When the flow rate of the fluid is known, the size of the pipe diameter depends on the allowable flow rate or the allowable frictional resistance (pressure drop). When the flow rate is high, the pipe diameter is small, but the pressure drop value is increased. Therefore, when the flow rate is high, the pipeline infrastructure investment can be saved, but the energy consumption of the operation of power equipment such as pumps and compressors increases. In addition, if the flow rate is too large, there may be some other adverse factors. Therefore, the pipe diameter should be determined according to the comprehensive consideration of construction investment, operating costs and other technical factors.<br><br>The connection method between the pipe and the pipe connection, the pipe connection, the valve and the inlet and outlet nozzle on the equipment are determined by the nature of the fluid, the pressure and temperature, and the material, size and installation site of the pipe, mainly including threaded connection, flange connection, socket connection and welding and other four methods :① threaded connection: mainly suitable for small diameter pipes. When connecting, it is generally necessary to wrap fluorine plastic sealing tape or paint thick paint and wrap hemp and other sealing materials on the threaded connection part to prevent leakage. When the pressure is above 1.6 mpa, the gasket is generally added to the end of the pipe to seal. This connection method is simple and can be disassembled and reassembled, but the live joint must be installed in the appropriate place of the pipeline to facilitate disassembly. ② Flange connection: The applicable diameter range is large. When connecting, different flanges and sealing gaskets are selected according to the nature of the fluid, pressure and temperature, and bolts are used to clamp the gaskets to keep the seal. Flange connection is mostly used in the pipe section that needs to be disassembled frequently and where the pipe is connected with the equipment. (3) Socket connection: used for the connection between cast iron pipes, concrete pipes, clay pipes and their connections, only suitable for water supply, drainage and gas pipelines working under low pressure normal temperature conditions. When connecting, it is generally filled with hemp, cotton or asbestos rope in the slot of the socket, and then filled with asbestos cement or lead and other materials, and rubber sealing rings can be filled in the socket to make it have good flexibility and allow the pipe to move a little. ④ Welded connection: the strength and sealing of this connection is the best, suitable for all kinds of pipelines, save labor and materials, but the pipe and pipe connection must be cut off when disassembly.<br><br>In the city, the main lines of water supply, drainage, heating and gas pipelines and long-distance oil and gas pipelines are mostly laid underground, while the process pipelines in factories are mostly laid on the ground for easy operation and maintenance. The passage, support, slope and liquid exhaust, compensation, heat preservation and heating, anti-corrosion and cleaning, identification and painting and safety of the pipeline are all important issues for both above-ground and underground laying.