Above Ground Tanks
Large chemical and petroleum product storage tanks can be found at chemical processing plants, refineries, and industrial locations. They are huge metal structures 150 feet in diameter and 50-60 feet tall and can easily hold more than two million gallons of gas or other hazardous liquids. Most tanks are made of steel plate that is welded together to form the structure. The material and the welds are inspected for manufacturing defects when constructed but must also be periodically inspected throughout their service life for signs of damage. The carbon steel is prone to attack by corrosion and in some circumstances cracks can form over time. NDT personnel use visual, X-ray, ultrasonic and other inspection methods to search for flaws and service-induced damage.
Inspections and thickness measurements of the tank walls can be made manually with the inspector in a man lift or hanging down from the top. However a much safer way to make an inspection is to use a crawling robot. These robots have magnetic wheels that allow them to cling to the tank walls. Using remote controls, an operator guides them into positions and makes the necessary measurements. They work great on the side walls, however, getting to a tank floor is a different story since it is not accessible from the outside. The floor is particularly prone to thinning due to corrosion attack and tank owners must find the weak spots in the floors before they breach the tank’s integrity. This often involves the costly process of draining the contents, removing the layer of sludge from the bottom and cleaning the tanks so inspection personnel may enter it.
Not all tanks are built above ground. A very special inspection situation involves the tanks used to store industrial, medical, military waste. Many of these tanks have exceeded their original design life, and as the tanks age concerns about their leaks continue to increase.
What makes these inspections especially difficult is that the tanks are buried underground to help shield the radiation. Since the waste is particularly hazardous, special precautions must be taken to limit exposure to personnel to the radiation and chemical hazards. This extraordinary situation has resulted in many innovative technologies being developed to inspect the tanks. Visual examinations of the tanks are conducted using remote video cameras. These cameras are mounted on robotic arms that are lowered into the tanks.
The images to the right show what is know as the light-duty utility arm fitted with the video inspection system inside a tank and being deployed down into a tank from above. Some of the tanks are manufactured with two shells and there is a small space between the walls of the shells. The robotic crawlers can be lowered down into this space and used to perform ultrasonic inspections of welds and to make thickness measurements of the tank walls