I measured my burners and it is actually 15" from burner tip to kettle. I have a 3 tier sculpture with 25 gallon kettles, and 120qt cooler MLT. I ran a 3/4" flex line from the meter, and 3/4" pipe all the way to the burner. I was worried about running both burners full out at the same time. At the burner I used a bell reducer to the 1/2" going into the burner. I generally do 13-15 gallon batches and use a HERMS in my HLT. Make sure you have a good wind screen for dropping down the flame with this style of burner. My wind screen is just 1/8" plate in a 20" x20" square. I bought a cheap burner from a food supply for wok burners. I had a lot of trouble with soot. I ended up buying one burner from Tejassmokers. It was much more expensive, but was bench tuned. I found a bit that matched the orifice size of that burner. I believe it was 15 guage, and I drilled out the other burner. To answer your question:
These burners are designed to run full enough to draw air into the mix. This is why I have that 15" gap. IMO people using this style of burner put them way too close to the kettle. It forces you to run the burner at levels that hurt its designed performance. I crank the burner up when I start and the flame is nice with no soot. Literally the flame is up to the bottom of the kettle. It then gives me enough room to keep the flame strong enough to keep a good mix of air and NG going into the burner, even when dropping down the heat. I just use gas ball valves to control the flame. I have another burner in an old stand, much like a turkey fryer. It is useless at 6" from the kettle. I have to turn the flame so low that it blacks my pot with soot. I would highly recommend test burning your burner and looking at the flame height. I think you will find that it will burn best when you feed it a lot of gas. This style of burner works great for me. It didn't until I gave it some room. I have no trouble bringing 23 gallons up to boil and dropping the temp down controlling my boil intensity. Long and short, give your burners some room to work. Hope this helps you.