My feedback on main drains in an above ground pool...

[Anonymous [GDPR] European in the UK]

Yes, the total energy is less for a smaller orifice but the energy per unit area is higher directly in front so it is more focused and tends to push the water farther than the larger orifice. Same reason smaller spa jets feel more powerful.

Up close this is true, how about at a distance? The less overall power dissipates easier, the path decays. Because it's closer to the surface or pointing to the surface would help. It's like laminar flow jets all the rage but they are better than a conventional nozzle as that spray breaks up into the air because it's too turbulent to stay together. Just my observation.

Not quite. The 0.5 PSI is at 70 GPM (full speed). Pressure loss is proportional to GPM^2. Given your RPM and wattage, I suspect your flow rates are much much lower. For 10 GPM the PSI loss is only 0.01 PSI.

Yes true, my flow rate being around 31 US GPM would still be around 0.2 PSI (lazy guess, off to work) although my taper machined eyeballs probably less than the blunt end with smaller hole common to pool manufacturers.

First, cavitation only occurs in the impeller inlet. As water travels outward in the impeller pressure rises and the vapor bubbles burst. So all the damage occurs in the impeller and fairly close to the inlet. The volute has the highest pressure in the system so water could not boil there. But the reason most pools do not have caviation problems is because PBs tend to install plumbing systems that have much higher head loss in the return plumbing than in the suction plumbing and this keeps the pump from cavitating.

Yes, I wrote that badly, I meant the boiling temperature can be felt on the volute it's cavitation on the back of the impeller blades I was meaning, you put that so much better than me. I had to watch a YouTube video to re think what I had written. It was interesting to see the cavitation start so far ahead of the pump caused by the pressure drop in the valve the throttled down. https://www.youtube.com/watch?v=eMDAw0TXvUo
Because I have the pumps below the water line I don't need a self priming pump so the impeller design is different to a conventional pool pump even when the owners have the pump below water level they still use the "pool pump" cos that's what the label says......

Even running flat out at 2850 RPM I do not get cavitation, maybe because the pump increases rpm slowly it doesn't trigger the sudden pressure drop but also of course the dynamic head increases in proportion to the amount of water being moved
The massive savings in electricity, and less cleaning work with improved water clarity speak for themselves, Ben set out the Best guess idea and changed a lot of things, this is the first forum I have gone anywhere near explaining what I have done on pools but there would be a lot of very happy pool owners if they had the same system, running pools on a light bulbs worth of electricity! It's good for the co2 footprint as well, saving around 2 tons of co2 per season compared to a 3/4 HP motor and 1.5 HP probably double that.

[mas985]

Up close this is true, how about at a distance?

From my observations, it is true at a distance as well. The currents are stronger in the middle of the stream with the smaller orifice but off to the sides, the larger orifice has more flow. But I would expect this behavior since the more force you have in a particular direction, the further the flow should travel. You can actually prove this to yourself with a hose and an adjustable nozzle. The smaller the nozzle the further the water travels both above water level and below water level just to different extents due to the differences in density of air and water.

[Anonymous [GDPR] European in the UK]

Thanks Mark, something to pay a bit more attention to with the dye in the water. I did have some flow figures somewhere and there was also a slight reduction in the flow with the smaller orifice but that due to only a tiny amount of energy being used at the pump, icing on the cake really but good to examine.

By way of comparison, the other larger pool I mentioned runs at 125 watts, 1400 RPM moving 63 US GPM, 0.5 PSI on tank gauge. When needed ( to pass regulation) we can crank that to 100 US GPM, 464 watts 2150 RPM, gauge reading 2.7 PSI but the pump could do more, sadly the builders fitted 3 skimmers but on to one pipe so the max we can draw is 100 GPM. That is a standard setup but the M/D is plumbed on a 3 port to do inlet and outlet for heating or cleaning if required.