Presentation Report

On

Tubular Non Destructive Techniques

Amity School of Engineering & Technology

Submitted by: Submitted to:

Abhishek Thakur Dr Ravi Prakash

Akshay Mistri HOI, ASET

Sarthak Anand

CONTENTS

Acknowledgement………………………………………………………………...01

Abstract……………………………………………………………………………02

Introduction

Remote field testing (RFT).………………………………..03

Internal rotary inspection system (IRIS)…………………...03

Theory of Operation & Principle

RFT………………………………………………………....04-06

Signal Interpretation (RFT)………………………………...06-07

IRIS………………………………………………………...07-08

Result Interpretation (IRIS)………………………………...08-09

Benefits and Limitations

RFT…………………………………………………………10

IRIS…………………………………………………………10

References………………………………………………………………………….11

ACKNOWLEDGEMENT

I wish to express my sincere gratitude to Amity University and Amity School of Engineering

and Technology for giving me this great opportunity of learning. I truly admire this opportunity

to get in depth knowledge of the Non-Destructive Techniques subject. It helped me a lot. I truly

thank Dr Ravi Prakash, my faculty guide, who was more than a teacher. He provided his

valuable guidance and support. He is a great help and support throughout and a constant

guidance. He provided great guidance and knowledge and corrected my mistakes every time I

made one. Finally I would like to thank my friends for their understanding and support. Without

the help of the above mentioned people, I would have faced many difficulties.

Abhishek Thakur

Akshay Mistri

Sarthak Anand

ABSTRACT

Someone has rightly said that practical experience is far better and closer to the real world than

mere theoretical exposure. The practical experience helps the student to view the real world

closely, which in turn widely influences his/her perceptions and understanding of the real

situation.

Tubular Non Destructive Techniques, as the name suggests, is used to test tubes/pipes without

any kind of destruction. In this report, there are two tubular NDT techniques are discussed.

These two are Remote Field Testing (RFT) and Internal Rotary Inspection System (IRIS).

Remote Field Testing uses Electromagnetic induction to detect cracks in pipes/tubes. This

technique is limited to ferromagnetic materials.

The Internal Rotary Inspection system uses ultrasonic waves for detection of pitting, corrosion

and wall thickness in pipes. The advantage of this system is that it can be used for both

ferromagnetic as well as non-ferromagnetic materials.

Operation principle, equipments, theory of operation, result interpretation of both the

techniques is discussed in detail in this report.

Introduction

𝐑𝐞𝐦𝐨𝐭𝐞 𝐅𝐢𝐞𝐥𝐝 𝐓𝐞𝐬𝐭𝐢𝐧𝐠[𝟏]: It is an electromagnetic method used to detect defects in steel

pipes and tubes. A probe pushed in the tube to be tested, can detect inner and outer defects with

equal sensitivity. Basically the probe consists of an exciter coil which generates magnetic field

and a receiving coil which generates current due electromagnetic induction. As the probe is

moved, the changing magnetic field generates current in pipe/tube. Also, since the receiver coil

is moving with the probe it cuts the magnetic field lines generated by the current in the pipe

wall. The receiving/detector coil is placed two to three pipe diameters away from the exciter

coil. At places where there is metal loss, the field arrives at the detector with less travel time

(greater phase) and with greater amplitude due to reduced path through the pipe wall.

Fig.1 A RFT probe inserted in a tube.

𝐈𝐧𝐭𝐞𝐫𝐧𝐚𝐥 𝐑𝐨𝐭𝐚𝐫𝐲 𝐈𝐧𝐬𝐩𝐞𝐜𝐭𝐢𝐨𝐧 𝐒𝐲𝐬𝐭𝐞𝐦[𝟐]: It is an ultrasonic method for non-destructive

testing of pipes and tubes. The IRIS probe is inserted into the tube flooded with water and is

slowly pulled out. As it is being pulled data is recorded and displayed on the user interface. It

is normally not used for crack detection and is insensitive to cracks that are parallel to the

longitudinal axis of the tube. It is used for detection of pitting, corrosion and wall thickness.

Fig 2. An IRIS probe inside a tube flooded with water

Theory of Operation and Principle

Remote Field Testing: A probe as already explained is pulled through the tube. The exciter

coil is provided with a low frequency AC current. This produces a changing magnetic field

which produces magnetic field axially and radially. Due to this an eddy current is produced in

the tube surface which extends axially in the tube. This eddy current produces a secondary

magnetic field of its own which is less powerful than the field of exciter coil (primary field)

but is more spread out than the primary field. The receiving coil is placed at a distance where

the eddy current field becomes more powerful than the primary field. This can be seen from

the figure below.

Fig 3. Primary and Secondary magnetic fields

As the magnetic field due to eddy current is also changing, a voltage (EMF) is generated in

the receiving coil due to electromagnetic induction. By monitoring this induced voltage we

can detect changes in the specimen. As can be seen from the figure below, the magnetic

intensity at a point of defect seems to increase.

Fig 4. Magnetic field lines at a point of defect

Exciter coil

As can be seen above this method works on the principle of electromagnetic induction.

According to different magnetic field intensities area near the probe are divided into different

zones which are explained below[𝟑].

Fig 5. Field zones near a RFT probe

o Direct coupling zone: It is the zone where the alternating magnetic field from

exciter coil produces eddy currents in the tube wall. Due to high varying fields

this zone is of very less importance in the test.

o Transition zone: This zone lies just outside the direct coupling zone. Here

occurs a great interaction between the magnetic fields of the exciter coil and of

the eddy currents. The overall field intensity is drawn by green coloured line in

the graph. It can be seen that the field is strongest at the inner diameter of tube

and changes abruptly in the transition zone marked in the tube.

Fig 6. Field strength and Phase at various tube diameters

Also, by blue coloured line, where it abruptly changes in the transition zone

shows the region where the field from the eddy currents dominate over the field

of the exciter coil.

o Remote field zone: It is the zone where the direct coupling/interaction between

the exciter coil and the receiving coil is negligible. Coupling takes place

indirectly through the field generated by the eddy currents. This region is

generally two tube diameters away from the exciter coil. The amplitude of field

strength on outer diameter exceeds the strength at inner diameter at 1.65 tube

diameters approximately. Therefore, RFT is sensitive to inner defects as well as

outer defects.

Signal interpretation

The output of the probe i.e. the output voltage of receiving coil is given to an amplifier which

amplifies the output to a readable value because it comes out in microvolt range.

When all conditions are proper (no defect) then the phase of receiving coil’s voltage w.r.t phase

of exciter coil voltage is directly proportional to the sum of wall thickness at that location[𝟒].

Thus any change in wall thickness will result in phase and amplitude changes which can be

detected. This change can indicate corrosion, pitting or changes in wall thickness.

Fig 7. Resultant image when defect is wear scar

Fig 8. Resultant Image when defect is a hole

Internal Rotary Inspection System: As already discussed, it is an ultrasonic immersion pulse

echo technique to detect pitting and wall thinning in tubes. The probe of IRIS contains an

ultrasonic transducer which converts electronic energy into mechanical energy (in form of

sound waves). The frequency of the sonic waves generated by the transducer ranges from 100

kHz to 50 MHz depending upon the type of test needed[𝟓]. This sound beam is directed by the

transducer towards a mirror inclined at 45° with the tube axis. Due to this the waves strike

perpendicularly on the tube’s inner surface. Part of this beam is reflected back by inner surface

and part is transmitted through tube wall and reflected by the outer diameter of the tube.

Fig 9. IRIS probe inside a tube under test

Ultrasonic

Transducer

Probe (Body)

Mirror

Water

The time difference between the two reflected beams is used to measure the wall thickness. For

measurement of complete inner surface of the tube, the mirror is mounted on a water turbine

that rotates at a speed of 2000 RPM[𝟔]. As the probe is slowly pulled out (2.5 to 4 cm/sec)

measurements of full tube length can be made. Since the beam makes a spiral movement, for

complete coverage the probe has to be pulled slowly. On an average there are 150 reading per

revolution and 2400 revolutions per minute. Each pulse is marked on a horizontal scan line on

the user interface screen as shown in Fig 9.

Result Interpretation

With the help of modern interfaces, we can see the real time images of the tubes and the

flaws can be pointed out in real time. There are 3 different views of the tube obtained on the

user screen. These views are usually called as:

B Scan

C Scan

D Scan

are shown below.

Fig 10. 3 different views at desired section of tube

The C scan shows the plan view of the tube when it is being rolled out flat. Colour coding is

used to display the wall thickness at different locations of the tube. Different softwares use

different colour coding which can be better understood from the Fig 11 given below. The red

color spot shows metal loss at that point.

Fig 11. An IRIS viewer with colour coding

The B scan provides the 2D transverse view of the tube at any location along the tube length.

While the D scan provides the 2D longitudinal view of the tube length at any desired

circumferential location on tube.

Benefits and Limitations

Remote field testing

Benefits:

o Most suitable for ferromagnetic tubes used in boilers and heat

exchangers.

o Equally sensitive to internal and external flaws.

o Can inspect tube with 75 mm outer diameter and 5 mm thickness.

o Relatively insensitive to probe lift-off and wobble.

o Fast probe pulling speeds ranging between 6 to 12 inches/sec.

o The size of probe w.r.t the inner diameter is not so critical (fill factor).

Limitations:

o Not good in detecting gradual metal loss.

o Pits smaller than 6% of tube cross section cannot be detected.

o Examination of tubes with big diameters cannot be done.

o Examination of finned tubes has lot limitations.

Internal Rotary Inspection System

Benefits:

o Both ferromagnetic as well as non-ferromagnetic tubes can be tested.

o Provides very accurate readings of wall thickness. With 15 MHz

transducer frequency, accuracy of 0.1mm can be obtained.

o Can detect and size corrosion, pitting and gradual metal loss.

o It provides a 3D image of defect, providing its profile and depth.

o As image is provided, result interpretation is much easier in this

method.

Limitations:

o It is a slow technique, since the probe is pulled slowly. Only 80 to 90

tubes can be checked in a 12 hour shift.

o For successful test, the tube needs to be cleaned properly as compared

to other techniques.

o The tube is required to be filled with water for testing. This is not

possible in all cases.

o The probe must be centralized in the tube for accurate results.

References

1. Wikipedia, (2013) Remote field testing

http://en.wikipedia.org/wiki/Remote_field_testing [accessed 03/08/2014]

2. Wikipedia, (2013) Internal rotary inspection system

http://en.wikipedia.org/wiki/Internal_rotary_inspection_system [accessed 03/08/2014]

3. Anastos, J (2001) RFT Theory of operation

https://www.nde-

ed.org/EducationResources/CommunityCollege/Other%20Methods/RFT/RFT_Theor

y.htm [accessed 05/08/2014]

4. Anastos, J (2001) RFT Signal Interpretation

https://www.nde-

ed.org/EducationResources/CommunityCollege/Other%20Methods/RFT/RFT_Signal

s.htm [accessed 05/08/2014]

5. Olympus NDT, (2006) Design Characteristics of Transducers

https://www.olympus-ims.com/data/File/panametrics/UT-technotes.en.pdf [accessed

14/08/2014]

6. Innospection Ltd., (2008) Internal Rotary Inspection systems (IRIS)

http://www.innospection.com/pdfs/IRIS.pdf [accessed 15/08/2014]