QuoteAlso, my vent will be trenched 18" in the ground in flex
Do you mean the gas will be trenched in the ground?
Are we talking appliance vent or regulator vent clearances?
You would follow Clause 8.14.8 for appliance vent clearances. Regarding the appliance vent clearance to a window, propane and natural gas both require 1' for appliances 100,000 BTU's or less and 3' for appliances above 100,000 BTU's.
You would follow Table 5.2 for regulator vent clearances. Propane and natural gas both require 3' from the regulator vent to a building opening.
Only the clearances to an appliance intake and source of ignition are increased from 3' to 10' when dealing with propane versus natural gas.
I have never seen anything in the code that states the clearance from a regulator to grade, but I've been told it's 3'. I have seen many propane regulators installed less than 3' from grade so let me know if you find anything definitive.