Okay Paul,
Took some time to think about this again
as well as do some reading online. Amazing what is online once you find it LOL
I don't know if I would use the term "zero" head pressure, although I do agree with what you're getting at.
Equal static head pressure on each line at the pump - when the pump is at rest.
As I'm sure you found out in your prior research, installing the pump below the tank, and having the suction side full of water from above - can be called a "flooded suction" when talking in those techno-weenie pump terms.
If both discharge and suction sides are identical in height, and are both "seeing" the exact same fluid head pressures - as measured from the center of the pump impeller - then you would indeed have a flooded suction.
You have equal static heads applying pressure down each leg to the pump, with the no differential pressure between the two when the pump is off.
When you apply power to the pump, and the impeller begins to spin, you will see a drop in static head on the suction side.
From what I read, my understanding is that this occurs because as the impeller spins, in applys energy to the water at the far ends of each impeller blade, and at this point in the pump housing. The design of the pump allows this higher pressure water to be forced out the discharge outlet, and as a result, the suction side must give up it's energy - by losing some of it's pressure.
When ever there is flow - there must be a pressure drop.
Now as the water begins to travel up the return line to the tank, it's leg has a higher total pressure ( static pressure + flowing pressure ).
As this is happening, the suction side leg has a lower total pressure at the suction side of the pump ( static head - loss of pressure at intake ).
The Net Positive Suction Head is very high for sure, as compared to pulling liquid into the pump from a traditional "pump-in-sump" installation without the flooded leg above, Since you've flooded the suction side with static head from above, the pump will not cavitate, and the pump will run more efficiently.
Atmospheric pressure is pressing down on the tank surface, and this pressure is added to the total pressure in each leg.
Vapor pressure is just the pressure at which the saltwater will vaporize at a given temperature ( boil ).
In our discussion - the importance of both of these is not too great. We have two opposing sides - each seeing the same atmospheric pressure ~ each seeing the same vapor pressure.
They basically cancel each other out.
You still have head pressures your dealing with - it's just that when you look at - you could somewhat "cancel them out" as well ~ although they are still very much there.
Obviously friction comes into play as mentioned above ~ but on short distances such as these ~ it's minimal.
But again, these forces are there for sure.
I beleive most pump curves showing flow vs. head, are calculated without the available "flooded suction" head pressure being present. My guess is that since you have a gravity fed flooded suction side that has an equal head as the discharge side - that you would be able to get much more flow through the pump - than what is given on it's pump performance curves.
How much more - I would think it's a little different for each pump, but the rules would apply to all.
In any event - Just some new stuff I read about pumps and stuff.
Figured you like to hear some of it.
Maybe not LOL
BTW ~ I really like your set up. Very creative for sure.
Looking forward to seeing some more pics.
Is it in "action" yet ?
