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• James Bradford

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Temporary Pipeline Demagnetization

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Commercial Demagnetizing Equipment Pipelines commonly have residual magnetism following a magnetic cleaning or MFL inspection, causing problems when welding.

Commercial equipment is available on the market which is specifically designed to reduce the effects of residual magnetism during arc welding operations. This equipment can generally be obtained through local suppliers.

Finishing the inspection Pipelines often retain residual magnetism following a cleaning run by a Magnetic Cleaning pig, or an inspection by MagneScan or another magnetic flux leakage tool. If ignored, this magnetism can inhibit subsequent welding, making it difficult to repair the pipeline. A demagnetization process must be undertaken so that welding can take place. Moderate levels of magnetism, anything above 20 Oe, can deflect the welding arc from the pipe surface. This effect is termed ‘arc blow’, and it makes welding difficult and ineffective. The level of arc blow occurring in the welding operation depends on the welding technique applied. Generally, TIG welding is more sensitive than MIG or MMA, and AC techniques are less sensitive than DC. But depending on the level of residual magnetism in the pipeline, welding complications can arise regardless of the technique applied. The effect of residual magnetism can be overcome using proven procedures and proprietary equipment that have been applied to many pipelines, and have demonstrated their reliability over several years. The following document outlines two methods of Magnetic Field Compensation recommended by PII, depending upon the equipment available and the pipeline involved.

Typical magnetization levels can be categorized as follows:

Minor Magnetism (100 Oe) This level of magnetization often results after an inspection with high resolution MFL tools such as MagneScan or TranScan. Demagnetization is necessary before welding, regardless of the AC or DC technique applied.

The Removal of Detrimental Magnetic Fields at Pipeline Tie-ins. The American Gas Association, 1980. 1515 Wilson Boulevard, Arlington, Virginia 22209 USA

Listed below are a specialized PII Gauss Indicator and suitable Gauss Meters to be used in the demagnetization procedures described in this document.

Demagnetization Techniques for Weld Requirements. The Weld Institute, July 1984. Abington Hall, Abington, Cambridge, CB1 6AL UK

• PII Gauss Indicator, which can be purchased from any PII location • GM04 Gauss Meter and Transverse Hall Probe Magnetic Instrumentation 8431 Castle Wood Drive Indianapolis, Indiana 46250 Tel: (317) 842- 7500 Fax: (317) 849 - 7600

Metals Handbook Welding and Brazing Vol.6. American Society for Metals Demagnetization of Pipe Butt Joints Prior to Welding. K. Prosser, British Gas Standard Ref. KP/YS/2593/2, January 1978.

• Trilec 705 Gause Meter and Transverse Hall Probe Magnetic Developments Limited Unit 17, Highworth Industrial Estate Highworth, Swindon, Wiltshire SN6 7NA United Kingdom Tel: 011 44 1793 -766001 Fax: 011 44 1793-765576

Magnetic Arc Blow Pipe Joints. E.W.L. Norman B.A. M. Sc, The Welding Institute Research Bulletin, September 1981. Magnetism in the Welding of Pipelines. P.J. Blakeley, J. Simkin, Welding and Fabrication, Vol. 60, No. 10, December 1992.

• 7000 Series Gauss/Tesla Meter & Transverse Hall Probe FW Bell Products Division 6120 Hanging Moss Road Orlando, Florida 32807 Tel: (407) 678 - 6900 x213 Fax: (407) 677- 5765

Demagnetization of Pipe Ends Before and During Welding. H. Koster Praktiker, Vol. 38, No. 12 December 1986 (in German). Magnetic Arc Blow. Part 2: Effects and Solutions. E.W.L. Norman Metal Construction, Vol. 16, No.8. August 1984.

• Redcliffe Field 2 (formerly model 100) with Transverse Probe RC1DOT International MagnaProducts Inc. 3103 Cascade Drive Valparaiso, Indiana 46383 Tel: (219) 465 -1998 Fax: (219) 462 - 5145

Procedure to allow welding on magnetized line pipe TN 101

Residual magnetism in pipelines

References

Corporate headquarters PII Group Ltd.

PII North America Inc.

PII Canada Limited

PII Mexico

Atley Way, Cramlington,

7105 Business Park Drive

Calgary:

Tel: 52 5 559 8233

Northumberland

Houston, Texas 77041

Tel: (403) 262-7447

Fax: 52 5 575 5562

NE23 1WW, United Kingdom

Tel: (713) 849-6300

Fax: (403) 237-9693

Email: [email protected]

Tel: ++44 191 247 3200

Fax: (713) 937-0740

Toronto:

Fax: ++44 191 247 3101

Email: [email protected]

Tel: (905) 946-0173

Email: [email protected]

Fax: (905) 946-0262

www.piigroup.com

Email: [email protected]

Magnetic Field Compensation

Step by Step

Step by Step

1. With a PII Gauss Indicator or Gauss Meter and a suitable probe, measure the strength and polarity of the magnetic field in the root gap of the joint to be welded. Figure A shows how to measure the magnetic field.

Fig. 1: Diagram of coils layout for method 1

Method One: The two welding generator approach One method of reducing residual magnetism in pipe work is to induce a compensating magnetic field during the welding process with a second welding generator. This method is employed when welding cannot be completed quickly. By maintaining this compensating field throughout the welding procedure, the risk of arc blow is eliminated. Using this method of demagnetisation, the residual magnetic field is opposed by the induced field, reducing overall pipe magnetism to low levels. These are the minimum equipment requirements to carry out this operation: • Flexible welding cable, 300 amp. capacity. Aluminum cored cable is not recommended for this procedure. • 1x Standard DC Welding Generator for field compensation. It is important that the current output is smooth DC and controllable down to approximately 10 amp. • 1x Standard AC or DC Welding Generator for welding. • PII Gauss Indicator or Gauss Meter, range between 10 to 1000 Gauss. Do not attempt this procedure without one.

2. Wrap the welding cable around one pipe end to form a tight coil (approximately 10 -12 turns). With the same welding cable, form a coil around the other pipe end. The edge of the coils should be within 500 mm of the root gap. The coil should be wound in the same direction for both pipe ends. Figure 1 provides a diagram of the coils layout, and Figure B a picture. 3. Connect the cable ends to the DC welding generator. Then, using the Gauss Indicator or Meter, prepare to read the field in the weld gap. Figure C shows the equipment and coils layout. 4. Set the welding generator current to its lowest setting and switch it on, as shown in Figure D. Slowly increase the current supplied to the coils and observe the reading on the Gauss Indicator or Meter. The field should decrease as the current increases. 5. In the event that the field increases, reverse the lead connections to the generator. This changes the polarity of the induced magnetic field. 6. Continue to increase the current until the field in the weld gap is as close to zero as possible – at least below 20 Oe. This is the point that the induced magnetic field is of equal magnitude to the residual field, but of opposing polarity. Normal welding becomes possible at this point. 7. Throughout the welding of the root pass, ensure that the current to the coils remains on.

1. Before starting, ensure that all pipe components are fully prepared and sized for the proposed tie-in, as time is limited. Separate all components by at least 300 mm. Figure 2 provides a diagram of the pipeline layout; Figure A a picture.

A

Fig. 2: Diagram of equipment and coils layout for method 2

B

C

D

Method Two: When only one generator is available A second method of demagnetising a pipeline is to induce a magnetic field that temporarily disrupts the residual magnetism in the pipe prior to normal welding procedures. The advantage of this approach is that a single DC welding generator can do the whole job of demagnetising the pipeline and welding the root pass. However, the pipeline remains demagnetised for only a short period of time. If two generators are available, operators may prefer to use method one. This procedure does not permanently demagnetise the pipeline, but instead creates a temporary disruption of the fields to allow the welding process to proceed. After disrupting the field strength to a suitable level, the operator has about one hour to start the root weld. These are the minimum equipment requirements to carry out this operation: • Flexible welding cable, 300 amp. capacity. Aluminum cored cable is not recommended for this procedure. • Standard DC Welding Generator • PII Gauss Indicator or Gauss Meter, range between 10 to 1000 Gauss. Do not attempt this procedure without one.

2. The PII Gauss Indicator or Gauss Meter probe should be in gentle contact with the pipe and aligned such that it measures the field strength in line with the pipe axis. Identify the position around the circumference where the field is at its maximum, then mark the location and note and reading polarity. Figure B shows how to measure the magnetic field. 3. Wrap the welding cable around the pipe to form a tight coil (approximately 1820 turns). The edge of the coil should be within 80 mm from the pipe end being demagnetised. Figures 2 and C show the coil layout. 4. Connect the DC welding generator output to one end of the cable and complete the circuit by connecting the stick holder to the earthing clamp with a suitable piece of steel plate – see Figure D. This should be done such that it facilitates its use as a circuit breaker at a later stage. Figure E shows a picture of the equipment and coils layout. 5. Adjust the welding generator to a low current setting, somewhere less than 80 amps, and turn it on with the polarity switch. This will change the field strength shown on the Gauss Indicator or Meter. If the reading decreases or reverses, note the position of the polarity switch, for use in step 6, and turn the welding generator off. If the reading increases then repeat step 5, this time with the polarity switch reversed. Then follow the earlier instructions – note the position of the switch and turn the

6. Adjust the current setting control to zero. Turn on the generator with the polarity switch, and set it to the position established in step 5. Quickly increase the current supply to the coil to 300 amps and hold for ten seconds. Switch off by breaking the stick holder/earthing clamp joint to cause an immediate interruption in the current. Do not reduce the current gradually, as the interruption speed of the applied field produces the required changes. Switch off the welding generator in the normal manner, as shown in Figure F. 7. Observe the field strength and direction at the position noted in step 2. Compare this reading to the original. The field strength should be reduced or reversed. See Figure G. 8. If the reading is less than 20 Oe, then DC welding is possible without further demagnetisation.

9. If the reading is less than 100 Oe there is a choice of either AC welding or continuing further demagnetisation by repeating steps 2, 5 and 6 until the field falls below 20 Oe. 10. If the reading is greater than 100 Oe, further demagnetisation is necessary. If the polarity has reversed, then repeat steps 2, 5 and 6. If not, return to step 6. 11. If a constant minimum field strength reading is obtained after repeated demagnetisation then there is insufficient flux density created by the product of the weld current in amps and the number of turns on the coil. This is a common problem when working on thick wall pipe, usually in excess of 19 mm. To overcome this it is necessary to increase the number of turns on the coil, thereby increasing the amp turns. This, in turn, increases the demagnetisation penetration. Repeat the whole procedure from step 2, with the ampere product doubled.

A

B

C

D

Table 1. Minimum length of cable required for demagnetisation Normal Pipe Diameter

E

Minimum length of cable (m*)

mm

Inches

mm

Inches

100 150 200 250 300 350 400 450 500 550 600 750 900

4 6 8 10 12 14 16 18 20 22 24 30 36

9 12 16 20 24 28 32 35 39 43 46 58 69

7 10 13 16 19 22 26 29 32 36 38 48 57

generator off. * These lengths only refer to the coils – further cable is necessary to connect the coil to the welding generators. Note: Thick wall pipe may need more turns in the coil and so need more cable.

F

G