Hi All,
At our gas plant, I have seen some pipes connected to the underground drain with flange connections just inches aboveground (3-5"). Is this a good practice for installing pipe risers?
It is common because most of the time, buried pipe flange connection is not a good practice. The flange being a restrained joint wont give much flexibility from earth movement, thus will cause stress to the pipeline.
I have always seen face of flange set at least 12" above top of grade. That's for process or cooling water lines. Drain lines probably aren't as critical.
Exterior, underground compatible, pipe coating should extend up at least 12" above grade. Been a standard forever. The transition zone between water/air, or earth/air is the most critical attack point for corrosion.
I don't think you will find this under any code but is often referenced in various Client & Contractor Specification and additional to previous replies there has to be sufficient envelope to insert and remove bolting.