API 653 Tank Inspection Checklist (Annex C) Walkthrough

Short answer: API 653 includes sample checklists in Annex C (Appendix C in older editions). There is one for in-service inspection (C.1) and one for out-of-service inspection (C.2). They are lists of items to consider, not mandatory forms. The in-service list covers the foundation, shell, roof and appurtenances while the tank operates. The out-of-service list adds the bottom, the inside of the shell, the roof structure, floating-roof components and internal piping. The authorized inspector decides what applies and signs the report.

This page walks through the structure of both checklists section by section. For each group of items it explains what the inspector is looking for, which NDE or survey method confirms it, and who normally does that part. We summarize in our own words. Annex C is copyrighted, so we do not reproduce it; use your licensed copy of the current API 653 edition as the form. API 653 is in its 5th edition with addenda, and Addendum 4 (July 2025) revised text on tank-bottom thickness measurement. Section numbering below follows editions we could verify. Check the current edition before you copy a reference into a procedure.

Preparing for a tank outage? Ask for an NDE scope and quote for your out-of-service inspection. We reply within 24 hours.

What the Annex C checklists are, and what they are not

Direct answer: Annex C gives sample checklists of items to consider during in-service and out-of-service inspections. API 653 calls them samples. An owner can adapt them, combine them with site-specific items, or use its own forms, as long as the inspection covers what the code and the tank need.

The checklists help in three ways. They prompt the inspector to cover every component. They give a common structure for reports across a tank farm. And they show auditors that the inspection was systematic. But a ticked checklist is not an inspection report. API 653 expects a report with measurements, findings, calculations, recommendations and the next inspection dates. The checklist sits behind that report as a working record.

Three kinds of inspection use checklist items:

• Routine in-service inspection. A visual check from the ground by owner or operator personnel, at an interval of no more than one month. These people do not need to be authorized inspectors, but they must know the storage facility and the signs of trouble. Findings go to an authorized inspector for follow-up.
• External inspection. A formal visual inspection by an authorized inspector while the tank stays in service. The maximum interval is 5 years, or a shorter interval based on the shell corrosion rate. Ultrasonic shell thickness measurement runs on its own schedule.
• Internal (out-of-service) inspection. The tank is emptied, cleaned and gas-freed, and the bottom is examined. The interval comes from the bottom's minimum remaining thickness and corrosion rates, or from an RBI assessment, within the maximums the code sets.

Our guide to API 653 inspection intervals covers the interval rules in detail.

In-service checklist (C.1): foundation and shell

Direct answer: the in-service foundation items look for settlement, washout, cracking or spalling of the ring wall, and water pooling at the tank's edge. The shell items look for leaks, distortion, coating breakdown, corrosion and pitting, and problems at nozzles and welds. Most of this is visual. UT thickness readings back up any suspected thinning.

Foundation. The inspector walks the full perimeter. Typical items are: settlement visible around the tank; cracking or spalling of a concrete ring wall; voids under the bottom edge; erosion of the pad; vegetation and water pooling against the chime; and condition of the seal between the bottom projection and the foundation. Water trapped against the chime drives soil-side corrosion of the bottom edge. A drainage problem found from the outside often predicts floor damage found later from the inside. Where settlement is suspected, the inspector asks for a survey. API 653 Annex B covers settlement evaluation, explained in our article on tank settlement measurement and acceptance.

Shell. Items cover visible leaks; shell distortion, buckling and peaking or banding at welds; paint and coating failures; corrosion, pitting and grooving, especially at the bottom course and around the wind girder; condition of welds; insulation, if any, and its weather seal; and signs of corrosion under insulation. Nozzles and manways are checked for leaks, reinforcing-plate tell-tale holes, cracked welds and distortion from piping loads. Product-specific mechanisms, such as caustic cracking or corrosion at the liquid line, add items for the inspector to look for.

NDE behind the items. Visual inspection (VT) is the base method. UT thickness gauging confirms shell thinning and supplies the corrosion rate used for the external and UT intervals. Magnetic particle (MT) or penetrant (PT) testing confirms suspected cracks at nozzle welds or reinforcing-plate edges. Where the shell is insulated, the inspector may ask for insulation removal at suspect points or a screening method first.

In-service checklist (C.1): roof and appurtenances

Direct answer: the roof items cover deck condition, drainage, low spots, corrosion and, for floating roofs, seal condition and the level of the roof. The appurtenance items cover gauge wells, sample hatches, vents, emergency roof drains, ladders and platforms. Several items need a gas test before anyone steps onto a roof.

Fixed roofs. The inspector looks for deck corrosion and pitting, holes, low spots that hold water, and condition of the roof-to-shell joint. Before walking on a fixed roof, its condition is checked from the edge or with UT readings. A corroded deck is a fall hazard, and safe access is part of the inspection plan.

External floating roofs. Items include the primary and secondary seals (gaps, tears, wear and contact with the shell), the roof's level and buoyancy, pontoon condition, roof drains, the rolling ladder and its track, and standing water or product on the deck. Pontoon inspection hatches are opened to look for liquid in the pontoons, which indicates a leak.

Internal floating roofs. Annex C includes items for viewing through roof hatches and for gas testing the space above an internal floating roof. A flammable atmosphere there may indicate seal or deck leakage. Entry rules follow the owner's confined-space and hot-work procedures.

Appurtenances. Typical items are gauge wells and gauge poles; automatic gauge components; sample hatches; pressure-vacuum vents and flame arrestors; emergency roof drains; thief hatches; stairways, platforms and handrails; grounding and shunts; and cathodic protection test points where present. Many of these are operational as much as structural. A stuck vent or a blocked roof drain can damage a tank faster than corrosion.

NDE behind the items. VT, roof deck UT where walking is planned, and a gas test. Our page on aboveground storage tank inspection describes how these are scoped.

Out-of-service checklist (C.2): exterior, bottom and shell interior

Direct answer: the out-of-service list starts with an overview and the tank exterior, then focuses on what can only be seen with the tank empty. That means the bottom's product side, the inside of the shell, welds and the bottom-to-shell corner. Floor scanning and UT supply the numbers the inspector needs for the bottom calculation.

Overview and exterior. The list begins with records review: previous reports, design data, repair history and settlement data. It then repeats the external checks with the tank empty, which often gives better access to the chime and foundation.

Bottom interior surface. Items cover the condition of the bottom plates and lap welds; pitting and general corrosion on the product side; signs of soil-side corrosion, such as dishing or holes; coating or lining condition; bulges and settlement; the sump and its welds; and the critical zone near the shell. Key steps include:

• Floor scanning. Magnetic flux leakage (MFL) scanning screens most of the floor for product-side and soil-side wall loss. Indications are then sized with UT. See magnetic flux leakage testing and our comparison of MFL vs UT floor scanning.
• UT follow-up. UT measures remaining thickness at pits and confirms soil-side loss found by MFL. Edges, the sump and areas near the shell that a scanner cannot reach are covered by manual UT.
• Weld examination. Vacuum box testing finds leaks in bottom welds. MT or PT checks the shell-to-bottom weld and repairs.
• Calculation. The inspector uses the findings to calculate minimum remaining thickness for the next interval, explained in our guide to tank floor pitting and minimum remaining thickness.

Shell seams and plate. Inside the tank, the inspector checks shell plates for corrosion at the liquid line and the bottom course, weld condition, and cracks around shell penetrations. UT thickness readings and, where required, MT or PT on welds back these up.

Out-of-service checklist (C.2): roofs, floating roofs and common appurtenances

Direct answer: these sections cover the roof structure from below, floating-roof pontoons, legs and seals, floating-roof drains and vents, and internal nozzles, swing lines, diffusers, heating coils and access structures. Most items are visual, with UT and leak testing where the inspector needs a number.

Fixed roof and its appurtenances. From inside, the inspector looks at rafters, girders, columns and the roof plate's underside for corrosion, sagging and loose connections. Column bases on the floor are checked for corrosion and for whether the floor below them is sound. Vents, gauge wells and other roof nozzles are checked from inside.

Floating roof. Items cover pontoons (leaks, corrosion, liquid inside), deck plates and welds, support legs and their landing pads, seals and their mechanisms, and anti-rotation devices. Floating-roof appurtenances include roof drains and their flexible hoses or articulated joints, vents, the autogauge, and ladders.

Common appurtenances. Internal nozzles and their reinforcement, swing lines and their winches, diffusers, heating coils, mixers, water draw-offs and sumps. Access structures such as stairways, platforms and rolling ladders are also covered.

NDE behind the items. VT throughout; UT on roof plates, pontoons and nozzles where thinning is suspected; vacuum box on deck welds where leaks are suspected; and hydrostatic or pneumatic tests on roof drains as the owner specifies.

Checklist sections mapped to methods and roles

The table below links each checklist area to the method that confirms it and to who usually does the work. The authorized inspector decides scope and completes the report in every case.

API 650 vs API 653 checklists

Direct answer: API 650 governs design and construction of new welded tanks, so its "checklist" is a construction inspection and testing record. API 653 governs tanks after they enter service. Use API 650 items for a new build and API 653 Annex C items for in-service tanks. Repairs and reconstruction under API 653 refer back to API 650 for many requirements.

A construction inspection under API 650 typically records material verification, fit-up and dimensional checks, weld examination (including radiography of shell butt welds as the standard requires), vacuum box or other leak testing of bottom welds, and the hydrostatic test. The tank's shape, such as plumbness and roundness, is checked against API 650 tolerances. None of that is in Annex C, because Annex C assumes the tank already exists. Our comparison of API 650 vs API 653 explains where one code hands over to the other.

Smaller shop-built tanks are often inspected to STI SP001 rather than API 653, especially at facilities covered by EPA's SPCC rule. STI SP001 has its own checklists for periodic owner inspections. The SPCC rule (40 CFR 112.8(c)(6) for onshore facilities) requires integrity testing of bulk storage containers on a regular schedule following industry standards. Which standard you use depends on tank type and size. Confirm with your SPCC plan and Professional Engineer.

Common mistakes and the regulatory overlay

Direct answer: the most common checklist failures are treating the sample as the whole inspection, ticking items with no measurement behind them, skipping the monthly walkdown record, and not linking findings to follow-up. Regulators look for a programme that produces records, not just forms.

• Checklist without numbers. "Bottom OK" with no MFL map or UT readings gives the inspector nothing to calculate from.
• Undocumented monthly inspections. Operators often do the walkdown but keep no record. Keep a simple signed log.
• Findings with no owner. Each item that needs action should become a tracked recommendation with a due date.
• Old forms. Copies of the 2003-era checklist still circulate. Update to the current edition and addenda.
• Wrong standard. Using API 653 forms on a small shop-built tank, or STI forms on a large field-erected tank, causes confusion with auditors.

Regulatory drivers vary. EPA's SPCC rule covers oil storage at many facilities. OSHA PSM (29 CFR 1910.119(j)) applies where tanks are part of a covered process. PHMSA regulations under 49 CFR 195 reference API 653 for breakout tanks on hazardous liquid pipelines. States add their own rules. In Canada, provincial fire codes and environmental regulations govern storage tanks, and CSA and ULC standards apply to some tank types. Confirm the requirements with your jurisdiction.

How Atlantis supports this

Atlantis performs the NDE that backs the checklist items. Our ASNT-certified technicians, under ASNT Level III oversight, perform MFL floor scanning with UT follow-up, UT shell and roof thickness surveys, MT and PT on welds, vacuum box testing support and VT. We deliver data in the form your inspector needs for the bottom and shell calculations. Your API 653 authorized inspector stays inspector of record, completes the checklist and report, and sets the next intervals; Atlantis does not sign API 653 reports or approve repairs. See storage tank inspection and our API 653 inspection support. Request a tank NDE quote; we reply within 24 hours.

FAQ

Is there an official API 653 tank inspection checklist?

API 653 Annex C has sample checklists for in-service and out-of-service inspections. They are samples of items to consider, not mandatory forms. Owners can adapt them or use their own forms that cover the same ground.

What does the API 653 inspection checklist cover?

The in-service list covers the foundation, shell, roof and appurtenances. The out-of-service list adds the bottom, shell interior, roof structure, floating-roof components, internal nozzles and piping, and access structures.

Who can complete an API 653 checklist?

Owner or operator staff do the monthly routine in-service walkdown. External and internal inspections are done by an API 653 authorized inspector, who completes and signs the report. NDE technicians supply measurements.

Is there an API 650 tank inspection checklist?

API 650 covers new construction, so its inspection records focus on materials, welding, weld examination, leak testing of bottom welds, dimensional tolerances and the hydrostatic test. Use API 653 for tanks in service.

How often is the routine in-service inspection done?

At an interval of no more than one month, as a visual check from the ground by personnel familiar with the facility.

Can I download the Annex C checklist?

Annex C is part of a copyrighted standard. Buy API 653 from API or an authorized distributor and use the current edition's checklist. Avoid unofficial copies, which are often older editions.

Which NDE methods support an out-of-service checklist?

MFL floor scanning with UT follow-up, UT thickness on shell, roof and nozzles, vacuum box testing of bottom welds, and MT or PT on the shell-to-bottom weld and repairs.

Does the checklist set the next inspection date?

No. The authorized inspector calculates intervals from measured corrosion rates and code limits, or from an RBI assessment the owner commissions, and records them in the report.

Do shop-built tanks use the API 653 checklist?

Often not. Many shop-built tanks are inspected to STI SP001, which has its own periodic inspection checklists. Your SPCC plan should name the standard used.

What changed in API 653 Addendum 4?

Addendum 4 (July 2025) revised text on tank-bottom thickness measurement. Check the current edition and addenda in your licensed copy before updating procedures.

Scoping the NDE for an upcoming outage? Send us your tank list and last report and our Level III team will propose a method plan.

Related reading: API 653 tank inspection guide.