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Residual Magnetism in Gas Pipeline

Hi,
Recently I came across a peculiar case of magnetism in a gas pipeline
that is in use since 50 years.It is a 40",0.375"thk,API 5LX52 carbon
steel pipe,14KM long,laid underground,with cathodic protection.
Corroded sections of this line were being replaced with new flanged
pipe segmets,for which flanges had to be welded to the cut ends of the
existing old pipe.Theline was cut using cold cutting saw.
We experienced strong magnetic field at the cut ends,making it
impossible to weld the flanges on the cut ends of the old
pipe.Surprisingly,the magnetic field was found only upto about 6" from
the cut ends; beyond this the line was not magnetised.The cathodic
protection was already disconnected and hence we presumed that the
only source of the mag field induction could be the cold cutting
saw.In order to confirm this,one of the cut ends was futher cut with
gas torch;but,still the portion upto 6" from the cut end showed strong
magnetism as before.
CAN ANYONE PLEASE EXPLAIN WHY THE MAGNETIC FIELD IS CONFINED ONLY TO
THE CUT ENDS(UPTO ABOUT 6" FROM THE END)? IS IT THE RESIDUAL MAGNETISM
INDUCED BY THE CATHODIC PROTECTION - IF SO, WHY IT IS CONFINED TO THE
CUT ENDS AND NOT ALL THROUGH?
regards
Gopakumar
Reply to
Gopakumar
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My wild guess is the same reason a long bar or rod magnet pulls better at the ends. Also, how far are the ends of the pipe from each other?
You may be able to run some steel bars over the gap, alowing the magnetic feild to flow down the bar, away from the weld.
Les
Reply to
PIW
All metal structures will develop some magnetism if they remain in the same position for long periods of time. One example I know of is a ship having deviation problems with the magnetic compass after a long time docked in one position. Drillpipe also will develop magnetic properties. In your case the lines of flux will flow along the metal until they reach the end of the pipe. Air does not carry these "lines if flux" as well so a welding rod or any other metal will be attracted. A circular pipe will carry the lines of flux nicely around its circumference but not at the ends. This is a crude description. :'))) Couldn't resist. I suspect that the cathodic protection would aid in the pipe aligning itself with the earths magnetic field. Vibration is also known to aid in this process. Randy
Reply to
Randy Zimmerman
Mag field is throughout the length of the pipe, but you only see it strongly at the ends where it must exit the pipe into free space.
It has nothing to do with the cutting process (though it can... A torch cut will bring the material above the Curie point-the temp where ferromagnetic properties go away on heating and come back on cooling- and when it cools it will pick up the mag feild of the earth, or any other strong magnet nearby) but may be residual from manufacture, from previous welding, from long term exposure to the earths field, from strong currents flowing through the pipe, from friction of the material in the pipe, or from natural magnets near the pipe some distance away.
Not a lot you can do in a pipe this size. AC weld current will help a lot, if you have an acceptable procedure, and you may be helped by attaching multiple work leads to both pieces and frequent tacks to reduce the current density and give a more uniform current in the weld area (reduces the mag field from the weld current) But likely, it's just going to be a pain.
Reply to
e
How about placing a series of small bar magnets around the pipe oriented against the residual magnetic field to cancel it out during the welding process?
I believe by sliding them towards or away from the end of the pipe you may be able to position them to almost cancel the residual magnetic field at the end.
Good luck-
Paul T.
Reply to
Paul T.
You could also wind wire around the pipe near the end and pass an adjustable DC current varying the amount and direction until the field is seen to disappear (use a compass to test).
Alternate solution would be to swing pipeline to E-W direction (just kidding)
One other effect that you will see in pipe is that it magnetizes into distinct domains with the strength of magnetization varying along the pipe.
Graham P
Reply to
Graham Parkinson
I have no experience with your welding situation, however, I do know that a magnetic field decays with heat. Couldn't you just preheat the pipe before welding?
Eric D
Reply to
Eric D
Odds are good the whole pipe is magnetized. Heating a section will destroy the magnetic field inducing properties of that section, the steel will still conduct lines of force from the next door, non heated sections. This might reduce the problem but unlikely enough to make it worth the effort.
Eric D wrote:
Reply to
RoyJ
This is a common occurence, just wrap your ground lead around the pipe several times, experimentation will tell you which direction, and how many wraps.
JTMcC.
Reply to
JTMcC

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