Do not forget about this from the 2013 ed of 13
8.15.19 Return Bends.
8.15.19.1 Unless the requirements of 8.15.19.3, 8.15.19.4, or 8.15.19.5 are met, return bends shall be used where pendent sprinklers are supplied from a raw water source, a mill pond, or open-top reservoirs.