Every superhero has an arch nemesis. Superhero’s also have sidekicks to help them bring their enemy to justice. One of the biggest enemies of your steam system is oxygen in your feedwater. Among other issues, oxygen present in your boilers feedwater leads to “oxygen pitting” also known as “oxygen attack”.
The dearator or (DA), is your boilers sidekick, helping fight against oxygen attack. Oxygen pitting most commonly attacks boiler tubes – it looks like tiny pinholes on the surface of water tubes. When oxygen is in hot water and under pressure, it is highly corrosive. Even the smallest amounts of oxygen present in boiler feedwater can cause catastrophic failure. For all the chemists out there, oxygen attack is an electrochemical process that can be described by the following reactions:
► Anode: Fe è Fe2+ + 2e-
► Cathode: ½ O2 + H2O + 2e- è 2 OH-
► Overall: Fe + ½ O2 + H2O è Fe(OH)2
In this reaction a temperature rise provides enough additional energy to accelerate reactions at the metal surfaces, resulting in a rapid and severe corrosion.
How to guard against oxygen pitting: Maintain proper DA operation by preforming routine checks:
1. Integrity and functionality of pressure and temperature gauges and sensors
2. Visually check vessel welds for cracks and leaks
3. Visually check for debris and corrosion
4. Semi-annual testing of unit performance – measuring dissolved 02 without chemical feed
5. Evaluate sulfate levels – large swings can be an indication of spray valve malfunction
6. Visually check all valves and man heads
7. Calibrate all instrumentation on an annual basis
Regular checks of O2 levels coming out of the DA: When functioning as specified, your DA liberates the vast majority of O2 in the feedwater – but not all of it. A DA built to today’s industry codes will typically provide water with a dissolved O2 content between the range of 6-10 parts per billion. An oxygen scavenger is required to eliminate all dissolved O2 in the feedwater makeup.