Drying is the step people plan last and regret first.

It gets treated as cleanup, as the thing that happens after the real work. Then the dew point will not come down, the window closes, and suddenly the cheap afterthought is the reason the line is not in service. Here is how to think about it properly.

Why Drying Is Not Optional

Residual water in a commissioned line causes real problems, and they are not theoretical.

Water plus hydrocarbon plus pressure and low temperature gives you hydrates, which are solid, and which plug things. Water plus CO2 or H2S gives you acid, which corrodes from the inside where you cannot see it. Water in a cryogenic service freezes. None of these show up on day one. They show up later, expensively.

That is why the dew point spec exists, and why it is not a formality you can talk your way out of.

What the Dew Point Number Actually Means

Dew point is the temperature at which the water vapor in the line would condense back into liquid. If your line’s internal dew point is minus 40 degrees F, that means the gas inside is dry enough that water will not condense out until you get down to minus 40. The colder the number, the drier the line.

Common targets:

• Minus 20 degrees F. Adequate for a lot of general transmission service
• Minus 40 degrees F. Common spec for natural gas transmission, and a frequent contract number
• Minus 60 degrees F and below. Cryogenic, LNG, and specialty process service

The difference between minus 20 and minus 40 sounds small. In gas volume it is not. Each step colder takes disproportionately more dry nitrogen, because you are chasing the last of the moisture off the pipe wall.

How the Drying Actually Works

It is unglamorous. You flow dry nitrogen through the line. The nitrogen picks up moisture from the wall. That moisture leaves out the far end. You measure the dew point of what is coming out. When it hits spec, you are done.

The gas going in is dry, 99.5 percent N2 or better. The gas coming out starts wet and gets progressively drier as the wall gives up its moisture. Watching that curve flatten is how you know you are close.

What Drives How Long It Takes

Four things, in roughly this order of importance.

Internal volume. Not length. A 20 inch line holds vastly more than an 8 inch line of the same length, and volume is what you are exchanging.

How dry you need it. Minus 60 is not twice as hard as minus 30. It is much worse than twice.

How well the dewatering went. If the pig train left standing water, you are now trying to evaporate puddles with gas flow, which is a terrible way to spend a day.

Whether you have to stop. This is the one that kills schedules. Continuous flow gets you there. Stopping to refill does not just pause progress, it partially undoes it, because moisture re-equilibrates off the wall while you sit. Reaching minus 40 in 14 hours, like we did on one 18 hour window, is mostly a story about not stopping.

The Mistakes I See Most

Sizing gas against pipe length instead of internal volume. The most common one, and it always comes up short.

Skipping a proper dewatering run. Trying to dry a line that still has standing water in the low spots. You will be there a very long time.

Measuring dew point at the wrong place. You want it at the outlet, on the gas leaving the line. Measuring at the trailer just tells you your nitrogen is dry, which you already knew.

Not knowing the spec until the crew is on site. The dew point target determines how much gas the job needs. Finding it out on the day is finding it out too late.

Frequently Asked Questions