Deaerator Pressure and Temperature Changes: Why They Affect Dissolved Oxygen in Boiler Feedwater Systems

Deaerator Pressure and Temperature Changes: Why They Affect Dissolved Oxygen

During normal operation of a turbine unit, dissolved oxygen in boiler feedwater always fluctuates whenever the deaerator pressure or temperature rises or falls. But do you really understand how dissolved oxygen changes, and why? This article explains the mechanics behind these fluctuations and how to keep dissolved oxygen within the qualified range.

The Core Principle Behind Deaeration

First, it is essential to understand the core of deaeration: only by heating the water to the saturation temperature that corresponds to the current pressure can the dissolved gases inside the water be released. Pressure and saturation temperature have a one-to-one correspondence. However, the change of water temperature always lags behind the change of pressure, and this lag is the root cause of dissolved oxygen fluctuation in the deaerator.

Case 1: When Deaerator Pressure Suddenly Rises

When the deaerator pressure rises, the water temperature does not rise in sync. At this moment, the water temperature is lower than the saturation temperature at the current pressure. Because the water has not reached its saturation temperature, deaeration performance deteriorates, and the dissolved oxygen in the water rises.

• Immediate effect: Water temperature lags below saturation temperature → deaeration weakens → dissolved oxygen increases.
• After a period of time: Once the deaerator heats the water up to the saturation temperature at the new pressure, dissolved oxygen falls again and returns to the qualified range.

Case 2: When Deaerator Pressure Suddenly Drops

When the deaerator pressure drops, the water temperature decreases slowly. At this moment, the water temperature is higher than the saturation temperature at the current pressure, so the water is in an “oversaturated” state. In this state, the dissolved gases in the water are more easily released, and the dissolved oxygen decreases.

• Immediate effect: Water temperature lags above saturation temperature → gas readily escapes → dissolved oxygen drops temporarily.
• After a period of time: Once the water temperature gradually falls and matches the saturation temperature of the new pressure, deaeration capacity returns to normal and dissolved oxygen slowly climbs back.

Summary of Pressure Change Effects on Dissolved Oxygen

Conclusion

In short: when deaerator pressure rises sharply, the water temperature lags below the saturation temperature, so dissolved oxygen increases; when pressure drops sharply, the water temperature lags above the saturation temperature, gases are easily released, and dissolved oxygen temporarily decreases. Understanding this lag relationship helps operators anticipate dissolved oxygen fluctuation before it exceeds the qualified range, ensuring stable and reliable deaeration performance in boiler feedwater systems.