Introduction
Storage tanks are equipment with infrequent maintenance periods, so they require continuous external inspections, to have a better control of their conditions during the time they are in service, minimizing or avoiding unexpected failures that generate operational delays and Unplanned emergency maintenance.
For the major maintenance of a storage tank, the Final Work Scope (AFT) is the document or tool that indicates the inspection, maintenance and repair activities that must be carried out on the equipment and the quality of its content will allow achieving the better results regarding time and budget management, defining the effectiveness of said document.
A Final Scope of Work (AFT), is consolidated with the collection of information on lessons learned and recommendations from the last maintenance stop, also considering the information generated by changes in operational variables, provisional repairs, design alteration, new installations , among others, occurred from the last major intervention to the next general maintenance. This information is reviewed, analyzed and unified with the requests from the departments involved, such as: inspection, operations, process, maintenance, engineering and projects to develop the AFT as a final product.
Parameters to consider for performing the AFT
- Review the historical files of the tank, where you can learn about the design characteristics, materials, complementary accessories, previous inspections and repairs, corrosion data and possible alterations in its structure. In this part of the process, the results of the inspection plans applied to the tanks are also analyzed, using the API-RP-580, 581 risk-based inspection (RBI) methodology. The purpose of which is to control the equipment, where its different design and operational variables are analyzed and they are also inspected externally to detect the probabilities and consequences of equipment failures.
- Analyzing the reports generated by the custodian of the tank (operations), who is in charge of carrying out permanent tours, these personnel who are knowledgeable about the facilities and operational variables can provide important information regarding the conditions of the tank.
- Carry out field inspection, which must be carried out by qualified persons. At this stage, the tank is evaluated through the application of non-destructive tests (NDT) and standards such as API-653 and API 579-1/ASME FFS-1 that are necessary according to the requirements of each equipment, conventional NDTs are commonly applied. such as visual inspection (VT) and ultrasound (UT), however there are other NDT methods that can be applied to equipment that has specific needs; For example, tanks with difficult access to the roof, tanks that have suffered variations in their operational parameters according to their design, tanks that want to increase their operational run (working time), equipment that presents unexpected failures, equipment with thermal insulation, among others. These tests can be drones (remote VT), guided wave inspection (WGT), acoustic emission inspection (AE) and robotic ultrasound inspection, among others.
- After analyzing the items described above, the behavior of the equipment during its time in operation and its external conditions is known, which will allow generating an effective final scope of work (AFT), based on all the data collected, after the consolidation of this document, it must be reviewed and approved by the pertinent management, to then give continuity to the following processes such as: cost estimation, procurement of materials, engineering and planning.
Field inspection of tank components
Having knowledge of the design specifications, the history of maintenance and inspections and the operational behavior, the field trips are started to carry out the required evaluations.
In the final scope of work, limited equipment inspection and maintenance activities are specified until the first connection of the associated lines, except that the custodian or one of the departments involved requests an extension of the area to be evaluated, for modifications, redesigns or improvements required by the equipment and it is opportune to execute it during maintenance.
SETTLEMENT: It is important to monitor in a general way and prior to inspections, the settlement, verticality and roundness of the tank, in search of any unevenness or considerable deformation, which may derive from a failure caused by settlement of the equipment foundation.
BOTTOM: Of this component of the tank, we can only observe the section called flange or external projection of the annular plate. Through visual inspection, the existence of any indication of leakage and corrosion can be ruled out both in the corner weld and in the sections of the plate (flange).
The bottom of the tank has the peculiarity of being a component that cannot be visually inspected with the tank in service; therefore, a more accurate analysis of the mechanical integrity of the floor is required, prior to the preparation of the AFT. Unconventional inspection methods can be applied; such as, for example, the Acoustic Emission (EA), where the bottom is evaluated in an integrated manner, being able to detect perforations and other discontinuities, as well as their location and severity.
There is also the short-range Guided Wave technique (GWUT), in which between 0.5 and 1 meter in length is evaluated, from the tank wall to the center of the tank, focusing on the critical zone of the floor.
WALL: The wall of the tank is made up of a set of rings welded together. Depending on the design configuration of the equipment, the wall can be evaluated from the outside and inside. Its external side is continuously exposed to atmospheric conditions, while its internal side is exposed to the specifications of the product it stores. When the tank is a fixed roof type, we can only inspect the external side of the wall, which generates greater uncertainty when requesting activities in the AFT; In cases where the tank is a floating roof type, the internal condition of the rings that are exposed at the time of inspection can be observed.
The visual inspection on the external side of the tank wall is carried out in search of signs of leaks, deformation of the sheets and corrosion; these fault types are generally located in the upper rings. Inspection with conventional ultrasound (UT) is also applied in accessible areas such as the lower ring and the stairway area.
For the evaluation of the internal side of the wall, when the tank is of the floating roof type and is not at its maximum storage capacity, the conditions of the sheets (oxidation and corrosion) can be observed, thus obtaining a reference of the rings not observed.
In the case of thermally insulated tanks, it is difficult to assess the mechanical conditions of the equipment wall. If there is any detachment of the insulating material, the condition of the wall in that area could be observed, we can also evaluate the areas near the hull accessories, where specific sections of uncovered plates remain. For the most demanding case, where it is necessary to verify the condition of the sheets, the removal and installation of inspection windows (isolated segments) is planned to carry out the evaluation with VT and UT.
ROOF: There are fixed roof tanks, floating roof tanks and combined fixed roof tanks with an internal float (internal membrane). All ceilings are externally exposed to atmospheric conditions; while internally, those of the fixed ceiling type are exposed to vapors generated by the stored product, which rise and when they come into contact with the sheets and structures, they pass into a condensate phase that is sometimes potentially corrosive; Regarding the floating type roof, its internal part remains in contact with the stored product.
We must emphasize that the traditional visual inspection of any type of roof is subject to the mechanical conditions of the main tank ladder, which is the access to the top of the same.
The inspection of fixed ceilings is restricted, since it is only possible to assess the visible area from the ladder platform (for safety reasons, the ceiling should not be accessed when the tank is in service without guaranteeing safe access).
During the visual inspection, flaws in the paint coating, types of corrosion, perforations and sinking in the sheets are identified. Regarding the internal structure, sometimes we observe that the upper ring of the wall and the top angle, present deformations in the support points where the structures rest (gusset plates), which indicates a possible failure or misalignment of the same; When finding a case of this type, it is necessary to carry out the analysis of the structure and determine what activities to request in the AFT, these cases are particular to self-supported roofs.
When the inspection is directed to a tank with an aluminum geodesic dome roof, they are evaluated in the same way in the accessible areas; Usually, in this type of roof, the damages found are minimal, requesting maintenance in the areas that require it.
In combined fixed roof tanks, which have a floating (internal membrane), their general maintenance must be indicated in the AFT or even preventively request the availability of a percentage of material for possible replacements.
In thermally insulated roofs, inspections are further restricted; Knowing the mechanical integrity of the sheets is only achieved if there is any detachment of the insulating material, the condition of the roof could be observed. As in the wall, the removal and installation of inspection windows is planned to carry out the evaluation with VT and UT.
Floating-type roofs have various accessories installed on their structure, which need to be kept in reliable conditions for proper operation during their operational run. To carry out an effective inspection, it is necessary to go down to the float, where the security and operations personnel are in charge of granting access permission to it, because there are factors that generate risks to the personnel involved and must be previously controlled.
During the visual inspection, one of the most important points is the internal inspection of the pontoons, it is observed if there is any indication of leakage (presence of product); In the case of single deck type floating roofs, it is verified that there are no signs of leakage in the central area of the roof (area without pontoons), in addition to the conditions of the paint coating, signs of corrosion and the welded joints between sheets to rule out crack formation.
The mechanical seals are made up of two components (primary and secondary seal), during the inspection to generate the AFT, only the secondary seal can be observed, and if it presents detachment of any of its elements, the primary seal is inspected in that area. . At the time of evaluating the secondary seal, the conditions of the elements that comprise it (pads, screws, clip, thrust system and vapor barrier) are inspected, so that they do not present any faults. Depending on the conditions found in the components of the seal, maintenance or total replacement of the seal is requested.
It is important to check the condition of the guide tube, looking for deformation or corrosion failure. The roof guide (platen) is also checked, which is installed on the tank wall and it does not allow the roof to rotate during its operational process (filling and emptying). These elements must be replaced in cases of finding a critical condition. Evaluate the external connections of the roof drainage pipe (master pivot), verify the conditions of the check valve, the drainage collector and the system outlet valve, that the latter does not present a leak, because it would be indicative of a leak of flexible joint.
Non-traditional inspection techniques
In addition to the inspections with conventional non-destructive tests that are carried out on the roof of the tanks, there are techniques that yield more precise results. In cases where partial or general evaluations are required, such as the Robotic Ultrasound technique, also applied to evaluate tank walls, where the use of these techniques generates a better approach when requesting activities in the AFT.
Continuing with the new techniques to evaluate the walls and ceilings of tanks, we also find the use of Drones (remote VT), which is a method used mainly for areas with difficult access for visual inspection; From this method we can say that not only is drone inspection safer, it is faster and even leads to potential cost savings.
PIPING SYSTEMS: In the inspection of the pipes, the conditions of the paint coating and indications of pitting or generalized corrosion are verified, the welded joints, the screws, packing and alignment of the flanged joints and the minor connections are evaluated, these last two points are commonly attacked by corrosion; the support points with the supports are verified and the laying of the pipe is observed in search of any unevenness or misalignment.
In the process valves, the external conditions are inspected and it is verified if there are indications of leakage through flanged joints and packing press. The operations staff is in charge of indicating if the valve has any internal passage and it needs to be removed for maintenance in external workshops.
For thermally insulated pipes, the conditions of the insulating material are evaluated and regarding the inspection of the body of the pipe, the inspection technique with Guided Wave UT can be applied, which allows it to be evaluated in its entirety.
METALLIC STRUCTURES: The access ladder to the roof is inspected to locate possible corrosion or coating failures in profiles and steps. If the ladder is of the spiral type, all support brackets with the hull must be evaluated.
The rolling ladder (only applies to floating roof type tanks) and is a component that requires maintaining reliable operability during the service time of the equipment, as it is part of the support that the roof requires to stay in position, therefore, it is maintenance, generation or replacement of its elements is necessary, depending on the condition that they present.
The wind girder or wind beam is a component mostly used in floating roof tanks, which is installed in the upper ring and is intended to counteract the effects of the wind and help maintain the circumference of the equipment. During the inspection, corrosion was mainly detected on the sheets of the walkway and on the welded joints between the hull and the structures.
FIRE FIGHTING SYSTEM: These systems are necessary to control fires safely and effectively. For tanks, there are different types of fire-fighting systems, where we mostly find cooling systems (water sprinklers) and foam systems.
In these systems, it is necessary to evaluate the pipes, foam chambers, discharge deflector sprinklers and other accessories, looking for signs of corrosion and perforations; The pipes of these systems normally present internal corrosion, because they are not kept permanently in service (empty), they are only used when tests are carried out or when an event occurs.
HYDROSTATIC OR SEALING TEST: The request for this test in the AFT is subject to or will be determined by the maintenance and repair activities that are indicated in the AFT or that are carried out during maintenance. Item 12.3 Hydrostatic Testing, of the API 653 standard, indicates the various parameters that must be taken into account so that this test is carried out at the end of tank maintenance.
Bibliographic references
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