Why are cast iron valves used in sewage systems?

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In municipal sewage treatment, industrial wastewater treatment, and pipeline transportation systems, cast iron valves (especially ductile iron valves) have long held a dominant position, with their application scope far exceeding that of stainless steel valves, cast steel valves, or plastic valves. This phenomenon stems not from technical conservatism, but from a comprehensive consideration of complex wastewater conditions, strict cost control, and high requirements for long-term operational stability. From medium properties and material characteristics to engineering suitability, cast iron valves meet the core requirements of sewage systems in various aspects, which makes them the standard choice in this industry for their practicality, durability, and cost-effectiveness. The operating environment in which sewage systems operate is usually quite harsh, with complex medium compositions that possess a high tendency to cause damage. Wastewater often contains large amounts of solid suspended particles such as sediment, slag, fibers, and sludge, which continuously scour the inner walls of the valve and its sealing surfaces as it flows ； At the same time, wastewater commonly contains weakly corrosive substances such as hydrogen sulfide, organic acids, and chloride ions, which can cause chemical or electrochemical erosion of metal components over time. Furthermore, valves are often installed in underground chambers or humid outdoor environments, making them susceptible to soil corrosion and external environmental impacts; moreover, the systems usually require continuous operation over long periods with infrequent maintenance needs. The characteristics of the aforementioned operating conditions require that sewage valves possess five key capabilities: wear resistance, resistance to mild corrosion, structural stability, cost control, and ease of maintenance. Cast iron material exhibits good suitability in these aspects. In terms of material properties, cast iron possesses several advantages that make it naturally suitable for use in sewage applications. Gray cast iron and ductile cast iron (mainly QT450-10) contain a uniformly distributed graphite structure. On the one hand, the presence of graphite endows the material with good compressive strength and rigidity; as a result, the valve body is less likely to deform. It can withstand the normal pressures in sewage pipelines (1.0–1.6 MPa) as well as the water hammer effects generated during the start-up and shutdown of water pumps, thereby ensuring structural stability over long periods of operation. On the other hand, graphite possesses a certain self-lubricating effect, which can reduce the frictional damage caused by solid particles to the inner walls of the valve and to the sealing surfaces, thereby improving wear resistance. Compared to ordinary steel, it is better able to withstand the risk of leakage at the sealing surfaces resulting from long-term erosion by particles such as sediment and slag. In terms of corrosion resistance, cast iron performs better than ordinary carbon steel in the mildly corrosive environments commonly found in sewage. A relatively dense oxide film can naturally form on the surface of cast iron in atmospheric or humid environments, which can slow down the process of electrochemical corrosion to a certain extent. When combined with conventional anti-corrosion coatings such as epoxy resin and asphalt, it can effectively resist the long-term effects of weak corrosive agents in sewage, such as hydrogen sulfide and organic acids; its actual service life is generally 15 to 20 years. Compared to carbon steel, cast iron has a relatively lower corrosion rate in humid and mildly corrosive environments, and its corrosion products do not tend to flake off and cause blockages in the flow channels. Compared to stainless steel, although cast iron has inferior corrosion resistance in highly corrosive environments, sewage usually exists in a weakly corrosive condition with a pH of 6–9. After anti-corrosion treatment, cast iron can meet the requirements of engineering applications, and its material cost is significantly lower. From an engineering application perspective, cast iron valves possess irreplaceable advantages in terms of cost-effectiveness and suitability. The pipe diameters in sewage systems range over a wide spectrum, with applications from small-diameter branch pipes of DN50 to main pipes of DN2000 and above. The cast iron casting process is well-developed, offers good shapeability, and is relatively easy to work with; it makes it suitable for manufacturing valve bodies with large diameters and complex structures. Its manufacturing cost is approximately 60%–70% of that of cast steel valves and 30%–50% of that of stainless steel valves. At the same time, cast iron valves have a relatively simple structure with fewer components; combined with soft sealing materials suitable for sewage such as nitrile rubber (NBR) and ethylene propylene diene monomer rubber (EPDM), they can achieve reliable sealing and smooth operation. Even when left idle for long periods, it is less prone to sticking, requires less frequent maintenance, and its servicing is straightforward, which helps reduce the operational costs over the entire lifecycle of the wastewater system. Furthermore, the stability and versatility of cast iron valves meet the requirements for the large-scale use of sewage systems. Its material properties are less affected by temperature fluctuations, making it suitable for the typical sewage temperature range of -30°C to 120°C. It can be widely used in various applications such as underground pipelines, pump stations, biological treatment tanks, and sludge pipelines. The industry features a high degree of standardization; installation, replacement, and procurement of spare parts are all straightforward, as no special processes or tools are required. It is suitable for various industrial wastewater treatment systems, including municipal sewage, chemical wastewater, and metallurgical wastewater, and represents a reliable valve material that has been proven effective through long-term practical use in engineering projects. In summary, the use of cast iron valves in sewage systems is a rational choice that balances performance suitability, cost optimization, and long-term operational stability. Ductile iron offers higher strength and toughness, while gray cast iron has advantages in terms of cost control. With appropriate anti-corrosion coatings and sealing structures, it can effectively address engineering challenges such as wear, corrosion, and clogging caused by sewage. Unlike the potential overdesign associated with high-cost materials, cast iron valves achieve safe, efficient, and long-term operation of sewage systems at a favorable cost-performance ratio; this is precisely why they remain the dominant choice for sewage valves amid continuous technological advancements in the industry.