Feedwater Tanks
The feedwater tank stores treated water for the boiler, often doubling as a deaerator in smaller plants. Learn about tank design, temperature control, and common problems.
Purpose of a Feedwater Tank
The feedwater tank serves three functions: it stores water to buffer demand fluctuations, it provides a location for water treatment chemical injection, and it can serve as a simple deaerator by heating feedwater with steam sparging to reduce dissolved oxygen.
In smaller boiler plants, the feedwater tank may be the only water treatment step between the water supply and the boiler. In larger plants, a dedicated deaerator vessel is used, and the feedwater tank is the storage section beneath it.
Design Considerations
- Capacity: Typically sized for 20 to 30 minutes of boiler operation at maximum load. Provides buffer for makeup water system response.
- Temperature: Feedwater should be as hot as possible (80 to 90 degrees C) to reduce thermal shock to the boiler and reduce oxygen solubility. Steam sparging or heating coils maintain temperature.
- Material: Steel with internal coating, stainless steel, or fibreglass. Must withstand feedwater temperature and chemical environment.
- Level control: Float or conductivity probe controls makeup water valve. Prevents tank from running dry or overflowing.
- Venting: Tank must be vented to release flash steam and stripped gases. Vent pipe should route to a safe location.
Common Problems
- Cold feedwater: If makeup water is added faster than the heater can warm it, cold water enters the boiler, causing thermal shock and higher dissolved oxygen. Solution: size the heating system for maximum makeup rate.
- Oxygen corrosion: Cold water in the tank absorbs oxygen. Without adequate heating and venting, oxygen passes to the boiler. Solution: maintain temperature above 80 degrees C and ensure the vent is open.
- Level control failure: A stuck makeup water valve can flood or starve the boiler. Install high and low level alarms in addition to the primary level control.
- Chemical injection point: Chemicals should be injected below the minimum water level, not above it. Dosing into the air space wastes chemicals and causes fumes.
Hot feedwater saves fuel
Every 6 degrees C rise in feedwater temperature saves approximately 1% in boiler fuel. If you can raise your feedwater temperature from 25 to 85 degrees C through condensate return and steam heating, that is roughly 10% fuel savings.
Maintenance
- Inspect the tank interior annually for corrosion and scale buildup.
- Clean the tank every 2 to 3 years, removing sludge and debris.
- Test the level controller and alarms monthly.
- Check the heating system (steam sparger or coil) for leaks and blockages.
- Inspect the vent for blockages (insects, debris, ice in cold climates).
- Verify chemical dosing pumps are delivering the correct dose.