CONTROL OF VIBRATION WITH DYNAMIC ABSORBER

Size: px

Start display at page:

Download "CONTROL OF VIBRATION WITH DYNAMIC ABSORBER"

Maryann Elliott
5 years ago
Views:

1 CONTROL OF VIBRATION WITH DYNAMIC ABSORBER Ry Beebe Senior Lecturer Monsh University School of Applied Sciences nd Engineering Summry: High beting vibrtion ws evident in the structure of lrge boiler structure nd ssocited buildings. The level becme high enough to be nnoying to workers in the vicinity. Vibrtion nlysis ws used to identify the frequency of the exciting vibrtion nd its likely source. It ws concluded tht the vibrtion ws close to the resonnt frequency of the structure, but reduction t the source of vibrtion ws neither sufficient nor prcticble to control it. A vibrtion bsorber system ws designed. The pper describes the investigtion, the identifiction of the source, nd the successful control mesure - design nd instlltion of series of dynmic bsorbers. The feture used on lter designs to prevent this problem is lso described. Keywords: Vibrtion nlysis, structurl resonnce, vibrtion bsorber. INTRODUCTION High beting vibrtion ws evident in the structure of lrge boiler structure nd ssocited buildings. Although the mplitude ws well below tht considered to be uncceptble for mchines for their integrity, the level incresed to becme high enough to be nnoying to workers in the vicinity. Vibrtion nlysis ws used to identify the frequency of the exciting vibrtion nd its likely source. As the vibrtion ws concluded to be t the resonnt frequency of the structure, reduction of the source ws not prcticble ws to control the vibrtion. A vibrtion bsorber system ws designed. This pper describes the investigtion, the identifiction of the source, nd the successful control mesure - design nd instlltion of series of dynmic bsorbers. The feture used on lter designs to void this problem is lso described.. THE PROBLEM An nnoying vibrtion ws present in the boiler steelwork structure of lrge power genertion unit. Although brely noticeble initilly, this incresed over some months to rech uncceptble levels from the viewpoint of its effect on people. Initilly, this only ffected chemicl instrument room, where lbortory glsswre left on bench would eventully bounce off nd brek on the floor! Eventully, the vibrtion cused nnoynce to the shift operting stff in the plnt control room. The mplitude of vibrtion ws low, nd t no stge indicted dnger to the plnt itself. However, vibrtion sufficiently high to be nnoying to people is one of the vlid resons for eliminting or reducing it. (Ref ) Vibrtion ws mesured t selected plces on the structure, nd nlysed with mnully-tune vibrtion nlyser. The vibrtion ws found to be t frequency of close to 8Hz, nd would behve in cyclicl fshion, incresing to mximum, then decresing to lmost nothing, over two-minute period. Such behviour is typicl of clssicl beting behviour, where two frequencies tht re very close together move into phse nd out of phse, s described in texts on vibrtion such s Ref. Hving red tht the nturl frequencies of lrge steel frmed structures cn be s low s in this rnge, the investigtor suspected tht this structure, some 90m tll, ws being excited to vibrte t its nturl frequency. A serch for the likely source of the exciting vibrtion commenced. The regulr vibrtion condition monitoring nd previous investigtions of the low speed fn-type col mills ment tht their nominlly constnt speed of 465r/min, or 7.8Hz, ws well known, nd ccordingly, they were quickly identified s the likely source. The unit hs eight such mills rrnged round the boiler. These mills provide the gs hndling cpcity to drw hot gs from the boiler t furnce exit level t temperture of the order of 000 C. The col (lignite) is dded just before entry to the mill, where it mixes with the hot gs s it impcts the bldes of the mill wheel. The fn ction then conveys the pulverised col nd hot wet gs mixture t pproximtely 60 C to the burners. Shown opened up through its entry door in Figure I, the impeller Presented t ICOMS00, Interntionl Conference of Mintennce Societies, Brisbne

hs mss of bout 0t, nd wer occurs on the impeller bldes in fshion tht is not unevenly distributed round the wheel. Unblnce therefore increses with service.

Ech of these rfts lso supported two of the fn-type mills. This foundtion rrngement ws not redily evident to observers (mechnicl engineers!

2 hs mss of bout 0t, nd wer occurs on the impeller bldes in fshion tht is not unevenly distributed round the wheel. Unblnce therefore increses with service. Figure I: fn-type col mill, showing impeller Study of the plnt construction drwings reveled tht ech of the four corner columns of the min steel structure ws supported on concrete rft. Ech of these rfts lso supported two of the fn-type mills. This foundtion rrngement ws not redily evident to observers (mechnicl engineers!), s the only visible floor concrete ws clerly not connected structurlly to the mill foundtions. The drwings showed tht this reinforced concrete pving hd been lid over the rfts nd the re between them, s in Figure II, nd ws designed to crry only the mintennce lods such s vehicles nd crnes. Figure II: min corner column nd the two djcent mills It ws therefore likely tht the vibrtion from two corner-djcent mills would be very well trnsmitted into the boiler steelwork vi the shred mill nd column supports. It ws decided to investigte further t its source how the vibrtion vried with time. Therefore, vibrtion trnsducer ws fitted t the sme loction on the bering csings of pir of these djcent mills. The time trces of the vibrtion were rrnged to be viewed on twin-bem oscilloscope. The time trces were clerly shown s sine wves of pproximtely 8Hz. As the time trces moved into phse, the structurl vibrtion ws t mximum, nd vnished when the trces were in nti-phse. This ws the clssic cse of beting s suspected. The forces from the residul unblnce in ech of the two impellers pprently blnced ech other when in nti-phse, but dded together when in phse to provide lrge exciting force to the structure. It ws lso evident tht the speeds of the mills vried slightly in service, possibly due to slight vritions in the intensity of col flow.

3 The levels of vibrtion s mesured t the berings of these mills were not high, nd not of concern for the condition of the mills themselves, ccording to vilble Stndrds (Ref 3). A common mesure to reduce excessive vibrtion is to reduce the source, which in the cse of rotting mchinery usully mens blncing. Blncing of these mills in-situ ws possible, s it hd been successful on smller mills of similr design, but here it ws considered too lengthy nd cumbersome process. The stte of blnce would lso be expected to decy s blde wer progressed, nd frequent blncing ws not prcticble. Given the lrge structure, the other common control mesures of chnge in system mss or stiffening were lso not prcticble. It ws decided to try the use of vibrtion bsorber. 3. THE SOLUTION A vibrtion bsorber consists of mss connected to the originl system by springs, such tht is free to vibrte independently of the originl structure. The springs cn be of ny type. An bsorber is useful where structure or mchine tht cn be considered to hve single-degree-of-freedom vibrtes t its nturl frequency, usully due to some forcing or exciting vibrtion. The bsorber dds second mss-spring combintion to the system, converting in into two-degree-of-freedom system. This new combined system now hs two new nturl frequencies, one bove nd one below the originl nturl frequency. When the bsorber system is tuned to the driving frequency nd hs reched stedy stte, the force provided by the bsorber mss is equl in mgnitude nd opposite in direction to the originl disturbnce force. With zero, or close to zero, net force cting on the primry mss, it does not move nd the motion is bsorbed by motion of the bsorber mss. For the method to be successful, the hrmonic excittion must be known nd not chnge much from its constnt vlue. Dmping must lso be negligible. These conditions were considered to pply in this cse. The theory behind the use of the vibrtion bsorber is given in textbooks on vibrtion nlysis, such s Ref. Defining these terms: Rtio of mss of bsorber to originl system mss: μ (usully chosen in the rnge of 5% - 5% of the originl system mss, where this is known) Nturl frequency of the originl system: p (ssumed here from the symptoms to be close to 7.8Hz) Nturl frequency of the bsorber system before it is ttched to the originl system k m = = Stiffness of bsorber springs Mss of bsorber Rtio of frequency of bsorber system to tht of originl system: β = (which for tuned system =) The eqution relting these gives the vlues of the nturl frequencies of the new system normlised to the frequency of the bsorber system (Ref ): p = + β β ( + μ) ± β 4 β ( + μ) β ( μ) + The vlues tht indirectly specify the mss nd stiffness of n bsorber system cn be seen s the rtio of the bsorber mss to the initil system mss, nd the rtio of the nturl frequencies of the bsorber mss system nd the originl system. The greter the bsorber mss compred with the mss of the originl system, the wider the useful frequency rnge of the bsorber. In the more usul pplictions of vibrtion bsorber, where the msses of both originl system nd bsorber re known, nd hence μ is redily found, the useful frequency rnge of the new system cn lso be clculted equting the following expression to, nd lso to (Ref ). This results in four roots, the centrl pir of which define the useful frequency rnge. As long s the exciting force remins inside the useful frequency rnge, the bsorber will be effective. (If this expression is greter thn unity, the bsorber will in fct increse the force to the originl system). 3

4 With dr s the frequency of the driving force: [ / ] ( ) dr [ ( / ) ] μ ( / ) [ + μ( / ) ( / ) ] dr p p dr p As in this cse the effective mss of the totl system ws not known, nd ws too complex nd too lrge to consider single bsorber t ny one position, some initil experimenttion ws done t point of reltively high vibrtion in the chemicl instrument room. A combintion of msses nd springs tuned to s close to 7.8 Hz s could be obtined ws tried nd eventully the correct combintion ws found to minimise the mplitude of vibrtion t tht point. The sme experimenttion ws pplied t mid-spn of the steel floor supporting bems under the control room. The finl mss/spring system ws designed s frme holding lengths of scrp steel ( supply of worn mill lining pltes ws redily vilble). Severl of these rrngements were instlled in loctions t bout mid-spn of the bems underneth the control room, with compression springs resting on the bottom flnge of the bems s shown in Figure III. Figure III: typicl bsorber supported on lower flnge of control room bems Another of simple tble design plte with springs underneth - ws instlled in the chemicl instrument room which is on the sme level s the control room. The systems were successful in controlling the vibrtion in the res where it hd been cusing nnoynce. Detiled mesurements of the mplitudes were not necessry, s the criteri for success ws to remove the nnoynce, nd this ws chieved. The systems hve been operting successfully for some yers. 4. THE FUTURE DESIGN The design of future boiler-turbine units ws well dvnced, nd it ws not possible to redesign the structures, which in ny cse were of completely different rrngement to the erlier plnt. The new mills were lso to be vrible speed. It ws decided to instll the fn-type mills on vibrtion isolting foundtions, designed by specilist compny. Ech mill ws mounted on its own concrete block foundtion, but insted of being supported directly connected with the min structurl system s in the plnt described in this cse study, the foundtion blocks re supported on severl lrge springs. Lrge simple dmpers re lso provided. The effectiveness of these proprietry systems in service is redily pprent when stnding with one foot on the mill block nd the other on the djcent floor. It ws not prcticble to modify the initil plnt to such n isolting system. 5. CONCLUSION An bsorber system ws successful pplied on n unusully lrge structure. Such n ppliction should encourge wider use of this vibrtion control mesure where the conditions require nd llow it. 4

5 6. ACKNOWLEDGEMENT Following the initil identifiction of the problem, the work of collegues, in prticulr Mr Crl Ross, in the experimenttion nd design of the system were pprecited. 7. REFERENCES () Beebe, RS Mchine condition monitoring MCM Consultnts 00 reprint () Inmn, DJ Engineering vibrtion Prentice Hll 996 (3) AS65, Prt Rotting nd reciprocting mchinery mechnicl vibrtion Mesurement nd evlution of vibrtion severity of lrge mchines in situ Pper presented t the Interntionl Conference of Mintennce Societies (ICOMS), Brisbne, 00. 5

Fig. 1. Open-Loop and Closed-Loop Systems with Plant Variations

Fig. 1. Open-Loop and Closed-Loop Systems with Plant Variations ME 3600 Control ystems Chrcteristics of Open-Loop nd Closed-Loop ystems Importnt Control ystem Chrcteristics o ensitivity of system response to prmetric vritions cn be reduced o rnsient nd stedy-stte responses