Ultrasonics Examination: Part 3

The previous article dealt with the manual scanning method of ultrasonic examination stating that accurate determination of weld flaw size and position - to within ±2mm -  was difficult, if not impossible, in most circumstances. Methods developed now enable flaw sizes to be determined with accuracy better than ±1mm. This article will look at two of these techniques; time of flight diffraction (TOFD) and phased array ultrasonic testing (PAUT).

Conventional manually scanned ultrasonic testing normally uses a single fixed angle and frequency probe; the position and size of a flaw being determined by the amplitude of the signal reflected from the flaw and presented on an oscilloscope screen (Fig 3 in Job Knowledge 128). This is a somewhat unreliable method as the amplitude of the signal and therefore an estimate of its size depends on the orientation of the flaw. TOFD uses two angled compression wave probes mounted on a frame so that they are a fixed distance apart; one a transmitter, the other a receiver. The probes are positioned either side of a weld as shown in Fig 1. In a flaw-free weld two sound waves will be detected by the receiver – one that travels along the surface of the weld, the lateral wave, and one reflected from the back wall. When a flaw is present (for example a crack as shown in Fig 1) the pulse emitted by the transmitter is diffracted or scattered from the tip of a flaw and this diffracted signal is picked up by the receiver. The time of flight of the signal is measured and compared with that of the lateral wave, a simple calculation enables the position of the tip of the flaw to be determined. Moving the probes in a predetermined scanning pattern enables the other end of the flaw to be detected so the flaw size can be established in both the trough thickness and longitudinal directions. The calculations are performed automatically by the software program within the equipment and the flaws displayed as a black and white A scan image.

Figure 1: TOFD

Various scanning patterns may be used so that the results can be presented as A-scan, B-scan, looking along the weld length or D-scan, a side view. (For a description of A, B, C and D scans see Job Knowledge No. 127).

TOFD is regarded as the best method for the detection and sizing of planar, through-thickness flaws. One limitation is the detection of small surface breaking flaws on the scanning surface as these tend to be lost in the lateral wave response, although this may be not too significant as most surface breaking defects can be readily detected using MPI or liquid penetrant methods.

The rapid progress of electronics and computing power has enabled complex methods of scanning and data processing to be developed. This has culminated in phased array ultrasonic testing (PAUT) which, as the name suggests, uses an array of small transducers unlike the conventional manual A-scan probe with only one transducer. A single PAUT probe may contain between several tens and several hundreds of transducers. These small transducers are computer controlled and can be pulsed independently in a set sequence or phase; the pulses of sound interfering with each other to produce a sound beam of a certain angle, see Fig. 2. By varying the time and pattern of the pulses, the angle and shape of the beam can be varied so that the beam can be steered electronically, sectorial scanning or S-scan.

Figure 2: Illustration of the sector scan composed of many A-scans from the beams being steered through a range of angles. Note that, in addition to steering the beam, the focal law may also be focusing the sound field to improve defect detection and resolution

The benefits of this technology compared with conventional single transducer scanning are that the beam can be steered and focused with a single probe. Beam steering enables the beam to be swept through an object without moving the probe, the reflected data being processed to provide a visual image of a cone shaped slice through the object. Moving the probe enables a large number of slices to be assembled to provide a three dimensional image – a good example is the use in medical diagnostics to examine the functioning of the heart in real time.

For the non-destructive examination of welds this ability to inspect a weld with multiple angle beams from a single probe means that the probability of detecting flaws is greatly increased. It is also possible to focus the beam electronically at multiple depths to improve the ability to accurately determine the size and position of weld flaws.

Figure 3: Phased array results showing A-scan, B-scan and S-scan of a nozzle to shell weld

The small probe size and the ability to manipulate the beam without moving the probe enables inspections in limited access or of components of a complex shape. Cost is also a factor – although the probes and the processing/display units are more expensive than the single transducer equipment, the time to perform a scan can be substantially reduced. Work carried out by TWI suggests that a phased array scan can take 20% of the time for a conventional scan with better coverage although the off-line interpretation of the results may take longer.

The results may be presented as S-, A-, B- or C-scans, enabling better interpretation. The results of a phased array examination of a single sided nozzle to shell weld is given in Fig. 3. The weld shape is given by the red lines superimposed on the S-scan display. This is a single sided weld, the lower half of the image being a mirror of the weld. Whilst the scanning operation may be performed automatically by mechanised manipulating equipment and the accuracy may be better than ±1mm, the interpretation must be carried out by experienced and skilled personnel trained specifically in the interpretation of phased array scanning results. An investigation by TWI showed that the skill of the individual carrying out the interpretation was by far the most important factor in producing reproducible and accurate results. 

Scanning can be performed manually or with the probe attached to a carriage. A typical application is the examination of pipe butt welds using orbiting crawler tractors

Such dedicated and robust mechanised equipment is readily available for site use, replacing radiography and giving benefits in terms of cost, flaw detection and health and safety issues.

An excellent example of the capabilities of PAUT is given in the article ‘Measuring the crack growth rate (da/dt) of a fatigue crack using phased array ultrasonics’ by Channa Nageswaran, Principal Project Leader at TWI.

This article was written by Gene Mathers.

Ultrasonic Examination Part 2

The previous article (127) explained the basic principles of ultrasonic examination. As to determine accurately the size and position of a feature it is necessary, with any measuring equipment, to calibrate the ultrasonic examination system.

The type of calibration blocks (there are varying shapes and sizes to be used), depend on the application and the form and shape of the subject being tested. The calibration block should be made the same as the material being inspected and the artificially-induced flaw should closely resemble the actual flaw of concern. The best calibration block for calibrating ultrasonic testing equipment is one in the same grade of material and heat treatment condition as the production items and with a weld containing genuine flaws such as slag entrapment, porosity, lack of fusion, cracks etc. Techniques developed enable flaws of known sizes to be introduced into a welded joint. Such calibration blocks can be produced to validate the ultrasonic test method but are expensive and tend to be used only in applications such as nuclear vessel manufacture and critical offshore/process plant fabrication.

A number of standard calibration blocks are available with the shape and dimensions being specified in international standards such as ISO 2400, ISO 7963, ASME V and ASTM E164. Calibration of a compression wave probe used to measure thickness is simple and carried out using a stepped wedge calibration block. These calibration blocks have smooth, machined features and are not therefore truly representative of flaws in a welded component.

For calibrating equipment to be used to interrogate welded joints the calibration block needs to be more complex than a simple step wedge, with probably the two most common types illustrated in Fig. 1, the ISO 2400 Number 1 block and the ISO 7963 Number 2 block. These are machined from steel to very closely controlled tolerances and contain a number of features that can be used to calibrate the ultrasonic equipment. The standard Number 1 block is 300mm long and 25 or 50mm thickness with a 100mm radius machined on one end. The test block also contains two drilled holes, 50 and 1.5mm in diameter and a flat bottomed machined notch.

Fig 1: Number 1 and Number 2 calibration blocks

Smaller lighter blocks are useful for site use, and may be used to calibrate both compression and shear-wave probes for beam angle, time base, resolution and sensitivity. Sensitivity and resolution are terms frequently used – sensitivity is the ability to detect small flaws within the weld, resolution the ability to locate and separate individual flaws.

Weld discontinuity acceptance criteria are initially based on the height of the signal displayed on the oscilloscope screen. This is not as simple as it may appear since the ultrasonic beam is influenced by the microstructure of the metal through which it is propagating, becoming scattered and diffused - similar to car headlights in fog! As a general rule the larger the grain size the greater the scattering effect, the reflected beam becomes attenuated or decreased in strength the further away the reflector is from the ultrasonic probe. This must be taken into account when accepting or rejecting flaws within the weld – a 4MHz signal would lose some 0.02–0.03db per mm in steel. Fig 2 illustrates this decrease in amplitude or signal height with distance.

Before calibrating the operator must select the frequency of the transducer as this determines the wavelength of the sound. The frequency has a significant effect on the ability to detect a flaw – a rule of thumb is that a flaw must be larger than one half the wavelength to be readily detectable. 

The ultrasonic operator will select a calibration block with some feature of known dimensions, often a 3mm diameter flat bottomed hole and the appropriate ultrasonic probe, these generally being specified in the relevant application code or standard. The height of the reflection at known distances from the probe would be determined and from this data would be drawn a distance amplitude correction (DAC) curve by joining the tips of the signals that can be seen in Fig 2. This provides a means of establishing a ‘reference level sensitivity’ as a function of distance from the ultrasonic probe and allows the signals from similar reflectors to be evaluated.

The characteristics of an ultrasonic probe vary according to the size of the piezo-electric transducer and its frequency. It is therefore essential that each probe to be used to examine a welded component is individually calibrated and a DAC curve established for each different situation.

The contract specification, application code or acceptance standard specifies the relevant ultrasonic acceptance standard of height, length, position etc of the reflector. It is unwise to refer to a visual or radiographic acceptance standard in the absence of a relevant ultrasonic acceptance standard. An ultrasonic acceptance standard will state which reflectors are acceptable or unacceptable based on the amplitude of the signal compared with a DAC curve or other ultrasonic specific acceptance criteria. One such specification that refers to the DAC curve is ISO 11666 ‘NDT of welds – Ultrasonic testing – Acceptance levels’ which defines four levels:

• the reference level ie the amplitude of the DAC curve at the relevant distance
• the evaluation level ie the amplitude at which the reflector must be examined more closely to determine through thickness height and length of the discontinuity
• the recording level ie amplitude at which the size and position of the discontinuity must be recorded
• the acceptance level above which the discontinuity must be rejected – this may be above or below the DAC curve. Any reflector with a signal below the evaluation level would be ignored
  

Fig. 2 The reduction in amplitude with distance

If, as the ultrasonic testing (U/T) probe is scanned across the surface of the component, and the amplitude of the signal exceeds the specified evaluation level, the U/T operator would need to investigate the reflector in detail to determine its size, orientation and position within the component. If the probe is moved transverse and parallel to the weld and rotated slightly, a skilled and experienced operator can often identify the flaw type – crack, lack of fusion, etc – by observing the changes in the shape of the pulse-echo on the oscilloscope screen.

To enable the operator to identify the position of a flaw it must be possible for the path and width of the beam to be visualised. Accurately dimensioned diagrams of the weld-cross section superimposed on what would be the beam path are required. This may be unnecessary in many situations but provides additional confidence in critical applications and may be a mandatory part of a written U/T procedure.

The size of a reflector is generally determined by the ‘6db drop method’, as illustrated in Fig. 3.

The operator moves the probe backwards and forwards at right angles to the axis of the reflector until the maximum amplitude response is found. This point is noted and the scanning continued until the amplitude of the signal has dropped by 6dB, this point also being recorded. From this the length or height of the reflector can be determined (Fig 3). If above the recording level this would be recorded on the U/T report before being compared with the acceptance standard for either acceptance or rejection.

It is impossible to measure accurately the size of a reflector using a manual scanning technique for a number of reasons. The speed of the sound within the component may vary due to changes in the microstructure and the cleanliness of the parent metal; the probe will be made to within dimensional tolerances, as will the calibration block and these will affect the accuracy of calibration; the beam width may vary; the couplant and surface condition of the component will affect the coupling and hence sound transmission; the surface of flaws within the weld are generally not flat, smooth reflective surfaces oriented at 90 degrees to the beam; the probe movement is measured manually with a rule or tape measure. The most important factors in achieving accurate, consistent and reproducible results are the skill, competence and integrity of the operator.

The accuracy of conventional manually-scanned pulse-echo ultrasonic examination carefully performed by a competent operator is around ±2mm. Such inaccuracy can be important when carrying out a fitness for service analysis, where the through thickness of a flaw is of critical importance. Some methods of achieving greater accuracy will be dealt with in the next article.

This article was written by Gene Mathers.

Ultrasonic Examination Part 1

Ultrasonic examination uses the same principles as the sonar used for the detection of submarines – a sound wave is emitted from a transmitter, bounces off any objects in its path and is reflected back to a receiver, somewhat similar to shining the beam of a torch at a mirror. Knowing the speed of sound in the material enables the distance of an object to be determined by measuring the time that elapses between the transmission of the sound pulse and detection of the “echo”. In welded components the examination is generally performed by moving a small probe containing both a transmitter and receiver over the item and displaying the echo on an oscilloscope screen. This is shown in Fig. 1 which illustrates a simple pulse-echo angle probe examination.
The oscillator sends pulses of electricity to a piezo-electric crystal, the pulse generator, embedded in the ultrasonic probe which causes it to vibrate at a very high (ultrasonic) frequency, well above any audible frequency and typically between 1Mega Hertz(MHz) and 15MHz. Ultrasonic probes used for weld examination have frequencies generally between 2MHz and 5Mhz, the lower frequency probes being used for the examination of coarse grained material or on rough surfaces, the higher frequency probes for the detection of fine defects such as cracks or lack of fusion. The ultrasonic vibrations are transmitted into the material to be tested using a “couplant” such as grease, paste or water which helps transmission of the vibrations. The better the surface finish then the better is the coupling and the more searching the examination – hence there is sometimes a requirement to grind smooth the weld cap and remove the root penetration bead on welded joints.
Once in the material the vibrations travel in a predictable path as a beam of sound pulses until they encounter an obstruction or interface such as a line of slag, porosity or a crack when most of the sound will be reflected - remember the analogy of the torch and mirror. Depending on the angle at which the beam strikes the obstruction some or all of the sound beam will be reflected back to the receiver in the probe. Here it vibrates a piezo-electric crystal; the electric signal that is generated is amplified, rectified and displayed on an oscilloscope screen.
The sound beam when it enters the object being scanned has a cross section approximately that of the transmitter but, like the beam of a torch, will diverge as shown in Fig. 1. As the beam travels through the material it also loses energy – it becomes attenuated. These effects need to be taken into account when the position and size of a defect is to be accurately determined.

Fig 1. Schematic of Angle Probe Ultrasonic examination.

The oscilloscope screen in Fig. 1 shows on the vertical axis the signal height or amplitude and on the horizontal axis the time taken for the signal to return to the receiver and therefore distance from the transmitter. This method of examination is known as an “A scan” and is the most common method in use in industry for the examination of welded joints. In Fig 1 three signal peaks can be seen on the oscilloscope screen – one where the signal enters the sample, one reflected from the back face of the sample - the “back wall echo” - and, between the two, a reflection from some feature – a “reflector” such as a welding defect. The distance of this signal on the screen from the transmission pulse will give the distance of the reflector from the probe so a little simple geometry can be used to calculate the position and depth of the reflector within the block of material. Comparing the height of the signal with the signal from a known size of reflector enables the size of the feature to be determined.
There are two main types or modes of sound waves – longitudinal or compression waves which alternately compress and decompress the material in the direction of propagation and shear waves which vibrate the material at right angles to the direction of propagation. Which mode is produced depends upon the angle at which the sound beam enters the material. Probes that project the beam into the test piece at an angle normal (90degs) to the plate surface are known as compression probes and are ideally suited to the detection of defects such as plate laminations or for the measurement of plate/pipe thickness as shown in Fig 2.

Fig. 2. Compression Wave Probe

To obtain the strongest reflected signal the beam should ideally strike the feature at 90O – flaws that lie parallel to the beam may be missed. This means that to examine a weld that may contain flaws laying in any number of orientations within the weld a range of different angle probes and scanning patterns must be used. To do this both compression and shear wave probes may be used, shear wave probes projecting the beam into the test piece at an angle, as shown in Fig. 1. Probes with angles of 30^o, 45^o, 60^o and 70^o are commercially available. Examples of standard probes are illustrated in Fig. 3.The angle of the probe is often selected to give the strongest signal from the defect of interest and for very high integrity welds all five probe angles may be used.

Fig 3. A 2.5MHz 70 degrees shear wave probe and a compression wave probe.

As shown in Fig. 4 the sound beam can be made to scan the full depth of a weld by moving the probe back and forth. At the half skip distance the beam would readily detect lack of side wall fusion along the left hand fusion line but may miss lack of side wall on the right hand fusion line. Moving the probe to the full skip distance so that the beam reflects off the back face enables the right hand fusion line to be scanned with the beam at the optimum angle to detect lack of side wall fusion.

Fig 4. Schematic of “skip distances”

To examine completely the weld there needs to be a number of such scanning patterns longitudinal and transverse to the weld, from both sides of the weld, from both plate surfaces and from half to full skip distance.  If all of these scans are carried out using all five probe types and two frequencies then it becomes a very lengthy and costly exercise! Such detailed examinations tend to be confined to items such as primary circuit nuclear components and highly critical offshore applications.
Whilst many ultrasonic examinations are carried out with a manual operative moving the probe, viewing the results on the oscilloscope screen and manually recording the results the process can be mechanised with the probes mounted on a carriage and the results recorded electronically. This has become more prevalent as computing power has increased since the carriage may carry several probes and provides information on the carriage position and orientation. This data is then analysed and compared with an acceptance standard, enabling a weld to be sentenced automatically .
There are a number of advantages to ultrasonic testing:-

1. It is very good – and better than radiography - for the detection of planar defects such a lack of fusion and cracks
2. It can determine both the depth and position of defects.
3. It is readily portable and easy to use on site and in areas of restricted access.
4. Access is required to one side only.
5. There are none of the health and safety problems associated with radiography.
6. The result is immediately available.

But, as with any industrial process there are some disadvantages:-

1. Very skilled and conscientious operatives are required .
2. The manual examination process is slow, laborious and tiring for the operative.
3. Surface breaking defects are difficult to detect.
4. Accurate sizing of small (<3mm) defects is difficult if not impossible.
5. The root region in a single sided full penetration weld is difficult to interpret.
6. The geometry of the joint can restrict the scanning pattern and impede accurate interpretation.
7. Interpretation is subjective and depends upon the operative’s skill and experience.
8. With manual scanning no permanent and objective record is produced.

The A-scan mentioned above is one method for reporting the results of the scan – there are in fact four methods identified as A-, B-, C- and D-scan. The A-scan method is the conventional way of presenting the results – signal amplitude vs distance; B-scan is a view looking along the length of the weld; C-scan is a plan view and D-scan a view from the side of the weld. These are illustrated in Fig.5

Fig 5 Schematic of A-, B-, C- and D- scan results

This article was written by Gene Mathers.

About Robotic arc welding

Robotic arc welding

Introduction

The development of automated arc welding solutions continues to be driven by the requirement for higher product quality, productivity and reduced costs. In addition, good manufacturing system flexibility, which is essential for responding to the dynamic behaviours of the market and therefore keeping products competitive, has become a key development target for the manufacturing industries. As a result, robotic welding processes offer attractive alternative solutions to traditional manual operation and hard automation.
Since the first application of a welding robot in industrial production in the early 1960s, robotic welding has expanded across a range of manufacturing industrial sectors. Between 2010 and 2013 alone, over 600,000 industrial robots were commissioned globally [1]. Robotic welding has been recognised as the most popular industrial application of robotics worldwide [2]. It is estimated that approximately 25% of all in-service industrial robots are employed for welding operations [3]. Automotive manufacturing (Figure 1) represents the most active industry sector in terms of robotic welding adoption (approximately 40% of total global robot supply), followed by the electrical and electronics industry (approximately 20% of total global robot supply). Apart from resistance spot welding, the two most common robotised welding processes for production purposes are metal inert gas (MIG) welding and tungsten inert gas (TIG) welding respectively.

[ Zoom ]

Figure 1 Robotic welding used in automotive production (photo copyright @ ABB)

Basics of robotic arc welding

A basic robotic arc welding system is formed by two subsystems: the welding equipment delivering the energy from the welding power source to the workpiece, and the robot providing relative positioning of the heat source and the workpiece. Normally six-axis industrial robots comprising a three-axis lower arm and a three-axis wrist are used, since they enable the welding torch mounted at the wrist to achieve all the positions necessary for three-dimensional welding. Traditionally, general purpose industrial robots are employed, carrying arc welding torches as end effectors. Many robot manufacturers have recently developed arc welding-specific robots, which are smaller and less expensive. This reduction of the required capital investment has further increased the sale of robotic welding systems. Another recent development in welding robotics has been the introduction of seven-axis robots, which feature an additional axis in the lower arm providing additional flexibility and saving floor space (Figure 2). The major characteristics of industrial robots are summarised in Table 1.

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Figure 2 Seven-axis welding robot (photo copyright @ Motoman)

Table 1 Typical characteristics of an industrial arc welding robot

Payload	2 to 30kg
Axes	Six to seven
Velocity	Up to 5m/s
Acceleration	Up to 25m/s2
Repeatability	≥0.05 mm
Communications	Profibus, DeviceNet, CANopen, Ethernet/IP and serial channels
IO capabilities	Digital/analogue IOs

In a production environment, workpiece manipulators are frequently implemented as part of the robotised system. The devices extend not only a welding robot’s working range but also its accessibility, especially when welding complex and large geometries. The control of a manipulator is often integrated with that of the robot, which enables the synchronised and simultaneous control of the two mechanisms. The integrated control allows coordinating motion between the robot and manipulator (workpiece) which maintains the optimum welding positions (higher deposition rate and quality) and possibly increases the welding speed (higher productivity). Figure 3 shows a six-axis industrial robot integrated with a cold metal transfer (CMT) arc welding system and a two-axis workpiece positioner at TWI Cambridge.

[ Zoom ]

Figure 3 Robotic CMT welding cell at TWI (photo copyright @ TWI)

Process sensing

Robotic welding is a challenging combination of welding, robotics, sensor technology, control systems and artificial intelligence. Driven by the increasing demands of improved quality, productivity and flexibility, precise and adaptive control of the robotic welding processes has become a crucial target for the development of modern systems. Sensing technologies designed for welding and its automation are the essential elements for enabling this desired level of control. The sensors are applied to observe and measure process parameters, acting as the sources of input to the control system. By acquiring and analysing the input information from the sensors, the control system adapts output of the robotised welding process in accordance with the defined welding procedure specifications.
There are various types of sensors available for robotic arc welding applications. Depending on their functions, the sensors are classified into two categories: process and geometrical. The former measure the process parameters of the robotic welding process (eg arc voltage, current, wire feed speed and torch rotation), which determine the stability of the process. The latter measure the weld joint geometry (eg gap sizes, weld size changes, deviation from the nominal path and orientation changes) and are used for weld searching, seam tracking and real-time adaptive welding.
Table 2 Typical sensors used in robotic arc welding

Function	Sensor
Welding current measurement	Hall effect sensor
	Current shunt
Arc length control	Voltage sensor
Distance control	Capacitive sensor
Weld edge searching	Tactile (electrical contact) sensor
	Proximity sensor
Weld seam tracking	Tactile (mechanical contact) probe
	Eddy current sensor
	Through-arc sensing (weaving with electric measurement)
	Vision sensor
	Laser scanning (Figure 4)
Weld penetration monitoring	Infrared radiation sensor
	Ultrasonic sensor
Weld pool monitoring	Vision sensor
	Thermal imaging
Weld quality inspection	Eddy current sensor
	Ultrasonic sensor
	Laser scanning

[ Zoom ]

Figure 4 Laser scanning used in robotic arc welding (photo copyright @ Servo-Robot)

Key issues

The benefits of implementing robotic arc welding are evident but there are some issues associated with it which should not be ignored. Robotised technology is a good solution to fill the burgeoning skills gap in welding fabrication industry, but using and programming the industrial robots is still a complex and difficult task for regular operators. Despite the fact that modern sensing techniques for robotic arc welding are readily available and reasonably reliable, it is still challenging to effectively and efficiently apply them in some applications. The high temperature, intense light from arc, fume, high current, molten metal, spatter, and other factors involved in arc welding can interfere with the sensors. Developing a control system which can fully utilise the information obtained by the sensors and effectively translate it to the fabrication is still a difficult task.

Summary

Robotic arc welding is an essential component of today’s manufacturing plants. The primary benefit of robotic arc welding is the production of high-quality welds in a shorter cycle time, with manufacturing flexibility another major advantage. Through extensive application in many manufacturing industry sectors, robotic welding has been developed to a mature production method. Strong industrial need continues to drive the rapid development of robotic arc welding and associated technologies to overcome technical difficulties and expand their capability.
TWI has established good experience in robotic welding through its services in generic research, contract R&D, technical information, consultancy, standards drafting, training and qualification. For more information, please contact us.

References

1. International Federation of Robotics, 2014, World Industrial Robot Statistics 2014, http://www.ifr.org/industrial-robots/statistics/.
2. United Nations and International Federation of Robotics, 2000, World Industrial Robotics 1996: Statistics and Forecast, New York: ONU/IFR.
3.  J N Pires, A Loureiro and G Bölmsjo, 2006, Welding Robots: Technology, System Issues and Applications, London: Springer-Verlag.

TWI Group websites

A review of the application of weld symbols on drawings

A review of the application of weld symbols on drawings - Part 2

Part 1 of this article which appeared in the May/June issue of Connect, dealt with the most basic weld symbols as they appear on engineering drawings. As previously mentioned, it is essential that all concerned in any project are aware of which Standard is being applied.

Weld sizing

In order that the correct size of weld can be applied, it is common to find numbers to either the left or to the right of the symbol.
For fillet welds, numbers to the left of the symbol indicate the design throat thickness, leg length, or both design throat thickness and leg length requirements. Figure 1 gives examples of symbols used in different Standards.

Fig.1

For fillet welds:
Superseded BS499 Pt 2 gives

a = design throat thickness
b = leg length

ISO 2553/EN 22553 requirements

a = design throat thickness
z = leg length
s = penetration throat thickness

For butt joints and welds, an S with a number to the left of a symbol refers to the depth of penetration as shown in Fig.2.

Fig.2

When there are no specific dimensional requirements specified for butt welds on a drawing using weld symbols, it would normally be assumed that the requirement is for a full penetration butt weld ( Fig.3).

Fig.3

Numbers to the right of a symbol or symbols relate to the longitudinal dimension of welds, eg for fillets, the number of welds, weld length and weld spacing for non-continuous welds, as Fig.4.

Fig.4

On fillet welded joints made from both sides, a staggered weld can be shown by placing a 'Z' through the reference line ( Fig.5).

Fig.5

Supplementary symbols

Weld symbols indicate the type of preparation to use or the weld type. However, there may still be occasions where other information is required. The basic information can therefore be added to in order to provide further details as shown in Figs.6, 7 and 8.

Fig.6

Fig.7

Fig.8

Weld all round

For a Rectangular Hollow Section (RHS) welded to a plate, for example:

Weld in the field or on site

The box attached to the arrow can be used to contain, or point to, other information.

Welding process type

ISO 4063 gives welding processes specific reference numbers. As shown in Fig.9 the appropriate process number is placed in the tail of the arrow. Other processes are given a unique number. In this example, 135 refers to MAG welding.

Fig.9

There are a number of additional symbols given in the Standards ( eg ISO 22553) which refer to additional welding or joint requirements. Figure 10 shows the requirement for a sealing run.

Fig.10

Compound joints/welds

A compound weld could be a 'T' butt weld which requires fillet welds to be added to increase the throat thickness as shown in Fig.11.

Fig.11

The broken reference line

Fig.12

The main feature that distinguishes weld symbol standards is that for ISO 2553 and BS EN 22553, there is an additional feature of a broken reference line.
This method is used when a weldment or weld preparation needs to be specified on the 'other side' of the arrow as shown in Fig.12.
Any symbol that is used to show a joint or weld type feature on the other side of the arrow line is always placed on a dotted line.
BS 499 and AWS require symbols to be placed above the reference line (which indicate the other side) or below the reference line (indicating the arrow side).

Summary

Weld symbols are a very useful way of communicating welding requirements from the design office to the shop floor.
It is essential that the 'rules' of the standard used are correctly applied by drawing office personnel. However, it is also important that shop floor personnel are able to read and understand the details of weld symbols.
Much of this requirement can be met by reference to the standard being used within the organisation and by the drawing office personnel considering the needs of the end user such as the welders, welding supervisors, welding inspection personnel and welding engineers in order to minimise costly mistakes due to misinterpretation.
Training of all personnel in the correct use of weld symbol specifications also plays an important role in ensuring that weld symbols are both correctly applied and correctly read.
This article was written by Mark Cozens of Weld-Class Solutions.

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An intro to infrared thermography for mechanical applications

Infrared thermography is the process of using a thermal imager to detect infrared radiation (heat) that is emitted by an object. The technology allows operators to validate normal operations and, more importantly, locate thermal anomalies (abnormal patterns of heat invisible to the eye) which indicate possible faults, defects or inefficiencies within a system or machine asset.
A few examples of where infrared is applied in industrial situations include:

• Electrical Systems (faulty electrical connections or overloaded circuits)
• Mechanical Equipment (abnormally warm motors or possible bearing failures)
• Fluid Systems (line blockages, tank levels or pipe temperatures)
• Building Applications (detect missing insulation, air infiltration or moisture damage)
• Low-Slope Roofs (locate wet roof insulation)

While infrared imaging systems are relatively simple to use, interpreting images taken with this equipment can be far more complex and challenging. With training and experience, however, remarkable information can be extracted from an infrared camera that can help asset managers and maintenance personnel alike better maintain systems and equipment while monitoring or improving the quality of products.
When it comes to inspecting mechanical equipment with infrared, certain conditions are needed. It is also important for the thermographer to have a strong foundation of basic radiometry and heat transfer, knowledge of how the mechanical equipment operates, as well as a solid background on the infrared camera’s capabilities and limitations. As with any type of technology, proper training is essential to successfully operate these systems.

Figure 1: The causes of an abnormally warm motor bearing are not always obvious but infrared is a key means of finding those which are running hot.

Inspecting mechanical equipment with infrared thermography covers a wide variety of systems, everything from motors, rotating equipment, steam traps, refractory, and tank levels and more. Most of these inspections de-emphasize taking absolute temperature measurements and instead concentrate on comparing overall thermal patterns to understand the asset’s health.
Baseline inspections, where a thermographer captures an overall “thermal map” of a particular equipment type, is one of the more valuable uses of the technology, as subsequent inspections are then compared to the original “map” to detail any changes that may have occurred over time. The examples in this article represent only a few of the many areas where IR is utilized for inspecting mechanical equipment in an industrial facility.
While there are many ways to monitor the electrical and mechanical conditions of a motor, thermal imaging has proved itself to be useful mainly as a quick and efficient screening tool. We know that heat is created in any number of ways – by excessive friction, high electrical resistance, reduction of cooling air or fluids and problems with current flow in a motor. If an issue is detected, in the form of an abnormal thermal signature seen in the infrared image – an additional test method such as vibration analysis, airborne ultrasonics or motor circuit analysis – is typically requested to help pinpoint exactly what is causing an anomaly.

Figure 2: Couplings often exhibit strong thermal signatures when there is misalignment or related problems.

Abnormal heating, such as what might occur in a misaligned coupling, could precede a detectable vibration signature. Left alone, such an issue compounds itself as it continues to get worse or even wear on the motor bearing itself. That in turn can also typically be detected with thermography. A variety of issues that are related to the motor’s electrical system may also have thermal patterns associated with them. An internal fault because of the breakdown of insulation on the motor windings can increase the overall temperature of the motor including the motor housing. Often, when the cooling air flow pathways become blocked in a motor, the winding insulation also fails.

Figure 3: Infrared thermography is often used to locate suspected problems in bearings which can then be confirmed with other technologies.

Inspecting bearings is another useful application for infrared thermography. Abnormal friction within a bearing, generating heat, causes the bearing’s surface temperature to rise. This thermal signature, when detected, is an indication of a potential bearing problem, anything from under-lubrication, over-lubrication, poor maintenance or simply a bad bearing. Infrared is especially helpful for inspecting low-speed equipment, including overhead conveyors or idlers, quickly and easily.
Thermography is also used to monitor temperatures on high-temperature refractory insulation such as a rotating kiln, furnace or boiler. Thermal patterns detected in these cases can be used to monitor the condition of the insulation or even calculate the thickness of the remaining refractory. This type of monitoring is invaluable as it facilitates planned maintenance outages and repair of a problem area vs. a catastrophic, and expensive, unplanned failure.

Figure 4: Thermography is a perfect tool for inspecting boiler refractory and air leakage issues. The location of failing refractory can be trended and, when alarm temperatures are determined, repairs can be scheduled.

With the cost of energy increasing, steam traps are another application where infrared thermography can provide valuable savings for a facility. This is accomplished by detecting failures of traps that are stuck open. While airborne ultrasonics is considered a primary test method for this application, infrared inspections can be completed quickly and easily, even at some distance from the device. A trap that is functioning properly will typically have a large temperature difference between the steam and condensate sides. This temperature difference will temporarily disappear as it cycles. If both sides of the trap are warm it is an indication that the trap has failed open, a condition that can cost literally thousands of dollars in a year.
Both insulation and heat-tracing on process lines can also be inspected with infrared. Applications such as detecting line blockages, monitoring line temperatures, locating damaged or missing insulation can be accomplished with infrared with a few exceptions. Lines covered with very high levels of insulation are more problematic as are process lines that are wrapped with shiny aluminum or stainless steel sheet metal coverings. Inspecting in these situations can be very difficult due to the high thermal reflectivity/low emissivity of the surfaces encountered.

Figure 5: It is possible to locate levels in tanks and silos when they are in thermal transition, providing key information or confirming existing data.

Finally, one of the easier mechanical applications of thermography is to locate levels of gases, liquids, fluidized solids and even sludge in tanks, vessels and silos. Although most storage tanks have a gauge to detect levels, these sensors often fail or must be independently confirmed due to the critical nature of a process.
Similar to inspecting line blockages, the different thermal capacities of solids, liquids, and gases and the way heat moves in and out of them mean these materials change temperature at different rates during a transient heat flow cycle. Gases, having the lowest thermal capacity of them all, typically change most quickly. Solids, with varying capacitances do not change as rapidly because slower, conductive, heat flow controls the temperature next to the tank wall. Floating materials, including waxes, can usually be distinguished from the liquid due to capacitance differences. Liquids typically take the longest to change because of their high thermal capacitance.
In summary, mechanical infrared inspections can be challenging and rewarding both financially and professionally. The person performing the inspection must have an adequate working knowledge of the equipment they are inspecting as well as sufficient heat transfer theory training to interpret the thermal patterns they observe. As with any predictive maintenance technology, thermography is only one tool in the technician’s toolbox. All the applicable predictive tools should be utilized to give the best possible picture of an asset’s health.
CREDIT:About the authors:
Harold Van De Ven is an instructor and consultant and John Snell is a principal for The Snell Group. For more information on infrared thermography, visit www.thesnellgroup.com.

Detecting Fluid and Sludge Levels in Tanks using INFRARED THERMOGRAPHY

Detecting Fluid and Sludge Levels in Tanks

Infrared Thermography can be used to verify tank levels and determine any sludge build-up. This can generally be done at a distance with makes it a safer than manually checking levels.

IR of tank levels

Visual of tanks

Sludge in tank bottom

Tank visual

Tanks in inverted grey palette

Tanks in inverted grey

Railroad tank car

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Phone: (832) 465-1190
Email: info@advanced-ir.com

Infrared Thermography Test for High Temperature Pressure Pipe cont.................

3.2 Outside Cooling After Steam Heating After pipes heated a short time (from 30 seconds to 5 minutes), the stationary heat flow condition will be constructed. Temperature differences between areas of different wall thickness within a pipe are nonexistent. In order to get significant thermal contrasts for the defects, outside cooling should be performed to break the stationary heat flow condition. The outside cooling are tried by use of ice and refrigeration gas. The minimum detectable defect is a inside hole with diameter 6mm and depth 1mm for F114´4mm stainless steel pipe and a inside hole with diameter 10mm and depth 2mm for F140´5mm carbon steel pipe. Figure 8 shows the instantaneous thermal images of the defects for these two pipes after ice cooling. Fig 8: Thermal images of defects after steam heating and ice cooling. 4. ANALYSIS FOR THE TESTING RESULTS Through viewing the whole infrared thermal imaging process and analyzing the testing data listed in table 3, a serious of phenomena and laws are found as follows: Comparing the minimum detectable defects of F114´4 stainless steel and F140´5 carbon steel, it can be found that the testing sensitivity of stainless steel is much higher than carbon steel. For a F10mm opening hole defect, the duration time is about 2 minutes from emerge to disappear of the defect on thermal image for stainless steel pipe, but it is just 30 seconds for carbon steel pipe. The reason is that the thermal conductivity of stainless steel is 15W/(m.°C) and carbon steel is 48W/(m.°C). That is, the thermal conductivity of materials is a key factor affected the sensitivity of infrared thermography test. The lower the thermal conductivity is, the higher the sensitivity is and the longer the duration of defects emerging is. For defects of different diameters in the same pipe, the detectable depths of the defects are different. This phenomena show that the size of defects is another key factor affected the sensitivity of infrared thermography test. The larger the area of defect is, the higher the sensitivity of wall loss is. Comparing the testing results of 3 type carbon steel pipes with different thickness, it was found that the thinner the wall is, the smaller the size of detectable defects is. But the thicker the wall is, the longer the duration of defects emerging is. For a F10mm opening hole defect, the duration time is about 30 seconds for F140´5 pipe, 120 seconds for F168´16 pipe, and more than 200 seconds for F180´36 pipe. Thus it can seen that the thickness is also a key factor affected the sensitivity of infrared thermography test. The thicker the material is, the lower the testing sensitivity is, but the longer the duration of defects emerging is. Comparing the testing results of inside heating and outside cooling, it was found that the detecting sensitivity of inside heating is higher than outside cooling. The detecting sensitivity of refrigeration gas cooling is higher than ice cooling. Assuming that the operation pressures of the 4 type pipes in table 2 all are 3 MPa, fracture mechanics calculation shows that a 10mm diameter and 80% wall loss defect is safe for all the 4 pipes. That is, the sensitivity of infrared thermography test is much more over the accepted defects for safe operation of the pipes. 5. CONCLUSION The testing results show that infrared thermography test is a reliable non-destructive method for detecting wall loss defects produced by corrosion and flow erosion of high temperature pipe. The sensitivity of infrared thermography test is much more over the accepted defects for safe operation of the pipes. The thermal conductivity of materials is a key factor affected the sensitivity of infrared thermography test. The lower the thermal conductivity is, the higher the sensitivity is and the longer the duration of defects emerging is. The shape and size of defects is another key factor affected the sensitivity of infrared thermography test. The larger the area of defect is, the higher the sensitivity of wall loss is. The thickness is also a key factor affected the sensitivity of infrared thermography test. The thicker the material is, the lower the testing sensitivity is, but the longer the duration of defects emerging is. The excitation method breaking temperature balance is one of factors affected the sensitivity of infrared thermography test. The detecting sensitivity of inside heating is higher than outside cooling. The detecting sensitivity of refrigeration gas cooling is higher than ice cooling. The outside cooling method is suitable for on-line infrared thermography test for high temperature has very large potential in future. REFERENCE Edited by Xavier Maldague, Nondestructive Testing Monographs and Tracts Volume 7, Infrared Methodology and Technology, Gordon and Breach Science Publishers, 1994. Roderic K. Stanley, Patric O. Moore and Paul Mclntire eds, Nondestructive Testing Handbook, Volume 9, Special Nondestructive Testing Methods, American Society for Nondestructive Testing, 1995. T.-M. Liou, C.-C. Chen, T.-W.Tsai, Heat transfer and fluid flow in a square duct with 12 different shaped vortex generators, Transactions of the ASME. Journal of Heat Transfer vol.122, no.2, p.327-35, May 2000. Richard N. Wurzbach, David A. Seith, Infrared Monitoring of Power Plant Effluents and Heat Sinks to Optimize Plant Efficiency, Proceedings of SPIE - The International Society for Optical Engineering Thermosense XXII April 24-27, 2000, Orlando, FL, USA. R. Rozenblit, M. Simkhis, et al, Heat Transfer in Horizontal Solid-liquid Pipe Flow, International Journal of Multiphase Flow, Volume 26, No.8, 2000. Shigemichi Yamawaki, Toyoaki Yoshida, et al, Fundmental Heat Transfer Experiments of Heat Pipes for Turbine Cooling, American Society of Mechanical Engineers (Paper) Proceedings of the International Gas Turbine & Aeroengine Congress & Exposition June 2-5, 1997, Orlando, FL, USA. Yuwen Qin, Naikeng Bao, Thermographic Nondestructive Testing Technique for Delaminated Defects in Composite Structure, Proceedings of SPIE - The International Society for Optical Engineering Thermosense XVII, 1995, Orlando, FL, USA. Kenneth R. Maser, Mehdi S. Zarghamee, Proceedings of the Speciality Conference on Infrastructure Condition Assessment, August 25-27, 1997, Boston, MA, USA. Xavier Maldague, Pipe Inspection by Infrared Thermography, Materials Evaluation, Volume 57, No.9, 1999. © AINDT , created by NDT.net

Infrared Thermography Test for High Temperature Pressure Pipe

1. INTRODUCTION

High temperature pressure pipelines are extensively used in petrochemical plant and power station. The leakage and explosion accidents often take placed after a certain period of time because of media corrosion, cavitation erosion, welding defects cracking, stress corrosion cracking and materials deterioration. According to the statistic of large number of pipe leakage accidents, media corrosion and cavitation erosion is the main reasons lead to leakage and explosion accidents. The ratio is larger than 50% of the total accidents. In order to insure the safety operation of the pipes, it is necessary to search for some good NDT methods to assess wall thickness and then replace only critically damaged pipes. X-rays (or g-rays), ultrasonics, and infrared thermography are useful for such purposes. Due to infrared thermography test possess the advantages of being noncontact, fast, harmless, useful for either the reflective or transmissive method, and easy to deloy, it has the potential to test the pits and wall loss defects on-line for high temperature pressure pipes.
The discontinuities in solid materials can change the heat flow condition. The change of the heat flow condition can result in the fluctuation of the temperature on the surface of the materials. Both infrared testing and thermal image testing use this principle to measure the change of the surface temperature and then to deduce the discontinuity condition in the materials.
In the aspect of non-destructive test for industrial equipment, infrared thermography test is applied to operation condition monitoring of electrical equipment, power plant machinery and high temperature equipment [1-3]. In the aspect of non-destructive test for pressure pipe, just a few papers were found by use of international literature retrieve from INSPEC, EI, ISMEC, METADEX and etc. Almost of these papers investigated the insulation condition and heat loss for high temperature pressure pipes [4], heat transfer for heat exchangers [5-7], defects detect for composite pipes [4] and leakage test for underground concrete pipes [8]. Due to metals have very high thermal conductivity, few people try to test the wall loss defects for steel pipes by use of infrared thermography. It was just found that Maldague performed the infrared thermography test for a small pipe in laboratory [1, 9]. Two wall thinning defects on an elbow were detected. It is not found that systemic investigation was performed for different type of pipes with different wall thickness. We also can not find the sensitivity of the infrared thermography test for steel pipes with different wall thickness.
In order to get solution for such problems, a big pipe testing installation was constructed. A serious of infrared thermography testing experiments during heating and cooling were performed for 4 kinds of stainless steel and carbon steel pipes which are drilled different size holes on inner-surface.

2. TESTING EQUIPMENT AND INSTALLATION

2.1 TVS-2100 Thermal Video System
All the experiments were performed by the TVS-2100 Thermal Video System. The infrared camera head of this system is optically mechanical scanning type. The detector is InSb with 10X10 cell arrays. The detecting wavelength is 3~5.4mm. The operating ranges of temperature is -40^°C~950^°C. The minimum detecting temperature difference is 0.1^°C. at 30^°C and the sensitivity is 0.01^°C. The field of view is 10^°(V)´15^°(H). The field resolution is 2.2mrad. Table 1 lists the minimum resolving size for different view distance. The detection distances are from 20cm to infinity. The scanning speed is 30 frames per second. One floppy disc drive can save one frame per 10 seconds. There is an analogue video output, all the testing image can be saved in the tape recorder.


View distance (m)	0.2	0.4	0.6	0.8	1.0	1.2	1.4	1.6	1.8	2.0	3.0	4.0	5.0	10
Minimum size (mm)	0.44	0.88	1.32	1.76	2.20	2.64	3.08	3.52	3.96	4.4	6.6	8.8	11.0	22.0
Table 1: Minimum resolving size for different view distance of TVS-2100 thermal video system.

2.2 Pressure Pipe Testing Installation
Figure 1 is the pressure pipe testing installation constructed by 5 different type one meter length steel pipes and linking with a 150^°C steam pipeline. The 150^°C steam and normal temperature water can go into the pipe testing installation according to the testing requirement.


Fig 1: Pressure Pipe Testing Installation for Infrared Thermography Test.

2.3 The Preparation of Inside Defects of Pipes
First, 1 meters length one stainless steel pipe and three carbon steel pipes were vertically cut into 3 parts. Then, a serious of different diameter and different depth holes were drilled. Finally, the three parts were welded into a pipe again. Table 2 lists the size of all defects in the 4 type of steel pipes.


Type and Specification (mm)	Inside defects quantity and size (mm)
F114×4 stainless steel	24 defects: Diameter: (F2, F4, F6, F10)X Depth: (15%, 25%, 35%, 50%, 65%, 75%)
F140×5 20 Carbon Steel	24 defects: Diameter: (F2, F4, F6, F10)X Depth: (20%, 40%, 50%, 60%, 70%, 80%)
F168×16 20 Carbon Steel	12 defects: Diameter: (F4, F8, F12)X Depth: (10%, 20%, 40%, 60%) 12 defects: Diameter: (F5, F10, F15)X Depth: (20%, 40%, 60%, 80%)
F180×36 20 Carbon Steel	12 defects: Diameter: ( F4, F8, F12)X Depth: (10%, 20%, 40%, 60%) 12 defects: Diameter: (F5, F10, F15)X Depth: (20%, 40%, 60, 80%)
Table 2: The size of defects in 4 type of steel pipes.

3. INFRARED THERMOGRAPHY TEST FOR INSIDE DEFECTS OF PIPES

3.1 Inside Steam Heating
The pipe was passed through 150°C steam to heat from normal temperature. At the same time, the thermal image of the defects was looked and recorded in the floppy disc and tape recorder. The typical thermal images of inside defects for 4 type of pipes are shown from figure 4 to figure 7 respectively. Table 3 lists the minimum defects tested by infrared thermography for 4 type of pipes.

Type and Specification (mm)	The size of minimum defects tested (mm)
F114´4 stainless steel	F10´15%, F6´25%, F4´50%, F2´65%
F140´5 20 Carbon Steel	F10´20%, F6´40%
F168´16 20 Carbon Steel	F10´20%, F8´20%, F5´40%
F180´36 20 Carbon Steel	F12´20%, F10´40%, F8´60%
Table 3: The minimum defects tested by infrared thermography for 4 type of pipes.

Fig 4: F114×1000×4 stainless steel Pipe.

Fig 5: F140×1000×520 Carbon Steel Pipe.

Fig 6: F168×16 20 Carbon Steel Pipe.

Fig 7: F180×36 2020 Carbon Steel Pipe.