It is not just sap below the valve but mostly sap trapped inside the ball (ball valve). That is where 2 in series works, close the top one, drain the lower one, then close the lower one (ball now empty) and open the top one. It sounds like a lot of steps, and it only works if you can fully drain the bottom one before you close it, that means the air must get in from the bottom.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.