Flange, also known as a flange disk or boss, is a component used for connecting shafts to each other or for joining pipe ends. It is also used at equipment inlets and outlets, such as reducer flanges, for connecting two devices. A flange connection or flange joint refers to a removable joint that consists of a flange, a gasket, and bolts, forming a combined sealing structure. Pipe flanges are the flanges used for piping in pipe systems; on equipment, they refer to the equipment's inlet and outlet flanges. Flanges have holes, and bolts are used to tightly secure the two flanges together. Gaskets are used to seal between flanges. Flanges come in threaded (screwed) connections, welded flanges, and clamp flanges. Flanges are always used in pairs; low-pressure pipelines can use threaded flanges, while those above 4 kg pressure use welded flanges. Two flange disks are joined with a sealing pad and then bolted together. Flanges of different pressures have different thicknesses and require different bolts. Pumps and valves, when connected to pipes, have locally shaped flanges corresponding to these equipment components, also known as flange joints. Any connection piece that uses bolts to connect two flat surfaces around the perimeter and is sealed 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 device, like the connection between a flange and a pump, so it's not appropriate to call the pump a "flange-type part." Smaller components like valves can be called "flange-type parts."