Storage Tank Inspection

Storage Tank Inspection

In modern days, oil and gas storage tank's life cycle can be managed with the computing solution, which enables the construction and operational maintenance records to be archived permanently by electronic means. Electronic archiving is the alternative method to conventional data archiving which requires manual intervention and record keeping. These are now being replaced with NDE Life time records. Major benefits associated with electronic archiving are:-

> Information is preserved in its original quality i.e. does not degrade with time
> Computing solutions provide much more efficiency and cost savings
> Reduced physical storage space requirements
> Electronic retrieval and management

CIT's structured methodology of permanent archiving holds digital radiography records as well as film digitization records along with critical construction information. This information is multiplied to meet the QA requirements of associated project/product.

CIT holds a long-term track record of excelling in NDT radiographic testing systems and specialized services that are suitable for detection, identification of volume defects such as cavities and solid inclusions, and incomplete penetration where a gap exists. CIT's trained staff meets with the EN14015, EEMUA159, API653 and ISO9002 standards to conduct the inspection of storage tanks directly or via your existing established NDT sub contractors.

X-ray radiography is used as it can penetrate the double-walled structure of tanks that consists of an outer concrete layer and inner shell.

Storage tanks can be divided as Aboveground Storage Tanks (ASTs) and Underground Storage Tanks (USTs). While the ASTs have more capacity, they require frequent maintenance operations and inspection as compared to USTs that are of lower capacity.

These tanks are usually made of Carbon Steel/Stainless Steel or Aluminium. Where carbon steel is used, the concentration of carbon should be less than 0.3%. The material has to be decided carefully depending upon the costing and material that is to be stored in the tank. The construction basically involves welding the steel plates together to form the structure and proceeds in three stages:
> Stage 1: A concrete base is constructed upon which the tank ring walls reside. The first tank ring is erected, which is approximately 8 to 10 feet high. The only means of entry/exit is a manhole cut into one of the steel plates that comprises the ring.
> Stage 2: A second ring is erected on top of the first ring and welded into place. With this ring in place, the tank walls are 16 to 20 feet high. A "door sheet" is then cut out, creating an opening approximately 8 feet wide by 10 feet high. This opening provides access for personnel, equipment and material. Airflow is typically not a problem at this stage.
> Stage 3: The top is erected and forced air ventilation is used to assist in air flow.

Usually the roofs of ASTs are designed to be rounded so as to prevent it from buckling under wind loads. Now days, most tanks are built in with a floating fixed roof inside the rounded one, which rises and sinks according to the fluid level inside the container. This acts as a safety point as it decreases the vapor space above the liquid level.

Another safety requirement to prevent leakage is to build a dike around the container that holds the spilled liquid and prevents it from leaking out. The leakage is caused as a result of corrosive action, which may result in pits, cracks, crevices etc. Therefore, it is necessary to regularly inspect the tanks, keep records for tracking, plan out inspections and assess results for appropriate measures to be taken. Besides, it is also necessary to inspect the newly constructed tanks for welding defects.

CIT offers supply of system or services through partners for storage tank inspection that incorporates:-
> Digital Computed Radiography technology
> Radiograph Film Digitisation
> Corrosion Assessment
> Condition Management and Monitoring

CIT's digital computed radiography technology can help in NDT testing of welds (side walls, top and bottom) during construction and testing of side walls from outside. Besides, detecting the welds for cracks and pits, wall thickness assessment can also be done using CIT's DR equipment and software package. The film digitisation system can digitise the old radiographic films for electronic storage. These can be used as track records for integrity testing during the maintenance operations throughout the lifespan of the tank.

The LNG Storage Tanks are built to have a life expectancy of 10-20 years. These are special tanks that are used for storing Liquefied Natural Gas (LNG) at low temperature of -162 C. As LNG is a cryogenic, it is a strict requirement that the tanks should maintain a constant pressure by constantly releasing the boil off gas.

The above-mentioned CIT technologies and services can be used to inspect both Aboveground Storage Tanks (ASTs) and Underground Storage Tanks (USTs).

Contact CIT for further information and discussions.