NDE_Thermography

Introduction

All objects emit electromagnetic radiations. At ambient temperatures and above these are predominantly infra-red (IR) radiations. IR radiations are invisible to the eye but with the help of a suitable detector can be transformed into a visible image. Variations in the temperature of the surface of the object can be visualized in the form of a thermal image of that object. This enables temperature variations to be detected from a distance.

Principle

Thermography makes use of the infrared spectral band of the electromagnetic radiation. The infrared band is further subdivided into four smaller bands: (1) near infrared (0.75- 3 ľm) (2) middle infrared (3-6 ľm) (3) far infrared (6-15 ľm) and (4) the extreme infrared (15-100 ľm). The most commonly used band for commercial infrared imaging is between 0.75-15 ľm.

The transmission and absorption behavior of infrared light is different from that of visible light. Infrared radiations travel in a straight line outwards from the source. They can propagate in a vacuum and in certain solids, liquids and gases. They can be optically focused and directed by lenses or mirrors or dispersed by prisms. The basic factors affecting thermal measurements are:

  1. Emissivity: This is defined as the ratio of the radiance of a body at a given temperature to the corresponding radiance of a black body at the same temperature. For a black body the emissivity factor is 1.0. Emissivity is a critical parameter for quantitative measurement of the temperature of the object under examination.
  2. Surroundings: It is important to have the object surroundings free from other thermal radiation sources. Otherwise radiation from these sources would also be reflected by the object under examination, leading to erroneous values.
  3. Atmosphere: The effects of atmosphere are of importance when the object is far away. The atmosphere not only attenuates the radiation from the target, but also alters the spectral characteristics. These effects are negligible in cases when the object under investigation is quite close and the atmosphere is uncontaminated by vapors, smoke, hot gases etc. When critical measurements are to be made on an object in the presence of hot air/gases, as in the case of a furnace, suitable filters such as high temperature gas filters must be used along with the appropriate correction factors to take into account the ambient temperature and the attenuation by these filters. If the objects are situated at a large distance in the case of air borne thermography, atmospheric absorption plays an important role. Atmospheric absorption is a complex phenomenon and mathematical modeling is used for estimating the temperatures.

Techniques

Thermography can be classified into two categories:

  1. Passive: In the passive technique, the natural heat distribution is measured over the surface of a hot structure. This finds application in temperature monitoring.
  2. Active: In active technique, heating or cooling is induced or applied to the part or the complete surface and the movement and redistribution of temperature profile across the test surface is measured. This generally finds use in nondestructive evaluation.

Detectors and Equipment

The detection system for infrared imaging can be a contact (surface) system, such as cholesterol liquid crystal or a non-contact tele-system such as a thermo graphic camera. Surface systems tend to be less costly and simple and have high resolution. Camera systems are more expensive. Thermal imaging with tele-camera systems has wider engineering applications.

A non-contact thermography system consists of an infrared scanner, monitor, control unit and a calculator for field applications. The output can also be stored in a modified video thermal recorder, which can be analyzed later using a personal computer with image processing facilities. The infrared scanner consists of an optical system, scanning mechanism, infrared detector and associated electronics. The optical system collimates the incoming infrared radiation into the detector. The scanning mechanism scans the surface within the field of view. The heart of the thermography system is the infrared detector. There are different modes for obtaining a thermal image:

  1. Image converters: Here the thermal image is converted into an electron image through the use of converters, which is then converted into an optical image.
  2. Pyricon Based Devices: These are normal vidicon tubes with infrared sensitive faceplates.
  3. Mechanical Scanning devices: The image is scanned with a moving mirror across a fixed and cooled detector. This type of system is used extensively in NDT.

The advent of personal computers has revolutionized the field of thermography. Thermograms can be subject to image processing and enhancement to obtain minute details not visible otherwise. Software options for image processing include contrast stretching, spatial filtering, and thermal chopping.

Applications
  1. In the petroleum industry thermography finds application in the monitoring of stack temperature, maintenance of plant equipment such as reaction towers, refining furnaces, ducts and piping and detection of corrosion in oil tank shell and measurement of oil levels.
  2. Thermography is used for the inspection of transmission lines, substations and distribution systems. Hot spots indicating higher temperature regions can be seen in the photographs.
  3. Thermal imaging is used for condition monitoring of furnace tubes, fluid transfer lines and evaluation of heat resistant linings in refractory furnaces.