Select the gasket to be used with the flanges, if not already known or specified by the piping vendor. Align the flanges.
Center the gaskets if they are to be used. Bring the flanges and gaskets together.
Check flange alignment again. Insert the bolts.
Secure the flange bolts with sufficient force. Verify that the flanges are centered and the flange mating surfaces are aligned.
Check that flange bolts are not over-torqued.
Optional Procedural Steps
Flanges may or may not need to be welded after they have been bolted together. After the flanges are assembled, leak tests may be run.
Flanges may be stress tested to ensure that the flanges and bolts are not over-stressed. Inspection by a third party may be required.
The American Water Works Association (AWWA) has provided standards to the American National Standards Institute (ANSI) for proper water piping design and procedures for their assembly. According to "Ductile-Iron Pipe and Fittings" by the American Water Works Association, "ANSI/AWWA C110/A2110, C153/A2153 and C115/A2115 set forth flange dimensions, material requirements, facing, flange and bolt hole alignment" The American Society for Mechanical Engineers (ASME) issues standards for boilers and their piping.
According to "Practical Plant Failure Analysis" by Neville Sachs, "current industry practices and the ASME Boiler and Pressure Vessel Code call for flange alignment -- two mating flanges must be parallel to within 1 mm in 200 mm and the center lines of the two flanges must be within 3 mm maximum offset".