The correct answer is D) Only when chemical cleaning fails to restore distillate production capacity to normal. The reasoning behind this is that mechanical cleaning of feed water heater tubes should only be undertaken when chemical cleaning has failed to restore the distillate production capacity to its normal level. Chemical cleaning is generally the preferred method as it is less labor-intensive and can be done without disassembling the equipment. Mechanical cleaning should be the last resort, as it involves physically removing deposits from the tubes, which can be more time-consuming and potentially damaging to the equipment. The other options are incorrect because: A) A drop-off in distillate production capacity does not necessarily mean mechanical cleaning is required, as chemical cleaning may be sufficient; B) Increased steam flow to the feed water heater shell is not a reliable indicator for when mechanical cleaning is needed; and C) Mechanical cleaning should not be done during regularly scheduled maintenance unless it is specifically required.