Tankwall Inspection

The Tankwall Inspection system consists of a Four-Wheel Wall Crawler and the Tankwall Inspection Software running on the NB-2000, a laptop computer-based UT system. The system design was based on the needs of tank owners and inspection service companies with extensive experience in tank wall thickness measurement.

Four-Wheel Wall Crawler

The Four-Wheel Wall Crawler is a four-wheel drive, four-wheel independent suspension, magnetic wheel scanner designed for tank wall inspection. Large suspension travel, high clearance, and offset wheels make it tolerant of obstacles and irregularities on the tank surface such as welds, lap joints and rivets. It can be ordered with either 3-inch or 4-inch diameter wheels. With the 4-inch wheels, it routinely inspects riveted storage tanks with a 1-inch lap joint. A simple joystick controls speed, direction, and steering. The compliant transducer holder accommodates contact, delay line, and bubbler probes. It can be adapted to wheel probes if desired. Extensive use of aluminum alloy construction makes the crawler both rugged and light weight (approximately 12 pounds).

Others have attempted to inspect storage tanks with crawlers with the following features:

• fixed permanent magnets instead of magnetic wheels
• without all-wheel drive
• without independent suspension
• without offset wheels
  

These scanners may look good on flat smooth plate, however experience has shown that they cannot tolerate the normal obstacles encountered inspecting real tanks.

Fixed permanent magnets in the body of the scanner require low clearance between the body of the scanner and the tank since the magnetic force decreases exponentially with the distance. Low clearance means that welds, rivets, or lap joints higher than the clearance will stop the scanner. If the distance is increased when the wheels roll over the obstacle, the attachment force is rapidly decreased increasing the risk of the scanner falling off the tank.

All-wheel drive increases traction. Without all-wheel drive, traction and steering can be lost when the drive wheel is traversing an obstacle.

Without independent suspension, the scanner is less tolerant of obstacles. When one wheel encounters an obstacle, it lifts the entire scanner, affecting traction and steerign of the other wheels. In addition to stopping the scanner, this could also affect the magnetic attachment force, increasing the risk of the scanner falling.

Offset wheels prevent two wheels from encountering the same weld or lap joint at the same time. This reduces the driving force necessary to overcome the obstacle. It also reduces the lifting force on the scanner and assures good contact and traction from at least three wheels at all times.

A scanner without offset, no suspension, and fixed magnet instead of magnetic wheels is the least tolerant of common obstacles such as lap joints or welds. If the two front wheels can roll up onto the obstruction, they lift the front up increasing the magnet to surface distance and reducing the attaching force. This may cause a loss of traction in the front wheels or it may cause the scanner to fall off. If the front wheels make it over the lap joint, weld, or row of rivets, it may "high center" and get stuck.

UT System Software

The UT system is based on the NB-2000. This is a laptop computer-based automated UT system with high performance pulser receiver and digital A-scan recording that uses the familiar Windows graphical user interface. The Tankwall System Software simplifies system operation because it is designed specifically and only for tank wall thickness measurement.

Setting up and calibrating the UT instrument is typical of any UT scope using the graphical control panel. Either one or two gates may be set. With the single gate, a direct measurement to the first back wall echo is made. Two gates are used with bubbler or wheel probes. The first gate is used to detect the front surface. Measurements through paint are also made using two gates in order to eliminate the paint thickness from the measured value.

During scanning the operator sees the Inspection screen. The Inspection control panel allows him to set (or zero) his encoder position for each lift as well as read current position during the scan. The Raw Data display is a TOP view of the stacked A-scans. It allows him to monitor data quality for lift off and see internal reflections from inclusions that may affect the thickness readings during the scan. The B-scan display shows the wall thickness profile based on the gate data. This is shown full scale and scrolls with the current position of the scanner. If he detects a problem with data quality during the scan, he can reverse the crawler as needed to rescan only the affected area. The new data overwrites the old.

After scanning the inspector has access to the Data Display Screen. This gives him A-scan, B-scan, and C-scan views of the recorded data. Clicking on either the B-scan or A-scan makes the Mark Data window appear. This tool allows him to mark data to be included in each course. It also allows him to mark bad data to be excluded from analysis.

• Lift off at welds and lap joints
• Bad coupling due to blistered paint
• Inclusions in the plate that trip the gate and give a false low reading

The system automatically analyzes the data not excluded by the inspector. The A-scan display shows the gates used for the inspection. If the inspector determines that the gate position or threshold was not correctly set during scanning, he can set new gates in the A-scan display. He can then have the system make new C-scan and B-scan displays based on the revised gates. Thus the data can be analyzed using different settings without rescanning the tank.

The inspector uses the Tankwall Report screen to prepare the report. Check boxes allow him to select which report components to print. It also gives him a preview of the results.

For each course and each lift the results include:

• High reading
• Low reading
• Average reading
• Standard deviation
• Total number of readings
• Number of readings below the minimum specified thickness
• A strip chart plot of the wall profile

The strip chart plots show the elevation on the vertical axis and the thickness on the horizontal axis. A vertical reference line marks the minimum specified thickness for each course.