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API 653 RECOMMENDED INSPECTION INTERVALS

VISUAL EXTERNAL INSPECTION

1. All tanks shall be given a formal visual external inspection by an API certified inspector at least every 5 years, or at the quarter corrosion rate life of the shell, whichever is less. Tanks may be in service during this inspection.

IN SERVICE ULTRASONIC THICKNESS MEASUREMENTS OF THE SHELL

1. The extent of ultrasonic readings shall be determined by the owner/operator.
2. When used, ultrasonic thickness measurements of the shell shall be made at intervals not to exceed the following:
   • At five year intervals for existing tanks where the corrosion rate is not known.
   • When the corrosion rate is known, the maximum interval shall be the smaller of RCA/2N years (where RCA is the remaining corrosion allowance in mils and N is the shell corrosion rate in mils per year) or 15 years.

INTERNAL INSPECTION

1. Internal inspection intervals are determined by the bottom corrosion rate.
2. The actual inspection interval shall be set to insure that the bottom plate minimum thicknesses at the next inspection are not less than the following:
   • 0.10 inches if there is no means for detection and containment of a bottom leak.
   • 0.05 inches if there is a means for detection and containment of a bottom leak.
   • 0.05 inches if there is an applied bottom reinforced lining greater than 0.05 inches thick in accordance with RP 652.
3. In no case shall the inspection interval exceed 20 years.
4. When corrosion rates are not known and similar service experience is not available to determine the bottom plate minimum thickness at the next inspection, the actual bottom thickness shall be determined by inspection(s) within the next 10 years of tank operation to establish corrosion rates.

API 653 RECOMMENDED INSPECTION INTERVALS

So how should an owner apply these recommended inspection intervals to his tanks ? First, he must determine the dates and types of previous inspections. Were ultrasonic thickness readings taken on the tank floor, shell, and roof? What recommendations were made? Are there original construction records available for the tank? Were any repairs made and are there detailed repair records? Based on the data available, can corrosion rates be calculated for the floor and shell? If so, these corrosion rates can be used to apply the API 653 guidelines and determine the dates that the next inspections are due. If not, then API 653 states that shell ultrasonic inspections (if used and this is highly recommended) should be conducted within 5 years and floor inspections within 10 years. Have inspections been performed at these intervals, or are they overdue?

Do not overlook the API 653 recommended external visual inspections by an API 653 certified inspector every five years. These inspections are in addition to the daily, weekly, or monthly inspections that the owner and/or his personnel perform on a routine basis. API examines and certifies API 653 inspectors and requires renewal of this certification every five years. Ask your inspection company for a copy of their inspector’s certificate and maintain it in your tank inspection records.

Just what should an inspection report include ? What records should an owner maintain ? Inspection records form the basis for a maintenance/inspection program. API 653 recommends three types of records: construction records, inspection history, and repair/alteration history.

API 653 states that construction records include drawings, specifications, nameplate data, construction reports, and results of any tests and evaluations. Inspection history records include all measurements taken, the condition of all items inspected, and a record of all tests and examinations. Any unusual conditions noted should be recorded along with recommendations for repair. Calculations of corrosion rates and recommended inspection intervals should be included. Repair/alteration history records include all data about a tank since its construction; repairs; alterations; replacements; service changes such as product, temperature and pressure; and coatings and linings. Inspection reports include recommended repairs and reasons for them, sketches or computer drawings showing the location and extent of defects, steel thicknesses measured, conditions found (photographs are often included), repairs, settlement measurements, and recommendations.

If your inspection records or reports do not include all of the above, be sure to start collecting any data that may be available. Include these requirements in any requests for inspection quotes from inspection companies. Be sure the quotes you receive all include the same items and promise the same quality of report. Ensure that the reports you receive include all the data and information required. In other words, “Do It Right!”

MPE, Inc. is located in Hebron, Connecticut. They can be reached at 860-228-3636 or by e-mail at inspector@mpeusa.com.