One of the first applications of the borescope was for inspecting steam turbine rotor discontinuities in the 1920s. Today a wide variety of commercial borescopes and videoprobes can be used for access within HRSG components. These include both flexible and rigid borescopes and a variety of videoprobes and end effectors. Mirror or prism attachments can be used to get right angle views of tubes or pipe connections. Access can be via hand-holes or other penetrations.
Flexible video inspection probes employ high-resolution camera at the probe tip to capture detailed color inspection footage. Interchangeable lenses bring subjects both near and far into sharp focus. Variable-intensity fiber optic illumination floods hidden targets with cool, white light. Knobs on the handset control the tip’s up/down and left/right articulation. Rigid achromat borescopes transmit images using linear arrays of lenses rather than optical fiber, resulting in clearer, brighter images. Swappable objective sleeves convert standard forward-viewing instruments into right angle- or oblique-view instruments with directional control. When connected by a lightguide to a fiber optic lightsource, the borescope is capable of delivering large quantities of light to the inspection area.
Flexible fiberscopes negotiate bending passageways, delivering internal views of HRSG components unattainable with rigid borescopes. By transmitting images over optical fiber, a fiberscope fits through holes too narrow for standard video probes and also bends around tighter radii. The fiberscope is also capable of delivering large quantities of light to the inspection area, and levers on the handset control the tip’s articulation in a similar way to the video probe.
Principle of Operation
The following features of the object to be inspected define the specifications required for the test instrument :
- objective distance
- object size
- discontinuity size
- entry port size
- object depth and
- direction of view
The objective distance is the distance between the borescope and the object to be inspected. It defines the required illumination source and the focal distance for maximum magnification. The size of the object and the objective distance determine the field of view required and therefore the type of lens to access as large as possible a portion of the surface.
Discontinuity size specifies the size of features that need to be detected (for example, small cracks vs. large macroscopic damage). Reflectivity impacts on illumination requirements because darker surfaces require more light than lighter surfaces. Entry port size (for example, the diameter of a hand hole) defines the largest size of instrument that can be used. Direction of view determines the length and position of rigid borescopes.
Many special purpose borescopes are available commercially including:
- Calibrated borescopes to determine angles or sizes of objects in the field
- Panoramic borescopes to provide rapid panoramic scanning of the ID of tubes and pipes
- Ultraviolet borescopes for fluorescent magnetic particle and fluorescent penetrant tests
- Waterproof and vaporproof borescopes
- Water cooled and gas cooled borescopes for tests of furnace cavities and other high temperature applications.