Noise sources in centrifugal pumps

Transcription

1 Proceedngs o the 2nd WSEAS Int. Conerence on Appled and Theoretcal Mechancs, Vence, Italy, November 20-22, Nose sources n centrugal pumps BERND DÜRRER, FRANK-HENDRIK WURM ARTEC Research and Technology Center WILO AG Nortkrchenstraße 100, Dortmund GERMANY Abstract: - The reducton o nose emsson s o vtal mportance or pumps used n HVAC applcatons. Several components contrbute to the nose emsson o a pump, e.g. pressure pulsatons orgnatng rom the hydraulcs or electromagnetc orces n the electrcal drve. In order to reduce the nose emsson, t s necessary to denty the share o each source n the overall nose level. Ths can be acheved by vbraton measurements and spectral analyss on the pump and approprate models o the underlyng nose-generatng processes. In ths paper, the most mportant nose sources n centrugal pumps and ther electrcal drves wll be explaned. The eect o these sources wll be llustrated by measurement results o structure- and lud-borne sound power. Possble remedes and ther eect on nose reducton wll be dscussed. Key-Words: - Nose, vbraton, pulsaton, structure-borne sound, lud-borne sound 1 Introducton The nose emsson o pumps used n HVAC applcatons s an mportant qualty and comort eature. Especally the emsson o structure- and lud-borne nose has to be consdered, as these can propagate along the ppes to senstve locatons ar-away rom the nstallaton poston. The propagaton and radaton o nose depends on actors lke buldng structure and materals, ppe connectons and other components n the nstallaton (e.g. thermostatc valves and heatng radators). As these actors are beyond the control o the pump manuacturer, t s crucal to mnmze the exctaton o nose at ts orgn. In order to acheve a mnmum nose level, the man nose sources and exctaton processes must be known. These are, o course, related to the hydraulcs and the mechancal assembly, but also (especally n the case o pumps wth a canned motor) to the electrcal drve. For each o these sub-systems, the potental nose sources wll be dscussed. Only nose sources nherent to the desgn are ncluded whle nose caused by deects s dsregarded. 2 Hydraulc nose sources The hydraulcs o pumps conssts o one or several rotatng mpellers and restng dusers respectvely a gude casng. The mpellers convey energy accordng to the Euler equaton to the lud. In the duser resp. the gude casng the low s decelerated and the lud s guded to the next mpeller or the ppe system beyond the pump. The low nsde a pump s three-dmensonal, nstatonary and mostly very turbulent. Several processes n the pump result n pressure pulsatons and consequently excte lud-borne sound, that propagates n connected ppes, and also excte vbraton o the sold boundares that leads to the radaton o ar-borne sound [1]. 2.1 Instatonarty o low The nstatonarty o the low s caused by secondary lows n the mpeller due to rotaton, the nte number o blades and nte blade thckness, but also by eects o turbulence. The nte number o blades leads to a secondary low, caused by the asymmetrc outgong low o the mpeller. The nte blade thckness causes a notch n the wake low. Both eects result n a tme-dependent ncdent low on the restng parts (gude vanes o the duser, volute tongue o the gude casng) and consequently excte vbraton o these parts Pressure pulsatons Pressure pulsatons are detected at dscrete requences that are multples o the rotatng requency and the number o blades; these requences are also called blade passng requences (BPF): bp = n (1) The ampltude o these pressure pulsatons depends on a number o desgn parameters o mpeller and duser and operatng parameters. One o the most mportant parameters s the dstance between mpeller and the volute tongue. Smaller dstances typcally result n a much hgher ampltude o the BPF. However, ths b rot

2 Proceedngs o the 2nd WSEAS Int. Conerence on Appled and Theoretcal Mechancs, Vence, Italy, November 20-22, dstance also eects the ecency o the pump. Thereore, n ndustral practce oten a compromse has to be ound between hydraulc and nose speccaton. In addton to peaks at the BPF, there are sometmes peaks to be ound at lower multples o the rotatng requency. Ths s oten the case when the number o blades s not a prme number, e.g. an mpeller wth 8 or 9 blades. In such a case, the mpeller can be regarded as a superposton o several vrtual mpellers, each wth a number o blades that corresponds to a prme actor o the actual number o blades, or nstance an mpeller wth 9 blades can be regarded as a superposton o 3 vrtual mpellers wth 3 blades each. Measurements o such mpellers oten reveal BPFs at the 3 rd and 6 th multple o the rotatng requency n addton to the expected BPF at the 9 th multple. As the pressure pulsatons are generated by the wake low, ther ampltude s also dependent on the operatng pont o the pump. In general, t s neasble to operate a pump at partal load ar away rom the best ecency pont (BEP). Especally the pump s operated at low low rates, ths also means a hgher manometrc head,.e. hgher derental pressure. As the overall derental pressure o the pump s the mean o the luctuatng pressure, ths ncrease o the mean derental pressure typcally results also n an ncrease o the ampltude o pressure pulsatons, caused by an unsteady separaton o the low. Thereore, t s mportant durng the plannng o a new nstallaton to select a pump whose BEP s close to the speced operatng pont. In case, that several alternatves are avalable, the pump wth the lower hydraulcs characterstcs should be selected. Ths s especally mportant or nstallatons wth a varyng load prole (e.g. heatng systems) where the maxmum load s only rarely needed. In addton, also the radal hydraulc orces actng on the mpeller are mnmum at the BEP. As the radal hydraulc orce s transmtted va the shat and the bearng, t wll excte vbraton o the pump housng and urther propagate as structure-borne sound along the connectng ppes. The ampltude o the pressure pulsatons can be reduced by a desgn o mpeller outlet and volute tongue that s optmzed wth respect to acoustcs, as the pressure pulsatons are nluenced by the nteracton o the wake low and the volute tongue. I the dstance between mpeller outlet and volute tongue can not be ncreased due to the hydraulc requrements, nose generaton can be reduced by mprovng the geometry (or nstance, the shape o the tralng edge o the blades). Sharp edges and bends at the volute tongue should be avoded. In addton, t s oten advantageous to desgn the volute tongue and the mpeller blades n such a way that tralng edges o the blades and the leadng edge o the volute tongue are tlted aganst each other. Ths can ether be acheved wth mpeller blades that are twsted n axal drecton or by a volute tongue that grows n axal drecton so that the leadng edge rses towards the duser outlet. By ths tlt o the edges, the smallest gap between mpeller and volute tongue wll occur at derent angular postons, wth respect to the rotatonal axs. As a result, the pressure oscllaton wll be smeared over tme, reducng the ampltude o ts tonal nose. For nstallatons wth varyng load, t s recommendable to use a pump that can adapt to these condtons. For example, n heatng nstallatons wth thermostatc valves, the low resstance changes wth the postons o the valves. In case o mostly closed valves, a pump wthout control wll provde hgher derental pressure that may result n unwanted whstlng nose at valves and radators. An electroncally controlled pump recognzes the lower power need o the crcut and wll adapt to the new state o the system Turbulence The mpact o turbulence on nose exctaton s stll a subject o basc research. Prncpally, the low n pumps (ncludng the sound eld) can be calculated by solvng an equaton system ormed by equatons or the conservaton o momentum (Naver-Stokes equatons) and the conservaton o mass [2]. The resultng system o partal derental equatons has not been solved analytcally untl today. Numercal soluton s only possble or a lmted number o cases (e.g. relatvely smple geometres, small Reynolds numbers) due to the extreme varety o derent szes o eddes that have to be taken nto account. Thereore, or engneerng purposes oten the tme-averaged equatons are solved numercally. By ths method, the mpact o turbulence s taken nto account statstcally. The representaton o small eddes, that may contan sgncant energy, s extremely smpled. The eect o ths approach on the perormance o acoustc calculatons s a eld o research nowadays. However, t may be hypotheszed that even small energy-rch eddes have sgncant mpact at least on the generaton o lud-borne sound. Another major hydraulc source o nose s cavtaton n the pump. Yet, as or most applcatons cavtaton s a orbdden state o operaton, ts eects are not urther dscussed here. 3 Nose sources n the electrcal drve The electrcal drve has to be ncluded n the analyss o nose sources o pumps: Ths s obvous n the case o canned motors where the electrcal drve cannot be separated rom the pump, but also or glanded pumps, where the coolng an o the motor s a domnant source o ar-borne nose and the motor tsel an mportant source o structure-borne nose. Nose n the electrcal

3 Proceedngs o the 2nd WSEAS Int. Conerence on Appled and Theoretcal Mechancs, Vence, Italy, November 20-22, drve can be excted by mechancal orces (e.g. due to mbalance), magnetostrctve orces and radal electromagnetc orces. The nose exctaton process also depends on the type o electrcal drve: Though typcally an asynchronous motor s used, electroncally commutated motors (ECM) become more and more mportant due to ther much lower energy consumpton. 3.1 Mechancal orces As or all rotatng machnery, mbalance o rotor components can be a source o unwanted vbraton. The eect o the mbalance s a revolvng centrugal orce that can be detected n the vbraton spectrum at the rotatng requency. Ths eect can be avoded by proper balancng o rotatng parts. Deects n the mechancal assembly (e.g. bearng aults) may lead to addtonal unwanted nose and vbraton, but these are not dscussed here. 3.2 Magnetostrctve orces The magnetc lux n the lamnaton stack causes a relatve elongaton (known as magnetostrcton) that s proportonal to the squared lux densty B n the stator [1]. As the lux densty s proportonal to the supply current, the elongaton changes perodcally, causng vbraton o the stator at requences that are multples o twce the supply requency. For a supply requency o 50Hz, vbraton wll occur at 100Hz, 200Hz, 300Hz, etc. (Fg. 1). Reducton o nose caused by magnetostrctve orces can be acheved by usng materal wth low magnetostrcton and unorm pressng o the lamnaton stack. [m/s²] 1 300m 100m 30m 10m 3m 1m Autospectrum(Körperschall) - Input 800Hz Workng : Input : Input : FFT Körperschall - 800Hz number o rotor and stator slots, the permeance n the ar gap depends on the angular poston. In addton, the permeance may vary due to eccentrcty or dssymmetres o rotor and stator and magnetc saturaton o the lamnaton stack [4]. By multplyng the permeance wth the magnetomotve orces o rotor and stator, the magnetc lux densty n the ar gap s obtaned. The magnetc lux n the ar gap causes a radal orce, that s proportonal to the squared magnetc lux densty. As the ampltude o ths radal electromagnetc orce changes perodcally wth the rotaton, vbraton o the stator s excted at partcular requences that depend on the type o electrcal drve, ts desgn parameters and the rotatonal speed Inducton motors For an nducton motor, t can be shown, that even the motor s ree rom mperectons lke eccentrcty, rotor and stator dssymmetres and saturaton, a radal orce wll occur at requences that depend on the supply requency 1, the number o rotor slots Z rt, the slp s and the number o pole pars p [4]: Z (1 s) p 1 rt = ± 2 1 (2) Addtonal sde bands wll appear at multples o twce the supply requency (Fg. 2). The vbraton mode that s excted depends on the number o stator slots Z st, Z rt and p. In the presence o other adverse actors lke eccentrcty, the ampltude o ths radal orce may sgncantly ncrease, resultng n notceable nose and vbraton. [m/s²] 1 300m 100m 30m 10m 3m 1m 300u Cross-spectrum(Vbraton #4 - axal vorne,vbraton #2 - axal hnten) - Current (Magntude) Nr. 17-2,013µF mea : St.max 1,0mh 2,013µF : Input : FFT Vbraton Z rt rot ± u Fg [Hz] Vbraton spectrum showng strong peaks at multples o 100Hz caused by magnetostrcton 3.3 Radal orces Radal orces are caused by the man magnetc lux n the ar gap o an electrcal machne. Due to the nte Fg k 1.2k 1.4k 1.6k 1.8k 2k [Hz] Vbraton spectrum caused by radal orce n a sngle-phase nducton motor In the case o a sngle-phase asynchronous motor (that s oten used or small crculaton pumps wth a canned motor), these eects can be ampled by the unbalance between the magnetc elds o man and auxlary wndng [5]. The nose caused by radal orces n nducton motors can be mnmsed by adjustng the

4 Proceedngs o the 2nd WSEAS Int. Conerence on Appled and Theoretcal Mechancs, Vence, Italy, November 20-22, desgn parameters (number o rotor and stator slots and pole pars) n such a way that only hgher lexural modes o vbraton are excted and that the exctaton requences do not concde wth resonant requences o the stator core [6]. For the mechancal assembly, t s o vtal mportance to acheve optmum concentrcty: Otherwse, the eects o the non-unorm ar gap wll lead to a sgncant ncrease o nose exctaton [7]. Deects n electrcal components lke broken rotor bars or wndng aults may also lead to nose and vbraton, but are not dscussed here Electroncally commutated motors (ECM) ECM drves are more and more oten used or pumps due to ther consderably lower energy consumpton: As the magnetc eld o the rotor s not created by nducng current n the rotor conductors, but by permanent magnets n the rotor, the nternal losses o the motor are dramatcally reduced. Whle the rotatonal speed o nducton motors s lmted by the supply requency, the ECM drve can be desgned or hgher speeds. Ths means that sze and weght o the pump can be reduced whle provdng comparable hydraulc power. Smlarly as or the nducton motor, the exctaton o vbraton depends on the magnetc eld n the ar gap. Agan, the non-unormty o the ar gap s a major source o nose exctaton. For the ECM drve, eccentrcty o rotor and stator wll result n nose and vbraton exctaton at specc requences related to the number o rotor poles p and stator slots s [8]. Two extreme cases o eccentrcty can be dstngushed: statc and dynamc eccentrcty. In the case o statc eccentrcty, the rotor s concentrc wth ts own axs o rotaton, but ths axs does not concde wth the center o the stator. Ths results n a radal orce drected to the mnmum ar gap that oscllates as the rotor poles pass by durng a revoluton. Consequently, apart rom a constant component, a radal orce wll occur at requences that are multples o the rotatng requency tmes the number o rotor poles p: Though the two cases o statc and dynamc eccentrcty provde some nsght nto the exctaton o vbraton n an ECM drve, t s not possble to generalze rom these two academc cases to typcal vbraton phenomena ound n practce. Actual vbraton measurements o ECM drves oten show peaks at any multple o the rotaton requency (Fg. 3). Obvously, such a result cannot be explaned by a smple superposton o the spectra caused by statc and dynamc eccentrcty. Nevertheless, such a spectrum s the eect o a combnaton o both statc and dynamc eccentrcty: Dependng on the angular poston o the rotor, the eects o statc and dynamc eccentrcty wll ether partally compensate or amply each other durng a revoluton o the rotor. Ths means that the radal orce wll become maxmal when the rotor reaches a poston where the ar gap becomes mnmal, whereas the radal orce wll become mnmal when the center o the rotor s closest to the center o the stator: In ths case, the devatons o the wdth o the ar gap rom ts nomnal wdth around the crcumerence reach ther mnmum, resultng n a mnmal radal orce. In addton to the act that the radal orce vector s rotatng, ts ampltude s oscllatng between these two extremes. The magntude o ths oscllaton s typcally bgger than or the cases o statc or dynamc eccentrcty alone, as the varaton o the magnetc lux s bgger than that caused by the dscrete nature o stator slots and rotor poles alone. rot = p (3) rot For dynamc eccentrcty, the axs o rotaton s concentrc wth the stator, but not wth the center o the rotor. Ths means that the poston o the mnmum ar gap s also rotatng, resultng n a radal orce vector that s revolvng at the rotatng requency. Addtonally, ths orce wll oscllate as the mnmum ar gap passes by the stator slots durng a revoluton. Consequently, apart rom the rotatng requency, a radal orce wll occur at requences that are multples o the rotatng requency tmes the number o stator slots s: = s (4) rot Fg. 3 Vbraton spectrum o a ECM drve runnng at 3800RPM: Vbraton s excted by radal orce at multples o the rotaton requency. The exctaton requences can be calculated by developng the progresson o radal orce over revoluton angle nto a Fourer seres: As the luctuaton o the radal orce s perodc wth the revoluton o the rotor, the spectrum may contan any multple o the rotaton requency (Fg. 3). The ampltude o each requency depends on the devatons o rotor and stator rom ther center and the proporton o these devatons to each other.

5 Proceedngs o the 2nd WSEAS Int. Conerence on Appled and Theoretcal Mechancs, Vence, Italy, November 20-22, Furthermore, t would be an overly smpled assumpton that the eect o eccentrcty can be regarded as a two-dmensonal case where the axes are only shted but stll parallel. In realty, the axes o rotor and stator may also be tlted aganst each other, meanng that the radal orce wll not only depend on the angular poston but also wll vary along rotatonal axs. Ths s especally mportant n the case o pumps where hydraulc orces act on the mpeller, causng one-sded addtonal mbalance that s also load-dependent. The nose exctaton can be avoded by achevng optmum concentrcty o rotor and stator. Agan, deects n electrcal components (e.g. wndng aults, uneven magnetzaton o the rotor) may lead to addtonal nose and vbraton, but are not dscussed here. 4 Conclusons The overall nose level o a centrugal pump s determned by a lmted number o tonal components that can be related to underlyng perodcal processes n the hydraulcs and the electrcal drve. It s thereore possble, to denty the contrbuton o each o these processes to the overall nose level by a spectral analyss o pressure pulsaton and vbraton measurements. Based on the known desgn parameters o hydraulcs and electrcal drve, the orgn o domnant peaks n the spectrum can be deduced and approprate counter measures can be taken. Consequently, nose and vbraton measurements and accordng result analyss provde mportant clues or the contnuous reducton o nose emsson o pumps. Reerences: [1] Guelch, Bolleter, Pressure pulsatons n centrugal pumps, J. Vb. Ac., Vol. 114, 1992, pp [2] Gülch, Kreselpumpen, Sprnger Verlag, Berln, [3] Heckl, Müller, Taschenbuch der technschen Akustk, Sprnger Verlag, Berln, [4] Yang, Low-nose electrcal motors, Clarendon Press, Oxord, [5] Yang, Acoustc nose rom small 2-pole sngle-phase nducton machnes, IEE Proc.-Electr. Power Appl., Vol. 122, No. 12, 1975, pp [6] Fnley, Nose n nducton motors Causes and treatments, IEEE Trans. Ind. App., Vol. 27, No. 6, 1991, pp [7] Ellson, Yang, Eects o rotor eccentrcty on acoustc nose rom nducton machnes, Proc. IEE, Vol. 118, No. 1, 1971, pp [8] Hartman, Lormer, Undrven vbratons n brushless DC motors, IEEE Trans. Magn., Vol. 37, No. 2, 2001, pp