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.
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