Dissolved oxygen control
Dissolved oxygen control
What sort of dissolved oxygen control is exercised to avoid boiler feedwater corrosion?
Re: Dissolved oxygen control
Dissolved oxygen in boiler feedwater is primarily controlled through a two-pronged approach: mechanical deaeration to remove the bulk of the oxygen, followed by chemical oxygen scavenging to eliminate remaining traces.
1. Mechanical Deaeration
Mechanical removal is the first and most critical line of defense, reducing dissolved oxygen to low parts-per-billion (ppb) levels before the water ever enters the boiler.
Deaerating Heaters (Deaerators): These operate on the principle of stripping gases by heating the feedwater to its saturation temperature (boiling point) using steam, per Henry's Law (which states that the solubility of a gas in a liquid decreases as temperature increases and partial pressure of the gas above the liquid drops).
2. Chemical Oxygen Scavenging
Because mechanical deaerators cannot remove 100% of dissolved oxygen and to protect the system against air in-leakage downstream chemical oxygen scavengers are fed continuously into the deaerator storage tank or feedwater line. These compounds react chemically with residual oxygen to form inert products.
1. Mechanical Deaeration
Mechanical removal is the first and most critical line of defense, reducing dissolved oxygen to low parts-per-billion (ppb) levels before the water ever enters the boiler.
Deaerating Heaters (Deaerators): These operate on the principle of stripping gases by heating the feedwater to its saturation temperature (boiling point) using steam, per Henry's Law (which states that the solubility of a gas in a liquid decreases as temperature increases and partial pressure of the gas above the liquid drops).
2. Chemical Oxygen Scavenging
Because mechanical deaerators cannot remove 100% of dissolved oxygen and to protect the system against air in-leakage downstream chemical oxygen scavengers are fed continuously into the deaerator storage tank or feedwater line. These compounds react chemically with residual oxygen to form inert products.
Re: Dissolved oxygen control
Monitoring is important to adjust chemical feed rates dynamically.
Use of Online Oxygen Analyzers placed immediately downstream of the deaerator and at the economizer inlet to provide real-time ppb measurements. Target levels are typically maintained below 5 to 10 ppb for high-pressure systems.
Then pH and Oxidation-Reduction Potential (ORP) Control which is often paired with oxygen scavenging to ensure the feedwater environment remains alkaline (typically pH 8.8–9.2 for mixed metallurgy systems or higher for all-ferrous systems using volatile amines like ammonia or morpholine), preventing general acid corrosion alongside pitting.
Use of Online Oxygen Analyzers placed immediately downstream of the deaerator and at the economizer inlet to provide real-time ppb measurements. Target levels are typically maintained below 5 to 10 ppb for high-pressure systems.
Then pH and Oxidation-Reduction Potential (ORP) Control which is often paired with oxygen scavenging to ensure the feedwater environment remains alkaline (typically pH 8.8–9.2 for mixed metallurgy systems or higher for all-ferrous systems using volatile amines like ammonia or morpholine), preventing general acid corrosion alongside pitting.