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Quality Inspection of Nuclear Grade Valve Manufacturing Process

The quality inspection items, contents and methods in the manufacturing process of nuclear-grade valves are introduced, and the precautions in the inspection process are pointed out.

1 Overview

Nuclear-grade valves are pressure-bearing parts with very strict quality requirements, and it is very important to ensure the quality of their products. The product quality of the valve is guaranteed by the whole process of design, production and use. Doing a good job in valve manufacturing and quality inspection is of great significance to the economical and safe operation of nuclear power valve equipment. In the valve manufacturing process, the quality inspection process runs through the design, material selection, processing, assembly and debugging, which is an essential measure to ensure the valve quality.
2. Inspection items and content

The inspection items of the valve include material inspection, manufacturing accuracy inspection, non-destructive inspection, corrosion inspection, marking and painting inspection, cleanliness inspection and performance inspection.

2.1. Material inspection

The material inspection of nuclear-grade valves is an important part of ensuring the quality of valves and is related to the safety of nuclear power valves. Material inspection work is generally divided into material surface quality inspection, material internal quality inspection and material chemical composition and physical properties inspection.

(1) Surface quality

The surface quality inspection of valve materials can remove unqualified materials before feeding and processing, so as to prevent the loss of feeding process and ensure product quality. In the process of rolling, forging and casting of the blank, as well as in the process of storage and transportation, steel often produces some damage and defects, among which the common surface defects such as irregular material marking, surface cracks, oxide scale rust layer and surface corrosion, wrinkles And heavy skin, mechanical damage and shape and size deviation, etc., should pay attention to inspection and control.

(2) Internal quality

For important valve parts, in addition to the appearance and surface quality inspection, the internal quality inspection of the material should also be carried out. The internal defects of the material mainly include non-metallic inclusions, interlayer cracks, white spots, pores, delamination or uneven structure, composition deviation and coarse grains. When the above defects exist in the material, it will affect the mechanical properties and structural strength of the valve, shorten the life of the valve, and cause accidents in serious cases. Attention should be paid to the inspection and discovery of internal defects, and the use of such materials as valve pressure-bearing parts should be stopped.

(3) Chemical composition and physical properties of materials

For the selection of general general valve materials, a certificate of conformity and a material specification are required, and qualitative inspection methods such as spectrum and spark identification can be used by sampling. However, for nuclear industry valves of radioactive medium and key valves of major projects, materials must be selected according to relevant technical standards or drawings, and chemical composition analysis of materials must be carried out according to technical requirements. The material testing department can only use it if the material re-inspection qualification report is issued. When testing the chemical composition and physical properties of the materials, it is necessary to pay attention to the completeness of the picking procedures, the correct sampling of samples, the commissioning of the samples, the reporting of the samples, the material marking and transplantation work, and the strict material substitution procedures.

2.2. Manufacturing accuracy inspection

The manufacturing accuracy inspection of valve parts mainly includes tolerance and matching size inspection, surface roughness inspection, shape and position tolerance inspection.

(1) Tolerances and fitting dimensions

When inspecting the tolerances and matching dimensions of valve parts, it is first necessary to clarify the meaning of the tolerances and matching dimensions on the drawings, and make accurate inspection conclusions. The standards related to tolerance and coordination issued by our country can be queried and applied during inspection.

In the process of valve manufacturing or maintenance, it is important and the most basic requirement to correctly master the measurement method and use the measuring instrument accurately. These include the correct selection of measuring instruments, the correct analysis of the causes of measurement errors, and the correct processing of measurement data.

Reasonable selection of measuring instruments is the main condition for obtaining measurement results with required accuracy, ensuring product quality, improving measurement efficiency and reducing costs. The general requirement is that in the mass production of valves, advanced and high-efficiency special measuring tools should be used, and general-purpose measuring tools should be used in small batch production and maintenance of valves. When selecting, a suitable measuring tool should also be selected according to the shape of the valve part to be measured, so as to prevent the measurement from being hindered by the shape of the object, such as the measurement of the internal size of the valve. In order to ensure the reliability of measuring the dimensions of parts, GB/T 3177-2009 stipulates the measurement of the dimensions of smooth workpieces and the selection of measuring tools.

In the inspection of valve parts, the measurement error exists objectively, but it should be controlled within the smallest possible range, especially for valve parts with optional or single matching, more attention should be paid to controlling the measurement error, which is conducive to improving production efficiency and ensure product quality.

(2) Surface roughness

The inspection of the surface roughness of valve parts should first master the relevant technical standards. There are many inspection methods for surface roughness, and the surface roughness that has high requirements on the surface or needs to be subjected to arbitration inspection can be measured by instruments (such as profilometers, etc.) by the metrology department. In the processing site, the inspection can be carried out according to the visual macroscopic experience, and the surface roughness sample can also be used for comparison and identification. Commonly used surface roughness inspection methods include comparison method, light section method, needle tracing method of interference method, etc.

(3) Shape position tolerance

During the inspection of the shape and position tolerances of valve parts, the relevant standards and measurement techniques for the shape and position tolerances should be correctly understood and accurately mastered, and they should be strictly implemented. The opening and closing parts are the moving parts that play a closing role in the valve, which are used to cut off, adjust and change the flow direction of the medium. All opening and closing parts have one or two sealing surfaces that match the sealing surface of the valve seat with high precision. The sealing surface of opening and closing parts is the main leakage source of the valve. Therefore, the geometric tolerance of the opening and closing parts should be strictly controlled during the manufacturing and inspection process. The inspection of other shape and position tolerances, such as the inspection of the cylindricity of the contact part between the valve stem and the packing, and the inspection of the valve stem trapezoidal thread tolerance, shall be carried out in strict accordance with the requirements of the drawings and relevant standards.

2.3. Non-destructive testing

The non-destructive inspection of nuclear grade valves shall be in accordance with the provisions of EJ/T 1039-1996, RCC-M or ASME standards. Some important valve parts must undergo non-destructive inspection during the manufacturing or maintenance process. At present, the most widely used inspection methods in production are radiation, ultrasonic, magnetic particle and penetrant.

Non-destructive testing only adds a certain physical quantity to the object to be tested, and then uses a specific detection device to detect changes in the penetration, absorption, reflection, scattering, leakage, penetration and other phenomena of this physical quantity, so as to check whether the object to be tested exists. abnormal. Due to the limitations of non-destructive testing methods, the influence of equipment errors, human factors, environmental factors, etc., and the comprehensive characteristics of abnormal parts of the measured object, the accuracy of non-destructive testing is biased.

In order to improve the reliability of the test results as much as possible, the test must be carried out in strict accordance with the relevant technical standards of non-destructive testing. Select the detection method suitable for the detection of abnormal parts, the non-destructive testing personnel should hold the "NDT personnel technical qualification certificate", the non-destructive testing equipment should be adjusted accurately, the inspection situation should be recorded in detail and the conclusion report should be made accurately.

2.4. Corrosion resistance

There are two main aspects to the corrosion resistance inspection of the valve, one is the corrosion resistance inspection of the chemically treated parts on the valve surface, and the other is the corrosion resistance inspection of the valve stainless and acid-resistant steel materials.

(1) Surface treatment parts

When testing the corrosion resistance of nitrided parts or surface chemically treated parts of valves, the parts are required to be quenched and tempered before nitriding or electroless nickel plating, and the decarburized layer metal should be cut off. The corrosion resistance inspection requirements are specified in the drawings and relevant standards. .

(2) Stainless acid-resistant steel

Through the material test piece, it is mainly to check the quality change of the stainless steel intergranular corrosion test piece due to the corrosion of the medium. The degree of change depends on the concentration, temperature and pressure of the medium, and also on the tissue state of the test piece itself. The basic methods are copper sulfate sulfuric acid boiling test method (L method), copper filings, copper sulfate sulfuric acid boiling test method (T method), nitric acid boiling test method (X method), oxalic acid electrolytic etching test method (C method) and fluorine Sodium nitric acid constant temperature test method (F method). In the process of valve manufacturing or maintenance, the inspection of stainless steel valves is selectively carried out according to drawings or technical conditions.

2.5. Valve marking and painting

(1) Logo

There should be signs on the surface of the valve, including valve pressure level, nominal size, medium flow direction, material, trademark, nuclear safety level, smelting or forging (casting) furnace number and tracking number, etc. It is required that the valve logo should be obvious, clear, neatly arranged and symmetrical, and the font should be regular. The factory name or factory logo of the manufacturer should be marked on the parts that are easy to see, such as valve body, bonnet, handle, wrench, hand wheel spokes etc. on the parts.

(2) Identifying paint

The nuclear-grade valve identification paint should meet the requirements of EJ/T, RCC-M and ASME and other relevant standards and documents. Depending on the valve body material, paint the valve body with the corresponding color paint. The valve sealing surface material should be identified and painted on the transmission handwheel, handle or wrench. The painting of valve electric, pneumatic, hydraulic and gear transmission shall comply with the specifications and requirements of nuclear power valves. The paint layer should be durable, resistant to radiation corrosion, beautiful and uniform, and ensure that the signs are clear and clear.

2.6. Cleanliness

The cleanliness of nuclear-grade valves is a very important indicator, and cleanliness inspection is an indispensable content, which should meet the requirements of RCCMF6000 and EJ/T or ASME related standards. The valve must be cleaned prior to the cleanliness test. Class A Cleanliness Valves—All specification grade stainless steel valve parts must be cleaned with Class A water. Class B Cleanliness Valves—All specification grade carbon and alloy steel valve parts must be cleaned with Class B water. The water quality of Class A or Class B water shall comply with the regulations of RCC-M F6000 or EJ/T1022.18.

In general, the washing and cleaning method should be used. The cleaning process must not alter the properties of the base material or contaminants that cause damage. Generally, the tank dip method, spray method or scrub method can be used, and it can only be carried out with stainless steel wire, nylon brush or clean cloth that has not been used and lint-free. After cleaning, dry it with oil-free air at 60-80°C, and inspect and accept according to the regulations of RCCMF6000 or the regulations of EJ/T 1022.17.

2.7, valve performance

The factory inspection of nuclear grade valves mainly includes the strength performance of the shell, the strength performance of the closing parts, the sealing performance and the action performance test. The prototype must also be functionally tested.

After the valve is assembled and debugged, the necessary test and inspection methods should be used to verify whether the valve meets the basic performance and technical standards. Inspection methods and acceptance criteria shall comply with the provisions of NB/T 20010.9, RCC-M, ASME or test program.

(1) Test pressure of nuclear grade valve

The shell strength test is 1.5 times the maximum allowable pressure of the valve at room temperature, the disc strength test is 1.1 times the maximum allowable pressure of the valve at room temperature, and the sealing test is 1 times the maximum allowable pressure of the valve at room temperature. Nuclear grade valves are not allowed to leak externally, and internal leakage and pressure holding time are in accordance with relevant standards or test programs.

(2) Test medium

The test medium is water, steam, air and inert gas. Stainless steel valves are classified according to their cleanliness. In the hydrostatic test, Class A water is used for Class A water, and Class B water is used for Class B water. Carbon steel and alloy steel valves are classified according to their cleanliness. In the hydrostatic test, Class B water is used, and Class C water is used for Class C water. Corrosion inhibitors or other anti-rust measures can be used during hydrostatic testing. Using steam as the test medium has a direct effect on the steam valve, and can find defects that are difficult to find in the hydraulic test. For example, the steam safety valve should be tested with steam. Using air as the test medium has sufficient gas source and low cost. The test medium used in general gas valves should pay attention to safety during the test. Inert gases include nitrogen and argon, which are safe and reliable, but have a high cost, and are usually used in the sealing test of nuclear-grade valves. Argon gas tightness tests are usually performed for valves in containment vessels. Nitrogen as the test medium is mainly used for safety valves and some important valves. After the valve test is completed, the test medium in the valve should be removed in time and dried.

3. Conclusion

The quality and reliability of nuclear-grade valves determine to a large extent the reliability of the entire nuclear power plant. Manufacturers of nuclear-grade valves must establish a complete quality assurance system, formulate a corresponding quality assurance program and relevant procedures and documents for quality control. Strictly implemented in the manufacturing process, each step has rules to follow, well documented.

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