Tank integrity testing is a critical element of every SPCC plan. Under 40 CFR 112.8(c)(6), facilities must assess the integrity of oil storage containers using testing methods and intervals established by industry standards. The two primary standards are STI SP001 for shop-fabricated tanks and API 653 for large field-erected tanks. This guide explains both standards, the testing methods they require, how inspection intervals are determined, and how SPCC software tracks it all automatically.
Common Integrity Testing Challenges
STI SP001: Shop-Fabricated Tanks
Overview and Scope
STI SP001 (Standard for the Inspection of Aboveground Storage Tanks) was developed by the Steel Tank Institute for shop-fabricated ASTs, typically under 50,000 gallons. It establishes a systematic approach to evaluating tank condition and determining appropriate inspection intervals based on the tank's construction, protection features, and historical performance.
Inspection Categories and Intervals
STI SP001 assigns tanks to inspection categories based on their level of protection. Each category has a defined maximum inspection interval:
- Unprotected tanks (bare steel, no corrosion protection) - Inspection interval of 5 years
- Tanks with corrosion protection (coating systems, cathodic protection, or corrosion-resistant materials) - Inspection interval of 10 years
- Tanks with corrosion protection AND release prevention (double-wall construction, release detection, or equivalent) - Inspection interval of 15 years
- Tanks with cathodic protection AND release prevention - Inspection interval up to 20 years
These are maximum intervals. The actual interval may be shortened based on inspection findings, corrosion rates, or site-specific risk factors.
API 653: Field-Erected Tanks
Overview and Scope
API 653 (Tank Inspection, Repair, Alteration, and Reconstruction) applies to large field-erected welded storage tanks built to API 650 or its predecessor API 12C. These are typically large-diameter tanks (often 50,000+ gallons) used in refineries, terminals, and bulk storage facilities. API 653 establishes requirements for both external and internal inspections.
External Inspection Intervals
API 653 requires external inspections at intervals not exceeding 5 years or one-quarter of the remaining corrosion life based on the measured corrosion rate, whichever is less. External inspections evaluate the tank shell, roof, foundation, appurtenances, and secondary containment from the outside. They do not require the tank to be taken out of service.
Internal Inspection Intervals
Internal inspections require the tank to be emptied, cleaned, and confined-space entry procedures followed. The maximum interval is determined by the corrosion rate calculation:
- Measure remaining shell and floor thickness via ultrasonic testing
- Compare to the minimum required thickness for the tank design
- Calculate the corrosion rate (mils per year lost)
- Determine remaining life: (current thickness - minimum thickness) / corrosion rate
- Set next internal inspection at no more than one-half the remaining life or 20 years, whichever is less
Testing Methods
Visual Inspection
The foundation of every integrity assessment. External visual inspections check for corrosion, coating degradation, structural deformation, foundation settlement, leaks, and weld condition. Internal visual inspections (when the tank is emptied) check floor plate condition, bottom-side corrosion, internal coating integrity, and sediment accumulation. Visual findings guide the selection of additional testing methods.
Ultrasonic Thickness Testing (UT)
The most widely used non-destructive testing method for tanks. A handheld probe measures the remaining wall or floor thickness at specific grid points. Results are compared to the original or minimum required thickness to calculate corrosion rates and remaining life. UT is used during both STI SP001 and API 653 inspections and requires ASNT Level II-certified technicians.
Magnetic Flux Leakage (MFL)
A robotic or manual scanning method used primarily for tank floor inspection. An MFL scanner magnetizes the floor plate and detects anomalies (pitting, corrosion, thinning) by measuring changes in the magnetic field. MFL can scan large floor areas rapidly and is commonly used during API 653 internal inspections of large tanks.
Hydrostatic Testing
The tank is filled with water to its maximum operating level and held for a specified period (typically 24-72 hours) while monitoring for leaks, settlement, or structural distress. Hydrostatic testing is typically used for new tank commissioning, tanks returning to service after repair, or when other methods cannot adequately assess tank integrity.
Additional Testing Methods
- Vacuum box testing - Applies a vacuum to weld seams and floor-to-shell junctions while applying soapy solution. Bubbles indicate leaks. Used for weld integrity verification
- Acoustic emission (AE) testing - Sensors detect stress waves from active corrosion, cracking, or leaking while the tank remains in service. Useful for screening and prioritization
- Radiographic testing (RT) - X-ray or gamma-ray imaging of welds to detect internal defects. Used primarily for weld quality verification on critical joints
Documentation Requirements
Ecesis SPCC Software
SPCC Software
STI SP001 and API 653 logic built in for automated testing schedule management.
Inspection Software
Schedule, document, and track all tank inspection types and findings.
Preventive Maintenance
Track tank repairs, coating maintenance, and cathodic protection systems.
Employee Training
Track inspector certifications (STI SP001, API 653, ASNT NDT levels).
Compliance Obligations
Manage testing deadlines alongside all other regulatory obligations.
Mobile EHS App
Record inspection findings and testing results from the field.
