Also, could concealed pipes thereby block light? It's a bit jarring, and also breaks mob farms, when the fans and the pipes behind them let in light.
Speaking of fans, while I like that they don't use much steam, I think their buffers - or maybe those of the pipes - are a little too big. I've got a system with twenty fans pushing mobs toward a drop pit, and after cutting the steam flow, they take a LONG time to stop running. Even using another valve pipe as a steam purge takes several minutes. While it makes perfect sense that pipes and devices have capacity, I think the distribution could stand to be skewed somewhat toward the boilers and tanks. As it is, valve pipes may help vent dangerous overpressure, but they aren't nearly responsive enough to do any meaningful control.
Moving onto other things: One of the big things that's kept me from really getting into FSP in the past has been the inability to automate it. Block placers, hoppers, and a redstone clock help greatly by allowing the rock smasher to be automated (and with that done, it's actually one of the faster means of ore processing, albeit not very efficient with default settings compared to other tech mods; I appreciate having that be configurable). I've been trying to think on how some more automation could be implemented while keeping a more steampunk feel.
First: a redstone-sensitive valve pipe would be really nice. It would make sense to be somewhat tricky to craft - a weeny little redstone signal isn't going to turn those valves all by itself, not without some clever amplification - and that would be in order to keep it from trivially replacing rupture discs. It would also allow for somewhat easier remote control of steam mechanisms, although I'm not unwilling to change my pipe layout to have that be my "signal" process - except that casting a lot of brass for long runs gets tedious. I could see this being in the form of a wye pipe, where one end is always open and the signal changes which of the others is open and which is blocked; this would allow it to act as an automatic steam purge if placed appropriately.
Speaking of the tedium of casting a lot: a system for automating crucibles. A little bit of metalwork around crucibles/nether crucibles and molds could produce variants of them intended to link up to a cam shaft. This shaft would run along a line of the things, or possibly in the same block as / part of the modified molds. Put a device on the end that accepts steam to turn the camshaft, and it will rhythmically work the crucibles, then, some seconds later, the molds, spitting the ingots out opposite the crucibles to be collected by hoppers or whatever other mechanism. It'd still take player action or ingenuity to deliver ingots to the crucibles, especially in an appropriate ratio for alloys, but it would take care of the more labor-intensive and boring part.
Steam-driven conveyors would be nice as a more controlled, if more complicated, option for item transport than the fans, and would work really nice for such an automated smeltery, too. Automatically pouring metals from the crucibles to the molds, then parting the molds once the ingots have had time to cool, dumping the ingots onto a conveyor belt, which brings them over to a hopper - I think that'd be a very suitably steampunk-y factory setup.
Speaking of hoppers, a variant that only places one item could be really handy for making a regulated boiler. Perhaps a rotary arm mechanism - feed it steam, and it'll grab an item from the container behind it (or from its own internal inventory, relying on hoppers etc to feed it), placing it in the one in front if the input slot there is empty. This could also sensibly double as a block placer if the input item is a block and the destination space is empty air, meshing well with the rock smasher while being a decent stylistic fit for the mod. It might drain a little steam constantly even if it's trying but failing to place (either because there's something in the way or because there's nothing to place), which would encourage clever use of it - or at least putting it behind a valve pipe and turning it off when not in use. Could have a configurable cycle time and corresponding steam cost by way of the wrench.
Sorry if you've already got plans, or don't intend to introduce this sort of thing at all; I just thought I'd put a little more work into the methods than just saying something like "you should put in some automation." I like the feel of the mod, I just don't like the amount of time spent standing around cycling crucibles and molds once a system gets big.
All blocks that quality can be used for concealment. Chisel blocks fail one of the checks, though no idea which one.