Flat welding and butt welding are the welding way of the flange and pipe connections. Flat welding just need one-side welding don't need welding the inside the mouth of the pipe and flange connections. Butt welding need flange double-sided welding. So flat welding flanges are used for low and medium voltage pipe, butt weld flange for medium and high pressure pipe flanges connection. And because one more procedure than flat welding flange, the mounting cost, labor cost, complementary material fee of welding flange will be much higher.