Overview
HDPE pipe usually refers to PE80 (including steel skeleton pipe), PE100, and ultra-high molecular weight polyethylene pipe. It has good resistance to high and low temperatures; It has the advantages of environmental stress cracking resistance, impact resistance, aging resistance, corrosion resistance, and convenient installation. It is a new type of pipe popularized and applied in China in recent years. It is especially suitable for pipeline laying in areas with harsh environments such as a salt lake, swamp, Gobi, and desert.
Several Problems Encountered in Pipeline Design & Construction and Their Solutions
Hydraulic Calculation and Design Flow Rate
The inner surface of the HDPE pipe (especially the UHMWPE pipe) is very smooth and the fluid resistance is small. In hydraulic calculation, some experts and data suggest Williams formula, with a C value of 145 ~ 160 (160 for UHMWPE pipe); The recommended formula in 4.3 of technical specification for outdoor polyethylene water supply pipeline engineering (discussion draft) is: I = 8.75 ╳ 10-4 ╳ q1.761/dj4.761. The calculation results of the two formulas are similar, and the latter is safer. After one year's operation, the measured operating conditions of the long-distance water transmission pipeline in Lop Nur, Xinjiang are basically consistent with the calculated results.
The local head loss of the pipeline is designed as 5% ~ 10% of the loss along the way. When the pipe diameter is small, take 10%.
The design flow rate is mainly determined through technical and economic comparison. When the pipe diameter is small, a lower flow rate can be adopted, and when the pipe diameter is large, a higher design flow rate can be adopted. From the perspective of safe operation, it is recommended that the design velocity of a long-distance non-metallic water transmission pipeline should not be greater than 1.5m/s. In the long-distance water transmission project of our institute, the velocity of the DN200 pipe is 0.7m/s, and that of the DN600 pipe is 1.23m/s. If the flow velocity is too large, the head loss increases rapidly, the pressure grade of the pipe and the project cost increase accordingly, and the probability of water hammer increases at the same time. For example, when the flow velocity of DN600 pipe is 1.23m/s, the head loss of 100km has reached 170m. When the water pipeline is not long (within 20 kilometers), it is also reasonable to appropriately increase the design flow rate.
Selection of Water Delivery Pipe Materials in Areas With Complex Engineering Geological Conditions
The long-distance water transmission project in the west water source area of Nalinggele, Qinghai passes through desert, swamp, salt lake, and other no man's land. Our institute has made a detailed technical and economic comparison of the pipe selection scheme. The comparison of the selection of ductile iron pipe, welded steel pipe, HDPE pipe, and other pipes shows that the scheme of using HDPE pipe saves about 20% of the total investment. The soil corrosives in this area are strong. If metal pipes are selected, the external wall must strengthen grade anti-corrosion. Moreover, it is extremely difficult to transport and install metal pipes in the desert, swamp, and salt lake areas, and some sections cannot be constructed at all.
In the HDPE pipe series, the selection of PE80, PE100, steel skeleton plastic composite pipe, and ultra-high molecular weight polyethylene pipe shall also be determined according to the engineering geological conditions of the pipeline laying section. UHMWPE pipe or PE100 pipe is generally selected in the design because of their high-cost performance. Ultra-high molecular weight polyethylene pipe and steel skeleton plastic composite pipe should be selected in sections crossing swamps and rivers.
Long-distance metal pipelines usually use a kind of pipe with wall thickness. The pressure grade of HDPE pipe with the same outer diameter is different, and the price varies greatly. In order to save investment, the pipeline can be divided into several pressure sections in the design of a long-distance HDPE water transmission pipeline. According to the pressure bearing condition of the pipe section, pipes with different pressure levels shall be selected by sections. In the practice of long-distance water transmission projects, we usually select pipes with 3 or 4 pressure grades, ranging from 0.8 ~ 1.6Mpa.
Discussion on Setting Expansion Device for Buried Pipe
When the polyethylene pipeline is laid in swamp or river bottom with small soil friction, the longitudinal deformation caused by temperature change shall be considered for calculation. According to the formula △ L = 0.13l (△ T), the theoretical calculation value is large in the long-distance pipe section. Our Institute conducted a test on the long-distance (57km) water transmission project in Lop Nur, Xinjiang. The pipeline passes through the salt marsh with a length of 16km, adopts steel skeleton plastic composite pipe, without an expansion device, and the buried depth is 1.3m. The pipes in other sections are equipped with sleeve-type expansion devices. After more than one year of operation, no problems have occurred. Engineering practice has proved that when the pipeline crosses sections with complex geological conditions, it should be analyzed in detail. When the pipeline is buried deeply, the setting of the expander may not be considered.
Maximum Spacing of Exhaust Valves and Setting of Special Valves
Usually, an exhaust valve is set at the convex part of the pipeline. However, in the design of a long-distance HDPE water transmission pipeline, the vent valve shall be set at a certain distance in a relatively flat section to vent in time to ensure the stability of the system. In swamp and saline-alkali land, if the exhaust valve is set too tightly, it may increase the potential safety hazard due to the leakage of the exhaust valve. There are little domestic data on the allowable maximum spacing of exhaust valves. From the point of view of many long-distance pipelines that have been put into operation designed by our institute, the maximum spacing is 4km, and there is no problem. In sections with large local slopes, the maximum spacing of exhaust valves is 2km.
Controlling pipeline overpressure is the key to ensure the safe operation of the system. It is also necessary to set adjustable pressure reducing and stabilizing valve in the pipe section where overpressure may occur in the water transmission pipeline to control the possible pressure fluctuation at the most unfavorable point.
Setting of Buttress
For long-distance water transmission with HDPE pipe, thrust pier, fixed pier, and anti-skid pier should be set according to relevant pipeline technical regulations. The concrete strength of the abutment shall not be less than C15, and the outer wall of the pipe shall be wrapped with a 3mm thick elastic buffer layer. In particular, a fixed pier must be set at the valve in the pipeline to anchor the valve to the bottom plate of the well. When the vertical gradient of the pipeline is greater than 10 degrees, a concrete anti-skid pier should be made. The anti-slip abutment foundation must be cast in the undisturbed soil excavated under the pipe foundation. The anchorage between the anti-skid pier and the upper pipe is made of cast concrete.
Pipeline Foundation And Soil Backfilling Under Special Geological Conditions
In several long-distance HDPE pipe water transmission projects designed by our institute, the pipeline laying conditions are poor. It passes through desert, swamp, Gobi desert, and salt lake. It is difficult to find backfill soil that meets the requirements of relevant pipeline technical regulations dozens of kilometers near the pipeline. The design requires that the foundation surface in contact with the pipeline shall be free of hard blocks and other protrusions. During construction, it is mainly controlled that the compaction coefficient of the pipe top and the backfill area on both sides of the pipeline shall meet the design requirements, the pipe top shall not be less than 0.85 and the two sides of the pipeline shall not be less than 0.95. Generally, the dry sand gravel in Gobi and desert cannot meet the above requirements, so watering and other measures must be taken. For pipe laying in the Salt Lake area, the hard salt shell (generally 0.2 ~ 0.6m) on the surface shall be stripped and backfilled with fine-grained saline soil at the lower part.
As one of the new pipes, HDPE pipe is more and more used in long-distance water transmission projects. How to ensure the safety, economy, and rationality of the system and timely compile and publish the corresponding engineering technical regulations is an urgent task.
The successful application of HDPE pipe in long-distance water conveyance projects is inseparable from the product quality, design quality, construction quality, and scientific management of the pipe itself. The optimized design scheme is the key to the safety, economy, and rationality of the project.