Inspection of Tanks according to API (American Petroleum Industry) Association
Tank inspections according to API (American Petroleum Institute) Standards comply with a set of internationally accepted requisites and must be performed by authorised API inspectors.
The following are the features of this type of inspection:
OBJECTIVE: The Tank’s INTEGRITY is guaranteed
FREQUENCY of Inspection. Factors to be considered:
- The Product contained in the tank
- The results of previous visual inspections
- Corrosion rates
- Systems for Preventing Corrosion / Detecting Leaks
- The Tank location: Isolated or in areas of Risk
- The Risks of Contamination
- Changes to Operating Modes (filling / emptying)
- Service Changes
- Double Bottom or Spill Prevention Barriers
THE INTERVAL BETWEEN INSPECTIONS (INTERNAL / EXTERNAL)
- Depending on the Service RECORD
- If there is no record of service for the tank, that of a SIMILAR tank in service may be considered, if possible on the same site
- Local legal requisites may affect the intervals and frequency of inspections: Limits on steam emissions, the state of seals, Leaks permitted,
- Repair Procedures, etc.
Inspection OUTSIDE the tank
- EXTERIOR Inspection of the tank
- A Visual Inspection performed by an Authorised API Inspector.
- Frequency: Every 5 years or ¼ of the remaining tank life, whichever is lower.
- Measurement of Thicknesses using Ultrasounds
- The number of measurements must be decided by the Owner
- Corrosion Speed UNKNOWN: 5 years
- Corrosion Speed KNOWN: Half the remaining tank life, or 15 years, whichever is lower.The corrosion rate can be estimated on the basis of SIMILAR tanks in service, with thicknesses measured at not more than 5 year intervals. Internal Inspection of the Surround, with the tank not in service, can be replaced by UT measurement of thicknesses externally, provided that that the Int Insp. Interval ≤ Ext Insp. Interval (Corr. Rate KNOWN)
INTERNAL tank Inspection
- To ensure that there is no Severe Corrosion or Leakage in the Bottom
- To compile data to evaluate minimum Bottom and Surround thicknesses
- To identify and assess settling in the Bottom of the Tank
- Visual Inspection directed by an Authorised API Insp.
- An Authorised API inspector will gather the results of other NDTs and guarantee the quality and the integrity of the data.
- This can be done during service, using robots and non-intrusive NDT techniques, as long as the data secured make it possible to assess the following: thickness, corrosion rate and the integrity of the bottom. In these circumstances, an Authorised API Inspector and an Expert in non-intrusive NDTs must guarantee the quality and integrity of the data obtained.
- Inspection INTERVALS
- Based on corrosion speeds measured in previous inspections of the tank, or anticipated on the basis of tanks with a similar service record.
- Normally, the bottom corrosion rate will condition inspection intervals.
- The real Interval is established on the basis that minimum bottom thickness at the time of the next inspection is not less than the values indicated on table 6.1 of API 653. This interval shall under no circumstances be more than 20 years.
- If the Corrosion Rate is not known, and information on similar tanks in service is not available → The inspection Interval must not exceed 10 years.