Intro Predictive Maintenance 2 Part 6 ppt

Techniquesinclude vibration analysis, ultrasonics, ther-mography, tribology, process monitoring,visual inspection, and other nondestructiveanalysis methods.Maximum frequency Broadband an

Axial Of, on, around, or along an axis (straight line

about which an object rotates) or center ofrotation

Bearing cap The protective structure that covers bearings

Boundary condition Mathematically defined as a requirement to

be met by a solution to a set of differentialequations on a specified set of values of theindependent variables

object relative to a reference point, usuallymeasured in mils

Dynamics, operating Deals with the motion of a system under the

influence of forces, especially those thatoriginate outside the system under consider-ation

Fast Fourier Transform (FFT) A mathematical technique used to convert a

time-domain plot into its unique frequencycomponents

ac-celerate Quantitatively, it is a vector equal tothe body’s time rate of change of momentum

component in a machine-train’s vibrationsignature

Frequency Frequency, f, is defined as the number of

rep-etitions of a specific forcing function orvibration component over a specific unit oftime It is the inverse of the period, , of thevibration and can be expressed in units ofcycles per second (cps) or Hertz (Hz) Forrotating machinery, the frequency is oftenexpressed in vibrations per minute (vpm)

Frequency, circular Another measure of frequency measured in

radians (w = 2p f)

Frequency, natural All components have one or more natural

frequencies that can be excited by an energysource that coincides with, or is in proximity

to, that frequency The result is a substantialincrease in the amplitude of the natural fre-quency vibration component, which is

referred to as resonance Higher levels of

1

T

input energy can cause catastrophic, nearinstantaneous failure of the machine or struc-ture.

Frequency, primary The base frequency referred to in a vibration

analysis that includes vibrations that are monics of the primary frequency

har-Gravitational constant The constant of proportionality in the

English system of units, gc, which causes onepound of mass to produce one pound of forceunder the acceleration of gravity, equal to32.17 lbm-ft/lbf-sec2

Harmonic motion A periodic motion or vibration that is a

sinu-soidal function of time, that is, motion along

a line given by equation x = a cos(wt +f),

where t is time, a and w are constants, and f

is the phase angle For example, X = X0sin(wt + f) where X is the displacement, X0isthe amplitude, w is the circular frequency,and f is the phase angle

Harmonics Multiples of the primary frequency (e.g., 2¥,

3¥)

a frequency of n hertz if in one second it goes through n cycles.

mechani-cal and/or a force imbalance Mechanimechani-calimbalance is when there is more weight onone side of a centerline of a rotor than on theother Force imbalance can result when there

is an imbalance of the centripetal forces erated by rotation and/or when there is animbalance between the lift generated by therotor and gravity

rolling-element bearings, impellers, andother rotors, that turn with a shaft

Machine-train A series of machines containing both driver

and driven components

Maintenance management program A comprehensive program that includes

pre-dictive maintenance techniques to monitorand analyze critical machines, equipment,

and systems in a typical plant Techniquesinclude vibration analysis, ultrasonics, ther-mography, tribology, process monitoring,visual inspection, and other nondestructiveanalysis methods.

Maximum frequency Broadband analysis techniques, which are

used to monitor the overall mechanical dition of machinery, are based on the overallvibration or energy from a frequency range

con-of zero to the user-selected maximum quency (FMAX)

Moment of inertia The sum of the products formed by

multi-plying the mass of each element of a body

by the square of its distance from a

specified line Also known as rotational inertia.

unin-terrupted rhythm

Periodic motion A motion that repeats after a certain interval

Phase angle The difference between the phase of a

sinu-soidally varying quantity and the phase of asecond quantity that varies sinusoidally at the

same frequency Also known as phase ence.

differ-Piezoelectric Describes a crystal or film that can generate

a voltage when mechanical force is applied

or produce a mechanical force when avoltage is applied

Predictive maintenance The practice of using actual operating

condi-tions of plant equipment and systems to mize total plant operation Relies on directequipment monitoring to determine theactual mean-time-to-failure or loss of effi-ciency for each machine-train and system in

opti-a plopti-ant This technique is used in plopti-ace of tropti-a-ditional run-to-failure programs

called time trace or waveform) or

frequency-domain vibration curves

Radial Extending from a point or center in the

manner of rays (as the spokes of a wheel areradial)

two radii and an arc joining them, all of thesame length A circle consists of 2p radians

alternately

Spring constant The number of pounds tension necessary to

extend the spring one inch Also referred to

as stiffness or spring modulus.

Thermography Use of heat emissions of machinery or plant

equipment as a monitoring and diagnosticpredictive maintenance tool For example,temperature differences on a coupling indi-cate misalignment and/or uneven mechanicalforces

such as a shaft in order to sustain tion/load requirements A twisting loadimparted to shafts as the result of inducedloads/speeds

accelera-Transducer Any device or element that converts an input

signal into an output signal of a differentform

design and operation Predictive maintenancetechnique that uses spectrographic, wear par-ticle, ferrography, and other measurements ofthe lubricating oil as a diagnostic tool

Turbulent flow Motion of fluids in which local velocities and

pressures fluctuate irregularly and randomly

Ultrasonic analysis Predictive maintenance technique that uses

principles similar to those of vibration sis to monitor the noise generated by plantmachinery or systems to determine theiractual operating condition Ultrasonics isused to monitor the higher frequencies (i.e.,ultrasound) that range between 20,000 Hertzand 100 kiloHertz

analy-Vector A quantity that has both magnitude and

direction, and whose components transformfrom one coordinate system to another in thesame manner as the components of a dis-placement

Velocity The time rate of change of position of a body

It is a vector quantity with direction as well

as magnitude

displace-ment with respect to a fixed reference Themotion will repeat after a certain interval

Vibration analysis Vibration analysis monitors the noise or

vibrations generated by plant machinery orsystems to determine their actual operatingcondition The normal monitoring range forvibration analysis is from less than 1 up to20,000 Hertz

Thermography is a predictive maintenance technique that can be used to monitor thecondition of plant machinery, structures, and systems It uses instrumentation designed

to monitor the emission of infrared energy (i.e., temperature) to determine operatingcondition By detecting thermal anomalies (i.e., areas that are hotter or colder thanthey should be), an experienced surveyor can locate and define incipient problemswithin the plant

8.1 I NFRARED B ASICS

Infrared technology is predicated on the fact that all objects with a temperature aboveabsolute zero emit energy or radiation Infrared radiation is one form of this emittedenergy Infrared emissions, or below red, are the shortest wavelengths of all radiatedenergy and are invisible without special instrumentation The intensity of infraredradiation from an object is a function of its surface temperature; however, tempera-ture measurement using infrared methods is complicated because three sources ofthermal energy can be detected from any object: energy emitted from the object itself,energy reflected from the object, and energy transmitted by the object (Figure 8–1).Only the emitted energy is important in a predictive maintenance program Reflectedand transmitted energies will distort raw infrared data Therefore, the reflected andtransmitted energies must be filtered out of acquired data before a meaningful analy-sis can be completed

The surface of an object influences the amount of emitted or reflected energy A perfectemitting surface, Figure 8–2, is called a “blackbody” and has an emissivity equal to1.0 These surfaces do not reflect Instead, they absorb all external energy and re-emit

considerations of the actual emissivity of an object improve the accuracy of ture measurements used for predictive maintenance To help users determine emis-sivity, tables have been developed to serve as guidelines for most common materials;however, these guidelines are not absolute emissivity values for all machines or plantequipment.

tempera-Variations in surface condition, paint, or other protective coatings and many other variables can affect the actual emissivity factor for plant equipment In addition toreflected and transmitted energy, the user of thermographic techniques must also con-sider the atmosphere between the object and the measurement instrument Water vaporand other gases absorb infrared radiation Airborne dust, some lighting, and other vari-

Figure 8–1 Energy emissions All bodies emit energy within

the infrared band This provides the basis for infrared

imaging or thermography A = Absorbed energy R =

Reflected energy T = Transmitted energy E = Emitted

Figure 8–2 Blackbody emissions A perfect or blackbody

absorbs all infrared energy A = Absorbed energy R =

Reflected energy T = Transmitted energy E = Emitted

energy.

ables in the surrounding atmosphere can distort measured infrared radiation Becausethe atmospheric environment is constantly changing, using thermographic techniquesrequires extreme care each time infrared data are acquired.

8.2 T YPES OF I NFRARED I NSTRUMENTS

Most infrared-monitoring systems or instruments provide special filters that can beused to avoid the negative effects of atmospheric attenuation of infrared data; however,the plant user must recognize the specific factors that will affect the accuracy of theinfrared data and apply the correct filters or other signal conditioning required tonegate that specific attenuating factor or factors

Collecting optics, radiation detectors, and some form of indicator are the basic ments of an industrial infrared instrument The optical system collects radiant energyand focuses it on a detector, which converts it into an electrical signal The instru-ment’s electronics amplifies the output signal and processes it into a form that can bedisplayed Three general types of instruments can be used for predictive maintenance:infrared thermometers or spot radiometers, line scanners, and imaging systems

ele-8.2.1 Infrared Thermometers

Infrared thermometers or spot radiometers are designed to provide the actual surfacetemperature at a single, relatively small point on a machine or surface Within a pre-dictive maintenance program, the point-of-use infrared thermometer can be used in conjunction with many of the microprocessor-based vibration instruments to monitorthe temperature at critical points on plant machinery or equipment This technique

is typically used to monitor bearing cap temperatures, motor winding temperatures, spot

Figure 8–3 Graybody emissions All bodies that are not

blackbodies will emit some amount of infrared energy The

emissivity of each machine must be known before implementing

a thermographic program A = Absorbed energy R = Reflected

energy T = Transmitted energy E = Emitted energy.

checks of process piping temperatures, and similar applications It is limited in that thetemperature represents a single point on the machine or structure When used in con-junction with vibration data, however, point-of-use infrared data can be valuable.

8.2.2 Line Scanners

This type of infrared instrument provides a single-dimensional scan or line of ative radiation Although this type of instrument provides a somewhat larger field ofview (i.e., area of machine surface), it is limited in predictive maintenance applications

compar-8.2.3 Infrared Imaging

Unlike other infrared techniques, thermal or infrared imaging provides the means toscan the infrared emissions of complete machines, process, or equipment in a veryshort time Most of the imaging systems function much like a video camera The usercan view the thermal emission profile of a wide area by simply looking through the instrument’s optics A variety of thermal imaging instruments are on the market, ranging from relatively inexpensive, black-and-white scanners to full-color,microprocessor-based systems Many of the less expensive units are designed strictly

as scanners and cannot store and recall thermal images The inability to store and recallprevious thermal data limits a long-term predictive maintenance program

Point-of-use infrared thermometers are commercially available and relatively pensive The typical cost for this type of infrared instrument is less than $1,000.Infrared imaging systems have a price range from $8,000 for a black-and-whitescanner without storage capability to more than $60,000 for a microprocessor-based,color imaging system

inex-8.3 T RAINING

Training is critical with any of the imaging systems The variables that can destroythe accuracy and repeatability of thermal data must be compensated for each timeinfrared data are acquired In addition, interpretation of infrared data requires exten-sive training and experience

Inclusion of thermography into a predictive maintenance program will enable you tomonitor the thermal efficiency of critical process systems that rely on heat transfer orretention; electrical equipment; and other parameters that will improve both the reli-ability and efficiency of plant systems Infrared techniques can be used to detect prob-lems in a variety of plant systems and equipment, including electrical switchgear,gearboxes, electrical substations, transmissions, circuit breaker panels, motors, build-ing envelopes, bearings, steam lines, and process systems that rely on heat retention

or transfer

8.4 B ASIC I NFRARED T HEORY

Infrared energy is light that functions outside the dynamic range of the human eye.Infrared imagers were developed to see and measure this heat These data are trans-

formed into digital data and processed into video images called thermograms Each

pixel of a thermogram has a temperature value, and the image’s contrast is derivedfrom the differences in surface temperature An infrared inspection is a nondestruc-tive technique for detecting thermal differences that indicate problems with equip-ment Infrared surveys are conducted with the plant equipment in operation, soproduction need not be interrupted The comprehensive information can then be used

to prepare repair time/cost estimates, evaluate the scope of the problem, plan to haverepair materials available, and perform repairs effectively

8.4.1 Electromagnetic Spectrum

All objects emit electromagnetic energy when heated The amount of energy is related

to the temperature The higher the temperature, the more electromagnetic energy itemits The electromagnetic spectrum contains various forms of radiated energy,including X-ray, ultraviolet, infrared, and radio Infrared energy covers the spectrum

of 0.7 micron to 100 microns

The electromagnetic spectrum is a continuum of all electromagnetic waves arrangedaccording to frequency and wavelength A wave has several characteristics (Figure

8–5) The highest point in the wave is called the crest The lowest point in the wave

is referred to as the trough The distance from wavecrest to wavecrest is called a length Frequency is the number of wavecrests passing a given point per second As

wave-the wave frequency increases, wave-the wavelength decreases The shorter wave-the wavelength,the more energy contained; the longer the wavelength, the less energy

For example, a steel slab exiting the furnace at the hot strip will have short lengths You can feel the heat and see the red glow of the slab The wavelengths have

wave-Figure 8–4 Electromagnetic spectrum.

become shorter crest to crest and the energy being emitted has increased, entering thevisible band on the spectrum By contrast, (infrared energy) when the coil comes off

of the coilers it has been cooled Energy is lost The wavelength have increased crest

to crest and decreased in frequency

8.4.2 Heat Transfer Concepts

Heat is a form of thermal energy The first law of thermodynamics is that heat given

up by one object must equal that taken up by another The second law is that the fer of heat takes place from the hotter system to the colder system If the object iscold, it absorbs rather than emits energy All objects emit thermal energy or infraredenergy through three different types or modes: conduction, convection, and radiation

trans-It is important to understand the differences among these three forms

RADIO

INFRARED VISIBLE ULTRA- VIOLET

X-RAYS

GAMMA RAY

Figure 8–5 Wavelengths.

Radiation is the transfer of heat by wavelengths of electromagnetic energy The mostcommon cause of radiation is solar energy Only radiated energy is detected by aninfrared imager If the aforementioned motor were sitting outside in the slab storageyard with slabs stacked around it, the electromagnetic energy from the sun and fromthe slabs would increase the temperature

The purpose of the previous example was to make the thermographer aware that othercauses of the thermal energy could be found or not found In this case, was the motorhot because of a bad bearing or because of solar radiation? Was the motor missed andfailed later because of the fan blowing on it and causing convection cooling? Con-duction is the only mode that transfers thermal energy from location to location within

a solid; however, at the surface of a solid or liquid, and in a gas, it is normal for allthree modes to operate simultaneously

Emissivity

Emissivity is the percentage of energy emitted by an object Infrared energy hits anobject; the energy is then transmitted, reflected, or absorbed A common term used in

infrared thermography is blackbody A blackbody is a perfect thermal emitter Its

emis-sivity is 100 percent It has no reflection or transmittance The objects you will bescanning will each have a different emissivity value A percentage of the total energywill be caused by reflection and transmittance; however, because most of your infraredinspection will be quantitative thermography, the emissivity value will not be asimportant now

8.5 I NFRARED E QUIPMENT

Listed as follows are the criteria used to evaluate infrared equipment It is important

to determine which model best fits your needs before a purchase is made Some ofthese points will be important to you and others will not You will know more aboutyour needs after you have finished reading this book

• Portability How much portability does your application require? Does

weight and size of the instrument affect your data collection? What kind ofequipment will you be scanning?

• Ease of Use How much training is required to use the imager? Can it be

used easily in your environment?

• Qualitative or Quantitative Does it measure temperatures? If yes, what

tem-perature range will be measured? Will you need more than one range?

• Ambient or Quantitative Measurements What are the maximum upper and

minimum lower ambient temperatures in which you will be scanning?

• Short or Long Wavelengths Long-wavelength systems offer less solar

re-flection and operate in the 8- to 14-micron bandwidth Short-wavelengthsystems offer smaller temperature errors when an incorrect emissivity value

is entered The operating bandwidth for a short-wave unit is 2 to 5.6 microns

• Batteries What is the weight and size of the batteries? How long will

they last? Will you need additional batteries? How long do they take tocharge?

• Interchangeable Lenses Do the ones available fit your application? What

are their costs?

• Monitor, Eyepiece, or Both Will you need to show a live image to others

while performing an inspection?

• Analog or Digital How will you process the images? Does the imager have

analog, digital, or both capabilities?

• Software Can the software package produce quality reports and store and

retrieve images? Do you require colonization and temperature editing?

8.6 I NFRARED T HERMOGRAPHY S AFETY

Equipment included in an infrared thermography inspection is almost always gized Therefore, a lot of attention must be given to safety The following are basicrules for safety while performing an infrared inspection:

ener-• Plant safety rules must be followed at all times

• Notify area personnel before entering the area for scanning

• A qualified electrician from the area should be assigned to open and closeall panels

• Where safe and possible, all equipment to be scanned will be online andunder normal load with a clear line of sight to the item

• Equipment whose covers are interlocked without an interlock defect anism should be shut down when allowable If safe, their control coversshould be opened and equipment restarted

mech-8.7 I NFRARED S CANNING P ROCEDURES

The purpose of an infrared inspection is to identify and document problems in an trical or mechanical system The information provided by an inspection is presented

elec-in an easily and understandable form A high percentage of problems occur elec-in nation and connections, especially in copper-to-aluminum connections A splice or alug connector should not look warmer than its conductors if it has been sized prop-erly All problem connections should be dismantled, cleaned, reassembled, or replaced

termi-as necessary

8.8 T YPES OF I NFRARED P ROBLEMS

There are three basic types of thermal problems:

• Mechanical looseness

• Load problems

• Component failure

8.8.1 Mechanical Looseness

Mechanical looseness occurs most often A loose connection will result in thermalstress fatigue from overuse Fuse clips are a good example because the constant heat-

up and cooldown creates a poor connection An accurate temperature measurement,

or use of an isotherm, will identify a loose condition When the isotherm is broughtdown to a single pixel, or temperature, it will identify the source of the loose condition

8.8.2 Component Failure

Understanding the nomenclature of the problem can identify component failure.Specifically, the actual component will be the heat source For example, a heat-stressedfuse in a three-phase assembly will appear hotter than the other two fusses

8.8.3 Common Problems Found and What to Scan

Following are examples of what to scan while performing an infrared survey to easilydetect common problems

Motor Control and Distribution Centers

Have the switchgear panel covers opened or removed by qualified personnel beforeinspection Scan cable, cable connections, fuse holders, fuse circuit breakers, and bus

Main Secondary Switchgear

Have the switchgear panel covers opened or removed by qualified personnel beforeinspection Scan cables, cables connections, circuit breakers (front and back), and bus

Circuit Breaker Distribution Panels

Covers on small circuit breaker panels do not have to be removed for scanning Circuitbreakers and conductors are very close to the metal covers Defective components areusually detectable by the heating of the cover in the area of the problem If a problemexists, remove the panel cover to locate the problem Only remove panel covers thatcan safely be removed

tions, and rotors Bearing problems can be found by comparing the surface ture of like motors Overheating conditions are documented as hot spots on the CRTand are usually found in comparing equipment, end bell and end bell (same type bear-ings), and stator to end bell.

tempera-Transformer—Oil-Filled

Scan transformer, transformer fins, cable connections, bushings, and tap changer Onall transformers, the oil level should be inspected during the survey During theinfrared survey, if a transformer appears exceptionally warm, the cooling radiators arenear ambient temperature, and the transformer is above 50 percent of full load, the oillevel is too low to circulate the oil and cooling is not taking place Oil in the trans-formers is cooled by convection; as the load increases, the oil expands and the levelincreases until it then circulates in the cooling radiators As a result of repeated oilsamples and oil leaks, the reduced volume of oil causes the winding to overheat, thusreducing the life of the transformer Plugged cooling heaters, isolated radiators, andplugged individual cooling fins can also be detected

Transformers—Dry-Type

Scan transfers, cable connections, bushings, and tap changer Enclosure covers on dry-type transformers should be removed only if there is safe clearance between thetransformer connections and the enclosure panels Some models, especially the newerones, have screened openings for ventilation Use these openings for your scanningsurvey

The iron in these transformers is hot It will heat the bus work and cause substantialinfrared reflection By increasing the temperature scale and adjusting the level control

on the imager, you will be able to get uniform images, which will show hot spots inthe secondary bus or the iron A hot spot in the iron usually indicates a short Makecertain that reflection is not a factor

Compare all windings If temperatures are over a winding, but there is a difference intemperature of two windings, there may be an unbalanced load A hot spot on awinding may point to a shorted turn

Transformer Bushings

As a scanner moves upward on the transformer main tank and tap changer ment, the bushings, lighting arresters, and their bus connections should be observed.This area is also critical because the integrity of the transformer, substation, or thecomplete system depends on proper installation and maintenance of each component

compart-A survey of the transformer bushings, comparing one to the other, will reveal anyloose connections or bushing problems With the scanner, you can determine if theconnection is loose internally or externally

A capacitor has two conductive surfaces, which are separated by a dielectric barrier.Capacitors usually function as power factor correctors When energized, all unitsshould have the same temperature if the size is the same A high uniform temperature

is normal A cold capacitor usually indicates a blown fuse or bad cell Isolated spots showing a high temperature on a surface of the capacitor may indicate a badcapacitor

High-Voltage Switchgear

Scan lighting arresters, insulators, cables, cables connections, bussing, circuit ers, and disconnect switches

break-Load Break Switches

In the switch, two metal surfaces act as conductors when they are brought into contact.Usually, problems are restricted to the contact surface Poor contacts usually show up

as hot spots

Fuses

A fuse is a metal conductor, which is deliberately melted when an overload of current

is forced on it Major problems affected are loose mechanical stab clips that cause hotspots, corroded or oxidized external contact surfaces, and/or poor internal connec-tions, which are bolted or soldered

Circuit Breakers

Circuit breakers serve the same function as a fuse It is a switching device that breaks

an electrical circuit automatically Problem areas are caused by corroded or oxidizedcontact surfaces, poor internal connections, poor control circuitry, and/or defectivebushings

Conductors

The melting points and current-carrying capacity of conductors are determined by thesize and base material of the conductors During a survey, compare between phasesand between conductors and connections An unbalanced load will account for somedifferences between conductors Use metering devices already installed to check thedifferences

The type of load will affect whether the load is balanced Three-phase motor loadsshould be balanced; lighting and single-phase loads may be unbalanced

Other Problems

• Broken strands These hot spots are found at the support and at the cable

termination

• Spiral heating This is found on stranded wire, which is heavily oxidized.

The problem will show up as a hot spiral from one connection to another.There is a load imbalance between the strands, which results in a poor connection

• Ground conductor Usually there are no hot spots on a ground conductor.

They do show up, however, as hot spots when there is abnormal leakagecurrent to the ground Be suspicious about such spots Always point themout in the inspection report

• Parallel feeders A cold cable indicates a problem when parallel conductors

are feeding the same load

A PPENDIX 8.1 Abbreviations

DT Delta temperature The delta notation represents the difference in two

temperatures

m Electrical units for ohms Also used to describe microns in the

infrared electromagnetic scale

°C Degrees Celsius

°F Degrees Fahrenheit

A PPENDIX 8.2 Glossary

A/D conversion The conversion of continuous-type

electri-cal signals varying in amplitude, frequency,

or phase into proportional, discrete digitalsignals by means of an analog–digital converter

Absorptivity Ratio of the absorbed to incident

electro-magnetic radiation on a surface

Ambient temperature Ambient temperature is the temperature of

the air in the immediate neighborhood ofthe equipment

contrasted with digital data having discretevalues

Atmospheric absorption The process whereby some or all of the

energy of soundwaves or electromagneticwaves is transferred to the constituents ofthe atmosphere