Corrosion of metal valves and application of synthetic material valves

Corrosion of metal valves and application of synthetic material valves

Corrosion of the valve is usually understood as the damage to the metal material of the valve under the action of a chemical or electrochemical environment. As the phenomenon of "corrosion" 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 span, reliability and service life of the valve. The effect of mechanical and corrosive factors on the metal greatly increases the total amount of 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 oil, natural gas, and oil layer water makes the metal surface more destructive and quickly loses its ability to work.　　

Since the chemical corrosion of metals depends on the temperature, the mechanical load of the friction parts, the sulfide contained in the lubricating material and its acid resistance, the duration of contact with the medium, the catalysis of the metal on the nitriding process, and the corrosion resistance The conversion speed of the molecules of the substance to the metal and so on. 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.

1. 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 process of use.　　

The method of metal valve anticorrosion 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 anticorrosion.　　

There are four main methods of anti-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 hindered 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 flow of water to prevent corrosion.) 　　

Second, the application of material valves　　

Synthetic material valves are superior to metal valves in many corrosive working conditions. First is the corrosion resistance, and second is the net weight. As for its strength, it depends on the shape, arrangement and quantity of the reinforcing fiber. (Generally speaking, the greater the percentage of fiber, the greater the strength of the synthetic material.) In valve applications, the weight content of the fiber basically ranges from 30% to 40%, and its chemical stability is mainly determined by the potting in the final product The resin body characteristics of the fiber are determined. In synthetic material valves, the solid polymer body can be either thermoplastic (such as PVC-polyvinylidene fluoride, PPS-polyphenylene sulfide, etc.) or 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 heat resistance of synthetic materials is called 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 performance of the most widely used synthetic materials with the performance of corrosion-resistant metals.　　

Glass reinforced vinyl Graphite reinforced vinyl Graphite reinforced PPS 316SS Hast-C

Tensile strength (Psi) 4000 6000 26000 85000 90000

Bending strength (Psi) 8000 12000 35000 NA NA

Compressive strength (Psi) 14000 18000 21000 85000 90000

Heat distortion temperature (°F) 295 7320 520 NA NA

Coefficient of thermal expansion (in/in/°F) 15×10-6

15×10-6

13×10-6

6.3×10-6

9.9×10-6

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 level 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 the corrosive working conditions of many metal or glass fiber reinforced plastic piping systems. In the 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 (which can increase the service temperature of the valve from 400°C to 1200°C or higher), plastics (with the characteristics of rust and corrosion resistance), synthetic materials (with the characteristics of corrosion resistance, light weight, and high strength), memory alloys (using the shape Reversibility and high elasticity of memory alloy or temperature memory alloy to manufacture valves) and so on. With new materials, a large number of new valve products with high performance are being developed. The development of new technology has made it possible for various engineering materials to be used in the valve manufacturing industry.