Corrosion is the dominant threat to boiler system integrity. Left unmanaged, dissolved oxygen in boiler feedwater attacks internal metal surfaces, causing pitting that weakens tubes, drum shells, and economisers from the inside. The damage is not always visible until it fails. Managing oxygen corrosion in boiler water requires two things: removing the oxygen from the feedwater before it reaches the boiler, and verifying that the removal has been effective. Those two functions require two distinct measurements, and confusing one for the other is a costly mistake that many operators make.
What Dissolved Oxygen Tells You
Dissolved oxygen (DO) is oxygen that has dissolved into the feedwater from the atmosphere or from steam condensate returning to the system. Even at concentrations measured in parts per billion, dissolved oxygen is aggressive towards boiler steel. It drives pitting corrosion that creates deep, localised damage at the base of the pit, which is far more dangerous than the uniform surface corrosion that general oxidation produces.
The primary method of oxygen removal in industrial boiler systems is mechanical deaeration. A deaerator heats the feedwater to drive dissolved gases out of solution before the water enters the boiler. When a deaerator is functioning correctly, dissolved oxygen in the feedwater leaving it should be very low. When it is not functioning correctly — due to temperature shortfall, venting problems, or excessive condensate return — dissolved oxygen levels rise.
Measuring dissolved oxygen at the deaerator outlet and at the boiler inlet tells you directly whether your mechanical oxygen removal is working as intended. It is a real-time check on a critical piece of equipment.
What Sulphite Tells You
Sulphite is a chemical oxygen scavenger, most commonly sodium sulphite or sodium metabisulphite, dosed into the feedwater after the deaerator as a secondary defence against any residual dissolved oxygen. The sulphite reacts preferentially with oxygen to form sulphate, neutralising the oxygen before it can reach and attack the boiler metal.
Measuring sulphite residual in the boiler water tells you whether your chemical scavenging programme is maintaining an adequate protective excess of scavenger. The target is not zero — you want a residual to ensure that any dissolved oxygen that escapes the deaerator is captured chemically. Too little residual means inadequate protection. Too much residual is wasteful and, in high-pressure boilers, can introduce sulphate contamination that affects steam purity.
Sulphite measurement does not tell you how much dissolved oxygen is present. It tells you whether your chemical response to potential oxygen is calibrated correctly.
Why Both Measurements Are Necessary
Operators who measure only dissolved oxygen are verifying their mechanical deaeration but have no visibility on whether their chemical scavenging programme is correctly dosed. A deaerator that is performing well may still allow trace oxygen through, and without sulphite monitoring there is no confirmation that the chemical programme is catching it.
Operators who measure only sulphite residual know that scavenger is present in the water, but have no direct confirmation of how much oxygen the system is actually dealing with. If the deaerator develops a problem, sulphite consumption will increase — but by the time that shows up as a depleted residual, oxygen may already have been in contact with boiler surfaces for some time.
The two measurements are complementary. Dissolved oxygen tells you the load your chemical programme has to handle. Sulphite tells you whether your chemical programme is handling it adequately. Neither measurement alone gives you the full picture.
Continuous Monitoring vs Periodic Grab Sampling
Many boiler operators manage both parameters through periodic grab sampling and manual testing. The problem with this approach is that boiler system conditions are not static. Steam demand fluctuates, condensate return rates change, and deaerator performance can vary with load. A grab sample taken once per shift captures a single point in time. It will not detect a deaerator performance drop that occurs and recovers between sampling intervals, and it will not flag a sulphite residual depletion event that happens overnight.
Continuous inline monitoring eliminates these blind spots. With sensors installed at the appropriate measurement points, operators receive a real-time signal for both dissolved oxygen and sulphite residual. Alarms can be configured to alert when either parameter moves outside acceptable limits, enabling immediate corrective action rather than discovery after the fact.
The Pyxis range includes inline sensors for both dissolved oxygen and sulphite measurement, designed for continuous boiler water monitoring in industrial and commercial steam generating systems. Together, they provide the complete monitoring picture that neither parameter can deliver alone.
To discuss which Pyxis sensors are appropriate for your boiler system configuration, contact us at info@autoflotechnology.com.