Condenser and Deaerator Dissolved Oxygen Exceedance: Causes, Hazards and Corrective Actions

Condenser and Deaerator Dissolved Oxygen Exceedance: Causes and Corrective Actions

Dissolved oxygen refers to the oxygen dissolved in water, measured in µg/L. During operation, if the oxygen content in condensate or feedwater exceeds the value specified in the operating procedures, it is considered dissolved oxygen exceedance. The dissolved oxygen in the condenser and deaerator must be kept within limits; once it exceeds them, adjustment should be made immediately.

Hazards of Dissolved Oxygen Exceedance

1. Oxygen causes oxidation corrosion of metal equipment such as pipes, valves, feedwater pumps, high- and low-pressure heaters, economizers and water walls, producing rust and impurities.
2. Impurities clog filters and valves, affecting feedwater flow.
3. Impurities deposit on heating surfaces, causing scaling, reducing heat-exchange efficiency and potentially leading to tube-wall bursting.
4. Consumption of deoxygenation chemicals increases, raising unit operating costs.
5. Long-term exceedance seriously threatens the safe and stable operation of the unit.

Causes and Treatment: Condenser Dissolved Oxygen Exceedance

• Check whether outside air leaks into the condenser — inspect condenser vacuum, shaft seal pressure, and whether pumps and valves admit air.
• Low unit load, low condensate temperature and low ambient temperature make oxygen dissolve into water very easily.
• Properly increase the sealing water pressure of the condensate pump and adjust the sealing water to trickle evenly onto the condensate pump packing.
• Check the dissolved oxygen of the chemical makeup water; if it exceeds the limit, contact the chemical department to make adjustments.

Causes and Treatment: Deaerator Dissolved Oxygen Exceedance

1. Check whether condensate dissolved oxygen exceeds the limit (same checks as above).
2. Check the opening of the deaerator exhaust-to-atmosphere valve; if too small, open it appropriately.
3. Check whether high-pressure heater drain is normal; a leaking valve can cause oxygen to dissolve into the water.
4. Check whether a large amount of cold water is being supplied to the deaerator — common at low load when boiler slag-cooling water is excessive.
5. Check the deaerator heating steam source; if extraction steam pressure and temperature are low, the water is not heated sufficiently and oxygen is not fully released.
6. Internal structure damage of the deaerator — blocked or fallen nozzles, internal breakage, etc.
7. Large load fluctuation with the deaerator water level too high or too low causes exceedance and incomplete deaeration.
8. Air leaking from the feedwater pump sampling valve — common when the sampling valve is not closed in time during feedwater pump shutdown for maintenance.

Summary of Common Causes

Equipment	Common Cause	Corrective Action
Condenser	Air in-leakage, low load / low temperature, uneven sealing water	Check vacuum and seals, raise sealing water pressure, adjust makeup water
Deaerator	Small atmosphere exhaust opening, low load with low water temperature, leaking drains	Open exhaust valve, raise heating steam, check drains and internals
Feedwater pump	Sampling valve admits air after shutdown	Close the sampling valve in time

Conclusion

Condenser and deaerator dissolved oxygen exceedance can lead to unsafe events, so it must be adjusted immediately once detected. In daily operation, the most frequent causes of deaerator dissolved oxygen exceedance are low water temperature at low load, low ambient temperature, uneven adjustment of condenser sealing water, a small deaerator atmosphere exhaust opening, and air admitted through the feedwater pump sampling valve after shutdown. Correctly identifying the cause allows operators to restore dissolved oxygen to within the qualified range quickly and keep the unit running safely.