With mine I used an 18" nipple.
This is well below the exaust and seems to work well.
Also made my riser 1" while the other two intakes are 3/4".
This was suggested to me By H20Loo.
I did get a couple of puffs of smoke out of it while seasoning the drum "wide open", but during my cook last sunday I had one 3/4" intake open and the ball valve 1/2 to 3/4 open and had no trouble at all with it.
I might even change it to a 12" riser just for comparison soon.