Deaerator Performance
Introduction:A deaerator preheats the boiler feedwater and assists in the removal of non-condensable gases, such as oxygen and carbon dioxide. These gases are undesirable because they promote corrosion in the feedwater system. Deaerators are generally of three basic designs; (1) the spray type, (2) the tray type, and (3) the spray-tray type. There are some vacuum deaertors in use today but they are not very common, and they are not as efficient. These common types mentioned above use-saturated steam to raise the temperature of the feedwater and to provide a scrubbing action for release of the dissolved, non-condensable grease. The trays and sprays are for the purpose of breaking the water up into small droplets and/or mist, thereby exposing more surface area and making the scrubbing action of the steam more efficient.
Proper Operation
When a deaerator is working properly, the water exiting the unit should be within 1°F of the saturated steam at its use pressure, the oxygen content should meet the specifications of the deaerator manufacturer (usually 5 to 7 ppb) and there should be no evidence of corrosion in the feedwater system. A continuous 18-inch plume of escaping steam above the deaerator vent line is recommended. Adjusting the air outlet valve on the vent system can control this plume. Caution should be exercised so as to avoid throttling the valve too much, because air and non-condensable gases will accumulate in the deaerator. This will result in decreased heating and incomplete deaeration. This condition is known as air blanketing and can contribute to a wide variation between the saturated steam temperature and the deaerated water temperature. Before chemicals can economically complete the oxygen removal requirement, the deaerator must be operating properly.
Trouble Shooting
Troubleshooting is important in keeping the deaerator functioning efficiently. Therefore, six of the more common problems will be discussed below.
- Insufficient Heat – This condition is said to exist when the difference between the saturated steam temperature at use pressure and the outlet water temperature is in excess of 3°F. When this condition exists, the first priority should be to make sure the pressure and temperature gages are working properly. Having satisfied this requirement, the following causes and remedies should be investigated:
- Insufficient venting – Increase the vent rate by opening the manually operated air vent valve.
- Steam pressure - Reducing valve operating improperly – Check to see that the valve operates freely, lubricate if necessary – make sure all instrument lines to the valve controller are connected properly.
- Improper spray from nozzle – Check nozzle for partial pluggage from sediment – check for deposits on the seat – check for a broken spring.
- Excessive air in the deaerator – Check stuffing boxes for leaks on pumps upstream of the deaerator – repair leaks or seal with water.
- High or Low Water Level – Improper operation of the inlet water control valve – adjust as required.
- High Pressure – Check the operation of the steam pressure-reducing valve – check the pressure relief valves on the deaerator and in the main steam supply system, for proper operation.
- Low Pressure – Check the operation of the steam pressure-reducing valve and adjust as necessary – check the steam pressure of the main supply system.
- Excessive Steam Pressure Losses Through The Deaerator – Check the trays for sediment and deposits – remove and clean if necessary.
- Vent Plume Disappears Periodically – This could be creating a vacuum each time the plume disappears, thereby pulling outside air into the deaerator – check to see if large amounts of cold makeup water is entering the unit during the time the plume disappears – this kills the steam, reduces the temperature and contributes to inefficient deaeration – the cause could be a periodic loss of condensate, an erratic steam demand, a malfunctioning steam reducing valve or a sticking water level control valve.