Exploration • Drilling • Field Services
Around the pipe in 80 seconds
Magnetism stops welding. Then the old ‘black magic’ begins. John Anderson reviews the causes and some solutions without recourse to witch doctors.
M
agnetic arc blow causes welding job delays, poor weld quality, welder frustration and unexpected costs – particularly frustrating when everything is in place yet
the project is completely stalled. Now we get to the point where ‘black magic’ is invoked to make the magnetism disappear: heat it up, hit it with a hammer, trail the weld set cables over it ... wait till the sun goes down? Like most old wives’ tales, there is often a spark
of truth in there somewhere. For example if you could heat the whole pipe to the Curie point temperature (~600C) then the magnetism would be removed. However, so much of such lore does not have any real practical validity and the magnetism problem needs to be addressed with proper engineering rigour. Tere are several solutions to the arc blow problem and here we identify the advantages and disadvantages of each.
Field nulling
Te first method is to use an automatic or manual nulling method. Te idea here is to induce a reverse field in the pipe (or other steel part to be welded) so that the net field is zero. With this in place welding can take place as if there were no magnetism gremlins at all (Fig. 1). When the welding is completed, the reversing field can be removed and magnetism in the pipe returns – but this is now of no consequence. Tis patented technique was developed over 10 years ago in conjunction with a national welding institute and the resulting demagnetiser, Zeromag ZM100A is now made by Diverse in the UK. Te technique can be fast to deploy and the result is effectively instantaneous. Te demagnetiser is designed specifically for this application, and can operate in one of two ways, manual or automatic: the choice is determined by the cause of the magnetism. Te variety of possible causes of the magnetism
result in some complexity with this approach. If the magnetism has for example, been induced end to end by the earth’s field, then the field emanating from the pipe end surface will be homogenous and the field can be easily nulled all around the circumference with a single manual setting. Alternatively, if the magnetism has been induced by
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industrial processing such as NDT, magnetic lifting or pigging, then the field at the end of the pipe will not be homogenous and the reverse nulling field will need to change dynamically in magnitude and sign as the welding progresses. Tis is accomplished with the demagnetiser in automatic mode.
Degaussing
Classic degaussing is another approach to this problem. Te technique removes the magnetic field by down cycling which excites the magnetic field to a very high positive value then progressively decreases the magnitude of the field to zero while alternating its sign – a process called down cycling (Fig. 2). Te result is that through this wizardry, all the magnetism is removed! Tis approach has several practical problems. It can be slow taking hours or even days to do the job. Worse it may not be effective at all if the strength of the degaussing field is too low (below the saturation
Fig. 1. Welding with the magnetic gremlins banished.
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