Anti-corrosion Application of Metal and Synthetic Material Valves
The corrosion of the vacuum valve is usually understood as the damage to the metal material of the valve under the action of a chemical or electrochemical environment. Since the “corrosion” phenomenon occurs in the spontaneous interaction between the metal and the surrounding environment, how to isolate the metal from the surrounding environment or use more non-metallic synthetic materials has become a common concern.
It is well known that the corrosion damage of metal has a considerable influence on the life, reliability and service life of the valve. The effect of mechanical and corrosive factors on metals greatly increases the total wear on the contact surface. The total amount of wear on the friction surface of the valve during operation. During the operation of the valve, the frictional surface is worn and damaged due to the simultaneous mechanical action and the chemical or electrochemical interaction between the metal and the environment. For valves, the climatic conditions of their pipelines are complicated; the presence of hydrogen sulfide, carbon dioxide and certain organic acids in media such as oil, natural gas, and oil layer water increases the destructive power of the metal surface and quickly loses its ability to work.
Because the chemical corrosion of metals depends on the temperature, the mechanical load of the friction parts, the sulfide contained in the lubricating material and the stability of its acid resistance, the duration of contact with the medium, and the catalytic effect of the metal on the nitriding process, corrosion and erosion The conversion speed of the molecules of the substance to the metal, etc. Therefore, the anti-corrosion methods (or measures) of metal valves and the application of synthetic material valves have become one of the themes of current research in the valve industry.
- The anti-corrosion of metal valves can be understood as coating the metal valve with a protective layer (such as paint, pigment, lubricating material, etc.) that protects it from corrosion, so that the valve can be manufactured, stored, transported or It is not corroded during the whole use.
The anti-corrosion method of metal valves depends on the required protection period, transportation and storage conditions, valve structure characteristics and materials. Of course, it is suitable to consider the economic effect of removing anti-corrosion.
There are four main methods for preventing corrosion of metal valves and their parts:
1. Put the volatile corrosion inhibitor into the steam atmosphere (wrap it with barrier paper, blow to inhibit air from passing through the product chamber, etc.).
2. Use blocked water and alcohol solution.
3. Apply a thin layer of anti-corrosion (protective) material to the surface of the valve and its parts.
4. Apply the hindered film or polymer thin layer to the surface of the valve and its parts.
(Note: The current valve manufacturers widely use lubricating materials and water to block the melt flow to prevent corrosion.)
2. Application of material valve
Synthetic material valves are better than metal valves in many corrosive conditions. First, corrosion resistance, and second, net weight. As for its strength, it depends on the shape, arrangement and quantity of reinforcing fibers. (Generally speaking, the greater the percentage of fiber, the greater the strength of the synthetic material.) In valve applications, the basic weight content of fiber ranges from 30% to 40%, and its chemical stability is mainly determined by potting in the final product The resin body characteristics of the fiber are determined. In a synthetic material valve, the solid polymer body can be either a thermoplastic (such as PVC-polyvinylidene fluoride, PPS-polyphenylene sulfide, etc.) or a thermosetting resin (such as polyester, vinyl and epoxy resin) Wait). Thermosetting resin maintains its strength at medium temperature better than thermoplastics (ie, thermosetting resin has a higher heat distortion temperature). (Note: In the operating conditions, the measurement of the thermal resistance of synthetic materials is called the heat distortion temperature.)
At present, the most commonly used synthetic material for chemical process valves is vinyl epoxy resin (thermoplastic material), and its reinforcing fibers are chopped glass fiber (1/4 inch long) and chopped graphite fiber (1/4 inch long). ). The following table compares the properties of the most widely used synthetic materials with those of corrosion-resistant metals.
Glass reinforced vinyl graphite reinforced vinyl graphite reinforced PPS316SSHast-C
Tensile strength (Psi) 40006000260008500090000
bending strength (Psi) 80001200035000NANA
Compressive strength (Psi) 1400018000210008500090000
Heat distortion temperature (°F) 2957320520NANA
Coefficient of thermal expansion (in/in/°F) 15×10-6
Note: NA is not applicable (or invalid)
From the surface analysis, compared with metal, glass and graphite reinforced synthetic materials have lower tensile strength. Therefore, the parts of the synthetic material valve with greater stress should have a thicker section and be attached with reinforcing ribs to achieve the same as metal performance.
Due to the corrosion resistance, high strength and light weight of the synthetic material valve, it becomes an economical alternative valve product in many corrosive working conditions of metal or glass fiber reinforced plastic piping systems. In chemical process conditions, the prospects for the use of synthetic material valves are excellent.
In the valve industry, the use of organic materials and synthetic materials is no longer the traditional concept of metal and non-metal valve materials. Ceramics (can increase the service temperature of the valve from 400℃ to 1200℃), plastics (with the characteristics of anti-rust and corrosion resistance), synthetic materials (with the characteristics of corrosion resistance, light weight and high strength), memory alloys (using shape The reversibility and high elasticity of memory alloy or temperature memory alloy are used to manufacture valves). With new materials, a large number of new valve products with high performance are being developed. The development of new technology makes it possible to apply various engineering materials to the valve manufacturing industry.