PetroStreet

What is the minimum number of tubes to be inspected for a heat exchanger when it is made available for inspection?
Is there any criteria available?

There is a criteria mentioned in some of the standard but I do not recall now.
Remember our maintenance colleagues along with inspection make sure of that minimum tubes covered.

Minimum tubes to inspect = 10% (if ≤100 tubes) or 5% (if >100 tubes, but ≥10 tubes).
This reference is from ASME PCC-2 Article 312, over an above which you can apply risk based adjustments as per API 572.
Like what we do in critical services (flammable, toxic, or high-pressure systems), 20–30% minimum or even 100% of tubes, depending upon history, and the inspection test results.
No one wants to let the opportunity go wasted, and at the same time, overdoing would give nothing in return.

Thanks Irish for this reference.

The reference which we have come across most of the times is from API RP 572 which emphasizes risk-based inspection in this case.
The number of tubes inspected should be adjusted based on:

• Service severity (e.g., corrosive fluids, fouling tendency).
  Past inspection history.
  Criticality of the exchanger in the process.

Thanks. The references especially worked.