I finally completed my project which involved a massive pump stack, bringing magma from level 6 to level 146. I found the basic pump stack iffy for such a large project, since severely delays construction that no pump can be completed until the pump below it is done. The dependencies are a headache, so I went with an alternating approach where every pump rests on solid ground, and power transfer is done with gears. This adds 50% to the power requirements, but is well worth it since all the pumps can be built in parallel.
This of course required massive power, so I ended up with 17 standard dwarven reactors, generating some 2800 excess power. I experimented with some alternate designs which drove multiple banks of water wheels with a single, separate pump, but this proved unreliable and hence inefficient. The fluid simulation in DF being what it is, what should be a channel with a reliable, rapid current often isn't. I may try different designs in some future fort.
By the time I was done, the fort was really at a crawl. I'm running on a Q6600, and my 220 dwarf fort was running at about 6 FPS. It's purely computation, since I get 100+ FPS on embark, and only get slowdowns when the population climbs into the 50+ range. I threw the lever, engaging my 3 banks of reactors with the magma pump stack power train....
... and FPS dropped from 6 to 2 to 1 to 0. Which wasn't actually stopped, just less than 1.
I immediately threw the lever back, and after a few painful minutes, the pump stack shut down and the frame rate climbed back up to that pathetic 6. I think it's the pumps themselves that eat CPU cycles. The amount of new fluid movement was tiny compared to the waves that were already going on from the partially-drained cavern lake that provided the working fluid for my reactors, and of course the reactors themselves had been going full blast for some time.
I think the only solution, if you want to accomplish such a long-distance lift, is to do it in stages. Pump it up 20 levels or so to a reservoir, stop the first set of pumps, and then pump it up another 20 levels to a second reservoir, etc.
- Gus