Introduction
Aboveground storage tanks dedicated to water storage are used most often for fire protection and potable water. Water tanks for fire protection are used in numerous applications including fire sprinkler systems supply, water source for firefighting, to create a fire perimeter, and as a source of water at fire departments to fill up trucks. Potable water storage tanks have many applications in commercial and residential circumstances. Water tank inspections are generally conducted in accordance with National Fire Protection Association (NFPA) – 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems and American Water Works Association (AWWA) M42 Steel Water Storage Tanks. Compared to aboveground storage tank inspections of petroleum product tanks, water tank inspection is typically less complex and expensive since the water is not a hazardous material, non-flammable, and clear. Water clarity allows for excellent in-service tank testing with remotely operated vehicles (ROVs) that provide high quality video of the tank internal coating, shell, and nozzle conditions. However, precautions must be made that any ROV used for water tank inspection does not contaminate the water. Well defined rigorous sanitation processes must be applied to all tools entering the tank before water tank inspections. These guidelines are outlined in AWWA C652 – Disinfection of Water Storage Facilities. This article discusses some of the in-service water tank inspection guideline and the non-destructive testing (NDT) technology to support water tank testing.
Introduction to Water Tank Testing
The videos above and below show typical water tank testing video footage, from the interior of water tank, acquired from a tethered ROV. In the first clip, the ROV is approaching a 12” diameter outlet to assess condition. The water tank ROV assesses the coating condition clearly and effectively with illuminated high-resolution capabilities. The second video demonstrates the ROV’s capability to detect subtle paint blistering and coating failure.
There are a variety of fire protection and potable water tank types that require periodic in-service and out-of-service tank inspection, typically in 3-5 year inspection frequencies. Fire protection tanks are often ground level above ground storage tanks made from welded steel, riveted steel or concrete. Additional configurations for potable water tank inspections include single pedestal, leg tank, standpipe, fluted column and composite elevated. Figure 1 shows a water tank that was inspected in-service by TKS in accordance with NFPA 25 - Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems. The 250,000 gallon capacity heated tank, 35’ high tank – 37; diameter tank, was equipped with insulation.
Figure 1 In-service fire water tank testing to NFPA 25 - Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems
In accordance with NFPA , the interior of steel tanks without corrosion protection shall be inspected every 3 years while the interior of all other types of tanks shall be inspected every 5 years. The tank interior shall be inspected using visual, ultrasonic, magnetic flux leakage (MFL), acoustic emission testing, or other non-destructive testing methods for pitting, corrosion, spalling, rot, other forms of deterioration, waste materials and debris, aquatic growth, and local or general failure of interior coating. Ultrasonic testing is used to gage the wall thickness of the water tank floor and shells, to estimate annual corrosion rates based on historical data, and predict the remaining life of the asset. MFL tank floor testing is used to search for tank floor underside corrosion that would otherwise be undetected by ultrasonic testing and visual inspection. Acoustic emission testing of tank floor bottoms may be deployed on tanks that must remain in-service. Tank bottom acoustic emission testing locates water leak activity and potential corrosion areas remotely from the outside of the tank.
Remotely Operated Vehicle (ROV) Water Tank Inspections
Remotely operated vehicle (ROV) inspection of water tanks eliminates the need for emptying and cleaning the tank prior to inspection which are the largest tank inspection cost drivers. ROV water tank inspection utilizes a sanitized underwater ROV with a high resolution video camera, lighting, and recording equipment to inspect all surfaces of the tank below the water level. The visual inspection process is applicable to several industry standards including NFPA 25 and is an excellent solution, compared to diving inspection, for evaluating coating integrity without removing the tank from service. TKS inspection videos and associated reports help the tank owner determine the overall condition of the interior and exterior of the tank, assisting in identifying and budgeting for any necessary repairs. Example high resolution still shots acquired using the water tank ROV are shown below in Figures 2 and 3. In Figure 2, many areas of water tank bottom paint blistering and total coating failure are observed. In Figure 3, the ROV water tank inspection detected advanced corrosion on the outside diameter of vent piping. The coating breakdown and corrosion are observed over a 2” diameter area close to the girth weld joining a straight section to an 90 degree elbow.
Figure 2 Paint blistering and corrosion discovered during ROV water tank testing
Prior to inspecting a potable water tank, the remotely operated vehicle (ROV) and tether is disinfected in accordance with AWWA C652 – Disinfection of Water Storage Facilities. The standard includes disinfection procedures for underwater inspection and/or cleaning of potable-water-storage facilities but does not describe the technical aspects of underwater inspection and/ or cleaning. As the water tank interior is inspected, the operator observed the water tank inspection in real time. All video is stored permanently for further analysis and the final tank inspection report and to support repair recommendations.
Figure 3 Advanced corrosion discovered during ROV water tank testing
The ROV has proven itself to be tremendously useful in providing underwater video inspections in pipes, intakes, tanks, and dams. The ROV using for water tank testing has a maximum dimension of 10”. The same ROV used for water tank testing can adequately access and maneuver in a 24" diameter pipe. The ROV's major advantage is its size, and video storage capabilities. From a logistics perspective, ROV tank inspection is completed with half the labor resources to assess water tank interiors with a dive team. The ROV facilitates access into tanks since the tethered vehicle is light enough to be lowered easily into the tank independent of water level. Also, the requirement for confined space entry (CSE) permits is largely eliminated.
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