·Home ·Table of Contents ·Methods and Instrumentation

Computerized Eddy Current Flaw Detector VD-89NM with Higher Reliability of Detection and Danger Level estimation of Stress-corrosion cracks when Inspecting Gas Pipeline under Stress-Corrosion V.F.Muzhitsky, V.A.Karabtchevski MSIA "Spectrum", Russia S.V.Karpov VNIIGAZ, Russia Contact

Since 1995 multichannel and computerized eddy current flaw detector VD-89NM is used for stress-corrosion cracks parameters evaluation mainly after getting results of inpipe pigs (magnetic flaw detector shell) using for gas pipeline of 1420mm. Modern model of flaw detector is provided with matrix of probes allowing to get necessary view resolution and high speed of tests. Additionally flaw detector can be used with one probe or matrix of two probes for more precise testing of cracks. Stress-corrosion crack depth is evaluated by microprocessor with formal operation of its software, which eliminates personal influence of an operator to the results. Special software for IBM computer and methods been developed by VNIIGAZ allow evaluation and qualification of stress-corrosion defects with its level of danger. That software and methods are developed in conjunction of "Instruction on stress-corrosion defects qualifying with its level of danger."
High volume of investigations was fulfilled. In 1999 900m of pipes were changed in gas pipeline in Bashkiria, and the documentation of results allows to make clear special characteristics of stress-corrosion defects.
One speciality of the defects is that many cracks are often found out on the same defect part of surface. So the crack depth evaluation gets high error D of influence of neighbouring cracks on the inspected surface.
It could be calculated as: D=(Amlt. - Amon.)/Amlt. (1), here Amlt. - flaw detector data for group of cracks, Amon. - flaw detector data for alone crack.
We use a mathematical model of maximum of tangential magnetic field - Hmon. - above the alone crack [1]: Hmon. = Hmax./[2p(m-1) Hinit.] ={z/(x²+z²)-(z+h)/[x²+(z+h)²]} (2), where Hmax. - magnetic field maximum above the crack, Hinit. - magnetic field coming from a source, m - magnetic properties parameter, z - gap between probe and surface of an object, x - distance between probe position and crack position on the surface, h - depth of the crack.
Fig.1a...1c show an interference of magnetic field numbers of two cracks on the surface. The result is that field maximum is changing from 0,87Hmon. up to 1,8Hmon. with changing of Dx - distance between positions of cracks. The same way flaw detector data depended from magnetic field are changing their numbers of Dx. It could be either negative or positive. If it is negative, so the real number of crack depth could appear much higher, defect danger estimation will rise up in that case. If error D is positive, so it is possible to get smaller number of crack depth evaluation and, consequently, to get longer time for using gas pipeline without repair. It coud be seen from fig. 1, that important parameter of defects magnetic field interference is a width of area of positive field numbers. It defines if error D is positive or negative. Summary magnetic field of two equal depth crack is presented on fig.1b, so the maximum field of each other is not influenced by field of another crack. Distance between that two cracks is half a width of positive field meanings area. From (2) it could be calculated as:

Fig 1: Summary magnetic field Hmlt. of two cracks,Hmon.-magnetic field of alone crack for cases,when: a.)-cracks are on a distance more than x0;b.)-cracks are on distance x0(-half a width of area of positive magnetic field of alone crack);c.)-cracks are on a distance less than x0.


Error D (1) were calculated for case of equal cracks for different z, h, Dx. Results are presented on fig.2. It looks that, if distances between cracks are small, error D is positive and could be more than 80%. With that distance Dx been increased error D becomes negative but limited by 25%. In all cases with z been increased error D becomes positive and high. So it could be recomended to use small numbers of z.

Fig 2: Error D of summary magnetic field evaluation of two equal cracks under different z for the cases of different Dx:a.) Dx = 0,9mm;b.) Dx = 2,5mm;c.) Dx = 5,4mm;d.) Dx = 10,3mm.


Defect field dependence of x,z,h (2) might be approximately changed by triangle dependence with Hmax in top of it and 2x₀ - bottom width. If it is possible to evaluate real numbers of z and D x, error D number (1) might be significantly decreased by H1₀(H2₀)been approximatly calculated as: H1₀(H2₀)={H1(H2)-H2(H1)[1-D x/z]} / {1-[1-Dx/z]²} (4), here H1 and H2 are crack magnetic field number influenced by another crack. After using that algorithm, calculating shows that error D becomes limited by 15% for bigger crack and 25% for smaller one. Best condition of test for using that algorithm is provided with D x/z = 0,3...0,6.
That approaches were used for real crack depth evaluation by flaw detector VD-89NM.
When real cracks with depth 4,5; 5; 6,5mm were detected - fig.6,- with gap 3...5mm, by real distance between cracks from fig.2 the error could be determined as about 15%.
We compare maximum field number for spot of equal cracks with the same for alone crack of the same depth. As it is seen from fig.3 under the only number of gap that two numbers of field are equal. Under the more gap the field maximum of spot of cracks Hmlt. is more than the field of alone crack Hmon. and under the smaller gap the field maximum of spot of cracks is smaller than the field of alone crack..

Fig 3: Magnetic field maximum H0 of alone crack dependence to compare with magnetic field maximum Hs of equal cracks dependence of gap number z.

Fig 4: Gap number z dependence of crack number h,for pare of which magnetic field maximum of alone crack is equal to the same of spot of equal cracks with changing of Dx : Dx1 > Dx2 > Dx3.


There were calculated some dependences on what z number will suit to h number for field maximum to be the same on spot of equal cracks and on the alone crack, it looks on fig.4.
And here D x is a parameter.
Using the same approach as for formulae (4), summary magnetic field for spot of equal cracks might be calculated as: (5),
and for z>>h approximately H_Õ=H₀(z/Dx ) (6). So, if Dx is evaluated, H₀ might be determined approximately with coefficient n: n=H_Õ /H₀=z/Dx (7),
which depends on z, evaluated by flaw detector VD-89NM. It could provide error D decreasing down to ± 20% as for alone crack detection, so as for spot of equal cracks.
Calculations show that n from (7) might be of 15% more or less, than real n, calculated from (2), in condition of 1<z/Dx<N/2, where N - number of cracks of the spot. It is a condition of approach acceptance. Fig.5 shows topography and profyle of crack depth estimations distribution on part of surface of real pipe of main pipeline. So that results are corrected by methods which are offered here. Error was commonly decreased down to 15%.

Fig 5: Topography and profyle of crack depth estimations distribution on part of surface of real pipe of main pipeline

Conclusions:

1. With using a magnetic field of a coil turn with electric current as crack magnetic field model [1] error of crack depth evaluation is calculated for the case of interference of magnetic fields of neighbouring cracks. For cracks detected by inpipe pigs and other means it gives a possibility to determine crack danger level more reliable.
2. Approaches used for calculations allow to write simple equations to take in account interference of closely posed cracks.
3. It is possible to evaluate crack depth with error less than ±20% by VD-89NM for cracks closely disposed to each other.

References:

1. V.F.Muzhitsky, Model of surface defect and calculation of the topography of its magnetostatic field. Soviet Journal of Nondestructive Testing, Vol. 23, No. 3, pp. 166-172 (Nov.1987).

© AIPnD , created by NDT.net

|Home|    |Top|