Topics on this blog

  1. Theory of Deaeration
  2. Spray and Tray Cascading System
  3. Deaerator Trouble Shooting
  4. Preventive Maintenance of Deaerator

Deaerator is an equipment used for the removal of impurities from feed water like dissolved oxygen, carbon monoxide, carbon dioxide and content of Sulphur.  In addition to removing free oxygen and dissolved gases, a Deaerator provides the advantage of heating boiler feedwater. Expansion and contraction of heating surfaces and the thermal shock are reduced and also a reduction in fuel consumption by adding hot feedwater to a boiler.

 

THEORY OF DEAERATION

  • Dissolved gas is removed by reducing the partial pressure of that particular gas in the surrounding atmosphere.
  • The solubility of any gas dissolved in any is directly proportional to the partial pressure of gas above the liquid.
  • The heating system is provided with trays, steam header inside the Deaerator.
  • The first row of trays are designed to act as a condensing stage which allows only the non-condensable gases to escape and vent losses of steam is appreciably controlled.

The Deaerator is made of two parts:

– In the upper tank, deaeration is done and the water is heated

– In the lower tank, deaerated feed water is stored.

SPRAY & TRAY CASCADING SYSTEM

  • In order to accomplish a high degree of efficient deaeration, water is sprayed through various spray nozzles arranged below the water header.
  • When water enters the nozzle at pressure, fine mist generated by whirling action of vanes.
  • Mist so created passes through various stages of trays arranged in rows.
  • Incoming steam not only heats the tray but also removes dissolved gases.

By the time water reaches

  1. Stage 5 – Devoid of O2& CO.
  2. Stage 7 – Co2
  3. Stage 8 – So2
  • Condensation of steam is reduced to the negligible degree because of already preheated water and most of the steam remains at vapour.

 

DEAERATOR TROUBLESHOOTING/ PROBLEMS IN DEAERATOR

Why Oxygen O2 Level is High in Deaerator?

A heater is specified to ensure minimum O2 levels of 7 ppb.

The Oxygen level can go high in the Deaerator if there are any loose fittings which result in the air infiltration.

The other reasons for High O2 can be because of Trays not installed properly in place, O² inlet not in accordance with specified designed conditions, Water inlet temperature can be too low, Spray valves not installed correctly.

Even if the stabilization period is insufficient the O2 levels will be high. The improper venting can also cause High Oxygen level in a deaeration system.

 

Why the outlet temperature of Deaerator low?

The calibration of the thermometer can be wrong which can show the incorrect Thermometer reading.

If the steam flow is insufficient, Heater will be flooding or if the Inlet Flow is not piped correctly the temperature can indicate low temperature.

The ratio of the Steam and water should be correct for the correct outlet temperature.

If the Pipe and Valve sizing in not correct the spray valves will not function properly to give proper temperature.

It is good to check all valve and control settings, and inlet flows and temperatures periodically to prevent damage.

 

 

Why there is Excessive Pressure fluctuation in Deaerator?

Steam Pressure Reducing Valve, downcomer, and equalizer may be improperly sized or calibrated.

The Inlet steam pressure can be too high or too low or there could be excessive inlet temperature variation to cause fluctuation.

Even if the heater is flooding there could be excessive pressure fluctuation. The design should be within the design range to avoid pressure fluctuation.

 

Why Water Hammering in Deaerator?

Water hammering can happen when the designing is not as per the parameters.

The inlet flows mixing just prior to Deaerator inlet can cause water hammering it is better to design to mix flows farther upstream of Deaerator.

High inlet velocities and Improper pipe design is also the reason for water hammering.

It is good to redesign as per the HEI parameters.

 

Why Carbon Di Oxide CO2 is high in Deaerator?

High CO2 at the inlet then the CO2 would be higher in the Deaerator.

If the ph level of incoming water is High the CO2 will be higher, than, it is good to lower the ph level by controlling the level of dozing in the DM plant to reduce the CO2.

CO2 will be high if the venting is improper.

 

Why Iron Oxide in Deaerator?

A possible reason for Iron oxide in Deaerator could be due to incoming Condensate or system corrosion which can happen if positive pressure is not maintained in the Deaerator.

Frequent shutdowns can cause Iron Oxide in Deaerator.

Higher Oxygen levels in Deaerator can also result in Iron Oxide.

The vent piping should be short and as vertical as possible to avoid Iron Oxide in Deaerator.

The vent flow should be proper to avoid water carryover which could also be the reason for Iron oxide.

 

Why Sudden/ unexpected Deaerator storage tank level excursions?

The reason for sudden storage tank level excursions can be due to the malfunctioning of the control systems.

It is also good to check the overflow level and boiler feed pump operation to rectify the malfunctioning overflow or improper boiler.

Even if there is a Pressure fluctuation the Deaerator tank would be flooding.

 

Why Deaerator Tray Upsets/ Tray Abuse?

Tray abuse is created when much colder condensate suddenly enters the feed box and is sprayed in the spray chamber, rapidly lowering pressures just above the trays. This results in flashing and both steam and water want to move-up rapidly or even violently through the trays.

The typical tray design features a series of level trays having multiple slots with raised lips. In this arrangement water falling vertically in a thin film to the tray below hits the tray surface and must travel horizontally before falling to the next tray.

The Tray Upset problem may arise when the partial pressure above the water in the storage tank drops enough to permit flashing of the stored water allowing a mixture of steam and water to shoot quickly.

 

PREVENTIVE MAINTENANCE OF DEAERATOR

  1. Check the ph level in DM plant
  2. Check for the Thermometer calibration
  3. Check the fittings all valve and control settings.
  4. Check the blocks in vents
  5. Check all inlet flows and temperatures
  6. Check steam supply
  7. Check Steam and water ratio and check for restrictions
  8. Check all valve and control settings
  9. Check Pipe and valve sizing
  10. Check flow level and boiler feed pump operation