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NDT.net - December 1998, Vol. 3 No. 12
MFL and PEC Tools for Plant Inspection
J.H.J. Stalenhoef, J.A. de Raad Röntgen Technische Dienst bv P.O. Box 10065, 3004 AB Rotterdam, The Netherlands telephone +31 10 2088208, fax +31 10 4158022, Corresponding Author Contact: Paul van Rooijen AIB-Vinçotte Nederland B.V., P.O.Box 6869, NL 4802 HWBREDA, The Netherlands Email: firstname.lastname@example.org TABLE OF CONTENTS Introduction Inspection with Magnetic Flux Leakage (MFL) Tools Inspection of Storage tanks Inspection of Piping and Vessels Inspection with Pulsed Eddy Current (PEC) Inspection of insulated components Inspection of corroded, coated or hot surfaces References
Table of Contents ECNDT '98 Session: Pipeline InspectionIntroduction
Increasingly, pipeline and plant operators apply non-destructive testing (NDT) as an economic tool to establish the condition and integrity of their equipment and components. Results are used for predictive maintenance and life-time extension programmes. This approach, unless risk based inspection (RBI) is used, requires more surface coverage of the inspected locations than can be achieved bycommon spot-check inspection techniques . Full-surface coverage with NDT inspection is often prohibited due to unsuitable methods which are slow and hence too expensive. This created the demand for the so called screening tools with which suspect areas can be detected very rapidly. In recent years several of these methods have entered the market. In this paper two methods will be discussedthat can be applied on carbon steel components.
The first method is based on magnetic flux leakage (MFL) and is suitable for inspection of non-insulated components. The second method is a Pulsed Eddy Current technique called INCOTEST which can be applied on insulated components.
Inspection with Magnetic Flux Leakage (MFL) Tools
MFL tools are suited for inspection of exposed bare, painted orthinly coated carbon steel components. Nowadays MFL is a well-known matured technique, extensively used for example in intelligent pigs. RTD launched this technique in the 1980's to inspect the floors of storage tanks. The family of scanners now includes the Floorscanner and Mini Floorscanner for inspection of storage tanks, the Pipescan for inspection of piping and the Wallscan for inspection ofvessels.The principle of MFL is shown in Figure 1. A strong magnet is used to establish a magnetic flux in the material to be inspected. When there is no defect the uniform flux remains in the metal as illustrated in Fig. 1. In contrast, Fig. 1. also illustrates the flux leakage that occurs when there is a defect due to corrosion or erosion. Near the defect the flux "leaks" out of the metal. Sensorswhich can detect this flux leakage are placed between the poles of the magnet and generate an electric signal proportional to the magnetic leakage flux. Defects causing a leakage flux exceeding a predetermined threshold are detected. The amount of leakage flux is dependent on depth, orientation, type and position (topside/bottom-side) of the defect. Defects exhibiting various combinations ofvolume loss and depth can produce the same flux leakage level. Therefore truly quantitative depth information can not be obtained by this method. It is a qualitative not a quantitative inspection tool. For accurate sizing of the defects ultrasonics can be applied after detection.
Fig 1: Principle of Magnetic Flux Leakage for detection of wall loss
Inspection of storage tanks Typically storagetanks suffer from corrosion at the underside of the floor bottom. Often this type of corrosion will be localised (pitting), sometimes larger areas may be corroded, for instance at places close to the tank shell. Having access only to the top surface of the floor this corrosion can not be found by visual inspection. Therefore an inspection method is needed that can detect underside corrosion from...