Fluids Transmission Pipe must have a hollow cross section, it can be round, square, triagular or any other shape. In some special condition, you must use a rectangular shape pipe. However, round shape fluids transmission pipe is the most common type due to its minimal perimeter in all geometric cross section/area ratio which has the ability to get the maximum internal cross section area within the same amount of material condition. However, the round fluids transmission pipe also has some limitations, for example, under the condition by plane bending, square pipe and rectangular pipe have larger flexural strength. This advantages allows square and rectangular pipe are widely used for the skeleton of some farm machinery, steel furniture. 

Steel pipe is widely used in the market due to its low cost and high strength. The steel pipe production process can be divided into two major categories which are seamless steel pipe and welded steel pipe. While welded steel pipe is divided into ERW, SSAW and UOE. Seamless steel pipe is the only type to be used in fluids transmission. With the development of material science, molding, machine equipment, the quality of steel pipe has been greatly developed. Welded steel pipe has the advantages such as wall thickness uniformity, high precision, low energy consumption, high production efficiency etc. In the past, API standard oil and gas line pipe is only use seamless steel pipe, but today over 95% of API line pipe has been raplaced by welded steel pipe specially in Japan, Europe, United States. 

When the welded steel pipe is used as fluids transmission pipe, the steel pipe will have different requirements:
1. Leakage is not allowed. Ultrasonic Testing and Hydrostatic Testing are required to ensure the quality of the welding seam.
2. Internal anti-corrosive coating is necessary when conveying corrosive chemical solution. 
3. The pipes must remove the burr to reduce  fluids transfer resistance.