A pump on a flat surface acts like a 1 tile wall. It draws from the level bellow, not the level it's on.
For insane pumping speed, us...
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Each pump faces inwards, pushing water into the center tile. If this center tile has a hatch at the bottom, then opening the hatch pushes water down very fast, because a pump will actually allow water to push faster than it will naturally flow. If you stack this several layers deep, then 4*n layers will produce a lot of pressure and push a lot of water. That's important, because it means that a pump's speed is reduced by the speed that water flows. It creates empty space faster than water will refill it. If you can fill the area the pump is drawing from, then the pump will push water much much faster. For example...
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This will push the water to the right, far faster than a single pump would. Three pumps are feeding into the one pump, ensuring that the one pump has a constant source of water to draw from. This can be useful if you only have 1 tile to use as an outlet, but need a lot of fluid to push through.
Generally, any method of bottlenecking water, and putting a lot of pressure on it, mechanically or with normal pressure, will allow a faster flow through a smaller area, but for sheer volume per frame, you'll do better with several pumps pushing water into a wide corridor.