TL;DR
Fluids are not lost when deconstructing pipes or tanks, and buildings with fluid inputs will eject any stored fluids if their recipe is changed or cleared, but if you deconstruct a building without clearing the recipe first (or if it has no recipe to clear, e.g. flamethrowers), any fluids are simply lost.What?
When you deconstruct a pipe segment or storage tank (or marked one for deconstruction), any fluid inside will be moved into the rest of the attached fluid system, as long as there's enough room. Similarly, if a building has a recipe that uses fluid ingredients, clearing the recipe will cause any stored fluids to be pushed back out into the attached fluid system (again assuming there's room, and that the attached system doesn't already contain some other fluid). But when you deconstruct a building without clearing the recipe first, all fluids inside are just destroyed.Worse, there are a number of entities which consume fluid inputs but don't have any recipe to clear, such as boilers, heat exchangers, steam engines/turbines, and flamethrowers. For these entities, there is no way to extract the ingested fluids at all. This is particularly egregious in the case with flamethrowers, since the fluids they consume are relatively valuable (unlike boilers and heat exchangers, where you're only losing water) and they consume it so slowly (relative to their storage capacity) and only when there are enemies about, so it's not practical to just cut off the input and wait until they use it up.
This shouldn't happen; a building which is deconstructed or marked for deconstruction should attempt to eject its fluidic contents if possible just as it would if the recipe is cleared, even (especially!) if it doesn't have any manually-set recipe which can be cleared.