Nitrogen and Drying

Dew point criteria and the soak test rebound

Drying reads as a falling outlet dew point punctuated by rebounds. The rebounds are not failures; they are the measurement working correctly.

The criterion

Acceptance is stated as an outlet dew point — a temperature at which the gas leaving the line would begin to condense water. Lower is drier. Criteria are set by what the line will carry and by the operator’s specification.

A criterion without a measurement method and location is not a criterion. Where the probe sits, and at what pressure the dew point is referenced, both change the number.

Why the soak test exists

Sweeping gas through a line dries the surfaces the gas touches first. Water held in low points, under debris or in dead legs migrates outward only slowly.

Stop the sweep and let the line stand — the soak — and that trapped moisture redistributes into the gas. Resume the sweep and the dew point reads higher than when you stopped. That is the rebound.

Reading the rebound

A large rebound means substantial removable moisture is still present. Keep drying.
A diminishing series of rebounds means the removable moisture is being consumed. This is the signature you want.
A rebound that will not diminish suggests a source that the sweep is not reaching — a pocket, a dead leg, or an ingress.

The pattern across successive soaks carries the information, not any single reading.

What final acceptance requires

Two things together: the drying must cross the criterion, and the final post-soak reading must remain within it.

Crossing the criterion during an active sweep and then declaring success is the common error. The sweep is holding the number down; the soak is what tests whether the line is actually dry.

Write the soak duration inA soak long enough to be meaningful is specified in advance. Left to the schedule, it shrinks to whatever time remains.

Records

Dew point traces, soak start and end times, ambient conditions and probe locations all belong in the record. Drying is a phase that gets questioned later, and the trace is the evidence.

Standing noteThis is educational reference material, not engineering advice. Regulations are revised continually; the editions in force at the time of a project govern, and every project requires its own qualified engineering analysis and the operator’s governing specifications.

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