causes of vibration for steam turbine

7/27/2019 Causes of Vibration for Steam Turbine

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CAUSES OF VIBRATION

GENERAL

There are many causes of vibration, and an accurate analysis must be made in order to find the

stimulus before attempting to perform any corrective action. Some of the most common causesof vibration are as follos!

". Rubbing#. $isalignment

%. &il 'hip

(. Thermal )nstability

*. +ouble re-uency ibration

/. 0ore ibration

1. &ther 0auses of Generator ibration

2. $echanical 3nbalance

Let us loo4 at each cause and determine the course of action to ta4e if the situation occurs on aturbine5generator 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 locali9edheating on the shaft surface. 0ircumferential temperature gradients develop because 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 severeeffect is the "2:; movement of the through the critical speed

range. This interesting effect is illustrated in igure ".

igure ". $ovement of


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CAUSES OF VIBRATION

As the rotor accelerates, it develops a high spot in line ith the unbalance, or center of mass =see

igure ">. This is called .


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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-uipmentill 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


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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 changethe 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 ofsensitivity 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 thechanges are made. There is very little value in using a balance program to reduce vibration that is

caused by misalignment.

&)L ' and most of 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 tothe oil hip fre-uency. )n these cases, the resonant fre-uency ill often determine hipfre-uency.

igure #. 6earing &il 'edge

5 ( 5 $ (#*


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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


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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@tbe eliminated by balance or4. .

&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


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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 ofcalculating a balance shot using the Bone shotC and the Bmeasured effectC techni-ues.

A ord of arning should be mentioned here. 6e 5fore beginning any balance program, it is

e?tremely essential that the cause of vibration is correctly identified as a mechanical unbalancecondition that can be corrected by adding balance eights.


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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 balanceinstrument, ta4e accurate readings at the right time and location, and record and plot the data in a

neat and accurate manner.


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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 andcouple 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 &Aand &6represent the unbalanced condition. This data is resolved into static and

couple components by completing the parallelogram :5A50565& =see igure (>. This is done bydraing 60and A0e-ual and parallel to &Aand &6, respectively. 60and A0

intersect at 0 and therefore complete the parallelogram. Ne?t, dra the diagonals of the

parallelogram vectors &0and A6. These to vectors, or diagonals, bisect each other at point

+.

ector &0represents 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 A6represents the couple component. To measure and locate this component, transposeA6so that it intersects point : and call it E. Note that vector +Ais 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


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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 generaluse. 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 laststage 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 at the critical or

operating speed of a unit.

5 ": 5 $ (#*


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CAUSES OF VIBRATION

E?ample!

G5% Turbine5Generator

L8+ KA@ Rotor

&perating speed = %/:: rpm

LS6 %%.*C


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CAUSES OF VIBRATION

igure *

GRA8


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CAUSES OF VIBRATION

igure /. Graphical Representation

igure / represents the graphical representation of the problem. 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 referencemar4 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 ? (./ mils =from

vector analysis> "2.( ounces at the :; rotor angle on the I* bearing balance plane. Since thisis a couple correction, the eight ="2.( ounces> is to be placed at the :; M "2:; #1:; rotor

angle on the I/ bearing balance plane.

5 "% 5 $ (#*

Stator Angle


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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 puM mlM =%.:5 :;

shot desired location to place eight

resolved phase angle reading =)R+ mar4>

pu location of pic4 up

ml )R+ constant

5 =/::> M =::> M =%.:5#.(> ::

shot /&/::M::M*(:

shot ::

The eight sensitivity is still ( ounces7mil. on the stator angle. 6e sure to hold the previously positioned heel stationary =see igure 2>.

rom the previous methods e obtain a is lined up opposite the e-uipment constant =:;> on the pic4up heel.

Remember to hold the positions of all heels constant, or else the anser is meaningless. =See

igure .>

5 "( 5 $ (#*


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CAUSES OF VIBRATION

igure 2. 8ic4up Angle

The B'C indication =igure > on the


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CAUSES OF VIBRATION

E?ample!

G5% Turbine5Generator

L8+ BAC Rotor 5 %/:: rpm

%%. *C LS6

:;

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 arethe same. The O'gtof the General Electric TS) e-uipment is normally e?pected to be ithin Q#:;

from the O'gtobtained using the )R+.

5 "/ 5 $ (#*


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CAUSES OF VIBRATION

S is usually located /:; above the right side hori9ontal Doint, hile the

reference probe => is usually located #:; belo the right side hori9ontal Doint. 'ith the collarnotch 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 "#:;

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 ,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.

5 "1 5 $ (#*


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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 shoton 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 ithinacceptable 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. .


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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


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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 belo1 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.


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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 thevibration 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.

Trip immediately if shaft

vibration e?ceeds!

Less than 2:: rpm * 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.

Trip immediately if shaft

vibration e?ceeds!

Less than 2:: rpm 2 mils

2::5#::: rpm ": mils for # minutes "( mils

#:::5running speed ": mils for "* minutes "# mils

#" $ (#*

causes of vibration for steam turbine

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