1. Home
  2. ⚙ Mechanical Engineering
  3. Residual Magnetism in Gas Pipeline

Residual Magnetism in Gas Pipeline

Hi,

Recently I came across a peculiar case of magnetism in a gas pipeline that is in use sine 50years.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
Loading thread data ...

Dear Gopakumar:

1) How did you decide that the end was magnetic? 2) What problem does this present, that prevents you from welding?

Note that heating with a torch will destroy any non-current based magnetism, so I suspect you've got a different problem than you think... perhaps static charge, such as from an active cathodic protection system, can attract small particles too. Did you kill the protection system on

*all* connected piping? Just nearby disconnection might not be enough.

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)

You might have a magnetic field further in than you think. How did you measure the magnetic field? It may be that what you are measuring is cancelled out by the previous section of pipe you've "passed".

Dave

Reply to
David Harper

//

Easy: At the cut ends the field is NOT confined, but spreads. Elsewhere it is concentrated in the steel walls.

To reweld flanges onto the cut ends, it is necessary to wind a convenient number of turns of heavy gage insulated wire at a convenient distance back from the weld site on the pipe, and connect this neutralizing coil to a variable current power supply.

The objective is to adjust the current until a pocket compass is not deflected from magnetic north, near the ends, during the weld.

Brian Whatcott Altus OK

Reply to
Brian Whatcott

Any ferromagnetic material that is magnetized longitudinally will exhibit a strong field measurement at the two ends. These are the so called north and south poles. Field strength is always strongest at the poles, therefore, that's why it appears that the pipe, in your case, is only magnetized 6" from the end, no matter how much you cut off. What is highly probable is that the whole length of the pipe is one big bar magnet. It would be necessary to demag the whole length to remove all residual magnetism. One can demag the ends for welding purposes to prevent arc blow. However, over time, the magnetic field will redistribute itself through the weld and into the next pipe section. CP can potentially leave a very strong residual field even after discontinuation of the current.

Reply to
John Dendy, NDT Level III

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.