Penetrant testing of sealing welds between steam generator tube sheet and tube hole

Penetrant testing of sealing welds between steam generator tube sheet and tube hole

This paper introduces the penetrant testing process and operation of the sealing weld between the steam generator tube sheet and the tube hole, analyzes on the common defects of the penetrant testing of the sealing weld, which can provide experience for the penetrant testing and quality control of the sealing weld between the tube sheet and the tube hole of the main equipment of the nuclear island in China, and provide a reference for the peers.

The steam generator is one of the key equipments in the nuclear island of the nuclear power plant, which generates steam through heat exchange, drives the turbine to do work, and plays the role of isolating the first circuit from the second circuit. The steam generator is a nuclear level 1 equipment, the material of the tube plate is 18MnD5, the surface of the tube plate is welded with nickel-based alloy (ENiCrFe-7), and the material of the heat transfer tube is NC30Fe. the heat transfer tube and the nickel-based weld layer of the tube plate are sealed by positioning expansion + sealing welding + hydraulic expansion. If the seal weld is damaged or defective, it will cause the erosion of the nickel-based stacking layer of the tube plate by corrosive media, which will affect the safe operation of the whole nuclear power equipment.
Penetrant inspection is used to detect surface defects in the seal welds and has good results in detecting pinhole-like defects and sparse defects [1]. The process and operation of penetrant inspection are introduced, and the defects commonly detected by penetrant inspection of seal welds are analyzed and summarized.

1. Penetrant inspection process

Penetrant inspection process with reference to the standard RCC-MMC4000 (2007 version). Pipe hole sealing weld specifications (diameter × wall thickness) of 17.73mm × 1.01mm, the material NC30Fe, according to its structure (see Figure 1) characteristics, the use of solvent removal type coloring penetration detection method for its 100% detection, detection time for the completion of welding.

Figure.1 Pipe hole sealing welding weld structure schematic
Heat transfer tube seal welding welding without opening bevel, welding method for the automatic tungsten inert gas shielded welding without wire filling. The weld throat size (a) is the radius of the circle centered at the root of the weld and completely tangent to the largest circle in the weld, a>0.66e (e is the heat transfer tube wall thickness). In the case of pores or slag, a>∑di+0.66e, where ∑di=d1+d2 (∑di is the sum of the diameters of pores or slag, d1,d2 are the diameters of pores or slag).

2. Penetration testing process

2.1 Selection of materials for permeation testing

Penetrant inspection materials include: UP-T penetrant; UR-T cleaning agent; UD-T developer; Type I test block (plating layer thickness of 30μm, 50μm).
Each batch of penetration testing material products should be determined in accordance with the method of Appendix FV of RCC-MF6000 (2007 version) for the contaminant content, and an official test report should be issued. The content of the harmful element sulfur in the permeation detection materials should be less than 0.02%, and the total content of element fluorine and chlorine should be less than 0.02%.

2.2 Selection of testing equipment

Testing equipment includes white light illuminance meter, pyrometer, 5-10 times magnifying glass, measuring tape, clean white paper or white cloth without hair removal, etc.

2.3 Process and operation

2.3.1 Surface preparation and pre-cleaning

Before the implementation of penetration testing, tube plate sealing weld using acetone for degreasing, and evaporate at least 10 min. the surface of the weld to be tested and the adjacent area of at least 25 mm should be treated as clean, dry, and should be free of rust, oxide, flux, spatter, grease, paint, fiber, dust and other foreign substances that interfere with the penetration or block the opening of the defective surface.
The weld to be measured after pre-cleaning, natural evaporation drying or hot air, cold air drying. Drying time of not less than 5min to ensure that the surface of the sealed weld has been completely dry and clean before the application of penetrant.

2.3.2 Application of penetrant

Apply penetrant by brush method. Using a brush without hair, brush the penetrant evenly on the surface of the weld. For the application of penetrant at the internal edge of the pipe hole sealing weld, the brush width should be not less than 25mm to ensure that the surface of the weld is covered with all wetting of the penetrant. During the whole penetration testing process, the temperature of the pipe hole weld product and the penetrant used should be kept at 10℃-50℃. The product during the penetration process is shown in Figure 2.

Fig.2 Product during infiltration

2.3.3 Removal of excess penetrant

Excess penetrant on the surface of the pipe hole sealing welds should be wiped with a clean, dry cloth or absorbent paper until the vast majority of the penetrant is removed, and then lightly wiped with a cloth or absorbent paper moistened with cleaning agent to remove the residual penetrant. In the wiping process, should be wiped along the tube hole clockwise or counterclockwise single direction, not reciprocal wipe, so as not to cause weld surface contamination.

2.3.4 Drying

Solvent removal type penetration detection can take the natural drying method for drying. Drying treatment must prevent the drying of penetrant in the surface defects, the surface temperature of the specimen should not exceed 50 ℃.

2.3.5 Imaging

In order to make the developer evenly cover the surface of the pipe hole weld, the developer must be shaken well before spraying, spraying needs to be carried out in all directions up, down, left and right. Developer nozzle should be 300-400mm away from the surface of the weld to be inspected, spraying direction and the inspected surface angle of 30 ° -40 °. After spraying developer, the inspected surface should form a thin and uniform layer of developer film. The imaging time of the pipe hole sealing weld is 30-60 min. the product of the imaging process as shown in Figure 3.

2.3.6 Observation and recording

Observation of the image results should be carried out immediately after the developer dries, and only after reaching the specified image time can the observation finally end. The observation is carried out using a strong flashlight illumination (illumination >500lx), and the outer surface of the sealed weld and the inner side of the pipe mouth are observed in all directions.

Figure.3 Products in the process of developing the image

2.3.7 Display assessment and quality acceptance

No display is allowed on the weld metal of the pipe hole sealing weld. Any display with a size greater than 1.5mm on the base material within 5mm from the edge of the weld, and all displays with a size less than 1.5mm but whose spacing is less than 3mm, are unqualified.
When a defect is found, the number, size, type and location of the defect shall be recorded in the penetration information record sheet (identified and confirmed based on the alphanumeric steel mark number on the tube plate).

2.3.8 Post-treatment

Use a lint-free cotton cloth dipped in cleaning agent to wipe the surface of the pipe hole welds after the test is completed to remove residues from the penetration testing process.

3. Detection results and analysis

Defects are mostly porous, showing a circular display or linear display, scattered or concentrated on the surface of the weld circumference [2] (see Figure 4). The causes of defects are mainly related to cleanliness and are analyzed as follows.
(1) The internal cleanliness of the pipe plate overlay layer is not high. The welding method of the pipe plate is submerged arc with pole overlay + manual welding, and there may be tiny impurities inside the overlay layer, especially in the area with pole closing arc. The poor cleanliness inside the tube bundle base material will affect the welding quality of the tube bundle – tube plate weld.

Figure.4 Defects shown
(2) Improper cleaning in the process of tube threading and positioning expansion. Individual tube holes and tube bundle to be welded area is not completely clean, in the presence of impurities and other foreign matter, will lead to defects in the seal weld.

4. Conclusion

The use of solvent removal type coloring penetration inspection method can effectively detect defects such as porosity, to ensure the quality of steam generator tube plate tube hole sealing welds. Through the penetration detection process and operation of the experimental study to determine the reasonable process parameters, the seal weld penetration detection provides a useful reference for the domestic nuclear island main equipment tube plate tube hole seal welding quality control experience.

Source: China Tube Sheet Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

References:

• [1] Gu Wenbin. Comparative study of the reliability of nondestructive testing methods for pressure vessel welds [J]. Wan Agricultural Machinery, 2015(1):47.
• [2] Zhang M, Nantong. Penetration testing of titanium alloy tube plate seal welds in equipment cooling water heat exchangers [J]. Nondestructive testing, 2019,41(12):72-74.