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Introduction to thermographic analysis © SKF Group

1.1.1.

Introduction to thermographic analysis

Summary

Thermographic analysis is an effective predictive maintenance tool to use in conjunction with other

types of condition-monitoring processes. The greatest benefit of thermography is realized when it is

used to identify a range of possible problems based upon the condition of various types of machines.

This article explains the process of thermography and discusses the advantages and disadvantages

of this type of analysis.

SKF @ptitude Exchange

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JM02008

Jason Michael Mais

14 Pages

Published April 2002

Revised Oct, 2012



Introduction to thermographic analysis © SKF Group

Table of contents

1. Overview of the Process........................................................................................................3

2. Thermography......................................................................................................................3

2.1. Advantages and Disadvantages ...................................................................................................................5

3. The Uses of Thermography...................................................................................................6

3.1. Electrical Systems ..........................................................................................................................................6

3.2. Hydraulic Systems....................................................................................................................................... 11

3.3. Electronic Systems......................................................................................................................................11 3.4. Energy Systems...........................................................................................................................................12

3.5. Refractory Insulation................................................................................................................................... 12

3.6. Structures.....................................................................................................................................................13

4. Conclusion ..........................................................................................................................13

5. References .........................................................................................................................14



Introduction to thermographic analysis 3(14)

© SKF Group

1. Overview of the Process

Thermographic analysis is a technique in which

an infrared camera or device is used tophotographically portray the surface

temperatures of a component or machine, based

on the radiation emitted by the object.

Thermographic analysis provides a high

resolution, non-contact means of monitoring the

condition of electrical and electromechanical

equipment. The primary concern of

thermography is to monitor the transfer of

infrared heat radiation from an object. The

development of this technology replies upon

sensing the differences in surface temperaturesand displaying those differences in color images

that are displayed on a monitor, LCD or

television. These images, or thermograms, can

then be copied, photographed or recorded to

further analyze the patterns of heat gain or loss.

Figure 1: Example of a handheld thermal imaging

camera with LCD display

Thermographic analysis is an effective predictive

maintenance tool to use in conjunction with other

types of condition-monitoring processes. In

general, maintenance strategies are placed into

three major categories with adjoining parameters:

Breakdown (failure based)

Regular planned (time based)

Predictive (condition based)

2. Thermography

The use of a non-contact means of monitoring

the condition of electrical and electromechanical

equipment is valid for several reasons:

Contact between surfaces is avoided

Non-hazardous to the environment

Resistant to electromagnetic noise

Explosive environment approved

Conduct as a real-time process Reliable due to the semi-infinite lifetime

expectancy

The greatest benefit of thermography is realized

when it is used to identify a range of possible

problems based upon the condition of various

types of machines. Thermography should be

used as an additional technology that can aid in

providing further information to a maintenance

program. A solid foundation, such as a vibration-

monitoring program, should be provided to whichthermography can then be an added benefit.

Table 1 on the following page indicates a list of

many of the types of conditions found when

assessing a manufacturing environment as well

as contributing factors that can be monitored. As

can be seen from the table, thermography

(temperature monitoring) is a well-matched

addition to vibration analysis. The difference

between thermography and temperature

monitoring is that thermography gives an

indication of varying temperature across a given

area. Temperature monitoring only assess a

temperature at a given point where the

temperature sensor is placed.




© SKF Group

Machine Fault Temperature Pressure Flow Oil Vibration

Electrical Machine Faults

Machine – cooling systems,

earth faults, circulating

currents, lamination, cracking

insulation

X X

Mechanical misalignment and

rubX X

Commutators, brushes and

slip ringsX X

Ancillary equipment – fuses,

loose connections, overload orunbalanced load, pitted relay

contacts, switchgear,

distribution boards,

transformers

X

Mechanical

Misalignment, bent shaft X X

Damaged rolling element

bearingsX X

Damaged gears X XInadequate or insufficient

lubricationX X

Damaged journal bearings X X X X X

Loose components X X X

Energy Systems

Boilers, steam systems, flues,

heat exchangers and

regenerators

X X X X X

Refractory insulation,buildings and roofing

X

Electronic Systems

Discrete components, printed

circuit boards and bondingX

Table 1: Indication of measurement type and ability to detect




© SKF Group

With the use of thermal images the user is

provided with data that evaluates the process and

displays the results in a fairly short time.

It is important to point out that the ability to store

and retrieve data is crucial to the development of

a predictive maintenance program.

2.1. Advantages and DisadvantagesFrom an advantage standpoint it is important to

note that thermography can be implemented in

many areas of industry. These industries range

from condition-monitoring, predictive

maintenance to using the data to plan a

maintenance strategy more effectively. The use

of thermography can be a key contributor in the

success of a maintenance program.

The consideration of the disadvantages must alsobe considered. Many of the disadvantages that

use to be prevalent have been address by the

implementation of better software and hardware

packages. The only major disadvantage that still

must be addressed is that of operating the

camera in an industrial environment. The

operation of the camera can be somewhat

cumbersome and takes some development time

until the user is comfortable. A summary of the

advantages and disadvantages are noted in Table

2 and 3 below.

Design

Process Plant : Steam and water lines, heating units, kilns, process pipes, containers, ducts, vents,

exhaust stacks, flue pipes, insulation (refractory)

Intelligent Machine Design : Cooling design on electrical motors

Plant and Machine MaintenanceMaintenance planning, procedures and reporting : implementing timely, appropriate maintenance on

mechanical and electrical plant and machine

Efficiency monitoring : cooling towers, doors, windows, ventilation, heat exchangers, steam traps, foam

and refractory insulation

Machine and Component Failure

Analysis of mechanical component condition : bearings, seals, gears, actuators, hydraulic rams

Analysis of electrical component condition : fuses, switches, insulators, relays, bus bars, commutators,

brush gearTable 2. Advantages of a thermography program




© SKF Group

Cost

Hardware : Cameras and lenses can be expensive initially

Software : Software limitations on some systems

Practical Considerations

Object Source : Emissivity, transmittion and size of detail. The objects emissivity must be known.

Object surroundings: The object’s surroundings should have a homogeneous (ambient) temperature

and should not include hot areas so positioned that the object can reflect the radiation.

Atmospheric influences/attenuation : Distance, composition and ambient temperature can affect the

quality of detail.Table 3. Disadvantageous of a Thermography program

3. The Uses of Thermography

There are numerous advantages to using a

thermography program within a variety of

industries. The follow is a list of the key areas

of contribution for thermography:

Electrical

Mechanical Electronic

Energy

Refractory Insulation; and

Structures

Based upon these defined areas, the

considerations are as follows:

3.1. Electrical Systems

With a plant, electrical systems are consideredto be among one of the most critical areas.

Electrical systems are based upon several key

formulas. One of these key formulas is Joule’s

Law. Joule’s Law states:

2 I P = R (watts)

where

P = heat generated (watts)

I = load (amps)

R = resistance (ohms)

In many instances, the element that is expelling

the greatest amount of energy (heat) is due to

a loose, oxidized or corroded electrical

connection. This extemporaneous heat is an

indication of a problem and is a key sign when

conducting a predictive maintenance program.

In electrical systems, educated guesses can be

based upon a change in resistance causing a

doubling effect on the current. This isespecially prevalent in systems that are not

fully loaded. In addition, cold areas “spots” can

be an indication of an open circuit. This is due

to a blown fuse and can often go undiagnosed

for several days.




© SKF Group

As an electrical system is activated, two

conductors interface to form the circuit. In this

mergence, the surfaces of the contacts

intersect at a certain number of points called“elementary contacts”. These contacts are

limited due to the characteristics of the

contacts.

As these two conductors interface, wear begins

to be exhibited. When the wear on these

contacts develops substantially, an increase in

electrically resistance will develop and therefore

will produce excess heat and a thermal “hot

spot”. This hot spot can usually be identified

easily using thermography.

To develop a viable idea of the rate at which

this hot spot is deteriorating, “trending” must

be used to evaluate the system.

Trending is a common practice in many types

of condition-monitoring programs. Trending

most often involves plotting a value against

time. In this instance, the vertical access is

temperature. Based upon regular intervals ofcollection, a clear picture of the status of the

equipment can be developed. Where the load

is variable, ideally the temperature

measurements should be taken in conjunction

with current assessments. This system of

measuring will allow the correlation between

the rise in temperature and the current

measurement to be established.

The following formula relates the correction of

the temperature rise to that of the reference

temperature (cooling by natural convection and

radiation)):67.1)(

m

r

ms

I

I T T Δ=Δ

Correction of temperature rises to a reference

current (cooling by forced convection and

radiation):

2)(m

r

ms

I

I T T Δ=Δ

Where

sT - temperature rise

mT - measured temperature rise

I - reference current

Im - measured current

One of the many advantages of trending is that

it requires a method and process therefore

establishing required measurements that must

be taken and recorded. In addition, it relies less

upon the analysis of a specific measurementbut, in turn, can analyze a particular set of

measurements over time. An example of a

temperature trend is noted below.




© SKF Group

Figure 2: Trend plot of temperature increasing over time (courtesy of SKF Condition Monitoring).

In general and for simplicity, electrical

components can be classified into two

categories:

1. Low current devise, which are covered by

electronics and microelectronics

engineering

2. High current devices such as fuses,

busbars, switchgear, cables, insulation,

transformers and isolators.

All of these types of components have been

successfully monitored using IR (infrared)

techniques. The most common problems

facing electrical systems are:

Loose connections

Load imbalances

Corrosion resulting from resistive heating

Many types of technological advances such as

thyristors, that are used to control motor speed

in large motors, are connected in parallel and

causer a masking affect making it difficult to

detect a problem.

Some other examples of common measuringtechniques are to measure electrical imbalance

between electrical phases. An unequal

temperature in this situation may indicate an

imbalance in a three-phase motor. Other

examples are:




© SKF Group

Inspection of high-voltage transformers

Inspection of high-voltage power lines

Blown or damaged fuses or fuse holders

Overheating power factor capacitors Switchgear, control panels, isolators, circuit

breakers, relay contacts and connections.

3.1.1. Identifiable Electrical Failures

With regard to electrical machinery failure,

identifiable failures include:

Rotor body defects

Rotor winding faults

Water coolant faults

Stator winding faults

Winding insulation defects and

Stator core defects

If these failures are propagated into the

condition-monitoring arena, there are three

main sources of problems:

Mechanical sources, which include bearing,

rotor unbalance, looseness, misalignment,end-winding damage, brushes and brush

components.

Aerodynamic sources which involve

turbulence, blade-passing frequency and;

Electromagnetic sources such as static air-

gap eccentricity, dynamic air-gap

eccentricity, air-gap permeance variations,

open or shorted windings, unbalance

current phase, broken rotor bars, torque

pulses and magnetostriction.

As discussed previously, there are well-

established condition-monitoring parameters

such as vibration and motor speed. When

thermal condition monitoring is considered,

such parameters as below can be added:

Machine Enclosure

o Overheating and cooling

o Defective cooling system

o Poor electrical connections Frame Overheating

Rotor Body and Winding Overheating

Stator

o Stator core – lamination

o Stator windings

o Stator end winding portion – cracking

insulation

o Bearing and Seals Overheating

One of the major advantages of non-contact

thermal monitoring is that it does not requireisolation of the electrical machine yet still

provides useful information of the machine’s

component. The following figure shows a

thermal image of the drive side of a motor.

Figure 3: Thermal image of an overloaded

circuit or fuseIn addition to electrical systems, a consideration

of mechanical systems is equally important.




© SKF Group

Mechanical SystemsIn a plant environment, mechanical systems

represent a large potion of a plant’s assets.

There are many types of rotating equipmentwithin a plant that can be monitored using

thermography. Thermography can be used to

monitor both simplistic machinery as well as

machines comprised of numerous components.

It is a commonality among machines that rotate

or reciprocate that friction occurs between

interfacing components. This interface causes

heat to be generated and in turn wear to occur.

Friction, if left unattended or unresolved, can

lead to catastrophic failure.

A combination of trended wear data with theindication of supporting vibration data can

prove to be very accurate in assessing the

health of a machine or component. Both of

these measurements are most useful when

trended.

Figure 4. Thermal image of an overheated bearing on a belt driven fan

Some common reasons for mechanical failure

may include:

An increase in loading on a bearing cause the

bearing to wear prematurely

An increase in the stresses of the machineleading to premature fatigue problems

An increase in forces that are applied to the

machine, such as loose components or

footing

The effects of inertia leading to imbalance of

a component or rotating shaft.

Some of the most common forms of mechanical

deterioration of a system are imbalance,

misalignment, looseness, damaged components

such as impellers in a pump or vanes of a fan,

damaged bearings, gears, etc.

The value added with the use of thermography isthat it allows the user a tool to better assess the

condition of the mechanical systems in the plant.

In addition to assessing mechanical systems,

thermography can aid in monitoring hydraulic

systems.




© SKF Group

3.2. Hydraulic Systems Though the use of thermography when assessing

hydraulic systems is not as common as its’ use

for mechanical system, thermography can beused to analyze the changes in temperature of

the system. These changes can be indications of

problems like:

Leakage

Clogging of the system

Component failures

Improper installation

An example of this occurred when assessing the

effectiveness of a seal with a small axial inclusion.

With the use of thermography, an indicated “hot

spot” appeared on the image due to the increased

pressure 35 kg/m2 (~500 psi) of the fluid

escaping from around in inclusion.

It is important to note that care must be taken

when visually assessing hydraulic systems

because defects of a mechanical system may

coincide with defects in the hydraulic system,

therefore causing some confusion as to the causeof the problem. The use of addition techniques

should be used to further clarify the situation.

3.3. Electronic SystemsProbably one the areas that has benefited the

most from the use of thermography is that of the

electronic systems. Electronic and

microelectronic systems such as printed circuit

boards (PCBs) and their components being the

items affected the greatest. This affect has been

realized due in part to the design of PCBs. PCBscontain many small components that are difficult

to monitor with conventional methods.

Therefore, the use of thermography has aided

greatly.

The development of temperature measurement

devices has progressively migrated from pattern

type measurements to a formidable device that

uses a complex computerized Thermographicsystem to automatically inspect items such as

PCBs.

A common contributor of reduced service life in

electronic components is high operating

temperature. An indication of this can be

explained in the following equation:

C at rateFailure

T at rateFailure

T °=

75π

Another form of failure is premature component

failure of new components. This type failure can

be conceptualized using a “bathtub” curve. A

bathtub curve is based upon reliability studies

and indicates a high-probability of failure during

the “running-in” potion of the system – some

cases may be caused by poor installation

problems. The probability of failure is then

reduces for an extended period of time –

indicative of its’ normal operating conditions.

Then, the probability of failure is increased –representing the components wear out condition.

In this area, the greatest concentration on

detecting a components condition is

concentrated.




© SKF Group

Figure 5: Representation of a Bathtub Curve (A).

In addition, other patterns of failure are shown.

Thermography can be used to inspect specific

components within a system so that “thermal

run-away” can be avoided and thus a possible

catastrophe.

In addition, thermography can be used in non-

destructive test inspections of integrated circuit

boards. In this instance, the induced heat creates

a thermal pattern that can then be diagnosed.

Care must be taken in this type of testing toaccount for the positioning and geometry of the

components and their acceptable limits.

This allows for proper diagnoses once the data is

collected. Concerning electronic systems, other

energy sources can be considered when

conducting a thermography program.

3.4. Energy SystemsEnergy systems are being considered more often

than not as the world migrates to an energyefficient mentality. This migration causes

management and maintenance personnel to

consider conserving more of their resources

when it concerns the use of energy.

Thermography can be positioned as a key

contributor in assessing the performance of a

system. Non-contact thermal monitoring can be

used to detect the area in which resources arebeing wasted. When concerned with the use of

an energy system , there is a heavy burden placed

upon ensuring that proper insulation and

adequate maintenance of the insulation is

achieved. Faulty insulation and leaks in the

system are readily visible with the use of

thermography. These areas appear as increases

in temperature output. An example of a system

where this type of leak occurred is noted below.

3.5. Refractory InsulationRefractory systems such as furnaces operate at

temperatures as high as 1500ºC (2732ºF). The

use of thermography to inspect these items

during operation is of great value.

Some of the more common uses of

thermography in these types of environment are:

Monitoring product parameters such as the

temperature of steel strips within the furnace Integrity of insulation, joints or brickwork

within a system

Monitoring the burner operation; or

The operation of water-cooled elements

By monitoring these types of parameters, the

characteristics of the system can be well

documented and analyzed.




© SKF Group

Figure 6. This thermal image of a furnace clearly indicates an abnormality in the surface heat

distribution.

3.6. StructuresThe use of thermography to detect losses in

structures from poor insulation, poor sealing or

poor structural integrity is vital in achieving a

thermally sound structure.

The use of a thermal imager can easily produce

a pattern that is associated to heat loss

therefore identifying the problem areas. Some

of the most commonly detected or identifiable

losses are:

Detection of a leak in the roofing systam

based upon solar loading

Leaks in chimneys or vents; also

Leaking uindows or door areas

4. Conclusion

The Use of thermography to evaluate the

operation and conditions of items quch as

electrical boards, process equipment and

insulation integrity has increased substantially

over the past few years. The industry is

expected to continue this trend based upon the

ability to impart a cost savings in their facility.

Moreover, many influential issues such as:

Market awareness and acceptance Application diversity

Advancements in Equipment

Development of Standards; and

Development in training

All of these issues are key contributors to

growth in this technology. But, in addition to

purchasing the technology needed to

implement a successful program, you must also

recognize the following key aspects:

Planning the implementation phase,

Providing proper training, and

Supporting the system that is established




© SKF Group

5. References

Industrial Maintenance (1998)

Thermography gives maintenance insight

Thomas, R.A. (1999) Thermography.

Mobley, R.K. (1990) An introduction to

Predictive Maintenance

skf_introduction to thermal imaging analysis_237241[1]

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