Exploration • Drilling • Field Services
Degaussing is a preferred choice as there are no issues with the type of magnetism or restriction on the number of weld positions. Tere are two drawbacks: the magnetism remains in the rest of the pipe and will return to the region to be welded over time; and whether it will be successful is not known in advance. Classic degaussing removes the magnetic field
by exciting it to a very high positive value then progressively decreasing the magnitude of the field to zero while alternating its sign – a process called down cycling (Fig. 2). Te success of this technique depends on two crucial factors: the maximum field supplied; and the rate of the down cycle. Is the field applied large enough? To successfully degauss the applied field must be large enough to magnetically saturate the steel (i.e. reach the region of the BH graph in Fig. 2 where the slope is flattening).
Different grades of steel have varying saturation fields and their BH curves are often not readily available. For different materials with similar geometry, one may be successfully degaussed whereas for another field nulling is required. Is the downcycle frequency slow enough?
Down cycle frequency determines how far the magnetic field penetrates into the pipe wall. Many industrial degaussers operate at 50Hz, but the field at this frequency only penetrates a few mm (again this is material dependent). At frequency of 1Hz penetration rises to at least 10-20mm. So for thick walled pipe degaussers that can operate at lower frequencies are required, such as the ZM150 degausser operated with Zeromag. Selecting the best route to tackle magnetism depends on the details of the job. Field nulling will work for all jobs but can place restrictions on the number of welders that can work simultaneously. Whether manual or automatic nulling is used will be governed by the type of magnetism present. Joint and pipe end
Fig. 2. BH curve showing classic degauss down cycling.
If the maximum field does not saturate the steel then, following the degauss cycle, the pipe will be left in a magnetised state. A number of factors affect the field that can be applied, including the number of turns applied to the pipe and the geometry, and the maximum degaussing current. Pipe size is a factor that is defined for a particular job and this determines the maximum number of turns that can be applied. Fig. 3 shows how the degaussing field drops as the pipe size increases (for a fixed current of 100A and 100m length of demag cable). For large pipe sizes the degauss field is unlikely to saturate the material and in this case degaussing will not be successful. For a given conductor diameter, degausser power
is important because it determines the maximum degaussing field (ampere turns) that can be achieved. Te maximum degauss field for a specific scenario can be calculated, however, whether this is sufficient for effective degaussing depends on the material.
degaussing solutions do not place any restrictions on the job but whether they work for a particular job depends on a number of factors. So to answer the question originally posed, it depends! l
For more information ✔ at
www.engineerlive.com/iog
Fig. 3. Nominal degauss field for 100A through 100m demag cable wound over 15cm.
Dr Steve Foulds is a magnetic technical specialist at Diverse Technologies, Cambridge, UK. www.diverse–
technologies.net
www.engineerlive.com 33
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