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[Quantum Drop]

Having embarked upon a sand-rich volcano surrounded by angry tree-huggers, can anyone explain to a relative newbie to magma projects how to do this without accidentally screwing everything up?

[vjek]

If you can build a magma-safe pump stack, you can likely build the described lava sprinkler. 
It's just drawing it from the source, pumping it up the stack and pushing it out at (ideally) 2 Z above your targets.
Armok help you if there's grass though, because such a thing will light your entire embark level on fire, and spread.

Personally?  I would build a contained/dug magma-filled hallway or even the old "pressurized magma above a door" trick, with multiple ingress paths and grated surrounding floors for drainage.
In my experience, the challenges when dealing with magma automation and/or magma traps have been related to:

- finding a magma source that refills fast enough
- possibly building a cistern large enough
- having a drain that's large/fast enough
- non-evaporating magma blocking enemy pathing for extraordinarily long durations
- buggy enemy & ally pathing over tiles on which magma has evaporated
- partial (1/7) versus complete (7/7) magma depth, on the target(s)/tile(s)
- arranging power/axles/gears to be adjacent (vertically/horizontally) without compromising access/security/pathing
- doors remaining open after enemy vaporized by magma behind or above the door
- goblinite item movement and retrieval
- teleportation/pushing of 'swimming in magma' enemies through fortifications full enough to permit magma flow, but causing no damage to the enemy

But happily, all of those problems have solutions. 

[PatrikLundell]

I'd add "0 FPS" to the list. That's what happened the single time I tried a magma pump stack and a magma trap. The game reported FPS was 0, but it was possible to get a dorf to pull the lever to disable the stack, even though it took quite some time with an FPS rate of 0-1.

Fire starting where you didn't want it is probably the greatest danger. Vegetation growing to where it can be set on fire, enemies running around while on fire, etc.

[Thisfox]

Water is a better experimental medium. It pays off to find a heavy aquifer, start a fort there, and play with fluid dynamics in that fort, using water. Then return to the magma fort later, with some newfound knowledge which only drowned and did not vaporise your mechanics and miners.

[anewaname]

Regarding the mentions of "if you have grass on your map you might have a problem", a fire can sneak underneath a closed door if the door was built over grass.

[PatrikLundell]

Regarding the mentions of "if you have grass on your map you might have a problem", a fire can sneak underneath a closed door if the door was built over grass.

It can also pass under a closed drawbridge, as a number of cavern fires caused by fire spewing FBs locked away beyond those bridges have demonstrated. I'm still not sure if some of them also got under the walls or not, so I've taken to build a floor along the walls to hem in any potential fires, in addition to building a floor before I build the drawbridge.

[Moeteru]

I've got a successful magma-based trap in the entrance to my current fort fed by a 100z pump stack so I can explain a few of my design decisions and how to avoid common pitfalls.

If you're tapping a volcano then the walls of the magma tube will be vertical. If you don't mind using exploits then the best way to breach it is by digging diagonally upward through a bridge, as described in this thread. This is 100% safe for your miners if you do it correctly.

The next problem you might encounter is how to get the magma to flow quickly. Use a pump. Have the inlet of the pump as close to the volcano as possible because that's the only part of the magma's path where it isn't under pressure. Make sure the pump is situated at least on the same z-level as your entrance tunnel, but having it higher isn't a problem.

If you're building a magma pump stack, make sure you use the FPS-saving design from this thread. It prevents a lot of the temperature recalculations which cause so much lag with normal designs.

One issue I ran into with my early designs was the time taken for the magma to evaporate. You can be waiting 6 months or more for your entrance tunnel to be usable by wagons if you rely on evaporation alone. There are two solutions to this problem, and I've actually used both in my current fort.

• The simpler solution is to make sure wagons have another path into your fort. Leave this path open most of the time, but when siegers show up just pull the lever and force them into the longer trapped tunnel. Just be careful of two things: 1) make sure there's enough distance between the map edge and the bridge to give your dwarves time to close it, and 2) make sure that when you fire your magma trap none of the magma can spill into your main entrance tunnel. Use bridges to seal off the magma trap before activating it, and wait for it to drain before opening it up again.
• The more complex solution is a retracting floor. Have retracting bridges along the entire length of your magma trap with a large evaporation/drainage tank below. Once you've finished burning the organic material away from that precious goblinite, just pull the lever and let everything drop into the pit below (maybe check there aren't any dwarves underneath first). Pull the lever again to reset the trap to a clean, wagon-pathable state.

To dispose of the magma it's best not to rely solely on evaporation, just in case you end up with a 2/7 or deeper layer over your whole evaporation chamber. I always use either a map-edge drain or, more recently, a pit containing an iron minecart on a trackstop set to dump towards a wall. If you consider those too exploitative, then you could always dig a drainage shaft into the caverns. Also, it's obviously essential to make sure that there are at least some magma-safe doors between the areas which may get flooded with magma and the rest of your fort.

For reference, here are some screenshots of my system. It's currently in the cleanup phase after a goblinite delivery.

Spoiler: top layer (click to show/hide)

Spoiler: middle layer (click to show/hide)

Spoiler: bottom layer (click to show/hide)

It's not clear from the images but there are lever-linked hatches holding back the magma in the top layer. The drain on the bottom layer is underneath the green glass floor grates in the north west corner.
Also if I was building this again from scratch I'd make sure all the path lengths from the pump stack to each magma release hatch were the same. With this current design the north end of the trap fills up with 7/7 magma while there are still enemies alive in the south end.