Controlling Oxygen Levels in Water to Maintain Boiler Corrosion Protection

Oxygen is a gas that is present in the atmosphere and when dissolved in water can lead to fast failure of industrial boilers systems through oxygen pitting corrosion. Here, Mark Simkiss, Business Development Manager, will explain the background to this in more detail and discuss ways to keep boiler systems protected.

What is oxygen and what is pitting corrosion?

Oxygen is a gas that makes up approximately 21% of the atmosphere. Although critical for life and combustion, oxygen in boiler water systems can be an issue. Oxygen presence allows pitting corrosion to take place. This is an especially dangerous type of corrosion for boiler systems. General corrosion removes an even layer off the top of the metal surface, whilst pitting corrosion happens quickly in isolated spots. Think of it like the difference between sanding the top of a plank of wood evenly all over compared to taking a drill to several small spots. Needless to say, pits going deep into the metal can quickly lead to leaks and, in the worst case, catastrophic failure of the boiler.

How do we stop this from happening?

In order to stop pitting corrosion from happening we need to remove the oxygen or convert it to a safe form. Oxygen naturally dissolves in water. Tap water contains 7-8 parts per million (ppm)pit of dissolved oxygen. Before adding this to the boiler the oxygen is removed from the water. 

To explain this I’m going to use an example. Imagine a can of fizzy drink chilling in the fridge. Inside the drink is gas. In order for the gas to get out the can needs to be opened. If the can is shaken vigorously the gas comes out quicker.

This is the same for water being used in a boiler. To get the gas out of the water, before it is transferred to the boiler, a hotwell or deaerator is used. These need to be vented so that the gas can escape. They also use temperature increase to help remove the gas quicker. This is like shaking the bottle as it causes the gas molecules in the liquid to move faster and allow the gas to get out.

Hotwell temperature for low-pressure boilers is normally controlled to >85℃. This maximises the oxygen removed mechanically and raising it too much higher will risk the cavitation of the feed water pumps. This means there will still be some dissolved oxygen present. In order to remove this remaining oxygen, a chemical is added. The primary chemical used for this is Sodium Bisulphite. This reacts with the oxygen to remove it by forming sodium sulphate.

Na₂SO₃ + ½O₂ → Na₂SO₄

In order to ensure the boiler is protected, the level of the chemical is maintained with a reserve so that if any oxygen transfers through to the boiler, there is a surplus ready to remove the oxygen. This is a key parameter that boiler operators or water treatment providers will monitor on a regular basis to ensure there is enough chemical there. For sulphite, this is normally maintained at a level of 30-70 ppm. 

There are several other options for oxygen removal such as DEHA, Tannin and carbohydrazide. All these chemicals have pros and cons. The chemical that is best for your system will depend on the type of system and its operation. It is best to discuss this with a water treatment expert.

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