Fluid system

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Fluids are non-solid items, such as water and oil. They can normally only exist inside entities for fluid handling (like pipes), and buildings that have fluids as input ingredients or products (like an oil refinery).

Fluids

"Fluids" tab on the production statistics screen.

The following fluids are available in-game:

Water

Crude oil

Petroleum gas

Light oil

Heavy oil

Lubricant

Sulfuric acid

Steam

Mechanics

Fluids cannot be carried by the player, moved using inserters, dropped on the ground, nor stored in chests, unless the fluids are stored in barrels. They cannot be spilled or even dumped in a lake, and are counted in continuous fractions, rather than discrete integers.
When the player picks up a structure that contains fluids, the contained fluid will try to flow into connected structures and any excess fluid that does not fit is destroyed.

Storage

In the game, fluid is held in entities that behave as vessels (fluid boxes) of a defined size (volume). The vessels automatically connect to each other if their inputs/outputs are adjacent (pipes connect to all directions) and allow fluids to flow between them.

The volume of fluid contained in a fluid box is a value between 0 and the maximum volume. For instance, the pipe can hold 100 units of fluid, therefore the value in the pipe can be a number between 0 and 100. The level of fluid in a given entity is manifested by a percentage of the entity's maximum volume that is being occupied by a fluid. It can be observed in pipes and tanks; they have windows through which the fluid is seen at a certain level, or perhaps even as just a small trickle.

Fluid mixing

GUI of a pipe inside a fluid system containing mixed fluids. Clicking the trash can icon next to a fluid will flush that fluid from the system.

The game will prevent players from accidentally mixing fluids when placing most buildings, e.g. pipes containing different fluids cannot be placed directly next to each other. However, not every possible case of fluid mixing is considered, so the player may still mix fluids accidentally or by purposely working around the building restrictions. A fluid system containing mixed fluids can be cleared of the undesired fluids in the GUI of a pipe or storage tank that is part of that fluid system. Fluids that are flushed from pipes or storage tanks are deleted permanently.

In a fluid system that contains mixed fluids, the alt-mode fluids icons on its pipes/storage tanks will reflect the fluid that is contained in that specific structure. So, it may look like a fluid system only contains one fluid when looking at the alt-mode icons, when in reality it contains multiple fluids which are located in some other connected structures. The GUI of a connected pipe or storage tank will always show all fluids contained in the fluid system.
This means that if the game disallows connecting two pipes that look like they contain the same fluid with "Cannot connect systems with different fluids", one of the fluid systems contains multiple fluids. This can solved easily by opening the GUIs of the pipes that the game refuses to connect which will show the extra fluid and allow flushing it from the fluid system.

Flow

All connected tanks and pipes are treated as a single vessel in that the level of fluid must be equal in all parts, to even out pressure exacted by a higher fluid level on smaller ones. This is why level is also often referred to as pressure, even though pressure is actually caused by a difference in level between two entities. All flow of fluid that happens between pipes is to achieve this balance (pumps practically ignore it and buildings disrupt it; more on that further below). The flow rate between pipes is dependent on pressure (the difference in level between the adjacent entities), it becomes slower as pipes even their levels out.

Coming back to how the 'level' is defined, this also means that all connected pipes and tanks attempt to even out to the same percentage of their respective volumes. For example, if 12,550 units of fluid are left to flow into a storage tank of 25,000-unit capacity with one pipe of 100-unit capacity connected, there will be 12,500 units in the storage tank and 50 units in the pipe, both being filled to the same percentage (50%) of their capacities, even though the amounts themselves are unequal.

Machines that produce fluids put them in their output slots, which are related to a specifically labeled output pipe socket somewhere on the machine (pressing Alt reveals the labels). The slot will attempt to empty itself into the entity connected to the machine's socket, unless it is full, or contains a non-matching fluid. Machines that consume fluids also have an accordingly labeled pipe input socket. If an entity containing the correct fluid is connected to it, the machine will start behaving like a pipe that can never be filled, meaning the fluid from connected pipes and tanks drains into the machine at a fixed rate, until the machine's input slot is full. There may be machines that have pipe sockets for both input and output (like a drill placed over uranium ore). They then drain the fluid for themselves first, and once full, behave as a regular pipe that attempts to even out its level with adjacent entitites. If there are multiple output/input sockets for one fluid on a machine, their activity is distributed to them equally unless some of them are blocked/full.

Temperature

Temperature is currently only relevant in heating water as a medium for power generation. Even though all fluids in the game have a temperature value, it is generally the default 15°C.

Energy, whether harnessed from fuel in boilers, or from nuclear power through heat exchangers, can be used to turn water to steam, being a liquid form of work. Steam holds energy at a ratio of 0.2 kJ per °C per unit. In other words: 0.2 kJ of work is necessary to heat a unit of steam by one °C. Since steam/water is set to have a maximum temperature of 1000°C and minimum of 15°C, the most work that can be done on one unit is 197 kJ.

In practice, this is barely utilized in a great variety: Boilers only output steam of 165°C temperature, and heat exchangers only output 500°C hot steam, never hotter, never colder; if insufficient energy is supplied, the heaters do not output steam altogether. The steam also does not grow colder over time. Using the 165°C steam in a steam engine has the same effect as using it in a steam turbine, although it is impractical, since turbines are made to consume 500°C (superheated) steam, generating proportionally more power. All of this makes for no need of exact calculations.

Transport

Fluids can be transported through pipelines, barrels, or railway. It is generally practical to use piping for short-distance distribution to machines (or barrelling, if there is need to use belts), and railway transportation for longer distances.

Pipelines

Eight pipelines, each carrying a different fluid.

Pipes are the most basic way to channel fluids from A to B. They automatically connect to any adjacent pipe and can do so to all four cardinal directions simultaneously. Underground pipes only work in two opposite directions, linking to another underground pipe on one side, and to another entity on the other. If a pipe section becomes too long without using pumps, all fluid inside it will be "spread thin", resulting in very slow flow and preventing machines to use its contents effectively. Tanks behave the same as pipes, except their volume is much greater, which can cause this inconvenience over a much smaller distance if multiple tanks are used. Underground pipes can help alleviate this issue; although they can connect a distance of up to 10 tiles, their volume is always equivalent to two pipes.

Pumps use electrical power to transfer fluids in one direction very quickly. They also block any back-flow, which means they can pressurize a section of piping, filling it as much as possible. This is very useful to counteract the "thin spread" outlined above, among other things. They can also be disabled using the circuit network which stops fluid flow through the pump.

The table below shows how fast will fluid flow in a pipeline with a certain frequency of pumps. If a higher flow rate is desired, pumps should be placed more frequently. Because underground pipes only count as 2 regular pipes in terms of volume, a full-length section only counts as two pipes in this table, if a pump is placed between each underground section. Placing a filled storage tank before a pump ensures maximum possible flow rate and is therefore a suitable start of any pipeline.

Number of pipes between two pumps	Maximum flow  (u/sec)
0 (pump to pump)	12000
0 (tank to pump)	12000
0 (pump to tank)	12000
0 (pump to boiler to pump)	12000
0 (pump to 2 boilers to pump)	6000
1	6000
2	3000
3	2250
4	1909
5	1714
6	1588
7	1500
8	1434
9	1384
10	1344
11	1312
12	1285
17	1200
20	1169
30	1112
50	1067
100	1033
150	1022
200	1004
201	999
261	799
300	707
400	546
500	445
600	375
800	286
1000	230

Note that all fluid throughput numbers listed above are not directly calculated by the game, but are a result of the underlying fluid mechanics.

Approximate formula for throughput by number of pipes between two pumps:

1 <= pipes < 197:
    flow = 10000 / (3 * pipes - 1) + 1000
pipes >= 197:
    flow = 240000 / (pipes + 39)

The above formula is empirically inferred from the actual measurements, and is not an accurate representation of what happens in the game code. Starting with pipe lengths above 166, its result may be 1 too high compared to in-game measurements with pumps.[1]

Barrels

Barrels are used by Assembling machines to effectively "bottle" fluids into an item that can be handled like any other item; carried in an inventory, placed in chests and handled by Inserters. This allows the player to transport fluids via the belt transport system and the logistic network (as well as the railway, although fluid wagons are also an option on rails). Assembling machines are also used to empty the barrels, depositing their contents to pipes and leaving an empty barrel for another use.

Railway

Railway is another method of transporting fluids, and can be conducted in two ways: Either the fluids are directly pumped into a fluid wagon, or they are poured into barrels and loaded into cargo wagons. Both methods have their advantages:

Advantages of using fluid wagons

• Faster loading and unloading
• Higher capacity (50k vs 20k)

Advantages of using barrels in cargo wagons

• A single cargo wagon can transport multiple types of barrelled fluid (and regular items at the same time)
• Cargo wagons, as opposed to fluid wagons, don't need to be perfectly aligned to be (un-)loaded, allowing for more flexible train station designs

See also

• Oil processing
• Power production
• In-depth post about fluid mechanics
• Another in-depth examination of fluid mechanics