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STI SP001 Internal Tank Inspections: What You Need to Know

Learn more about this inspection standard for shop fabricated and small field-erected aboveground storage tanks.


For steel tanks used for storing various products and chemicals, including flammable or combustible liquids, the interior condition is critical to ensuring tank integrity. This is where internal inspections per the STI SP001 standard come in.

Regular internal inspections are vital to evaluate a tank’s true structural health and identify potential issues before they lead to catastrophic failures. If your tanks fall under the STI SP001 standard, here is what you need to know about these important internal inspections.

What is the STI SP001 Standard?

The STI SP001 inspection standard is a set of guidelines for the inspection of aboveground steel storage tanks used for storing flammable and combustible liquids. It was developed by the Steel Tank Institute (STI). The standard establishes inspection and evaluation procedures to determine if a steel aboveground storage tank can continue safe operation until the next scheduled inspection. The core purpose of performing inspections per SP001 is to thoroughly assess the current overall condition of the AST and identify any changes that have occurred since the previous inspection.

What type of tank requires an STI SP001 Internal Inspection?

The STI SP001 Standard is relevant for aboveground storage tanks (ASTs) that are up to 30 feet in diameter and up to 50 feet tall. It is most commonly used for the inspection of welded carbon or stainless-steel shop fabricated tanks with a capacity of up to 75,000 gallons, but may also include field-erected tanks within the same dimensions and up to 264,000 gallons. The ASTs can be horizontal or vertical, and built with single or double walls. The tanks are typically used to store liquids used in industry, including flammable, combustible, or hazardous materials.

If your tank has a manufacturer tank plate that states that it was manufactured to the Underwriters Laboratory Standard (UL) or some smaller American Petroleum Institute (API) standards, it likely could be inspected under the STI SP001 standard. You can also check for a standard certification plate. Owner purchase records should also help you determine if your tank is shop-fabricated or field erected.

What is the Purpose of the Internal Inspection?

The purpose of the internal inspection is to identify any corrosion or defects that could compromise the structural integrity of the tank bottom. This helps to ensure the safety of the tank and the people who work around it.

Conducting STI SP001 inspections also supports regulatory compliance for tank owners. The EPA's Spill Prevention, Control, and Countermeasure (SPCC) rule requires facilities to implement measures that prevent oil spills from aboveground storage tanks. Internal and external inspections per STI SP001 allow tank owners to thoroughly evaluate the condition of their ASTs and make repairs that reduce spill risks. Following the SP001 standard demonstrates due diligence in keeping tanks compliant with SPCC spill prevention regulations.

How often is a STI SP001 Internal Inspection Required?

Tank size and category helps determine internal inspection intervals. For example, Category 1 tanks with spill control and a continuous release detection method (CRDM) are never required to have an internal inspection in their lifetime.

Category 2 tanks with spill control, but no CRDM, are required to have internal inspections, but tank size will dictate the inspection intervals. For example, tanks that fall into Category 2 and are between 5,001 and 30,000 gallons require an Internal Inspection every 20 years, while tanks between 30,001 and 50,000 gallons are required every 15 years. 

If you have tanks that fall under Category 3 (no spill control, no CRDM), your inspection intervals increase significantly due to greater risk factors such as having the tank bottom touching earth or not having secondary containment and a leak detection system in place.

Formal external STI SP001 inspections are part of the formal internal inspection process. This is because the inspections are complementary and provide a more comprehensive assessment of the tank's condition.

Who conducts the inspection?

An internal inspection is a formal inspection that should only be conducted by certified personnel meeting the requirements outlined in STI SP001. They must hold proper confined space entry certifications and be an inspector, certified through Steel Tank Institute on inspection procedures.

Preparing and Planning the Inspection

Safety is paramount. When entering a confined space to conduct an internal inspection, personnel are required to follow OSHA’s 29 CFR 1910-146 “Permit Required Confined Space Entry” protocols. The team must utilize proper protective equipment and monitor the tank atmosphere for oxygen levels, toxicity, and other atmospheric hazards. Lighting equipment and cameras should be explosion-proof.

Before entry, the man way cover shall be removed and the tank shall be properly cleaned following OSHA safety guidelines.

Inspection Process

The inspection process will begin with a comprehensive visual examination of all external and internal surfaces. This visual inspection checks for corrosion, pitting, cracking, holes, leaks, discoloration, deformation or any damage.

Inspectors will conduct Ultrasonic Thickness Testing (UTT). This test precisely measures steel thickness in various areas. These measurements are compared to the minimum acceptable thicknesses based on design specifications. This shows if steel loss has occurred over time.

Detailed documentation is vital. Inspectors will take notes, make sketches, and take photographs and videos if necessary to record the tank condition. This information will be compiled by the inspector and will be utilized when creating their report of findings.

Areas of Focus

While inspecting the entire interior visually, the team pays particular attention to the following areas:

  • Floor: Inspect floor plates, weld seams, and any piping for any visual corrosion or imperfections. UTT testing will be conducted to determine any potential metal loss.
  • Shell & Heads (Horizontal Tanks): Inspect the tank shell plates and heads for corrosion, pitting, cracks, bulges, settlement, and deformities.
  • Shell (Vertical Tanks): Inspect the tank shell plates and heads for corrosion, pitting, cracks, bulges, settlement, and deformities.
  • Roof & Structure: Inspect roof plates for any indications of corrosion, holes, and cracks. Check weld seams and bolted connections.
  • Appurtenances: Check condition of vents, fittings, ladders, fasteners. Look for leaks at nozzle connections.
  • Welds/Seams: Check for cracks, pinholes or signs of failure at welds and seams throughout.

Inspection Findings & Repairs

Inspection findings are categorized based on severity. Minor damage includes surface corrosion and deficiencies that do not compromise integrity. Major damage impacts integrity, such as metal loss >10% of thickness. Critical damage represents an imminent tank failure risk if not addressed.

For major or critical damage, STI SP001 provides criterion on required repairs. This may involve corroded section replacement, reinforced patching, seam welding and more. Verification testing demonstrates repairs meet the design standard.

With detailed documentation, tanks can be certified as fit for continued service after successful repairs.

Importance of Routine Internal Inspections

Internal inspections per STI SP001 provide vital periodic evaluation of a tank’s structural stability and risks. Issues not apparent externally can be detected early and repaired. This significantly reduces the likelihood of catastrophic failures.

Conducting and documenting these inspections also meets regulatory compliance requirements in many regions related to aboveground storage tanks.

Optimizing tank life, reliability and performance requires a robust internal inspection program. This proactive approach minimizes disruptive emergency repairs and protects your assets.

The Bottom Line

For critical infrastructure like steel storage tanks, internal inspections deliver valuable insights that support integrity. STI SP001 provides the blueprint for qualified personnel to thoroughly examine the interior and identify any needed repairs. By sticking to routine internal inspections, tank owners can address problems early and ensure the safety of their operations.


-- Genesis Environmental Solutions provides API 653 and STI SP001 storage tank inspections throughout Illinois, Iowa, Kansas, Missouri, Nebraska and Oklahoma. We provide AST services for all types of tanks, including fuel oil tanks. All work is carefully documented and compliant with federal, state and local regulations. Contact us today for more information.

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