A summary of API’s 510 Inspection Frequencies:
(emphasis by me)
6.4 External Inspection
6.4.1 Unless justified by an RBI assessment, each aboveground pressure vessel shall have a visual external inspection at an interval that does not exceed the lesser of FIVE (5) years or the required internal/on-stream inspection. It is preferred to perform this inspection while the vessel is in operation. The interval is established by the inspector or engineer in accordance with the owner-operator’s QA system.
6.5.1 Inspection Interval
6.5.1.1 Unless justified by an RBI assessment, the period between internal or on-stream inspections shall not exceed one-half the remaining life of the vessel or 10 years, whichever is less. When the extent of thinning can be detected or effectively monitored externally, an internal inspection is not required at one-half the remaining life. Whenever the remaining life is less than four (4) years, the inspection interval may be the full remaining life up to a maximum of two years. The interval is established by the inspector or engineer in accordance with the owner operator’s QA system. Refer to 6.5.2 for guidance on when an on-stream inspection may be used to satisfy (in lieu of) an internal inspection requirement.
6.5.1.2 Unless justified by an RBI assessment, the period between thickness measurement inspections shall not exceed the lesser of one-half the remaining life of the vessel or 10 years. Whenever the remaining life is less than four (4) years, the inspection interval may be the full remaining life up to a maximum of two years.
6.5.1.4 For pressure vessels in noncontinuous service, the interval is based on the number of years of actual service (vessel in operation) instead of calendar years, provided that when idled, the vessel is:
a) isolated from the process fluids, and
b) not exposed to corrosive internal environments (e.g., inert gas purged or filled with noncorrosive hydrocarbons). Vessels in noncontinuous service and not adequately protected from corrosive environments may experience increased internal corrosion while idle. The corrosion rates should be carefully reviewed before setting the internal or on-stream intervals.
6.5.2 On-stream Inspection in Lieu of Internal Inspections
6.5.2.1 At the discretion of the inspector, an on-stream inspection may be substituted for the internal inspection in the following situations:
a) when size or configuration makes vessel entry for internal inspection physically impossible;
b) when vessel entry for internal inspection is physically possible and all of the following conditions are met:
1) the general corrosion rate of a vessel is known to be less than 0.125 mm (0.005 in.) per year;
2) the vessel remaining life is greater than 10 years;
3) the corrosive character of the contents, including the effect of trace components, has been established
by at least five years of the same or similar service;
4) no questionable condition is discovered during the external inspection;
5) the operating temperature of the steel vessel shell does not exceed the lower temperature limits for the
creep rupture range of the vessel material referenced in API 579-1/ASME FFS-1;
6) the vessel is not subject to environmental cracking or hydrogen damage from the fluid being handled;
7) the vessel does not have a nonintegrally bonded liner such as strip lining or plate lining.