Flat welding and butt welding are both the welding methods when the flange is connected to the pipe. Welding neck flange is a type of steel pipe welding neck flange, sometimes referred to as WN flange for short. It has a better sealing, has been widely used in high pressure and other pipelines or equipment instrumentation like valves, pumps, or other equipment, for the field of chemical, petroleum, power industry and other where required high pressure, high temperature, flammable, explosive, toxic, harmful, toxic medium.
Flat welded flanges with neck, also referred to as slip-on welding flange, SO flange for short. SO flange and WN flange looks similar from their appearance and some people think they can be replaced, actually they according to with the same specification like ASME B16.5, ASME B16.47, ISO 7005-1-2011, BS 4504-3.1, DIN EN 1092-1 and so on. Today here we will introduce the difference between them.
Different welding forms: The welding joints of the flat welding neck flange and steel pipe is a fillet weld, while the neck butt welding flange is a girth weld. Flat welding welds can not be radiographic inspection, but butt-welding can. The welding seam of the butt-welding flange with neck and steel pipe belongs to class B seam, and the welding seam of flat welding flange with neck and pipe belongs to class C seam, which is tested by different non-destructive methods.
Different material: Flat neck welding flange is machined from ordinary steel plate, and neck butt welding flange material is machined from forged steel parts, of which strength is higher and not easy to leak.
Different nominal pressure: the nominal pressure of the neck flat welding flange is 0.6-- 4.0Mpa and the nominal pressure of the neck butt welding flange is 1- 25Mpa. Obviously, the latter is more adaptable to higher levels of stress than the former.
Different connection methods: Flat welding flange can only be connected with the pipeline, but can not be directly connected with the butt-welding pipe fitting; Butt-welded flanges can be directly connected to all butt-welded pipe fittings (including elbows, tees, reducers, etc.), as well as pipes.
Different costs. The flat welding flange with the neck has a large inner diameter, small weight, and low cost. Flat welded flanges do not need to weld the pipe and flange inner connection. Butt-welded flanges need to be welded on both sides of the flange, which costs more in installation, labor and materials. Butt-welded neck flanges with a nominal diameter greater than 250mm should be tested, while SO flanges are not required.
In general, flat welding flanges are generally used for low and medium pressure pipelines, the butt-welding process is to reduce stress concentration and butt-welding flanges are used for the connection of medium and high-pressure pipelines, general butt-welding flanges are mostly with neck flanges, also called long neck flanges.