If you’re an Engineer or Facilities Manager, you will probably already know that steam boilers can suffer from corrosion and that this can mean serious problems – blockages, failures, leaks, and even complete failure. It can cost you in downtime and repairs.
Fortunately, the answers to reducing corrosion so that your system life is maximised can be surprisingly simple.
WHAT CAUSES CORROSION?
There are many potential causes which are described very well here https://www.suezwatertechnologies.com/handbook/boiler-water-systems.
We are going to focus on the main causes – OXYGEN and ACID
Water contains oxygen and if you think about it this makes sense, otherwise living organisms such as fish and plants wouldn’t survive in rivers and oceans. The problem is that oxygen causes corrosion of metal in water systems.
Fortunately, you can remove the oxygen from the water before it enters the steam boiler by:
1. Physical de-aeration – this just means heating the water.
This graph shows the oxygen content of water at different temperatures. The British Standard recommends 80˚C – 85˚C.
This is achieved using steam injection and condensate return so you will need a feedtank of the correct design.
2. Chemical de-aeration – adding chemicals which remove the oxygen (called oxygen scavengers). The most commonly used chemical is sulphite but there are others with different advantages and disadvantages.
But what about the costs?
There is a cost in energy of raising the temperature of the feedtank, but this is off-set by the reduced water temperature increase in the boiler. The main cost is in chemicals but (here’s the most important part) – increasing the feedwater temperature reduces chemical usage by a significant amount. Raising the feed tank temperature from 60˚C to 85˚C could reduce costs by 47%!
Here’s the calculation:
Other than the oxygen content, the main thing to focus on is the pH/Alkalinity of the water in the boiler. To get this right, you need the boiler water pH to be between 10.5 and 12. This allows a protective film of magnetite to form inside the boiler. To achieve this, you dose an alkalinity builder chemical (caustic) to the system.
HOW DO YOU REDUCE CORROSION?
We recommend you keep these 2 points in mind:
- Make sure you are reducing the oxygen in your system as much as possible by heating the feedtank (hotwell) to 80˚C – 85˚C.
- Regularly test the boiler water pH and Alkalinity – dose caustic to increase the pH if needed.