Weld neck flange is the same diameter and wall thickness of the joint end as the pipe to be welded, just as the two pipes are welded. Slip on flange is a concave platform at the interface which is slightly larger than the outside diameter of the pipe. The pipe is inserted and welded inside. Butt welding has better welding performance and less corrosion.

1.Flat welding and butt welding are welding methods when flanges and pipes are connected. When welding slip on flanges, only one side welding is required, and the inner joints of welded pipes and flanges are not required. The welding of the weld neck flanges requires double side welding. Therefore, flat welding flanges are generally used for low and medium pressure pipelines, butt weld flanges are used for medium and high pressure pipelines. Butt weld flanges are generally at least PN2.5 MPa. Butt welding is used to reduce stress concentration. The welding flange is mostly a neck flange and also a nipple flange. Therefore, the installation cost of the welding flange, labor costs and auxiliary materials costs are higher, because there is more than one process.
2.Weld neck flange and slip on flange are the two most commonly used flanges. The weld neck flange is necked and the slip on flange is not. If it is for safety reasons, it is best to use weld neck flange. If it is normal medium and the pressure is not high, slio on flanges can be considered to use.
3.The weld neck flange is more expensive than the slip on flange, and is more susceptible to bumping during transport due to its shape.