Pipes and pressure

Every pipe you connect together forms one network with one shared pool. A network carries one medium at a time, either a gas (air, steam or exhaust) or water, and it holds one temperature and one pressure over its whole length. Machines are not part of the run: a boiler, an engine or a pump sits beside it and pushes into or draws out of the pipe that meets its port face.

The numbers

fact	value
capacity per pipe	30 L at 1 atm
gas pressure	stored volume divided by capacity, read as gauge atm
iron pipe burst pressure	5 atm
steel pipe burst pressure	10 atm
grace at burst pressure before a pipe lets go	30 s
gas bled by an open end	8 L/s from the whole run
water drained by an open end	10 L/s
gas drawn by a chimney on a passthrough or outlet	16 L/s per chimney
evaporation from a water run	50 L per in-game day

Pressure is the stored volume divided by the run's capacity, and the capacity is 30 L per pipe. A ten-pipe run reads 1 atm at 300 L and 2 atm at 600 L; an empty run reads 0. The look-at line marks it (g), a gauge reading, so steam temperature is taken one atmosphere higher: 100 C times the fourth root of the reading plus one, which is 119 C at 1 atm, 132 C at 2 atm, 157 C at 5 atm and 190 C at 12 atm.

Water behaves differently. It cannot be packed past the run's capacity, so a water line reads its fill ratio while it is filling and jumps to the delivery head of the pump feeding it once it is brim full, 1 atm from the Manual Fluid Pump, 1.5 atm from the Mechanical Fluid Pump, and the engine's inlet steam pressure times 0.75 from the Fluid Pump.

A gas run that nothing is drawing from cools 2 C per second toward 20 C, and a leaking one cools 5 C per second on top of that.

Bursting, and the passthrough limit

Only the four plain shapes take part in over-pressure failure: Piping (Straight), Piping (Bend), Piping (T-Junction) and Piping (X-Junction). The weakest of them sets the ceiling for the whole run, so one iron segment caps an otherwise steel line at 5 atm. Hold that ceiling for 30 s with nowhere to vent and one pipe at that rating, chosen at random, lets go: it drops its materials, its cell becomes air, and the run splits in two at the gap. Any relief that drops the pressure resets the 30 s.

Everything else on a run is exempt by design. Pipe Passthrough (Straight), Pipe Passthrough (Bend), Pipe Outlet, Piping (Valve), Piping (Pressure Valve), Fluid Intake and the tuyeres and ports of Steelmaking Expanded neither burst nor cap the pressure. A run built only from those has no rating at all and will never burst, whatever it reaches; a run with one plain pipe in it is limited by that pipe, wherever in the line it sits.

That matters most on a steam main. A Lancashire Boiler chokes at 12 atm, above the 10 atm of steel pipe and far above the 5 atm of iron, so a Lancashire line that is left with nowhere to send its steam will burst its own pipes. Keep a consumer or a pressure valve on it.

Open ends

Any connector left facing air leaks. Gas costs the run 8 L/s and water 10 L/s, and the rate is for the whole network, not per opening, so a second hole does not double it. A leaking run is also capped at 1 atm however hard it is fed: a producer is held to that ceiling while any end is open, so a Watt Engine, which engages at 2 atm, never starts on a line with a hole in it. A run fed slower than it leaks runs down to empty instead, and that is the usual reason a line reads 0 atm while a boiler is plainly boiling.

A chimney is the exception. Stand an ordinary chimney on the open top connector of a passthrough, a passthrough bend or an outlet and the network vents 16 L/s through it instead of leaking 8 L/s, smoking while it does. That is the way to exhaust a firebox: 16 L/s is exactly what a boiler makes.

Laying a run

1. Choose the material for the pressure you expect, not the pressure you have today. Iron carries 5 atm, steel 10 atm, and mixing them means the iron decides.
2. Place pipes toward the machine. Straights, bends and junctions orient themselves to their neighbours as you place them.
3. Turn a piece that guessed wrong with a wrench. The wrench cycles only the orientations that are valid in that cell, so a piece with one valid orientation will not turn at all: break it, place the neighbour that decides its shape, and place it again.
4. Butt the last pipe straight against the machine's port face, or end the run in a Pipe Outlet against it.
5. Cap or connect every other end. A run with an open end will not pressurise.
6. Check the result with .exmod network hi. Every network is tinted its own translucent colour, the colours hold still as you work, and the highlight refreshes about four times a second, so a run that is secretly two networks shows up at once. .exmod network unhi turns it off.

What goes wrong

The whole run reads 0 atm or 1 atm and the machines will not engage. Something is open. Walk the run with the highlight on and look for the connector nobody built against: a valve with nothing past it, an outlet facing a wall it does not touch, a junction with a spare arm.

Two runs that should be one are two colours. A pipe between them has no connector facing the other, or a closed Piping (Valve) sits in between. A closed valve severs the network at its own cell by design.

The run carries the wrong thing. A network claims its medium from the first producer and holds it until it empties, and a water run refuses gas outright. Where two gas runs merge, exhaust outranks air, so joining a firebox line into a blast line turns the lot into exhaust. Split them with a valve or a pressure valve.

Pipes burst on login or after a restart. A pressure spike on chunk load has been reported to put a whole plant past 20 atm and burst pipes and boilers that had relief valves fitted. There is no fix as of 2026-09-14. Vent and stop a pressurised run before a restart rather than trusting it to survive one.