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the service magazine of the PRÜFTECHNIK Group

No. 10 – Focus: Centrifugal pumps

In this edition:

Reducing pump vibration

Beating in a pumping station

Online monitoring at theBerlin Wasserbetriebe

Detecting cavitation in pumps

Analyzing sources of noise

Aligning vertical pumps correctly

The toughest vibration standardfor centrifugal pumps

News

Centrifugal pumps can be found inalmost every industrial application. Inthese pumps, the fluid is swept along bya rotating impeller and forced outwardonto a circular path. The absorbed ki-netic energy is converted to pressureenergy in the outlet area of the pumphousing. To achieve the necessary differ-ential pressures in the various media,flow rates of 10–100 m/s are standard,which can generate strong vibrations. In

addition, centrifugal pumps are increas-ingly being operated at variable speedsto save energy. This further increasestheir tendency to vibrate.

We have therefore dedicated this issueof the Telediagnose.com magazine tocentrifugal pumps. We will show howthese pumps can be monitored to detectdisturbance vibrations at an early stage,enabling the implementation of specificmeasures to reduce vibrations.

PRÜFTECHNIK News

Reducing pump vibration

If two vibrations of a similar frequencyare superimposed, beating occurs. Thisphenomenon is clearly audible in theacoustics. The tone, which is made up oftwo individual tones of slightly differentfrequencies, changes its volume at thebeat frequency, rhythmically becominglouder and softer.Due to the high amplitudes that this cancause, beating generated by two neigh-boring machines can lead to enormousvibration damage and even machine fail-ure. Because the cause of beating cannotalways be eliminated, it is important tokeep its damaging effects as small aspossible.

The principle of beating and the beatfrequency are illustrated in Fig. 6 andcan be demonstrated in a simulation.Beating can also be detected in thedisplay of FFT vibration analyzers. Therecorded time signals of the vibrationvelocity and other measured quantitiesmust be of sufficient length. If this typeof analyzer is not available, you shouldbecome sceptical if amplitudes fluctuatewildly.

This was the case at a pumping sta-tion. The operator detected conditionswith strongly fluctuating machine andbuilding vibrations. The source wasthought to be the pump and an inade-quate building foundation. In the con-trol room, the sense was that the vibra-tion intensity was linked to how the

pump was operated and that the vibra-tions only occurred during high pumpoutput. But why? Systematic vibrationanalyses were contracted with thePRÜFTECHNIK Machinery Service. Thebasic measurements revealed a numberof unfavorable conditions that led topronounced building vibrations.

The pumping station conceptIn the pumping station, the pressure

increase is achieved in three consecutive

centrifugal pumps that run at differentspeeds. Pump 1 is the slowest, pump 2runs somewhat faster, and pump 3 is thefastest. On the measurement day, pumps2 and 3 had a relatively high vibrationlevel when running at a high deliveryrate. It dominated the first order in thevibration velocity.

Measurements of the building re-vealed additional beating in the vibra-tion velocity. Beating changed as thepump speed increased. The operating

Fig. 1: Vibration measurement on a pump

Condition Monitoring Service

Beating in a pumping stationDr. Edwin Becker

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the service magazine of the PRÜFTECHNIK Group

Fig. 5: Beating in the pump foundations ofpumps 2 and 3

Fig. 2: Vibration measurement with VIBXPERT®

Fig. 6: Simulated beating of two neighboringfrequencies (from the ISO-certified vibrationseminar that PRÜFTECHNIK offers in Ismaningor on customer premises).

Fig. 7: Order spectra – measured on threepump units

Fig. 3: Protruding key in a coupling

Fig. 4: Beating in the building with the pumpsbeing run in different modes

Fig. 8: Employee of PRÜFTECHNIK MachineryService working at precision balancing themachinery

PreviewOur 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– Measuring load resonance curves

without shutting off the exhaustfan.

mode of the pumps also had an influ-ence on the beating. It could be shownthat the beating was influenced by thepressure ratio of the pumps to eachother. When only pumps 1 and 3 wereoperated together, the beating was re-duced (see Fig. 4). However, this wasnot the solution the operator was look-ing for. On the contrary, increasing thedelivery rate further is the order of day.Therefore, avenues were sought to ac-tively reduce vibration.

ImbalanceThe vibration analyses showed that

relatively strong rotational vibrationsarose as the motor speed increased(1st order). The balance condition waschecked. In the process, it was noticedthat keys in the couplings protruded orwere missing altogether. One of the firstmeasures was to correct this situation.But this was not the only issue.

Critical speedThe motor type in use was not free of

natural frequencies in the operatingspeed range for which it was designed.At approx. 3600 rpm, the motor even

passed through a bending critical speedthat led to a marked increase in ampli-tude peaks on the motor. Phase jumpswere also found in the coast downcurves.

Precision balancingA precision balancing procedure was

performed. This reduced the vibrationamplitudes and beating was no longer

noticeable in the building. Ultimately,beating in the area of 6 mm/s remainedwhen operating at critical speed, whichis why this speed could not be clearedfor continuous operation. Extensive re-construction or the use of a motor with-out natural frequencies was a recom-mendation PRÜFTECHNIK had to make.It was also recommended that the newpump motors meet the requirements ofEN ISO 13709.

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the service magazine of the PRÜFTECHNIK Group

Fig. 1: Condition Monitoring application at the Berline Wasserbetriebe

Fig. 2: Causes of vibration excitation that are monitored by VIBNODE®.

Misalign-ment

Electricalfaults

Imbalances

Bearingdamage

Hydraulicproblems

Waterworks

Pumping stations

Wastewater treatment plant

Pumping stations(wastewater disposal)

Plant operation

Process control

Diagnosisinformation

Diagnosis reportsRecommendations

Condition Monitoring Application

Vibration-based Online Condition Monitoringat the Berlin WasserbetriebeMatthias Luft

The task of these water and wastewa-ter utility companies is to provide a safeand reliable drinking water supply andto dispose of wastewater in an environ-mentally friendly manner. Typical appli-cation areas for Condition Monitoringinstrumentation 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-ter disposal and impair production sta-bility.

To minimize the risk of machine fail-ures, the Berlin Wasserbetriebe have formany years employed Condition Moni-toring of machines and systems on thebasis of vibration measurements, initial-ly only with mobile data collectors andmore recently, since 2008, using perma-nently installed Condition MonitoringSystems.

Today, 44 machine units, mostly largepumps in sewage pumping stations andtreatment plants, are monitored withVIBNODE® (Fig. 1). More machinetrains will be equipped this year.

VIBNODE® is an Online ConditionMonitoring System for diagnostic vibra-tion monitoring. The objective is to de-tect damage development at an earlystage. Damage becomes visible in chang-es in machine vibration and noise, andgradually increases over days, weeks ormonths. The following characteristicvalues are measured every few minutes:

Vibration severity as per DIN ISO10816-3, for monitoring low frequen-cy machine vibrations up to 1 kHz=> levels increase if there is animbalance or alignment errorVibration acceleration, for monitor-ing machine noise up to 10 kHz =>levels increase in the event of lubri-cation problems, bearing wear andgear tooth damageSpecial roller bearing characteristicvalues (characteristic band values ofthe envelope spectrum) => levelsincrease with raceway damage in theroller bearingIn addition, VIBNODE® measures en-

tire vibration spectra for in-depth diag-nosis on a daily basis and when thresh-old values have been exceeded.

The machine units are equipped withpermanently installed vibration sensorson the main bearings. In addition, thePLC obtains important operating charac-teristic values via a field bus coupling inorder to correlate the vibration-baseddiagnostic data with machine operatingstates (Fig. 3):• RPM• Suction pressure• Pump discharge pressure• Performance• Flow rate

Because waterworks, intermediatepumping stations und wastewater treat-ment plants are distributed over a largearea, it was decided to network theVIBNODE® systems by means of dataeMails that transfer data by one of thefollowing routes:• By modem if there is a telephone line• Using an Ethernet connection if there

is a LAN• By GPRS modem if there is no com-

munications structure.The data are automatically trans-

ferred to the ‘Diagnosis’ server PC andarchived there in a diagnosis databaseby OMNITREND®, the central softwarefor vibration diagnosis. Specialists fromthe ‘Central maintenance’ department

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the service magazine of the PRÜFTECHNIK Group

Fig. 1: High frequency acceleration spectrumwith two cavitation occurrences

Fig. 4: Trend on a damaged NDE-bearing of a turbo blower until itis changed

Fig. 3: Example: Monitoring of a recirculating pump for an aerationbasin in a wastewater treatment plant

Centralcontrol

unit

Intranet

VIBNODE®

Vibration accelerationMotor outputRPMPressureFlow rate

NDE-bearing

Alarm,threshold violation

Bearinggreased

Motor replaced

evaluate the measurement data usingvibration diagnostic methods. Violationsof thresholds are immediately and di-rectly sent to the control unit and dis-patched to the personnel on call in analarm eMail. At any time, the operatorcan determine the current state of themachinery by viewing the diagnosis re-ports.

How important Condition Monitoringis for machine units was clearly demon-

strated in a recent incident at a waste-water treatment plant (Fig. 4). After arelatively brief increase in roller bearingcharacteristic values, VIBNODE® sig-naled an alarm at the NDE-bearing of adrive motor. After relubrication did notlead to significant improvement and thevibration diagnosis signals pointed toadvanced bearing wear, the motor wasreplaced. An examination of the bearingshowed considerable wear due to poor

lubrication caused by resinified or car-bonized grease. A sudden failure of thismachine train would have disturbed thesensitive biological equilibrium in thesettling tank, interfering with the perfor-mance of the treatment plant. As afollow-up measure, the lubricatinggrease was changed to optimize lubrica-tion at high bearing temperatures.

Condition Monitoring Basics

Detecting cavitation in pumpsMarcel Kenzler

Cavitation refers to the formation ofcavities and bubbles in fluid. Centrifugalpumps are subject to gas and vaporcavitation. In gas cavitation, gas bubblesarise during fluid entry if local pressuredrops below the saturated vapor pres-sure. Vapor cavitation occurs when thevapor pressure of the fluid is reached inthe pump working chamber. Vapor bub-bles not only reduce the rate of delivery,but they implode in areas of higherpressure to cause pressure surges. Thiscan cause cavitation erosion or evendestroy the pump. Cavitation can bedetected in acceleration FFTs measuredat high frequency. Usually, the frequencyspectra will exhibit very broadband exci-tations as shown in Fig. 1. It cannot becalculated exactly where the cavitation

vibrations are located. Online vibrationmonitoring or trial runs, however, makeit possible to determine the frequencyranges. If the frequency bands and theassociated cavitation types are known,equipment such asVIBNODE® can beused to monitorcavitation occur-rence online. Awarning is issued assoon as a vapor orgas cavitation takesplace. PRÜFTECH-NIK has accompa-nied pump manu-facturers as they al-located frequencyranges such as these

and has defined discrete cavitation char-acteristic values.

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the service magazine of the PRÜFTECHNIK Group

Glossary of termsDid you know?

Sound: Vibrations of media at frequencies from16 Hz to 20 kHz are audible to the human ear.Sound can be classified as air-borne sound,structure-borne sound and water-borne sound.

Pitch, timbre: Pitch is indicated in the frequencyspectrum by a single line. Timbre consists ofmultiple partial tones whose frequencies com-bine to a harmonic whole as perceived by thehuman ear.

Noise: Noise consists of randomly combinedtime functions and/or non-harmonic frequencies.It may be perceived as a nuisance or as a sourceof discomfort. The individual threshold for noisetolerance cannot be measured.

Personal noise exposure: A measure of thetotal noise received at the workplace averagedover an 8-hour shift or a 40-hour work week.

Sound pressure: The sound pressure is a time-dependent alternating pressure superimposed onthe equilibrium pressure. The so-called A-weight-ing describes the hearing response.

Sound particle velocity: The sound particlevelocity refers to the speed with which materialparticles oscillate in the sound medium.

Sound intensity: The product of the soundparticle velocity and the sound pressure equalsthe sound intensity. It is expressed in units ofpower per unit area and provides a clear repre-sentation of the sound gradient.

Level: Because the values of the sound parame-ters extend over multiple factors of ten, a log-arithmic scale is used in technical acoustics. It isexpressed in decibels (dB). The equation used tocalculate the sound level values has the follow-ing basic form:

(Measured value relative to the reference valuex0 )

Sound power level: A machine-specific mea-sure of the total radiated sound power of asound source under constant operating conditi-ons. The sound power level is dependent on themeasurement room, distance and other noisesources.

Frequency: The frequency expresses the pitch ofthe sound in Hz. A doubling of the frequency isan octave or three third-octave steps. The hea-ring range covers ten octaves.

Volume: The sensitivity of the human ear varieswith the frequency. Sounds of the same pressurebut with different frequencies are perceived tohave different volumes. To nevertheless obtainan adjusted noise characteristic value, the soundpressure is weighted as a function of the fre-quency. DIN 45633 mandates use of the A-weighting curve, identified by dB(A), for ma-chinery construction and operation.

Spectrum: The spectrum displays the frequencycomponents contained in a signal. It is used toanalytically characterize a sound. There are oc-tave, third-octave and narrow band spectra.

Figs. 2+3: Third octave spectrum of the sound pressu-re level of the above pumps with pronounced excita-tions at the vane pass frequency

Fig. 1: Reference box as perEN12639:2000

Fig. 4: Frequency spectrum with log. frequency axis

Fig. 5: Order spectra of the vibration velocity

L = 10 . log — [dB]xx0

Condition Monitoring Application

Analyzing causes of noiseMisel Tanasijevic

The noise radiation of a centrifugalpump can be used as a measure of itsrunning characteristics. When unusualnoise occurs, the noise levels and thethird octave and narrow band spectrashould be measured. To do so,VIBXPERT® and a microphoneare used to measure the sound

pressure level on an envelopingsurface at a distance of 1 mabove the centrifugal pump.The resulting mean sound pres-sure level can then be calculat-ed using DBSPECTRA® andcompared with the agreednoise levels. Dominant exciterscan be identified in the thirdoctave band or on the basis ofnarrow band spectra in OMNI-TREND®.

Example in apower plant

A variable speed centrifugalpump in a new power plantdrew attention because it gen-erated additional noise whenoperating in the range of 700rpm. Measurements were takenat several speed levels and thecustomer’s findings were veri-fied.

Result of acceptancemeasurement

At 700 rpm, the levels in-creased markedly and the 125Hz, 250 Hz and 500 Hz thirdoctave bands rose dispropor-tionately. The cause was thevane pass frequency, as shownby the narrow band frequencyspectra and order spectra. Thisindicated that there was turbu-

lence in the pumping medium. Ourrecommendation to the pump manu-facturer was to check the lines, theintake cross-sections and the suctionpressures in each case to reduce thevane excitation.

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the service magazine of the PRÜFTECHNIK Group

Fig. 4: Alignment result at machine C

Fig. 2: ROTALIGN® Ultra laser transmitter at-tached with a magnetic bracket

Fig. 5: Principle of plumbness measurementwith INCLINEO®

Fig. 3: Sensor attached with a magnetic bracket– equipped for wireless data transmission

Fig.1: Pumped storage power plant Hohenwarte

Fig. 6: Plumbness measurement on a verticalshaft with the INCLINEO®

Alignment Application

Aligning vertical pumps and checking for plumbnessBernardo Quintana

With eight pumped storage units anda rated power output of 320 MW, thepumped storage power plant at Hohen-warte II in Thuringia is the largest pow-er plant on the Saale River. In times ofexcess power generation – generally atnight – water is pumped into the elevat-ed storage tank to make it available tothe turbines for power generation dur-ing peak consumption.

Vertical alignmentwith ROTALIGN® Ultra

As part of an inspection of machine C,PRÜFTECHNIK Machinery Service wascontracted to align the shaft. The equip-ment of choice was ROTALIGN® Ultra,ideally suited for this type of machinewith its functions for aligning verticalmachines and for the wireless trans-mission of data between computer andsensor.

First, the gear coupling between theturbine and pump was disassembledand the laser and sensor were attachedto the flange faces using magneticclamping equipment (Figs. 2 and 3). Totake the measurements, both shaftswere rotated to the clock positions at0(=elevated water storage)–3–6–9.

In the next set of measurements takenat the second coupling between the tur-bine and generator, the bearing clear-ance of the journal bearings had to betaken into account. The air gaps at theclock positions were filled with 0.4 mmshims.

The machinery was aligned with theaid of hydraulic presses by insertingshims at the radial bearings under thepump. Fig. 4 shows the alignment re-sult. Alignment is now well within thetolerance specified for a speed of 428rpm.

Plumbness measurementusing INCLINEO®

The relative alignment of the shafts toeach other is not the only criterion forlarge vertical machines. The plumbnessof the shafts – i.e., the relationshipbetween the rotating centerline to gravi-ty – is also important. The tolerance

here is 0.02 mm/m. If the shafts are notplumb, this will result in imbalances,

vibrations, higher temperatures, ashorter bearing life span and/or lowerefficiency.

The measurements were taken withthe new INCLINEO®, a high-precision,electronic inclinometer. It is simple toattach to the shaft with its magneticbase. The value for the plumbness ofthe shaft was obtained by comparingtwo measurements at opposite posi-tions.

The measurements were taken intwo directions as well as at the 45°positions to ensure repeatability. Thesemeasurements were repeated at every

section of the shaft, thus determiningthe position of the entire shaft relativeto the plumb line.

If you require support for similarmeasurement tasks, our globally activeMachinery Service will be glad to assistyou.

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the service magazine of the PRÜFTECHNIK Group

Fig. 3: Vibration thresholds for horizontal pumps that rotatefaster than 3600 rpm and with a power input of over400kW/stage.

Taken from “Centrifugal pumps for petroleum, petrochemical and natural gas in-dustries (ISO/DIS 13709:2007)”

Fig. 2: Measurement locations for vibrationmeasurement on centrifugal pumps of theOH1 and BB1 types

Explanation:1 P = 3000 kW/stage2 P = 2000 kW/stage3 P = 1500 kW/stage4 P = 1000 kW/stage5 P = 700 kW/stage6 P = 500 kW/stage7 P = 300 kW/stage

Fig. 1: Typical measurement locations for mobile measurements

Condition Monitoring Recommendations

The toughest vibration standard for centrifugal pumpsDr. Edwin Becker

After a suitable pump is selected, therunning and operating behavior shouldbe checked for vibration because oftenthe machines that exhibit the least vi-bration are the better machines. Butwhat measurement locations and accep-tance criteria should be used for thevibration measurements? DIN EN ISO13709 provides challenging answers. Inthe experience of PRÜFTECHNIK, thisstandard, entitled “Centrifugal pumpsfor petroleum, petrochemical and natu-ral gas industries”, represents one of thetoughest vibration standards for pumpmanufacturers.

Type classesand measurement locationsThe standard subdivides centrifugal

pumps into 18 different type classes (seeTable 1). In addition to the requireddimensioning, it makes technical recom-mendations on housing design and indi-cates where vibration sensors should bemounted and where measurementsshould be taken. Fig. 2 shows accep-tance-related measurement locations forpumps of type class OH1 and BB1. Whileit is sufficient to take measurements onthe collar bearing in a horizontal, verti-cal and axial direction for the OH1 type,two additional measurement locationsneed to be used for the BB1.

Allowablevibration valuesIn Chapter 5.9.3, ISO

13709 presents allow-able vibration velocities(5 to 1000 Hz) that arepower- and speed-relat-ed and specifies the fre-quency range in whichthe FFT spectra shouldbe measured. For hori-zontal pumps workingin the rated operatingrange, the vibrationsmay not exceed a valueof 3.0 mm/s at poweroutputs of less than 300 kW and speedsof up to 3600 rpm. At higher speeds andoutputs, the nomogram shown in Fig. 3should be used. Vibrations greater than4.5 mm/s are not allowable on horizon-tal pumps.

For vertical pumps, 5.0 mm/s is theupper limit. It should also be noted that,outside of the operating range for bothtypes of pumps, a vibration increase ofonly 30% is tolerated.

Aside from the allowable vibrationvalues, ISO 13709 also stipulates specif-ic requirements for the structural andresonance behavior of centrifugalpumps and the necessary acceptancemeasurements.

More details can be found in thevibration specifications. ISO 13709 isavailable in English from the Beuth-Verlag (EUR 280).

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the service magazine of the PRÜFTECHNIK Group

Dates

News

ContactPRÜFTECHNIKCondition Monitoring GmbH85737 Ismaning, GermanyFax: +49 89 99616-0Fax: +49 89 99616-341eMail: info@pruftechnik.com

PRÜFTECHNIKAlignment Systems GmbH85737 Ismaning, GermanyTel: +49 89 99616-0Fax: +49 89 99616-100eMail: info@pruftechnik.com

www.pruftechnik.com

Information on all trade fairs, semi-nars and other important events of thePRÜFTECHNIK Group can be found onour website at www.pruftechnik.com

VIBXPERT®

now with coast-down spectraWhen a pump is switched off, natural

frequencies can be determined as it iscoasting down. Instead of coast-downcurves, VIBXPERT® can now be used tomeasure full coast-down spectra. Theycontain even more information.

ME’scope connectionODS is the ‘fine art’ of Condition

Monitoring. VIBXPERT® and ME’scopeVes 5.0 now work together to be able toperform OOOOOperation DDDDDeflection SSSSShape.

VIBNODE®

measures time recordsVIBNODE® now has the facility to

record time waveforms of the vibrationvelocities or accelerations – a uniquefeature for a low-cost CMS.

OMNITREND® with vibrationcode, diagnosis code and cor-

rection codePumps can be systematized using ma-

chine codes and described by means ofvibration, diagnosis and correctioncodes. PRÜFTECHNIK originally devel-oped these tools for wind turbines. Vi-bration codes are used to evaluate vibra-tion amplitudes. Diagnosis codes can beused to make standardized conditionstatements independent of language.Correction codes contain specific recom-mendations for the machine manufactu-rer or operator on how to improve ma-chine availability.

New laser speed sensorPRÜFTECHNIK has introduced a new

laser speed sensor onto the market. Itcan be used to measure speeds on veryslow and very fast machines. A speedmarker is attached as a reflector. Thevisible laser beam can be pointed to thereflector from distances of up to onemeter and deliver precise speed values –when the equipment is being balanced,during resampling and for coast-downmeasurements. The package is roundedout with a quick-mount stand with a balljoint.

WEARSCANNER®

For force-lubricated machines (alsofor pumps), the change in particle sizedistribution in oil is an important mea-sure of machine condition. On the basisof eddy current technology, PRÜFTECH-NIK has developed an autonomous sen-sor for determining particle sizes in upto 8 size classes. WEARSCANNER®®®®® isconnected to the monitoring system orthe system control unit via ModBus TCP.

New service:noise measurement

The PRÜFTECHNIK Service and Dia-gnosis Center now performs noise mea-surement anywhere in the world andoffers recommendations on noise reduc-tion.