Application of PP-R and HDPE pipes in Nangang coking plant

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The use of PP-R and HDPE pipes in Nangang’s coking operations began with experimental applications of these plastic pipes in the ammonium sulfate mother liquor crystallization system of the ammonium sulfate production line at Nangang Coking Plant in September 1999; since then, they have become widely used in the coking industry, delivering excellent results in systems related to ammonium sulfate production, desulfurization, biochemical processes, and water supply. A set of PP-R and HDPE plastic pipe systems in Nanjing Iron and Steel’s ammonium sulfate system has been operating stably since its installation, with virtually no maintenance required. 1 Pipe characteristics ⑴ Features of PP-R pipes. Made from type 3 polypropylene, it boasts excellent heat and corrosion resistance, making it highly suitable for use in drinking water, heating, and applications requiring high levels of corrosion protection: — Light weight. With a density of 0.89~0.91 g/cm3, it can significantly reduce transportation costs and the intensity of installation work. ——It has good heat resistance. It can be used for a long time at a working water temperature of 65°C; a temporary use temperature of up to 95°C is possible, with a softening temperature of 140°C. ——Long service life. PP-R pipes can have a service life of 50 years at an operating temperature of 65°C and an operating pressure of 1.0 MPa. It has good corrosion resistance. It has no chemical reaction with most ions and chemicals; it does not rust, corrode, or support bacterial growth. ——Low thermal conductivity. The thermal conductivity at 20°C is 0.23–0.24 W/mK, which is much lower than that of steel pipes (40–52 W/mK), hence it offers good thermal insulation. —— The tensile modulus is low. The expansion force resulting from temperature changes is low, making it suitable for direct burial within wall and floor coverings. —— The pipeline has low resistance. The inner wall of the pipe is smooth, preventing scaling; its friction coefficient is only 0.007, which is much lower than that of metal pipes. —— The pipe fittings are firmly connected. Due to the excellent heat welding properties of polypropylene, heat welding allows pipes and fittings made from the same material to be joined together into a single unit, thereby eliminating the risk of leaks. ——Recyclability. It causes no environmental pollution during production, construction, or use; its waste can be recycled repeatedly, making it a green product. —— PP-R pipes have a price comparable to galvanized pipes; they are easy to install, require low costs, and result in an even lower overall cost. ⑵ Features of HDPE pipes. Made of polyethylene, it offers good cost-effectiveness, and boasts a range of advantages such as stable and reliable connections, material resistance to impact, cracking, aging, and corrosion. Compared to traditional piping materials, HDPE pipeline systems have the following advantages: — Reliable connections. Polyethylene pipeline systems are connected using electrofusion, and the strength of the joints is higher than that of the pipeline itself ； Polyethylene pipes are connected to other pipes using flanges, which is convenient and fast. —— It has good low-temperature impact resistance. Polyethylenes have an extremely low temperature of brittle failure, allowing them to be used safely within a temperature range of -60 to 60°C. During winter construction, the pipes do not crack due to the materials’ good impact resistance. —— It has good resistance to stress cracking. It features low notch sensitivity, high shear strength, and excellent scratch resistance, with very outstanding resistance to environmental stress cracking as well. —— It has good resistance to chemical corrosion. HDPE pipes can resist corrosion by various chemical substances, and the chemicals present in soil do not cause any degradation of the pipes. Polyethylene is an electrical insulator, so it does not suffer from rot, rusting, or electrochemical corrosion ； Furthermore, it does not promote the growth of algae, bacteria, or fungi. —— Resistant to aging, with a long service life. Polyethylene pipes containing 2–2.5% of uniformly distributed carbon black can be stored or used outdoors for 50 years without being damaged by external radiation. —— It has good wear resistance. Comparative wear resistance tests between HDPE pipes and steel pipes show that the wear resistance of HDPE pipes is 4 times that of steel pipes. —— It has good flexibility. The flexibility of HDPE pipes allows them to bend easily; in engineering applications, the direction of the pipes can be adjusted to bypass obstacles. In many cases, this flexibility reduces the need for fittings and lowers installation costs. —— The water flow resistance is low. HDPE pipes have a smooth inner surface, with a Manning coefficient of 0.009. The smooth surface and non-stick properties ensure that HDPE pipes have a higher transport capacity compared to traditional pipes, while also reducing pressure losses and energy consumption in transportation. 2 Pipe specifications: PP-R pipes and HDPE pipe systems are divided into two main categories: pipes and fittings. PP-R pipes comply with enterprise standards, equivalent to DIN8077/78 ； Each pipe is 4 meters long, and can also be customized according to customer requirements ； The color is green; it can also be customized according to customer requirements ； They are divided into three main categories based on the temperature and pressure of the medium used: cold water pipes, hot and cold water pipes, and hot water pipes. The size of the pipe is determined by its outer diameter, which corresponds to the respective series of steel pipes. The specific production specifications are shown in Table 1. Table 1: Specifications and Dimensions of PP-R Pipes
Nominal outer diameter, mm | Nominal wall thickness, mm | Corresponding steel pipe series | Cold water pipe | Hot and cold water pipe | Hot water pipe
S5 | SDR11 | S4 | SDR9 | S3.2 | SDR7.4 | S2.5 | SDR6
Nominal pressure, PN, 0.1 MPa | 10 | 12.5 | 16 | 20 | D20 | 1.9 | 2.3 | 2.8 | 3.4 | DN15 | D25 | 2.3 | 2.8 | 3.5 | 4.2 | DN20 | D32 | 3.0 | 3.6 | 4.4 | 5.4 | DN25 | D40 | 3.7 | 4.5 | 5.5 | 6.7 | DN32 | D50 | 4.6 | 5.6 | 6.9 | 8.4 | DN40 | D63 | 5.8 | 7.1 | 10.3 | 12.5 | DN65 | D75 | 6.8 | 8.4 | 10.3 | 12.5 | DN65 | D90 | 8.2 | 10.1 | 12.3 | 15.0 | DN80 | D110 | 10.0 | 12.3 | 15.1 | 18.3 | DN100

Due to the high plasticity of plastic pipes, more support is required compared to steel pipes. Nanjing Yuangu Factory produces a composite pipe made of polypropylene and fibers (F-PP); its maximum diameter is D63. Its size series is the same as that of PP-R pipes, and it has better rigidity, thus requiring less support. However, the price of F-PP pipe is 20% higher than that of PP-R pipe. The HDPE pipes produced by this manufacturer comply with the national standard GB/T13663-2001 ； Each pipe is available in lengths of 6m, 9m, and 12m, or can be customized according to customer requirements ； Colors are black, blue, and black with co-extruded blue stripes ； Based on the material grade, it is divided into PE80 and PE100, with PE80 being generally used. The specific specifications of the PE80 pipes produced include a nominal outer diameter ranging over 20 series, from D20, D25, D560 to D630 mm. There are many specifications for pipe fittings, including straight pipes, reducers, 90° elbows, 45° elbows, etc. The use of PP-R pipes and HDPE pipes in coking plants generally requires consideration of the properties of the medium, the temperature of the medium, the pressure of the medium, and the installation conditions. Currently, those that are being used successfully in coking plants include: PP-R pipes – for ammonium sulfate crystallization, ammonium sulfate mother liquor, desulfurization liquid, wastewater, and water supply; HDPE pipes – for ammonium sulfate mother liquor circulation, ammonium sulfate mother liquor return, concentrated sulfuric acid addition, desulfurization liquid, wastewater, biochemical aeration air ducts, and water supply. Ammonium sulfate yields the best results in terms of application effectiveness. The original ammonium sulfate pipeline system used ordinary stainless steel pipes, which required replacement every six months on average. The use of 316L pipes improved the situation slightly, but the problem of weld leaks remained unresolved; repairs and welding were needed almost on a daily basis, and sometimes production had to be halted for such repairs. With the use of PP-R and HDPE pipes, pipeline maintenance is essentially no longer necessary. The first PP-R pipe installed at Nangang’s coking plant (referred to as Nangang’s “first coking pipe”) was removed as part of renovation work between September 1999 and October 2003, and no problems occurred throughout its use; it was in pretty good condition when taken out for inspection. 3 Key usage points: (1) Plastic pipes are relatively sensitive to temperature. PP-R and HDPE pipes function properly when the temperature of the fluid is below 60°C. As the operating temperature increases, the pressure resistance of these pipes decreases; generally, for every 10°C increase in temperature, the pressure resistance drops by 13%. ⑵ Plastic pipes have relatively strict requirements regarding support spacing, which should generally not exceed 2–3 meters ； Continuous support is preferred at individual locations ； If possible, it is best to use a buried installation. ⑶ For small pipe diameters (≤D110), PP-R pipes are generally used, as their overall performance is superior to that of HDPE pipes. HDPE pipes can only be used for concentrated sulfuric acid pipelines. ⑷Homogeneous pipes are connected by heat fusion, while heterogeneous pipes are connected using flanges. ⑸There is a significant difference between fittings and metals, and this must be fully taken into account during design to avoid installation problems.