Flat welding and butt welding are welding methods used when connecting flanges and pipelines. When welding flat welding flanges, only single-sided welding is required without welding the inner port of the pipeline and flange connection. Welding installation of butt welding flanges requires double-sided welding of the flange. So flat welding flanges are generally used for low and medium pressure pipelines, while butt welding flanges are used for connecting medium and high pressure pipelines. The flanges for butt welding are generally at least PN2.5Ppa, and butt welding is used to reduce stress concentration. Generally, butt welding flanges are mostly neck flanges, also known as nipple flanges. So the installation cost, labor cost, and auxiliary material cost of welding flanges are higher because there is an additional process.