The outline of a socket weld flange is similar to a slip-on flange, but the hole is counter-bored to accommodate pipe. Smaller diameters of high pressure pipes often use socket welding pipe flanges. In high pressure systems like hydraulic machinery and steam lines, socket weld flanges are frequently employed. Inserting the pipe into the socket end of the pipe flange and applying fillet welding to the hub of the flange is how socket weld pipe flanges are connected. This enables a smooth bore and improved passage of the liquid or gas inside the pipe. In high stress applications, an internal weld is an optional addition.