Designing an industrial gas pipeline: sizing materials and safety

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.

The word that does the work in it is decades. A pipeline is the one part of a gas installation that nobody revisits. Cylinders are inspected on a cycle, regulators get replaced, plant gets overhauled, while the pipe is buried in a ceiling void and forgotten. Design decisions you make in an afternoon will be lived with by people you will never meet, using equipment that does not exist yet, at flows nobody has told you about. That is the standard to design against, and it is why almost every rule below is about leaving margin rather than meeting today's duty exactly.

Size on flow at the furthest outlet against the pressure drop you can afford getting there. It sounds obvious and it is routinely got wrong, because the number everyone has to hand is the plant's output, not what arrives at the last bench.

Pressure drop rises sharply as bore falls — far faster than the change in diameter suggests — and every bend, valve, tee and fitting adds its own. A run with many fittings can lose more pressure to them than to the straight pipe.

The case to design for is all outlets drawing at once. Assume it unless you positively know it cannot happen, because the day it does happen is the day somebody is trying to finish a job.

The commonest error is undersizing the main run, and its signature is distinctive: everything works during commissioning, when one outlet is tested at a time, and the complaint arrives weeks later when the shop is busy. On most gases you can recover by raising supply pressure. On acetylene you cannot — which is the subject of the last section, and the reason it is worth reading before you size anything.

Pressure rating is the easy half. The gas decides the rest.

Acetylene — no copper, and that includes the alloys. Acetylene forms copper acetylide, which is explosive and shock-sensitive, and it forms slowly and invisibly inside the pipe. A line that has run without incident for years may be accumulating it. IS 14814 is blunt: containers, pipes, valves and fittings of copper or copper alloys — brass and bronze included — are not to be used. Where a standard does permit brass in a specific component, the copper content is capped; IS 13497 puts that cap at 65 percent for the fusible plug on a dissolved-acetylene cylinder, which is a useful benchmark for anything brass that must touch the gas.

Oxygen — everything in contact must be rated for oxygen service. Not "suitable for gas", not "same as we used on the nitrogen line". Seals, gaskets, thread compounds and valve internals all count, and a single general-purpose component substituted during a repair undoes the whole specification.

Moist gases make corrosion resistance a design input rather than an afterthought, and any line that can condense needs a fall and a drain designed in from the start.

Everything — no adaptor improvised to make one thread meet another. Thread standards differ by gas deliberately, so that the wrong regulator cannot be fitted. An adaptor defeats a safety feature that somebody designed on purpose.

Pressure test. Leak test. Purge. Record all three, with dates and readings.

The record matters as much as the test. In fifteen years the question will not be "was this line tested" but "prove it", and an installation with no commissioning file is, for practical purposes, an untested one.

Purge fuel gas lines with nitrogen, never air. Acetylene is flammable in air from 2.5 percent right up to 82 percent, so an air purge does not dilute a hazard, it walks the pipework straight through the middle of the flammable range. Nitrogen purge points should be designed in rather than improvised, and the purge connection must not be shared with anything else.

At intervals along the run, at every branch, 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 whoever designed it, and no drawings anybody can find. What is written on the pipe will be their only information. An unlabelled gas line is a hazard handed to a stranger, and the stranger may be one of your own people after a refurbishment.

Colour alone is not labelling. It is a secondary check at best, it differs between the industrial and medical schemes, and paint fades, gets overpainted and gets patched with whatever was in the store.

Most of the above applies to any industrial gas. Two do not fit the general case, and both need reading before you size anything.

Acetylene is limited by bore, not just by pressure. IS 15190 Part 1 covers acetylene pipelines to a maximum working pressure of 155 kPa g at up to 60 °C, and Part 2 picks up from there for high-pressure runs inside a plant. But the constraint that catches out engineers who know other gases is the diameter: the code restricts bore to DN 50 as the general case and treats DN 100 as an exception requiring justification, and it sets the pressure class of a whole system by its widest section.

The reason is physics, not caution. Acetylene can decompose without any oxidant, and the wider the pipe, the lower the pressure at which that decomposition will sustain itself and travel. The literature records it propagating at and below atmospheric pressure in large-bore pipe. At Hüls in 1954 a decomposition ran through more than two kilometres of 15, 30 and 60 cm main at barely above atmospheric, wrecking plant and injuring people.

So on acetylene the instinct that serves you everywhere else — solve pressure drop with a bigger pipe — is the wrong one. You solve it with shorter runs, fewer fittings, manifolds near the point of use, and flashback arrestors dividing the system into sections that are then designed independently. Get it wrong at design stage and the only fix is re-piping. The full rules are here.

Oxygen must be clean, and slow. It has to be cleaned for oxygen service — free of oil, grease and particulate — and velocity controlled, because contamination and particle impact can ignite the pipe itself. That last point surprises people: the hazard is not the gas burning, it is the pipework burning in the gas. Velocity control often makes an oxygen line larger bore than flow alone would require, which is the opposite of the acetylene rule and a good reason never to size the two from the same spreadsheet. Oxygen line cleanliness is covered here.