Flange, also known as a flange disc or boss, is a component used for connecting shafts or for joining pipe ends. It is also used at equipment inlets and outlets, such as reducer flanges, for connecting two pieces of equipment. A flange connection or flange joint refers to a removable connection consisting of a flange, a gasket, and bolts, forming a combined sealing structure. Pipe flanges are the flanges used for piping in pipe installations, or the inlets and outlets of equipment. Flanges have holes, and bolts secure them tightly. They are sealed with a gasket. Flanges come in threaded (screwed) connections, welded flanges, and clamped flanges. Flanges are always used in pairs; low-pressure pipelines can use threaded flanges, while those above 4 kg of pressure use welded flanges. Two flange discs are joined with a sealing pad and then bolted together. Flanges of different pressures have different thicknesses and require different bolts. When pumps and valves are connected to pipelines, the local parts of these equipment items are also made into corresponding flange shapes, also known as flange connections. Any connection piece that uses bolts to connect two flat surfaces and is sealed simultaneously is generally called a "flange," such as the connection of ventilation pipes. Such components can be referred to as "flange-type parts." However, such connections are only part of a single device, like the connection between a flange and a pump, so it's not appropriate to call the pump a "flange-type part." Smaller items like valves can be called "flange-type parts."