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Knowledge Bank

Capability of modern tank floor scanning with Magnetic Flux Leakage

Presented at the WCNDT conference 2016 by Jim Costain

This paper focuses on state-of-the-art Magnetic Flux Leakage (MFL) technology for the inspection of storage tank floors. The primary advantage of the MFL approach is the ability to locate and estimate the size of defects over large areas in a quick and efficient manner. As with any inspection approach, there are limitations which can influence the consistency and reliability of reported defects. However, MFL sometimes appears to be considered as a simple technology screening approach where any competent inspector can interoperate their signals. However, there are misconceptions of the approach that might simply be due to a lack of awareness. Some perceived limitations will be outlined together with suggestions to reduce their effects. To assist with overcoming these parameters further, technological advancements such as a combination of high-resolution scanners and a complementary approach to top and bottom defect discrimination and lead more consistent and reliable inspections.



Efficient inspection from measurement collection through to report

Presented at the WCNDT conference 2016 by Wayne Woodhead

Ascertaining the integrity of large steel structures such as storage tanks, pipes and vessels is a complex task. Silverwing (UK) Ltd has developed an inspection solution that can inspect, manage, present and generate reports. The inspection data generated from these large assets can be large, requiring gigabytes to even terabytes of storage and meticulous analysis. And while it is normally accepted that more data is always better, the ability to handle, analyse and report findings with vast amounts of information efficiently becomes a challenge. In this paper, examples are given towards illustrating how volumes of information is technically handled and how it can improve the efficiency of the overall inspection process from the measurement gathering stage to the report and how this can benefit the inspection company, asset integrity engineer and asset owner..



Bandwidth of MFL in steel plate inspection

Presented at the WCNDT conference 2016 by Dr. Neil Pearson

Magnetic flux leakage (MFL) continues to be a widely used approach to detect defects caused by corrosion in applications with large areas. The principal merit of MFL is that large areas can be covered relatively quickly making it beneficial for example in the inspection of asset components that are costly to expose and in large area; two good examples are the floor of above ground storage tanks or exceedingly large surface areas such as pipelines. Though rapid, MFL is continually reported to have limitations when estimating the geometry of defects. In this paper we introduce and define the frequency response (FR) of MFL in an attempt to understand the relationship between MFL and defect geometry. This novel approach describes the relationship between MFL and defect shape using simulated sinusoidal defects to reveal important fundamental characteristics of MFL.



Factors that affect the defect sizing capabilities of the Magnetic Flux Leakage Technique

Presented at the BINDT conference 2014

This paper is concerned with investigating inherent Magnetic Flux Leakage (MFL) technology variables that affect the reliability, repeatability and accurate sizing of defects. External defect sizing factors such as a clean inspection environment are not considered. With such extraneous variables removed the effects on the MFL signal due to magnetic saturation, the calibration process and of defect geometry can be investigated. The results presented herein confirm that an under-saturated inspection surface is a major limiting factor in defect sizing. Consequently to overcome the limitations presented by under-saturation a new calibration procedure is proposed and investigated. Further suppositions that pertain to defect sizing, due to defect geometry, are also explored and verified empirically.



Improving Confidence in Tank Inspection

Published in the Tank Storage Magazine September / October 2014

Tank inspection is an important part of effective asset management, from both regulatory and operating efficiency perspectives. A failure of a tank can be catastrophic, but probably more likely is degradation resulting in slow loss of stored product and possibly contamination of the environment. ...



Discrimination of top and bottom discontinuities with MFL STARS Technology

Presented first in MENDT 2012

Discontinuities due to corrosion can occur on the top and bottom surface of above ground storage tank (AST) floors and can, if unknown increase the likelihood of a leak as inadequate repair strategies may be applied. Although surface discrimination can in some cases be achieved visually, low lighting conditions and in particular ASTs with opaque coatings on the top surface of the floor make the task of identifying top surface discontinuities dicult. This paper presents a novel approach to the problem of discriminating top and bottom discontinuities, an approach that has been incorporated into the next-generation of Floormap MFL scanner (Floormap3D) developed and manufactured by Silverwing (UK) Ltd...



Tank assessment depends on the accuracy of measurements as well as the probability of detection

Published within Tank Storage Magazine September 2012
Storage tank corrosion inspection is an essential part of asset management, being used not only to certify safe operation, but also to predict expected lifetime, devise a repair strategy and assess the impact of different contents stored...



The challenge to asset owners and operators is to use new systems to reduce out of service time

Published within Tank Storage Magazine Vol 8 Issue 2 March 2012
Storage tanks are an essential part of the distribution network and are vital in buffering the varying demands of end users. They are most commonly built from steel with thickness from 5-15 mm. Steel is a relatively cheap and strong material that can easily fabricated on site but does have inherent problems of corrosion over time...



The influence of maintenance on the life cycle of above ground storage tanks

Published within INSIGHT magazine June 2012 edition

In supply chains such as those in the petrochemical industry, the above ground storage tank (AST) plays an important role in ensuring a continuous flow of product and these like other components must undergo regular maintenance. While maintenance on the majority of surfaces of the typical AST can be conducted when in its normal operational condition, the AST floor presents a particular challenge because of its inaccessibility. As a consequence the tank has to be periodically emptied and made out-of-service to conduct inspections and repair work deemed necessary. This is a costly activity both in terms of loss of earnings and the maintenance operation itself...



Practical limits of MFL in steel plate inspection

Presented by Neil Pearson at the 18th World Conference for Non Destructive Test (18th WCNDT)

Magnetic flux leakage (MFL) is a widely used approach to detect corrosion in applications where large areas are to be inspected in short time scales. A particularly good example is in above ground storage tanks (ASTs) within the petrochemical industry where tank floors are inspected periodically, calling for the AST to be taken out-of-service and emptied. This makes maintenance times that much more expensive and calls for techniques that are both reliable and fast. MFL is widely used in the context because of its inherent speed. Magnetic flux leakage (MFL) is a widely used and accepted technology for locating defects on a tank floor. While MFL signals are often linked to the volume of a defect, its depth is perhaps the most dicult to estimate and the most critical dimension since it indicates the closeness of a potential leak and if misinterpreted can lead to erroneous repair strategies with costly outcomes. Therefore, accurately determining the geometry of defects is pivotal if an optimum repair strategy is to be formulated...



A study of MFL signals from a spectrum of defect geometries - Poster

Presented by Neil Pearson at the 18th World Conference for Non Destructive Test (18th WCNDT)



What preparation is required before a magnetic flux leakage (MFL) tank inspection takes place?

General questions and answers tank preparation prior to an MFL inspection



Magnetic Flux Leakage Technology - JC Drury

Magnetic Flux Leakage (MFL) technology has been used in the monitoring of Underfloor or Far Side (FS) corrosion for about 11 years. Silverwing (UK) Ltd entered the field ten years ago, in 1991. As with all Non Destructive Testing methods, MFL has both advantages and disadvantages, as well as pitfalls for the unwary. This paper attempts to explain the underlying principles of the method and highlights the advantages, disadvantages and pitfalls...



A comparison of the magnetic flux leakage & ultrasonic methods - JC Drury

Presented at the 15th World Conference in Rome 2000

Magnetic Flux Leakage (MFL) and manual Ultrasonics (UT) have been used extensively for the detection and sizing of corrosion pits in ferrous plates and pipes. Users and providers of these inspection services may have different perceptions and expectations of the sensitivity and accuracy of the methods. This paper discusses the underlying principles of the methods and their effect on Probability of Detection (POD) and accuracy. It addresses the limitation on accuracy of the ultrasonic method due to pit shape and reflectivity, and also, for the first time, takes a quantitative look at the effect of pit volume on MFL results...



Corrosion Monitoring and Thickness Measurement - What are we doing wrong ? - JC Drury

During the last three years my interest in the use of ultrasonics for the detection of corrosion and the measurement of remaining wall thickness has been re-awakened. The method has been extensively used to verify and quantify Magnetic Flux Leakage (MFL) results. Discrepancies between ultrasonic and MFL results have usually been put down to "Another MFL false - call." However, having watched many ultrasonic tests being carried out, I have come to believe that the boot is often on the other foot and that we place too much faith in the ultrasonic method for corrosion monitoring. The techniques used by many practitioners give rise to serious shortcomings in both probability of detection, and accuracy of remaining wall assessment. These shortcomings are not confined to corrosion monitoring of flat plate, but apply equally to pipe and vessel inspection. This paper looks at the root causes of poor performance with ultrasonics and suggests some methods to improve the situation...



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