Tube Inspection [PDF]

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TUBE INSPECTION

THE BEST NDT METHOD FOR YOUR  HEAT EXCHANGER, FEEDWATER HEATER, CONDENSER, BOILER AND AIR COOLER INTEGRITY ASSESMENT

PT TIGA SEKAWAN ENJINIRING tse-groups.com

+62 (21) 222 31548

[email protected]

E L B A T

13 15 17 19 20

I N T R O D U C T I O N

T U B E

S E L E C T I O N

E D D Y

C U R R E N T

T E S T I N G

E D D Y

C U R R E N T

A R R A Y

R E M O T E

N E A R

F I E L D

F I E L D

M A G N E T I C

I N T E R N A L

T E S T I N G

T E S T I N G

F L U X

S Y S T E M

V I S U A L

E P I L O G U E

L E A K A G E

R O T A R Y

I N S P E C T I O N

R E M O T E

C H A R T

I N S P E C T I O N

CONTENTS

F O

3 5 7 9 11

TUBE INSPECTION

PAGE | 03

INTRODUCTION

The petrochemical and oil/gas industry depends on many heat exchangers and boilers for efficient operation. A heat exchanger is a system used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes. Heat exchangers are vital components in numerous process plants. Failing or leaking heat exchangers can lead to lower efficiency, unplanned shutdowns or even accidents which in turn can result in damage to equipment, environment or people. In general there are two main locations where leakages will occur in a heat exchanger: 1. At the tube/tube sheet joints 2. Along the length of the tubes

PAGE | 04

TUBE INSPECTION

Examination and condition monitoring of heat exchangers are performed using a variety of techniques. Selection of the best suited technique depends on whether the objective is fabrication control, preventive in-service inspection, or inspection due to failure as for example leakage and depends on the type of defects which is expected to be detected by the examination. Widely applied techniques are: 1. Eddy Current Test 2. 3. 4. 5. 6. 7.

Eddy Current Array Remote Field Testing Near Field Testing Magnetic Flux Leakage Internal Rotary Inspection System Remote Visual Inspection

Each of the NDT techniques has its advantages and limitations. Selection of the right technique depends on tube material, expected defect types and the purpose of the inspection. Often more than one technique will be applied to increase the level of confidence of an inspection. This booklet is provided to you by PT Tiga Sekawan Enjiniring and is to assist plant inspectors in selecting the right techniques for their specific inspection needs.

TUBE INSPECTION

TUBE

PAGE | 05

INSPECTION

SELECTION

CHART

Tube Inspection Selection Chart legend (Table A, B, C) The test method has proven results for the specific application The test results obtained from the test method can be interpreted reasonably Test method is either not suitable for the sought application or non-reliable in terms of repeatability

Table A.

Tube Testing Technique Suitability According to Material

Material type/Tech

NonFerromagnetic

Low Ferromagnetic

Tube Integral finned Tube Integral finned Tube

Ferromagnetic

Integral finned Alumunium finned

ECT

ECA

RFT

NFT

MFL

IRIS

RVI

PAGE | 06

Table B.

TUBE INSPECTION

Tube Testing Detection Capabilities According to Defect Type

Defect / Tech

ECT

ECA

RTF

NFT

MFL

IRIS

RVI

ID Pitting OD Pitting Axial cracking Circumferential cracking ID corrosion OD corrosion At tubesheet

Table C.

Tube Testing Sizing Capabilities According to Defect Type

Defect / Tech ID Pitting OD Pitting Axial cracking Circumferential cracking ID corrosion OD corrosion At tubesheet

ECT

ECA

RFT

NFT

MFL

IRIS

RVI

TUBE INSPECTION

EDDY

CURRENT

PAGE | 07

TESTING

(ECT)

Eddy Current Testing (ECT) is an effective way of assessing the condition and lifespan of tubes, particularly in the power generation, petrochemical, chemical, fertilizer and air conditioning industries. It is a high-speed inspection method and one of the major advantages is that it can be performed through paint and coatings. This technique is only suitable for non-ferrous material such as Brass, Copper, Copper-Nickel and Austenitic Stainless steel. ECT can pinpoint minute flaws using sophisticated software and detection probes. Analysts can scan for anomalies on a surface and subsurface level with ease and accuracy. ECT can detect both internal and external defects and can distinguish between them. Cracks can be detected depending on their size and orientation. By applying Multi frequencies, defects under support plates can be detected and to some extent quantified.

Theory

Eddy current testing uses electromagnetic induction to identify defects in tubing. A probe is inserted into the tube and pushed through the entire length of the tube. Eddy currents are generated by the electromagnetic coils in the probe and monitored simultaneously by measuring probe electrical impedance. The information revealed by the probe will detail the tube defects. The scanning data will be recorded by the software and kept us as a backup for future reference.

PAGE | 08

TUBE INSPECTION

Advantages

Disadvantages

Fast (450-700 tubes per day)

Limited to only non-magnetic tube

Overall wall-loss and local defects can be detected High sensitivity to small defects

material Application is limited to 3 inch tube sizes and 0.125 wall thickness

(dia. holes/pits > 0.5 mm) Accurate sizing of defects possible Possible to distinguish between inand external defects Possible to detect and quantify defects under support plates Cracks can be detected depending on size and orientation

Pin holes are difficult to detect and evaluate Discontinuities adjacent to end sheets are difficult to detect Tubes must be cleaned

Only basic required

cleaning

Permanent records obtained on test results

of can

tubes be

TUBE INSPECTION

EDDY

CURRENT

PAGE | 09

ARRAY

(ECA)

Eddy Current Array (ECA) is the supercharged version of Eddy Current Testing (ECT). They consist of arrays of coils that activate in sequences in order to eliminate interference between them. The array slides on top of surfaces, offering an overall wider coverage and increased sensitivity to defects compared to ECT. The Eddy Current Array technology is able to not only detect surface-breaking defects but to some degree also subsurface defects. Most conventional Eddy Current flaw detection techniques can be reproduced with an ECA inspection. With the benefits of single-pass coverage, and enhanced imaging capabilities, ECA technology provides a remarkably powerful tool and significant time savings during inspections.

Theory

Traditional Eddy Current and Eddy Current Array technology follow the same fundamental principle. When alternating current is introduced in a coil, a magnetic field is produced (in blue). When the coil is positioned over a conductive part, opposite alternating currents or eddy currents (in red) are created. The path of the eddy currents (in yellow) is disturbed by the defects in the part. ECT is a simple and accurate inspection method used to identify surface as well as near-surface defects in conductive material. The method can be employed to calculate the electrical conductivity of materials and can even be used to measure nonconductive coating. In addition, ECA testing covers a huge area in a single pass and considerably reduces inspection time. It enhances reliability and probability of detection and reduces the requirement for mechanical and robotic scanning systems.

PAGE | 10

TUBE INSPECTION

Advantages

Disadvantages

Larger area can be scanned in a

Limited to only non-magnetic tube

single-probe pass, while maintaining a high resolution Easier analysis because of simpler

material Application is limited to 3 inch tube sizes and 0.125 wallthickness

scan patterns Improved flaw detection and sizing with C-scan imaging Inspection of complex shapes using probes customized to the profile of the part being inspected More sensitive to circumferential cracking

Tubes must be cleaned

Can distinguish between defect signal and tube sheet signal If only testing tube ends 5 sec per tube vs 40 sec per tube for rotating techniques

TUBE INSPECTION

REMOTE

FIELD

PAGE | 11

TESTING

(RFT)

Remote Field Testing (RFT) is primarily used to inspect ferromagnetic tubing since Conventional Eddy Current techniques have difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials. This technique is very suitable for detection and quantification of overall wall-loss. Local defects can be detected and quantified provided that they have some volume (diameter pit >5 mm). Remote Field Technique can detect both internal and external defects but it is not possible to distinguish between them. Defects under or close to the tube sheet are hard or not possible to detect. Only a basic cleaning of the tubes will be sufficient.

Theory

The probe used in Remote Field Technique examination contains a send and a receiver coil. In the bigger send coil an alternating magnetic field is generated. This field is indirectly coupled to the receiver coil as a direct coupling between the two coils is shielded by the strong magnetic fields originating from the eddy currents that are being generated in the tube. At a low enough frequency the shielding will lose some of its strength allowing the exciter field to penetrate the tube wall in axial direction. Once the magnetic field reaches the exterior of the tube it will spread rapidly along the tube with little further attenuation. Research found that a portion of the magnetic field rediffuses back through the pipe wall to the interior of the tube at a certain location. At this position the smaller receiver coil is placed to detect the remaining field. Now the indirect coupling path between send and receiver coil is complete. The magnitude and the phase of the received magnetic field depend on the amount of material that was crossed in the indirect coupling path. If wall-loss occurs in a tube there will be less attenuation and delay of the exciter field before it reaches the receiver coil. The signal on the computer screen represents the change in the received magnetic field, and thus the condition of the tube. During signal analysis, the signals acquired during a Remote Field inspection will be compared to the signals from reference defects. Reference defects are defects with known depth and shape and are machined into a calibration standard. The calibration standard needs to be of the same material and dimensions as the tubes to be examined.

PAGE | 12

TUBE INSPECTION

Advantages

Disadvantages

Can inspect ferromagnetic tubes

Some limitation to distinguishing

up to 3.5 inches in diameter with 0.125 inches wall thickness Inspection speed (up to approx. 40

ID from OD defects Evaluation of small flaws such as pits can be difficult

feet per minute) Can detect large area discontinuities such as steam erosion and baffle wear Amplitude changes in the signals sensed are not speed-sensitive Flexible probes can be used to inspect and travel through U-bend

Examination of finned tubes has a lot of limitations unless fins are in axial direction Instrumentation and test probes can be very expensive Tubes must be cleaned

areas Permanent

records

obtained on test results

can

be

TUBE INSPECTION

NEAR

FIELD

PAGE | 13

TESTING

(NFT)

Near Field Testing (RFT) is a rapid and cost-effective solution intended specifically for fin-fan carbon-steel tubing inspection. This technology relies on a simple driver-pickup eddy current probe design providing very simple signal analysis. NFT is specifically suited to the detection of internal corrosion, erosion or pitting in carbon steel tubing. The NFT probes measure lift-off or ‘fill factor’ and convert it to amplitude-based signals. Because eddy-current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tube. NFT is specifically suited to detecting corrosion, erosion, and pitting inside carbon steel tubing. NFT is perfect for fin-fan tube heat exchangers because eddy currents do not go through the wall of the tube. NFT is also much more sensitive to defects close to structures such as support plates and tubesheets.

Theory

NFT technology uses two coils, a transmitter and a receiver. Typically the receiver coil is close to the transmitter coil, taking advantage of the transmitter’s near-field zone — that is, the zone where the magnetic field from the transmitter coil induces strong eddy currents, axially and radially, in the tube wall. NFT probes operate within the same frequency range as Remote-Field Testing (RFT) probes. Remote field is a send/receive through the wall technique that uses the send and the receive coils separated by approximately two and a half tube diameters, or there may be two send coils the same distance on both sides of the receive coils. Near Field has the sensor coils close together and is not a through wall transmission technique. The signals that result from the RFT are dependent upon the condition of the tube near the send coils, near the receive coils, and to some degree, in between the coils. Also, as the RFT travels outside the tube wall, the signals are also very dependent upon support structure, such as support plates and tube sheets that may be between the send and receive coils. On the other hand, the signals that result from NFT are dependent only upon the condition of the tube close to the coils and as the signal barely penetrates outside the tube wall, is only slightly dependent upon support structures, such as support plates.

PAGE | 14

TUBE INSPECTION

Advantages Fast

when

used

as

screening

technique Possible to examine carbon steel (finned)tubes Both pits and overall wall-loss can be detected (dia. Pits > 3 mm) Detection of internal thinning and pitting based on sensitivity of eddy current lift off No need for an external reference coil Cleaning of tubes less critical The results is unaffected by support plates and Tube sheets

Disadvantages Cannot detect external defects Sizing defects limited (based on signal volume) Less suitable as stand alone technique

TUBE INSPECTION

MAGNETIC

FLUX

PAGE | 15

LEAKAGE

(MFL)

Magnetic Flux Leakage is a technique used for the inspection of tubes made of ferritic materials. This technique will normally be applied as a fast screening technique if small diameter pitting is expected. Because of limitations to its sizing abilities the technique is not often used as a stand-alone technique. Verification by other techniques is recommended. MFL can also be used on airfin cooler tubes. MFL is sensitive to sharp type defects like pits and grooving. In- and external pits can be detected. Depending on probe configuration MFL can distinguish between in- and external defects and can detect gradual wall-loss. For ID/OD discrimination the probe needs to be equipped with a second coil and to detect gradual defects a Hall-effect sensor in the probe is needed.

Theory

The probe consists of a magnet and two flux leakage sensors, which set up a flux field in the tube wall as it passes through the tube. The field fluctuates when it encounters a flaw. The flux rate fluctuation effect is picked up by the coils and displayed on the display apparatus and chart recorder. A Hall effect element can be added as a combined-type probe, which is used to detect absolute flux such as gradual wall loss. The output of the Hall effect detector depends on the orientation of the sensor in the probe relative to the discontinuity and whether the location of the discontinuity is on the inside or outside surface.The output of the magnetic flux leakage coils is related to the change of flux caused by the discontinuity but not the discontinuity size.

PAGE | 16

TUBE INSPECTION

Advantages Fast

when

used

as

Disadvantages

screening

technique Possible to examine steel (finned) tubes

Less

suitable

as

stand-alone

carbon

technique Very sensitive to inspection speed, accuracy of test results can

Distinguishes ID from OD flaws Can inspect ferromagnetic tubes up to 3.5 inches in diameterand 0.120 inches wall thickness Non volumetric defects like crackscan be detected depending on size, shape and orientation Defects under support plates can

fluctuate with probe speed Sizing of in- and external defects limited (based on signal volume) Probe speed dependant Can not inspect U-bend tubes Cleaning of tubes very critical because use of big fill factor probe is required

be detected and extent quantified

to

some

Instrumentation can withstand adverse field conditions Permanent records can be obtained on test results

TUBE INSPECTION

INTERNAL

ROTARY

PAGE | 17

INSPECTION

SYSTEM

Internal Rotary Inspection System (IRIS) is an ultrasonic method for testing of pipes and tubes. The ultrasonic beam allows detection of metal loss from the inside and outside of the tube wall. It is a fairly sensitive technique. The sensitivity achieved will depend on tube dimensions and tube cleanliness. Both ferromagnetic and nonferromagnetic tubes can be inspected. With IRIS the remaining wall thickness of tubes can be accurately measured. Defects under support plates can be measured without any limitations. IRIS is more accurate than other tube inspection techniques and has the advantage of presenting information about the geometry of defects.

Theory

This examination method employs an ultrasonic immersion pulse echo technique. The ultrasonic transducer is contained in a test head, which fits into and is centered in the tube to be inspected. The ultrasonic pulses are emitted along a path parallel to the tube axis. A rotating 45-degree mirror then reflects these pulses so that they are directed radially on to the tube wall. Reflections from the inner and outer walls follow the same path back to the transducer. The time interval between the first echo – from the internal surface of the tube – and the first echo – from the outside surface of the tube – can be used to represent the tube wall thickness. As the mirror rotates, the ultrasonic beam is traversed around the tube circumference and each successive pulse is mapped out as a horizontal scan line on the instrumentation screen. A typical system can generate approximately 190 readings per revolution and approximately 2400 revolutions per minute.

PAGE | 18

TUBE INSPECTION

Advantages

Disadvantages

100% tube inspections converge

Low inspection speed (test speed

(end to end) Actual wall thickness accurately be measured

can

is approximately 15 feet/minute), High cost Wall thicknesses lower than 0.8

Wall loss and pit detectability’s accuracy and sizing plus orminus 0.002 inch Can examine both ferromagnetic and non-ferromagnetic tubes Both internal and external defects can be measured Can distinguish between in-and

mm cannot be accurately measured Difficult to detect external defects when inner wall is heavily corroded Water is needed on site to function as couplant Cannot detect circumferential cracks

external defects Can inspect tube sizes up to 3.0

Cleaning of tubes is very critical

inches with wall thickness up to 0.25 inches Can inspect U-bend tubes with some radius limitations Defects under support plates can be detected and sized Not influenced by changes in permeability or conductivity that can cause false indications using electromagnetic methods Gives information about geometry of defects

TUBE INSPECTION

REMOTE

VISUAL

PAGE | 19

INSPECTION

Remote Visual Inspection (RVI) is the application of visual inspection at a position remote from the position of the operator performing the inspection. As well as allowing inspection of inaccessible areas, RVI is a safer alternative to an operator entering a confined space to conduct visual inspection. RVI requires the use of optical equipment to access the remote inspection site. Pan, tilt and zoom (PTZ) cameras have evolved to a more compact size in order to access extremely confined spaces. Software developments have enabled objective data to be viewed on screen, recorded for analysis and reporting later. Continued advancements in the field of robotics has empowered inspection cameras to travel into previously inaccessible areas. RVI is used to examine a wide range of infrastructure. When utilized as a standalone tool or part of a remote crawler system, RVI cameras can deliver high definition video with advanced imaging options for detailed assessment of any industrial structure with limited or unsafe access.

Advantages Can be used on every material Removes humans from potentially unsafe conditions Gives information about geometry of defects Visualization of defects gives high level of confidence

Disadvantages Sizing of defects not possible Only the internal tube wall can be examined High level of cleanliness required

PAGE | 20

TUBE INSPECTION

EPILOGUE

For all Electro Magnetic techniques (ECT, ECA, RFT, NFT, MFL) that were discussed, a calibration standard is essential for good results. During signal analysis, the signals acquired during a heat exchanger inspection will be compared to the signals from reference defects. Reference defects are defects with known depth and shape and are machined into a calibration standard. The calibration standard needs to be of the same material and dimensions as the tubes to be examined. For IRIS examination it is recommend to use a reference standard to verify equipment response prior to an inspection carried out. In conclusion, no single technique can be applied for testing all heat exchangers or boiler tubing materials. Perhaps in some applications, more than one technique is needed. Also proper technique selection leads to reliable tests and achievement of accurate results. Note: This book is intended for informational purposes only and all information in this book is based on capability Olympus MS 5800 ER1U with fully loaded for tube inspection duty. Named values have been generalized and depend on specific situations. To the best of our knowledge, the information in this publication is accurate however the publisher does not assume any responsibility or liability for the correctness of such information.

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