API 510 may provide some assistance. Section 6.3 gives information regarding external inspection intervals. The outer shell should be inspected at five year intervals. The equipment inside is usually inspected at ten year intervals, the maximun allowable by the 510 standard.

API will give you guidance on this, also check with your local jurisdiction to see if they have any special requirements for cold boxes. Normally you would only inspect the equipment inside a cold box if you open it for maintenance or repair (as evidenced from frost on the outside or LEL's from your probes). Vessels inside a cold box normally have welded manways and the aluminum plate exchangers can only be externally visually inspected or leak tested; LPI is futile. The only piping that you normally get concerned with are the aluminum/stainless transition joints (usually mfg. by Tube Turns). We have a VERY large box and have repaired it twice and modified it once in the last ten years. The failures were from thermal cracking in the exchanger cores causing cross over leaks between the passages and leakage into the box proper. This is the predominant type of failure in cold boxes as they are in a non-corrosive service. Mercury contamination can cause havoc in the aluminum plate exchangers from LME but normal plant design eliminates mercury sources except for raw feed. Contact the supplier of your cold box (Linde/Lotopro, Air Products, etc.), they'll have good info on inspection. If you need more info on plate exchangers, ALPEMA (aluminum plate exchanger manufacturers association) has some good literature. Good luck!