In the corrosive medium operating conditions of valves, anti-corrosion chemical equipment is the most critical areas, if not the correct choice of chemical industry valve metal materials, a little carelessly, ranging from damage to equipment, while in causing the accident or even lead to disaster . According to relevant statistics, chemical equipment, about 60% of the damage caused due to corrosion, so the selection of equipment in the chemical industry should pay attention to when the first selection of the scientific. Usually there is a misunderstanding that the stainless steel is "Universal", no matter what the media and environmental conditions Pengchu stainless steel, this is not true, and it is very dangerous. The following media to talk about for some commonly used chemical selection points:
1. Sulfuric acid medium, as one of the strong corrosive medium, sulfuric acid is to use a very wide range of important industrial raw materials. Different concentrations and temperature of the sulfuric acid corrosion of materials vary greatly, for more than 80% concentration, the temperature is less than 80 ℃ in concentrated sulfuric acid, carbon steel and cast iron has good corrosion resistance, but it is not suitable for high-speed flow of sulfuric acid , not used as a valve material; ordinary stainless steel, such as 304 (0Cr18Ni9), 316 (0Cr18Ni12Mo2Ti) is also limited use of sulfuric acid. Therefore, transport of sulfuric acid pump and valve are usually high-silicon cast iron (casting and machining difficult), high alloy stainless steel (alloy No. 20) manufacturing. Fluorine plastic has good resistance to sulfuric acid properties, using fluorine Valve (F46) is a more economical choice. If the pressure is too large, temperature, plastic valve with the points on a hit, you can choose more expensive than it has a ceramic ball valve.
2. Hydrochloric Acid must have patience to sit through most of the hydrochloric acid corrosion of metallic materials (including a variety of stainless steel material), containing molybdenum is also only be used for high-Si-Fe 50 ℃, 30% hydrochloric acid below. And metal materials, the contrary, the vast majority of non-metallic materials have good corrosion resistance, hydrochloric acid, so rubber lined pumps and plastic pumps (such as polypropylene, fluorine plastic, etc.) is the best choice for conveying hydrochloric acid. But this kind of medium, if the temperature exceeds 150 ℃, or the pressure is more than 16 kilograms, any plastic (including polypropylene, fluorine plastic or even PTFE) will not be competent, while not currently on the market have very good the valve, but you can try now emerging ceramic ball valve, this valve has the advantage of self-lubrication, torque force is small, non-aging, life span is much longer than the average over the valve, its disadvantage is that the price more higher than the plastic valve.
3. Nitric acid media, most of the general metal corrosion in nitric acid was rapidly destroyed, stainless steel is the most widely used nitric acid-resistant materials, all concentrations of nitric acid at room temperature have good corrosion resistance, it is worth mentioning that the molybdenum-containing stainless steel ( such as the 316,316 L) of nitric acid corrosion resistance is not only no better than ordinary stainless steel (eg 304,321), and sometimes even better. For high-temperature nitric acid, commonly used titanium and titanium alloy materials.
4. Acetic acid medium which is corrosive organic acid, one of the strongest material, ordinary iron and steel in all of the acetic acid concentration and temperature will be severe corrosion, stainless steel is an excellent acid-resistant materials, 316 stainless steel containing molybdenum can applicable to high temperature and dilute acetic acid vapor. For the high temperature and high concentrated acetic acid, or contain other corrosive media such as stringent requirements may make use of high alloy stainless steel or fluorine plastic pump.
5. Alkali (sodium hydroxide) steel is widely used in 80 ℃ below 30% within the concentration of sodium hydroxide solution, there are a number of petrochemical plants in 100 ℃, 75% is still below the ordinary iron and steel, although the corrosion increased, but the economyä½³. Ordinary alkali corrosion resistance of stainless steel for iron, compared with no obvious advantages, as long as the media were mixed to allow a small amount of iron, stainless steel is not recommended. For the high-temperature lye to use more titanium and titanium alloy or high alloy stainless steel.
6. Ammonia (ammonium hydroxide) Most of the metal and non-metal in liquid ammonia and ammonia (ammonium hydroxide) in the corrosion is very slight, only the copper and copper alloy unfit for use.
7. Chlorine (chlorine) Most of the chlorine in the corrosion resistance of metal valves are very limited, particularly in the case of chlorine with water, including various alloy valves, this case, the PTFE valve is a very good choice, but the production of chlor-alkali chemical plants will find: PTFE valve used for a long time for a little torque force increases, the problem of aging will be highlighted PTFE out, this case is fatal leak occurred in a . May consider using PTFE lined ceramic ball to replace the original core of common PTFE lined valves, the use of ceramic self-lubricating and corrosion resistance of PTFE will have perfect results.
8. Salt water (seawater) General steel in sodium chloride solution and sea water, salt water corrosion rate is not too high to be used in general coating protection; all kinds of stainless steel also has a very low uniform corrosion rate, but may be due to chloride ions caused partial corrosion, usually 316 stainless steel well.
9. Alcohols, ketones, esters, ethers common alcohol media are methanol, ethanol, ethylene glycol, propanol, etc., ketones media are acetone, methyl ethyl ketone, etc. A variety of esters of medium ester, ethyl ester, etc., ethers media are ether, ethyl ether, butyl ether, etc., they are basically no corrosive materials can be used to apply a specific use should also be under the relevant requirements of the medium properties and make a reasonable choice. Also noteworthy was the ketones, esters, ethers of a variety of rubber are soluble, the choice of sealing materials to avoid making mistakes.
There are many other media can not be introduced in this 11, in short, when you must not be arbitrary in the selection and blindness, should be more access to relevant information or learn from mature experience.
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