On Ray's machines and maybe many others, the open side of the U shaped cups face down and they seal when the pressure in the housing increases, that pressure forces the 2 sides of the U shaped seal tight against the membrane housing and against the very top of the membrane. It makes no difference which way the sap is flowing, the housings only have one end that is removable, that's where the seal needs to work, the other end only has outlets for permeate or concentrate or both, no removable 4" plug. As the pressure increases the seals form a perfect seal if not damaged.
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.