The use of tell-tale holes acts as an early detection and safeguard for accelerated or unpredicted thinning of tubes. The tell-tale holes minimize the effect of leaks associated with a heater tube failure. The tell-tale drilling practice is regarded as a safeguarding measure in loss prevention and should not be considered a substitute for, or a relaxation of, good inspection and quality control practices.
a. Tell-tale hole diameters and drilling depths:
1. Tell-tale hole drill diameters will be 1/8 in. (0.32 cm).
2. Drilling depth tolerance shall be + 0 in. – 1/64 in. (+ 0 cm – 0.04 cm).
3. Heater tube drilling depths will be calculated according to the retirement thickness in API Std 530.
b. Tell-tale hole locations:
1. The location of tell-tale holes is based on the corrosion type and the heater tubes configuration. Typical drilling patterns are located at heat-affected zones, 180-degree bends, and areas with potential for accelerated corrosion, and random areas on straight run lengths.