ABOVE GROUND STORAGE TANK INSPECTION
Since API 653 was introduced, operating companies have adopted a more comprehensive approach to the inspection of their storage tanks. The API 653 document prescribes the minimum requirement for the maintenance of storage tanks. Traditionally, all of this inspection was performed using manual techniques. However, the availability of new technology to scan large areas of tank surfaces is altering the inspection methodology.TANK FLOOR INSPECTION:
Traditionally, tank floors were inspected by cutting
coupons from the floor to establish a baseline for the
underside condition of the floor. Since the introduction
of a magnetic floor scanner, during 1989, the integrity
of the floor no longer needs to be destroyed by the
removal of coupons. A number of floor scanning tools are
now available, MATRIX has chosen the MFE Enterprises tool
for our Tank Inspection Program.
 | Twenty Four active sensors |
| Battery Operated |
| Inspects floors up to 0.375" thick |
| Inspects through coatings up to 0.100" thick |
| No umbilical cord required |
This innovative MFE tool enables us to screen a tank floor for underside corrosion at a rate of one thousand square feet per hour. The only preparation requirements are that the floor is clean and free from loose debris, such as scale. The MFE tool detects corrosion whether it is located on the top or the underside of the tank floor. The magnetic field does not discriminate between the location of the flaws. The Magnetic field detects the material volume loss and so, on occasion, a small diameter deep pit may have the same magnetic signal as a large diameter pit of less depth. Since the data from the tool is qualitative, we have to perform additional inspection to quantify the depth of corrosion. This is done by using a mechanical pit gauge for top side corrosion, and by performing ultrasonic inspection.
Ultrasonic Inspection: Ultrasonics have been used to perform thickness measurements for more than forty years. In the early days, flaw detectors used bulky non solid state devices whose performance tended to drift as the unit warmed up. Today's devices are solid state and remain stable in a variety of temperature environments.
Probes have also improved from the original quartz based devices. Polarized ceramics provide vastly improved performance characteristics that enable many different materials to be inspected with varying impedence characteristics.
THE INSTRUMENT:
Three forms of instruments are used for corrosion measurement; the Thickness Meter, the Flaw Detector and the Multi-Channel Automated Ultrasonic Scanning System.
| Thickness Meter |
This device can be provided in a number of forms. Most of the meters today are digital and are very compact battery operated devices. Many of the units available have a data-logging capability and most use dual probes.
| Flaw Detector |
The portable Flaw Detectors or Scopes are also relatively small and light weight (less than 6 pounds). Some of these have digital read out capability, in addition to a time base trace, which may be either analog or digital. These instruments can be used for thickness testing with conventional compression probes or can be used with angle beam probes for flaw definition or weld inspection.
| Automated Ultrasonic System |
Many varieties of these instruments exist from the single channel to multiple channel (typically 4-8 channel) devices. Some of the systems digitize the complete wave form and others select the key wave features to minimize the file size. The purpose of these instruments is to provide reliable, repeatable data in a form, which enables a secondary analysis to be performed.
Probes:
Probes are the other key component to ensure a successful test. Their importance is to make sure the test is a success. In general, two forms of probe are used for this work, either single or dual crystal units. Probes can be constructed to be wide band, narrow band, have varied damping, and even be focused to a certain depth.
| Single Crystal Probes |
They are generally used in immersion applications or in a situation where thick sections are being tested. A major advantage of the single crystal probe is that we can use the multiple echoes generated to eliminate the effect of surface coatings. One limitation of Single Crystal probes is the detection of localized thinning on thin sections, such as, tank floors which are generally less than 0.25" thick.
| Dual Probes |
They generally have a wedge between the crystal and the plate surface. The wedge enables the crystals in the probe to be angled toward one another simulating a line focus. These probes are unable to measure variances in plate coating thickness. However, the focus allows the probe to detect small corrosion pits which leave greater than 50 mils remaining. Dual Probes cannot compensate for coating thicknesses. If surfaces are coated a fixed correction has to be made.
Automated Ultrasonic - Multi-Probe Testing:The data can be collected by deploying multiple probe units from a motorized robotic scanner. The scanner is configured to provide either line or raster scans. A typical scanner format is provided in the Illustration shown as Figure 1.
The benefits of using the Multiple Probe arrangement are that this enables data collected from the transducers to be assembled to form a series of plots that describe the location, geometry and size of the reflectors detected. The industry has developed a nomenclature associated with the plots as indicated in the illustration. The Scan types are normally referred to as "A", " B", "C", and End View "B" - Scans as shown in Figure 2.
Shell Inspection:
The tank floor is just one component of the tank which requires inspection. In addition, the shell, roof, foundation, and appurtenances require Visual and Ultrasonic inspection. A comprehensive check list ensures that all aspects of the inspection are completed by the inspector.
The report documents all the inspections performed and includes a drawing of the tank's floor, roof, and shell which highlights all the areas requiring remediation.