Motherwell Bridge

Floor Tester for Storage Tanks

The reporting software allows the production of storage tank floor templates, into which the Floormap generated data can be imported. This allows the production of inspection reports at three levels of resolution, namely, Tank level – This allows a quick overview of the condition of the tank floor with each plate colour coded to indicate the worst case detected level of corrosion. Plate level – This allows plates to be analysed individually, at a 250 mm x 250 mm resolution. Track level – This allows a more detailed (15 mm x 15 mm resolution) corrosion map of individual plates to be produced. Plates up to 12.5 mm thickness can be inspected in full mapping mode with sensitivity to defects of 20% loss or better. Plate thickness’ greater than 12.5 mm and up to 20 mm can be inspected in analogue mode with reduced sensitivity. Inspection can be carried out through coatings up to 6 mm thick depending on the plate thickness. It is often necessary to inspect tanks with extensive top surface pitting. In the early days of MFL tank floor testing this was considered impracticable, since the top surface pits would mask any underflow corrosion. However, the sensor arrangement on the Floormap is slightly more sensitive to underfloor than to top surface pitting. Floormap will store and report all indications above a predetermined threshold. Severe defects can be chosen for ultrasonic or visual confirmation first, and the threshold varied as necessary at the reporting stage, depending on the results of this correlation. Because of the irregular shape of annular plates, they are difficult to inspect in mapping mode. Annular plates are usually inspected in manual mode (amplitude threshold with auto-stop) or monitor mode (computerised depth and position estimation, but no mapping), and indications marked up for subsequent confirmation. Results are then colour coded at the tank level only.

The magnetic / sensor carriage is also designed so that it can operate in manual mode, remote from the main body of the Floormap on extension leads. This is particularly useful where access is restricted, for example, when heating coils or internal pipework are present. In manual mode an LED array allows the operator to locate areas of corrosion as they are traversed. In addition, an auto-stop device is incorporated which automatically stops the rear edge of scanning head over defects, thus ensuring the maximum possible confidence in the inspection. The areas identified are then visually and ultrasonically examined and coded for top surface or underfloor corrosion. The floor should be free from debris and product residues, but it is not always necessary to grit blast. In addition to the Floormap, Torch Integrated Inspection Service can also provide design advice, fabrication and repair facilities, heat treatment, welding and metallurgical consultancy. These facilities are all available within the Motherwell Bridge Group and can be activated on a single source basis, thus giving savings on both time and financial aspects.

The effective inspection of large storage tanks is becoming more and more important as environmental awareness and the associated costs of leakage increase.

The cost of leakage includes:

Environmental pollution
Clean up costs and penalties
Loss of product
Temporary loss of storage capacity
Adverse affect on company reputation.

In the past, underfloor corrosion has been difficult to monitor in a cost effective way and ultrasonic spot readings cannot be regarded as an adequate method of ensuring that no critical defects are present in the tank floor. Torch Integrated Inspection Services is now able to offer a cost effective service for tank floor corrosion monitoring based on the tried and proven Floormap. Floormap is based on magnetic flux leakage principles, and relies on the fact that underfloor corrosion pitting will produce a measurable leakage field on the top surface when a sufficiently strong magnetic field is applied to the floor plate. Magnetisation of the plate is produced by very strong horse shoe type, rare earth magnets. The leakage fields are detected by an array of Hall effect sensors centred between the poles of the magnet bridge and stretching the full scanning width of the system (250mm). The scanner is motorised with a forward speed of 0.5m/s. The Floormap incorporates an on-board ruggedised computer and data acquisition circuitry. During inspection, the digitised leakage field signals are converted into estimated percentage wall loss by comparison with an amplitude based correlation curve, produced with signals obtained previously from a reference calibration plate of the same thickness as those being inspected. The artificial calibration defects used have been found, by trial and error, to give a good approximation of natural defects.