MadTownMax said:
...and can be valved back to desired flow without harm to the pump." so If they're not pressure-rated, someone should let dolphin know
It's not a question of wether or not the pump can survive operating under back-pressure without damage... "pressure rated" means the pump/motor is designed to push water against back-pressure, without losing as much velocity. Something to do w/ the type/speed/power of motor they use, and impeller construction (more fins on the impeller, I think?). The Ampmaster line can't pump 20 feet vertically, or it will shut down (17.5, actually). Yet under 0 head pressure, it cranks out an amazing 3000 GPH. A "Pressure rated" Iwaki100 by comparison, only is capable of moving 2/3rds that volume at 0 head (2000 gph), yet can shoot a stream of water over 36 feet in the air... That's what they mean by "pressure rated" when referring to pumps in the aquarium trade. Wether it can operate in an environment containing backpressure, is irrelevant, it's still not a pressure rated pump. The slope of the curve of that pump against head pressure, will be a very different shape, than that of a pressure rated pump. I would make sure that RC's calculator is setup for that kind of pump (and the forumla has the proper slope programmed into it), and not a pressure rated slope.
Quite honestly, any
quality pump, should be able to operate with a certain degree of backpressure. The fact that Dolphin is confidently stating it won't harm their pump, means they're sure they've built a quality product (and from what I've heard from other user's, they have).
For closed loops and situations with minimal back-pressure, non-pressure rated pumps are usually a better option - they use less energy, and typically produce more flow (and are cheaper, too!). However you'd never get an Ampmaster to pump a return upstairs to the first floor, if you sumps were in the basement. For that type of task (or pushing water through a Venturi/Beckett injector in a skimmer, or chiller, or other filtration device), you're far better off using a pressure rated pump.
- Mac