Stainless steel pipe has become the United States nearly 25 years to build the city wastewater treatment plant (WWTP) standard timber. Since the late 1960s, there have been more than 1,600 municipal wastewater treatment plants in the United States that use stainless steel aeration pipes, digestion tank gas pipelines and sludge pipelines, as well as stainless steel mobile bridges, valves, ponds, bolts, handrails and Other equipment. The initial selection of stainless steel instead of galvanized and painted carbon steel is to reduce maintenance and replacement costs, these materials repair and replacement costs are higher because of their poor corrosion resistance.
Low-carbon steel grades 304L and 316L are used for welded construction. The purpose of using these low-carbon steels is to resist corrosion after corrosion without heat treatment. Higher carbon content can increase the strength of the steel, mainly for the stem and pump shaft, these components do not need welding, high strength can make the design optimization. Molybdenum-containing stainless steel is better resistant to localized corrosion, preferably in a more aggressive environment or in order to prevent unexpected accidents that may occur at any time. The corresponding steel products have corrosion resistance and similar mechanical properties.
304 / 304L and 316 / 316L stainless steel are the main materials of sieve, grit chamber, overflow weir, bolts, mobile bridge, aeration tank, digestion tank and sludge pipeline in urban wastewater treatment equipment. In the past 20 years , According to the International Nickel company conducted field tests, these materials have become the standard construction materials. 304 and 316 stainless steel weight loss corrosion rate <0.1 mil / year. Corrosion rate <0.1 mil / year indicates that the useful life of this material is expected for more than 20 years. These very low corrosion rates mean that there is no need to leave a “corrosion margin” on the wall thickness when designing stainless steel equipment, but corrosion margins must be left for carbon and ductile iron. So the use of stainless steel in terms of weight / material can save much more.
Service performance of stainless steel
Experiments and experience show that 304 and 316 stainless steel has the following advantages:
(1) resistance to atmospheric corrosion of steel to add chromium to improve the corrosion resistance. The amount of chromium required to make the stainless steel “no rust” may vary depending on the weight loss of the steel content. Chromium content greater than and equal to 12.5% of the steel in the marine atmosphere exposed to 25 years after the weight loss is quite small. These “pure chrome” stainless steels (ferrite and martensite) and the surface layer have a layer of corrosion, which is not suitable for use in many atmospheric environments where the appearance is particularly important. In contrast, nickel and nickel containing 8% to 10% of the austenitic steel grades 304 and 316 are more resistant to corrosion and are exposed to the atmosphere for many years. The H2S gas produced in the digestion tank penetrates into many parts of the wastewater treatment plant, accelerating corrosion of copper alloys, aluminum and carbon steels. In humid H2S, 304 and 316 stainless steel corrosion rate <0.1 mil / year (<0.0025mm / year). This corrosion rate is negligible. 304L stainless steel is the digestive tank gas pipeline standard material, and the use of good results. Although people do not want to expose the steel to wet chlorine, but in the chlorine storage area near the well ventilated limited position, the appearance of the pipe did find some general pitting. 304L and 316L stainless steel in the wet chlorine accumulation and condensation of the non-ventilated atmosphere is easy to rust and the occurrence of general pitting. Proper ventilation can eliminate these phenomena, you can also regularly spray the surface of stainless steel, can rust and pitting can be reduced to a minimum.
(2) Chlorine-Ozone Stainless steel is usually resistant to the general concentration of chlorine gas in the wastewater treatment equipment. According to 1010 carbon steel, cast iron, 304 and 316 stainless steel samples exposed to chlorine in a certain residual chlorine concentration of fresh water exposed data, 304 and 316L in the residual chlorine content of up to 2ppm of well water corrosion resistance is good, and Carbon steel and cast iron corrosion rate is twice as high. If the continuous exposure to 3 ~ 5ppm residual chlorine, the measured data show that 304 / 304L prone to crevice corrosion, insurance should use 316L, stainless steel is not suitable for chlorine spray system, where the chlorine content of up to 50 to hundreds of ppm. Ozone is an increasingly popular oxidant substitute that can be used alone or in combination with chlorine. Although the initial use of ozone generator 316L selection without the use of 304L background and lack of data, but 316L has become the best material for the manufacture of ozone generator.
(3) Other chemical additives Ferrous sulfate is a chemical additive that is often added to wastewater treatment plants: De Renzo says 304 and 316 stainless steel samples are in ferrous sulfate solution containing sulfuric acid (waste acid solution) with pH value of 1 to 2.5 Corrosion resistance is very good, corrosion rate of less than 0.8 mil / year (<0.02mm / year). In the wastewater treatment plant, there is generally no sulfuric acid, therefore, stainless steel in the acid-free ferrous sulfate corrosion rate can be ignored. Ferric chloride is sometimes used in the conditioning tank to flocculate, the sludge is further concentrated and dehydrated in the conditioning tank, and then burned and drained. The 304 and 316 samples placed on the activated sludge position 6 have both pitting and crevice corrosion in the presence of ferric chloride content of 250 to 300 ppm. 316 stainless steel Corrosion resistance to ferric chloride slightly better than 304 stainless steel, can be used as a more conservative choice for ferric chloride injection point after the higher content of ferric chloride. Although the test data is not ideal, however, 304 stainless steel pipe is generally used for conveying the effluent after treatment.
(4) microbial corrosion (MIC) standard stainless steel in the stagnant and slow flow of water, the weld and weld around sometimes prone to microbial corrosion. The presence of microbial corrosion on a frame made of 409 (11% chrome) stainless steel in a rotary bio-compression device (RBC’s) tested in a wastewater treatment plant is an example. Microorganisms are roughly stacked at the welds of the frame. Rotary bio-compression device stainless steel frame to 2 rev / min speed in the sludge when the time out for the microbial corrosion provides the ideal environment. There is little microbial corrosion in other parts of the wastewater treatment plant.
Stainless steel design for light components and piping systems has been very cost effective. Although the initial investment cost of stainless steel may be higher than other materials, but its long life, the total maintenance costs are low, the total life expectancy reduced costs. The European Stainless Steel Development and Information Group (EUROINOX) has completed a European project for a wastewater treatment plant in Italy. Although the initial installation cost of stainless steel is more than 25% higher than that of carbon steel, only the life cycle cost of carbon Steel flat. This analysis includes capital costs and maintenance costs, and the choice of stainless steel for design, saving weight using a conservative estimate of 10%. Stainless steel devices include mechanical guards, mobile bridges and handrails, these parts are commonly used galvanized steel.
304L and 316L stainless steel pipes have been very successful for use in pipes and other uses in wastewater treatment plants. The high design performance and low corrosion rate of stainless steel allow the use of lightweight construction. Good cleaning and maintenance of the factory will further improve the excellent performance of stainless steel.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)