Guide
Acetylene pipelines: the one number that governs everything
IS 15190 Part 1 covers acetylene pipelines to a maximum working pressure of 155 kPa g, about 1.5 kgf/cm² g, at up to 60 °C. It recommends restricting bore to DN 50, and caps it at DN 100. Bore is the governing variable, because the permitted pressure falls as the pipe gets wider.
- Maximum line pressure
- 155 kPa g
- Prohibited material
- Copper and high-copper alloys
- Flammability range in air
- 2.5 % to 82 %
- Required at every take-off
- Flashback arrestor and non-return valve
- Purge medium
- Nitrogen only
One number governs the whole design
Most gas pipeline design is a balance of flow, pressure drop, diameter and cost, with pressure as the free variable. If the far end is short of pressure, you raise the supply.
Acetylene removes that option. IS 15190 (Part 1) : 2002 covers acetylene pipelines to a maximum working pressure of 155 kPa g (about 1.5 kgf/cm² g) at up to 60 °C, and Part 2 picks up from 155 kPa g to 2550 kPa g for high-pressure runs inside a plant. That is not a design target to be negotiated upward.
Above roughly that pressure, free acetylene can decompose explosively into carbon and hydrogen, with no oxidant present anywhere in the system. No amount of pipe wall thickness helps, because there is nothing to contain: the energy is already inside the gas.
Rules that matter
You cannot solve pressure drop with pressure. And this is where acetylene catches out engineers who know other gases: you cannot simply solve it with bore either.
On any other service, a wider pipe is the safe answer to pressure drop. On acetylene, a wider pipe is the more dangerous answer: the wider the bore, the lower the pressure at which a decomposition will sustain itself and travel. IS 15190 Part 1 writes that into the design rules directly:
What that means in practice
- Bore is generally to be restricted to DN 50. DN 100 is the ceiling, and the code permits it "only wherever special requirement calls for their use"
- The pressure class of the whole system is set by the section with the largest internal diameter — one wide leg derates everything
- Install a flashback arrestor between sections and each section counts as a separate pipeline system, designed on its own. That is the sanctioned way to keep a wide leg from derating the rest
So the levers that remain are:
- Shorter runs, and fewer of them
- Fewer fittings, bends and restrictions, each of which costs pressure you cannot replace
- Manifolds located close to the point of use rather than centrally for tidiness
- Arrestor-separated sections, so bore can vary without derating the whole system
The most common design error in acetylene distribution is sizing the line as though it were any other fuel gas, then discovering at commissioning that there is no headroom left to fix it. At that point the choice is re-piping or living with poor flow.
Copper is prohibited
Acetylene reacts with copper and high-copper alloys to form copper acetylide, which is explosive and shock-sensitive.
It forms slowly and invisibly inside the pipe, which is what makes it dangerous: a line that has worked without incident for years may be accumulating it.
IS 14814 : 2023 is blunt about it: containers, pipes, valves or fittings made of copper or copper alloys (brass and bronze included) should not be used. Where a brass component is unavoidable and a standard permits it, the copper content is capped: IS 13497 : 2017, which governs the fusible plug on a welded dissolved-acetylene cylinder, requires that the copper content of the brass shall not exceed 65 percent. That is the only place in the acetylene standards where a number is put on it, and it is a useful benchmark for anything brass that must touch the gas.
And, emphatically, it rules out any adaptor somebody found in a drawer and made fit.
Rules that matter
What has to be on the line
- Flashback arrestor at every take-off, and at the source
- Non-return valve with each arrestor, so the other gas cannot enter the line at all
- Traps and drain points at low spots, because acetylene carries acetone and a long or cold run will condense it
- A layout that falls toward the drains, not away from them
- Isolation valves that someone can actually reach in an emergency
- Nitrogen purge connections, because the line will need purging and it must never be purged with air
Liquid solvent collecting in a dead leg eventually reaches a torch, contaminates the flame and damages equipment. Drainage is not a detail on an acetylene line.
Purging
Nitrogen only. Never air.
Acetylene ignites from 2.5 % to 82 % in air, the widest range of any common industrial gas, so purging with air can build an explosive atmosphere inside the very pipework you are trying to make safe. Nitrogen displaces the acetylene without forming a combustible mixture.
This is a requirement, not a preference, and it is the same rule that governs the plants that generate the gas.
Where this sits
The governing code is IS 14814, Acetylene — Code of Safety, alongside IS 11006 for the arrestors. The general principles that apply to any gas line — sizing, materials, pressure drop, testing — are on industrial gas pipeline design. Everything on this page is what changes when the gas is acetylene.
Acetylene pipelines — frequently asked questions
What pressure may an acetylene line actually run at?
For a distribution pipeline, IS 15190 Part 1 covers up to 155 kPa g — about 1.5 kgf/cm² g — at temperatures up to 60 °C, and Part 2 covers 155 kPa g to 2550 kPa g for high-pressure work inside a plant. Note that this is the PIPELINE code. IS 14814 : 2023 cl 7.2.1, the general safety code, is more conservative about free acetylene outside the cylinder: 0.612 atm normally, and 1.021 atm — exactly 15 psig — only with clearance from the Chief Controller of Explosives. Design to the pipeline code, and treat the safety code as the floor for anything that is not an engineered, arrestor-protected pipeline.
How do I get more flow if I cannot raise pressure?
With bore. Every other gas service solves a pressure-drop problem by raising supply pressure; acetylene cannot, so the pipe has to be larger, the run shorter, or the fittings fewer. This is the single most common design mistake — sizing an acetylene line as though it were an oxygen line and then discovering there is no headroom to correct it.
Why is copper prohibited?
Acetylene reacts with copper and high-copper alloys to form copper acetylide, which is explosive and shock-sensitive. It forms slowly and quietly inside the pipe, so a line that has worked for years can be accumulating it. This rules out copper pipe, brass fittings above a defined copper content, and any adaptor someone found in a drawer.
Where must flashback arrestors go?
At every take-off point, along with a non-return valve, and at the source. The arrestor stops a flame travelling back up the line; the non-return valve stops the other gas entering it in the first place, which is what makes a flashback dangerous rather than merely alarming.
How is an acetylene line purged?
With nitrogen, never with air. Acetylene forms a flammable mixture with air from 2.5 % to 82 %, so an air purge can create an explosive atmosphere inside the very pipework you are making safe. Nitrogen displaces the acetylene without forming a combustible mixture.
What about condensate and solvent carry-over?
Acetylene carries acetone with it, and a long or cold run lets it condense. The line needs traps and drain points at low spots, and a layout that falls toward them. Liquid collecting in a dead leg will eventually reach a torch, contaminate the flame and damage equipment.
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