Debasis, Your question is not clear if you mean demagnetizaton procedure to be followed for removing an already magnetized component?. I guess your question relates to : Which among the two - circular or longitudinal field generation for de-magnetization is easier to resort for removal of an already magnetized component? Magnetization is of two varieties - temporary or one of permanent category. Question : Which among this needs attention from service related or operational problems from long term use? Temporary field generation using hand held Yoke or coil type MT testing where surface magnetization of < 2-3 mm is only possible. For this demagnetization is not needed. Fields gnerated are weak and disappears after some hours. If your question is related to demagnetization of large turbine or compressor rotors or turbine or compressor casings, Yes after service run of several thousand hours, high horse power and high speed turbine / compressor rotors / casing become magnetized which requires a well thought out de-magnetizing procedure to remove the effects of deep magnetc fields set - which affect the rotor and casing performance and also that of bearings used for turbo machinery. De magentization is done on both circular and longitudinal fields in high speed turbo-machinery and casings. In some cases, if additional weights have been added or removed by welding or grinding from dynamic balancing needs at low rpm or in some cases where heat treatment has been done on the rotor during repairs to replace shrink-fit assembly components like sleeves or spaceers or an inadvertant use of connecting to the turbine or compressor casing from near by structure and use of welding generators near such critical equipment, then deep irreversible magnetic fields are generated in the equipment . Such unwanted magnetc fields create problems in operation of high speed turbo-machinery or frequent bearing failures or down time in turbo machinery or for the plant De-magetization is done before installing or re-starting the rotor for service. Longtiduinal fields (magnetic fields) are easier to remove than circular fields - which are again related to depth of magnetization in these rotors or casings after several years of service or after repairs (HT after shrunk-fit sleeves or spacers introduction) or additonal welding or grinding effect for dynamic balancing or after inadvertent connection a D.C welding generator near by the turbine or compressor. In some complicated cases, you may have to carry out de-magnetization procedure four or five times after a gap for successful de-magnetization - whether ciruclar or longitudinal field removal. This is done for large high speed and high horse power turbo-machinery and casing to remove the effects of deep magnetic fields. May I suggest that instead of a general theoretical question / doubt, you may think of asking a specific question on problem faced while adopting or using a particular technique. Discussion forum may benefit from sharing of such application or problem faced and users' experience in solving similar problems. Trust this will help you C.V.Srinivasan Nishi Engineers Pvt Ltd India June 29, 2007 E-mail: nishi@vsnl.com >It is usually advised and practised that in MT NDT >Longitudinal Magnetisation is done as last operation where >both Circular & Longitudinal Magnetisation Technique are >done simultaneously as testing techniques, where >demagnetisation is a mandatory requirement. This is because >of the fact that longitudinal magnetisation field is both >easy to detect and demag compared to circular field. > >Any one can discuss/ critics on the statement pl. >regards