How to "Top Up" CMSHow to Top Up (Refill) Carbon Molecular Sieve (CMS) in N2-GEN TS, CS, and S Series Nitrogen Generators

Applies to: N2-GEN TS, CS, and S Series industrial PSA nitrogen generators 
Your unit may or may not have bleed valves behind the bed gauges. Both cases are covered in the steps below.

Carbon molecular sieve (CMS) settles and attrits slightly over time, which can open a small void at the top of a bed and reduce nitrogen output or purity. Topping off restores full bed packing. If you've measured a void in your CMS and have received additional material, follow the steps below to safely refill the sieve beds.

Topping up vs. complete replacement

Two different jobs keep a PSA generator's sieve beds healthy, and it's important to know which one you need. Topping up (this article) adds a small amount of fresh CMS to fill the void that forms at the top of a bed as the media settles. The bulk of the CMS is still good, so you are simply restoring full bed packing. Complete replacement removes the entire CMS charge and replaces it with fresh material, and is the right call once the media itself has aged, broken down, or been contaminated, because at that point no amount of topping up will bring purity back.

The quick rule of thumb: if you've measured a void but purity is still near spec, top up. If purity won't hold even after a top-up, or you're seeing heavy carbon dust or the beds have taken on moisture, the media is spent and needs to be replaced.

	Top Up (this article)	Complete Replacement
What it is	Add fresh CMS to fill a settling void at the top of the beds.	Remove and replace the entire CMS charge.
When to do it	You've measured a void, but purity is near spec and the media is otherwise healthy.	Purity won't hold even after topping up, you see heavy carbon dusting, the media is aged or exhausted, or the beds were exposed to moisture.
Typical trigger	A routine level check finds headspace; gradual, minor purity drift.	End of CMS service life, a contamination event, or performance a top-up can't recover.
Frequency	Check levels annually; top up as needed (often on the order of every 36 months).	Infrequent, and driven by media condition rather than a set schedule.
Effort & downtime	Short; a small amount of material.	Larger job; full bed evacuation, disposal per the CMS SDS, and more downtime.
Procedure	Follow the steps in this article.	See Large-Scale PSA CMS Removal and Replacement Procedure.

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⚠ Warning — Stored Pressure Sieve beds hold pressure even after the generator is switched off. Never loosen an inspection port plug or open the unit until the bed's pressure gauge reads 0 PSI. Pulling a safety relief valve releases high-pressure gas and can eject debris, so wear eye protection and keep your face and hands clear of the vent.

Before you begin:

• Isolate the feed-air supply at the inlet valve and lock it out. Where your site policy requires it, also lock out the electrical disconnect.
• Have fresh, factory-sealed CMS of the correct grade and quantity on hand. Keep the container sealed until you are ready to pour.
• Wear safety glasses, gloves, and a dust mask. CMS is a fine adsorbent and produces dust when handled.
• Work in dry conditions and keep each bed open for as little time as possible. CMS rapidly adsorbs moisture from the air, which degrades performance.
• Have a clean funnel ready.

Refill Instructions:

1. Close the inlet valve supplying feed air to the nitrogen generator.
2. Turn off the generator using the system controller.
3. Depressurize the sieve beds:
   • If your unit has valves on each sieve bed, open the valves located behind the pressure gauges until the pressure reads 0 PSI, then close the valves.
   • If your unit does not have these valves, carefully pull up on the safety relief valve at the top of each bed to release the pressure.
4. Slowly remove the inspection port plug on the top of Sieve Bed A, allowing any remaining pressure to escape.
5. Using a funnel, carefully fill the bed with CMS until the material reaches the top of the threads.
6. Reinstall the plug on Sieve Bed A.
7. Repeat steps 4 through 6 for Sieve Bed B.
8. Turn the generator back on.
9. Open the feed air inlet valve.
10. Allow the unit to run for approximately 1 hour.
11. Repeat steps 1 through 10 until no additional CMS material can be added to either bed.

▲ Attention — Sealing the Ports Before reinstalling each inspection port plug, clean any CMS granules from the threads so the plug seats and seals properly. After the unit is back up to pressure, check each port for leaks with a soap solution and re-seat any plug that shows bubbles.

Tip: Topping off may take multiple cycles of depressurization and running to allow the CMS to settle.

ℹ Note — Purity After Refill Fresh CMS needs a short period of cycling before it reaches full performance. After restart, monitor nitrogen purity (or the O₂ reading) and vent the generator to atmosphere until purity returns to your setpoint. Do not send off-spec nitrogen to your process line. If purity does not recover after several cycles, open a support ticket.

Related Articles

• Large-Scale PSA CMS Removal and Replacement Procedure
• How Frequently Should I Measure My CMS Levels and What's the Process?
• How Long Does CMS Last in a PSA Nitrogen Generator?
• N2-GEN TS, CS, and S Series Preventive Maintenance
• Industrial PSA Nitrogen Generator Service Cheat Sheet

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