Industrial Gas Pipeline Design

Designing an industrial gas pipeline: sizing materials and safety

A gas pipeline is sized on flow and allowable pressure drop, built from materials compatible with the gas, and fitted with isolation, relief and non-return devices. 2 gases break the general rules entirely: acetylene, capped at 1.5 bar g, and oxygen, which demands cleanliness and velocity control.

What you are actually designing

A gas pipeline has one job: deliver the required flow, at a usable pressure, to the furthest outlet, safely, for decades. Everything else follows from that sentence.

Sizing

Size on flow at the furthest outlet against the pressure drop you can afford getting there.

Pressure drop rises sharply as bore falls, and every bend, valve, tee and fitting adds to it. The worst case is all outlets drawing at once — design for that unless you positively know it cannot happen.

The commonest error is undersizing the main run, discovered only when several outlets are used together. On most gases you can recover by raising supply pressure. On acetylene you cannot, which is the subject of a whole separate rule below.

Materials

Specify against the gas, not the pressure alone:

Acetylene — no copper, no high-copper alloys, ever. They form explosive, shock-sensitive acetylides inside the pipe, slowly and invisibly.
Oxygen — materials, seals and gaskets rated for oxygen service, and nothing general-purpose substituted anywhere.
Moist gases — corrosion resistance becomes a real design input rather than an afterthought.
Everything — no adaptors improvised to make one thread meet another.

Safety devices

Isolation valves somebody can actually reach in an emergency, not behind a machine
Pressure relief wherever a section can be isolated and then heated
Non-return valves wherever two gases could meet at shared equipment
Flashback arrestors at every fuel gas take-off
Drains and traps at low points, where condensate or solvent will collect

Commissioning

Pressure test. Leak test. Purge. Record all three.

Purge fuel gas lines with nitrogen, never air — acetylene ignites from 2.5 % to 81 % in air, so an air purge can build an explosive atmosphere inside the pipework being commissioned.

Label everything

At intervals along the run and at every outlet, naming the gas.

Somebody will work on this system in fifteen years with no knowledge of how it was built, no access to the designer, and no drawings that anyone can find. What is written on the pipe will be their only information. An unlabelled gas line is a hazard handed to a stranger.

The two gases that break the rules

Most of the above applies to any industrial gas. Two do not fit the general case:

Acetylene — capped at 1.5 bar g, absolutely. You cannot solve a pressure-drop problem by raising pressure, so acetylene lines are sized with bore and short runs instead. Get this wrong at design and the only fix is re-piping.

Oxygen — must be cleaned for oxygen service and velocity controlled, because contamination and particle impact can ignite the pipe itself. This often makes an oxygen line larger bore than flow alone would require.

If your installation includes either, read those two pages before sizing anything.

Sources

Designing an industrial gas pipeline — frequently asked questions

How is a gas line sized?

On the flow required at the furthest outlet and the pressure drop you can afford getting there — not on the size of the pipe that happens to be in stock. Drop rises sharply as bore falls, and every bend, valve and fitting adds to it. Size for the worst case, which is all outlets drawing at once, unless you know that cannot happen.

What is the most common mistake?

Undersizing the main run, then discovering it only when several outlets are used together. On most gases you can recover by raising supply pressure. On acetylene you cannot, because the 1.5 bar g ceiling leaves no headroom — which is why acetylene lines are sized generously from the start.

Does the material matter?

Very much. Copper and high-copper alloys are prohibited on acetylene because they form explosive acetylides. Oxygen requires materials and seals rated for oxygen service. Moisture in the gas makes corrosion resistance a real consideration. Specify materials against the gas, not against the pressure alone.

What safety devices does a gas line need?

Isolation valves that someone can reach in an emergency; pressure relief where a section can be isolated and then heated; non-return valves wherever two gases could meet at shared equipment; and flashback arrestors on every fuel gas take-off. Drains and traps at low points where condensate or solvent can collect.

How should a new line be commissioned?

Pressure test it, leak test it, purge it, and record all three. Purge with nitrogen rather than air on any fuel gas line. Then label every run and outlet with the gas it carries — an unlabelled gas line is a hazard that will be inherited by someone who was not there when it was installed.

Should the line be labelled and colour coded?

Yes, at intervals along the run and at every outlet, with the gas named. Someone will work on this system years from now with no knowledge of how it was built, and their only information will be what is written on the pipe.