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Technical Discussions
Peter
Singapore, Joined Jan 2012, 119

Peter

Singapore,
Joined Jan 2012
119
12:02 Apr-07-2014
MPI AC/DC Coil Calibration

Dear All,

Can somebody guide me on this AC/DC coil yearly calibration. How to perform inhouse calibration for this coils? Any idea or if you have any guideline kindly forward to me for my reference.


Peter

 
 Reply 
 
Tim
Tim
15:07 Apr-07-2014
Re: MPI AC/DC Coil Calibration
In Reply to Peter at 12:02 Apr-07-2014 (Opening).

What are your requirements or what standards (specifications) are you working?

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
16:40 Apr-07-2014
Re: MPI AC/DC Coil Calibration
In Reply to Peter at 12:02 Apr-07-2014 (Opening).

Peter,
I can't say anything about AC coils, but DC coils used in the inspection of oilfield tubulars such as drill pipe and drill collars are discussed in TH Hill DS-1, Fearnley Proctor NS-2 and API RP7G-2 (ISO10407-2)

In DS-1, DC coils used to magnetise rotary connections shall produce a magnetising force of 1200 amp-turns per inch of component diameter.

NS-2 is rather more complex.
When the component OD is less than 8 inches, the coil shall produce a magnetising force (in amp-turns) = (440 + 65Dp) Dc, where Dp is the component OD and Dc is the average coil diameter.
Alternatively, the magnetic field in air measured at the centre of the coil bore shall not less than (200 + 30)Dp) gauss.

For components 8"OD and larger the amp-turns shall be 960(Dc) or the magnetic field at the centre of the coil shall be not less than 440 gauss

API RP7G-2 provides a table (C.1/D.1) specifying both required amp-turns and minimum gauss in air at the centre of the coil for a range of component ODs

To certify that a coil meets any or all of these standards requires a calibrated DC ammeter capable of measuring the DC current flowing in the coil windings.
Every coil should be marked with it's number of turns, so when the current is measured, it's a simple matter of multiplying turns x amps to get the magnetising power of the coil.
When certifying coils to either of these standards it's important to calculate the maximum component OD that coil can magnetise and still meet the requirements of the standard.
For example, under DS-1, a 10" ID coil which produces 10000 amp-turns is only capable of magnetising components up to 8.33 inches OD (10000/1200 = 8.33)
A 10" ID coil which produces 12500 amp-turns can magnetise components up the maximum size of the coil (12500/1200 =10.41)

It may be that it is not possible to get a calibrated ammeter in series with the coil windings. In that case a calibrated electronic gauss meter and axial hall probe is required.
Using a gauss meter and probe when certifying to NS-2 and API RP7G-2 is easy since these standards provide the minimum gauss value required for any size of component.
For DS-1 certification a little maths is needed
The magnetic field (B in gauss) at the centre of a coil = (12.57 x NI)/Dav
Where N is the number of turns in the coil
I is the value of DC current flowing through the coil
Dav is the AVERAGE diameter of the coil
This equation can be re-arranged to solve for the coil current I
I= ( B x Dav)/(12.57 x N)
Using this equation, and knowing the number of turns in the coil and the average diameter of the coil, you can calculate the amps flowing through the coil and hence calculate the amp-turns for the coil

Hope that helps

Best Regards

Bruce McPherson

 
 Reply 
 
George Hopman
Consultant,
NDE Solutions, USA, Joined Apr 2009, 44

George Hopman

Consultant,
NDE Solutions,
USA,
Joined Apr 2009
44
16:45 Apr-07-2014
Re: MPI AC/DC Coil Calibration
In Reply to Peter at 12:02 Apr-07-2014 (Opening).

Peter, to the best of my knowledge, no prime manufacturer specifies a "calibration" of the magnetizing coil. On a bench unit, electronic devices such as the ammeter and timer are calibrated. A magnetizing coil has nothing to be adjusted, so it cannot be calibrated.

For a "system performance" test, I believe Rolls Royce specifies a check of the coil utilizing a test piece available in the UK or in the United States manufactured by Magwerks of Indianapolis, Indiana.

I trust that this information is of some help to you.

George Hopman
ASNT Level III #15776
ASTM E07.03 Subcommittee Chairman

 
 Reply 
 
Halim Mohamed
Engineering, QA/QC
Egypt, Joined Jan 2013, 2

Halim Mohamed

Engineering, QA/QC
Egypt,
Joined Jan 2013
2
05:44 Apr-08-2014
Re: MPI AC/DC Coil Calibration
In Reply to Peter at 12:02 Apr-07-2014 (Opening).

Dear Peter,
to calibrate the coil, for ex. coil should give 1200 amp-turns per inch as per DS-1and for calibration you should check the amp as per the size of coil and compare the resistant value with the resistance when it was New and connect the coil to your electrical source and measure the amperes that get into the coil, and calculate "Ampere-turns = measured Ampere X coil's turns number" = it should give 1200 amp-turn/per inch as per the ID of the coil.
or use guess meter to measure it, and you can use the equations in NS2

I hope it is clear
anything about calibration email me

 
 Reply 
 
Peter
Singapore, Joined Jan 2012, 119

Peter

Singapore,
Joined Jan 2012
119
12:10 Apr-08-2014
Re: MPI AC/DC Coil Calibration
In Reply to Halim Mohamed at 05:44 Apr-08-2014 .

Dear Mr Bruce McPherson,


We have to comply with DS-1 and API RP 7G-2 requirements. Kindly clarify below:-

1. Can we use clamp meter to measure the current?
2. Do we need to perform this test without placing any part in the coil?
3. Can we use below calculation for Hollow and Solid bar?


To certify that a coil meets any or all of these standards requires a calibrated DC ammeter capable of measuring the DC current flowing in the coil windings. Every coil should be marked with it's number of turns, so when the current is measured, it's a simple matter of multiplying turns x amps to get the magnetising power of the coil.
When certifying coils to either of these standards it's important to calculate the maximum component OD that coil can magnetise and still meet the requirements of the standard.

CALCULATION
For example, under DS-1, a 10" ID coil which produces 10000 amp-turns is only capable of magnetising components up to 8.33 inches OD (10000/1200 = 8.33)
A 10" ID coil which produces 12500 amp-turns can magnetise components up the maximum size of the coil (12500/1200 =10.41)

 
 Reply 
 
Tim
Tim
14:43 Apr-08-2014
Re: MPI AC/DC Coil Calibration
In Reply to Peter at 12:10 Apr-08-2014 .

It is apparent from this discussion that the term calibration has many different meanings.

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
21:53 Apr-08-2014
Re: MPI AC/DC Coil Calibration
In Reply to Peter at 12:10 Apr-08-2014 .

In answer to your questions:

1. You must measure the DC current flowing through the coil windings - NOT the AC current coming from your wall socket into the coil.
If the clamp meter can measure DC current and is calibrated, then you use it by clamping around one of the wires carrying DC current into the coil.
However, most clamp meters only measure AC current and this is not acceptable
2. All the measurements should be made with no metal objects in the coil
3. The calculation comes from the requirements of DS-1 which are designed around the inspection of pin and box rotary connections. DS-1 says nothing about magnetising solid or hollow bars

What you are trying to do is NOT "calibration" - you are attempting to show that the coil conforms to one or more standards. If you can do that successfully, then you could issue a "Certificate of Conformance" for the coil, but definitely NOT a "Certificate of Calibration"

Regards

Bruce

 
 Reply 
 
Peter
Singapore, Joined Jan 2012, 119

Peter

Singapore,
Joined Jan 2012
119
06:57 Apr-17-2014
Re: MPI AC/DC Coil Calibration
In Reply to Bruce McPherson at 21:53 Apr-08-2014 .

Dear Mr Bruce McPherson,

Thanks for your kind advice. As per your advice we are planning to use our HIRST gauss meter to perform the testing. Kindly let me know below steps correct or not.

1)There is little measurement function available in our equipment as below and for this test I planned to choose DC Peak. Am I correct?

a) DC: DC magnetic field measurement.
b) DC PEAK: Maximum positive peak reading of the DC field.
c) AC RMS: True RMS (Root mean Square) of input signal.
d) AC RMS MAX: Maximum true RMS.
e) AC PEAK: Maximum positive peak value.


2)There are 2 probes available with this equipment .One is axial probe and one is transverse probe. I am choosing axial probe.

3)As per your advice we are planning to measure at the center of the coil and calculate the current using this formula I= ( B x Dav)/(12.57 x N) . Can we apply the same formula if we measure the gauss value at inside wall of the coil?

4)Maximum OD

As per DS-1 requirements 1200 ampere turns is the requirements.

a)Center of the coil
Current obtained from formula I= ( B x Dav)/(12.57 x N) divided by 1200. So I can get maximum tubular OD I can inspect using the coil if I placed at centre of the coil.

b)Inside Wall of the coil
Current obtained from formula I= ( B x Dav)/(12.57 x N) divided by 1200. So I can get maximum tubular OD I can inspect using the coil if I placed at inside wall of the coil.


Peter

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
11:14 Apr-17-2014
Re: MPI AC/DC Coil Calibration
In Reply to Peter at 06:57 Apr-17-2014 .

Peter,

1. Use DC rather than DC peak
2. Axial probe is the correct choice
3. No, the formula only works if magnetic field is measured at the centre of the coil. Readings at the inner wall of the coil will be higher, but should NOT be used to calculate coil current
4. The maximum OD for the coil is based on measurements made at the centre of the coil, not at the inside wall of the coil.

Best Regards

Bruce

 
 Reply 
 
Andreas
NDT Inspector, Specially in the oilfield
Tuboscope Vetco(Deutschland)Gmbh, Germany, Joined Oct 1999, 130

Andreas

NDT Inspector, Specially in the oilfield
Tuboscope Vetco(Deutschland)Gmbh,
Germany,
Joined Oct 1999
130
08:32 Apr-19-2014
Re: MPI AC/DC Coil Calibration
In Reply to Bruce McPherson at 11:14 Apr-17-2014 .

Hallo Bruce ,
would it be not a good thing , if the manufacturer of the coil to the measuring of the coil current and then measuring the Ampere/m (Gauss is obsolete)in the middle of the coil.But everything on the label of the coil.
Now it would be easy to see any change on the field strength of the coil.
Ampere/turns is not a good to calculate the field of a coil.


Regards

Andreas

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
11:52 Apr-19-2014
Re: MPI AC/DC Coil Calibration
In Reply to Andreas at 08:32 Apr-19-2014 .

Andreas,
In principle, I agree that manufacturers should mark their coils with the field strength at the centre, and I also agree that field strength at the centre of a coil (whether measured in gauss or A/m) is a much better guide to the magnetising power of a coil rather than amp-turns
However, many coils are operated from variable current power supplies and any marking of field strength would only be correct at one specific current.
Amp-turns and gauss may not be SI units, but they are not obsolete.
I suggest you check table C.1 (SI units) of ISO standard 10407-2 where you will see that amp-turns and gauss are the units used to define the performance requirements of exactly the type of DC coils discussed in this thread.

Best Regards

Bruce

 
 Reply 
 
Andreas
NDT Inspector, Specially in the oilfield
Tuboscope Vetco(Deutschland)Gmbh, Germany, Joined Oct 1999, 130

Andreas

NDT Inspector, Specially in the oilfield
Tuboscope Vetco(Deutschland)Gmbh,
Germany,
Joined Oct 1999
130
00:08 Apr-20-2014
Re: MPI AC/DC Coil Calibration
In Reply to Bruce McPherson at 11:52 Apr-19-2014 .

Hallo Bruce ,
there are srill some unit accepted and used like gauss etc , but it is not a SI Unit.
If use a external power supply it must have then a cal. ampere meter , then everything fits. So the coil should have with this ampere so much field strength measured at the middle of the coil. I forget the size of cable and length between coil must also specified. To get comparable value you need the temperature to , resistance of coil change with temperature.

Regards

Andreas

 
 Reply 
 
Kumar H M
Kumar H M
10:17 Jan-04-2016
Re: MPI AC/DC Coil Calibration
In Reply to Andreas at 00:08 Apr-20-2014 .

What is the difference between AC and DC Coil using for MPI Inspection and Why only DC Coil is acceptable to check Oil field tubular products like Drill Collar and BHA

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
10:47 Jan-04-2016
Re: MPI AC/DC Coil Calibration
In Reply to Kumar H M at 10:17 Jan-04-2016 .

An AC coil only produces a magnetic field on the surface of the item being inspected, whilst a DC coil produces a magnetic field which penetrates into the material being inspected.
Drill collars and BHA normally have internally and externally threaded connections which must be inspected (pins and boxes)
Using an AC coil over the outside surface of a box end will NOT magnetise the internally threaded area of the connection because the magnetic field is limited to the outer surface of the component
A DC coil, because it produces a penetrating magnetic field CAN magnetise the internally threaded area.

Best Regards

Bruce

 
 Reply 
 

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