Detecting Thinning in Pipe Walls  Using 
Infrared Thermography

By: Ron Newport Academy of Infrared Thermography

Reliability of a Piping System of a Processing Plant or Distribution Systems is a key consideration for safe and reliable operations.  

Besides leaks at joints and valves, etc, it is important to know the thickness of the pipe walls.  Conventional techniques generally consist of Radiography, Eddy Current, Ultrasonic, and Visual Inspection.  

The most common method used to determine wall thickness is Ultrasonic thickness measurement.  Ultrasonic measurements are based on a grid system of an entire elbow, fitting or pipe length, and is very time consuming.  As well, the inspection must be performed on suitable surfaces free from roughness, scale, weld spatter, and other conditions which may interfere with the transmission of ultrasound.

Another technique which holds much promise is Infrared Thermography.  It is much faster than ultrasonics and requires very little surface preparation. 

Infrared Thermography has been used for a multitude of applications for condition monitoring of industrial processes and facilities.  Most of these applications are “passive” applications.  This means the object being inspected have a normal thermal signature that will indicate the operating condition of the component.  Beyond the regular applications there is a big demand for an effective non-contact diagnostic method to determine pipe thinning and corrosion.

For the assessment of corrosion or thinning of the walls of piping, an active method must be used.  This means the pipe must be thermally activated in order to identify a thermal anomaly, which would then be further investigated.  This technique has been successfully used in many other applications such as composite delaminations and sub-surface corrosion identification in the aerospace industry.

Conventional methods of performing Thermography inspections utilize the surface temperature distribution of an object.  If the surface temperature is uniform, with no temperature variations, there is no indication of a problem.  As a result, for some applications it has limitations.

A new diagnostic procedure for Thermography has been developed recently and is expanding greatly in application.  It is called “Active Thermography”.  This method subjects the outer (or inner) surface of the object to forced thermal energy in effort to find subsurface anomalies on the inner portion of the object.  The Infrared camera catches the unsteady phenomenon as thermal images.

Diagnostic Procedure For Piping

 The procedure for thickness evaluation on piping is:

1.  The outer surface of the pipe is uniformly heated by using a Radiant Heater, Heat Gun, Flash Lamps or Heat Blanket.  The time to heat the pipe using a 1kw heater is 2 - 10 seconds.  This will vary with inside and outside surface temperatures.  The heater must be of large enough capacity so the shape of the defects will emerge distinctly and the object must be given a large temperature difference within a short period of time.
   

2. After completing the heating cycle, remove the heat source quickly and with an IR Camera capture sequenced images to obtain the surface temperature distribution of the heated zone.
   

3. The temperature distribution will indicate a larger temperature increase on the thin pipe versus the normal portions of the piping due to the difference of heat capacity due to the difference in wall thickness.  Time is critical because the thermal signature will equalize quickly.  Depending on the IR instrument used, integration processing may have to be used.
   

4. Comparing the thermal distribution to the various parameters such as - original pipe thickness, heating power, and inner and outer heat transfer coefficient of the pipe; the estimated thickness can be displayed.
   

5. Ultrasound can then be used as a verification method and to determine thickness.

 Results of Experiments

 Under experimental conditions using various sized rectangular shaped reductions in the inner pipe wall, the following was noted:

1. The method was successful in detecting reduction in the thickness of piping.
   

2. The smallest defect detectable up to 20% reduction in thickness was 30mm wide.  40% reduction in thickness was the limit for a 20mm wide defect, and 60% for a 10mm wide defect.
   

3. The method was capable of differentiating circular and rectangular shaped defects as long as the measurement is taken immediately after the object has been heated.
   

4. In order to detect defects by Thermography, the temperature difference between the normal and thinning portion should be 0.5^oC or greater.  If you can increase the temperature difference or the thermal resolution of the IR Camera, an improvement in detection limitations can also be expected.

IR Technique as Compared to Conventional

1. Since the system is capable of taking two dimensional surface measurement, it has a high diagnostic efficiency, and it is easy to carry out the test on the whole surface.
   

2. Since the technique is non-contact, pipe can be diagnosed at high temperatures.
   

3. Overlooking a serious defect is avoided since the larger the thickness reduction area and the deeper the thickness reduction, the easier it is to detect.
   

4. Special qualifications are not required for handling Infrared equipment as, for instance, X-ray.  The work is safer.
   

5. It is not necessary to have a high level of knowledge and experience to evaluate the results.
   

6. The technique is not limited to a particular pipe size or the type of fluid flowing through the pipe.
   

7. The overall diagnosis can be carried out in a short period of time.  Therefore, the time to evaluate pipes is shortened.

 Conclusion

 The active technique using an Infrared Camera can be used for a broad range of applications such as diagnosis of fouling and clogging piping, detection of cavitation erosion, inspection of inner surface of side walls and bottom plates of steel tanks, cracks developing under coated films of structures, plus others.

Current Issue

Archives

E-mail Us