File: PVE-3856, Last updated June 6 2017 LRB / CT


Requirements for inspection openings are based on UG-46 with additional code passages, code cases and interpretations.

ASME VIII-1 Inspection Opening References

UG-46 should be read in its entirety – a few brief points:

  • UG-46(a) – Vessels with compressed air  or with corrosion, or with erosion, or with mechanical abrasion need inspection openings.  Exemptions:
    • Air dried to atmospheric dew point -50°F or lower, 
    • Shell side of fixed tube heat exchangers
    • Non-corrosive service
  • UG-46(b) – use of telltale holes instead of inspection openings
  • UG-46(c,d&e) – opening requirements and exemptions based on the diameter of the vessel
  • UG-46(f&g) – what qualifies as an inspection opening based on size of the vessel and the opening size
  • UG-46(h,i&j) – design of the opening

Other VIII-1 code passages related to inspection openings:

  • UG-32(o) and UG-33(j) – use the rules of UG-36 through UG-46 when designing openings in heads
  • UG-42(e) – threaded inspection openings size vs threaded nozzle size
  • UG-45 – exemption to mininum nozzle thickness requirements for inspection openings
  • UCI-36(a) and UCD-36(a) – UG-46 applies to inspection openings in cast vessels with thickness restrictions
  • UCL-25(b) – telltale holes instead of inspection openings in clad vessels
  • ULW-16(a) and ULW-18(a&c) – inspection openings in layered vessels
  • App 9-4(b) – inspection openings in jackets
  • M-2(b) – inspection openings shall be accessible

Code Cases

Code Case 2634 refers to exemptions to inspection openings:

  • 2634 – Exemptions to inspection opening requirements when the vessel is contained in a sealed container


Some interpretations from  Refer to the actual interpretations for the full text.  Caution- old interpretations remain even after they have been superseded by code changes that render them obsolete.

  • VIII-81-56 and VIII-77-11 – it is the users job to determine if a service is corrosive (caution: VIII-1-04-15 and VIII-1-07-63 define under what conditions air is considered non-corrosive)
  • VIII-1-04-15, VIII-1-07-63 and VIII-1-89-93 – exemption per UG-46(a) for air, dried to -50°F and not
  • VIII-1-86-31 – inspection openings are not required for non-corrosive service
  • VIII-1-83-264 – if openings are to be used for inspection, the plugs must be supplied by the manufacturer
  • VIII-1-83-336 – any size vessel can be non-corrosive and flexibility in sizing of openings
  • VIII-1-83-358 – what qualifies as an inspection opening (this interpretation is probably superseded by code changes made in 1987)
  • VIII-77-101 – inspection openings for heat exchangers and telltale holes (very old, probably superseded by code changes to UG-46 directly related to heat exchangers and use of telltale holes)
  • VIII-80-93 – configuration of threaded openings
  • rec 14-2223 – inspection nozzle size limits for cold-stretched vessels 
  • rec 15-748 – understanding UG-46 opening size requirements
  • VIII-1-01-122  – corrosive service requires inspection openings even if the corrosion allowance is zero
  • VIII-1-92-136, VIII-79-12 and VIII-1-95-115 – substitution of inspection nozzle sizes, and non-corrosive service
  • VIII-1-95-46 – if a corrosion allowance is specified, then the service is corrosive unless otherwise stated on the MDR


“ASME Section VIII – Division 1 Example Problem Manual” PTB-4-2013 provides example E4.5.5 which briefly discusses exemptions to UG-45 rules for inspection only openings.

Disclaimer: This material is provided for educational uses only. Only ASME can make code interpretations.