Selecting and Qualifying a Borescope for Internal Tube Inspection
A heat-exchanger tube bundle (tube ID 25 mm, length 4 m) must be visually examined for pitting and erosion at the tube-to-tubesheet transition and along the first 300 mm. Access is from the channel head only. Available equipment: (A) a rigid borescope, 6 mm diameter, 0° forward view, 90° field of view, with a 5-inch working length; (B) a flexible video borescope, 5.5 mm diameter, 4-way articulation, 2 m working length, with a 640×480 camera and LED ring illumination; (C) a fiberscope, 4 mm, 1 m working length, with a separate light source. The specification requires detection of pits 0.5 mm deep and documentation of findings.
1. Evaluate each candidate against the inspection requirement and recommend a primary and backup instrument.
Read the worked answer
Requirement: reach 300 mm into a 25 mm ID tube, view the transition region at an angle (the tube wall is not directly ahead), detect ~0.5 mm pits, and produce documented images.
- (A) Rigid borescope, 5 in (~127 mm) working length, 0° forward view: cannot reach 300 mm, and a forward-view optic looking straight down a tube sees mostly the far wall - poor for the transition and wall pitting. Rejected.
- (B) Flexible video borescope, 2 m, 4-way articulation, 640×480: sufficient reach, articulation allows the tip to be turned toward the tube wall and the transition, and the camera provides recorded documentation. Diameter 5.5 mm fits a 25 mm tube easily. Primary.
- (C) Fiberscope, 1 m, 4 mm: reach is adequate for 300 mm but image is relayed through fibers (lower resolution, honeycomb artifact), no articulation stated, and no integrated recording - documentation would depend on an external camera. Backup only.
Recommend B as primary, C as backup for regions where a smaller diameter is needed.
2. What resolution and demonstration checks should be performed before accepting the chosen instrument for 0.5 mm pit detection?
Read the worked answer
The instrument must be demonstrated capable, not assumed capable:
- Resolution demonstration: image a reference target containing features at and below the detection size (e.g., a resolution/line-pair target or a machined pit of known depth) at the working distance used in the tube. Confirm the 0.5 mm feature is discernible and repeatable.
- Working-distance check: resolution degrades with distance; verify the target is resolved at the actual standoff (tube wall to tip) expected at the transition, not at the closest possible distance.
- Illumination check: confirm the LED ring provides adequate, uniform illumination at the wall - insufficient light at the far side of the tube is a common failure.
- Articulation reach: verify the tip can actually be angled to view the transition geometry within the 25 mm tube without binding.
- Image quality for documentation: confirm recorded images are sharp enough to show the pit and to allow later measurement/scale reference.
Record the demonstration results as part of the procedure qualification.
3. The flexible scope's camera is 640×480 and the tube is 25 mm ID. Explain why pixel count alone does not guarantee pit detection and what else governs it.
Read the worked answer
Pixel count sets an upper bound on sampling, but detection is governed by the optical chain:
- Field of view vs. working distance: if the 640-pixel width spans, say, 20 mm of tube wall, each pixel covers ~0.03 mm - nominally enough - but only if the optics are sharply focused at that distance. Defocus blurs the pit across many pixels.
- Depth perception: a 0.5 mm deep pit is detected largely by shadow/contrast, which depends on illumination angle. A ring light shining straight on can wash out shallow pits; angled illumination improves contrast.
- Surface condition: scale, staining, or product residue can hide a shallow pit regardless of resolution.
- Display and observer: the displayed image, monitor quality, and inspector's visual acuity and training affect what is actually perceived.
So the acceptance basis must be a demonstrated detection capability (task 2), not a pixel-count specification.