Inspecting Above Ground Storage Tanks

Whenever you have an above ground storage tank, it is important to make sure it is in good condition. This is especially important when the tank contains liquids that may contain toxic elements. There are a few ways to check for leaks and other problems, including acoustic emissions testing, C-scans, magnetic flux leakage, and other methods.

Acoustic Emission Testing

Using the Acoustic Emission (AE) method in in-service inspection of aboveground storage tanks is a reliable and efficient way to detect micro-leaks in the tank floor. It is an alternative to conventional inspection methods, such as X-rays, ultrasonics, or gas testing. AE also allows the identification of active corrosion damage.

During the AE test, signals from the bottom of the tank are recorded simultaneously with background noise signals. The number of AE events per hour within a specified reference area is then calculated. This density is used to estimate leak potential. A high number of events indicates a higher probability of leakage, while low activity indicates a lower likelihood of corrosion. The level of acoustic emission in the tank depends on the diameter of the bottom and the load forces acting on the bottom.

The AE method is suitable for the examination of vertical above ground storage tanks. Its benefits include rapid and nondestructive testing, a high state of security, and the ability to locate micro-leaks. However, acoustic emission methods do not meet the API 653 requirements for integrity testing of tank bottoms.

An investigation was conducted to determine the noise levels in three crude oil tanks. The measurements were performed on the first two tanks, and a third tank was tested for corrosion characteristics. Each tank was assessed for AE activity and for the presence of significant sources of AE. The results showed that each of the tanks had a slightly different noise level. The noise level was lower in tank 3 than in the other two tanks.

The acoustic emission sensors were installed on the tank shell at an average height of 1 m above the floor. A row of sensors was installed along the circumference of the tank, with sensors arranged in a standard pattern, and each sensor was placed a minimum of 15 m from the other sensors. Depending on the position of each sensor, the distance between the source and the sensor could vary. Ideally, the ambient conditions would be those of no wind and no direct sunlight. If the tank was sludge filled, the location error may be larger. The overall AE activity of the structure's bottom is estimated from the average density of the localized AE events from the tank floor.

The acoustic emission signal was evaluated for its amplitude and frequency response. For the first tank, the noise level was measured at a threshold of 12.4 dB. For the second and third tanks, the threshold was 11.6 dB.

The AE testing method is beneficial to both owners and operators of above ground storage tanks. It is quick and non-intrusive, and it has a small effect on tank operations. However, a more detailed inspection is necessary to determine corrosion or a possible leak. A number of factors should be considered when performing the AE test, including the number of sensors, the parameter settings, and the data processing method.

Magnetic Flux Leakage

Detecting magnetic flux leakage is a useful inspection method used to inspect steel structures. This is especially useful for checking the internal and external metal loss due to corrosion. Magnetic field leakage is a non-destructive testing technique that uses electromagnets to measure the leakage magnetic field. This method is usually used on above ground storage tanks and pipelines. It is also used in the oil and gas industry, for detecting defects in mild steel plates and used pipes.

The most important aspect of the magnetic field leakage detection method is the signal processing, which includes data acquisition, noise reduction, and data storage. In addition to these, an analysis of the signal waveform is required to determine defect status. It is possible to perform statistical recognition, but it may be necessary to use other NDT methods to evaluate the severity of the defect.

The axial peak of the leakage field is a good indicator of flaw depth. The length and width of a defect are related to the magnitude of the leakage field and the features of the magnetic leakage field. This is a well-established relationship. However, the magnetic leakage field can be distorted by debris, welds, and other factors. In addition, sensitivity drops dramatically as the defect size increases.

Using magnetic leakage as an indirect method to assess the risk of a defect is a well-established practice in the industry. Indirect methods include mapping methods, signal classification, and iterative methods. All of these methods are based on approximation of the detection signal and can be further improved by the aid of adaptive learning software programs.

The best type of detector for leakage magnetic fields is a Hall sensor. This type of sensor has favorable stability and temperature characteristics. Its optimum performance can be expected at thicknesses of about 10-15 mm. Other types of sensors can be used, but their performance varies depending on the magnetization and distribution of the probe. Typically, permanent magnets are used to create the magnetic field. The strength of the field can be adjusted to match the strength of the material. Alternatively, a combination of electromagnets and permanent magnets is also used to produce superimposed magnetisation.

An interesting feature of the magnetic leakage field is its ability to detect large non-axial oriented cracks. These are difficult to identify with the traditional magnetic flux leakage tool. A trained analyst can recognize the horseshoe-shaped sign, which indicates a dent.

The magnetic leakage field has been shown to have a significant effect on the depth of a defect. Various combinations of the volume of material lost, the orientation of the defect, and the position of the defect are required to get the same depth of defect. The inverse square law of the number of signals can be applied to calculate the exact depth of a defect.

C-scans

Performing C-scans when inspecting above ground storage tanks can provide valuable information about the condition of a tank. However, there are a few important things to keep in mind when completing this type of inspection. The primary objective of a visual inspection is to look for indications of degradation on the shell or supporting structure. If there are any concerns, you should take additional measurements. These include the location, nature, size, and type of defect.

A physical inspection of the tank can determine whether there are structural defects such as gouges, bulges, and cuts. You should also look for swelling or brittleness. If you suspect a defect, you should remove the tank from service. This is especially true if there is a visible sign of discoloration or deterioration.

A conventional ultrasonic thickness measurement is often used to assess the thickness of the materials in the tank. This type of test is performed in accordance with API 653 or ASME V. You will need to have a good understanding of how the test is performed to properly calculate your tank's thickness. The results of the measurement may vary depending on the equipment and the inspector's experience.

A C-scan is also a good way to identify small defects in the tank. This is a more thorough inspection method that will allow you to make a more detailed assessment of the condition of the tank's interior. The most common methods of conducting this type of inspection are with a remote-access UT crawler or a continuous scanner. These systems can operate automatically or manually. They can perform line scans on the tank's shell surface and a variety of other areas.

Another method of identifying defects is with a magnetic flux leakage array (MFL). The MFL is a type of non-destructive testing technology that uses guided plate waves to detect defects. This type of technique is performed by a probe with a thin titanium blade that slides under the aboveground storage tank's annular rings. The device then sends long-range guided waves into the base of the AST. When this happens, acoustic emission events are detected in the center of the tank floor. This data is then used to determine the severity of the corrosion. It is also used to determine the leak potential of the tank.

If you are looking for a high-quality solution for detecting smaller defects in a tank, you might want to consider the FloormapX scanner. This system is equipped with SmartMAGNET technology, onboard lighting, and precision active steering. You can also take advantage of its interactive laser guide, which correlates the physical locations of tank floor defects with the C-scan images.

A post-scan inspection report provides more information about the thickness of the plates and the remaining plate thickness. The report will identify the specific location of each plate and will highlight any areas of the bottom that need to be repaired. This can be used to set the corrosion rate of the tank and to determine the timing of any future repairs.