01-Introduction to Contion Monitoring

7/27/2019 01-Introduction to Contion Monitoring

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Introduction to

Condition

Monitoring

AbstractThis lecture introduces the predictive maintenance concept ofcondition monitoring for industrial rotating machines.This makes it easier to understand how important the need forcondition monitoring is.

Lecture Note

Prepared byEssam Abdel-Halim Moustafa


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Condition monitoring (CM), by definition, is simply a technique for routinelyevaluating the condition of equipment. This simple definition, however, hastremendous applications in improving the life of many industries and evenour day-to-day existence. It is instrumental in increasing industrialproduction and profitability, improving product quality, reducingenvironmental pollution, improving safety, and reducing the waste of ourlimited natural resources. Spectacular gains can be made in all of theseareas, if CM is properly employed. If not, it could equally generate losses.

CM can be done on almost any kind of device from micro-sized circuitboards to huge 1000 MW hydroelectric turbines. Nearly every imaginableindustry can use or is using some form of CM.

This lecture will touch on several important topics which concern CM, butthe focus will be on industrial applications using rotating machinery.

Introduction toCondition

Monitoring


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Maintenance has evolved from run-to-breakdown methods, to time-basedmethods to modern predictive maintenance practices. This has resulted inless spares and manpower to maintain machines, and higher machineavailability.

Types of Maintenance

CorrectiveMaintenance(Run-to-breakdown)

Repair it when it fails

Break-down

Time

Cost

PreventiveMaintenance(Time Based Maintenance)Maintenance at regular intervals

Time

Cost

Predictive

Maintenance(On Condition Maintenance)Problem detected before predictedfailure.Maintenance planned ahead

Time

Cost


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Which is historically the first maintenance strategy employed. A machineis repaired only after a failure has occurred. This is a very expensivemaintenance management scheme, since it requires: high spare partsinventory, high machine downtime, high overtime labor costs, and lowproduction availability.

Corrective Maintenance-Run to Breakdown-

Corrective Maintenanceleads to:

Secondary damage

Safety risk

Unplanned downtime

Unplanned

maintenance

Product waste

Spares inventory

Corrective Maintenanceleads to:

Secondary damage

Safety risk

Unplanned downtime

Unplanned

maintenance Product waste

Spares inventory

Notrecommended

for criticalmachines

Time

Cost

Break-down


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Where important machines are not fully duplicated or where unscheduledproduction stops can result in large losses, maintenance operations are oftenperformed at fixed time-intervals. The advantage of this maintenancestrategy is that it is planned strategy and is based on previous experienceand the mean-time-between-failure (MTBF) statistic for the machine. Thedisadvantage of this maintenance scheme is that it is not based on thecondition of the machine, but rather on the time elapsed since the previousmaintenance occurred.

Thus a failure may occur before a maintenance is performed, as in Run-To-Breakdown maintenance, or a perfectly operating machine may be

maintained with a consequent waste in labor and material .This system is therefore called Time-based Predictive Maintenance.

Preventive MaintenanceTime Based Maintenance

Not

recommended

for critical

machines

Time-based PreventiveMaintenance involves:

More frequent overhauls

Risk of early failures

Tampering with goodmachines

Time consuming overhauls

Experts needed for each

overhaul

Time-based PreventiveMaintenance involves:

More frequent overhauls

Risk of early failures

Tampering with goodmachines

Time consuming overhauls

Experts needed for eachoverhaul


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In which maintenance is performed on the basis of the machinecondition. This is done by monitoring the machine condition. Any changein condition is detected, and the time to Failure is estimated . This isalso accompanied by diagnosing the cause of the fault to actually pinpoint the defective components.

With this method each machine is considered individually by makingfixed-interval condition measurements to obtain a quantitative value ofthe Health of the machine.

In this way maintenance is only allowed when measurements show it tobe necessary.

Higher plant availability,performance and reliability

Greater safety

Better product quality

Attention to environment

Longer equipment life

Greater cost effectiveness

Higher plant availability,performance and reliability

Greater safety

Better product quality

Attention to environment

Longer equipment life

Greater cost effectiveness

Predictive MaintenanceOn-condition -Maintenance

Monitor the condition of themachine and predict when it

would fail Plan maintenance ahead of

time and save money

Repair the machines onlywhen they need to

Focus overhauls only on faultyparts

Monitor the condition of themachine and predict when itwould fail

Plan maintenance ahead oftime and save money

Repair the machines onlywhen they need to

Focus overhauls only on faultyparts

Recommended

for critical

machines


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Comparison of Maintenance Strategies

Run-to-breakdown Preventive Predictive

Method Failure based Time based Condition Based

Premise No maintenance MTBF As-Needed

Advantages No maintenancecost

Planned Maintenance.

Structured program

Lower maintenance costs .

Fewer machine failures.Less repair downtime.Reduced inventory.Longer machine life.

Increased production.Improved operator safety.Verification of new equipment

condition.Improved overall profitability.

Failure may occur before

scheduled maintenance.Maintenance may be perf-

ormed unnecessarily.Maintenance may cause

failure.

UseUsed only on cheap,

abundant and insignificantcomponents

Disadvant-

ages

High spare parts inventoryHigh machine downtime

High overtime labor costslow production availability

Used on all machines Used on all machines

Initial investment in equipment


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Why Make Condition Monitoring ?

Longer time between overhauls

Reduced repair duration

Reduction of spare-part stock

Less unexpected breakdowns

Elimination of secondary damageReduction in business i nterruptionand damage insurance premiums

Selection and purchase ofinstrumentation

Initial investigations, selection ofmonitoring points, establishmentof limits

Training

Yes NoConditionMonitoring


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Predictive Maintenance Flow Chart

Start

Trouble-

shooting

Chart

Machine

Specs.

&

Drawings

NoNo

Fault

correction

YesYes

Fault

diagnosis

Regular

Measurements

Fault

detected

Referencecreation


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Machine Potential Failures Analysis


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Why Vibration ?

Vibration is used as the fault detection parameter simply because it cangive an early warnings of fault development for a wider variety of typicalrotating machinery faults.

Other detection techniques, if used in isolation, limit the variety of faults,and so unexpected breakdown by a fault type not included, is a realpossibility.

Temp.

Temp.

P

ressure

P

ressure

Flow

Flow

Oila

naly

sis

Oila

naly

sis

Curr

ent

Curr

ent

V

ibra

tion

V

ibra

tionParametersParameters

Yes

Yes Yes

Yes Yes Yes Yes Yes

Yes Yes

Yes

Yes Yes

Yes

Yes

Out of BalanceOut of Balance

Misalignment /Misalignment /

Bent ShaftBent Shaft

Damage ofDamage of

journal bearingjournal bear ing

Damage ofDamage of

gearboxesgearboxes

MechanicalMechanical

LoosenessLooseness

MotorMotor

ProblemsProblems

Type ofType of

machine faultmachine fault

Belt ProblemsBelt Problems

ResonanceResonance

Damage of ro llingDamage of r olling

elements bearingselements bearings

Parameters Used for Detection of machine Faults

Yes Yes Yes


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Force

Mobility

Vibration

Freq.

Freq.

Freq.

Forces caused by-Imbalance - Friction

-Shock -Acoustic

Structural Parameters:-Mass -Stiffness

-Damping

Vibration Parameters:-Acceleration -Velocity

-Displacement

x

=

InputForces

SystemResponse(Mobility)

Vibration

x

=

What is Vibration

What is Vibration?Vibration is mechanical oscillation about a reference position. Vibration isan everyday phenomenon, we meet it in our homes, during transport andat work. Vibration is often a destructive and annoying side effect of auseful process, but is sometimes generated intentionally to perform atask.

Vibration of machines

Vibration is a result of dynamic forces in machines which have movingparts and in structures which are connected to the machine. Differentparts of the machine will vibrate with various frequencies and amplitudes.

Vibration causes wear and fatigue. It is often responsible for the ultimatebreakdown of the machine.


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Time

RMS AveragePeak

Peak-

Peak

Amplitude

T

Crest Factor :Peak

RMSdt)t(x

T

RMST

= 021 dt)t(x

T

verageAT

= 01

Signal level Descriptors

Signal Level Descriptors

The level of vibration signal can be described in different ways. Peak andpeak-to-peak values are often used to describe the level of a vibration signalsince they indicate the maximum excursion from equilibrium position. TheRMS (Root Mean Square) level is a very good descriptor, since it is ameasure of the energy content of the vibration signal.


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AverageRMS

Amplitude

Time

PeakPeak-Peak

dt)t(xTRMS

T

= 021

dt)t(xTverageA

T

=0

1

Crest Factor :

Peak

RMS

Time Signal Descriptors

Time Signal Descriptors

These descriptors are not only used in conjunction with a single sinusoidalsignal but also with normal machine vibration signals which are composed ofmany sinusoidal vibration components.


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Mass and SpringOnce a (theoretical) system of a mass and a spring is set in motion it willcontinue this motion with constant frequency and amplitude. The systemis said to oscillate with a sinusoidal waveform.

The Sine Curve

The sine curve which emerges when a mass and a spring oscillate can bedescribed by its amplitude (D) and period (T). Frequency is defined as thenumber of cycles per second and is equal to the reciprocal of the period.By multiplying the frequency by 2 the angular frequency is obtained,which is again proportional to the square root of spring constant k dividedby mass m. The frequency of oscillation is called the natural frequency fn.The whole sine wave can be described by the formula d = Dsin nt,where d = instantaneous displacement and D = peak displacement.

FFT transformation

D

d =D sinnt

m

k

T

Time

Displacement

Frequency1T

Period, Tn in [sec]

Frequency, fn= in [Hz =1/sec]1

Tn

Displacement

k

mn= 2 fn =


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We see that the longer the period of the sine wave, the lower thefrequency.

The magnitude of the peak in the spectrum corresponds to the energycontent of the sine wave (RMS).

Spectrum Analysis


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F = c v

VelocityDisplacement Acceleration

dv a

k

c

m

m

F = k d F = m a

Vibration Parameters

Mechanical Parameters

Before going into a discussion about vibration measurement and analysis,we will examine the basic mechanical parameters and components andhow they interact.

All mechanical systems contain the three basic components: spring,damper, and mass. When each of these in turn is exposed to a constantforce they react with a constant displacement, a constant velocity and aconstant acceleration respectively.


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M

M

a

M

dv

va

d

m

m

a

d

v

m

a

v

d

Velocity

Displacement

Accelerat ion

Time

(Simple vibration)

Frequency

(real machine)

Acceleration, Velocity and Displacement

Vibration may be measured in acceleration, velocity or displacement.

For higher frequencies the velocity is higher than the displacement, and acceleration is higher than velocity.

Note that the peaks in the three spectra from a real machine are situated at the same frequencies.

For very high frequencies, the peaks may not be seen in the displacement spectra due to noise.

Which Parameter to Choose?

If the type of measurement being carried out does not call for a particular parameter to be measured e.g. due tosome standard, the general rule is that the parameter giving the flattest response over the frequency range ofinterest should be chosen. This will give the biggest dynamic range of the whole measurement set up. If thefrequency response is not known start by choosing velocity.

An advantage of the accelerometer is that its electrical output can be integrated to give velocity anddisplacement signals.

This is important since it is best to perform the analysis on the signal which has the flattest spectrum.If a spectrum is not reasonably flat, the contribution of components lying well below the mean level,

will be less noticeable. In the case of overall measurements, smaller components might passcompletely undetected.

Use the Flattest Spectrum

In most cases this will mean that velocity is used as the detection parameter on machinemeasurements. On some occasions acceleration may also be suitable, although most machines willexhibit large vibration accelerations only at high frequencies. It is rare to find displacement spectrawhich are flat over a wide frequency range, since most machines will only exhibit large vibrationdisplacements at low frequencies.

In the absence of frequency analysis instrumentation to initially check the spectra, it is safest to makevelocity measurements (but still using the accelerometer, of course, since even the integratedaccelerometer signal gives a better dynamic and frequency range than the velocity transducer signal).


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+

+

+

Man-Machine Link

Correct &Correct &

EffectiveEffective

MaintenanceMaintenanceDecisionDecision

M / CInformation& Operation

Diagnostic

Knowledge


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Example of Machine Information table


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Example of Machine ComponentExample of Machine Component


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Vib.

r.p.m or (Hz)

Frequency Spectrum

MotorMotor SpeedSpeed : 600600 r.p.m

Rigid CoRigid Coupling-1: No. of bolts 44

First gear :First gear :5050 teeth

Second gear :Second gear :2020 teeth

Rigid CoRigid Coupling-2: No. of bolts 66

Fan bladesFan blades : 55


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MotorMotor SpeedSpeed : 600600 r.p.m

Rigid CoRigid Coupling-1 : No. of bolts 44

First gear :First gear : 5050 teeth

Second gear :Second gear : 2020 teeth

Rigid CoRigid Coupling-2 : No. of bolts 66

Fan bladesFan blades :5 blades5 blades

Vib.

r.p.m or (Hz)

Frequency Spectrum interpretation

600600 r.p.mr.p.m

(10 Hz)(10 Hz)

4Bo

ltX60

0

2400

rpm(40H

z)Gear Meshing Freq.

50 teeth x600

Or 20 teeth x 1500

30,000 rpm(500Hz)

1500

r.p.m

(25Hz)

6Bolt

X1

500

900

0r

pm(150Hz)

5Bla

desX

150

0

7500

rpm(125

Hz)

Motor Running Speed Freq. : 600 r.p.m (10 Hz)

First Coupling defect Frequency : 600 r.p.m x 4 bolt = 2400 r.p.m(40 Hz)

Gear Meshing Frequency : 600 r.p.m x 50 teeth=30,000 r.p.m(500Hz)

: or 1500 r.p.m x 20 teeth=30,000 r.p.m (500 Hz)

Second shaft speed : 600x50/20 =1500 r.p.m

Second Coupling defect Frequency :1500 r.p.m x 6 bolt = 9000 r.p.m(150 Hz)

Blade Passing Frequency : 1500 r.p.m x 5 Blade=7500 r.p.m (125Hz)


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Measuring Points


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r.p.m or (Hz)

Vib.

Reference Spectrum


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Vib.

r.p.m or (Hz)

Overall Alarm

Frequency Spectrum With Overall AlarmAlarm & DangerDanger


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Vib.

r.p.m or (Hz)

Frequency Spectrum With Band AlarmAlarm & DangerDanger

Band Alarm


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Vib.

r.p.m or (Hz)

Band No. Definitions

Band 1BAND Freq. MIN

BAND Freq. MAXBAND ALARMBAND DANGER

Suggested Setup

Band 2BAND Freq. MINBAND Freq. MAX

BAND ALARMBAND DANGER

!?

!?

!?!?

Example of Bands alarm Sheet


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Vib.

r.p.m or (Hz)

Frequency Spectrum With Profile AlarmAlarm & DangerDanger

Profile or Narrow band alarm


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Vib.

r.p.m or (Hz)

Overall Analysis


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Fan

Date

1

Frequency

Vibration

1

2

2

33

4

4

55

2

2

3

3

4

4

55

Frequency Spectrum Overall Level

1

Gearbox

1Vibration

DateFrequency

Frequency Spectrum or Overall Level

Frequency Spectrum or Overall LevelTo decide whether monitoring or testing of the overall level is sufficient ora complete frequency spectrum is required, the engineer must know hismachine and something about the most likely faults to occur or whichpart of the object is of interest.

The illustration shows two different situations in monitoring, but it mightas well be testing:

Monitoring of a fan: The most likely fault to occur is unbalance, which willgive an increase in the vibration level at the speed of rotation. This willnormally also be the highest level in the spectrum. To see if unbalance isdeveloping, it is therefore sufficient to measure the overall level at regular

intervals. The overall level will reflect the increase just as well as thespectrum.

Monitoring of a gearbox: Damaged or worn gears will show up as anincrease in the vibration level at the tooth meshing frequencies (shaftRPM number of teeth) and their harmonics. The levels at thesefrequencies are normally much lower than the highest level in thefrequency spectrum, so it is necessary to use a full spectrum comparisonto reveal a developing fault.

A general rule is overall measurements are permissible for simple, noncritical machines, while more complex, more critical machinery requiresspectral analysis.


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01-Introduction to Contion Monitoring

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