The most fundamental classification of storage tanks is based upon whether they are above or below ground. Robotic inspection is limited to aboveground storage tank, which has most of its structure above ground. The bottom of the tank is usually directly placed on an earthen or concrete foundation. Aboveground storage tanks are used to store chemicals and petroleum products. They are usually easier to construct, cost less, and can be built with far larger capacities than underground storage tanks and they are constructed with steel plates welded together. Most ASTs range from 3 to 100 m in diameter and 2 to 14 m high. The roof of the tank can be used to classify types of tank designs (Myers, 1997).
• Fixed roof tank
It can be vertical or horizontal; the vertical tank is cylindrical with a fixed roof. The roof shape varies which can be conical, dome shaped or flat. The drawback of this type is that they are not suitable for high vapour pressure storage unless vapour recovery is used which means that it allows evaporation of the stored liquid. Unlike the vertical tank, the horizontal shape is elliptical; it is found in both above-ground and underground tanks.
• External Floating roof tank
Consists of an open topped cylindrical steel shell with a floating roof that moves relative to the change of liquid level. There are two main types of roof: the pontoon roof is typical for floating roofs and is formed with a diameter approximately 10 to 30 m. The other type is the double-deck roof consisting of two layers which is built for very small tanks with a diameter of 10 m. The purpose of the external floating roof tank is to reduce the volatility of stored material. The main problem with floating roof is that the rain water can accumulate on the roof.
• Internal floating roof tank
These tanks are vertical with cylindrical shells just as fixed root tank but with a cover that floats on the surface of the liquid. The floating interior cover has sufficient buoyancy to ensure that the roof will float under all conditions, the floating roof and the tank shell have a gap of between 20 to 30 cm.
• Domed roof tank
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floating roof tank and has a welded deck and self-supporting fixed roof.
The NDTBOT is designed to be inserted into storage tanks with a fixed roof or floating roof through minimum manhole sizes of 300 mm for underside corrosion inspection of tank floors, Figure 3. 1 and Figure 3. 2 shows aboveground storage tank.
Figure 3. 1: Aboveground storage tank manhole (Georgia, 2017)
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3.1.2 Corrosion of tanks
Exposure of storage tanks to the environment results in corrosion in different areas of a tank such as an external surface wall, the external surface under the tank bottom, and the vapour space between liquid level and the roof of a tank. The NDTBOT is designed to inspect the tank bottom underside to which access from the outside is not possible. The corrosion is usually caused by salinity in the groundwater and other factors such as ground chemistry. The tank underside corrosion varies significantly based upon the site, design, foundation, drainage condition and other factors. Figure 3. 3 shows the corrosion locations in the storage tank.
Figure 3. 3: Tank corrosion modes (Myers, 1997). 3.1.3 Explosion in storage tanks
Storage tanks in refineries and chemical plants contain large volumes of flammable and hazardous chemicals. A small accident may lead to a million-dollar property loss and a many days of production interruption. In the last 50 years, trade organizations and engineering societies such as the American Petroleum Institute, American Institute of Chemical Engineers, American Society of Mechanical Engineers, and the National Fire Protection Association have published strict engineering guidelines and standards for the construction, material selection, design and safe management of storage tanks and their accessories. (James and Cheng-Chung, 2005) reported that 242 tank accidents occurred in the last 40 years due to static electricity, equipment failure and operational error. The three main ignition sources presenting high risk when working in a hazardous area are specified with some examples in Table: 3.1
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Table 3. 1: Classification of sources of ignition
Ignition Sources Examples
Hot Surfaces
Surfaces heated by coils, resistors, lamps, brakes, or hot bearings. Hot surface ignition can occur at the Auto- Ignition Temperature (AIT) or spontaneous ignition temperature at which a hazardous substance will spontaneously ignite without further energy.
Electrical Sparks
Occur when circuits are broken, or static discharge takes place. In low voltage circuits, arcs are often created through the making and breaking of electrical contacts
Friction and Impact Sparks
When casings or enclosures strike other tank surfaces
3.1.4 Intrinsic safety protection of NDTBOT
The robot operates in a non-defined zone when submerge in liquid. However, before getting to the liquid, the NDTBOT goes through zone 0, therefore, it needs to meet the requirement for zone 0 environments. The required protection for the region method and standard are shown in Table 3.2.
Table 3. 2: Zone definition (Victor, 2003)
Zone 0
PROTECTION TYPE
STANDARDS PROTECTION METHOD
Intrinsic Safety EN 60079-11
Where the design limits the ignition spark energy to below that which, will ignite the explosive gas. Safe even with two simultaneous faults.
Special protection
EN 60079-26 Special construction normally based on the use of two independent types of protection both individually.
Encapsulation EN 60079-18
Integral components which can potentially ignite an explosive gas are encapsulated allowing the isolation of these components from the explosive atmosphere surrounding them. This allows the strict control of surface temperatures under normal and fault conditions.