南京博克纳自动化系统有限公司
Application of new technology for detecting defects in heat exchanger tubes
        来源:南京博克纳自动化系统有限公司    日期:2017-10-09    关键词:涡流探伤,涡流探伤仪    
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In the power and petrochemical industry uses a lot of heat exchanger as the medium to transmit energy, its interior is composed of a large number of tubes, due to the long-term in the corrosive medium and alternating stress, often cause corrosion and abrasion, so the regular inspection of internal heat pipe is an important measure to ensure the equipment running of the.

Usually heat from the material can be divided into two categories, one is non ferromagnetic materials such as austenitic stainless steel, Aluminum Alloy, titanium alloy and copper alloy, conventional eddy current testing technology with its fast detection speed, high sensitivity, the defect signal analysis method, the defect detection of non ferromagnetic metal pipe has been widely used in thermal power, petrochemical and other sectors. However, the extensive use of the heat exchange tube of another type of carbon steel and low alloy steel of ferromagnetic materials in electric power and petrochemical industry, which is characterized by metal magnetic permeability of r>>1, due to the skin effect is strong with conventional eddy current detection method, such as the following formula to determine the conductor eddy current standard penetration depth:
.
8 =5.033 * (r o f more) -1/2

The frequency of type f more - Detection

Micron R -- relative permeability of materials

Conductivity of materials

For the non ferromagnetic materials, R =1, for the strong ferromagnetic materials, R =100 ~ 1000, according to the calculation formula of eddy current, strong ferromagnetic material penetration depth of only non ferromagnetic material 1/10 ~ 1/30, the vortex tube wall penetrating into the outer wall of the tube is very difficult, such as the detection frequency is 10kHz. In steel standard eddy current penetration depth of delta =0.051mm, if the interpolation type probe measuring wall thickness of 2mm tube, the inner surface of the eddy current amplitude by 100% penetration into the outer surface only:

Jx=2/Jx=o=J2/J0=e-2/ Delta =9.3 x 10-18

Such a low value of ordinary eddy current flaw detector can not be measured, that is, the conventional eddy current method can not check the outer wall defects of steel pipe.

On the other hand, the tiny change of the pipe diameter, the uneven composition of the pipe and the rust (ferromagnetic Fe3O4) on the wall of the pipe after a period of running will cause electromagnetic noise. These factors are the main reason causing the signal-to-noise ratio decreased.

In the absence of external magnetic field, the orientation of the spontaneous magnetization vector of each domain in ferromagnetic materials is different, but the external effects cancel each other, so the whole object does not exhibit magnetism. When the external magnetic field is insufficient, the magnetic moment of some magnetic domains in ferromagnetic materials turns to the external magnetic field, and it changes. Eddy current examination will produce magnetic noise. So the conventional eddy current testing technology can not meet the requirements of testing of ferromagnetic heat exchanger tubes.

In the regular inspection method of existing heat pipe heat exchanger is the most core is drawn out after cleaning, the artificial eye, subjective evaluation to get abandoned, but this method can only see the outer tube bundle distribution situation of internal control absolutely ignorant of, and there is a big random eye evaluation. A further method is to use endoscopy, but this is a very slow and troublesome method, and can only observe the corrosion of the inner wall, can not adapt to a large number of bundle inspection. So in the power of petrochemical industry has long heat in service of ferromagnetic material tube detection problem.

This paper introduces the technology of remote field eddy current (Remote field eddy current technique) is a new technology in the far field eddy current detection based on a pipeline, it has the advantages of conventional eddy current, without using the method of magnetic saturation and auxiliary of ferromagnetic pipe, can be directly inserted probe to detect cracks and corrosion pits and abrasion on the tube wall thinning defect detection technology is considered to be a most promising pipeline.

1. far field eddy current effect and mechanism

Far field eddy current technology is a pipeline detection technique based on a special physical phenomenon -- far field eddy current effect. The far field eddy current detection probe original is shown in Figure 1, it consists of two coaxial solenoid coil, excitation and detection coil, excitation coil with low frequency alternating current detection coil must be placed away from the excitation coil 2 to 3 times the pipe diameter at the far field region". Figure 2 shows that the phase difference between the potential and the exciting current in the detection coil varies with the distance between the two coils Ded (the multiples of the inner diameter of the tube Di), and the signal distance characteristic is called the characteristic of the signal distance. The characteristics can be qualitatively divided into the following three regions.

(1) when the Ded<1.8Di region induced potential is greatly reduced with the distance increasing, the phase change is not big, this is because the detection coil and the excitation coil coupling loss caused by eddy current testing in accordance with the theory of general, said the near field or direct coupling region.

(2) when Ded is increased to (2 ~ 3) beyond Di, the amplitude and phase were decreased at low rate, and the tube inside and outside the same, the phase lag is approximately proportional to the tube wall through the thick, can be approximated by one-dimensional phase calculation of skin effect:

=2 delta theta sigma PI Mu f more B.

Theta in phase -- phase lag of induced potential

Thickness of tube wall

F more - excitation frequency

Permeability of tube wall material

Conductivity of tube wall materials

This region is called the far field region, and the traditional eddy current concept can not be explained in this region. The special phenomenon appearing in the far field is called the far field eddy current effect.

(3) the transition region between the near field and the far field is called the transition zone. In the transition region, the decrease rate of the induced potential decreases, sometimes even slightly increases, and the phase difference changes dramatically.

The famous American scholar Schmidt TR pointed out in 1984 two, the main effect of the far field eddy current phenomenon depends on the tube in place, is a shielding effect along the inside of the pipe to the excitation coil directly coupled magnetic flux; two is the existence of two energy through the non direct coupling path of pipe wall. It originates from the excitation coil near the tube wall to the eddy current induction week, circumferential vortex quickly spread to the outer wall of the tube, and the amplitude attenuation and phase lag, to reach the outer wall of electromagnetic field and diffusion to the outside of the tube, the tube field attenuation attenuation is in direct coupling region is much slower speed, so the tube field produces in the vortex tube induction, diffusion through the pipe wall to the inside of the tube, and again produce amplitude attenuation and phase lag, which is far field signal received by the detection coil.

The far field eddy current effect is rapidly proved by the field finite element numerical simulation.


Characteristics of 2. far field eddy current testing technology

Is the use of conventional eddy current near the coil tube wall by direct magnetic coupling to pickup signal changes and vortex flow field, the far field eddy current to measure a distance through the magnetic field in the tube along the tube axis transmission back to the tube wall, the detection coil must be in from 2 to 3 times the diameter of the excitation coil far beyond field, therefore, the detection coil can receive far field signal is very weak, usually microvolt magnitude, but also a lot of mixed outside stray electromagnetic interference, its magnitude can be hundreds of times larger than the far-field signal, it brings difficulties to the signal processing and instrument making.

Due to the different field eddy current mechanism, the detection sensitivity of the inner and outer wall defects is low, the filling coefficient is low, and the eccentricity of the probe in the tube is very small. Its biggest advantage is to check the thickness of ferromagnetic pipe, the maximum detectable wall thickness of 25mm. This is not possible for conventional vortices. Secondly, it is for a wide range of wall thickness defects with high sensitivity and accuracy, precision can reach 2% ~ 5%, the defects on small volume, such as corrosion pits, high detection sensitivity depends on the material, thickness, permeability uniformity, detection frequency and probe out speed and other factors. In the petrochemical industry, the sensitivity of the carbon steel tube with a diameter of 80% and a diameter of 2.8 is detected on a 25 * 2.5 carbon steel tube.

The far field eddy current detection using frequency is much lower than that of conventional eddy current, in order to ensure that each cycle excitation signal collected, and missed, the detection speed is limited, usually only conventional eddy current testing method of 1/3 ~ 1/5, in the range of 10m ~ 20m/min, a working class pipeline 8h can check 200 ~ 500 the root length of 10m.

3. application examples

A heat exchanger in a petrochemical plant has been affected by the leakage of the pipe for several times. The test method was used to check the leakage during the overhaul, and the leakage of the pipe was also found. In order to thoroughly test the heat exchanger, understand its actual situation and evaluate its service life, the tube is inspected by far field eddy current testing method. Its specifications are 25mm 2.5mm, and the material is No. 20 steel.

Figure 3 is a bar chart of a heat exchange tube of the actual test results. It can be seen from the figure that the phase and amplitude curves are shifted to the right at S1 ~ S7, which is the signal of the baffle plate. In B3, B4, B5 and B6 phase and amplitude curve has shifted to the left, and there is a certain length, in the position of a large range of peripheral wall thinning by map coordinate calculation and compared with standard sample, determine the wall thinning depth of wall thickness of 13%. According to the field observation and understanding, the wall thickness reduction may be caused by the improper mechanical damage during the manufacturing process, and the thickness of the wall is further aggravated due to the stress concentration in the process of use. After testing, the heat exchanger and the number of tubes with the same.

The test of a total of three sets of the same high pressure heater, the structure shown in figure 5. Test result:

The detection result is heat exchangers are mostly mild corrosion, the most

The large wall thickness below 13%, only five or six heat exchangers have serious wall thinning. When the wall thickness of the heat exchanger is thinner than >20%, the tube is put into operation and has been running safely for one year. There is no planned shutdown due to the leakage of the pipe.

Another example is the test of the high pressure heater steel pipe of a six hundred thousand KW unit in a thermal power plant. The tube specification of the heater is U 16 2.1mm. The material is low carbon steel. At the beginning of 1990, it was put into operation in September 1998. After 35 thousand hours of operation, all kinds of leakage occurred. According to the analysis of the carbon steel pipe used in the heater, there are four main reasons for the leakage: bending stress, thermal stress, erosion thinning and corrosion.  They will lead to tube cracks, pits or even fracture, its harm is quite large, not only the heat efficiency, coal consumption increased, long and it will also lead to corrosion, even cause heating tube blasting. The test uses RQ-01 portable far field eddy current flaw detector, the probe uses the outer diameter 10.5mm of the far field differential and the absolute probe, the detection frequency is 350Hz. The calibration tube for the same specification, steel tube of the same material processing diameter 1.2mm hole and hole diameter of 2mm.

Inspection assessment of defects to comply with the following principles:

(1) the defect signal amplitude more than 1.2mm diameter hole for record standard, regardless of the size of the defect depth.

(2) the defect signal amplitude more than 2mm diameter holes for plugging the standard regardless of size and depth of defect.

According to the above principles, in the process of flaw detection, the signals found above the pipe plugging standard show that the flaw detection personnel can not be identified as non relevant signals, all should be sentenced to scrap.

The far field differential and absolute probe testing technology using RQ-01 remote field eddy current instrument and the diameter of phi 10mm, the detection frequency of 350Hz.

1# heater is not found more than record standard and standard tube plugging.

2# heater suggested blockage of 5 root, root (more than 5 Supervision record standard).

3# heater suggested blockage of 3 root, supervise the operation 2.

There are many defects on the same pipe section, but most of the defects on each tube segment are located at about 3mm away from the pipe mouth.

High pressure heater tube leakage and its running status is closely related to its operating characteristics:

(1) the impact of water flowing into and out of the tube plate causes a larger bending stress of the tube plate, which causes the crack initiation at the expansion joint between the nozzle and the tubesheet and extends into the tube.

(2) because of the poor drainage, the corrosion of the pipe section is particularly serious in the hydrophobic area, especially in the steam water two-phase mixture region.

(3) the water inlet pipe section chamber orifice inward 300mm the existence of water erosion thinning and cracking.

4. concluding remarks

The far field eddy current technology of the latest development of steel tube defects detection of the extensive use of electric power and petrochemical industry provides a feasible rapid and reliable method, application examples in the article confirmed the practicality of this method. The future task is how to improve and perfect this technology, expand the application in the field of power and petrochemical, and strengthen the basic and practical research.

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