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[Dorf3000]

Ok so I thought I'd ask here instead of digging everything and having to revert if it doesn't work.  I want to create a magma moat that is filled from underneath, by using a long tunnel from the magma source and having a pump there on the same z-level as the highest point that I want the moat to fill.  Like this:


|_moat__ _|      | |       | _pump_
       | |_______| |_______| |    |___source
       |_____________________|


I know this should work and fill the moat to the right level, and if the pump is shut off then the magma won't drain back.  The tricky part is that in the middle (it will be a long tunnel) I want to put in access for a magma channel to workshops, and I may need to add more channels in the future without a) draining the current ones and b) cooking some dorf biscuits.

It's my understanding that if the pump is turned off, a tunnel can be dug over the existing transfer pipe and channeled into, and the magma won't rise up and roast the unlucky miner. Even though other parts of the system are at a higher level the magma is 'chunky' and isn't pressurised at that point.  Then when the pump is restarted, it should fill the new channel without affecting the levels in the other parts.

Have I missed anything important in this plan? Will I have crispy dwarves or beserk Armorsmiths when their magma forge turns off during their moods?

[Quietust]

Your setup should be safe - if your pictured setup was completely filled with 7/7 magma and you dug a new shaft between the middle shaft and the pump, it would not fill up until the pump was activated and it would not drain the others.

[Ironhand]

You know what, I don't think this will work with magma.

I don't think magma stays pressurized like water does, so it won't fill up to the pump's level in the parts that are fed from underneath.
But I could be wrong.

[Flaede]

Magma gains pressure from pumps, it just doesn't pressurize normally all by itself, else magma pipes would be rather more dangerous than they are.

I didn't know the pressure "went away" if you shut the pump off. that is good news for me!

[Ironhand]

Oh, wow. In that case, this is awesome!
I will be utilizing this in my future forts.

[Flaede]

just be careful not to pressurize the volcanic pipe.

[Quietust]

Technically, there are two forms of pressure.

One form of pressure comes from water sitting directly on top of other water, causing it to teleport downward and outward to the nearest open spot on any Z-level below it (but not on the same Z-level, causing the familiar U-bend behavior). This is what causes water to rapidly rush out of large tanks, and it never affects magma.

The other form of pressure comes from actual sources of liquid, such as "river source" tiles at the beginning of a brook or stream, "waterfall" tiles at the beginning of an underground river, locations on the map edge where rivers flow in, and active screw pump output tiles. In these cases, the liquid is created at the tile, and if there isn't any room, it will teleport outward to the nearest open tile on the same Z-level or below and appear there instead (which is why damming a river will cause any U-bends to overflow). Since this only happens on the instant when the liquid is actually generated (screw pumps create liquid in their output tiles, if possible, and then delete the same amount of liquid from the input tile), the pressure is not persistent. This type of pressure applies to both water and magma, though with magma it only ever happens when using pumps (since there are no natural "magma source" tiles).

[Flaede]

WOw. thank you for the detailed explanation of all that. I understand a whole lot better what I thought I knew, but wasn't 100% understanding the WHYs of.

but to be a stickler:

(since there are no natural "magma source" tiles).

I've heard magma "appears" at the top of magma pipes, and that this is why it "rains" down from above when you drain a magma pipe. Obviously it doesn't create pressure, so I'm a little curious how that mechanic works.

[Angellus]

I thought it appeared from the bottom level...

[gtmattz]

I thought it appeared from the bottom level...

It technically 'appears' at the top of the pipe and 'falls' onto the magma in the pipe.  If you drain off a few levels of your magma pipe and watch the top level you will see 7/7 'stacks' of magma appearing on the top layer and then spreading out.  The magma will only spawn within the area of the top filled level of the pipe, so in some cases where you have a pipe with an extremely small top layer you can have refilling issues, especially if you have drained the pipe down past a layer which has a large 'floor' area, as the magma tends to want to spread out and evaporate on the floor.

[Magua]

It looks like it appears from the top, but magma definitively fills the pipe from the bottom.  This is trivial to test by draining two or three levels of the pipe, and then covering one of the drained z-levels with floor.

If magma filled from the top, you should see magma spawn on the floor, but you won't.

[bluea]

Quietust, that was a -very- clear description. It seems superior - or at least complementary - to the Wiki entry on Pressure. Could it be wedged in there somewhere?

[Kanddak]

(Hell yeah, someone besides me giving the correct theory of pressure. High fives for Quietust.)

So here's the thing about magma pipes. The "magma appears from the top and rains down" explanation is a wacky myth. Maybe it was true in an earlier version or something; I don't know how else it would have gotten so popular.

Basically, the magma flow tiles at the bottom of pipes find a path to the lowest empty space in a vertical line to create globs of 7/7 magma.
I think gtmattz basically has the idea, if when he says "top level" you understand he means the current top of the magma in the pipe, not the absolute top of the pipe itself. If you've drained all but 5 levels of your pipe, the new magma will appear on the sixth level and then fall into any empty space on the fifth. So there's this nice bubbling action in a refilling magma pipe. This is a fine opportunity to build a platform in the middle of your partly-drained pipe and chain elven virgins and kittens on it.
Note also that if you channel into certain places in the pipe for a pump intake, a magma flow tile can be aligned to put magma up through that pump intake channel when you wouldn't normally expect magma to flow upward like that. I'll leave the details of how I discovered this to the reader's imagination.

[Firnagzen]

http://www.bay12games.com/forum/index.php?topic=32453.0

With helpful diagrams!

[MC Dirty]

Note also that if you channel into certain places in the pipe for a pump intake, a magma flow tile can be aligned to put magma up through that pump intake channel when you wouldn't normally expect magma to flow upward like that. I'll leave the details of how I discovered this to the reader's imagination.

Hmm, let me guess: The metal industry part of the fortress got submerged in magma?