Cylindrical steel tanks

ABOVEGROUND STEEL TANKS
 

Types of steel tanks

Stiffening of membrane of single deck floating roof

Methods of erection

Typical damages of aboveground steel tanks

Inspection of steel tanks

Realized projects of tanks

Typical drawings of tanks

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INSPECTION OF ABOVEGROUND STEEL TANKS


The vertical cylindrical steel tanks for oil and oil products storage are dangerous facilities. During the long period of their exploitation the different process decrease tank's safety and increase the risk of accidents. For example such processes are settlement of foundation, corrosion, fatigue of material, deviation of the construction in comparison of project form .
Accidents with steel tanks cause big financial loss (the product stored in the tank is much more expensive then the facility itself), the pollution of environment, the risk of fire and victims.
The periodical inspection and diagnosis is a must in order to decrease the risk of partial or full distraction of tanks and to avoid all the problems that are the consequences of destruction. The inspection and diagnosis include several activities, related to the inspection of tanks assessment of the fitness of different elements for further exploitation, determination of zones joints, connections or construction elements in which the repairing works are a must, determination of safe way of exploitation assessment of residual resource.
According to specialists that made inspection of aboveground steel tanks, it is necessary to be carried out regular inspections of steel tanks in service. Various ways of tanks inspection with specific scope are:

a) external inspection – it could be done by personnel who exploits the facility and who is especially trained for this purpose. It includes examination of external surface of tank for discovering of:
- leaks from bottom and / or shell;
- various damages and serious deformations in the shell;
- symptoms of settlement of the foundation;
- damages of heat insulation.

Results from external inspection have to be formed as report and written in diary of the tank.
The period between two external examinations depends on real conditions but must not exceed one month.

b) partial inspection – it must be done by qualified persons – inspectors . Exploitation of the tanks could not be stopped. The inspection includes:
- introducing with technical documentation;
- visual control of construction of tanks and welding joints;
- determination of the kind and the degree of corrosion damages upon the
accessible  surface of the shell, joint between bottom and the shell supporting joint and tank roof;
- measuring of the real thickness of elements of roof and shell;
- measuring of deviations of geometrical form of the shell;
- check of status of foundation;
- measuring settlement of the foundation.


Discovered defects and damages must be entered in sketch that becomes part of tank documentation.
After analysis of collected data about tank's condition must be done design calculations of tank's structure and assessment of residual resource of the facility. Real condition of the tank must be evaluated and to be written instructions to further exploitation, repair or rejection.
The period between two partial inspections could be smaller of 5 years or 1/4 of residual corrosion life of the shell

c) full inspection – it must be done by qualified personal and purpose is full assessment of technical condition of the tank. In this kind of inspection tanks must be out of service. The facility must be empty. Gas must be cleaned with the control of concentration of carbon vapors and cleaned. The following kind of works is necessary to be done:
- introducing with technical documentation for exploitation, technology and design of steel tank;
- inspection of the surface outside and inside of all constructive elements;
- measuring of real thickness of all constructive elements of the tank;
- measuring of geometrical characteristic of all constructive elements of tank which measure exceeds the determined limits
- check of status of foundation;
- determination of kind and value of settlement;
design calculations of structure of the tank and lose of stability of the shell, including earthquake responsibility;
- assessment of residual resource of the facility and determination of next inspection.

The intervals between two full inspections shall be determined by the corrosion rates measured during previous inspections or anticipated based on experience with tanks in similar service and assessment of residual thickness of tank's bottom The actual inspection interval shall be set to ensure that the bottom plate minimum thicknesses at the next inspection are not less than the values as follow:
- 2 mm when is not present leak detection system;
- 1,3 mm when leak detection system is present.

The maximum period between two full inspections could not be bigger than:
- when corrosion rates is unknown - 10 years;
-
when corrosion rates is known - 20 years.

The technical report and conclusion must be prepared according to the results of inspection and it must be integral part of the documentation of the tank. Referring to this report the repairing work must be carried out aiming to increase the safety and to make longer the period of their exploitation.
When the damages are especially serious or irreparable, when the tank has not residual resource, the exploitation of the tank must be stopped.