What Is a Deaerator, and Why Does It Matter?

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Water and steam are everywhere in a refinery. They drive turbines, strip hydrocarbons, and carry heat across the plant. But before water can safely become steam, it needs to be cleaned of something invisible and surprisingly destructive: dissolved oxygen.

That is the job of the deaerator, a piece of equipment that looks simple on a block flow diagram but plays a critical role in protecting your boilers and steam system from corrosion.

The Problem: Oxygen in Water

All water contains dissolved oxygen (DO). At low concentrations it seems harmless, but at boiler temperatures oxygen becomes highly corrosive, attacking metal surfaces and shortening equipment life. Carbon dioxide (CO2) is another troublemaker, it dissolves into condensate to form weak acids that drive down pH and accelerate corrosion throughout the steam and condensate system.

The deaerator removes both. It is the last line of defense before boiler feed water (BFW) enters the boilers.

How It Works: Steam Stripping

The most common type is the tray deaerator, which uses stripping steam to physically drive dissolved gases out of the water. Here is the basic sequence:

1. Boiler feed water enters the top section of a smaller vertical vessel, dispersed through spray valves that break the flow into fine droplets. Smaller droplets mean more surface area, and more surface area means more efficient gas removal.

2. Stripping steam flows upward through the same section, contacting the water droplets as they fall across a series of trays. The trays slow the water down and increase contact time between the steam and the liquid.

3. The steam carries the oxygen and CO2 upward and out through a vent to atmosphere. You can identify a deaerator in the field by the characteristic steam plume rising from the vent.

4. The de-aerated boiler feed water collects in a larger horizontal vessel below, which serves as surge capacity for the BFW pumps that supply the boilers.

Oxygen Scavengers: The Backup

A properly operating deaerator can reduce dissolved oxygen to around 7 parts per billion (ppb). That is already quite low, but refineries often go further by injecting an oxygen scavenger chemical into the BFW downstream of the deaerator.

The most common scavenger is sodium sulfite (Na2SO3). It reacts directly with dissolved oxygen to form sodium sulfate (Na2SO4), a water-soluble, non-damaging product that simply passes through the system. Sodium sulfite is inexpensive, easy to handle as a liquid injection, and the reaction products do not create new problems downstream.

Oxygen scavengers act as a safety net: they catch whatever traces of oxygen the deaerator misses, or compensate if the deaerator is underperforming.

What to Look For in the Field

Deaerators have a distinctive profile on any refinery skyline. Look for a larger horizontal drum sitting low, with a smaller vertical vessel mounted on top. The steam plume rising from the top vent is a reliable visual indicator that the unit is running. No plume can be a sign of a problem worth investigating.

The Bottom Line

The deaerator is one of those units that does not get much attention until something goes wrong downstream. Protecting boiler feed water from dissolved oxygen is unglamorous work, but it is essential. Corroded boilers and steam lines are expensive and dangerous. A well-operated deaerator, backed up by oxygen scavenger injection, keeps that risk in check.