No.

10 – Focus: Centrifugal pumps

the service magazine of the PRÜFTECHNIK Group

PRÜFTECHNIK News

Reducing pump vibration In this edition:

Centrifugal pumps can be found in addition, centrifugal pumps are increas-
almost every industrial application. In ingly being operated at variable speeds Reducing pump vibration
these pumps, the fluid is swept along by to save energy. This further increases Beating in a pumping station
a rotating impeller and forced outward their tendency to vibrate.
onto a circular path. The absorbed ki- We have therefore dedicated this issue Online monitoring at the
Berlin Wasserbetriebe
netic energy is converted to pressure of the Telediagnose.com magazine to
energy in the outlet area of the pump centrifugal pumps. We will show how Detecting cavitation in pumps
housing. To achieve the necessary differ- these pumps can be monitored to detect
Analyzing sources of noise
ential pressures in the various media, disturbance vibrations at an early stage,
flow rates of 10–100 m/s are standard, enabling the implementation of specific Aligning vertical pumps correctly
which can generate strong vibrations. In measures to reduce vibrations.
The toughest vibration standard
for centrifugal pumps
Condition Monitoring Service
News
Beating in a pumping station
Dr. Edwin Becker

If two vibrations of a similar frequency pump was operated and that the vibra- centrifugal pumps that run at different
are superimposed, beating occurs. This tions only occurred during high pump speeds. Pump 1 is the slowest, pump 2
phenomenon is clearly audible in the output. But why? Systematic vibration runs somewhat faster, and pump 3 is the
acoustics. The tone, which is made up of analyses were contracted with the fastest. On the measurement day, pumps
two individual tones of slightly different PRÜFTECHNIK Machinery Service. The 2 and 3 had a relatively high vibration
frequencies, changes its volume at the basic measurements revealed a number level when running at a high delivery
beat frequency, rhythmically becoming of unfavorable conditions that led to rate. It dominated the first order in the
louder and softer. pronounced building vibrations. vibration velocity.
Due to the high amplitudes that this can Measurements of the building re-
cause, beating generated by two neigh- The pumping station concept vealed additional beating in the vibra-
boring machines can lead to enormous In the pumping station, the pressure tion velocity. Beating changed as the
vibration damage and even machine fail- increase is achieved in three consecutive pump speed increased. The operating
ure. Because the cause of beating cannot
always be eliminated, it is important to
keep its damaging effects as small as
possible.
The principle of beating and the beat
frequency are illustrated in Fig. 6 and
can be demonstrated in a simulation.
Beating can also be detected in the
display of FFT vibration analyzers. The
recorded time signals of the vibration
velocity and other measured quantities
must be of sufficient length. If this type
of analyzer is not available, you should
become sceptical if amplitudes fluctuate
wildly.
This was the case at a pumping sta-
tion. The operator detected conditions
with strongly fluctuating machine and
building vibrations. The source was
thought to be the pump and an inade-
quate building foundation. In the con-
trol room, the sense was that the vibra-
1 tion intensity was linked to how the Fig. 1: Vibration measurement on a pump
 the service magazine of the PRÜFTECHNIK Group

mode of the pumps also had an influ- passed through a bending critical speed noticeable in the building. Ultimately,
ence on the beating. It could be shown that led to a marked increase in ampli- beating in the area of 6 mm/s remained
that the beating was influenced by the tude peaks on the motor. Phase jumps when operating at critical speed, which
pressure ratio of the pumps to each were also found in the coast down is why this speed could not be cleared
other. When only pumps 1 and 3 were curves. for continuous operation. Extensive re-
operated together, the beating was re- construction or the use of a motor with-
duced (see Fig. 4). However, this was Precision balancing out natural frequencies was a recom-
not the solution the operator was look- A precision balancing procedure was mendation PRÜFTECHNIK had to make.
ing for. On the contrary, increasing the performed. This reduced the vibration It was also recommended that the new
delivery rate further is the order of day. amplitudes and beating was no longer pump motors meet the requirements of
Therefore, avenues were sought to ac- EN ISO 13709.
tively reduce vibration.

Imbalance
The vibration analyses showed that
relatively strong rotational vibrations
arose as the motor speed increased
(1st order). The balance condition was
checked. In the process, it was noticed
that keys in the couplings protruded or
were missing altogether. One of the first
measures was to correct this situation.
Fig. 7: Order spectra – measured on three
But this was not the only issue.
pump units

Fig. 4: Beating in the building with the pumps

being run in different modes

Fig. 8: Employee of PRÜFTECHNIK Machinery

Service working at precision balancing the
Fig. 5: Beating in the pump foundations of machinery
Fig. 2: Vibration measurement with VIBXPERT®
pumps 2 and 3

Critical speed
The motor type in use was not free of
natural frequencies in the operating
speed range for which it was designed.
At approx. 3600 rpm, the motor even

Preview
Our next issue will focus on fans
– Temporary Telediagnosis Service
– When the kiln fan trips
– Acceptance measurements on coo-
ling tower fans
– Monitoring bearing lubrication us-
ing vibration measuring equipment
Fig. 6: Simulated beating of two neighboring
frequencies (from the ISO-certified vibration – Measuring load resonance curves
Fig. 3: Protruding key in a coupling seminar that PRÜFTECHNIK offers in Ismaning without shutting off the exhaust
2 or on customer premises). fan.
 the service magazine of the PRÜFTECHNIK Group

Condition Monitoring Application

Vibration-based Online Condition Monitoring

at the Berlin Wasserbetriebe
Matthias Luft
The task of these water and wastewa-
ter utility companies is to provide a safe Plant operation
and reliable drinking water supply and
to dispose of wastewater in an environ- Diagnosis reports
Recommendations
mentally friendly manner. Typical appli- Waterworks
cation areas for Condition Monitoring
Process control
instrumentation are illustrated in Fig. 1.
Especially the failure of one of the cen-
trifugal pumps can endanger the reli-
ability of the water supply and wastewa- Pumping stations
Diagnosis
ter disposal and impair production sta- information
bility.
To minimize the risk of machine fail-
ures, the Berlin Wasserbetriebe have for
Pumping stations
many years employed Condition Moni- (wastewater disposal)
toring of machines and systems on the Wastewater treatment plant
basis of vibration measurements, initial-
Fig. 1: Condition Monitoring application at the Berline Wasserbetriebe
ly only with mobile data collectors and
more recently, since 2008, using perma- tire vibration spectra for in-depth diag-
nently installed Condition Monitoring Vibration severity as per DIN ISO nosis on a daily basis and when thresh-
Systems. 10816-3, for monitoring low frequen- old values have been exceeded.
Today, 44 machine units, mostly large cy machine vibrations up to 1 kHz The machine units are equipped with
pumps in sewage pumping stations and => levels increase if there is an permanently installed vibration sensors
treatment plants, are monitored with imbalance or alignment error on the main bearings. In addition, the
VIBNODE® (Fig. 1). More machine Vibration acceleration, for monitor- PLC obtains important operating charac-
trains will be equipped this year. ing machine noise up to 10 kHz => teristic values via a field bus coupling in
VIBNODE® is an Online Condition levels increase in the event of lubri- order to correlate the vibration-based
Monitoring System for diagnostic vibra- cation problems, bearing wear and diagnostic data with machine operating
tion monitoring. The objective is to de- gear tooth damage states (Fig. 3):
tect damage development at an early Special roller bearing characteristic • RPM
stage. Damage becomes visible in chang- values (characteristic band values of • Suction pressure
es in machine vibration and noise, and the envelope spectrum) => levels • Pump discharge pressure
gradually increases over days, weeks or increase with raceway damage in the • Performance
months. The following characteristic roller bearing • Flow rate
values are measured every few minutes: In addition, VIBNODE® measures en- Because waterworks, intermediate
pumping stations und wastewater treat-
ment plants are distributed over a large
Bearing
damage area, it was decided to network the
VIBNODE® systems by means of data
eMails that transfer data by one of the
Hydraulic following routes:
problems
• By modem if there is a telephone line
• Using an Ethernet connection if there
Misalign- Electrical
ment faults is a LAN
Imbalances • By GPRS modem if there is no com-
munications structure.
The data are automatically trans-
ferred to the ‘Diagnosis’ server PC and
archived there in a diagnosis database
by OMNITREND®, the central software
for vibration diagnosis. Specialists from
the ‘Central maintenance’ department
3 Fig. 2: Causes of vibration excitation that are monitored by VIBNODE®.
 the service magazine of the PRÜFTECHNIK Group

NDE-bearing

Central
control
unit
Alarm,
threshold violation
Vibration acceleration
Motor output
RPM Intranet
Pressure
Flow rate
Bearing
VIBNODE® greased

Motor replaced

Fig. 3: Example: Monitoring of a recirculating pump for an aeration Fig. 4: Trend on a damaged NDE-bearing of a turbo blower until it
basin in a wastewater treatment plant is changed

evaluate the measurement data using strated in a recent incident at a waste- lubrication caused by resinified or car-
vibration diagnostic methods. Violations water treatment plant (Fig. 4). After a bonized grease. A sudden failure of this
of thresholds are immediately and di- relatively brief increase in roller bearing machine train would have disturbed the
rectly sent to the control unit and dis- characteristic values, VIBNODE® sig- sensitive biological equilibrium in the
patched to the personnel on call in an naled an alarm at the NDE-bearing of a settling tank, interfering with the perfor-
alarm eMail. At any time, the operator drive motor. After relubrication did not mance of the treatment plant. As a
can determine the current state of the lead to significant improvement and the follow-up measure, the lubricating
machinery by viewing the diagnosis re- vibration diagnosis signals pointed to grease was changed to optimize lubrica-
ports. advanced bearing wear, the motor was tion at high bearing temperatures.
How important Condition Monitoring replaced. An examination of the bearing
is for machine units was clearly demon- showed considerable wear due to poor

Condition Monitoring Basics

Detecting cavitation in pumps

Marcel Kenzler
Cavitation refers to the formation of vibrations are located. Online vibration and has defined discrete cavitation char-
cavities and bubbles in fluid. Centrifugal monitoring or trial runs, however, make acteristic values.
pumps are subject to gas and vapor it possible to determine the frequency
cavitation. In gas cavitation, gas bubbles ranges. If the frequency bands and the
arise during fluid entry if local pressure associated cavitation types are known,
drops below the saturated vapor pres- equipment such as
sure. Vapor cavitation occurs when the VIBNODE® can be
vapor pressure of the fluid is reached in used to monitor
the pump working chamber. Vapor bub- cavitation occur-
bles not only reduce the rate of delivery, rence online. A
but they implode in areas of higher warning is issued as
pressure to cause pressure surges. This soon as a vapor or
can cause cavitation erosion or even gas cavitation takes
destroy the pump. Cavitation can be place. PRÜFTECH-
detected in acceleration FFTs measured NIK has accompa-
at high frequency. Usually, the frequency nied pump manu-
spectra will exhibit very broadband exci- facturers as they al-
tations as shown in Fig. 1. It cannot be located frequency Fig. 1: High frequency acceleration spectrum
4 calculated exactly where the cavitation ranges such as these with two cavitation occurrences
 the service magazine of the PRÜFTECHNIK Group

Condition Monitoring Application

Analyzing causes of noise

Misel Tanasijevic
The noise radiation of a centrifugal lence in the pumping medium. Our
pump can be used as a measure of its recommendation to the pump manu-
running characteristics. When unusual facturer was to check the lines, the Glossary of terms
noise occurs, the noise levels and the intake cross-sections and the suction Did you know?
third octave and narrow band spectra pressures in each case to reduce the
should be measured. To do so, vane excitation.
Sound: Vibrations of media at frequencies from
VIBXPERT® and a microphone 16 Hz to 20 kHz are audible to the human ear.
are used to measure the sound Sound can be classified as air-borne sound,
structure-borne sound and water-borne sound.
Pitch, timbre: Pitch is indicated in the frequency
spectrum by a single line. Timbre consists of
multiple partial tones whose frequencies com-
bine to a harmonic whole as perceived by the
human ear.
Noise: Noise consists of randomly combined
time functions and/or non-harmonic frequencies.
It may be perceived as a nuisance or as a source
of discomfort. The individual threshold for noise
tolerance cannot be measured.
Personal noise exposure: A measure of the
Fig. 1: Reference box as per total noise received at the workplace averaged
EN12639:2000 over an 8-hour shift or a 40-hour work week.
Sound pressure: The sound pressure is a time-
dependent alternating pressure superimposed on
pressure level on an enveloping the equilibrium pressure. The so-called A-weight-
ing describes the hearing response.
surface at a distance of 1 m
Sound particle velocity: The sound particle
above the centrifugal pump. velocity refers to the speed with which material
The resulting mean sound pres- particles oscillate in the sound medium.
sure level can then be calculat- Sound intensity: The product of the sound
particle velocity and the sound pressure equals
ed using DBSPECTRA® and the sound intensity. It is expressed in units of
compared with the agreed power per unit area and provides a clear repre-
sentation of the sound gradient.
noise levels. Dominant exciters
Level: Because the values of the sound parame-
can be identified in the third ters extend over multiple factors of ten, a log-
octave band or on the basis of arithmic scale is used in technical acoustics. It is
expressed in decibels (dB). The equation used to
narrow band spectra in OMNI- calculate the sound level values has the follow-
Figs. 2+3: Third octave spectrum of the sound pressu-
TREND®. re level of the above pumps with pronounced excita- ing basic form:
Example in a tions at the vane pass frequency
x
power plant L = 10 . log — [dB]
x0
A variable speed centrifugal
pump in a new power plant (Measured value relative to the reference value
drew attention because it gen- x0 )
Sound power level: A machine-specific mea-
erated additional noise when
sure of the total radiated sound power of a
operating in the range of 700 sound source under constant operating conditi-
ons. The sound power level is dependent on the
rpm. Measurements were taken
measurement room, distance and other noise
at several speed levels and the sources.
customer’s findings were veri- Frequency: The frequency expresses the pitch of
fied. the sound in Hz. A doubling of the frequency is
Fig. 4: Frequency spectrum with log. frequency axis
an octave or three third-octave steps. The hea-
Result of acceptance ring range covers ten octaves.
measurement Volume: The sensitivity of the human ear varies
At 700 rpm, the levels in- with the frequency. Sounds of the same pressure
but with different frequencies are perceived to
creased markedly and the 125 have different volumes. To nevertheless obtain
Hz, 250 Hz and 500 Hz third an adjusted noise characteristic value, the sound
pressure is weighted as a function of the fre-
octave bands rose dispropor- quency. DIN 45633 mandates use of the A-
tionately. The cause was the weighting curve, identified by dB(A), for ma-
chinery construction and operation.
vane pass frequency, as shown
Spectrum: The spectrum displays the frequency
by the narrow band frequency components contained in a signal. It is used to
spectra and order spectra. This analytically characterize a sound. There are oc-
tave, third-octave and narrow band spectra.
indicated that there was turbu-
5 Fig. 5: Order spectra of the vibration velocity
 the service magazine of the PRÜFTECHNIK Group

Alignment Application

Aligning vertical pumps and checking for plumbness

Bernardo Quintana
With eight pumped storage units and In the next set of measurements taken
a rated power output of 320 MW, the at the second coupling between the tur-
pumped storage power plant at Hohen- bine and generator, the bearing clear-
warte II in Thuringia is the largest pow- ance of the journal bearings had to be
er plant on the Saale River. In times of taken into account. The air gaps at the
excess power generation – generally at clock positions were filled with 0.4 mm
night – water is pumped into the elevat- shims.
ed storage tank to make it available to The machinery was aligned with the
the turbines for power generation dur- aid of hydraulic presses by inserting
ing peak consumption. shims at the radial bearings under the
Vertical alignment pump. Fig. 4 shows the alignment re-
with ROTALIGN® Ultra sult. Alignment is now well within the Fig.1: Pumped storage power plant Hohenwarte
As part of an inspection of machine C, tolerance specified for a speed of 428
PRÜFTECHNIK Machinery Service was rpm. here is 0.02 mm/m. If the shafts are not
contracted to align the shaft. The equip- plumb, this will result in imbalances,
ment of choice was ROTALIGN® Ultra, vibrations, higher temperatures, a
ideally suited for this type of machine shorter bearing life span and/or lower
with its functions for aligning vertical efficiency.
machines and for the wireless trans- The measurements were taken with
mission of data between computer and the new INCLINEO®, a high-precision,
sensor. electronic inclinometer. It is simple to
First, the gear coupling between the attach to the shaft with its magnetic
turbine and pump was disassembled base. The value for the plumbness of
and the laser and sensor were attached the shaft was obtained by comparing
to the flange faces using magnetic two measurements at opposite posi-
clamping equipment (Figs. 2 and 3). To tions.
take the measurements, both shafts The measurements were taken in
were rotated to the clock positions at two directions as well as at the 45°
0(=elevated water storage)–3–6–9. positions to ensure repeatability. These
measurements were repeated at every
Fig. 4: Alignment result at machine C

Plumbness measurement
using INCLINEO®
The relative alignment of the shafts to
each other is not the only criterion for
large vertical machines. The plumbness
of the shafts – i.e., the relationship
between the rotating centerline to gravi-
ty – is also important. The tolerance
Fig. 2: ROTALIGN® Ultra laser transmitter at-
tached with a magnetic bracket

Fig. 6: Plumbness measurement on a vertical

shaft with the INCLINEO®

section of the shaft, thus determining

the position of the entire shaft relative
to the plumb line.
If you require support for similar
measurement tasks, our globally active
Machinery Service will be glad to assist
you.

Fig. 3: Sensor attached with a magnetic bracket Fig. 5: Principle of plumbness measurement
6 – equipped for wireless data transmission with INCLINEO®
 the service magazine of the PRÜFTECHNIK Group

Condition Monitoring Recommendations

The toughest vibration standard for centrifugal pumps

Dr. Edwin Becker
After a suitable pump is selected, the Allowable
running and operating behavior should vibration values
be checked for vibration because often In Chapter 5.9.3, ISO
the machines that exhibit the least vi- 13709 presents allow-
bration are the better machines. But able vibration velocities
what measurement locations and accep- (5 to 1000 Hz) that are
tance criteria should be used for the power- and speed-relat-
vibration measurements? DIN EN ISO ed and specifies the fre-
13709 provides challenging answers. In quency range in which
the experience of PRÜFTECHNIK, this the FFT spectra should
standard, entitled “Centrifugal pumps be measured. For hori-
for petroleum, petrochemical and natu- zontal pumps working
ral gas industries”, represents one of the in the rated operating
toughest vibration standards for pump range, the vibrations
manufacturers. may not exceed a value Fig. 1: Typical measurement locations for mobile measurements
Type classes of 3.0 mm/s at power
and measurement locations outputs of less than 300 kW and speeds More details can be found in the
The standard subdivides centrifugal of up to 3600 rpm. At higher speeds and vibration specifications. ISO 13709 is
pumps into 18 different type classes (see outputs, the nomogram shown in Fig. 3 available in English from the Beuth-
Table 1). In addition to the required should be used. Vibrations greater than Verlag (EUR 280).
dimensioning, it makes technical recom- 4.5 mm/s are not allowable on horizon-
mendations on housing design and indi- tal pumps.
cates where vibration sensors should be For vertical pumps, 5.0 mm/s is the
mounted and where measurements upper limit. It should also be noted that,
should be taken. Fig. 2 shows accep- outside of the operating range for both
tance-related measurement locations for types of pumps, a vibration increase of
pumps of type class OH1 and BB1. While only 30% is tolerated.
it is sufficient to take measurements on Aside from the allowable vibration
the collar bearing in a horizontal, verti- values, ISO 13709 also stipulates specif-
cal and axial direction for the OH1 type, ic requirements for the structural and
two additional measurement locations resonance behavior of centrifugal
need to be used for the BB1. pumps and the necessary acceptance Fig. 2: Measurement locations for vibration
measurements. measurement on centrifugal pumps of the
OH1 and BB1 types

Explanation:
1 P = 3000 kW/stage
2 P = 2000 kW/stage
3 P = 1500 kW/stage
4 P = 1000 kW/stage
5 P = 700 kW/stage
6 P = 500 kW/stage
7 P = 300 kW/stage

Fig. 3: Vibration thresholds for horizontal pumps that rotate

faster than 3600 rpm and with a power input of over
400kW/stage.

7
Taken from “Centrifugal pumps for petroleum, petrochemical and natural gas in-
dustries (ISO/DIS 13709:2007)”
 the service magazine of the PRÜFTECHNIK Group

News
VIBXPERT® New laser speed sensor
now with coast-down spectra PRÜFTECHNIK has introduced a new
When a pump is switched off, natural laser speed sensor onto the market. It
frequencies can be determined as it is can be used to measure speeds on very
coasting down. Instead of coast-down slow and very fast machines. A speed
curves, VIBXPERT® can now be used to marker is attached as a reflector. The
measure full coast-down spectra. They visible laser beam can be pointed to the
contain even more information. reflector from distances of up to one
meter and deliver precise speed values –
when the equipment is being balanced,
ME’scope connection during resampling and for coast-down
ODS is the ‘fine art’ of Condition measurements. The package is rounded
Monitoring. VIBXPERT® and ME’scope out with a quick-mount stand with a ball
Ves 5.0 now work together to be able to joint.
perform O peration D eflection S hape.
WEARSCANNER®

For force-lubricated machines (also

VIBNODE® for pumps), the change in particle size
measures time records distribution in oil is an important mea-
VIBNODE® now has the facility to sure of machine condition. On the basis
record time waveforms of the vibration of eddy current technology, PRÜFTECH-
velocities or accelerations – a unique NIK has developed an autonomous sen-
feature for a low-cost CMS. sor for determining particle sizes in up
to 8 size classes. WEARSCANNER® is
OMNITREND® with vibration connected to the monitoring system or
code, diagnosis code and cor- the system control unit via ModBus TCP.
rection code
Pumps can be systematized using ma-
chine codes and described by means of New service:
vibration, diagnosis and correction noise measurement
codes. PRÜFTECHNIK originally devel- The PRÜFTECHNIK Service and Dia-
oped these tools for wind turbines. Vi- gnosis Center now performs noise mea-
bration codes are used to evaluate vibra- surement anywhere in the world and
tion amplitudes. Diagnosis codes can be offers recommendations on noise reduc-
used to make standardized condition tion. Contact
statements independent of language. PRÜFTECHNIK
Correction codes contain specific recom- Condition Monitoring GmbH
mendations for the machine manufactu- 85737 Ismaning, Germany
rer or operator on how to improve ma- Fax: +49 89 99616-0
chine availability. Fax: +49 89 99616-341
eMail: info@pruftechnik.com
PRÜFTECHNIK
Alignment Systems GmbH
Dates 85737 Ismaning, Germany
Tel: +49 89 99616-0
Information on all trade fairs, semi-
Fax: +49 89 99616-100
nars and other important events of the eMail: info@pruftechnik.com
PRÜFTECHNIK Group can be found on
8 our website at www.pruftechnik.com www.pruftechnik.com