Flanges, also known as raised face plates or flanged disks, are components used to connect shafts to each other, typically for joining pipe ends; they are also used at equipment inlets and outlets, such as reducer flanges. 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 flanges used in piping installations; when used 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 the flanges. Flanges come in threaded (screwed) connections, welded flanges, and clamp flanges. Flanges are always used in pairs; low-pressure piping can use threaded flanges, while pressures above four kilograms require 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 connections. Any connection piece that uses bolts to connect two flat surfaces around the perimeter and is sealed is generally called a "flange."