CAUSES OF VIBRATION GENERAL There are many causes of vibration, and an d an accurate analysis must be made in order to find the stimulus before attempting to perform any corrective action. Some o f the most common causes of vibration are as follos! ".
Rubbing
#.
$isalignment
%.
&il 'hip
(.
Thermal )nstability
*.
+oub +oublle rere-ue uenc ncy y i ibrat bratiion
/.
0ore ibration
1.
&the &therr 0aus 0auses es of Gene Genera rato torr ibrat bratio ion n
2.
$echanic nical 3nbala alance
Let us loo4 at each cause and determine the course of action to ta4e if the situation occurs on a turbine5generator that you are or4ing on.
R366)NG Rubbing occurs hen the rotating element comes in contact ith the stationary element. )n a turbine, the rotating element is the rotor hile the stationary elements are usually the oil deflectors and steam and7or diaphragm pac4ing. 8ac4ing and7or oil deflector rubs cause locali9ed heating on the shaft surface. 0ircumferential temperature gradients develop beca use the rub is usually more severe on one side of the shaft and the rotor gradually bos toard the high spot. The effects of rubs on vibration are more severe hen the rotor is operating close to, or belo, the first critical speed than hen the shaft is rotating far above it. &ne reason for this severe effect is the "2:; movement of the
through the critical speed range. This interesting effect is illustrated in igure ".
igure ". $ovement of
5"5
CAUSES OF VIBRATION
As the rotor accelerates, it develops a high h igh spot in line ith the unbalance, or center of mass =see igure ">. This is called , and moves against rotation. The center of mass assumes an intermediate position hich is somehere beteen its original location and the location caused by the effect of the bo. This This changes .
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CAUSES OF VIBRATION
Steam Temperature ariation luctuating steam temperatures can cause a rub because of stationary component distortion. This is especially true in a reheat section. The customer@ s steam temperature monitoring e-uipment ill generally indicate this condition as ell as the turbine metal temperature recorders. The logical corrective measure is to 4eep the steam temperature from varying too much. Rapid E?haust
$)SAL)GN$ENT Turbine5generator misalignment should be suspected as a cause of vibration hen there is evidence of oil hipping, vibration instability, apparent change in critical speed range, unu sual and e?ceptionally high critical speed variation, or here the critical speed vibration varies over a ide range of machine rpm =over *:: rpm>. Also, abnormal bearing metal temperatures may indicate misalignment in the unit.
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CAUSES OF VIBRATION
$isalignment by itself produces little vibration stimulus unless it is severe enough to unload bearings to the point here oil hip occurs. )f this happens, very large lo fre-uency shaft vibration amplitudes ill clearly identify the hip. $isalignment may also significantly change the response characteristics to the e?isting unbalance stimulus. or e?ample, in some instances the stationary vibration levels of the last turbine bearing have improved mar4edly due to small alignment changes to the generator hich did not affect either the stimulus or the shaft vibration. $odern calculation procedures produce a Bfle?ibility inde?C hich permits a rapid estimation of sensitivity to misalignment. This inde? can be made available to field engineers by consulting Turbine Engineering if misalignment is suspected as a cause or contributor to unit vibration. )f maDor alignment alterations are re-uired, Turbine Engineering should be consulted before the changes are made. There is very little value in using a balance program to reduce vibration that is caused by misalignment.
&)L '<)8 &il hip is caused by an unstable oil stimulus in the bearing oil film. )t develops as a comple? function of Dournal peripheral speed, oil viscosity, bearing shape, radial bearing load, and Dournal attitude angle. $isalignment can contribute to the hip condition by changing the bearing loading. The oil hip ill usually appear and disappear suddenly as operating conditions change . The resulting shaft vibration amplitude is generally very high =":5"* mils> and most o f the displacement ill occur at appro?imately half operating speed fre-uency. 'hip conditions have been clearly observed at fre-uencies that vary from "(:: to ##:: cycles per minute for different types of %/:: rpm rotors. &il hip instability develops hen the high pressure oil edge =igure #> creates a lifting force that is greater than the Dournal eight causing the Dournal to momentarily lift for relief. Repeated periodically, this effect amplifies, especially in the presence of a rotor resonance that is close to the oil hip fre-uency. )n these cases, the resonant fre-uency ill often determine hip fre-uency.
igure #. 6earing &il 'edge
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CAUSES OF VIBRATION
0orrective action for oil hip includes the folloing! ".
Additional Dournal loading by an alignment change
#.
Reduce oil edge pressure by additional drainage or lea4age area
%.
Loer oil viscosity
(.
3se of special bearings that add to unit stability
The design details of these special bearings are outside the scope of this section. elliptical and the tilting pad bearing.
T
Shaft heat sensitivity
#.
'ater or oil in the shaft bore
%.
3neven heat transfer beteen rotor parts
(.
Loose heels or pin bushings
*.
&ther loose or poorly fitted parts =i.e., buc4ets>
/.
3nsymmetrical ventilation
1.
Short circuited turns in field coils
)t may be generally said that thermal instability is characteri9ed by mar4ed changes in vibration levels as operating conditions vary. These vibrations ill almost alays e?ist in operating speed fre-uency. 0hanges in load, steam conditions, field temperature, ater and steam seal adDustments, along ith other variables may cause a gradual change in overall vibration level. )f there are large variations in operating conditions, ma4e one correctional move at a tileF evaluate the move and ma4e another if re-uired. This is the best ay to properly diagnose these phenomena. 3sually, the responsible factory engineering group is consulted before underta4ing an e?tensive study of this nature. The magnitude of vibration changes caused by certain forms of thermal instability may sometimes be reduced by balance or4. The important concept here is to reach a compromise beteen the various conditions that cause thermal instability and produce acceptable vibration levels during all normal operating conditions. The apparent changes in vibration level -uite often become smaller as the balance refinement progresses. )f step changes in vibration levels are itnessed hen inlet steam conditions are altered, it may be the signal that something drastic has happened to the turbine. 0rac4ed rotors have been indicated in this manner.
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CAUSES OF VIBRATION
+&36LE RE3EN0 )6RAT)&N +ouble fre-uency vibration =vibration at tice operating fre-uency> sometimes occurs on to pole generators because the generator rotor cross5section is unsymmetrical. This situation can@t be eliminated by balance or4.
0&RE )6RAT)&N A generator ill occasionally develop obDectionable vibration levels due to magnetic forces acting on loose portions of the core. This vibration occurs at tice operating fre-uency and may be transmitted from the core to the stator frame. 0ore vibration is often accompanied by a noticeable bu99ing sound hich can be readily identified because the sound ill -uic4ly disappear hen the field current is removed h ile the unit is at synchronous speed. 6alance or4 has no effect on this type of vibration. This problem is rare because of the use of spring mounted 4ey bars and cutting cross slots on the generator pole faces =# pole fields>. &ther sources of core vibration stimuli include irregular Dournals and periodic steam or electrical forces. 'hile these and other special cases have a high degree of importance, their effects are self5evident and ill not be discussed here.
&T
$E0
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CAUSES OF VIBRATION
adding small eights to the rotor. These eights change the location of the center of mass in order to ma4e it coincide ith the center of rotation. )n the folloing paragraphs e ill find out ho to obtain, interpret, and plot accurate balance data. Also, e ill discuss the methods of calculating a balance shot using the Bone shotC and the Bmeasured effectC techni-ues. A ord of arning should be mentioned here. 6e 5fore be ginning any balance program, it is e?tremely essential that the cause of vibration is correctly identified as a mechanical unbalance condition that can be corrected by adding balance eights.
8L&TT)NG )6RAT)&N +ATA )t is generally desirable to confer ith colleagues, the district office, or Turbine Engineering before placing a balance shot in a rotor. or these discussions, an established convention must be used throughout the balance program in order to avoid confusion. 8olar coordinate graph paper that is similar to General Electric orm N5"/% is used to plot the vibration data. Heep in mind that the convention plots phase angles as they are on the rotor. . The right side of the hori9ontal a?is is designated : ;, the top vertical a?is is :;, and so on. $agnitude is mar4ed on any convenient linear scale beginning ith 9ero hich is located at the cross section of the hori9ontal and vertical a?es. See igure %.
igure %. 8olar Graph 8aper 8lotting 0onventions
A separate sheet of polar graph paper should be used to plot data obtained for each rotor. )f these angular conventions are used, the effect of a eight move plotted on the graph paper ill be in the same direction as a eight move on the shaft. Thus, eight rotation on paper coincides ith eight rotation on the rotor. 'hen plotting data for a particular balance run, the average of four consecutive readings that ere obtained after the unit had reached steady state conditions is used. This value is mar4ed on the appropriate sheet in the form of a small arro that points radially outard from the center of 515
$ (#*
CAUSES OF VIBRATION
the graph paper. The bearing number and shot number are mar4ed near the tip of the arro in the folloing manner! I":, I"", I"#, etc.
)NTER8RET)NG +ATA 'hen performing any balance program it is very important to correctly set up the balance instrument, ta4e accurate readings at the right time an d location, and record and plot the data in a neat and accurate manner. . After the rotor and type of correction has been selected, the eight si9e must be determined. The unit si9e, speed. amount of unbalance, and location of unbalance determine the amount of corrective eight needed. inally, the angular location of the eight must be ascertained. This can be accomplished by using the previously discussed high spot theory along ith the e-uipment phase angle. A high spot number an d eight sensitivity chart contains this information for most large steam turbines. =See addendum.> The obDective of balance or4 is to properly locate corrective eights on a rotor so as to nullify the unbalanced condition. After the original shot is installed, another balance run is made and the effect of the shot is measured. urther changes in angular position and amount of eight can be deduced from this data if they are re-uired. After corrections have been made and ma?imum benefits gained, other corrections of another type may be implemented, or another rotor in the combination can be improved.
525
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CAUSES OF VIBRATION
E0T&R RES&L3T)&N & +ATA 'hen operating at their rated speed, the vibration levels of most turbine rotors are influenced by to critical speeds. Therefore, the data recorded during operation is resolved into static and couple components and plotted on polar graph paper. See igure (. E?ample! The folloing readings are recorded! BAC bearing = (./ mils at *:; B6C bearing = #.2 mils at "":; ectors &A→ and &6→ represent the unbalanced condition. This data is resolved into static and couple components by completing the parallelogram :5A50565& =see igure (>. This is done by draing 60→ and A0→ e-ual and parallel to &A→ and &6→, respectively. 60→ and A0→ intersect at 0 and therefore complete the parallelogram. Ne?t, dra the diagonals of the parallelogram vectors &0→ and A6→. These to vectors, or diagonals, bisect each other at point +. ector &0→ represents the total static unbalance in this e?ample. )t is e-ual to /.( mils at 1#;. Since many rotors are balanced in to planes =near the Dournal bearings>, vector &+→ represents the static unbalance effect at each bearing and e-uals %.# mils at 1#;. ector A6→ represents the couple component. To measure and locate this component, transpose A6→ so that it intersects point : and call it E →. Note that vector +A→ is e-ual and parallel to &E→, and vector is e-ual and parallel to &→. The couple effect on BAC bearing is # mils at "#;, hile the couple effect on B6C bearing is # mils at "#;.
B&NE S<&TC $ET<&+ 3S)NG AN )R+ The folloing approaches apply to calling a balance shot using the Bone shotC method! ".
Graphical Approach
#.
Analytical Approach =based upon the graph>
%.
3se of the B&ne ShotC 0alculator The folloing information is also re-uired!
The folloing information is also re-uired! ".
8ic4up angle
#.
6alance instrument used to obtain data
%.
The importance of the first to items is stressed in another section. 'eight sensitivity and the high spot number are functions of the speed of the unit in relation to its critical speed. The closer the unit speed is to the critical, the more sensitive the unit ill be to a mechanical unbalance stimulus. 0onversely, if the unit speed is further from the critical, the less sensitive the unit is to the same stimulus. or e?ample, a rotor moves through the first critical speed range and reaches a pea4 vibration amplitude of ": mils. This can be considered to be a pure static component. At operating speed the static portion of the pea4 amplitude is ( mils. 55
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CAUSES OF VIBRATION
igure (. ector 8lots The mechanical unbalance stimulus is the same for both situations. )f #: ounces of corrective eight as used at the critical speed, the eight sensitivity at critical speed is =#: ounces 7 ": mils>, or # ounces7mil. Similarly, the eight sensitivity at operating speed is =#: ounces 7 ( mils>, or * ounces7mil. )n each case the corrective eight totals #: ounces. or most units that ere built in Schenectady, a high spot number chart is published for general use. This chart is added as an addendum to this section for your reference, and it represents the accumulated data on many similar units. )f the turbine code, rotor type, rated speed, length of last stage buc4et =L8 rotors>, and type of e?haust hood are 4non, you ill be able to determine high spot numbers and eight sensitivity =ounces7mil> hen vibration data is ta4en a t the critical or operating speed of a unit.
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CAUSES OF VIBRATION
E?ample! G5% Turbine5Generator L8+ KA@ Rotor &perating speed = %/:: rpm LS6 %%.*C
The e-uipment phase angle for an )R+ B%"(C is :;F the .
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CAUSES OF VIBRATION
igure *
GRA8<)0AL $ET<&+ Locate the couple component of the I* bearing end plane =see igure *>. The rotor is stopped ith the )R+ mar4 at the couple component angle for I* bearing hich is /; on the stator reference. Ne?t, measure against rotation from the shaft pic4up using the folloing steps. ".
E-uipment phase angle is :;. This point on the shaft is the dynamic high spot.
#.
An <.S. correction of =%.:5 ? :; is re-uired. =%.:5#.(> ? :; *(;. This is the position for the couple correction for I* bearing, An additional "2:; correction for I/ bearing is re-uired because this is a couple.
5 "# 5
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CAUSES OF VIBRATION
Stator Angle
igure /. Graphical Representation igure / represents the graphical representation of the p roblem. The e-uipment constant of the )R+ must be considered in order to locate the high spot. :; *(; in order to locate the re-uired eight hich is against rotation from the high spot. The rotor position must no be located relative to the stator reference mar4s. rom the previous vector resolution the I* bearing couple component is located at /;. 3sing the :; stator reference mar4 as the initial point and measuring against rotation as positive, e find that the eight location against rotation from the )R+ mar4 is M:; M /; M *(; 5 /:; :;.
The first shot ould entail installing a eight of ( o9.7mil =from <.S. charts> ? (./ mils =from vector analysis> "2.( ounces at the :; rotor angle on the I* bearing balance plane. Since this is a couple correction, the eight ="2.( ounces> is to be placed at the :; M "2:; #1:; rotor angle on the I/ bearing balance plane.
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CAUSES OF VIBRATION
ANALT)0AL RE8RESENTAT)&N The graphic method as shon on igure / can also be e?pressed mathematically by the folloing e-uation. ∆shot
∆ 5 ∆ pu M ∆mlM =%.:5 :;
∆shot
desired location to place eight
∆
resolved phase angle reading =)R+ mar4>
∆ pu
location of pic4 up
∆ml
)R+ constant
high spot number
∆shot
=/:> 5 =/::> M =::> M =%.:5#.(> ::
∆shot
/&/::M::M*(:
∆shot
::
The eight sensitivity is still ( ounces7mil.
B&NE 5S<&TC 0AL03LAT&R The same results can be obtained by using the Bone shotC calculator. )n order to determine the location of the couple correction for I* bearing, let the B:C mar4 on the rotor angle represent the )R+ mar4. 8lace this B:C rotor mar4 at the resolved phase angle for the couple correction =/; 5 see igure 1>.
igure 1. Rotor vs. Stator Angle Ne?t, place the arro on the pic4up heel opposite the /:; =position of actual vibration pic4up> on the stator angle. 6e sure to hold the previously positioned heel stationary =see igure 2>. rom the previous methods e obtain a <.S. #.( and the )R+ e-uipment constant of :;. The high spot number =#.(> is lined up opposite the e-uipment constant =:;> on the pic4up heel. Remember to hold the positions of all heels con stant, or else the anser is meaningless. =See igure .>
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CAUSES OF VIBRATION
igure 2. 8ic4up Angle
The B'C indication =igure > on the <.S. scale is opposite the :; position on the rotor heelF hence, the "2.( ounces is placed at :; on number * bearing and :; M "2:; #1:; on number / bearing.
igure .
E-uipment 0onstant vs.
S<&T 0AL03LAT)&N ')T< GENERAL ELE0TR)0 TS) )n order to determine the eight location for a balance shot using data recorded by General Elec5 tric@s TS) e-uipment, the folloing formulas should be used! O'gt
=
location of corrective eight
O :;
=E-uation ">
Therefore, O'gt
O$ P ORG M O0al 5 :; M =%5:;
=E-uation #>
5 "* 5
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CAUSES OF VIBRATION
E?ample! G5% Turbine5Generator L8+ BAC Rotor 5 %/:: rpm %%. *C LS6
I* bearing *.: mils at "%*;
I/ bearing (. * mils at #2*; Resolving this data yields the folloing corrections! Static! I* bearing ".# mils at #::; I/ bearing ".# mils at #::; 0ouple! I* bearing (. / mils at "#"; I/ bearing (. / mils at %:"; The solution of eight location is as follos! O'gt
J
O$
I* bearing (. / mils at "#";=given>
ORG
:
O0al
*;
#.( =see <.S. 0hart>
Therefore! O'gt
"#"; 5:; M*; 5:; M =%.: P #.(> :;
O'gt
:; for I* bearing
O'gt
:; M "2:; #1:; for I/ bearing
The above e?ample is the e?act duplicate of the e?ample given for the )R+ B%"(C e-uipment. The initial readings ere different due to the difference in instrumentation, but the ansers are the same. The O'gt of the General Electric TS) e-uipment is normally e?pected to be ithin Q#:; from the O'gt obtained using the )R+.
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CAUSES OF VIBRATION
S<&T 0AL03LAT)&N ')T< 6ENTLE NEA+A TS) The 6entley5Nevada TS) unit does not re-uire an e-uipment constant correction. &nce the data is ta4en, resolved, and corrected for the proper
igure ":.
8lacing 6alance 'eight on 6entley5Nevada TS)
The rotor is vieed from the turbine end, hence it is rotating in a countercloc4ise direction. The vibration probe => is usually located /:; above the right side hori9ontal Doint, hile the reference probe => is usually located #:; belo the right side hori9ontal Doint. 'ith the collar notch located at the reference probe, the corrective eight is installed O degrees against rotation =0'> from the vibration probe. E?ample! Resolved data for I" bearing (. : mils at "#:; :; "(1;
$EAS3RE+ EE0T )t is very important to plot the measured effect of the first shot, hether or n ot it improves the unit vibration. The measured effect is a vector that represents the effect of the shot on the rotor that is being balanced, and from it e can deduce hat the theoretical vector should have been for that particular correction. The measured effect method =igure ""> has been the bac4bone of the balancing techni-ue for many years. )f large enough eights are used, it is almost alays possible to understand the effects of a particular shot in any given rotor in one or to balance runs. )n reference to igure "", the reading for shot : as (./ mils at /;, and for shot " it as %.: at 1:;. The vector beteen : and " represents the effect of the installed eight =dotted line>. )f the eight as rotated countercloc4ise by the angle θ,the vector ould be in a direction e?actly opposite the vector at :. Therefore, vector :"→ ould cancel vector :, e?cept that it is slightly less in magnitude. , and a very slight increase in eight.
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CAUSES OF VIBRATION
'hen surveying the response of an entire unit, as many as eight or ten plots may be re-uired. 8roper analysis of these plots can become very comple?. The 4ey to a successful balance program lies in improving the highest vibration levels, ma?imi9ing the effect of a balance shot on those bearings nearest the corrective eights, and often compromising a great deal. )t may be necessary to increase optimum vibration levels in one rotor in order to gain acceptable vibration limits in adDacent rotors. )n the end, all components of the entire unit ill operate ithin acceptable vibration limits. )t is e?tremely desirable that each bearing Bcalling forC a move is not adversely affected by the move. 'hen both bearings react favorably, chances are that the right rotor and correction as selected for the trial eight.
)6RAT)&N L)$)TS ibration limits are necessary in order to decide hether or not a balance program is necessary for a given unit. Also, limits are necessary for units that vibrate due to causes other than me5 chanical unbalance =i.e., oil hip, rubbing>.
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CAUSES OF VIBRATION
igure "". $easured Effect
or units operating under steady state conditions at high loads, vibration belo levels in Table " is considered satisfactory and no immediate ba"ance programs are re-uired. TA6LE ". Steady State 0ondition 5
Shaft ibration
6earing 8ed. ibration
0ouplingibration
0ollector Ring. ib.
%:::5%/:: rpm
( mils
" mils
/ mils
/ mils
"*::5"2:: rpm
/ mils
" mils
2 mils
2 mils
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CAUSES OF VIBRATION
Allied ith Table ", vibration at critical speed is satisfactory if the levels are belo those stated in Table #.
TA6LE #. 0ritical Speed ibration Levels 3nit
Shaft
6earing 8edestal
%:::5%/:: rpm
2 mils
% mils
"*::5"2:: rpm
": mils
* mils
A unit is considered ell balanced and refinement attempts are not Dustified if the steady state vibration levels are less than, or e-ual to, the levels outlined in Table %. Allied ith Table %, the critical speed vibration limit for all ell5balanced units ould be belo 1 mils on the shafts and % mils on the bearing pedestals.
TA6LE %. ibration Levels for a 'ell 6alanced 3nit 3nit
Shaft
6earing 8ed.
0oupling
0ollector Ring.
%:::5%/:: rpm
# mils
mils
% mils
( mils
"*::5"2:: rpm
% mils
" mils
* mils
/ mils
)t is difficult to establish absolute ma?imum steady state vibration limits because some Dudgment is involved hich is dependent on several variables.
TA6LE (. $a?imum ibration Levels 3nit
Shaft
6earing 8ed.
0oupling
0ollector Ring.
%:::5%/:: rpm
/ mils
# mils
2 mils
2 mils
"*::5"2:: rpm
2 mils
% mils
"# mils
"# mils
)n conDunction ith Table (, critical speed vibrations e?ceeding "# mils on the shafts are sufficient to arrant unit balancing.
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CAUSES OF VIBRATION
)6RAT)&N TR)8 L)$)TS ibration limitations are dependent upon speed, length of time at specified vibration levels, and the cause of the vibration. )n order to prevent possible damage to turbine parts, Table * gives the vibration trip limits relative to speed and time.
TA6LE *. ibration Trip Limits Relative to Speed and Time %:::5%/:: rpm units Speed
Trip after Shaft ibration e?ceeds! mils for mm.
Less than 2:: rpm
Trip immediately if shaft vibration e?ceeds! * mils
2::5#::: rpm
1 mils for # minutes
": mils
#:::5running speed
1 mils for "* minutes
": mils
"*::5"2:: rpm units Speed
Trip after Shaft ibration e?ceeds! mils for mm.
Less than 2:: rpm
Trip immediately if shaft vibration e?ceeds! 2 mils
2::5#::: rpm
": mils for # minutes
"( mils
#:::5running speed
": mils for "* minutes
"# mils
5 #" 5
$ (#*
