iso 7919-1 part 1 general guidelines

7/25/2019 ISO 7919-1 Part 1 General Guidelines

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STANDARD

econd edition

1996 07 1

5

Mechanical vibration of non reciprocating

machines easurem ents on rotating

shafts and evaluation criteria

Part I

General guidelines

Vibrations mecaniques des rnachiaes

orr

alternatives Mesurages sur

les arbres tourpants ei crireres dlevi luation

Partie : Directives ienerales

L

.d-d:..-.-

THAI

INDUSTRI L STANDARDS

INSHTUTE

T l l ibrary

S0099f4

~ a u a u _ a u x o s ~ ~ u

Tna: ::lr.lstr~al andards Insl~lule T SI

Rama v l R c R a j a l h e v e ~Bangkok

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Tel 202-55.0

0 a h .

2 j j i

I S0 7919-1 1996 : Mechanical

vibration of non-reciprocatiinu

* p f :

0000326

RO

Reference numbel

SO. 791 9-1 1996 E)


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Draft lnternational Standards adopted by tha technical: committees are

circulated to th e mem ber bodies for voting. Publication as an lnternational

Standard requires approval by at least 75 of the m emb er bodies casting

a vote.

lnternational Standard IS 0 7919-1 was prepared by Technical Com mittee

ISOflC 108, Mechanical vibrstion and shock Subcommittee SC 2,

i Measurzm ent and ev2luation of mechanical vibration and shock as ap plied

o mmechines veh icles and struc tures.

This second edition of IS0 79191 cancels and replaces the first edition

( IS0 791 9 1 19861, which has been technically revised.

IS 0 791 9 consists o f th e following parts, under the general title

Mecharr-

ical vibration of non-reciprocating machines easurements on rotating

shafts and evaluation criteria:

an 1: General guidelines

fJ

?

kf

an : Large land-based steam turbine generator sets

art 3: Coupled idu st ria l machines F l c 2

an

:

Gas turbine sets

an

5:

Machine sets in hydraulic power generating and pumping

plants

Annex A form s an integral part of this part of IS 0 7919 Annexes B, C, D

and E are for informa tion only.

IS0 1996

All riahts reserved. Unless otherwise s~ecif ied . o Dart of this ~ub lic ati onmav be reoroduced

or u6ized in any form or by any means electronic or mechanida~ ncluding photocopying and

:

microfilm w~th out ermission in writing from the publisher.

lnternational Organization for Standardization

Case Postale 56 CH-1211 Geneve 20 Switzerland

Printed ~nSwitzerland


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ISO.

ntroduction

Machines are n ow being operated at increasingly high speeds 2nd loads,

and under increasingly severe operating conditions. This has become

possible, to a large extent, by the more efficient u se of m aterials, although

this has sq metimes resu ted in thera being less m argin for dssigr? and

application errors.

At present, it is not uncommon for continuous operation to be expected

and required for

2

or 3 years between maintenance operations. Conse-

quently, more restrictive requirements are being specified for operating

vibration values of rotating machinery, in order to ensure continued safe

and reliable operation.

IS0 10816 1 stablishes a basis for the evaluation of mechanical vibration

of machines by measgring the vibration respofise on non-rotating, struc-

tural memb ers only. There are many types of machine, however, for which

measu rements on structural m embe rs, such as the bearing housings, may

not adequately chsracterize the running condition of th e machine, although

such measurements ere usefui. Such machines generally contain flexible

rc~t or ha ft systems, and changes in the vibration condition m ay be de-

tectsd mo re decisively and more sensitively by measurements on the ro-

tatkg elements. Machines having relatively stiff and/or heavy casings in

comparison to rotor mass are typical of those classes of machines for

wh ich shaft vibratiofi me asureme nts are frequently to be preferred.

For machines such as steam turbines, gas turbines and turbo-

compressors, all of which may have several modes of vibration in the

service speed range, measurements or, non-rotating parts may not be

totally adeqilate. In such cases, it may be necessary to monitor the ma-

chine using measurements on the rotating and non-rotating parts, or on

the r ota ti ng ~ 3 r t s lone.

The guidelines presented in this part of I S 0 7919 are compleme nted by

.

those given in IS0 10816-1. If the procedures of both standards are ap-

plied, the one w hich is mo re restrictive generally applies.

Shaft vibration measurements are use d for a number of purposes, ranging

-

from routine operational monitoring and acceptance tzsts to advanced

I

experimental testing, as well as diagnostic and analytical investigations.

~nr:i : ;r~lfiydvb These various measurement objectives lead to many differences in

1 =. I e lo methods of interpretation and evaluation. To limit the number of these

i

mt/RI;.

: :7, . (; ; 871m7 f l

differences, this part of I S 0 7919 is designed t o provide guidelines pri-

L.. arily for operational monitoring and acceptance tests.

During the preparation of this part of IS 0 7919, it w as recognized that

there was a need to establish quantitative criteria for the evaluation of

machinery shaft vibration. H owever, the re is a significant lack of data on^

this subject at present and, consequ ently, this part of IS 9 7919 has been

structured to allow such data to be incorporated as it becomes available.


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.

1r r l~~R hlA T lO N A iTANDARD I S 0

Mechanical vibration of non reciprocating

machines easurem ents on rotating

shafts

and

evaluation criteria

Part :

General

y

uidelines

This par: cf I S 0 791 9 sets out general guidelines for

m e a su ri ~ g an3 e;laluating machinsry vibration b v

means 3f measurements made directly on rotating

shaits for the purpose ot determining sheft vibration

with rsgard to

a) changes in \librational behaviou r:

b l excessive kinstic load;

ements are found to be more suitablc, provided that the

*I

guidelines are respected. d

t

For the purposes of I S 0 7919, operational monitoring

It

IS considered to be those vibration rneasurernents

made during the normal operation of a ma chine.

:j

:SO 791 9 p ermits th e use of several d ifferent

measurement

quantities and m ethods, prov~cied hat

they are we ll defined and their limitations are set out,

so that the interpretation of the measurements will

ba well understood.

1

This part of I S 0 7919 does not apply to re cipr ~ca iing

:

machinery.

C) the monitoring of radial clearances.

.4 I

It

is applicable to measurements of bo th absoldte and

relative radial shaft vibration, but excludes torsional

and axial shaft vibration. The procedures are appli-

cable for bo th operational mcn itoring of machines and

to acceptance testing on a test stand 2nd 2fter instal-

lation. Guidelines are also presented for setting oper-

ational limits.

NOT S

1

Evaluation criteria for different classes of machinery will

be incl~dedn other pans cf

I S 0

7919 when they became

available. In the meantime, guidelines are given in

annex

A.

2

The term shaft vibration is used throughout I S 0 7919

because, in most cases, measurements

will

be mace on

mach~ne' hafts; however, I S 0 7919 is also applicable to

measurements made on other ro tat~ng lements

if

such

el-

2 Normative reference

The following standard contains provisions which,

through reference in this text, constitute provisions

of

this part of I S 0 7919. At the t im e of publication, the

edition indicated was valid. All standards are subject

to revision, and parties to agreements based on this

part of I S 0 7919 are encouraged to investigate the

possibility of applying the most recent edition of the

standard indicated below. Members of

IEC

and IS 0

maintain registers of currently valid International

Standards.

IS0 10816-1 :1995,

Mechanical vibration valuation

of machine vibration by measurements on non

rotating

p rts

art

I

General guidelines.


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Measurements

3 1

Measurement quantities

3 1 1 Displacement

The preferred measurement quantity for the

measurement

of shaft vibration is displacement.

The unit of measurement is the micrometer

(1

pr =

m).

NOTE

3

Displacement is

a

vector qua r~tity nd, therefore,

when comparing two displacements,

it

may be necessary

to

consider tbe phase angls betweec them (see zlsc

annex Dl.

Since this part of IS0

7919

applies to both relative

and absolute shaft vibration measurements, displace-

ment is further de fined as follows:

a) relative displacement, w hic h is the vibratory dis-

placement between the shaft and eppropriate

structure, such as a bearing housing or machine

casing; or

b) absolute displacement, which is the vibratory dis-

placement of the shaft with reference to an

inertial reference system.

NOTE 4 It should be clearly indicatzd whether displace-

ment values are relative or absolute.

Absolute and relative displacements are further de-

fined by several different displacement quantities,

each of which is now in widespread use. These in-

clude:

S

vibratory displacement peak-to-peak in the

direction of measurement;

S

maximum vibratory displacement in the

plane of measurement.

Either of these displacement quantities may be used

for the measurement of shaf: vibration. However, the

quantities shall be clearly identified so as to ensuie

correct interpretation of the measurements in terms

of the criteria of clause 5. The relationships between

each of these quantities are shown in figures B.l and

8.2.

NOTE

At present, the greater of the two values

for

peak-to-peak displacement,

as

measured in two orthogonal

directions, is used for evaluation criteria. In future, as rel-

e'vant

experience

is accumulated, the quantity

S~

de-

fined in figure

8.2, may be

preferred.

3 1 2 Frequency range

The measurement of relative and absolute shaft vi-

bration shall be broad band so that the frequency

spectrum of the machine is adequately covered.

3 2 Types of measurement

3 2 1 Relative vibration measurements

Relative vibration m easure men ts are generally carried

out with a noncontacting transducer which senses

the vibrato~y islj laceinent between the shaft and a

structural member (e.g. the bearing housing) of the

machine.

3 2 2 Absolute vibration measuremenrs

Absolute vibration measurements are carried out by

one of the following methods:

a) by a shaft-riding probe, o which a seismic trans-

ducer (velocity typ s or accelerometer) is m ounted

so that

it

measures absolute shaft vibration di-

rectly; or

b) by a non-contacting transducer which measures

relative shaft vibratim in combination with a seis-

mic transducer (velocity type or accelerometer)

which measures the support vibration. Both

tral~sducers hall be mounted close together

SG

that they undergo the same absolute motion in

the direction of measurement. Their conditioned

outputs are vectorially summed to provide a

measuremer~t f the absolute shsft motiorl.

3 3 Measurement procedures

3 3 1 General

It

is desirable to locate transducers at positions such

that the lateral movement of the shaft at points of

importance can be assessed. It is recomm ended that,

f o f both relative and absolute measurements, tw o

transducers sh ould be located at, or adjacent to, each

machine bearins. They should be radially mounted in

the same transverse plane perpendicular to the shaft

axis or as close as practicable, with their axes within

5 of a radial line. It is preferable to mount both

transducers 90

?

5 apart on the same bearing half

and the positions chosen should be the same at each

bearing.

A single transducer may be used at each measure-

ment plane in place of the more typical pair of

orthogonal transducers if

~t

s known to provide ade-

quate information about the shaft vibration.


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l t is recommended that special measurements bs

made in order to determine the total non-vibration

runout, which is caused by shaft surface metallurgical

non-homogeneities. local residual magnetism and

.shaft mechanical runout.

t

should be noted that, for

asymmetiic rotors, the effect of gravity can cause a

false runo ut signal.

~a com me nd atio ns or instrumentation are given in

annex C.

3.3.2 Procedures for relative vibration

meas~rernents

Relative vibration transducers of the noncontacting

type are normally m oun ted in tapped holes in the

bearing housing, or by rigid brackets adjecent to the

bearing housing. Where the transducers are moun ted

in the bearing, they should be located so as not to

interfere with the lubricaticn pressure wedge. How-

ever, special arrangements for mounting transducers

in other axial @ca tionsmay be m ade, but different vi-

bration criteria for assessment will then have to be

used. For bracket-mounted transducers, the bracket

shpll be free from natural frequencies w hich adversely

affect the capability of th e transducer t o measure ihe

relative shaft vibration.

The surface of the shaft at the location of the pick-up,

taking irlto account the total axial float of the shaft

under all thermal cm dition s, shall be sm ooth and free

from any ~ eo m e tr ic iscontinuities (such as keyways,

lubrication passages and threads), metallurgical non-

homogeneities and local residual magnetism which

may cause false signals. In some circumstances, an

electroplated or metallized shaft surface may be ac-

ceptable, but it should be noted that the calibration

may be different. I t is recommended that the total

combined electrical and mechanical runout, as

measured by the transducer, should not exceed

25

%

of the allowa ble vibration displacement, speci-

fied in accordance wit h annex

A,

or

6

pm, w hichever

is the greater. For measureme nts made on m achines

already in service, where provision was not originaliy

made for shaft vibration measurements, it may be

necessary to use other runout criteria.

shall be rigidly mo unted t o the machine structure (e.g.

the bearing nousing) close to the non-contacting

transducer SG that both transducers undergo the

same absolute vibration of the suppo rt structure in the

direction of measurement. The sensitive axes of the

non-contacting and seismic transducers shall be Far-

allel, so that their vectorially summed, conditioned

signals result in an accurate measure of the absolute

shaft vibration.

3.3.4 Procedures for absolute vibration

measurements using a shaft riding mechanism

with a seismic transducer

The seismic transducer (velocity type or acceler-

ometer) shall be mour~tedadially on the shaft-riding

rnschanisrr~.The me chan ism shall not cha:ter o: b ind

in a manner modifying the indicated shaft vibration.

The mechanism shall be mouvted as described for

transducers in 3.3.1.

The shaft surface against which the shaft-riding tip

rides, takiri,g into account the total axial float of the

shaft under all thermal condi?ions, shall be smooth

and free fro m shaft discon:inuities, such as keyways

and threads. It is recommended that th e mechanical

runout of the shaft should not exceed 25 of th e al-

lowable vibration displacement, specified in accord-

ance w ith annex A, or 6 pm, whichever is the greater.

There may be surface speed avdlar other limitations

to shaft-riding procedures, such as the formation of

hydrodynamic oil f~ lm s eneath the probe, which may

give false readings and, consec;uently, manufacturers

should be consulted about possible limitations.

3 4

achine ope rat ing condi t ions

Shaft vibrarion rrleasurements sho uld be made u nder

agreed coliditions over the operating range of the

machine. These measurements should be made after

achieving agreed thermal and operating conditions. In

addition, measurements may also be taken under

co nd it i hs of, for example, s low roll, warming-up

speed, critical speed, etc. However, the results of

these measurements may not be suitable for evalu-

ation in accordance w ith clause

5.

3.3.3 Procedures for absolute vibration

measurements using combined seismic and

non contacting relative vibration transducers 3 5 achine foundat ion and st ructures

If a combina tion of se ismic and non-contacting relative

The type of machine foundation and structures (for

vibration transducers is used, the absolute vibration is

example piping) may significantly affect the measure d

obtained by vectorially summing the outputs from

vibration. In general, a valid comparison of vibration

both qansducers. The mounting and other require-

values of machines of the same type can only be

me nts for the non-contac ting transducer are as made if the foundations and structu res have similar

specified in 3 3 2 In addition, the seismic transducer dynamic characteristics.


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6

Environmental vibration and evaluation

3

of measurement system

prior to measuring the vibraticn of an opersting ma-

-ine

a check wit h the same measuring system and

stations should be taken wit h the macl-' ne in an in-

operative state. When the results of such measure-

ments exceed one-third of the values specified for th e

operating speed, steps should be taken to eliminate

en, jronrnentdi vibraticn effects .

4

Instrumentation

The instrumentatior? used fo r the purpose of com pli-

cnce wi th this part of IS 0 7919 shall be so designed

~ take into account temDerature, humidity, the

presence of any corrosive atmospl~ere, haft surface

,peed, shaft material and surfsce finish, sperating

medium (e.g. water, oil, air or steam) in contact with

the transducer, vibration and shock (three major axes),

airborne noise, magnetic fields, inetallic masses in

pioximity tc the tip of the transducer, and power-line

voltage fluctuations a nd trar ~sients .

~t s desirable that the measurement system should

have provision for on-line calibration cf the readout

instrumentation and, in addition, have suitable isolsted

outputs tc perm it further a~ ialysis s required.

5 Evaluation criteria

5.4 There are two principal factors by which shaft

vibration is judged:

a) absolgte vibration of the shaft;

b vibration of the shaft relative to the structural el-

enlents.

5.2 If the evaluation criterion is the change in shaft

vibration, then

a

\vhen the vibration of the structure, on which the

shaft-relative transducer is mounted, is small (i.e.

ks s than 20 % of the relative shaft vibration), ei-

:her the re lative shaft vibration or absolute shaft

\,bration may be used as a measure of shaft vi-

:ration;

b) \vhen the vibration of the structure, on which the

shaft-relative transducer is mounted, is 20

%

or

niore of the relative s iaft vibrat~o n, he absolute

shaft vibration shall be measured and, if found to

la

larger than the relative shaft vibration, it shall

?c used as the measu:.e of shaft vibration.

5 3

If the evaluation criterion is the kinetic load on

the bearing, the relative shaft-v ibration shall be used

as the measure of shaft vibration.

5.4 If the evaluation criterion is statorlrotor clear-

ances, then

a) when the vibrat~on f the structure, on which the

shaft-reiative transducer is mounted, is small (i.e.

less than

20 %

of t he relative shaft vibratiov), the

relative shaft vibration shall be used as a measure

of clearance absorption;

b) wh en ?he vibration of the structure, on which the

shaft-reiative transducer is mounted, is 2C % or

more of the relative shaft vibration, the relative

shaft vibratior: measurem ent may still be used as

a measure of clearance absorption unless the vi-

bration of the structure, on which the shaft-

relative transducer is mounted, is not

representative of th e total stator vibration. In this

latter casa, special me asu ienie ~its wil l be re-

quired.

5.5

The shaft vibration associated with a particular

classification range depends on the size and mass of

the vibrating body, the characteristics of the mo unting

system, and the outpu t and use of the machine. It is

therefore necassav to take into account the various

purposes and circumstances concerned when speci-

fying different ranges of shaft vibration for

a

specific

class of machinery. Where appropriate, reference

should be made to the product specification.

5 6

General principles for evsluation of shaft vi-

bration on difterect machines are given in annex

A.

The eva uation criteria relete to both operational mon -

itoring and acceptance testing, and apply only to the

vibration produced by the m achine i t ~ e lf nd not to

vibration transmitted from outside. For certain classes

of machinery, the guidelines presented in this part of

IS 0 7919 are com plemented by those given in

IS 0 10816-1 for measurements taken on non-rotating

parts. If the procedures of both International Stan-

dards are applied, the one which is more restrictive

shall generally apply.

Specific criteria for different classes and types of ma-

chinery will be given in the relevant parts of I S0 7919

as they are developed.

5.7 The evaluation considered in this basic docu-

ment is limited to broad-band vibration without refer-

ence to frequency components or phase. This will in

most cases be adequate for acceptance testing and

operational monitoring purposes. However, in some


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cases the use of vector information for

sessment on certain machine types may

Vector change information is particuldrly useful in de-

ticular machine is satisfactory when measured under

tecting and defining changes in the dynamic state of

certain steady-state conditions it can become unsat-

a machine which in some cases could go undetected

isfactory if these conditions change.

wh en using broad-band vibratian m easuremen ts. This

is demonstrated

n

annex D.

I t is recommended that in cases where some aspect

of th e vibration sensitivity of a m achine is in question

The specification of criteria for vector changes is be-

agreement should be reached between the customer

yond the present scope of this part

of

I S 0

7919.

and supplier about the necessity and extent of any

testlng or theoretical assessment.

5 8 The vibration measu red on a particular machine

may be sensitive to changes in the steady-state op-

erational condition. In most cases this is not signif-


9/29

Annex A

normative)

General principles for adopting evaluation criteria for different types of

machine

e l the rotational frequency of the shaft;

other types. For example, evaluation criteria for a

annex B). No attem pt has been made t o specify

79 9 as they are developed.

Factors affecting

evacuation

criteria

f )

the bearing type, clearance and diameter;

g) the functioa, output and size cf the m ixk ine un-

der consideration;

h) the relative flexibility of the

bearings,

pedestals

and foundations;

a

i

the rotor mass and flexibility.

Clearly, this range of factors makes it impossible to

define unique evaluation criteria which can be applied

to ail machines. Different criteria, which have been

derived from operaticnal experience, are necessay

for different machines, but at best they can only be

regarded as guidelines and there- will be occasions

where machines will operate safely and satisfactorily

outside any general recomme~dations.

A.2 valuation criteria

Two evaluation criteria are used to assess shaft vi-

bration. One criterion c;onsiders the r~iagnitude f the

observed brcad-band shaft vibration; ihe second coa-

siders changes in magnitude, irrespective of whether

they are increases or decreases.

A.2.1

Criterion : Vibration magnitude at

r ted speed

ion criteria for shaft -vibration measu rements.

conditions

This criterion is concerned with defining limits for

the purpose for which the measurement is made

for example, the requirements for ensuring that

shaft vibration magnitude consistent with acceptable

running clearances are maintained will, in general,

dynamic loads on the bearing, adequate margins on

the radial clearance envalope of th e machine, and ac-

be different from those if the avoidance of ex-

ceptable

vibration transmission into

the

suppon

cessive kinetic load on the bearing is the main

structure and foundation. The maximum shaft vi-

concern);

bration magnitude observed at each bearing is as-

sessed against four evaluation zones established fro m

the type of measu rement made bsolute or

relative vibration; international experience.

F

the quantities measured see annex B);

FigureA.l shows a plot of allowable vibration, ~n

terms of peak-to-peak shaft vibration, against the op-

the position where the measurement is made;


10/29

crating speed range. It is generally accepted that lim-

iting vibration values will decrease as the operating

speed of th e machine increases, but th e actual values

and their rats of change with speed will vary for dif-

ferent types of machine.

A 2 1 1 Evaluation zones

The following typical evaluation zones are defined to

permit a qua litative assessm ent of the shaft vibration

on a given machine and provide guidelines or. possibls

actions.

Zone A: The vibretion of newly commissioned ma-

chines would ncrmally fall within th is zone.

Zone : Mach ines ith vibration with in this zone are

norma lly considered acceptable for unrestricted long-

term operation.

Zone C: Mach ines wi th vibration with in this zone are

normally considered unsatisfactory for long-term ccjn-

tinuous operation. Generally, the machine may be

operated for a limited period in this condition until a

suitable opportunity arises for remedial action.

Zone D Vibration values within this zone are norma lly

considered to be of sufficient severity to cause dam-

age to the machine.

A 2 1 2

Evaluation

zone

l imits

-Numerical vaiues sssigned to th e zone boundaries are

not intended to serve as acceptance specifications,

which shall be subject to agreement between the

machine manufacturer and the customer. However,

these values provide guidelines for ensuring that

gross deficiencies or unrealistic requirements are

avoided. In

certain cases,

there may be specific fea-

tures associated with a particular machine which

would require different zone boundary values (higher

or lower) to be used. In such cases, it is normally

necessary to e xplain the reasons for this and, in par-

ticular, to confirm that the machine will not be en-

dangered by operating with higher vibration values.

A 2 2 riterion il han ge in vibration

magnitude

This criterion provides an assessm ent of a change in

vibration magnitude from a previously established

reference value. A significant increase or decreass in

broad-band vibration magnitcde may occur which re-

quires so me a ction aven though zone C of Criterion I

- has not been reached. Such changes can be instan-

taneous or progressive with time and may indicate

-: that damage has occurred or be a warning of an im-

Pending failure or some other irregularity. Criterion I I

is specified on the basis of the change in broad-band

vibration magnitude occurring under steady-state op-

erating conditions.

Whsn Cri teicn

I1 is

applied, the vibration measure-

merits being compared shall be taken at the same

transducer location and orientation, and under ap-

proximately the same machine operating conditions.

Significant changes from the normal vibration magni-

tudes should be investigated so that a dangerous

sitllation may be avoided.

Criteria for assessing changes in broad-band vibration

for monitoring purposes are given ir other parts of

IS 0 7919. However, it should be noted that some

changes may not be detected unless the discrete

frequency components are monitored (see 5.7).

A 2 3 Operational lim its

For long-term operation, it is common proctice for

some machine types to establish operational vibration

limits. These limits take the for m of ALAR MS _and

TRIPS.

ALARMS: To provide a warning that a defined value

of vibration has been reached or a significant change

has occurred, at which remedial action may be

necessary. In general, if an ALARM situation occurs,

operation can continue for a period whilst investi-

gations are carried out to identify the reason for the

change in vibration and define any reme dial action.

TRIPS To specify the m agnitude of vibiatior~beyond

which further operation of the machine may cause

damage. If the TRlP value is exceeded, immediate

action should be taken to reduce the vibretion or the

machine

should be shut down.

Different operational limits, reflecting differences in

dynzmic loading and support stiffness, may be speci-

fied for different messuremen: positior~ s and di-

rections.

Where appropriate, guidelines for specifying ALARM

and TRIP criteria for specific machine types are given

in other parts of IS 0 7919.

A 2 3 1

etting

of ALP RMS

The ALARM values may vary considerably, up or

down, for different machines. The values clrosen w ~ l l

normally be set relative to a baseline value de ter-

mined from experience for the measurement position

or direction for that particular machine.

t

is recomme nded that the ALAR M value should be

set higher than the baseline by an a mou nt equal to

a


11/29

of the upper lim it of zone 0 If the baseline

.low, the ALARM may be below zone C.

w machine, the initial ALARM setting should

agreed acceptance values. After

period of time, the steady-state baseline value will

established and the ALA RM setting should be ad-

f the steady-state baseline changes (for e xample after

machine overhaul), th s ALARM setting should be

, reflecting differences i a dynamic loading

st i f f i~esses.

A 2 3 2

Setting o TRIPS

The TRIP values w ill gene rallyrelate t o the mechanica

integrity of the mschine and be dependent on any

specific design features which have been introduced

to enable the machine to withstand sbnormal dynemic

forces. The values used will, therefore, generally be

the same for all machines of similar design and wo uld

not normally be related to the steady-state baseline

value used for setting ALARMS.

There may, h3wever, be differences for machines of

different design and it is not possible to give guide-

lines for absolute TRlP values. In ganersl, the TRlP

value w i il be ~ i t h in one C or D.

Zone

I

I

Relevant speed r a n g e

I

I

J

I

I

R o t a t i o n a l f r e q u e n c y o f s h a f t

NOTE

he actual values for vibration at the zone boundaries and the relevant speed range

will

vary for different types of

machine. It i's important to select the relevant criteria and to avoid incorrect extrapolation.

igure

A l

eneralized example of evaluation criteria


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Annex B

(informative)

Derivation of measurement quantities

0.1

Mechanics

of

shaft

vibration

Ths vibration of d rotating shaft is characterized at zny

axial location by

a

kinetic orbit, which describes how

the position of the shaft centre varies with time. Fig-

ure B.l shows a typical orbit. The shape of the orbit

@ depsnds upon the dynamic characteristics of the

shaft, the bearings and the bearing supports or foun-

dations, the ax~alocation on the rotor and the form

of vibration exatation. For example, if the excitation

takes the form of a single-frequency sinusoidal force,

.

the orbit is an ellipse, which can in certain circum-

siapces b e a circle Dr straight line, and the time taken

for the shaft centre to complete one circuit of the el-

lipse is equal to th e pe riod of the excitation force. One

of the most important excitation forces is rotor un-

balance, in which rhe excitation frequency is

equal

to

t h e r o t a t ~ ~ n a lrequeccy of the shaft. However, there

are other forrns of excitat~ on, such as rstor cross-

section asymmetry, for which the frequency is equal

to multiples of t+e rotational frequency of the shaft.

Whe re the vibration arises as a resu lt of, for exa mple,

destabilizing self-excited iorces, the orbit will not

normally be of

a

simple shape, but will change

form

over a period of time and it will not necessarily be

harmonically related. In general, the vibration of the

shaft may arise from a number of different sources

and, therefore, a complex orbit will be produced,

which

IS

tha vectorial sum o i the effects of the indi-

vidual excitation forces.

6.2 Measurement of shaft

vibration

At any axial locarion, the orbit of the shaft can be ob-

tained by takina measurements with two vibration

transducers moir,?ed n different radial planes, separ-

ated by 90 ( t h ~ ss the preferred separation, but small

deviations froni ;his do n ot cause significant errors).

If the angle between the transducer locations is sub-

stantially differelt from 9 ; a vector resolution into

the orthogonal directions will be required. f the

transducers measu re absolute vibration, then the orbit

will be the abso .re orbit of the shaft indepe ndent of

the vibratory motion of the non-rotating parts. If the

transducers measure relative vibration, then the

measu rsd orbit will be relative to that part of the

structlJre upon which the transducers are m ounted.

8 3

Measurem ent quantities

6.3.1

Time integrated me n position

The mean values of the shaft displacement

x,y),

n

any two specified orthogonal directions, relative to a

reference position, as sh ow n in figure

B. l ,

are defined

by integra ~with respect to time, as

show^

in the

following equations:

where

x t )

and y [ t are the time-dspender~t lternating

values of displacement relative

t

ths reference

pos

ition, and

t , , )

is large relative to the psriod of the

lowest frequency vibration cornponelit. In the case of

absolute vibration m easurements, the refe ience pos-

ition is fixed in space. For ~e lative ibration m easure-

ments, these values give an indication of tha mean

position of the shaft relative to the non-rotating parts

at the axial lvcation where the measurements are

made. Changes in the values may be due to a num ber

of factors, such as bearinglfoundation movements,

changes in oil film cha racteristics, etc., which norm ally

occur slowly relative to the period of the vibration

components which make up the alternating values.

It should be note d that,

in

general, the time-inregrated

mean position

in

any direction differs from the pos-

ition defined by taking half the summation of the

maximum and minimum displacement values (see

figure

E.2) .

However, when the shaft vibration is a

single frequency and sinusoidal, t ie n the locus of th e

shaft centre will be an ellipse. In such circurnsrances,


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the time-integrated mean position in any direction of

measurement will be the same as the pcsition identi-

fied by taking half the summation of the maximum

and m inim um d isplacement values.

0.5.2

Peak to peak displacenrent

of

the

vibration

The primary quantities of interest in shaft measure-

ments are the alternating valces which describe the

shape of the orbit. Consider the kinetic shaft orbit

shown in figure B . 2 and assume that there are two

transducers A and B mounted 90" apart, which are

used to measure the shaft vibration. At some instant,

the shaft centre wil l be coincident with the point K or

the orbit and the corresponding instantaneous value

of shsft displacement fro m the mean position wil l b e

S , .

Hcwever, in the plene of the transducers A and

8, the instantaneous values of shaft displacement

from the mean position wil l be sAl and S,, , respec-

tively, where

The

values of S,, SAl and S,, will vary with time as the

shaft centrs moves around the orbit; the carrsspond-

ing waveforms measured by each transducsr are

&own. in figure B . 2 .

NOTE

If the orbit is elliptical, then these waveforms

Gouid be pure sine waves of the same frequency.

The peak-to-peak value of the displacement in the

plane of transducer A (SA(,)) is defined as the differ-

ence between the maximum and minimum displacs-

rnents of transducer A and similarly for

S

for

trbnsducer B . Clearly SA(,) and SB(,) values will not

be equal and, in general, they will be different from

similar mea surem ents m ade in other radiai directions.

Hence, the value of the peak-to-peak displacement is

dependent on the direction of the measurement.

Since the se measurement. quantities are independent

of the absolute value of the mean position,

it

is not

necessary to use systems which can measure both

rhe m ean and alternating values.

Peak-to-peak displacement is the u nit which has been

used most frequently for monitoring vibration of ro-

tating machines.

Whereas measurement of the peak-to-peak displace-

ment in any two given orthogonal directions is a sim-

.pie matter, the value and angular position of the

maximum peak-to-peak displacement show n in

figure

8.2

is difficult to measure directly. However, in

practice, it has been fou nd acceptable to use alterna-

iive measurement quantities which enable a suitable

approximation for the maximum peak-to-peak dis-

placement value to be obtained. For more precise

determinations, it is necessary to examine the sl-,aft

orbit in more detail, as for example with an

oscii loscope. The three most common methods for

obtaining satisfoctcry app roxim atio~ s re described in

B.3 .2 .1 t o B . 3 . 2 . 3 .

B.3.2.1

Method A Resultant value of the

peai< to peak displaceme~t alues measured in

two orthogonal directions

The value of S(w ,a, can be approximated from the

following

equat~on:

S pp)max

.

(B.4)

The use of equation ( B . 4 ) as an approximation when

the vibration is predominantly at rotational frequency

will generally overe stim ate the value of S@,),,, wi th

a m aximum error of approximately 40 %.

The maximum error occurs for a circular orbit and

progressivefy reduces as the orbit becomes flatter,

w i th

a

zero error for the degenerate case of a straight

line orbit.

6.3.2.2

Method

B:

Taking the maximum value of

the peak to peak displacement values measured

in two orthogonal directions

The value of

S .,),,,

can be approximated from the

following equat~on:

whichever is the greater.

The use of equation ( B . 5 ) as an approximation when

the vibration is predominantly at rotational frequency

will generally under-estimate the value of

S ,,),,,,

with a maximum error of app~oximately 0

%.

The maximum error occurs for a flat orbit and pro-

gressively reduces as the orbit becom es circular, wit h

a zero error whe n t he orbit is circular.

B.3.2.3

Method

C

Measurement of S,,,

The instantaneous value of the shaft displacement

can be defined by S , , as shown in figure

B.2,

which is

derived from the transducer measurements SAl and

S,, using equation

(8.3).

There is a point on th e orbit,

defined by point

P

in figure

B.2,

where the displace-

ment from the mean position is a maximum. The

value of S corresponding to this position is denoted

by S,,,, which is defined as the maximum value of

displacement


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15/29

Transducer

~ r a n s d u c e rwaveform

SA

2-p

wavef arm

Transducer

Fixed reference axes

Time integrated m ean pcsition of orbit

Timeinte grated mean values of shaft displacement

lnstantaneous position of shaft ceptre

Position of shaft for m aximum displacement from time-integrated mean po sitio~

I~istantaneo us alue of shaft displacement

Ma xim um value of shaft displacement fr om time-integrated m ean position

0

lnstantaneous values of shaft displacement in directions of transducers

A

and

B

Ma xim um value of peak-to-peak displacement

Peak-to-peak values of shaft displacement in directions

of

transducers

A

and

B

respectively

respectively

Figure B 2 inetic orbit of shaft efinition of displacement


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iso 7919-1 part 1 general guidelines

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