I too have the "single valve" (fast valve = FC) on our NH3 applicator. It's worked fine for quite a few yrs. The FC is a little cheaper to buy since you're only buying 1 of the electric control valves vs 2 in the standard setup.

However, there are some downsides to the single valve, I think. Ted Thisius always does a great job of explaining the differences....maybe he'll reply also. He normally works on Precision Talk pg. But, one of the downsides is the "startup" at the end of row after you've completed a turn on the headland & start back into the rows & start the application. The FC valve has to go from 0 (shut-off) & then quickly find it's setting to give you the desired flow for your speed & flow rate desired. Sometimes it takes a bit of hunting to do that. This hunting "may" show up later in the year as N supply may cause some plant color differences. The 2 valve system leaves 1 flow control valve in whatever position it was when you shut off prior to your pickup & turn, then after your turn, the On-Off valve quickly opens & allows that 2nd valve to again do it's flow control work.

There are also some downsides to the 2 valve system. I like to keep an extra valve on the shelf just in case of late afternoon "Saturday" (or Sunday?) breakdowns. So, w/the single valve system I have 1 extra valve on the shelf...that's all it takes. For the 2 control valve system, I'd have 2 valves sitting on the shelf. More cost. But, if you don't want to keep spare valves, then it doesn't make any difference either way.

Either system work fine, I'm sure.