Steel pipe fittings- known as couplings- come in straight, 30-, 45-, 60- and 90 degree angles. The reason for this is to help you run new water lines (pipes) around corners, or if you have to install around an object like an existing pipe. They require that they be soldered in place, so it's best to practice soldering a few spare fittings to some old steel pipe before starting the fitting installation--this will reduce the possibility of leaking joints.
Therefore how to install fittings on a steel pipe? Please looking at the following instruction.
At the first time, measure the length of pipe needed between one fitting and the next, using a tape measure. Mark the pipe with a felt tip pen where it needs to be cut. Place a tubing cutter over the end of the new pipe and move it down to the felt tip mark. Tighten the blade of the cutter down onto the mark. Rotate the cutter once, before tightening and rotating a second time. Repeat until the pipe is cut through.
Then, sand the end of the pipe with a piece of emery cloth, as well as the inside of the coupling. Apply a thin layer of lead-free soldering paste (flux) to both sanded areas, using a small brush. Push the coupling onto the end of the pipe. Continue installing all new pieces of pipe and pipe fitting couplings in the same fashion. When you are finished doing this, all pipe and couplings will be in place, and ready to be soldered.
Thirdly, uncoil 10 inches of lead-free solder from its spool, bending the last 2 inches 90 degrees. Turn on the propane torch and heat the first newly installed coupling by moving the flame from side to side. When you hear the flux sizzle, take the flame away. Touch the seam between the pipe and the fitting coupling with the tip of the end of the solder. If it melts, apply 3/4 inch of solder to the seam--capillary action will suck the solder down between the coupling and pipe.
In addition, wipe away all excess solder with a rag, but remember that the coupling will be hot. Solder the next coupling in like fashion, and continue until all couplings have been soldered in place to the new copper line.