Factorio Forums

What did you do?
Connected pipes to a foundry for the concrete casting recipe (the one that takes water and molten iron)
Connected the moltlen iron pipe to a source of molten iron (I used an infinity pipe)
Switched the foundry recipe to casting iron plates from molten iron.
Switched the foundry recipe back to casting concrete.

What happened?
The water pipe got molten iron in it.

What did you expect to happen instead? It might be obvious to you, but do it anyway!
For the water pipe to not get molten iron in it.

Does it happen always, once, or sometimes?
Always.
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Some background: I'm putting together a nice generic automall, and was looking at including the foundry and other assembler-type machines in it. I was planning to have a single machine be able to make all the "just liquid iron" recipies, all the "no fluid recipes" and concrete casting. With another single machine doing all the liquid copper recipes including low density structures. (Multiplied to many machines each, obviously) With similar setups for the Electronics Assembler, Biochamber and Cryochamber.

But since I can't garuntee that the fluid input pipes have actually got any fluid in them (due to construction orders and research timings) and given how the refinery works with inputs and outputs, I was expecting this to just work, not give me mixed fluids.

That is correct behaviour.
When the foundry switches recipes, it will try to push input fluids into connected pipes.
So when changing from iron casting, as both inputs are molten iron, it will try to push molten iron into the pipe you intended for water.

Duplicate of 119923 Changing Recipe on Foundry leaks molten fluids.

Also see 122182 Fluid deletion when changing/clearing recipes with full pipes.

My search-fu missed those, sorry.