What Is Inspection?
Storage tank inspection is the assessment of the structural integrity of tanks holding liquids or gases. It may be carried out externally (in-service) or internally after the tank has been emptied and cleaned (out-of-service); the scope differs markedly between the two.
Why Is It Done?
The aim is to prevent leakage and structural collapse, to estimate remaining life and to set the next inspection interval. On storage tanks the most critical area is the bottom plating, which cannot be observed directly.
Which Equipment Is Covered?
- Atmospheric storage tanks
- Fuel and chemical storage tanks
- Underground tanks
- Water storage tanks
- Pressurised storage vessels
Legal Basis and Interval
There is no single universal interval. The inspection interval follows the most restrictive of three sources: the maximum period set by the applicable work-equipment legislation, the instructions in the manufacturer's manual, and the operator's risk assessment. Heavy duty, corrosive environments or shift work shorten it. The next inspection date is stated in the report.
Scope of Inspection
- Shell plates: wall thickness, corrosion, deformation
- Floor plates: underside corrosion, settlement and deformation
- Roof: structure, fixed/floating roof seal, drainage
- Weld seams and nozzle connections
- Anchorage and foundation settlement, level surveys
- Venting and pressure/vacuum valves
- Cathodic protection system (if present) and its effectiveness
- Coating and paint system condition
- Secondary containment (bund) and drainage arrangement
- Level gauges and overfill protection
Related Standards
- ISO/IEC 17020
- EN 14015
- API 653
- EN 12285
- ISO 9712
Method and Equipment
- Ultrasonic thickness gauge
- Magnetic flux leakage (MFL) scanner (for floor plates)
- Vacuum box (for floor-seam tightness)
- Levelling and settlement measuring equipment
- Holiday detector (for coating inspection)
- Cathodic-protection potential meter
Inspection Process
- 1Preliminary Information and PlanningThe equipment inventory, capacity data, manufacturer's instructions and previous inspection reports are collected; the inspection date and site conditions are planned.
- 2Document ReviewThe declaration of conformity, user manual, maintenance records and, where applicable, records closing out earlier nonconformities are reviewed.
- 3Visual InspectionThe load-bearing structure, connections, guards and warning markings are checked visually; wear, deformation and corrosion are assessed.
- 4Function and Safety Device CheckSafety devices such as brakes, limit switches, emergency stop and overload protection are verified by operating them.
- 5Measurement and Load TestDepending on the type of equipment, a load test, pressure test or electrical measurements are carried out. Test values are set according to the manufacturer's limits and the relevant standard.
- 6Evaluation and ReportingFindings are graded as conforming / non-conforming / conditionally conforming; the report and the inspection label are issued.
- 7Follow-upIf nonconformities exist, a follow-up inspection is carried out once they have been corrected; the next inspection date is stated in the report.
Nonconformities
Shell or bottom thickness falling below the calculated minimum calls for repair or for restriction of the operating conditions. Loss of tightness in the bottom welds and excessive settlement are also serious findings.
Inspection Report
- Details of the inspection body and the inspection personnel
- Equipment identification: make, model, serial number, capacity, year of manufacture
- Standard applied and scope of inspection
- Measuring instruments used and their calibration data
- Findings of the visual inspection and function check
- Load / pressure test values
- Nonconformities identified and their severity
- Conclusion (conforming / non-conforming / conditionally conforming) and the next inspection date
Accreditation
Inspection activities are carried out under ISO/IEC 17020, which requires the impartiality of the inspection body, the competence of its personnel and the validity of the inspection methods. Under the impartiality rule, a party that maintains or repairs an item of equipment cannot carry out the independent inspection of that same equipment. EUROLAB holds internationally recognised accreditation under ISO/IEC 17020, granted by accreditation bodies that are signatories to the ILAC mutual recognition arrangement; because accredited scope is defined per inspection type, it is confirmed in writing at the quotation stage.
Scope and authorisationsRelated Inspections
Frequently Asked Questions
Can the tank be inspected without emptying it?
Partially. An external inspection assesses the shell thickness, the roof, the coating and the foundation; however, the underside of the floor plates and internal corrosion can only be assessed once the tank has been emptied and cleaned. The two types of inspection complement each other.
How is the next inspection date determined?
Remaining life is estimated using the measured thickness, the design minimum thickness and the calculated corrosion rate. The interval is set based on this estimate and the maximum limit of the applicable standard; if corrosion is rapid, the interval is shortened.
Who may carry out the inspection?
The inspection is carried out by an inspection body accredited under ISO/IEC 17020 and by inspectors authorised by that body. The impartiality of the inspection body is fundamental: the party that maintains the equipment cannot conduct the periodic inspection of that same equipment as an independent inspection.