This article focuses on rubber, PTFE, and PVDF materials sealing and their industrial applications, revealing the unique properties and limitations of each material. And how to choose a suitable corrosion-resistant regulating valve.
1. Corrosion-resistant Materials
1) Rubber has a certain degree of corrosion resistance and is inexpensive. Therefore, it is widely used in the regulation valves of corrosion-resistant equipment at low temperatures. In the 1950s and 1960s, diaphragm valves with rubber lining were developed. To regulate high-flow corrosive media, lined rubber butterfly valves were also developed. However, rubber is not completely corrosion-resistant, and its use is limited by temperature and pressure conditions.
2) Polytetrafluoroethylene (commonly known as PTFE or F4) is the best corrosion-resistant material today, known as the "corrosion-resistant king." It has excellent corrosion and heat resistance and can be used for a long time in temperatures ranging from -250 to 250°C. Except for corrosion by fluorine and molten alkali metals under high temperature and pressure, and slight expansion caused by certain halides or aromatic hydrocarbons, it is not affected by strong acids (including concentrated nitric acid and aqua regia), strong bases, strong oxidizers, oils, ketones, ethers, alcohols, etc., even at high temperatures. It can resist almost all chemical media without corrosion.
3) Polyvinylidene fluoride (PVDF) is a new type of material with excellent corrosion resistance and temperature resistance. It has high chemical stability and excellent resistance to chemical corrosion, such as resistance to strong acids, strong bases, and strong oxidizers. It also has outstanding heat and cold resistance and can be used for a long time in environments ranging from -00 to 250°C. Additionally, it has characteristics such as non-adhesion, non-absorption of water, and non-combustibility. Moreover, this material has good fluidity and can be used as a corrosion-resistant lining material on the surface of components.
2. Corrosion-resistant Valve Selection
1) Full PTFE corrosion-resistant single-seat control valve
The full PTFE corrosion-resistant single-seat control valve adopts a structure with a metal shell and embedded PTFE. The metal shell is generally made of carbon steel or stainless steel, and the parts (cavities, channels, throttling components, etc.) in contact with the medium inside the valve body are all made of PTFE. This isolates the valve body material from the corrosive medium, providing a thorough solution to corrosion issues, especially in media such as sulfuric acid, hydrochloric acid, and wet chlorine gas.
2) Lined PTFE O-type ball valve
Lined PTFE ball valves, also known as special ball valves for water treatment, are mainly used in two-position control devices for strong alkalis and water treatment (desalination water). They use an internal lining of PTFE to ensure that the parts in contact with various corrosive media are made of PTFE, isolating the corrosive medium from the valve body material. In terms of structure, the ball core of the ball valve is a stiff and reliable part, fundamentally solving the unreliability of diaphragm valves, improving reliability, and extending the service life. Therefore, lined PTFE ball valves are the best products to replace diaphragm valves.
3) Lined PTFE V-type ball valve
Lined PTFE O-type ball valves are only suitable for two-position control and shut-off. In applications with impurities in strong corrosive media, lined PTFE V-type ball valves are suitable. Its structure, installation dimensions, and lined PTFE O-type ball valves are the same. The core of the ball valve opens a V-shaped hole with adjustable characteristics, ensuring good anti-blocking performance and excellent performance when working at small openings. In media with particles, viscosity, suspended solids, etc., it is recommended to use this corrosion-resistant valve.
4) Lined PTFE butterfly valve
Lined PTFE butterfly valves are suitable for large-diameter, high-flow corrosive control applications. It uses the same process as lined PTFE ball valves, lining fluoroplastics inside the valve body to ensure that the material in contact with strong corrosive media is resistant to corrosion. Therefore, it also has excellent corrosion resistance. At the same time, the butterfly valve structure also has excellent anti-blocking performance.
5) Corrosion-resistant alloy valve
Corrosion-resistant alloy valves are suitable for applications with high pressure (greater than 2.5 MPa) and extreme temperatures (higher than 180°C, lower than -40°C). However, the use of high-priced materials makes the cost of this valve extremely high. It is recommended to use a straight-through single-seat valve structure for DN≤20, a full-function valve structure for 25≤DN≤150, and a butterfly valve structure for DN≥200. This can significantly reduce costs and improve the performance-to-price ratio.
3. Products Not Recommended for Use
Lined plastic diaphragm valve
Before the 1980s, due to the inability to find better corrosion-resistant valves, lined plastic diaphragm valves were widely used by many manufacturers for a long time. However, field feedback shows that this valve has two obvious defects. First, it has poor reliability and a short lifespan. The thin film made of fluoroplastics is easily folded and broken during operation. Second, it is cumbersome. As the trend in valve regulation moves towards miniaturization, light weight, and instrumentation, diaphragm valves have not been able to reduce their weight. Because the diaphragm is subjected to the pressure of the medium before and after the valve, its unbalanced thrust is very large, and the corresponding actuator thrust must also be very large, making it impossible to reduce its weight.
