An introduction to predictive maintenance - part 5 pptx

The electromagnetic spectrum contains various forms of radiated energy,including X-ray, ultraviolet, infrared, and radio.. In the electromagnetic spectrum, the term band refers to a spec

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

Atmospheric attenuation The process whereby some or all of the

energy of the soundwaves or netic radiation is absorbed and/or scatteredwhen traversing the atmosphere

electromag-Atmospheric emission Electromagnetic radiation emitted by the

atmosphere

Atmospheric radiance The radiant flux per unit solid angle per

unit of projected area of the source in theatmosphere

Atmospheric reflectance Ratio of reflected radiation from the

atmos-phere to incident radiation

from 0.4 to 0.5 microns) that is used for

radiate measurements The term channel is

also in common use, with the same meaning

as band In the electromagnetic spectrum, the term band refers to a specific frequency

range, designated as L-Band, S-Band, Band, and so on

con-ductors when air is stressed beyond its ization point without developing flashover

ion-Electromagnetic spectrum Electromagnetic radiation is energy

propa-gated through space between electrical andmagnetic fields The electromagnetic spec-trum is the extent of that energy rangingfrom cosmic rays, gamma rays, and X-rays

to ultraviolet, visible, and infrared radiation,including microwave energy

Emissivity Consideration of the characteristics of

ma-terials, particularly with respect to theability to absorb, transmit, or reflect infraredenergy

Emittance Power radiated per unit area of a radiating

surface

Far-infrared Infrared radiation extending approximately

from 15 to 100 micrometers

by a nucleus in a transition between twoenergy levels

excessive light on that part of the subject

lying between the extreme of the visible(approximately 0.70 micrometer) and theshortest microwaves (approximately 100micrometers)

Infrared radiation Electromagnetic radiation lying in the

wavelength interval from 0.7 to 1,000microns (or roughly between 1 micron and

1 millimeter wavelength) Its lower limit isbounded by visible radiation, and its upperlimit by microwave radiation

Isothermal mapping Mapping of all regions with the same

temperature

Microwave band The portion of the electromagnetic

spec-trum lying between the far-infrared and theconventional radio frequency portion.Although not bounded by definition, it iscommonly regarded as extending from 0.1

cm (100 microns) to 30 cm in wavelength (1

to 100 gigaHertz frequency)

Mid-infrared Infrared radiation extending approximately

from 1.3 to 3.0 micrometers and being part

of the reflective infrared Often referred to

as short-wavelength infrared radiation

(SWIR)

Near-infrared Infrared radiation extending approximately

from 0.7 to 1.3 micrometers and being part

of the radiative infrared

Qualitative infrared thermography The practice of gathering information about

a system or process by observing images ofinfrared radiation and recording and pre-senting that information

Quantitative infrared thermography The practice of measuring temperatures of

the observed patterns of infrared radiation

wave-lengths within the range of approximately

100 microns to 2 meters

transmitting energy through space orthrough some medium

of electromagnetic radiation designated asradio waves; approximately 4 to 9 Hz in frequency

Reflectivity The fraction of the incident radiant energy

reflected by a surface that is exposed touniform radiation from a source that fills itsfield of view

Spectral band An interval in the electromagnetic spectrum

defined by two wavelengths, two cies, or two wave numbers

frequen-Temperature gradient Rate of change of temperature with

distance

Thermal emittance Emittance of radiation by a body not at

absolute zero because of the thermal tion of its molecules

agita-Thermography The recording of the thermal qualities of

objects and surfaces by means of scanningequipment in which the infrared radiation ormicrowave radiation recorded can be con-verted into a thermal image

Transmittance The ratio of energy transmitted by a body to

that incident on it

Ultraviolet band That portion of the electromagnetic

spec-trum ranging from just above the visible(about 4,000 ang.) to below 400 ang., on theborder of the X-ray region

Visible band The band of the electromagnetic spectrum,

which can be perceived by the naked eye.This band ranges from 7,500 ang to 4,000ang., being bordered by the infrared andultraviolet bands

X-ray Electromagnetic waves of short wavelength

from 00001 ang to 3,000 ang

A PPENDIX 8.3 Electrical Terminology

Alternating current (AC) Electrical current that reverses direction

periodically, expressed in hertz (Hz) orcycles per second (cps)

current, which is internally rectified todirect current before being released

current-handling capacity of an electrical device

describing electrical current The totalamount of current (amperes) flowing in acircuit

elec-trical current

which electromotive force is produced,usually the rotor of a DC motor or the stator

of an AC motor

whose nonlinear impedance characteristicsprovide a path for high-amplitude transients

strength

when bearing against a commutator, slipring, or the like will provide a passage forelectrical current

electrodes and an intervening insulator,which is called the dielectric A device used

to store an electrical charge

Circuit (closed) An electrical circuit in which current flow

is not interrupted

Circuit (open) Any break or lack of contact in an

electri-cal circuit either intentional (switch) orunintentional (bad connection)

Circuit (parallel) An electrical system in which all positive

terminals are joined through one wire, andall negative terminals through anotherwire

Circuit (series) An electrical system in which separate

parts are connected end to end, to form asingle path for current to flow through

Circuit breaker A resettable device that responds to a preset

level of excess current flow by opening thecircuit, thereby preventing damage tocircuit elements

Circuit protector A circuit protector is a device that will open

the circuit if it becomes overheated because

of too much electricity flowing through

it Thus, it protects other components from damage if the circuit is accidentallygrounded or overloaded Fuses, fusiblelinks, and circuit breakers are circuit protectors

conductor, which combines the separatemagnetic fields of all the winding loops toproduce a single, stronger field

expressed in amperes Current refers to thequantity or intensity of electrical flow.Voltage, on the other hand, refers to thepressure or force causing the electricalflow

direction only Used to change alternatingcurrent to direct current A rectifier

Direct current (DC) Electrical current that flows consistently in

one direction

various current-using sites or devices.Outside the building, distribution refers tothe process of routing power from thepower plant to the users Inside the build-ing, distribution is the process of using

feeders and circuits to provide power todevices.

Electromagnetic interference (EMI) A term that describes electrically induced

noise or transients

passage of a certain frequency band whileallowing other frequencies to pass Filtersare designed to produce four differentresults: (1) a high-pass filter allows allsignals above a given frequency to pass; (2)

a low-pass filter allows only frequenciesbelow a given frequency to pass; (3) abandpass filter allows a given band of fre-quencies to pass while attenuating allothers; and (4) a trap filter allows all fre-quencies to pass but acts as a high-imped-ance device to the tuned frequency of thefilter

insulation between two conductors where ahigh current flow exists, with a high poten-tial difference between the conductors

when the current passing through itexceeds the rated value of the fuse A plug-

in protector with a filament that melts orburns out when overloaded

at which other portions of a circuit are referenced when making measurements Apower system’s grounding is that point towhich the neutral conductor, safety ground,and building ground are connected Thisgrounding electrode may be a water pipe,driven ground rod, or the steel frame of thebuilding

fun-damental frequency For example, 120 Hz

is the second harmonic of 60 Hz, 180 Hz isthe third harmonic, and so forth

Harmonic distortion Excessive harmonic content that distorts

the normal sinusoidal waveform is

har-monic distortion This can cause ing of circuit elements and might appear to

overheat-a device overheat-as doverheat-atoverheat-a-corrupting noise

alter-nating current The term hertz is

synony-mous with cycles per second

Impedance (Z) Measured in ohms, impedance is the total

opposition to current flow in a circuit inwhich alternating current is flowing Thisincludes inductive reactance, capacitivereactance, and resistance

Inductance This term describes the electrical properties

of a coil of wire and its resultant magneticfield when an alternating current is passedthrough it This interaction offers imped-ance to current flow, thereby causing thecurrent waveform to lag behind the voltagewaveform This results in what’s known as

a lagging power factor.

property of inductance It should be notedthat at very high radio frequencies, astraight wire or a path on a printed-circuitboard can act as an inductor

as porcelain, glass, or Bakelite, that is usedfor insulating wires in electrical circuits toprevent the undesired flow of electricity

it into AC power

the electrical environment of its input fromits output, while allowing the desired trans-mission to pass across the separation

Kilohertz (kHz) A term meaning 1,000 cycles per second

(cps)

Kilovolt-Ampere (kVA) An electrical unit related to the power

rating of a piece of equipment It is culated by multiplying the rated voltage ofequipment by the current required (or produced) For resistive loads, 1 kilovolt-ampere equals 1 kilowatt

cal-Lightning arrester A device used to pass large impulses to

ground

Mean time between failure (MTBF) A statistical estimate of the time a

compo-nent, subassembly, or operating unit willoperate before failure will occur

Megahertz (MHz) A term for 1 million hertz (cycles per

second)

Motor alternator A device that consists of an AC generator

mechanically linked to an electric motor,which is driven by utility power or by bat-teries An alternator is an AC generator

Motor generator A motor generator consists of an AC motor

coupled to a generator The utility powerenergizes the motor to drive the generator,which powers the critical load Motor gen-erators provide protection against noise and spikes, and, if equipped with a heavyflywheel, they may also protect againstsags and swells

system is the neutral conductor Sometimes

called the return conductor, it carries the

entire current of a single-phase circuit andthe resultant current in a three-phasesystem that is unbalanced The neutral isbonded to ground on the output of a three-phase delta-wye transformer

resistance

Ohm’s law A law of electricity that states the

relation-ship between voltage, amperes, and tance It takes a pressure of one volt toforce one ampere of current through oneohm of resistance Equation: Volts = am-peres ¥ ohms (E = I ¥ R)

distribution system and travels throughspace These signals often cause interfer-ence with other communication services

used to convert AC to DC

Relay An electromagnetic switching device using

low current to open or close a high-currentcircuit

Resistance (R) A term describing the opposition of

ele-ments of a circuit to alternating or directcurrent

Resistor A device installed in an electrical circuit to

permit a predetermined current to flow with

a given voltage applied

of a variable resistance

the stator and produces an electrical currentfrom induction by the electromagneticfields of the stator windings

SCR (semiconductor, or silicon, An electronic DC switch that can be

trig-controlled rectifier) gered into conduction by a pulse to a gate

electrode, but can only be cut off by ing the main current below a predeterminedlevel (usually zero)

electromagnetic and radio frequency interference

to form a parallel circuit, through which aportion of the current may pass, in order toregulate the amount of current flowing inthe main circuit

periodic oscillation that expresses the sine

or cosine of a linear function of time orspace, or both

Single-phase That portion of a power source that

repre-sents only a single phase of the three phasesthat are available

mag-netic core, which moves when the coil isenergized

armature in a DC generator)

Switch A device used to open, close, or redirect

current in an electrical circuit

voltage lines or legs, which carry wave waveforms that are 120 degrees out

sine-of phases from one another

AC circuit and/or isolate a circuit from itspower source

Resistance)

work A watt is the mathematical product

of amperes and volts (W= A ¥ V)

A PPENDIX 8.4 Materials List

Material °F °C Emissivity

Material °F °C Emissivity