A series of recommendations to analyze corrosion in this class of products and the actions to be carried out in terms of prevention and maintenance with coatings.
by Juan Manuel Álvarez*
The inspection of tanks or pressure vessels aims to determine the condition in which it is located as a structure. The state or condition in which it is found involves the reliability of the materials with which it was built. The condition assessment considers both resistance and appearance.
From the point of view of the most common inspection techniques used in tanks, the following can be cited:
1. Visual.
2. Penetrating liquids.
3. Ultrasound.
4. X-ray.
As for the condition by resistance, the inspection must be aimed at determining discontinuities such as cracks, cracks, pores, scratches and even dents considering that they have modified the compressibility of the material, among others. Which will affect its resilience in terms of loads and efforts is concerned.
On the other hand, the inspection for condition related to appearance has to do, for example, with stains that can be produced by dripping, runoff, splashing or apozamiento of fluids, or, stains produced by reaction or by temperature. The stains produced by reaction are usually those related to the formation of corrosion products, which can either be caused by contaminants of the fuels, or by the presence of free water in the fuel that when decanted at the bottom of the tank or at the bottom of the pipe lines, will lead to corrosion reactions. Depending on the amount or concentration of sediments present in the fuel and the temperature at which it is located, it can lead to the formation of stains that will eventually spread superficially as a reaction for the formation of corrosion products.
Thus, the inspection will be determined by the detection of reactive and non-reactive defects of the tank construction materials. Reactive defects will be related to the presence of corrosion, reaction with localized contaminants or temperature. While non-reactive defects will be related to fissures, cracks, pores, scratches, etc.
- In the inspection for tank integrity, the following should be taken into account:
- The construction sheets of the tank.
- Welding beads.
- Welded elements such as nipples, drains, valves, etc.
- The mechanical joining elements such as bolts, flanks, etc.
- Protective coatings.
To relate all the above with a view to the vision or objectivity of the inspection of fuel storage tanks, the following count will be made:
Whether they are horizontal or vertical tanks for fuel storage, these must be protected. The protection of the same can be based on paint coatings, cathodic protection or cathodic protection with paint coatings. In order not to delve into cathodic protection techniques, we will limit ourselves to protection with paint coatings.
Fuels and petroleum derivatives including the same crude always have the presence of water, the water that after a while tends to decant is called "free water", and will be the one that generates the need to design the drainage points in the tanks. There are rules and protocols related to the drainage of free water in storage tanks for fuel and petroleum products, including crude oil. This is due to the affectation of the integrity of the storage tanks due to the effect of the formation of corrosion products, as a reactive defect that consumes and deteriorates the metallic materials of the tank, finally affecting the integrity and resistance in that area or area of the tank.
In tanks protected with paint coatings, "free water" affects the coating when the frequencies and qualities in the drain are not properly met, which is why in scheduled maintenance of the fuel and petroleum storage tanks the condition of the paint coating will be inspected. It is common to see at the bottom of the tanks the affectation by apozamiento of the decanted water that when inspecting by continuity of film the paint coating will determine the presence of pores or after defects of coatings where the presence of corrosion products will be detected.
Drainage points are mandatory inspection points at any of the corrosion inspection frequencies in fuel and petroleum storage tanks.
Tank inspection checklists should consider the frequency of inspection, which allows to relate the points, areas or zones to be inspected and organized according to the inspection techniques to be met.
For example, the daily checklist inspects drainage points, flanks, manholes, vents, bolts, plugs and elements welded to the tank; inspection aimed at the condition of the paint coating and the presence of corrosion products. The above by visual inspection, mainly.
After the construction or repair of tanks, inspections must be carried out aimed at the final condition of the weld, this inspection can be based on a visual inspection by gauges or defectology, or inspection by discontinuities with penetrating liquids (fissures, pores, etc.).
When in major tank maintenance inspection corrosion has occurred at the bottom of the tanks, suggesting corrosion spread and material consumption, the remaining thickness of the affected sheet material in the corrosion removal area should be measured, after corrosion removal. This inspection will be done by ultrasound thicknesses.
If the storage tank is in service, X-ray inspections can be performed to determine the presence or formation of non-reactive discontinuities such as fissures or cracks.
Non-reactive discontinuities can be detected by penetrating liquids if they are superficial and uncoated. Non-reactive discontinuities found inside the material or that are coated can be detected by radiography.
Inspections of tank coatings are based on visual inspection for defectology, and on non-destructive inspections of dry film thickness and film continuity, and destructive adhesion testing.
Although there are generalities in relation to inspections, the particularities will be directed with respect to storage products and their qualities, which is why in another article tank inspections with other products will be discussed.
As always I wish you a good aging and many successes in your work.
* Juan Manuel Álvarez Rodríguez. Designer for Corrosion Control, Certified Coating Inspector, Corrosion Technician and Marine Coating Inspector (NACE International, the Corrosion Society) A.S.T.M. member of the committees of corrosion, paints and coatings, fuels, general aviation, adhesives, nondestructive testing and composites. Certified Instructor ICAO (International Civil Aviation Organization). Inspector in Non-Destructive Testing. Instructor, lecturer and columnist in Paints and Corrosion Specialist in Metal Structures and Composite Materials Juancorrosion.com [email protected] Advisory Services, Inspections, Training and Contracts for the Control and Prevention of Corrosion.
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