How to Purge and Leak-Check an Analyzer Sample Line
Before any process sample is allowed into a new or reworked analyzer sample line, the line has to be proven tight and cleared of air, both for measurement integrity and for safety with flammable or toxic streams. This procedure is for the technician commissioning or returning a sample line to service. It covers the integrity leak check that confirms the line neither loses sample nor draws in air, and the inert purge that displaces the air before process gas arrives. The order matters: prove it tight, purge it inert, then introduce sample, and never the other way around when the sample is hazardous.
Purge and Leak-Check a Sample Line in one line: To purge and leak-check an analyzer sample line, first leak-check the line for integrity by pressurizing or evacuating it to the specified test condition and confirming it holds without decay, repairing any leak found. Then purge the line with an inert gas such as nitrogen to displace the air and any moisture before process sample is introduced, which prevents an explosive mixture with a flammable stream and protects an oxygen-sensitive analyzer. Only after the line is proven tight and inerted do you slowly introduce the process sample.
Isolate the Line and Gather the Test Setup
Start by isolating the sample line so you can test it as a closed system. Close the isolation valve at the tap and at the analyzer so the section under test is defined, and confirm you know every branch, drain, and vent on that section, because a leak check only means something if the boundaries are known and closed. Gather the test setup the analyzer manufacturer or site procedure specifies: a regulated source of clean test gas or nitrogen for a pressure test, or a vacuum source for a vacuum test, plus a gauge sensitive enough to see a slow decay over the test interval.
Decide pressure or vacuum test based on how the line runs in service. A line that operates above atmospheric pressure is naturally tested by pressurizing it, since that is the condition it sees; a line that operates under vacuum, such as an aspirated or eductor-driven path, must be vacuum-tested because a leak that draws air in under vacuum may seal under pressure and hide. This same air-ingress concern is why an oxygen analyzer path, described in the note on a trace oxygen analyzer, is leak-checked so tightly, since an inward leak reads as a false high. Confirm the components in the line can take the test pressure or vacuum before you apply it.
Run the Integrity Leak Check
Apply the test condition and let the line stabilize, then watch for decay over the specified interval. For a pressure test, pressurize to the test value, isolate the source, and confirm the pressure holds without a measurable drop; a falling pressure means gas is escaping through a leak. For a vacuum test, evacuate to the test level, isolate, and confirm the vacuum holds without rising; a rising pressure means air is leaking in. Allow for a brief thermal settling period, because a gas that was just compressed or expanded changes temperature and the pressure will move slightly until it equilibrates, which is not a leak.
Locate and repair any leak before proceeding, working the joints systematically. Sample-line fittings are small-bore and easy to under-tighten, cross-thread, or leave with a missing ferrule, so the leak is usually at a connection rather than in the tubing. A leak-detection solution on fittings for a pressurized line, or a systematic re-check of each joint for a vacuum line, finds most leaks quickly. Do not accept a line that nearly holds; a slow leak that fails to seal a hazardous sample or that draws air into an oxygen measurement is a real defect, and the whole point of the check is to catch it now rather than in service.
Inert-Purge to Displace the Air
With the line proven tight, purge it before process sample enters, because a new line is full of air and introducing a flammable process stream into that air can create an explosive mixture inside the tubing. Flow an inert gas, typically nitrogen, through the line to displace the air, venting to a safe location, and continue the purge long enough to exchange the line volume several times so the residual oxygen is driven down. For a hazardous stream this inert purge is a safety step, not just a cleanliness one, and the site procedure governs how thoroughly it must be done.
The purge also protects the measurement and the analyzer. Displacing air clears the oxygen that would corrupt a first reading on an oxygen-sensitive analyzer and removes ambient moisture that could condense or bias a dew-point or moisture measurement. Where the analyzer is especially sensitive, the purge continues until the analyzer itself reads the inert gas cleanly, confirming the air is gone all the way to the cell. Keep the purge flowing until you are ready to introduce sample, so the line is not left open to back-diffusion of air.
Introduce Sample and Verify the Result
Only now introduce the process sample, and do it slowly and deliberately. Open the tap isolation gradually so the process stream displaces the inert purge rather than slamming into the line, watch the analyzer come alive on real sample, and confirm the reading settles to a plausible process value. A line that was tight, inerted, and then filled cleanly should give a first reading you can trust; a first reading that reads high on oxygen or shows unexpected moisture suggests the purge was incomplete or a leak was missed, sending you back to the leak check.
Record the test method, the hold result, and the purge as the commissioning evidence for the line. When the analyzer output and the sample-system diagnostics are trended in a monitoring platform such as Merobix, the clean first reading on a properly commissioned line becomes the baseline against which a later air-ingress leak or a moisture intrusion is recognized, because a line that starts clean and later drifts toward an oxygen or moisture offset is telling you a fitting has loosened or a component has failed. Proving the line tight and clear now is what makes that later signal readable.
Frequently Asked Questions
Should I leak-check a sample line with pressure or vacuum?
Test it the way it runs in service. A line that operates above atmospheric pressure is pressure-tested, since that is the condition it experiences, while a line that runs under vacuum, such as an aspirated path, must be vacuum-tested because an inward leak that opens under vacuum can seal under pressure and hide. The vacuum test catches exactly the air-ingress leaks that corrupt oxygen and dew-point measurements, so match the test to the operating condition rather than defaulting to pressure.
Why purge an analyzer sample line before introducing process gas?
A new or opened line is full of air, and pushing a flammable process stream into that trapped air can form an explosive mixture inside the tubing, so an inert purge with nitrogen displaces the air first as a safety step. The purge also clears the oxygen and ambient moisture that would corrupt a first reading on an oxygen or dew-point analyzer. You exchange the line volume several times, venting safely, before slowly introducing the real sample.
In what order do I leak-check, purge, and sample a new line?
Prove it tight, purge it inert, then introduce sample, in that order. Leak-checking first confirms the line neither loses hazardous sample nor draws in air. Purging with inert gas next displaces the air so a flammable stream cannot form an explosive mixture and so an oxygen-sensitive analyzer starts clean. Only after both are done do you slowly introduce the process sample. Reversing the order, especially sampling before purging a flammable stream, is exactly the hazard the sequence exists to prevent.
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