Comparison of hydrostatic test and eddy current test for steel tube
The water pressure test and eddy current test of boiler tubes are the dense performance test of materials, and they are equivalent in the test method; and the eddy current test of steel tube has the characteristics of fast, accurate and easy to realize automatic detection, etc., which is superior to the water pressure test method which is time-consuming, labor-consuming and less accurate in the test method. Therefore, the eddy current test method can be used completely It can not replace the densification test of eddy current flaw detection. It is of great significance to control the material quality of boiler tube, improve the quality of boiler tube manufacturing and ensure the safety and reliability of boiler. As eddy current inspection technology has a strong practicability in the quality inspection and control of boiler tube, it has a good application prospect and promotion value in the boiler industry.
Steel tube hydrostatic test unit
Quality problems of boiler tubes
Seamless steel tube for boiler (hereinafter referred to as boiler tube) is an important material for boiler manufacturing. Its quality will directly affect the quality of boiler manufacturing, installation and use. The quality of boiler tube should be guaranteed by the steel tube factory, but in the case of short supply, there are always some quality problems in the boiler tube provided to the boiler manufacturer. The main pressure parts of the boiler made of it, such as water-cooled wall pipe, convection pipe, superheater pipe, heat exchanger pipe and so on, are leaking or bursting, which has become a problem for the quality of boiler products This is a big problem for boiler manufacturers and users. Under the situation of the seller’s market, the boiler manufacturer has almost undertaken all the responsibilities including the material supplier. How to control the quality of the boiler tube has become a more and more concerned problem of the boiler manufacturer. The solution is no more than two: one is to carry out the hydraulic test one by one for the boiler tube; the other is to carry out 100% eddy current inspection for the boiler tube.
Defects and damages of boiler tubes
According to the viewpoint of materials science, excellent metal materials should be continuous, pure and uniform in chemical composition, physical properties and geometry. If these three aspects are insufficient or damaged, the metal material is considered to be defective. If there is discontinuity (i.e. non tightness or non compactness or non compactness) in the geometry of metal materials, such as crack, shrinkage cavity, peeling, pit, delamination, pinhole, slag inclusion, etc., it is considered that the metal materials have scars (referred to as wounds), which does not include the discontinuity of chemical composition or the discontinuity of physical properties. From here, we can see that defects include injuries. The boiler tube may also have defects and damages in smelting and rolling process. According to the introduction of the steel tube plant, the defects of the boiler tube (here mainly refers to the damage) are mainly on the surface, and the external surface is more than the internal surface. About 70% of these defects come from raw materials (billets), slag inclusions and shrinkage holes left in billets due to insufficient oxygen blowing. When steel tubes are rolled with them, there may be transverse cracks, interlayer, fold, double skin and other defects, and the longitudinal cracks are mostly caused by pulling during rolling. If these defects or scars appear in the boiler tube, it is considered that there is discontinuity in the material, the compactness of the material is damaged, water may leak during the hydrostatic test, and the pressure components of the boiler may leak or burst during operation. Because of this, in order to ensure the quality of boiler tube, the standards of seamless steel tube for boiler in China and foreign countries clearly stipulate that as the guarantee of process performance, the steel tube shall be subject to hydraulic test one by one.
The hydraulic test of boiler tube is a densification test
China’s national standard gb3087-82 seamless steel tubes for low and medium pressure boilers stipulates in the technical requirements that the steel tubes shall be subjected to hydraulic test one by one without water leakage or sweating. For 20 steel, the maximum test pressure is 9.8MPa, and the withstand time shall not be less than 5S. The pressure of hydrostatic test is calculated as follows:
Where: P — test pressure, MPa; s — wall thickness of steel tube, mm; D — outer diameter of steel tube, mm; t — 60% of specified yield point of steel grade, MPa
The same provisions are made in Yb (T) 33-86 “cold drawn seamless steel tubes for low and medium pressure boilers” recommended by the Ministry of metallurgy.
For example: in gb3087-82, 20 steel Φ 51 × 3 steel tube, the yield point is 245MPa, and the hydrostatic test pressure is:
The hydrostatic test pressure shall be 9.8MPa
According to the technical requirements of Yb (T) 32-86 cold drawn seamless steel tubes for high pressure boiler recommended by the Ministry of metallurgy of China, the steel tubes shall be subject to hydraulic test one by one, with the maximum pressure of 20MPa and the test time of no less than 10s. During the test pressure, the steel tube does not leak or sweat. Gb5310-85 seamless steel tube for high pressure has the same regulations.
Generally speaking, there are two purposes of water pressure test: one is technological water pressure test, the purpose of which is to check whether the material (or component) leaks, that is, to check the sealing performance of the material; the other is confirmatory water pressure test, the purpose of which is to check whether the strength of the material (or component) is sufficient. It can be seen from this that the hydrostatic test of boiler tube belongs to the technological hydrostatic test, which is the material compactness test to check whether the material is continuous and dense; it is not the test to verify the strength. According to the strength theory of material mechanics, seamless steel tube belongs to the thin and long component, its diameter is very small, even the thin pipe with thin wall can bear a lot of pressure. For example, in gb3087-82 standard, No. 20 steel Φ 51 × 3 steel tube is supposed to have a crack 1.5mm deep on its surface. The strength hydraulic test is carried out on it. When it reaches the burst or water leakage, its pressure is still very high (at this time, the tensile strength of material stress is σb = 392mpa).
This shows that the burst pressure is much higher than the hydrostatic test pressure. That is to say, when the steel tube reaches the test pressure, even if there is a deeper crack, it is impossible to have water leakage.
It can be seen from the calculation of this example that it is difficult to find the water leakage phenomenon in the technological hydraulic test of steel tube, so there may be the risk of leakage inspection for the deep buried defects. Asme-sa-450 “general specification for carbon steel tube, ferritic alloy steel tube and austenitic alloy steel tube” in the United States clearly emphasizes that ” Hydrostatic test is a test method provided by many product specifications. This kind of test can find out the situation of liquid leakage from the inner pipe wall, which can be judged by visual inspection or pressure drop. In the hydrostatic test, the defects that can not penetrate the pipe wall but are very tight, or the defects that penetrate into the wall thickness for a considerable distance but are not completely penetrated are found. ” The enterprise standard b-no440 of Sumitomo Metal Industry Co., Ltd., a famous steel enterprise in Japan, clearly stipulates that after eddy current flaw detection for each steel tube, the steel tube does not need to undergo hydrostatic test, instead of hydrostatic test, eddy current flaw detection. German standard din17175 seamless steel tube made of heat-resistant steel also stipulates that eddy current testing can be used instead of hydrostatic testing.
Eddy current testing of steel tubes is also a densification test
The Chinese GB5310-85 and Yb (T) 32-86 standards are very clear: “all steel tubes passing eddy current inspection can be exempted from hydrostatic test.” This is because eddy current testing is also a material compactness test, which is equivalent to water pressure test. According to the German steel test code sep1925-74 “eddy current densification test of steel tubes”, eddy current test is a kind of densification test, which is used instead of hydrostatic test – the hydrostatic test of specified internal pressure for hollow bodies of various shapes. ” Why is eddy current testing also a densification test? This also has to start with the basic principle of eddy current testing.
Principle of eddy current testing
Eddy current testing (ET) is one of the conventional non-destructive testing techniques. It is applicable to the testing of conductive materials such as ferromagnetic and non-ferromagnetic profiles and parts as well as graphite products. It can find surface and near surface defects such as cracks, folds, pits, inclusions, looseness, etc. generally, it can determine the location and relative size of the defects, but it is difficult to determine the types of defects. Eddy current testing is widely used in the detection, material separation, thickness measurement, physical properties of test pieces and so on.
Eddy current testing is based on electromagnetic induction theory. A simple eddy current testing system includes a high frequency AC voltage generator, a testing coil and an indicator. The high frequency voltage generator (or oscillator) supplies the detection coil with the excitation current, so as to form an excitation magnetic field around the test piece (pipe), which induces the eddy current in the test piece, which is called eddy current; the eddy current in the test piece and generates its own magnetic field, and the effect of eddy current magnetic field weakens or offsets the excitation magnetic field, so as to produce the change of magnetic field. This change depends on the distance between the coil and the tube, the geometric size of the tube, the conductivity and permeability, as well as the metallurgical and mechanical defects of the tube. When the pipe passes through the coil, the change of these parameters of the pipe will cause the change of electromagnetic effect and generate electrical signal. After amplification and transformation, the signal will alarm, record and sort, and finally the purpose of pipe flaw detection can be achieved.
Skin effect and skin depth
When the direct current flows through the conductor, its current density distribution on the conductor cross section is basically uniform, but when the alternating current flows through the conductor, its current distribution on the conductor cross section is uneven. The current density of the surface layer is the largest, and the current distribution decreases with the increase of the concentration from the surface, which is called the skin effect of alternating current.
The current density distribution of alternating current on the cross section of conductor is attenuated according to the law of exponential function, namely:
Where: IO — surface current density, a / m2; I — current density at the depth δ from the surface, a / m2; μ — permeability of conductor, Henry / M; σ — conductivity of conductor, 1 / ohm · m; F — frequency, Hertz; δ — skin depth, m; e — base of natural logarithm, e = 2.718 ;
The size of skin effect is described by skin depth δ, that is, the density when the current density is reduced to 1 / E = 37% of the surface current density, that is, when I／Io = I／e = e -1, then
The above formula shows that the skin depth δ is inversely proportional to the square root of frequency f, and the larger f is, the smaller δ is. In eddy current testing, the conductivity and permeability of the workpiece are constant, and the only thing that can be changed is the frequency of the exciting current. Therefore, defects of different depths can be detected by changing the frequency of the current.
In the actual eddy current testing, the physical meaning of the above formula has changed due to the needs of the testing process. For example, the permeability μ of the conductor is expressed with respect to the permeability μ R, if the ferromagnetic material is magnetized by saturation, μ R ≈ 1; the frequency f of the AC power supply is expressed with the excitation frequency FD; the conductivity of the conductor is expressed with the conductivity σ of the test piece, the unit is changed to 1 / micro ohm · cm, or the resistivity P of the test piece, P = 1 / σ. At this time, the standard skin depth d of eddy current flaw detection can be expressed as follows:
For example, in GB3087 standard, 20 steel Φ 51 × 3 steel tube is made of low carbon steel, and it can be seen from the table that P = 16.9 (micro ohm · cm); if 5KHz excitation frequency is adopted, the detection thickness in theory is:
It can be seen from the theoretical calculation that when the above flaw detection process is adopted, the total wall thickness of Φ 51 × 3 steel tube is within the range of effective flaw detection, and it will not be missed in theory.
In eddy current testing, the interference of magnetic field and eddy current on the adjacent edge caused by the sharp change of workpiece geometry (edge) will cover up the detection of defects in a certain range. This phenomenon is called end effect. Due to the existence of end effect, when the end (head and tail) of the pipe enters or leaves the detection coil, the sensitivity of the defect near the end of the pipe will be lost, and there is usually a blind area at the end of the pipe. Therefore, the eddy current inspection of steel tubes is carried out in the whole piece. In the production process, the eddy current inspection is carried out first, and then the pipe is cut and cut.
Eddy current testing coil
The detection coil is the sensor of eddy current detection. Its main function is to establish a magnetic field on the conductor workpiece, to excite the eddy current, and to transmit the detection information. There are three basic types of detection coil: through type, interpolation type and point type. The through type is that the coil surrounds the outside of the workpiece to let the workpiece pass through freely. Pipe flaw detection is mainly through type. This kind of coil is more suitable for rapid and automatic detection, and point coil can also be used to make it spiral relative to steel tube.
It should be pointed out that the sensitivity of eddy current testing decreases with the increase of the buried depth of the defect (or the gap between the coil and the test piece). In order to improve the detection sensitivity, the gap between the coil and the steel tube should be minimized. However, if the gap is too small, it will prevent the steel tube from freely passing through the coil or damage the coil. The gap between the coil and the steel tube can be expressed by the filling coefficient η of the through coil, which is the ratio of the cross-sectional area of the test piece to the effective area of the measuring coil, and it is generally considered that η ≥ 0.7.
Sensitivity of eddy current testing to detected defects
China’s national standard GB7735 eddy current testing method for steel tubes is applicable to eddy current testing of round seamless steel tubes for boilers, ships, petroleum, chemical and other equipment. According to the standard, when the steel tube is subject to full surface flaw detection, the through type coil shall be used, the maximum outer diameter of the steel tube to be detected shall not be greater than 180mm, and the artificial defect shall be drilled.
Because eddy current inspection is not an absolute measurement method of defect depth, but a relative detection method, that is to say, the determination of detection results is to determine the defects of steel tubes by means of the amplitude comparison method of the display signals of artificial defects and natural defects of the comparison samples, that is, the equivalent comparison method. There are two types of artificial defects, one is the hole through the pipe wall and perpendicular to the pipe surface. The other is a groove parallel to the longitudinal axis of the steel tube and the side. The artificial defects in drilling can best simulate the dents, short and serious peeling, transverse cracks and other defects or scars on the steel tube surface. Therefore, the artificial defects in drilling are mostly used in eddy current testing instead of hydrostatic testing. The notch defect can simulate the natural longitudinal crack and other defects.
During eddy current testing of steel tubes, it is necessary to prepare a comparison sample. The steel tube of the comparison sample shall have the same nominal size, chemical composition, surface condition and heat treatment state as the tested steel tube, that is, similar electromagnetic characteristics. The bending degree (straightness) of steel tube shall be no more than 1.5 ‰, the surface shall be free of oxide skin, and the length shall meet the requirements of flaw detection equipment.
There are five artificial defects on the comparison sample, three of which are in the middle part of the comparison sample, distributed along the circumference of each other for 120 °, the distance between the axes of each other is not less than 200 mm, and the other two are not more than 200 mm from both ends to test the end effect.
It can be seen that as long as the process of eddy current testing is proper, all kinds of defects in the thickness of metal section can be detected completely for thin-walled steel tubes. Therefore, it can be said that eddy current testing of steel tubes is also a material compactness test, which is equivalent to the hydraulic test of steel tubes. Gb7735-87 standard stipulates that class A is applicable to replace hydrostatic test.
Other nondestructive testing methods can not replace the densification test of eddy current testing
According to German steel code sep1925, “no other nondestructive testing method can replace the densification test of eddy current testing.” This is determined by the different principles of various flaw detection methods. At present, five kinds of conventional nondestructive testing methods have different application scope because of their different principles and testing objects.
Radiographic inspection method
X-ray inspection is based on the fact that the high-energy ray has a strong penetrability to the workpiece and the difference of blackness occurs after being absorbed in the material. It is more suitable for the flaw detection of the middle and thin plate welds with relatively fixed workpiece position. The single wall single shadow or double wall single shadow flaw detection method is often used for the pipe welds, but for the material compactness inspection of the straight and long seamless steel tubes, the radiographic detection is not suitable.
Ultrasonic testing method
The ultrasonic flaw detection method of seamless steel tube mostly adopts water immersion method or contact method, and the flaw detection process is quite complex, which is not as simple and convenient as eddy current flaw detection method. GB5777-86 “seamless steel tube ultrasonic testing method” clearly points out: “the flaw detection method described in this standard is mainly to inspect the longitudinal or transverse defects of steel tube, but it can not effectively detect layered defects.” Because it can not find the defects in the circular direction or short and deep defects, and these defects are exactly the defects that affect the compactness of steel tubes.
Magnetic particle testing method
Magnetic particle testing is only applicable to ferromagnetic materials, and it is applicable to crack spinning and other defects on or near the surface of the tested parts. It can not detect the deep defects in the workpiece, so it is not suitable for the densification test of steel tube.
Penetrant testing method
The penetrant inspection method is suitable for the inspection of the opening defects on the surface of various materials, and it can not detect the internal defects of the workpiece.
Eddy current testing method
Eddy current testing method is derived from the principle of electromagnetic induction. It can find surface defects or deep buried defects, especially short and sudden shape defects. In addition, it has the characteristics of high-speed, non-contact and no coupling agent, so it is especially suitable for the detection of pipes. This is the reason that other nondestructive testing methods can not replace the densification test of eddy current testing.
Characteristics of eddy current inspection
From the principle of eddy current generation, it can be seen that eddy current detection has the following characteristics:
- (1) Eddy current testing is only applicable to conductive materials. If it is non-conductive materials, eddy current can not be induced, nor can eddy current be used for testing.
- (2) Eddy current testing is especially suitable for surface and near surface testing of conductive materials.
- (3) No couplant is required for eddy current testing. The electromagnetic field excited in eddy current testing is only a kind of electromagnetic wave, which has the characteristics of fluctuation and particle. Therefore, the probe (coil) and workpiece can propagate without coupling, so it can be used for non-contact testing. This is better than ultrasonic testing. Because ultrasound is a mechanical wave, it can only spread in matter. Therefore, coupling agent must be added between probe and workpiece in ultrasonic testing.
- (4) Eddy current testing can realize fast and automatic testing.
- (5) Eddy current testing can be applied to the detection of high temperature metals, because metals have conductivity at high temperature.
- (6) Eddy current testing is also applicable to the testing of special-shaped materials, as long as the coil is made of various shapes, it can be tested.
Eddy current inspection can replace the hydraulic test of boiler tube
Based on the above analysis, it can be concluded that although hydrostatic test and eddy current test are both tightness tests of steel tube materials. However, eddy current testing can replace the water pressure test of boiler tube in both test method and test result.
As a matter of fact, the feasibility of hydrostatic test is very poor, which is time-consuming, laborious, low efficiency and poor accuracy. Not to mention that it is not easy for the boiler manufacturer to re inspect the steel tubes in the excellent products, even for the steel tube factory. In the past, the warranty provided by the steel tube factory only mentioned “ensure no leakage in the hydraulic test”, but no specific pressure test data is the proof. It is gratifying that most of the steel tube factories now use the eddy current testing technology, which greatly improves the quality of the steel tube. With the increasing demand of users to improve the quality of boiler manufacturing, the boiler manufacturer has listed the quality of boiler tube as an important issue in the business management. 100% eddy current inspection is carried out for the re inspection of boiler tube, as an important part of the quality assurance system, to ensure the quality and reliability of boiler manufacturing.
steel tube eddy current flaw detector
From April to June 2019, eddy current flaw detection was carried out on the Φ 51 × 3 boiler tubes produced by a steel tube factory, with a total number of 11658 pieces, 840 of which were found to have over standard alarm defects, with a defect rate of 7.2%. Among the 840 defective steel tubes, 71 (8.5%) were confirmed to have longitudinal cracks (including strains), 21 (2.5%) had transverse cracks, 577 (68.7%) had surface defects (scratches, pits, bumps, etc.), 65 (7.7%) had heavy skin, and 71 (8.5%) were confirmed to have longitudinal cracks (including strains) by repeated eddy current inspection, surface inspection and artificial sensory inspection There were 106 defects (internal defects or internal surface defects) not found on the surface after manual inspection, accounting for 12.6%.
Analysis: in the longitudinal cracks, there are 53 rolling strains, only 18 folding cracks. Because of the shallow longitudinal rolling scars and the shallow surface defects, they do not affect the actual use of boiler tubes, so the number of steel tubes with real damages is (18 + 21 + 65 + 106 =) 210; therefore, the unqualified rate of eddy current inspection in this batch of boiler tubes is 210 / 11658 = 1.8%. This test result is more in line with the actual quality situation of the steel factory, so it is credible. After eddy current inspection, these damaged parts were cut off, and the SZL boiler made of this batch of steel tubes did not have steel tube leakage during the overall pressure test after assembly, thus eliminating the hidden danger of material and improving the manufacturing quality and reliability of the boiler.
Through the above preliminary practice, we think that it is feasible to replace the boiler tube hydrostatic test with eddy current flaw detection, and the inspection result is credible, and it is effective to control the quality of the boiler tube, which is of great significance to improve the boiler manufacturing quality and ensure the safe and reliable operation of the boiler. Therefore, eddy current testing technology has a good application prospect and practical value in boiler manufacturing industry.
Source: China Stainless Steel Tube Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless steel tube. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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