Spheres come in three types: foam, solid-cast, and inflatable. Foam spheres are used for cleaning lines that other pigs cannot navigate, including those with sharp bends or other restrictions. They can be bare, coated, fitted with wire brushes, or equipped with studs. They can be lost in lines with unbarred tees and generally do not seal well. They are most often used when other pigs are unsuitable or in automatic launching systems where they roll through the equipment.