API 653 Inspection for Liquid Fertilizer and Agricultural Storage Tanks

Short answer: No single federal rule sets inspection intervals for liquid fertilizer tanks, but the industry benchmark is The Fertilizer Institute's (TFI) Recommended Mechanical Integrity Practices, which apply API 653 to large (100,000 gallon and up) non-pressurized fertilizer tanks with fertilizer-specific changes. TFI suggests a baseline API 653 inspection, monthly in-house walk-arounds, external inspection by an API authorized inspector at the lesser of five years or the corrosion-rate interval, and internal inspection at no more than ten years unless a risk-based evaluation supports longer. Several states add their own rules.

Liquid fertilizer terminals, agricultural retail sites and co-op storage yards often run tanks that were built for oil, converted from other service, or put up decades ago with riveted seams. API 653 was written for petroleum storage, but it is the only widely recognised in-service standard for welded and riveted steel tanks, and the fertilizer industry has adopted it as the backbone of its integrity programme. This guide explains how API 653 is applied to urea ammonium nitrate (UAN), ammonium polyphosphate and other liquid fertilizer tanks, what is different from petroleum service, which NDE methods are used where, and what owners should confirm with their state and their authorized inspector.

Why fertilizer tanks get API 653 inspections at all

The trigger was a run of costly fertilizer tank failures in the 1990s. TFI formed a Tank Integrity Work Group in 2000 and issued inspection guidelines in 2001. On November 12, 2008, a liquid fertilizer tank at Allied Terminals in Chesapeake, Virginia split open vertically and released roughly two million gallons of UAN solution; two workers were seriously injured and some of the product reached the Elizabeth River. The U.S. Chemical Safety Board (CSB) found that contractors had removed the tank's vertical riveted seams and replaced them with welded plates, that the welding did not conform to recommended industry practice, and that post-weld inspection had not been done before the tank was refilled to its maximum level. CSB recommended that TFI do more to promote its guidelines, and in December 2009 TFI issued the updated document, Aboveground Storage Tanks Containing Liquid Fertilizer: Recommended Mechanical Integrity Practices, which remains the reference most owners and inspectors work from.

The lesson from Chesapeake for anyone running a fertilizer programme is that the dangerous moment is often after a repair, not during routine service. A shell modification that changes joint efficiency, a new plate, or a seam conversion has to be examined to the extent the fill-height calculation assumes, before the tank goes back to full height.

What the TFI practices say, in plain terms

TFI's general recommendation is that new liquid fertilizer tanks should be designed and built to API Standard 650 and that existing tanks should be inspected based on API Standard 653, modified for the characteristics of liquid fertilizer. The practices cover flat-bottom, vertical, cylindrical, fixed-roof tanks of 100,000 gallons or more built from carbon steel, stainless steel or aluminum. They do not cover refrigerated anhydrous ammonia storage, which is a different class of equipment with its own standards.

Key points from the document:

• Unknown or non-API construction is not an excuse. For tanks of unknown design or built to criteria other than API 650, inspection should follow API 653 as far as possible, and an authorized inspector (or an authorized inspector working with an experienced storage tank engineer) may modify the inspection to account for construction details that differ from API 650. The result should be equivalent to an API 653 inspection.
• Jurisdiction governs. Where a state statute or regulation is stricter, it applies; where TFI's practices are stricter, TFI recommends following its practices.
• Repairs follow the original standard. Weld procedure qualification and shell repair materials should be based on the standard the tank was built to; where that is unknown, use API 653 with the impact toughness requirements of API 650.
• Specific gravity matters. Liquid fertilizer usually has a specific gravity above 1.0, and this must be accounted for when evaluating shell thickness and allowable fill height.
• Flammability provisions can be adapted. Because fertilizers are generally non-flammable, API 653 flammability provisions may be modified, except where hydrocarbons are used as an interface seal.
• Responsibility stays with the owner. TFI states that it is ultimately the tank owner and operator's responsibility, in consultation with an authorized inspector, to implement appropriate inspection and maintenance protocols.

Inspection tiers and intervals

The TFI practices set out an inspection framework intended to be consistent with API 653. The table summarises it; confirm against the current TFI document and any state rule that applies to your site, and remember that the actual interval for a given tank is set by the owner and the authorized inspector based on corrosion rates and condition, not by a table.

TFI also notes that monthly visual and yearly external checks can be done by an in-house inspector, while all formal internal and external API 653 inspections should be performed by an API authorized inspector. Under the definitions in the TFI document, an authorized inspector is employed by an authorized inspection agency, which can be the jurisdiction, an insurer, the owner's own inspection organisation, or an independent contractor using API-certified inspectors, as the local jurisdiction allows. For how API 653 sets intervals in petroleum service, see our guide to API 653 tank inspection intervals.

Where fertilizer tanks differ from petroleum tanks

A UAN or phosphate tank fails in different places from a crude or diesel tank, and the inspection plan should reflect that. The TFI practices highlight several fertilizer-specific concerns:

• Welds and heat-affected zones. TFI singles out the condition of welds and weld HAZs as being of particular concern because these areas tend to be more commonly affected by exposure to liquid fertilizers. Internal inspection should look closely at shell-to-bottom welds, vertical and horizontal shell seams in the lower courses, and bottom lap welds.
• Soil-side bottom corrosion. TFI calls soil-side corrosion a potential problem area worth additional inspection, and recommends avoiding gravel bases for new construction because they can accelerate soil-side attack; sand bases with impressed current protection or, preferably, concrete foundations are preferred. When the product side looks good, methods such as ultrasonic thickness testing, magnetic flux leakage floor scanning or test coupons are used to find underside loss.
• Roof and rafters. Vapours can corrode rafter-to-roof joints on internally supported roofs, so those joints should be inspected.
• Material compatibility. TFI advises against copper and brass in contact with liquid fertilizer, notes that aluminum is not compatible with some products (including phosphate-based fertilizers and those containing potassium chloride), and states that unlined carbon steel is generally not acceptable for acidic solutions below pH 6.0, with exceptions such as highly concentrated sulfuric acid. Always confirm compatibility with the product supplier.
• Insulation. Where tanks are insulated, moisture combined with chloride-bearing insulation can drive corrosion under insulation on carbon and stainless steels; jackets must be maintained.
• Riveted and older tanks. Many agricultural tanks are old riveted tanks or have been converted from other service. API 653 includes provisions for evaluating riveted shells, but conversions from riveted to welded seams are high-risk repairs that change the joint efficiency used in fill-height calculations, as Chesapeake showed.

Coatings, bladders and the internal inspection

Fertilizer tanks are frequently lined with coatings (epoxy systems, fibreglass and similar) or fitted with bladders, which are non-adhering synthetic liners attached to the tank to separate the product from the shell and bottom. Linings change what an internal inspection can see and how often it should happen.

TFI recommends that tanks receive an inspection by an API authorized inspector immediately before a coating is applied or a bladder installed, unless they are new and were inspected to the construction standard. Protrusions should be ground smooth and weld defects corrected before lining, because sharp edges and defects puncture bladders and cause coating failures. Lining records (manufacturer, resistance properties, installation date, attachment method, installer) should be kept in the permanent tank file.

For bladders, TFI's inspection criteria include checking flexibility, discoloration and surface cracking, the support system, interface connections at flanges, roof columns and manways, the sump boot and boot-to-bottom weld, leak monitoring port valves, and whether the operating level has exceeded bladder design height. For coatings, the criteria include cracks, blistering, peeling and discoloration, consideration of a holiday test to find defects not visible to the eye, and close attention to transitions between coated and uncoated areas and across dissimilar materials. After any bladder leak repair, TFI recommends flushing the bottom under the bladder with clean water to remove corrosive product.

A practical point for planning: lined tanks still need the steel behind the lining assessed. Where a lining is sound, owners may rely on external UT and floor scanning results plus lining inspection; where the lining has failed locally, the area should be cleaned to bare metal and the steel examined and repaired under API 653 before re-lining.

NDE methods used on fertilizer tanks

The NDE toolkit is the same as for petroleum tanks; what changes is where you aim it.

The bottom is where most fertilizer tank leaks start, so floor scanning deserves a careful scope: decide coverage (full floor, critical zone, annular ring), the threshold for UT prove-up, and how results will be fed into the authorized inspector's remaining-thickness evaluation. Our comparison of MFL vs UT for tank floor scanning and the guide to tank floor pitting and remaining thickness go further. If you need a floor scan and shell survey scoped before your next outage, ask for a floor-scan scope and quote. Note that API 653 Addendum 4 (July 2025) revised text on tank-bottom thickness measurement, so check your inspection plan against the current edition and addendum.

Suitability for service, fill height and records

TFI expects every fertilizer tank to be evaluated for suitability for continued service, considering inspection reports, shell thickness, joint efficiencies and the product's specific gravity. Because fertilizer is denser than water, the hydrostatic head on the lower courses at a given fill height is higher than for most petroleum products, so a tank that was adequate in diesel service may need a lower maximum liquid level in UAN. The API 653 shell evaluation, including the joint efficiency appropriate to the radiography actually performed, drives that limit. This is a calculation the authorized inspector or tank engineer performs; the NDE contractor's job is to deliver accurate thickness and weld examination data to support it.

TFI lists the records that should be part of the tank file when a tank is evaluated, repaired, altered or reconstructed. In summary: integrity and re-rating calculations (including liquid level and brittle fracture consideration), repair and alteration calculations and drawings, inspection reports with thickness readings, material test reports, test results, radiographs and their reports, original construction data, tank identification and description, design conditions (liquid level, specific gravity, allowable stress), shell material and thickness by course, perimeter elevations, completion records, the basis for any hydrostatic test exemption, and the foundation type. Leak detection monitoring records should be kept for the life of the tank. Perimeter elevation surveys feed the settlement evaluation described in our guide to API 653 settlement measurement.

State rules and the regulatory overlay

Liquid fertilizer is not oil, so the EPA Spill Prevention, Control and Countermeasure (SPCC) rule, which drives many petroleum tank programmes through STI SP001 or API 653, generally does not reach a fertilizer tank unless oil is also stored there. Instead, regulation sits mostly with the states, typically through the state department of agriculture or environmental agency, and covers some combination of secondary containment, tank construction standards, inspection frequency and record keeping. Rules vary widely by state, by tank size and by whether the site is a terminal, a manufacturer or a retail dealer. After Chesapeake, CSB recommended that the Commonwealth of Virginia regulate the design, construction, maintenance and inspection of these tanks.

Because the requirements differ so much, the practical advice is simple: before setting your programme, check the current rules of the state agriculture department or environmental regulator where each tank sits, check any requirements in throughput or storage contracts (TFI encourages members to write its practices into storage contracts), and confirm with your insurer. Where a state rule is stricter than the TFI practices, the state rule governs. Canadian sites should check provincial environmental and fire code requirements, which vary by province, and any industry codes their fertilizer association applies.

Common mistakes on fertilizer tank programmes

• Treating an old riveted or converted tank as "not API" and skipping the inspection instead of applying API 653 as far as possible.
• Returning a tank to full height after shell repairs without completing the weld examination the fill-height calculation assumed.
• Using a petroleum-service maximum liquid level without recalculating for the fertilizer's specific gravity.
• Inspecting the lining but not the steel behind it, or relining over unrepaired weld defects and sharp edges.
• Scanning the floor but not proving up indications with UT, leaving the inspector without usable remaining-thickness data.
• Letting leak detection monitoring lapse; TFI recommends at least annual monitoring with records kept for the tank's life.
• Assuming no federal rule means no rule; state requirements and contract terms often apply.

How Atlantis supports this

Atlantis NDT performs the NDE that fertilizer and agricultural tank programmes need: shell UT thickness surveys and scans, MFL tank floor scanning with UT prove-up, weld examination by MT, PT, radiography and ultrasonic testing after repairs, and visual examination, by ASNT-certified technicians under ASNT Level III oversight. We deliver the data to your API authorized inspector, who remains the inspector of record and makes the suitability, fill-height and interval decisions. We do not perform RBI evaluations. See our aboveground storage tank inspection services and API 653 NDE support. Quotes within 24 hours: request a fertilizer tank NDE quote.

Frequently asked questions

Does a liquid fertilizer tank need an API 653 inspection?

No federal rule requires it, but TFI's industry practices recommend API 653-based inspection for non-pressurized fertilizer tanks of 100,000 gallons or more, and several states have their own requirements. Many storage contracts and insurers also expect it.

What is API 653 fertilizer tank inspection?

It is an in-service inspection of a fertilizer storage tank carried out to API 653, with fertilizer-specific additions from the TFI practices: attention to weld and HAZ corrosion, soil-side bottom corrosion, linings and bladders, material compatibility and the product's higher specific gravity.

How often should an API 653 agricultural tank inspection be done?

TFI suggests monthly in-house walk-arounds, formal external inspection by an API authorized inspector at the lesser of five years or the corrosion-rate interval, and internal inspection at no more than ten years unless an RBI evaluation supports longer. State rules may be stricter.

Who can perform an API-653 fertilizer tank inspection?

Formal internal and external API 653 inspections and the baseline inspection should be done by an API authorized inspector. Monthly and yearly visual checks can be done by the owner's in-house inspector. NDE contractors provide the examination data.

Do the TFI guidelines apply to anhydrous ammonia tanks?

No. The TFI practices exclude refrigerated liquid ammonia storage tanks and cover non-pressurized liquid fertilizer tanks.

Can a riveted fertilizer tank be inspected to API 653?

Yes. TFI recommends applying API 653 as far as possible to tanks of unknown or non-API 650 construction, and API 653 contains provisions for riveted shells. Seam conversions and shell repairs need particular care.

How are coated or bladder-lined fertilizer tanks inspected?

Inspect the tank before lining, then inspect the coating or bladder within two years of installation and at five-year intervals if there are no failure indications, with internal API 653 inspections at the frequencies the code or an RBI assessment sets.

Why does specific gravity matter for fertilizer tanks?

Liquid fertilizer is usually denser than water, so the lower shell courses see more pressure at a given fill height. The maximum liquid level must be calculated for the actual product.

What NDE finds bottom leaks in fertilizer tanks?

MFL floor scanning with UT prove-up is the usual approach for product-side and soil-side pitting; TFI also mentions test coupons. Leak detection systems give early warning between inspections.

Planning a baseline or internal inspection? Send us the tank list and we will scope the shell, floor and weld NDE for your authorized inspector.

Sources: The Fertilizer Institute, Aboveground Storage Tanks Containing Liquid Fertilizer: Recommended Mechanical Integrity Practices (December 2009); U.S. Chemical Safety Board, Allied Terminals investigation; API 650 and API 653 (current editions, available from API). Confirm against current editions and your state's rules.