Carbon steel pipe comes in sizes from 1/8-inch outer diameter to 72-inch outer diameter. It comes in varying thicknesses depending on the purpose it is being used for. You can use carbon steel pipe to transport gas in your house, to move liquids around in an industrial setting or for pipelines moving huge amounts of of oil. It has even been used in geothermal applications.
Carbon steel pipe that has an interior diameter of less than 2 1/2 inches is usually threaded and joined with a carbon steel union of the same size with the same thread configuration. You will also find other carbon steel fittings such as threaded elbows, both at 45 degrees and 90 degrees. Caps are available if you need to shut off a line. Flanges and double release unions can help in awkward places in gas lines. There are also threaded reductions, such as 2 inch to 1 1/2 inch, or 1 inch to 3/4 inch, made of carbon steel.
Large industrial carbon steel usually has welded unions. One of the exceptions is when the pipe has some type of liner to help prevent corrosion. These liners are usually made of plastic or fiberglass products and can be damaged by the heat of welding. Large carbon steel flanges are available for applications with lined pipe. They also use carbon steel nuts and bolts which can be up to 1 inch in diameter. While these are more expensive to install than a simple weld, they radically reduce corrosion at joins.
You can buy T joins, both with threads, as well as flanges in carbon steel. These, as well as most other fittings, come in a variety of carbon steel alloys. Lap joins and slip on joins are also available for carbon steel pipe up to 4 inches in diameter. These fittings are used on carbon steel pipe used for everything from household gas lines to the transportation of toxic liquids in an industrial setting.