In this procedure we will try to list the steps to follow in the replacement of structural elements that have suffered significant deterioration over time and can no longer fulfil their function.
Tanks status
First, we will have to define which elements in the structure of the tank itself can be replaced. This will require a technical visit to check the condition of these elements from the inside and whether it is possible to expand the tank life by replacing the deteriorated parts or whether it should be completely replaced by a new one.
After the visit we issue a report (accompanied by a photographic report), including the repair options to guarantee the optimum tank condition.
Tank Polyurethane Resin
Resin can have several problems, mainly related to problems with rusting of the galvanised elements (mainly sheet metal and roof structure). These rust spots generally weaken the overall structure of the tank and must be replaced, although a repair can also be considered. The different situations are detailed below:
REPLACEMENT OF DAMAGED ELEMENTS
The parts to be replaced are identified and the feasibility of replacement is assessed.
Connections: they can be changed; the diameter can be modified and even the location of the connection can be changed, or new ones can be installed.
Plating: This is usually carried out at the highest levels as it is necessary to remove the upper elements to proceed with the replacement operation.
Roof: The upper part usually suffers from a high level of corrosion mainly due to the constant humidity atmosphere between the water and the closed roof.
REPLACEMENT PROCEDURE
The replacement of the roof and the top of the sheets requires a lifting platform (MEWP) to work on the top from the outside and scaffolding for the inside. A crane is also needed to remove the defective roof and sheet metal and to fit the new sheet metal and roof. A MEWP is also needed to repair or replace the top connections.
The first step is to remove the trapezoidal sheet metal from the roof by removing the self-drilling screws and store them in the upper part to remove them with the crane and take them to the ground. To keep track of the location of each sheet, a diagram is drawn and numbered so that the location of each one can be identified later. The fastening straps that support the trapezoidal plate are removed and are also identified on the diagram so that they can be put back in place. Finally, the upper connections are removed to begin with the removal by crane of the shell parts that we have marked for the change. Once everything has been removed, the area where the old sheet metal meets the new sheet metal is cleaned mechanically (grinder with a brush disc).
We will then start with the installation of the new plates by means of the crane, sealing the joint with a polyurethane resin and installing new screws. The necessary shell parts will be installed. It is possible to extend the height of a tank by adding more steel pieces to the top, but we will have to replace or extend the access ladder, as well as cover the connections that will no longer be used (if we do not replace the areas where they are).
Once the new shell areas have been installed, the upper connections shall be reinstalled and the roof shall be closed following the diagram made during disassembly, or a new roof shall be installed if required.
REPAIR PROCEDURE
The repair or recovery of a mastic tank is carried out with waterproofing resins (polyurethane or epoxy). It is possible to repair the galvanised sheet metal enclosure, the floor or both at the same time:
- Coating of concrete floor screed with polyurethane resin.
- Galvanised sheet metal coated with epoxy resin.
PVC tank bladder
The bladder can also suffer from rusting of galvanised elements such as metal sheet and roof structure, but we also have a new problem which is to determine the condition of the installed bladder, and which is often the main problem. The different situations are detailed below:
REPLACEMENT OF DETERIORATED ELEMENTS
The parts to be replaced are identified and it is assessed whether replacement is feasible.
Connections: they can be changed; the diameter can be modified and even the location of the connection can be changed, or new ones can be installed.
Steel Sheets: These are usually carried out at the highest levels as it is necessary to remove the upper elements to proceed with the replacement operation.
Roof: The upper part usually suffers a high level of oxidation mainly due to the constant humidity atmosphere between the water and the closed roof.
Bladder: There are options for repair or replacement, depending on its condition.
REPLACEMENT OR REPAIR PROCEDURE
The repair of a PVC bladder will always be done when it is in optimum condition and does not need to be replaced. To do this, it is necessary to have the tank empty and dry to enter through the manhole, located in the last shell ring, and try to visually locate the leaking area. Once the cut has been located, we will proceed to vulcanise it. It is important to have a dry tank as electrical machinery is used inside it. Once the necessary work has been done, the tank shall be closed again and filled up to the level of the leak to check it. After 24 hours, if no leak is detected, the tank shall be filled. The repair will never be recommended if there are doubts about the state of the bladder or if it belongs to another supplier different to EFP.
The replacement of the bladder requires logistical elements (MEWP and crane). If it is determined that bladder replacement is the only way to ensure the serviceability of the tank, it will be necessary to completely empty the tank. To order the correct spare bladder, the tank height and diameter as well as the distance between the bladder hanging bolts shall be confirmed.
Remove the lower connections and take them outside to begin with the partial removal of the roof (half of the roof is recommended). We will continue with the removal of the upper connections and with the crane we will introduce the new bladder inside the tank (if it is not possible to do it safely through the manhole). The removal of the old bladder is optional and can provide additional protection against contact with the outer sheet of metal. Once we have access to the upper perimeter of the tank, the bladder will be lifted by mechanical means (self- propelled crane or pulleys) and hinged on the perimeter bolts. We will go around the perimeter of the tank at the same time as we hang the bladder until completing the circle. We will stretch the bladder and make the upper and lower connections as we would do in a new tank. The roof shall be closed, and the tank is ready to be filled the next day.
