In the chemical industry, valves as key equipment, the scientific nature of its selection is crucial, especially when dealing with corrosive media. Wrong material selection will not only lead to equipment damage, but also may cause serious accidents. According to statistics, about 60% of chemical equipment damage is caused by corrosion. Therefore, in the selection, we must pay attention to the scientific nature of the selection of materials to avoid blind selection.
Many people in the selection of materials, there is a misunderstanding that stainless steel is the “universal material”, applicable to all media and environmental conditions. However, this is incorrect and dangerous. Different media on the corrosive properties of materials vary greatly, so we need to select materials for specific media.
The following are some common chemical media valve selection points to help U.S. chemical practitioners to make a more informed choice:
- Sulfuric acid medium valve
Sulfuric acid is a strong corrosive medium, widely used in industrial fields. For the concentration of more than 80%, the temperature is less than 80 ℃ concentrated sulfuric acid, carbon steel and cast iron valves have good corrosion resistance, but not for high-speed flow of sulfuric acid. Ordinary stainless steel valves such as 304 and 316 have limited corrosion resistance to sulfuric acid. Therefore, high silicon cast iron, high alloy stainless steel valves (such as alloy 20) or fluorine lined valves (F46) are usually chosen to handle sulfuric acid. Under high temperature and pressure conditions, ceramic ball valves may be a more appropriate choice. - Valves for hydrochloric acid media
Hydrochloric acid is strongly corrosive to most metallic materials, including various stainless steel materials. In contrast, non-metallic materials such as rubber and plastic have good corrosion resistance to hydrochloric acid. Therefore, rubber lined valves and plastic valves (e.g., polypropylene, fluoroplastic, etc.) are ideal for conveying hydrochloric acid. However, when the temperature exceeds 150°C or the pressure is greater than 16 kilograms, these materials may not be up to the task. At this point, ceramic ball valves may be a viable alternative, albeit at a higher price. - Valves for Nitric Acid Media
Nitric acid has a rapid corrosive effect on most metals. Stainless steel valves are the most widely used nitric acid-resistant materials, with good corrosion resistance to all concentrations of nitric acid at room temperature. It is worth noting that molybdenum-containing stainless steel valves do not have better corrosion resistance to nitric acid than ordinary stainless steel valves. For high temperature nitric acid, titanium and titanium alloy materials are usually used. - Acetic acid medium valve
Acetic acid is an organic acid, with strong corrosiveness. Ordinary steel in all concentrations and temperatures of acetic acid will be seriously corroded. Stainless steel is excellent acetic acid resistant materials, containing molybdenum 316 stainless steel valves are also suitable for high temperature and dilute acetic acid vapor. In the case of high temperature and high concentration of acetic acid or other corrosive media containing high alloy stainless steel valves or fluoroplastic valves. - alkali (sodium hydroxide) medium valve
Steel is widely used in sodium hydroxide solution below 80℃ and within 30% concentration. Although corrosion will increase, but the economy is better. Ordinary stainless steel valves for lye corrosion resistance compared with cast iron has no significant advantages. For high-temperature lye, more titanium and titanium alloy valves or high-alloy stainless steel valves.
6 ammonia (ammonia hydroxide) medium valve
Most metals and non-metals in liquid ammonia and ammonia corrosion are very slight, but copper and copper alloy should not be used. - chlorine (liquid chlorine) medium valve
Most metal valves have limited resistance to the corrosion of chlorine gas, especially when chlorine gas carries water. PTFE valves are a good choice in this case, but the torque force may increase and PTFE aging may occur after long-term use. At this time, the use of PTFE lined ceramic ball core can be considered to replace the ordinary PTFE lined valves to get better results. - Salt water (seawater) medium valve
Ordinary steel in sodium chloride solution, seawater and salt water in the corrosion rate is not too high, but usually need to use coatings for protection. Various types of stainless steel valves also have a very low rate of uniform corrosion, but may cause localized corrosion due to chloride ions. It is usually better to use 316 stainless steel valves. - Alcohols, ketones, esters, ether media valves
These media are basically non-corrosive, commonly used materials can be applied. However, ketone, ester, ether on a variety of rubber solubility, in the selection of sealing materials need special attention to avoid mistakes.
In the selection of chemical valves, we must make a reasonable choice based on the properties of the medium and related requirements. Do not be arbitrary and blind, should consult more relevant information or learn from mature experience. Through scientific selection, we can ensure the stable operation of chemical equipment and avoid potential safety risks.