 Table of Contents ECNDT '98 Session: Chemical, Petrochemical | NDT and Heat Exchanger TubesHelle H. Rasmussen*, Hans Kristensen & Leif Jeppesen
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Heat exchangers are vital components in numerous process plants. With an increasing focus on operating economy and safety, the benefits achieved from baseline inspection and condition monitoring are evident.
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.
This paper will present a range of NDT methods and techniques which can be applied in the examination of tubes in heat exchangers or tubes in other heat transfer or processing equipment.
At FORCE Institute the four frequently used NDT techniques for tube examination are Eddy Current, Ultrasonic, Helium Leak test and Visual examination. The four techniques and their different applications will be presented below.
For examination of the individual tubes the eddy current technique is applied.
The eddy current equipment used by the FORCE Institute is compact and transportable. The eddy current instruments are multi-frequency units specially developed for tube inspection. The instruments have a build-in hard disk drive for recording of data. The probe drive units are fast and very stable mechanical devices specially developed by the FORCE Institute. Probes in a broad range of diameters are always on stock to be able to serve our customers even in urgent situations.
The examination is performed by use of an eddy current probe (bobbin probe) which is inserted into the tube via a probe drive system. When the probe is retracted the measurements are performed and the data are automatically recorded on strip chart or data disk.
 Fig 1: Eddy Current Examination of a heat exchanger |
By the use of different examination modes and multi-frequency technique it is possible to examine the tubes for detection of general and local corrosion or erosion. The technique can detect defects in the tube wall both at the straight part of the tube and at the u-bend as well as in the area below the tube supports. Even bimetallic tubes have been inspected for defects in the inner or outer tube or for defects in the bonding zone between the two materials. The eddy current technique has also been used for detection of unwanted foreign material located on the tubes, to be able to remove the material before any damage to the exchanger occurred.
For examination of the area below the tube sheet, the technique using a bobbin probe can be applied for detection of tube sheet corrosion or the rotating eddy current probe technique can be used for detection of smaller defects e.g. cracks.
At the rotating eddy current probe technique, a small surface probe which only covers a small area of the tube is pulled through the tube during constant rotation. The technique is used supplementary to the standard bobbin probes for locating and analysing small local defects.
Tubes of magnetic material can be tested by eddy current using the saturation technique or the RFET (Remote Field Eddy Current Testing) technique.
Examination is normally carried out by two inspectors and depending on the conditions of work, it is possible to examine up to 1000 tubes per day. The data are evaluated from the strip charts or the disk using a PC based evaluation and reporting software.
The results of the eddy current examination will be presented on a tubesheet map where different colours are used to visualise the results of the examination.
All the above Eddy Current Techniques are practised by the FORCE Institute.
For examination of tubes of all types of materials the ultrasonic technique is applied.
 Fig 2: Rotating Ultrasonic Examination, principle |
For examination of the tube wall the IRIS (Internal Rotary Inspection System) probes are used. These probes utilise the ultrasonic puls-echo technique. The ultrasonic transducer is rotated inside the tube and due to the frequency of the transducer and the rotation speed the entire circumference of the tube wall will be examined. When the probe is pulled through the tube a measurement of the wall thickness is performed.
The result of the examination is presented visually showing the unfolded tube wall. The wall thickness can be measured in any point of the tube circumference and at any position of the tube length. Defects will be displayed as they occur on the actual tube in size, propagation and location on the tube wall, i.e. internal or external.
The technique can be applied on straight tubes with internal diameters from 8 mm to 90 mm. For the larger tube diameters slight bends can be inspected as well.
For examination of the tube-to-tubesheet welds FORCE Institute have developed a special scanner based on our ultrasonic equipment, the P-scan system.
The scanner is inserted into the tube and the tube-to-tube sheet welds are examined from the inside.
The technique provides information of the welding-in height and detects lack of fusion as well as pores and inclusions. For all detected defects the length and position can be determined.
Both types of ultrasonic equipment used by the FORCE Institute are compact and transportable.
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| Fig 3: Ultrasonic P-scan Examination of tube-to-tube sheet weld, principle | ||
For examination of an entire heat exchanger or a single tube for detection of leaks the helium leak test is applied.
The examination can be performed as an overall test of the entire exchanger for determination whether leaks are present or not. This examination is fast and provides a quick survey of the exchanger.
The helium Leak Test method is sensitive to all types of leaks. If leaks are detected at the overall examination, each tube and tube-to-tubesheet weld can be tested individually by Helium Leak Test or by another NDT method.
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| Fig 4: Helium Leak Test Examination | |
For examination of single tubes or for verifying the results obtained by other NDE methods visual examination can be applied.
The visual equipment used by the FORCE Institute covers both small cameras and video endoscopes with diameters down to 6 mm.
At the examination the camera or endoscope is inserted into the tube and the examination of the tube is performed when the camera or endoscope is retracted. The examination can be documented on video tape if desired.
This type of examination will reveal the condition of the inner surface of the examined tubes. The extent of scale can be assessed and documented as well as the extent of internal corrosion, general or local. If a tube is blocked the visual examination can reveal the cause of the block up.
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| Fig 5: Visual Examination of tubes, ID approximately ø 15 mm | Fig 6: Visual Examination, example of equipment | |
As a supplement to the above mentioned NDT methods the Force Institute can offer a range of related applications.
Corrosion engineers can help in pinpointing the corrosion type to look for and the most critical components. Unexpected detection of corrosion damage or other defects can be followed up by failure analysis that may include chemical analysis of media, corrosion products, scale or the actual steel grade or alloy in which the damage is found. Analysis can be made either on samples taken to Force Institute laboratories (metallography, corrosion testing or chemical analysis) or performed on site by NDM (Non Destructive Metallography).
Other NDT methods can be applicable to special types of damage, for example PT (penetrant testing) will be more adequate to assess the presence of stress corrosion in the tube sheet.
If a heat exchanger is to be re-tubed, Force Institute can assist in the process of determining the best suited material for the heat exchanger in question, and giving a recommendation for welding procedures and tests.
Generally, the use of the above mentioned NDT methods reveal a considerable quantity of information regarding the condition of the examined heat exchanger. To summarise, some of the capacities of the NDE methods are listed below.
By the use of NDT techniques:
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