Location: Lethabo Power Station, Northern Free State
Owner and Specifier: Eskom
Project team: Specialist Diving Team from Allter Africa
Project purpose:
The Lethabo Power Station Potable water Head Tank is a 2 division tank which provides the power station with potable water and fire water which ensures there is fire water in the event of a fire hazard.
The potable water tank section combined with the fire water tank section was leaking badly due to corrosion which had taken place as the tank had aged over time.
This tank, however, cannot be emptied and taken out of service for maintenance to be carried out, without a bypass tank being installed at a major expense to the power station or the power station going into downtime at an even greater expense.
The constant and increased leakages caused the foundation and ground or surface on which the tank was erected, to be constantly wet and pooling with water, resulting in subsidence beginning to occur.
There was an urgency to stop the leakages in order for the ground to dry out and prevent the tank’s structure from being compromised by the subsidence. A quick and effective sealant solution had to be found, in order to seal the numerous leaks and our wet applied systems had the answer.
Background to the project:
The Lethabo Power Station Potable Water head tank is a Braithwaite type 24 metre X 24 metre square tank erected onto a 30 metre high steel structure and divided into two sections internally:
Potable Water section measuring 19,2m width and 4m length
Fire water section measuring 4,8m width and 20m length
Technical Information - Findings and Repair Procedures
There were two main areas with high volume leaks; these were at the inlet and outlet 450mm steel down pipes, on the north and south sides.
South Side Major Leak
The leak was around the gusset weld point. This was an extensive and constant leak which was causing ground subsidence due to constant water flowing down the pipe from the leak. The damage was identified and the area was sealed.
North Side Leak
This leak was around the weld gusset area between the pipe inlet and the tank wall.
Repair Process:
All equipment and divers were lifted to the roof of the tank, and access was gained from the roof of the tank where a panel was removed. All repairs were carried out under water, using a specialised system developed by Allter Africa, specifically for underwater repairs of steel tanks.
Various underwater repairs in process
Why our Products and Applications are Unique :
Our wet applied sealant solution system is unique because it is applied underwater by divers, to the leaking areas in the tank, thus repairing the damaged areas and sealing the leaks.
The repair can take place while the tank is on line, with its total capacity of water inside and no water needs to be removed for the repair to be carried out.
Our products are non-curing, do not dry out or age over time, adhere to the steel surface at a molecular level even under water and fill voids so are able to fill the areas of damage and seal the leaks in the tank. On areas to which our solution has been applied, the corrosion has been arrested so the damage will not be able to advance or degrade any further, so the damaged areas have been repaired.
Technical Developments and Benefit :
The products used for the tank repair are specifically designed for applications where surfaces cannot be dried, the products are designed, either to absorb moisture in order to perform the function for which they were designed, or to be tolerant to moist surfaces and each other. These products are patent protected and in South Africa we have found new and unique ways of using the products to achieve results not seen anywhere else. The tank repair application is a first in the world solution and we have take our technology to a new level in application development.
This tank repair was originally proposed as a short term repair in order for the ground and the foundation of the tank support structure to dry out so that a permanent repair could be carried out.
It was estimated that a timeframe of 8 months to a year would be required for Eskom to put a contingency plan in place and run a by- pass line in to a separate temporary tank so that they could take the tank out of commission in order to carry out a complete relining of the tank.
The “temporary repair” was done in December 2017 and to date the repair has not failed at all, so Eskom have not deemed it necessary to take the tank out of service in order to refurbish it in the short term.
This repair has been a huge cost saving for Eskom because they have not had to take the tank out of service, there was no down time at the power station and the ground on which the tank structure is anchored has completely dried out so there is no longer any safety issue or concerns that subsidence may compromise the tanks stability.
Since this repair was completed we have subsequently completed further underwater tank repairs.
In light of the development in the repair of Braithwaite type tanks that we have made, We believe that a whole new avenue has opened up for our clients that we currently supply in the panel tank industry and we have begun to promote this new development to them as an opportunity for their businesses to grow in the area of online underwater refurbishment of tanks, particularly where their clients have leaking tanks that are critical to the functioning or operating of their businesses and cannot be taken out of commission. The repair is also able to extend the lifespan of crucial but ageing leaking tanks at a much more cost effective rate than total replacement, or downtime while the tank is drained for a complete refurbishment.