lloydsregister thermography in condition assessment

7/31/2019 LloydsRegister Thermography in Condition Assessment

1/15

Lloyds RegisterTechnical Association

Thermography in condition

assessment

ABCDPaper No 2Session2002-2003


2/15

LLOYDS REGISTER TECHNICALASSOCIATION

Lloyds RegisterTechnical Association: Paper No. x, Session 2002-2003

The authors of this paper retain the right of subsequent publication, subject to the sanction of the Committee of

Lloyds Register of Shipping. Any opinions expressed and statements made in this paper and in the subsequent

discussions are those of the individuals and not those of Lloyds Register of Shipping.

Lloyds Register of Shipping 2002. All rights reserved. Except as permitted under current legislation no part of

this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast,

transmitted, recorded or reproduced in any form or means, without the prior permission of the copyright owner.

Enquiries should be addressed to Lloyds Register of Shipping, 71 Fenchurch Street, London EC3M 4BS,

England.

Web site: www.lr.org

Secretary, LR Technical Association

M. McMahon

[email protected]


3/15



Thermography in condition monitoring

Synopsis

Every surface with a temperature above absolute zero emits some infrared radiation. Thermography is aquick surveillance technique for determining the general health of an engineering plant. By detecting

the infrared radiation emitted by a body to produce a thermal map of its surface a thermographic camera

is an invaluable tool in condition monitoring. The image thus produced is called a thermogram. The

temperature variation is indicated in different colours or in shades of grey. It is a very useful condition

monitoring aid for both electrical and mechanical equipment when used to identify hot spots (or cold

spots in electric circuits). Identifying areas of equal temperatures (isotherms) in the baseline images and

detecting variations by subsequent trending can provide very early warning signs of equipment failure.

This paper explores the potential of thermography in monitoring and assessing the condition of

machinery on board ships.

Author

Gopinath Chandroth, CEng, MSc, PhD, MIMarEST


4/15



Table of Contents.

SYNOPSIS ................................................................................................................................................................ 3

AUTHOR .................................................................................................................................................................. 3

1. INTRODUCTION........................................................................................................................................... 5

2. THEORETICAL BACKGROUND............................................................................................................... 5

2.1. HISTORY OF DEVELOPMENT OF THE UNDERLYING THEORY ........................................................................... 6

2.2. REAL BODY (NON-BLACK BODY) RADIATION................................................................................................. 7

3. DEVELOPMENT OF THERMOGRAPHY EQUIPMENT....................................................................... 7

3.1. THERMOGRAPHY IN CONDITION MONITORING ............................................................................................... 8

3.2. FAULTS IN ELECTRICAL SYSTEMS .................................................................................................................. 9

3.2.1Heating effect........................................................................................................................................... 9

3.2.2Harmonic distortion of voltage............................................................................................................... 9

4. PRACTICAL APPLICATION...................................................................................................................... 9

4.1. TYPICAL RESULTS FROM A THERMOGRAPHIC SURVEY ................................................................................. 10

5. CONCLUSIONS ........................................................................................................................................... 14


5/15



1. Introduction

Every process, mechanical or electrical, is associated with changes in the thermal energy levelsof the associated equipment and system. Thermography or thermal imaging is a condition

monitoring technique, which exploits this fact. The infrared radiation from the surface of the

body is utilised to produces a thermal image which then can be used to identify hot or cold

spots on the body. By correlating expert domain knowledge of the concerned machinery or

system with the identified hot or cold spots, the condition monitoring practitioner can make

reasonable engineering diagnostic and prognostic decisions.

A thermal imaging camera is simple to use and does not require extensive training to achieve

good results. Neither does one need to be an expert in optics or understand the details of

construction of the camera. However it is important that one has a good grasp of the details of

the underlying processes in an engineering system, so as to come to useful conclusions. This

paper is based on the practical approach with particular emphasis on marine machinery. After abrief introduction to the theory of thermography based on a time-line approach, the practical

results are discussed.

2. Theoretical background

Sir William Herschel, in 1800, conducted an experiment to measure the temperatures of various

colour bands in the electromagnetic spectrum. He passed sunlight through a glass prism and

measured the heating effect of each colour. He accidentally discovered that the maximum

heating effect occurred at a point just beyond the visible red colour in the spectrum. This part

of the spectrum was later termed infrared. Infra is Latin for below or beneath. Forty years

later, Sir John Herschel, son of the former, captured a thermal image on paper. He called it a

'thermogram'. In 1880, Samuel Langley perfected his 'bolometer', an instrument with which he

could measure thermal radiation from an object 400 metres away.

Figure 1 The electromagnetic spectrum


6/15



The electromagnetic spectrum is shown in Figure 1. Out of the entire spectrum, thermal

radiation only takes place within the Ultraviolet, visible and Infrared ranges (0.1 micron UV to

10 microns IR). A thermal imaging camera uses only the infrared portion of the spectrum to

form a colour (or shades of grey) map of the surface under consideration.

2.1. History of development of the underlying theoryGustav Kirchhoff, in 1859, developed a general theory of emission and radation known as

Kirchhoff's law. It states that a substance's capacity to emit light is equivalent to its ability to

absorb it at the same temperature. In 1860 he formulated the concept of a blackbody which is

an idealised body that absorbs all of the radiation falling onto it and reflects none.

Joseph Stefan (1879) first determined the relation between the energy radiated by a body and its

temperature. Along with his student Ludwig Boltzmann, they derived what is popularly known

as the Stefan-Boltzmann fourth-power law [Ref. 2]

E = T4

= Stefan-Boltzmann constant = 56696 x 10-8

[Wm-2

K-4

] T = Absolute temperature, K

It states that the energy radiated by a blackbody radiator per second per unit area is proportional

to the fourth power of the absolute temperature. This law is fundamental to radiation

thermometry.

Wilhelm Wien in 1893 demonstrated that as the temperature increases, not only does the total

radiation increase (Stefan's law), but the wavelength of the emitted light also changes in inverse

proportion (changes from red to orange to yellow to white). This was known as Wien's

displacement law. While Wien's displacement law was applicable to high frequency blackbody

radiation, John Rayleigh developed the relationship for low frequency radiation.

Max Planck (1900), based on the findings of Stefan, Wien and Rayleigh, comprehensively

quantified the distribution of energy in the spectrum of the radiation from a blackbody.

Planck's radiation law describes 'blackbody radiation' as

2c2h

I (L,T) = -------------

L5

(ehc/LkT

1)

Where:

I is the intensity of radiation from a body at wavelength L and temperature T

c ~ 3x108

m/s is the speed of light. h ~ 6.626x10-34 (Planck's or black body constant in J/s)

k is Boltzmann's Constant ~ 1.380x10-23J/K

This is the underlying theory behind radiation thermometry, infrared thermography being just a

sophisticated use of radiation thermometry. Essentially it boils down to the fact that if the

radiation energy from the surface of an object can be measured, the temperature of the surface


7/15



can then be computed. However the blackbody is a hypothetical concept. We must now

consider real world objects.

2.2. Real body (non-black body) radiationA real body (as opposed to a blackbody) absorbs, reflects or transmits incident energy striking

upon it. The total radiation incident upon the body must be equal to the sum of the fractions

absorption , reflectance and transmittance . Thus,

+ + = 1

For an opaque body transmittance is zero and in a black body the transmitted and reflected

fractions are zero. Thus the blackbody has an absorption of unity. According to Kirchhoff's

law, it emits all of the absorbed radiation and hence a blackbody has an emissivity of unity.

Emissivity of any object is defined as the ratio of the radiant emittance E o of that body to the

radiant emittance Eb of a blackbody.

= Eo / Eb

In general for condition monitoring purposes, the emissivity factor is always less than 1 (a part

of the infrared energy is reflected). The Stefan law stated earlier must now take into account

the reduced emissivity of the surface. A highly polished surface would have an emissivity

factor approaching zero. While emissivity is a property of the material, emittance refers to the

properties of a particular object (shape, oxidation and surface finish). Thermal imaging

cameras have to be programmed with the emissivity factor of the object being measured. The

temperatures measured will only reflect the true temperature if the emissivity correction isaccurate. An emissivity factor of 0.80 to 0.90 gives reasonable results for painted surfaces on

machinery with the exception of 0.25 to 0.35 for shining aluminium paints. A quick on-site

method of determining emissivity is to take spot measurements on a strip of black masking tape

stuck on the surface and then repeat the spot measurements outside the masking tape.

Assuming an emissivity factor of 1 for the black tape, the approximate emissivity of the surface

concerned can be calculated.

3. Development of thermography equipment

Infrared radiation thermometers (or pyrometers) measure the energy being radiated from an

object in the 0.7 to 20 micron wavelength range. Being non contact instruments they consistsof an optical system which focuses the emitted energy from an object on to a radiation sensitive

detector. The detector absorbs infrared energy and converts it to an electrical signal. In

thermal imaging cameras Cadmium Mercury Telluride, Platinum Silicide or Indium

Antimonide are commonly used as detectors. Their main drawback is that they require cooling

which adds to the complication of the camera.


8/15



The FLIR (forward looking infrared imager) camera which uses electromechanical scanning in

the horizontal or vertical direction was the next development in thermal imaging cameras. By

moving a single array of sensors within the camera, a 2 dimensional thermal map in colour is

produced. The newer cameras consist of an entire matrix of sensors, which eliminates the need

for mechanical scanning movement (FPA or Focal Plane Arrays). Microbolometer detectors

eliminate the need for cooling. The size of a modern thermal imaging camera is comparable tothat of a small hand held video camera with smaller models increasingly coming onto the

market. A state of the art camera of 160 x 120 pixels with a 0.12o

C sensitvity at (25o

C) in the

7.5 to 13 m spectral range, weighing 700 gms including batteries and measuring 265 mm x 80

mm x 105 mm has recently been announced [Ref 5].

3.1. Thermography in condition monitoring

Thermography has been in use as far back as in 1965 even before the word condition

monitoring came into use. It is a versatile and useful tool while being easy to use. The most

important prerequisite of the data interpretation stage is that the user must have a good

understanding of the principles and the dynamics of the system being monitored.

The main advantages of thermography are as follows:

A non-contact measurement technique

Measurement can be made with the equipment under operational load.

Immune to electromagnetic noise

Real time measurement

Portable handheld instruments

Modern cameras are robust and reliable

The following constraints apply:

Cost Emissivity of an object must be known for accurate temperature measurement

Object surrounding should have a homogeneous background temperature

Direct heat sources should not be in the vicinity of the object being measured due to thepossibility of reflected radiation

Large separation between camera and object will result in reduced resolution images

High humidity (rain and snow) and ambient temperature effect on image

Glare effect on a bright day

Presence of insulation material between camera and target

Effect of object geometry (smaller than 60 degrees incident angle)

Electric circuit must be live and loaded in order to discover anomalies

In the context of condition monitoring within a ship's machinery space, none of the above

technical limitations pose a significant hurdle. In condition monitoring we are concerned with

the relative temperature difference within an image. We can also compare thermal images of

identical equipment in order to detect variations in the thermal patterns between them.

Absolute temperature measurement is rarely a requirement. As there is a reasonable physical

separation between machinery and heat sources (such as boiler furnaces and engine exhaust

systems which are well insulated), reflected heat radiation from a foreign source is minimal. In


9/15



practice in most cases, it would be possible to use the camera close to the equipment being

monitored. High humidity and low ambient temperature have minimal effect on the atmosphere

inside the engine room although care must be taken to avoid areas with heavy steam leaks and

in the vicinity of open hot-wells etc. Insulation material, covering steam traps or critical steam

valves, may be temporarily removed.

3.2. Faults in electrical systems

3.2.1 Heating effectJoule's law states that power or heat generated (P) is proportional to the square of the current

(I) flowing in the circuit and to the resistance (R).

P = I2R

As the condition of an electrical contact deteriorates, the cross-sectional area available for

current flow is reduced thus increasing the resistance to current flow. Resistive oxide layers

developed between the contacts further deteriorate the connection. Thermal hotspots are

formed which are easily picked up by thermal imaging cameras. Typically local imbalance in 3

phase supplies, loose connections and corrosion cause increased resistive heating. Busbars,transformers, fuses, relays, switchgear and other electrical equipment are amenable to this

condition monitoring technique.

Printed circuit boards fault diagnosis is another area of application of thermography to low

current devices.

3.2.2 Harmonic distortion of voltageFrequency components which are multiples of the 60 Hz fundamental electric frequency are

called Harmonics. LR's Rules (Part 6, Chapter 2, Section 1.7) provides guidance on acceptable

levels. They distort the sinusoidal voltage and current waveforms and cause overheating of

conductors, motor and generator windings, transformers and cables connected to the same

power supply with the devices generating the harmonics. They also cause 'nuisance' fuseblowing, frequent tripping of circuit breakers and premature failure of power capacitors [Ref. 3,

4].

Semiconductor static power converter equipment found in AC/DC conversion circuits,

converters associated with shaft generators, variable-speed motor drives for electrically

propelled vessels and even fluorescent lights are capable of injecting harmonic components into

the system. A thermal imaging camera can be used to detect overheating due to harmonic

voltage distortion at an early stage.

4. Practical applicationA thermographic camera is relatively easy to use and the results are easy to interpret. Howeverthere are several details that the user should be aware of in order to obtain the best results.

There are two ranges of the infrared band, which are utilised by typical cameras:

Long wave length 8 to 14 microns, suitable for low temperatures (to 10 C)

Short wave length 2 to 5 microns suitable for higher temperatures (to 400 C).


10/15



Short wavelength cameras used to be the most common till recently. However with thedevelopment of Focal Plane Array type of cameras, long wavelength (8 14 microns)

cameras are becoming the norm.

Extended measurement ranges from 40 to 2000 C are claimed by manufacturers.

The following are the most important definitions that a user of thermal imaging cameras needsto be aware of:

Thermal sensitivity: The smallest change in radiation level that the instrument is capable ofregistering, expressed in terms of temperature.

Temperature range: Temperature measurement from 40 to 2000 C is possible withmodern cameras

Environmental temperature: The range of temperature in which the camera may be safelyoperated

Thermal resolution: The smallest difference in temperature possible to be expressedbetween two measurements.

Spatial resolution: A measure of the fineness of detail directly proportional to the number ofpixels representing the image.

Accuracy: A measure of the difference between the true temperature and the measuredtemperature.

Spot size ratio: The ratio that expresses the maximum distance the camera can be from atarget of a given size and still maintain temperature measurement accuracy.

For further information on this topic the reader is referred to Ref. 1.

4.1. Typical results from a thermographic surveyThe images in this section, were captured during an exploratory thermographic survey

undertaken as part of a Condition Assessment Programme (CAP) on board a 20,302 GRT, 1982

build, crude oil tanker. LR does not require a thermographic survey as part of CAP, hence theimages were only taken at selected locations. The whole operation took about an hour to

complete.

Figure 2 is a classic illustration of faults in electric systems. The relays in the set shown in the

normal photograph (Figure 2) are in a circuit designed to remain energised at all times.

However, the relay on the extreme right corner (as seen in the thermal image) was found to be

operating at 52 C, 10 degrees above the others operating under identical duty and load.

Although 52 C is within the acceptable limits set by the manufacturer, the question arises

(from a condition monitoring perspective) as to why this particular relay should be operating at

a higher temperature than others which are performing a similar function under an identical

duty cycle (24 hour energised). The particular relay in question was the holding relay forming

part of the emergency shut down circuit, for a fuel pump.


11/15



Figure 2 Overheated relay in a bank of several holding relays (always on when the

equipment is operational)

Figure 3 is an example of the applicability of thermography to mechanical machinery condition

monitoring. The main engine main lubricating oil pump was found to be operating with an

overheated bearing at the pump drive end. Vibration measurements taken at this bearing

revealed excessive levels of vibration (around 12 mm/sec RMS velocity). Over-lubrication is

one of the main causes of overheating in pump bearings. Bearing misalignment or ball

skidding within the bearing can also cause overheating. It can be seen in Figure 3 that there

are 2 hot spots on either side of the bearing, which may be indicative of misalignment. Spectral

analysis of the vibration signal could be used to confirm the same.

Figure 3 Overheated Pump Bearing


12/15



Defective steam traps often malfunction on board ships and go undetected for long periods.

Figure 4 is a thermal image of a correctly functioning steam trap. It can be clearly seen where

the live steam flow ends and the condensate release begins.

Figure 4 Steam trap in good order

Exhaust gas leaks from diesel engine is a common occurrence causing an unhealthy working

environment in the machinery space. Figure 5 illustrates how leaks become obvious in thermal

images although in this particular case, the major leaks were already visible to the naked eye by

the discoloration of paint on the trunking. However, minor leaks detected by thermography

were not visible to the naked eye.

Emissivity factor was set at 0.95 for all the examples cited above. The temperature

measurements on the exhaust pipe were relatively much lower than expected because of the

incorrect emissivity adjustment (it should have been around 0.30). Since the objective was onlyto obtain a relative colour map, it was of no significance in this case.


13/15


14/15



Figure 6 Product build up at the bottom of a pipe (left) and Overheated pulley bearing in

a belt drive (right)

Figure 7 Localised overheating in a 3-phase electric supply (left) and printed circuit board

fault diagnosis (right)

5. Conclusions

Thermal imaging cameras can be used to scan the infrared emissions from any surface and

produce thermal maps of the scanned area. The complete thermal image is a particularly useful

tool in detecting local overheating in electrical equipment caused by dirt, loose connections,short circuiting, unbalance in 3-phase power supplies and overheating due to harmonic

distortion of voltage and current. Thermal imaging can be used to monitor mechanical

machinery for detecting uneven heat distribution caused by faulty bearings, uneven friction

between belts and pulley and several other deviations from normal mechanical functioning of

machinery. It is a useful aid in monitoring leakage from exhaust and steam systems as well as


15/15



in ensuring insulation integrity of refrigerated spaces and furnaces and detecting blockages in

heat exchangers.

Used in conjunction with standard condition monitoring techniques such as vibration and used

lubricating oil analysis, thermographic images add an extra dimension of information to the

state of health of the engineering system on board. Used as a quick surveillance tool, it isinvaluable.

References

Ref. [1] J. THOMAS, R.A., Machine & systems condition monitoring series,Coxmoor, 1999

Ref. [2] J. FOWLER, M., Black body radiation,University of Virginia website, 2002

Ref. [3] J. ABB, 'Guide to harmonics with AC drives', Technical Guide No. 6, ABB Automation Group,2000

Ref. [4] J. EVANS, I.C., Harmonic mitigation for AC thruster & small propulsion drives', Productliterature, Harmonic Solutions Co., 2002

Acknowledgement

The author would like to express his thanks to John Carlton (Head of department, TID) forsupporting the research of thermography in condition monitoring and to Brian Sharman (Head

of electrical engineering) for discussions on the electrical aspects.

Author's Biography

Graduated from Marine engineering college, Calcutta. In 1978 started as junior

engineer in the Shipping Corporation of India. From 1982 till 1993 worked on

foreign flag vessels, the final 3 years as Chief engineer. During the years 1993-

96, acquired an MSc degree in Software Engineering from the University of

Sheffield and worked as Software systems manager for Anchor Marine

International, Liverpool. From 1997 to 1999 worked as a Research Associate at

the University of Sheffield (Computer Science Department) simultaneouslyacquiring a PhD degree in the subject of Artificial Neural Networks in engine

fault diagnosis. In 1999 joined LR as a surveyor. Promoted to senior surveyor

in 2001. Currently working on an internal LR project for setting up a condition

monitoring consultancy service for the marine industry.

lloydsregister thermography in condition assessment

Documents