
the machine are known. Specific machinery problems are identified by relating
their vibration frequencies to the rotating speed (RPM) of the machine
components, along with other machine features such as the number of teeth on
gears, the number of blades on a fan etc., are shown in Table 7.1.
Table 7.1 Specification of centrifugal pump
1. Rotating speed of shaft - 2880 RPM 2. Type of bearings - Roller bearing 3. No of rolling element in each bearing - 7 4. No of impeller vanes - 6 5. Head range - 20-100 feet 6. Output range - 500- 1500 lpm
131
7.7 EXPERIMENTAL PROCEDURE FOR FAULT DIAGNOSIS Experimental setup used in this study for fault diagnosis of a
centrifugal pump is shown in Figure 7.2 (a). Centrifugal pump is rigidly fixed
to the foundation, so that the vibration caused due to the looseness with the
foundation could be avoided in experimental results. Accelerometers are
mounted at the inboard bearing of centrifugal pump as shown in Figure 7.2 (b).
(a) Major parts (b) Location of accelerometer
Figure 7.2 Experimental setup for fault diagnosis of centrifugal pump
Experimental setup is made to run as per the testing condition
standards. By changing the accelerometer orientation, data was recorded in
other two axes at the same bearing location.
The vibratory forces generated by the rotating components of a
machine are passed through the bearings. Vibration readings for both detection
and analysis are taken on the bearings whenever possible. Ideally, vibration
readings taken in horizontal and vertical directions are taken directly on or as
close as possible to the bearings with the accelerometer pointing towards the
centerline of the shaft. Axial vibration readings are taken on the bearing as
close to the shaft as possible. Adhesive mounting is used to mount the
132
accelerometer over the bearing which provides frequency response up to
540000 CPM (9000 Hz) that is sufficient for this experimentation. The things
that must be taken care while adhesive mounting is machine surface must be
flat, smooth and clean to ensure secure bonding.
The layer of adhesive should be kept as thin as possible to provide
maximum frequency response. Figure 7.3 shows the Lab VIEW block diagram for
this experimentation. NI USB DAQ 9461 with 4 input channels and delta type
DAQ is used to connect the accelerometer with Lab VIEW software. Initially, the
measurement was taken to check for background noise, which clearly indicates
that there is no significant level of surrounding as shown in Figure 7.4.
Figure 7.3 Lab VIEW block diagram
133
Figure 7.4 Lab VIEW measurement for surrounding noise level
Also, the frequency spectrum of the healthier pump was recorded, so this
Figure 7.5.
Then, the frequency spectrum of defected pumps was recorded. From the data
collected, the defects are identified through vibration analysis by finding the
difference in amplitudes. Also, according to pump standards the amplitude of
vibration is within 0.15 inches which is acceptable due to any cause of vibration
(Brain PG 2011).
Figure 7.5 Vibration level of a good pump
134
Few pumps were taken from pump industry which has customer
complaints for vibration analysis in order to find out the causes for complaints.
From Figure 7.6 (a) & (b), it can be clearly noticed that the peak amplitude of
vibration occurs at 50 Hz, which is clearly a characteristic of an unbalanced
impeller.
The magnitude of unbalance is little higher in case of second pump
when compared to first one. But in both cases, at frequency of 50 Hz,
amplitude of vibration is higher than the permissible value which has to be
addressed. Vibration was measured both in axial and radial direction.
(a) Radially
(b) Axially
Figure 7.6 Vibration measured for an unbalanced pump
135
The vibration levels measured in the radial and axial axes of the
pump are shown in Figure 7.7 (a) & (b). From the Figure 7.7 (b), it can be
clearly seen that the axially measured amplitude vibration of a pump is 700%
greater than that of radially measured vibration. Both are located at 100 Hz.
Also, since it has different magnitudes in different axes, it is found to be
directional. It shows that the amplitude at 100 Hz is clearly the characteristic of
misaligned shaft pump. Further, it can be understood that the vibration is at risk
because of the misalignment which has to be addressed.
(a) Radially
(b) Axially
Figure 7.7 Vibration measured for misalignment in a pump
136
Similarly a number of pumps were taken for experimentation to
study the effect of cavitation and hydraulic pulsation in various pumps.
Common pump defects and its corresponding frequency is shown in Figure 7.8
which will be helpful for further fault detection in defected or normal pumps.
Figure 7.8 Vibration characteristics of centrifugal pump with defects
7.8 VIBRATION CONTROL IN A PIPE LINE USING SMA
BASED ATDVA Vibration in the pipelines may be caused by various factors as
discussed above. However, the vibration due to the hydraulic pulsation is
caused when the impeller in a pump continuously transmits tiny pockets of
water into the pipeline. The hydraulic pulsation is calculated as given in
Equation 7.1.
Hydraulic pulsation frequency = NZ (7.1)
where N is the speed of the shaft and Z is the number of vanes on the impeller.
An attempt is made to develop an absorber system to control the amplitude of
vibration caused by the frequency which is the result of impeller rotation.
Hydraulic pulsation frequency caused by the speed is a result of operating
137
condition of the pump. The variation in the speed due to voltage fluctuation,
slippage in rotor and variation in load conditions that leads to increase in
vibration in the pipe line. This may increase the chances of causing damages to
pipeline, the seals and joints which results in leaks. The leakage of the toxic
chemical or gases may pollute the atmosphere or may even affect the
surroundings. Further, the pipe line produces sound and causes loss of the fluid.
These undesirable effects are produced due to the variation in hydraulic
pulsation frequency is addressed by the development of SMA based actively
tuned vibration absorber. Figures 7.9 & 7.10 show the centrifugal pump
apparatus and the experimental setup including the interfacing the system with
LabVIEW.
Figure 7.9 Experimental setup
Figure 7.10 Experimental setup with Lab VIEW
138
Based on the centrifugal pump selected for vibration analysis, the
hydraulic pulsation frequency was calculated by using Equation 7.1.
Speed of the motor (N) = 2880 rpm
Total no of vanes (Z) =7
Excitation frequency of the pump = 2880 x 7
= 20160 rpm (hydraulic pulsation frequency)
=20160/60
Excitation frequency of the pump = 336 Hz
When the vibration is measured on the pipe line, it is evident from
the Figure 7.11 that the peak occurs at nearly 336Hz and it is the frequency
which has the highest amplitude. This amplitude of vibration is due to
hydraulic pulsation (Excitation Frequency) which was considered for active
vibration control. But in the experimental results, it was found that the peak
frequency is varying between 336 Hz and 339 Hz. This may be due to the
voltage fluctuations, varying load conditions and slippage in rotor.
For the varying frequencies from 336 Hz to 339 Hz, the
conventional absorber has to be redesigned which is cumbersome. Also, the
SMA spring available may not offer the stiffness for the frequency around
336 Hz, Hence, it is decided to develop a combined absorber system with
conventional spring and SMA spring so the equivalent stiffness was considered
for calculation of excitation frequency.
139
Figure 7.11 Amplitude of vibration due to hydraulic pulsation
7.8.1 Development of conventional absorber
In this study, the conventional absorber is developed for the
excitation frequency of 336 Hz. The variation in excitation frequency due to
the impeller rotation is taken care by the SMA spring. Approximately a change
speed of 180 rpm (3 Hz) is accounted for reduction in amplitude of vibration.
Angular velocity x 3.14 x f
where f = 336 Hz
= 336 x 2 x 3.14
= 2110.08 rad/s
modulus E = 2.1x105 N/mm2 (spring steel)
=0.3
Wire diameter d= 1.8 mm
Mean diameter D=3.72mm
No of turns N=9
Stiffness of the conventional spring K =228757.1363 N/mm
140
7.8.2 Development of an absorber with conventional SMA spring
The stiffness offered by the SMA spring is not sufficient for
controlling this very high excitation frequency. Hence, to improve the stiffness,
a parallel system of springs has been used which consists of both the
conventional and SMA springs as shown in Figure 7.12.
Figure 7.12 DVA with conventional spring and SMA spring
K1 = stiffness of the conventional spring.
K2 = stiffness of the SMA spring.
Equivalent stiffness (Ke) = K1 + K2
In martensite state,
Equivalent stiffness of the parallel system = K1 + K2
K1=221533.7667 N/m
K2=1660.67 N/m in martensite state of SMA spring
Ke= 223194.436N/m and the corresponding frequency is calculated as
2 x x
where stiffness Ke = 223194.436N/m and suspended mass m=50g
rad/s.
Frequency f = 336.4307 Hz.
141
In austenite state,
K1 =221533.7667 N/m
K2=4863.281 N/m in austenite state for SMA spring
Ke = 226397.046 N/m and the corresponding controlling frequency is
calculated where
Ke=226397.046N/m and for the same suspended mass of 50g
2 = 226397.046/0.050
= 4527940.92
rad/s
= 2 x 3.14 x f
f= 339 Hz
Hence, the absorber was designed for controlling the frequency in the range of
336 to 339 Hz.
7.9 EXPERIMENTATION USING SMA BASED ATDVA
The excitation frequency of the pump is calculated as 336 Hz. But in
the experimental results, it was found to be varying between 336 and 339 Hz.
For the varying frequencies of 336 to 339 Hz, an SMA based actively tuned
absorber system has been developed to take care of this 3Hz change in
excitation frequency due to hydraulic pulsation. Figure 7.13 shows that the
peak occurs at 337 Hz with amplitude of 2.3mm. But the theoretical calculation
shows that the excitation frequency of the pump occurs at 336 Hz according to
the Equation 7.1. This variation in frequency may be due to voltage fluctuation,
varying load conditions and slippage in rotor.
An absorber designed with the help of SMA spring along with
conventional spring was used to control the amplitude of vibration due to the
142
change of 3 Hz in excitation frequency. The amplitude of reduction in vibration
at 337 Hz is shown in Figure 7.14. From this, it is evident that the amplitude of
vibration reduces from 2.3 mm to 0.8 mm which accounts for around 63.04 %
reduction of amplitude due to the SMAs stiffness changing ability. Also, the
experiments were carried out by intentionally varying the excitation frequency
within the permitted level of change in stiffness of SMA spring.
Figure 7.13 Amplitude of vibration without absorber at 337 Hz
Figure 7.14 Amplitude of vibration with absorber at 337 Hz
When the excitation frequency is varied to 338 Hz, the amplitude of
vibration is measured on the pipeline. Figure 7.15 shows that the peak
amplitude occurs at 338 Hz, owing to the variation in the excitation frequency.
The SMA spring will be supplied with required current by the control system,
143
in order to produce the desired stiffness to reduce the amplitude of vibration,
which is the result of change in frequency from 337 to 338 Hz. The amplitude
of vibration is reduced from 2.1 mm to 0.8 mm which is shown in Figure 7.16.
This results in 58.09% reduction in vibration of the pipe line.
Figure 7.15 Amplitude of vibration without absorber at 338 Hz
Figure 7.16 Amplitude of vibration with absorber at 338 Hz
When the excitation frequency is increased further to 339 Hz, it
tends to develop peak amplitude in pipe line at 339 Hz. The amplitude is found
to be 2.25 mm at 339 Hz as shown in Figure 7.17.
144
Figure 7.17 Amplitude of vibration without absorber at 339 Hz
This leads to 62.2% reduction in amplitude of vibration at the
frequency of 339 Hz which is depicted in Figure 7.18. If the excitation
frequency is reduced from 336 to 334 Hz. It is not possible to use the combined
DVA with conventional and SMA, because the conventional DVA was
designed to take care of the frequency of 336 Hz.
Figure 7.18 Amplitude of vibration with absorber at 339 Hz
The SMA spring is attached with conventional spring will increase
the stiffness leads to account only increase in excitation frequency for vibration
control. This problem can be addressed when only SMA springs of high
stiffness ranges can be combined parallel nature or co-axial in nature results in
more variation in excitation frequency range both below and above the
frequency of 336 Hz.
145
Figure 7.19 shows the reduction in amplitude of vibration in the pipe
line for the frequencies of 336 to 339 Hz. Table 7.2 shows the reduction in
amplitude of vibration with percentage.
Figure 7.19 Reduction in amplitude of vibration for the frequency range of
336 to 339Hz
Table 7.2 Comparison of percentage reduction in amplitude of vibration
for 336 to 339 Hz of excitation
Frequency (Hz )
Without absorber (mm )
With SMA spring (mm)
Percentage of reduction (% )
337 2.3 0.85 63.04
338 2.1 0.88 58.09
339 2.25 0.85 62.20
7.10 CONCLUDING REMARKS
Thus, the vibration due to different defects in centrifugal pumps has
been analyzed and the frequency at which the defects are happening has been
found out by the experimentation procedure. These frequencies would be
useful for the fault diagnosis of the centrifugal pumps without dismantling the
assembly. Experiments were further carried out with the help of SMA springs
0
0.5
1
1.5
2
2.5
337 338 339
Ampl
itude
(mm
)
Frequency (Hz)
Without absorber
With SMA spring(mm)
146
on centrifugal pump apparatus. In order to demonstrate the concept of actively
tuned dynamic absorber for varying excitation frequencies due to hydraulic
pulsation results in more amplitude of vibration in the pipe lines connected
with centrifugal pump. The change in excitation frequency in the inclining
trend was addressed with the help of ATDVA. The reduction in amplitude of
vibration around 50-65% was attained with the help of developed SMA based
actively tuned dynamic vibration absorber. The variation in excitation in
decreasing trend, which can be addressed only with SMA springs of high
stiffness ranges can be combined parallel nature or co-axial in nature resulting
in more variation in excitation frequency range in the decreasing trend.