RU2542684C1 - Method of increasing of validity in tightness control - Google Patents

Abstract

FIELD: test hardware.

SUBSTANCE: strain σ_o is article is measured to define the maximum strain σ_max in article so as to load the article and develop the strain from 1.25σ_o to 0.97σ_max and to release the strain completely for analysis of article tightness.

EFFECT: higher validity and reliability.

5 dwg

Description

FIELD OF THE INVENTION

The invention relates to methods for testing the condition of products within the framework of a quality control system, including preventive maintenance, in particular for evaluating product quality indicators based on the results of non-destructive testing of tightness. The invention can be applied in nuclear and traditional energy, shipbuilding, petrochemistry, in other areas of technology and engineering, which use heat exchangers and / or heat transfer pipes.

State of the art

The prior art there are a large number of ways to conduct leak testing.

The prior art method of tightness control, consisting in the use of a fluid to detect a leak (B. Lukasevich and others. "Steam generators of VVER reactor plants for nuclear power plants", M., 2004).

As the closest analogue, the method of tightness control, disclosed in the source of information RULES AND STANDARDS IN ATOMIC ENERGY, was chosen. “Leak control. Gas methods ”, PNAEG-7-019-89. The known method of tightness control is based on the use of test substances and registration of their penetration through defects in the structure using visual observation and / or various leak detector devices and other means for registering test substances. The disadvantage of this method is the limited sensitivity and reliability of the detected defects.

Disclosure of invention

The problem that this invention solves is to improve the operational properties of products based on increasing the reliability of identifying end-to-end defects of an operational nature such as fatigue cracks, stress corrosion cracking, technological defects with a small opening such as quenching, welding or other cracks or crack-like defects.

The technical result to which this invention is directed is that it allows you to control the quality of the product at the stage of factory outlet control or during operation of the product with higher quality and reliability, which will allow you to identify dangerous end-to-end defects in time and repair the product by the results of this control.

The above set of technical results is achieved by the fact that the method of increasing the reliability of the tightness control of products is that they determine the voltage σ _e acting during operation, in the product, determine the maximum allowable voltage in the product, σ _add , load the product and create a voltage of 1 or more in it , 25σ _e to 0.97σ _additional , dump the load completely and carry out the tightness control of the product.

A distinctive feature of this method is that before the inspection, the product is loaded with a load sufficient to reveal a hypothetical defect such as a crack at the inspection site, to a value that would ensure better penetration of gas or liquid through the through defect and thus increase the detection of defects.

Brief Description of the Drawings

Figure 1 shows the pneumohydraulic (aquarium) method for monitoring the tightness of the product.

FIG. 2 shows a diagram of stresses at the ends of a crack under tension in a plate of an elastoplastic material in an unloaded (a) and loaded state (b).

FIG. 3 shows the opening δ at the apex and the opening ν in the middle of the crack with a length of 2 s under the action of stresses σ and pressure p.

FIG. 4 shows a through crack in a pipe located along the axis of the pipe.

Figure 5 is a diagram illustrating the increase in the reliability of the pneumohydraulic method for monitoring the tightness of the heat transfer tubes of steam generators of nuclear power plants during monitoring in accordance with the prototype and in accordance with this method.

The implementation of the invention

The need for non-destructive testing of pipelines and pressure vessels is especially evident in many industries, for example, in the thermal and nuclear power industry in the operation of nuclear and thermal power plants, in the chemical and oil refining industries, as well as in engineering industries where heat exchangers are used. Particularly high demands are placed on parts in these industries for the strength, reliability and durability of their joints due to harsh operating conditions. The tightness control of structures and their assemblies is carried out in order to detect leaks due to the presence of through cracks, lack of penetration, burn through, etc. in welded joints and metallic materials.

The tightness control is based on the use of test substances and registration of their penetration through defects in the structure using visual observation and / or various leak detection devices and other means for registering test substances.

Depending on the properties of the test substance and the principle of its registration, control is carried out by gas or liquid methods, each of which includes a number of methods that differ in the technology for implementing this principle of registration of the test substance. At the same time, depending on the method used for leak testing, the location of the leak or the total leakage (degree of leakage) is determined.

The amount of leakage or total leakage is estimated by the air flow through the defect or all defects. Under the control system is understood the combination of certain methods and control modes and the method of preparing the product for control.

The threshold sensitivity of the control system is characterized by the minimum detected leaks or total leakage. Threshold sensitivity determines the level of confidence of the control and the probability of detecting a defect.

It is known from fracture mechanics that when a load is applied to an elastoplastic body with a crack, the magnitude of which is such that stresses arise in the body below the yield strength, however, plastic (i.e., irreversible) deformations arise at the crack tip. After removing the load, the body recovers its shape, with the exception of the crack tip zones, which ensures residual crack opening after the load is removed from the body.

The residual opening of the crack is the higher, the higher the stress created in the body with the crack, since the value of the opening δ at the mouth of the crack (FIG. 2) is equal to (see, for example, Panasyuk V.V. Limit equilibria of brittle bodies with cracks, Kiev, Naukova Dumka, 1968):

δ = (8R _P0,2 C) / πE ln [secπσ / (2RP _0,2 )],

where R _P0,2 is the yield strength, E is the elastic modulus, C is the half-length of the crack, σ is the effective stress.

The following describes the condition and the lack of existing methods of tightness control using the example of a pneumohydraulic (aquarium) method for monitoring cracks and crack-like through defects.

The pneumohydraulic control method is used to detect end-to-end defects. The essence of the method is that on one side of the pipe create a pressure of gas (or air), and on the other is a transparent liquid (for example, water) (FIG. 1). If there is a through defect in a pipe or other pressure vessel, then gas bubbles appear in the liquid, which determine the location of the through defect.

The reliability (probability of detection) of the pneumohydraulic control method depends on the size of the through defect and on the pressure p of the air under which the air is located.

If the defect is a crack, then the magnitude ν of its opening depends on its length c and on the working pressure in the pipe p _slave (pressure during operation), which creates stress in the crack region σ _e .

Under the influence of external pressure p _{air, the} crack decreases the opening and the reliability of the method decreases. When conducting tightness control by known methods, there are no actions aimed at increasing the crack (defect) opening (see, in particular, GOST 3242-79 "Welded joints. Quality control methods").

In real products, the probability of detecting through defects by the pneumohydraulic method is in the range from 0 to 50-60%, which is confirmed by cases of leaks through the through defects after performing eddy current control and pneumohydraulic control and jamming of all tubes with defect sizes exceeding 70% of the wall thickness. However, after the steam generators are put into operation, leaks through through defects are observed.

Thus, increasing the detectability of through defects in metals of parts and products of modern technical objects is an urgent task.

Before the start of operation and during operation of critical products, for example, in the field of nuclear energy, in accordance with regulatory documents, non-destructive testing of the state of products is carried out. Moreover, in operation, as a rule, during each inspection, defects of either technological nature or operational are detected. This is mainly due to the lack of reliability of non-destructive testing, including tightness control methods.

A way to increase the reliability of the tightness control is that before the tightness control is carried out, the product is loaded with a load sufficient to reveal a hypothetical crack type defect at the test site to a value that would provide a higher detectability of the through crack type defect.

A way to increase the reliability of the tightness control of products, which consists in determining the voltage σ _e in use during operation. This stress can be determined from the verification calculation of strength, which is mandatory for critical products.

Next, determine the maximum allowable voltage in the product σ _add . These stresses, as a rule, can also be taken from the verification calculation of the strength of the product. In the absence of a verification calculation, the maximum allowable voltage can be determined from the normative document for strength calculations, where safety factors are normalized. For example, in nuclear energy, there are the Norms for calculating the strength of equipment and pipelines of PNAEG-7-002-86 nuclear plants, in which the permissible stress σ _{e is} established for membrane stresses as the smaller of the two values:

R _P0,2 / n _t or R _m / n _m , where n _t - margin of yield strength, n _m - margin of ultimate strength R _m .

After that, the product is loaded and a voltage of 1.25σ _e to 0.97σ _add . The choice of this range is explained, on the one hand, by the need to increase crack opening, on the other hand, by the risks of destruction of the product material.

It was established experimentally that at a voltage less than 1.25σ _e , not all defects increase by a value sufficient for detection. On the other hand, with a voltage exceeding 0.97 σ _dop , the probability of destruction of the product material is too high.

After loading the product, the load is completely relieved and the tightness of the product is monitored.

According to the results of the inspection, repair the identified defects.

The invention is illustrated by the following example.

There are tubes made of austenitic steel, the geometric dimensions of which are shown in FIG. 4. During operation, cracks appeared in the tubes, including through ones (FIG. 4). The working voltage in the tubes is created by an internal pressure of 8 MPa and, in accordance with the Laplace formula, is 8 MPa × 7.5 mm / 1.5 mm = 40 MPA, the maximum allowable voltage is 120 MPA (determined from the ratio: Rp0.2 / pt = 180 MPa / 1,5).

In this case, the opening of the through defect with a length of 10 mm after removal of the load is approximately

After the tubes were loaded with an internal pressure of 24 MPa (the stress in the tubes was 24 MPa × 7.5 mm / 1.5 mm = 120 MPa), the residual crack opening increased and amounted to approximately

δ = 8 ∗ 180 ∗ 5 / 3.14 ∗ 2 ∗ 10E5 ∗ ln [1 | cos (3.14 ∗ 120/2 ∗ 180)] = 0.0067 mm,

that is, increased by about 10 times.

When an external pressure of 2 MPa was applied to the tubes, the crack opening decreased by 0.0012 mm (2v = 2σ × C / E = 2 × 0.0006 mm = 0.0012 mm), i.e. when using the prototype of the proposed crack method 10 mm or less closed and could not be detected. When pre-loaded with excess pressure, the residual opening of the crack with a length of 10 mm was 0.0067-0.0012 = 0.0055 mm, which ensured their full detection (FIG. 5). That is, the probability of detecting end-to-end defects has increased significantly and reached almost 100%.

After completion of the inspection, the defective tubes were repaired according to the inspection results.

Claims (1)

A way to increase the reliability of the tightness control of products, which consists in determining the voltage σ _e acting during operation, in the product, determining the maximum allowable voltage in the product σ _add , load the product and create a voltage in it from 1.25σ _e to 0.97σ _additional , drop the load completely and carry out a leak test of the product.

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